There is a grand tradition of scientifically minded science fiction authors creating not just the characters in their novels but also the brass tacks scientific details of the planets they reside on. This is the art and science of Worldbuilding.

Ignore this and you will be perpetually writing about adventures on Planet California.

Most notable are authors Hal Clement and Poul Anderson. You may want to read Clement's Mission of Gravity and Close to Critical. And Anderson's A World Named Cleopatra and How to Build a Planet.

The best handbook I've managed to find is World Building: A writer's guide to constructing star systems and life-supporting planets by Stephen L. Gillett, Writer's Digest Books, ISBN # 0-89879-707-1. The hardcover version is out of print but it is available in eBook format from ReAnimus Press.

One of the most impressive examples of worldbuilding was the collaborative effort Medea: Harlan's World.

For more Worldbuilding tutorials, be sure to try watching Artifexian's YouTube channel of instructional videos on the topic.

When you are trying your hand at worldbuilding, please try to avoid ice planets, desert planets, swamp planets, farm planets, volcano planets, and other single-biome planets. The pejorative term for this mistake is Monocosm (term invented by Roz Kaveney). Jerry Pournelle parodied this trope with the phrase "It was raining on Mongo that morning"

Worldbuiding can use top-down or bottom-up design, or even a combination of the two.

The major elements of worldbuilding are:

The elements at the bottom are more important, the author can decide how high up they want to go on the list.

The most important guiding principle is Internal Self-Consistency. If the planet Trantor is totally covered with a single mega-city, you'd better figure out where the crops are being grown.

The quest for self-consistency brings with it the side task of always being on the lookout for Unintended Consequences.

Bill The Galactic Hero

Veneria ... a fog-shrouded world of untold horrors, creeping in its orbit around the ghoulish green star Hernia like some repellent heavenly trespasser newly rose from the nethermost pit. What secrets lie beneath the eternal mists? What nameless monsters undulate and gibber in its dank tarns and bottomless black lagoons? Faced by the unspeakable terrors of this planet men go mad rather than face up to the faceless. Veneria . . . swamp world, the lair of the hideous and unimaginable Venians . . .

From Bill The Galactic Hero by Harry Harrison (1965)


For the vast majority of science fiction worldbuilding, the major alteration to the laws of physics is allowing some species of faster-than-light propulsion for their starships. Others will add things like psionics/psychic abilities. But besides those, the rest of the laws of physics operate exactly as in real life. This makes life easier on the science fiction author, and on the reader. Note that the capabilities and limitations of your FTL drive have major implications on worldbuilding.

Others writers will go to the trouble of creating a single major alteration of physics because they intend it to be the center point of the entire novel. For example, in Arthur C. Clarke and Michael Kube-McDowell's novel The Trigger, a newly discovered law of physics allows the creation of a gadget that renders firearms useless. The novel is about the everybody coming to terms with a world without guns.

Finally there are the very few physicists or for-all-intents-and-purposes physicists who turn the physics-smashing up to 11 and actually re-write most of laws of physics. Readers will often find their minds blown. This is uncommon since most authors do not have the knowledge to do this, nor the inclination to graduate thesis levels of work just for a stupid novel. The authors that do apparently do so because they think it is fun.

In his novel Raft Stephen Baxter shows us the adventures of a group of humans who have entered an alternate universe where the gravitational constant is about a billion times as strong as in our universe. There are no planets (since they collapse under their own gravity), stars are only a mile across and have very brief life-spans, and gravity is the dominant force in chemistry and other atomic-scale events.

In his trilogy Orthogonal Greg Egan writes about a universe where rather than three dimensions of space and one of time, there are four fundamentally identical dimensions. He had to explain about the physics of that weird place in an article. Light has no universal speed, and its creation generates energy. So plants make food by emitting their own light into the dark night sky.

In their webcomic Unicorn Jelly Jennifer Diane Reitz shows daily life of some humans in an alternate universe where the laws of physics are slightly different, humans need to eat a plant called "Vlax" in order for their metabolism to work, even the periodic table is weird.

In his novel Celestial Matters Richard Garfinkle writes in a hard-science science-fiction universe. It is just that the science is Ptolemaic astronomy and Aristotelian physics. The Earth really does lie at the center of the universe, surrounded by crystal spheres which hold each of the planets, the sun and the moon, all enclosed in the sphere of the fixed stars.


The Galaxy

Most of the equations here came from World Building: A writer's guide to constructing star systems and life-supporting planets.

Depending upon how focused your novel or game is, you may or may not care about the area around the planet you are creating. You do care if it is important to know if the planet is at the hub of an interstellar trade route, occupies an unfortunately soon-to-be strategic location, depends upon off-planet imports for survival, or is smack-dab in the middle of the invasion path for the Blortch Extermination Fleet.

In that case, look into the page about making an interstellar map. If an interstellar empire or two is involved, do some calculations to establish the size and dimensions.

Our galaxy (and presumably other galaxies) have a Galactic habitable zone. In our galaxy stars that are further than about 10 kiloparsecs (32,600 light-years) from the galactic center are too poor in heavy elements to support the creation of life. Stars that are closer than 4 kiloparsecs (13,000 light-years) to the galactic core have their planets regularly fried by deadly radiation from nearby supernovae and gamma-ray bursters. The in-between section is the "Goldilocks Zone", with our Sol unsurprisingly right in the middle of it. The distances are different for larger or smaller galaxies, but details on the particulars are scanty.

Doing It The Easy Way

If you want to avoid doing all the math, there are several role-playing games that have systems to generate stars, solar systems and planets by rolling dice and looking up results on tables. Some of them are remarkably well-researched and certainly scientifically good enough if you are in a hurry. They are certainly better than just making everything up.

When it comes to scientific accuracy, both James Cambias and Jon Zeigler are names you can trust.

A good online solar system generator is Jochen Linnemann's Voyager.

A good tried-and-true standby is the ACRETE algorithm. StarGen is an executable for Windows, Mac OS and Unix implementing the ACRETE algorithm. There is an online version as well. It just assigns orbits and generates planetary sizes, you'll have to do the rest of the work yourself.

If you use any of these, you can skip the rest of this section. But you might want to skim it anyway to get a broad idea of what is going on.

The Primary Star

You will be very interested in the primary star of the solar system your planet inhabits. It will have a major influence upon the living conditions on your planet. Specifically you need to know the star's Bolometric Luminosity, spectral class, age, and whether or not it is a binary (or trinary) with other stars.

Oh, and you do NOT want the primary to be a variable star. Planets that are irregularly baked then frozen by their primary are not good prospects for habitability.

Our star Sol is spectral class G2V. Back when I was young it was thought that habitable planets could only be found around stars of spectral class early F or G to mid-K. Recently it was discovered that many spectral class M stars were also good.

Some Properties of Main-Sequence Stars
  • Spectral Class: spectral classification
  • B-V: B-V color index
  • Teeff: effective temperature (Kelvin)
  • AbsMag Vis.: absolute magnitude (visual)
  • AbsMag Bol.: absolute magnitude (bolometric)
  • Bolo. Corr.: bolometric correction (add to absMag visual to get absMag bol)
  • Lum: luminosity (Sol=1)
  • R: radius (Sol=1)
  • M: mass (Sol=1)
  • ρ: density (g/cm3)
  • Lifetime: approximate lifetime on the main sequence, assuming Sol's lifetime is 1010 years

Table is from the World Building book. Values not in the table can be calculated by using linear interpolation.

Slum is stellar luminosity, it is the Lum column on the table.

BC is bolometric correction, it is the Bolo. Corr. column on the table.

Steff is effective temperature, it is the Teeff column on the table.

These will be used in the equations below.

In Dole's Habitable Planets for Man, he estimates that an average planet requires about 3.0×109 years to develop life. This is important, since generally the only way a planet has oxygen in its atmosphere is by the action of native plant life. No life = no oxygen = not habitable for human beings. This means that the primary star needs a lifetime longer than 3.0×109 years or it will die before life has enough time to develop. This disqualifies stars with a spectral class of higher than F2. But keep in mind that Dole's 3 billion year figure is more of an educated guess than it is ironclad scientific fact.

Obviously even though K0 stars has a lifespan of 1.9×1010 years, a particular K0 might be a newborn under a billion years old, thus still not having any planets with life or oxygen.

A World Called Cleopatra


The sun was named Caesar, mythology having been used up closer to home. It is of type F7, which means it is hotter and whiter than Sol. Its mass is 1:2, its total luminosity 2.05 Sol. The diameter is little greater, but spots, prominences, corona, and output of charged particles (solar wind) are fewer. It is a younger star than ours, though by less than a billion years. Either because of this, or because of variations in galactic distribution, the proportion of heavy elements in it and its planets is somewhat more than for the Solar System.

From A WORLD CALLED CLEOPATRA by Poul Anderson (1974)

     The long-range significance of the Neuheimer scheme was far nastier than several gigacredits' loss to the merchant princes, Falkayn saw. Suppose it did succeed. Suppose the mighty Polesotechnic League was defied and defeated, and the Kraokan Empire was established. Well, the Kraoka by themselves might or might not be content to stop at that point and settle down to peaceful relationships with everybody else. In any event, they were no direct threat to the human race; they didn't want the same kind of real estate.
     But the Neuheimer humans—Already they spoke of themselves as crusaders. Consider the past history of Homo self-styled Sapiens and imagine what so spectacular a success would do to a bunch of ideologically motivated militarists! Oh, the process would be slow; they'd have to increase their numbers, and enlarge their industrial base, and get control of every man-useful planet in this neighborhood. But eventually, for power, and glory, and upset of the hated merchants, and advancement of a Way of Life—war.
     The time to squelch them was now…
     …"All right," Falkayn muttered. "Step One in the squelching process: Find their damned planetary system!"
     They couldn't hope to keep its location secret forever. Just long enough to secure a grip on this region; and given the destructive power of a space fleet, that needn't be very long. While it remained hidden, though, the source of their strength was quite efficiently protected. Hence their entire effort could go into purely offensive operations, which gave them a military capability far out of proportion to their actual force.
     Nonetheless, if the League should decide to fight, the League would win. No question about that. In the course of the war, the secret was bound to be discovered, one way or another. And then—nuclear bombardment from space—No!
     The Landholders were gambling that the League, rather than start an expensive battle for a prize that would certainly be ruined in the course of the fighting, would vote to cut its losses and come to terms. Antoran being hidden, the bet looked fairly good. But no matter how favorable the odds, only fanatics played with entire living worlds for stakes…
     …Okay, then, where was the silly star?
     Someplace not far off. Jutta had betrayed nothing by admitting that the constellations at home were almost like the constellations here. The ancient Kraoka could not have traveled any enormous ways, as interstellar distances go. Also, the home base must be in this territory so that its fleet could exploit the advantage of interior communications.
     And Antoran must be large and bright, no later in the main sequence than, say, G0. Yet . . . every possible sun was already eliminated by information the League had long possessed.
     Unless—wait a minute—could it be hidden by a thick nebulosity?
     No. There'd still be radio indications. And Jutta had spoken of seeing stars from her home.
     Aurora. Hm. She'd mentioned the necessity for certain villagers to migrate toward the poles, as her planet got too near its primary. Which meant their original settlements were a good bit further toward the equator. Even so, auroras had been conspicuous: everywhere you went, she'd said. This, again, suggested a highly energetic sun.
     Funny, about the eccentric orbit. More than one planet in the system, too, with the same problem. Unheard of. You'd almost think that—
     Falkayn sat bolt upright. His pipe dropped from his jaws to his lap. "Holy . . . hyper . . . Judas," he gasped.

     "You see," Falkayn said, "I know where Antoran is."
     "Heh?" Beljagor jumped several centimeters in the pilot chair he occupied.…
     …"Little items. They gave the show away, though. Like, Antoran isn't a planet but a star. And just one star hereabouts can possibly fit the data." Falkayn let Beljagor rumble for a moment before he pointed skyward and said, "Beta Centauri."
     At last Beljagor was sufficiently calm to stand in one spot, raise a finger, and say, "You unutterable imbecile, for your information, Beta is a type B blue giant. People knew before space flight began, giant suns don't have planets. Angular momentum per unit mass proved as much (stars with a spectral class between O5 and F5 rotate rapidly, F6 through M9 rotate slowly. Rapid rotation=no planets). After the hyperdrive came along, direct expeditions to any number of them clinched the matter. Even supposing, somehow, one did acquire satellites, those satellites never would get habitable. Giant stars burn hydrogen so fast their existence is measured in millions of years. Millions, you hear, not billions. Beta Centauri can hardly be ten million years old (currently thought to be 14.1±0.6 million years old, which is close enough for government work). More than half its stable lifetime is past. It'll go supernova and become a white dwarf. Life'd have no chance to evolve before the planets were destroyed. Not that there are any, I repeat. The reason for only the smaller suns having planets is understood. A big protostar, condensing from the interstellar medium, develops too intense a gravitational field for the secondary condensation process to take place outside it.
     "I thought even humans learned so much elementary astrophysics in the first grade of school. I was wrong. Now you know."
     His voice rose to a scream. "And for this you got me out of bed!"
     Falkayn moved to block the cabin exit. "But I do know," he said. "Everybody does. The Antoranites have based their whole strategy on our preconception. They figure by the time we discover Beta Centauri is a freak case, they'll control the whole region."
     "Here are the facts," Falkayn said. He ticked them off. "One, the Antoranite System was colonized by Kraoka, who couldn't and didn't settle on planets with suns as cool as Sol. Two, Antoran has six planets in the liquid-water zone. No matter how you arrange their orbits, that zone has to be mighty broad—which indicates a correspondingly luminous star. Three, the outermost of those six planets is too cold and weakly irradiated for Kraokan comfort, but suits humans fairly well. Yet it has brilliant auroras even in the temperate zones. For that, you need a sun which shoots out some terrifically energetic particles: again, a giant.
     "Four, this human planet, Neuheim, is far out. The proof lies in three separate facts, (a) From Neuheim, the sun doesn't have a naked-eye disc, (b) There are no solar tides worth mentioning, (c) The year is long, I figure something like two Earth centuries. (I calculate an orbital radius of a whopping 75 AU, when Pluto is only about 40 AU) I know the year is long, because Jutta let slip that her people had to shift some towns poleward a while back. Orbital eccentricity was making the lower latitudes too hot, maybe also too much UV was penetrating the ozone layer in those parts and making poisonous concentrations of ozone at the surface, like here. Nevertheless, the original human settlement was forty years ago. In other words, Neuheim's radius vector changes at so leisurely a rate that it was worth sitting down in areas which the colonists knew would have to be abandoned later. I suppose they wanted to exploit local minerals.
     "Okay. In spite of its enormous distance from the primary, Neuheim is habitable, if you don't mind getting a deep suntan. What kind of star can buck the inverse-square law on so grand a scale? What but a blue giant! And Beta Centauri is the only blue giant close by."
     Finally, tonelessly, Beljagor asked, "How could there be planets?"
     "I've worked that out," Falkayn replied. "A freak, as I remarked before, perhaps the only case in the universe, but still possible. The star captured a mess of rogue planets."
     "Nonsense. Single bodies can't make captures." But Beljagor didn't yell his objection.
     "Granted. Here's what must have happened. Beta was condensing, with a massive nucleus already but maybe half its mass still spread over God knows how many astronomical units, as a nebular cloud. A cluster of rogue planets passed through. Beta's gravity field swung them around. But because of friction with the nebula, they didn't recede into space again. Energy loss, you see, converting hyperbolic orbits into elliptical ones. Could be that there was also a secondary center of stellar condensation, which later spiraled into the main mass. Two bodies can certainly make captures. But I think friction alone would serve.
     "The elliptical orbits were almighty eccentric, of course. Friction smoothed them out some. But Jutta admitted that to this day the planets have paths eccentric enough to cause weather trouble. Which is not the normal case either, you recall. Makes another clue for us."
     "The planets would've exuded gases and water vapor in the early stages of their existence, through vulcanism, like any other substellar globes," Falkayn plowed on. "The stuff froze in space. But Beta unfroze it.
     "I don't know how the Kraoka of Dzua learned what the situation was. Maybe they simply didn't know that blue giants don't have planets. Or maybe they sent a telemetric probe for astrophysical research, and it informed them. Anyhow, they discovered Beta had five potentially good worlds plus one that was marginal for them. So they colonized. Sure, the planets were sterile, with poisonous atmospheres. But the ancient Kraoka were whizzes at environmental engineering. You can sketch for yourself what they did: seeded the air with photosynthetic spores to convert it, released other forms of life to consume the primeval organic matter and form the basis of an ecology, etcetera. Under those conditions, microbes would multiply exponentially, and it'd take no more than a few centuries for a world to become habitable."
     Falkayn shrugged. "Beta will blow up and destroy their work in five or ten million years," he finished. "But that's ample time for anyone, hey?"

From A SUN INVISIBLE by Poul Anderson (1966)

Bolometric Luminosity

If you are just making up the star out of your imagination, you can use any value you want. But if you are using a star known to science, the values can be calculated. As an example, we will use Ross 128. Yes, I know that the Wikipedia entry already has the bolometric luminosity listed but I'm going to show you how to calculate it anyway.

The values you absolutely have to have are the Apparent Magnitude, the Spectral Class, and the Distance. For Ross 128 these are apparent magnitude = 11.13, distance = 10.94 light-years, and spectral class = M4V.

The first two values are easy to find, but the Distance is often missing. If the star is not in the Tycho or Hipparcos star near star catalogs you are probably out of luck. Oh, and astronomers use parsecs for distance instead of light-years. There are 3.26 light-years in a parsec, so divide light-years by 3.26 to convert a distance into parsecs.

First you'll need to calculate the Absolute Magnitude:

M = m + 5 - (5 * log(p))


  • M = Absolute Magnitude
  • m = Apparent Magnitude, given
  • p = Distance from Sol (parsecs), given
  • log(x) = common logarithm of x(use the log key on your calculator)
Ross 128 Example

For Ross 128, apparent magnitude = 11.13. Light-years divided by 3.26 give you parsecs, so the distance is 10.94 / 3.26 = 3.35 parsecs. Absolute Magnitude is:

  • M = m + 5 - (5 * log(p))
  • M = 11.13 + 5 - (5 * log(3.35))
  • M = 16.13 - (5 * 0.52)
  • M = 16.13 - 2.63
  • M = 13.50

Next you'll have to calculate the Bolometric Absolute Magnitude. Look up the Bolometric Correction (Bolo. Corr.) for your star's spectral class in the table and apply it:

Mbolo = M + BC


  • Mbolo = Bolometric Absolute Magnitude
  • M = Absolute Magnitude
  • BC = Bolometric Correction for star's spectral class, from table
Ross 128 Example

The spectral class of Ross 128 is M4, so according to the table the bolometric correction is -2.3. So the bolometric absolute magnitude is 13.50 + (-2.3) = 11.2.

Finally calculate the Bolometric Luminosity:

SlumBolo = 2.52(4.85 - Mbolo)


Ross 128 Example

So for Ross 128:

  • SlumBolo = 2.52(4.85 - Mbolo)
  • SlumBolo = 2.52(4.85 - 11.2)
  • SlumBolo = 2.52-6.35
  • SlumBolo = 0.0028

You'll need the Bolometric Luminosity in the Geography section.

Stellar Mass

The mass of the primary star can be calculated using the Mass Luminosity Law.

Smass = SlumBolo0.2632


Ross 128 Example

So for Ross 128:

  • Smass = SlumBolo0.2632
  • Smass = 0.00280.2632
  • Smass = 0.213 solar masses

You'll need the Stellar Mass in the Geography section.

Stellar Diameter

Sdia = (Soltemp2 / Stemp2) * sqrt(SlumBolo)

SdiaKM = Sdia * 1,391,600 (diameter of Sol in kilometers)


  • Sdia = diameter of primary star (Sol = 1.0)
  • Sdia = diameter of primary star (kilometers)
  • Soltemp = effective temperature of Sol (Kelvin) = 5770 K
  • Stemp = effective temperature of primary star (Kelvin) from table
  • SlumBolo = bolometric luminosity of primary star
Ross 128 Example

So for Ross 128:

  • Sdia = (Soltemp2 / Stemp2) * sqrt(SlumBolo)
  • Sdia = (57702 / 32002) * sqrt(0.0028)
  • Sdia = (33,292,900 / 10,240,000) * 0.0529
  • Sdia = 3.251260 * 0.0529
  • Sdia = 0.172 solar diameters
  • SdiaKM = 0.172 * 1,391,600
  • SdiaKM = 239,360 kilometers

You'll need the Stellar Diameter in the Geography section.

The Solar System

A World Called Cleopatra

In general, the Caesarian System is a normal one. Besides asteroids, it contains eleven planets. In outward order, these have been christened Agrippa (small, hot, nearly airless); Antony (about Earth size, with an atmosphere, but not habitable by man); Cleopatra (the sole terrestroid member); Enobarbus (smaller than Earth, larger than Mars, ruddy like the latter); Pompey (a gas giant, somewhat more massive than Jupiter); four lesser giants (Lepidus, Cornelia, Calpurnia and Julia); and finally, remote Marius and Sulla (the latter really just a huge comet which has never moved into the inner system). There are two distinct asteroid belts separating Enobarbus, Pompey and Lepidus.

Seen from Cleopatra, Agrippa and Antony are morning or evening stars, though the former is usually lost in sun glare. The latter is brilliant, its iridescence often apparent to the naked eye as solar wind causes its upper atmosphere to fluoresce. Enobarbus glows red, Pompey and Lepidus tawny white. Pale-green Cornelia can occasionally be seen without instruments.

From A World Called Cleopatra by Poul Anderson (1974)



At some point you will want to create planets that humans like us can walk around in their shirtsleeves and not instantly die hideously. You'll want the planet to be quote "Habitable" unquote.

Beware, because in the literature on the topic, the word "habitable" has many disparate meanings.

There is "habitable" in the sense of "human-habitable", that is, human beings can survive there. Keep in mind that both the Gobi Desert and Antarctica are considered human habitable.

There is "habitable" in the sense that alien creatures whose biology is based on proteins dissolved in water can survive there (i.e., "life as we know it"). This means that the planet's average temperature is such that liquid water can exist (basically between the freezing point and boiling point of water). Some of these planets would kill unprotected humans.

And there is "habitable" in the sense that alien creatures whose biology is based on alien chemistries can survive there (i.e., "life as we don't know it"). Aliens with a biology based on poly-fluorocarbons dissolved in molten sulfur can survive temperatures between 113°C and 445°C at one atmosphere of pressure. Which is hot enough to melt zinc.

Solar System Regions

Solar System Regions

Solar systems are divided into various regions where the boundaries are set by the intensity of sunlight. The further from the primary star, the lower the sunlight intensity. Each boundary is a sphere centered on the primary star, though generally you can get away with just using the circle where the sphere crosses the system's ecliptic. And there are two boundaries set by the primary star's gravity.

System Inner Limit (gravity)BsystemInnerGrav = 0.2 * Smass
System Inner Limit (sunlight)BsystemInnerLight = sqrt( SlumBolo / 16)
Circumstellar Habitable Zone inner limitBhabZoneInner = sqrt( SlumBolo / 1.1)
Circumstellar Habitable Zone outer limitBhabZoneOuter = sqrt( SlumBolo / 0.53)
Snow LineBsnowLine = sqrt( SlumBolo / 0.04)
Liquid Hydrogen (LH2) LineBlh2Line = sqrt( SlumBolo / 0.0025)
System Outer LimitBsystemOuterGrav = 40 * Smass


All boundaries are in Astronomical Units. Multiply them by 149,000,000 to convert into kilometers.

Calculate both BsystemInnerGrav and BsystemInnerLight. Use the larger value for System Inner Limit. Protoplanets will not form closer than BsystemInnerGrav because the sun-star's gravity disrupts the orbit. Protoplanets will not form closer than BsystemInnerLight because the sun-star's heat will vaporize them. Usually BsystemInnerGrav is always larger, unless the sun-star is a white-hot spectral class O star.

  • System Inner Limit: radius of smallest possible (original) planetary orbit, start of terrestrial planet zone
  • Circumstellar Habitable Zone inner limit: start of the habitable zone (for water-based life)
  • Circumstellar Habitable Zone outer limit: end of the habitable zone (for water-based life)
  • Snow Line: end of terrestrial planet zone, start of the gas giant planet zone
  • System Outer Limit: radius of largest possible planetary orbit

What does this mean?

  • All planet orbits must be between the system inner limit and the system outer limit.
  • All habitable planet orbits must be between the habitable zone inner limit and the habitable zone outer limit.
  • Planets between the system inner limit and the snow line will be rocky terrestrial planets.
  • Planets between the snow line and the system outer limit will be gas giant planets.

There are exceptions.

Astronomers were very annoyed to discover Hot Jupiters closer to the sun-star than the system inner limit. Apparently these form beyond the snow line, then somehow migrate inwards to their current orbit.

The "snow line" marks the beginning of gas giants in a solar system. Planets further from the primary star than the snow line tend to be gas giants. This is because it marks the point where water ice remains frozen during the formation of the solar system, so protoplanets can enter the runaway growth ending in a huge gas giant planet.

The habitable zone is for water-based life-as-we-know-it. The general idea of the habitable zone is that a terrestrial planet whose orbit is totally inside the zone can have large amounts of water that is liquid (i.e, not either perpetually frozen solid or in the form of steam). As you can imagine, astrobiologists are bitterly divided over just where to draw the borders of the habitable zone. The values used here are the current "mainstream opinion".

Habitable Zones from Stephen Dole

Back in 1964 the standard for planet builders was the RAND study Habitable Planets for Man by Stephen Dole. Its focus was on trying to identify which nearby stars were likely to host planets that were human habitable.

Its parameters on the boundaries of the circumstellar habitable zone (Dole calls it the "ecosphere") are considered a bit dated and simplistic nowadays, but they are interesting.

First off was the lower limit on the primary star's spectral class. Stars of spectral class M have a luminosity of only 0.061. Due to the inverse square law, a habitable planet will have to be very close or it will be a frozen ball of ice. Unfortunately if the planet is too close it will become trapped by tidal locking so that one side of the planet always faces the primary star and the other always faces eternal darkness. Dole considered tidal locking to disqualify a planet from being considered human habitable.

In Dole's equation he calculates "h" as the maximum height of equilibrium tides caused in a planet by its primary star. By inspecting the various planets and moons in the solar system, Dole determined that if the square of h was larger 2.0, the planet or moon would undergo tidal locking.

That is represented by the h2 = 2 line in the chart above.

Given that, (and with Doles other assumptions about the mass of habitable planets) he calculated that the width of the ecosphere started narrowing due to tidal locking once the primary star mass dropped below 0.88. And below 0.72 the ecosphere vanishes entirely. This corresponds to spectral class K1.

Now, when Dole was writing, the planet Mercury was thought to be tidally locked to Sol. About the time the book was published astronomers discovered that the planet was actually in a 3:2 spin-orbit resonance. This means the planet rotates three times on its axis for every two revolutions it makes around Sol. The point is that such a resonance makes a planet much more habitable than if it was tidally locked. So Dole's spectral class limit is a bit dated.

Dole thought it vital that a human habitable planet have life it, mostly to supply the planet's atmosphere with oxygen. He figured that a star has to emit light and heat for a fairly constant rate for at least 3 billion years to give life enough time to evolve. It goes without saying that the primary star has to have a lifespan longer than 3 billion years or it is automatically excluded. Dole figures this means the primary star must has a mass of 1.43 or less (spectral class F2 and smaller).

That is represented by the horizontal line extending from the 1.42 tick mark on the "Mass relative to Sun's mass" scale in the chart above.

Bottom line is Dole is restricting the primary star to spectral class F2 through K1 (that is: F2, F3, F4, F5, F6, F7, F8, F9, G0, G1, G2, G3, G4, G5, G6, G7, G8, G9, K0, and K1). Spectral class O, B, A, and F0 & F1 do not have a lifespan of 3 billion years. Spectral class K2 through K9 and class M will tidal lock their habitable planets.

Finally Dole calculated that to have human-habitable temperatures on the planet's surface, it would need a top-of-the-atmosphere illumination between 0.65 and 1.35 (maybe to 1.90) Terran illumination. See the inclination chart.

This is represented by the Illuminance 1.90, 1.35, and 0.65 lines in the chart above.

So according to Dole, the ecosphere is the area bound by the h2 tidal locking line, the 1.43 star mass line, the 1.35 (or 1.90) illuminance line and the 0.65 illuminance line. It is shaded in the chart above.

To use, draw a horizontal line through the primary star's mass on the scale. Note where the line enters and exits the ecosphere zone. Trace the intersections down to the Distance from the Primary scale to see the start and end points of the ecosphere in AU from the primary star.

But again this chart is a bit outdated.

Alternate Habitable Zone Calculation

Recently there was a new scientific paper with a new (complicated) way to calculate "life-as-we-know-it" habitable zones. I'm currently trying to figure out how to adapt this for alien habitable zones, but it will take a while.

For this calculation, you will need primary star's Steff and Slum from the table above, and whether the planet's mass is closest to 0.1 Earth mass, 1.0 Earth mass, or 5.0 Earth masses.

Tempstar = Steff - 5780

Fluxeff = Suneff + (A * Tempstar) + (B * Tempstar2) + (C * Tempstar3) + (D * Tempstar4)

BhabZone = sqrt(Slum / Fluxeff)

Find the values for Suneff, A, B, C, and D in the table:

Variable0.1 Earth Mass
Inner Hab Zone
1.0 Earth Mass
Inner Hab Zone
5.0 Earth Mass
Inner Hab Zone
Outer Hab Zone

Find the inner limit for the habitat zone by using the values in the column for inner hab zone for the appropriate Earth mass. Find the outer limit by using the values in the column for All Outer Hab Zone.

Ross 128 Example

Let us figure the habitat zone for a planet around Ross 128 with 5.0 Earth masses.

  • Tempstar = Steff - 5780
  • Tempstar = 3200 - 5780
  • Tempstar = -2580

Use values from 5.0 Earth Mass Inner Hab Zone column.

  • Fluxeff = Suneff + (A * Tempstar) + (B * Tempstar2) + (C * Tempstar3) + (D * Tempstar4)
  • Fluxeff = 1.188 + (1.433×10-4 * -2580) + (1.707×10-8 * -25802) + (-8.968×10-12 * -25803) + (-2.084×10-15 * -25804)
  • Fluxeff = 1.188 + (1.433×10-4 * -2580) + (1.707×10-8 * 6,656,400) + (-8.968×10-12 * -17,173,512,000) + (-2.084×10-15 * 44,307,660,960,000)
  • Fluxeff = 1.188 + (-0.369714) + (1.13624748) + (0.154012055616) + (-0.09233716544064)
  • Fluxeff = 2.016
  • BhabZoneInner = sqrt(Slum / Fluxeff)
  • BhabZoneInner = sqrt(0.024 / 2.016)
  • BhabZoneInner = sqrt(0.012)
  • BhabZoneInner = 0.110 AU

Use values from All Outer Hab Zone column.

  • Fluxeff = Suneff + (A * Tempstar) + (B * Tempstar2) + (C * Tempstar3) + (D * Tempstar4)
  • Fluxeff = 0.356 + (6.171×10-5 * -2580) + (1.698×10-9 * -25802) + (-3.198×10-12 * -25803) + (-5.575×10-16 * -25804)
  • Fluxeff = 0.356 + (6.171×10-5 * -2580) + (1.698×10-9 * 6,656,400) + (-3.198×10-12 * -17,173,512,000) + (-5.575×10-16 * 44,307,660,960,000)
  • Fluxeff = 0.356 + (-0.1592118) + (0.0113025672) + (0.054920891376) + (-0.0247015209852)
  • Fluxeff = 0.238
  • BhabZoneOuter = sqrt(Slum / Fluxeff)
  • BhabZoneOuter = sqrt(0.024 / 0.238)
  • BhabZoneOuter = sqrt(0.101)
  • BhabZoneOuter = 0.318

So for a planet with 5.0 Earth masses, the habitable zone starts at 0.110 AU and ends at 0.318 AU.

Alien Habitable Zones

Alien Habitable Zones

If your planet is for alien life forms whose biology has a different chemical basis, you'll need a different circumstellar habitable zone. For creatures whose biology is based on poly-lipids dissolved in liquid methane you'd want a zone where the planet's average temperature allows methane to be liquid, not solid or gas (about -183.6°C to -161.6°C).

I've done some very shaky extrapolation using the planetary temperature equation. I've assumed that all planets have an albedo of 0.3 and have a greenhouse factor of 1.1, and calculated backwards to come up with these trick figures. Use them at your own risk.

BhabZoneInner = sqrt( SlumBolo / BsunlightInner)

BhabZoneOuter = sqrt( SlumBolo / BsunlightOuter)

Alien Habitable Zones
Fluorosilicone-Fluorosilicone Hab500°C52.0400°C29.9
Fluorocarbon-Sulfur Hab445°C38.7113°C3.2
Human Habitable21.8°C1.1-27.7°C??0.53
Protein-Ammonia Hab-33.4°C0.48-77.7°C0.21
Polylipid-Methane Hab-161.6°C0.023-183.6°C0.0094
Polylipid-Hydrogen Hab-240°C0.0025-253°C2.4×10-5

A cursory look at the chart tells me that the planetary temperature equation is not working in this case. I know for a fact that Mercury can get up to 430K, but the equation is putting the 450K Fluorosilicone-Fluorosilicone zone about 0.3 AU closer to the Sun. I also find it suspicious that the human-habitable zone is not a subset of the Protein-Water zone. Oh, well, back to the drawing board.

Behind The Equations

Behind The Equations

If you want to know the details of these equations, read on. Otherwise just skip to the next section.

The basic sunlight equation is:

Bdist = sqrt( SlumBolo / Bsunlight)

BdistKM = Bdist * 149,000,000


  • Bdist = distance of boundary from primary star (astronomical units)
  • Bdist = distance of boundary from primary star (kilometers)
  • SlumBolo = Bolometric Luminosity
  • Bsunlight = sunlight intensity at boundary (Terra's sunlight intensity = 1.0)
  • sqrt(x) = square root of x

and the basic gravity equation is

Bdist = Kgravity * Smass


  • Bdist = distance of boundary from primary star (astronomical units)
  • Smass = Stellar Mass
  • Kgravity = gravity konstant for boundary

The various boundaries are:

System Boundaries
Red Zone Outer Limit
Yellow Zone Inner Limit
System Inner Limit
Terrestrial Planet Inner Limit
Yellow Zone Outer Limit
Green Zone Inner Limit
Circumstellar Habitable Zone Inner Limit
Green Zone Outer Limit
Blue Zone Inner Limit
Circumstellar Habitable Zone Outer Limit
Terrestrial Planet Outer Limit
Jovian Planet Inner Limit
Snow Line
Blue Zone Outer Limit
Black Zone Inner Limit
Liquid Hydrogen (LH2) Line
Jovian Planet Outer Limit
System Outer Limit


Planet Orbit

Orbital Distance

Presumably the planet you are building is a habitable one. Therefore you should set its orbital distance by the intensity of sunlight you'd like it to receive. To ensure that the planet is inside the circumstellar habitable zone, chose a value for sunlight such that Psunlight is between 0.53 and 1.1.

Pdist = sqrt( SlumBolo / Psunlight)

PdistKM = Pdist * 149,000,000


  • Pdist = distance of planet from primary star (astronomical units)
  • Pdist = distance of planet from primary star (kilometers)
  • SlumBolo = Bolometric Luminosity
  • Psunlight = sunlight intensity (Terra's sunlight intensity = 1.0) given
  • sqrt(x) = square root of x
Ross 128 Example

So let's make our planet around Ross 128 have the same sunlight intensity as Terra. So Psunlight = 1.0

  • Pdist = sqrt( SlumBolo / Psunlight)
  • Pdist = sqrt( 0.0028 / 1.0)
  • Pdist = sqrt( 0.0028 )
  • Pdist = 0.053 AU
  • PdistKM = 0.053 * 149,000,000
  • PdistKM = 7,897,000 kilometers

0.053 AU is freaking close, since Mercury is 0.39 AU away from Sol. Ross 128 is really really dim.

Planetary Year

Planetary Year

What is the planet's year? We calculate it using Kepler's Third Law.

Pyear = sqrt( Pdist3 / Smass )


Ross 128 Example

So the planet's year is:

  • Pyear = sqrt( Pdist3 / Smass )
  • Pyear = sqrt( 0.0533 / 0.213 )
  • Pyear = sqrt( 0.000148877 / 0.213 )
  • Pyear = sqrt( 6.99×10-4 )
  • Pyear = 0.0264 year = 9.6 days

And you thought Mercury had a short year.

Apparent Size of Sun

SangDia = 57.3 * ( SdiaKM / PdistKM)


Ross 128 Example

Ross 128's angular diameter in the sky of the planet is:

  • SangDia = 57.3 * ( SdiaKM / PdistKM)
  • SangDia = 57.3 * ( 239,360 / 7,897,000)
  • SangDia = 57.3 * 0.0303
  • SangDia = 1.72°

Even though Ross 128 has a tiny diameter compared to Sol, it's planet is so much closer that the star has an angular diameter in the sky about 3.4 times as big as Sol in Terra's sky.

A World Called Cleopatra

Cleopatra moves around Caesar in an orbit of slight eccentricity, at an average distance of 1.24 astronomical units. Its year is 1.26 times that of Earth, about 15 months long, and the sun in its sky has only 0.87 the angular diameter of ours. Nevertheless, because of its brightness, Caesar gives Cleopatra 1.33 times the total irradiation that Earth gets. A larger proportion of this energy is in the shorter wavelengths; Caesar appears a bit more bluish white than yellowish white to human vision. The lesser apparent size is not particularly noticeable, since no prudent person looks anywhere near it without eye protection, let alone straight at it. Shadows on the ground tend to be sharper than on Earth and to have more of a blue tinge. All color values are subtly different, though one quickly gets used to this.

From A World Called Cleopatra by Poul Anderson (1974)

Planet Proper

A World Called Cleopatra

The planetary system lies in Ursa Major, 398 light-years from Sol. This causes certain changes in the appearance of the heavens. Northerly constellations are "spread out" and most of the familiar stars in them show brighter than at Earth, though some have left the configurations because, seen from here, they now lie in a southerly direction. Fainter stars in them, invisible at Earth, have become naked-eye objects. These changes are the greater the nearer one looks toward Ursa Major. It is itself modified quite out of recognition by the untrained eye, as are the constellations closest to it. The further away one looks, around the celestial sphere, the less distortion. Southern constellations are comparatively little affected. Those near the south celestial pole of Earth, such as Octans, keep their shapes the best, though they exhibit the most shrinkage in angular size. Various of their fainter stars (as seen from Earth) are now invisible—Sol is too—but they have been replaced by others which (as seen from Earth) "originally" were northern.

Thus to a native of the Terrestrial northern hemisphere the sky seems considerably changed around the Dippers, Cassiopeia, etc. But Orion, for example, is still identifiable; and the constellations that an Australian or Argentinian is used to are not much altered.

However—the celestial hemispheres of Cleopatra are not identical with those of Earth. In fact, the Cleopatran north pole points toward Pisces, which is almost 90° from the direction of the Terrestrial axis. ("North" and "south" are defined so as to make the sun rise in the east.) There is no definite lodestar, but Pisces turns around a point in its own middle, accompanied by neighbors such as Virgo, Pegasus, and Aquarius. The south celestial pole is near Crater. The constellations that Earthmen are accustomed to seeing high in either sky are here—insofar as they are recognizable—always low, and many are only to be observed at given seasons. Under these circumstances, it may be most convenient for colonists to redraw the star map entirely, making new constellations out of what they see. Or perhaps this will happen of itself in the course of generations.

From A World Called Cleopatra by Poul Anderson (1974)

Density, Radius, Mass, Surface Gravity

Pg = Pmass / Pradius2

Pg = Pradius * Pρ

Pgmss = Pg * 9.81

Pmass = Pradius3 / Pρ

PmassKg = Pmass * 5.972×1024

Pρ = Pradius3 / Pmass

PρKgm = Pρ * 5,500

Pradius = cubeRoot( Pmass * Pρ )

PradiusKm = Pradius * 6,371

PradiusM = Pradius * 6,371,000


  • Pg = acceleration due to gravity (g)
  • Pgmss = acceleration due to gravity (m/s2)
  • Pmass = mass of planet (Terra = 1) given
  • PmassKg = mass of planet (kg) given
  • Pradius = radius of planet (Terra = 1) given
  • PradiusKm = radius of planet (km) given
  • PradiusM = radius of planet (m) given
  • Pρ = mean density of planet (Terra = 1) given
  • PρKgm = mean density of planet (kg/m2)

Here you will have to play around with selecting various values for mass, density, and radius until you get results you like. They are closely interrelated. Habitable Planets for Man suggested that the maximum gravity for a human-habitable planet should be about 1.5g.

A World Called Cleopatra

Cleopatra is smaller than Earth. In terms of the latter planet, its mass is 0.528, its radius 0.78 (or 4960 km at the equator), its mean density 1.10 (or 6.1 times that of water), and its surface gravity 0.86. This last means that, for example, a human who weighed 80 kg on Earth weighs 68.5 here; he himself soon adjusts to that—though he is well advised to maintain a lifetime program of physical exercise to avoid various atrophies and circulation problems—but engineering is affected. (For instance, aircraft need less wing area but ground vehicles need more effective brakes.) An object falling through a given distance takes 1.07 times as long to do so as on Earth and gains- 0.93 the velocity; a pendulum of given length has 1.14 the period; the speed of a wave on deep water is 0.93 what it is on Earth.

From A World Called Cleopatra by Poul Anderson (1974)

Planetary Temperature

What's the average planetary temperature? That's hard to calculate. This equation will give you a rough idea, but it predicts a slightly incorrect temperature for Terra. When I said "rough" I meant it.

Ptemp = 374 * G * ( 1 - A) * Psunlight0.25


  • Ptemp = average planetary temperature (Kelvin)
  • G = greenhouse fudge factor (~1.1 for Terra, 0.0 if planet has no atmosphere)
  • A = planet's Bond albedo (~0.3 for Terra, 0.9 for Venus) given
  • Psunlight = sunlight intensity (Terra's sunlight intensity = 1.0), use same value as used above

You can convert from Kelvin to Celsius by using Google (search for "287.98 Kelvin in Celsius") or with one of the many conversion calculators.

This equation gives slightly inaccurate results. But on the whole you'll know that if the temperature is below 0°C (273.15 K) or above 100°C (373.15) the planet is probably outside of the habitable zone.

Habitable Planets for Man suggested that a human habitable planet should have an average temperature somewhere between 0°C (273.15 K) and 30°C (303.15 K). That means a planet with an average temperature of 360K is unsuitable for unprotected humans but might be just perfect for some weird alien life form.

Ross 128 Example

Since we decided that Ross 128 planet would have the same sunlight intensity as Terra, if it has the same albedo as Terra it will also have the same average temperature as Terra. Or the same slightly incorrect temperature the equation gives:

  • Ptemp = 374 * G * (1 - A) * Psunlight0.25
  • Ptemp = 374 * 1.1 * (1 - 0.3) * 1.00.25
  • Ptemp = 374 * 1.1 * 0.7 * 1.0
  • Ptemp = 287.98 K = 14.8°C = 58.7°F

In Habitable Planets for Man Stephen Dole concludes that with respect to temperature requirements a region is human habitable only if the mean annual temperature lies between 0°C and 30°C, if the highest mean daily temperature during the warmest season is lower than 40°C, and if the lowest mean daily temperature of the coldest season is higher than -10°C.

Dole is of the opinion that a planet can be considered habitable if the habitable region is 10% or more of the planet's surface area.

This is not just a comfortable range for human beings, but it is also the temperatures best tolerated by Terran agricultural crops and domesticated animals we use for food.

Sun Color

The Sun replaced with other stars. Distance is assumed to be 1 AU in all cases. Image by Halcyon Maps. scroll vertically to see rest of infographic



If something is shining as a blackbody, its temperature determines its color, because not only does the intensity of the radiated electromagnetic energy change with temperature, so do its wavelengths. The color tends from red toward blue with increasing temperature. Because of this, we often speak of “red stars” or “blue stars.” These names, though, are largely misnomers in terms of what the eye would actually see. They just refer to the wavelength bias. Even a “cool” “red” star is extraordinarily bright and hot by everyday standards. A typical “red” dwarf, for example, which make up the bulk of the stars in the universe, has a temperature of about 3000 K, about the same as a filament in an ordinary incandescent light. The star’s light won’t look red at all, and the eye is sufficiently adaptable that scenes will look normal, just as things look perfectly normal by incandescent light on Earth. The temperature of something truly “red”—a charcoal fire or glowing stovetop, say—is more like 1000 K. Quite a few science fiction stories have spoken of the lurid light of a red sun: Robert L. Forward in Rocheworld, Poul Anderson in Trader to the Stars, and many others. But this “local color” is just not true! Obviously, when Jerry Oltion and Lee Goodloe talked about the “bloody light” of dawn from a red dwarf star, in their novella “Contact,” the “star” was actually a brown dwarf. [To appear reddish the companion must be considerably cooler, and must be a so-called “brown dwarf”, a body that is not quite a star.]

Stars are also classified by spectral type, which is also due mostly to temperature. Different elements absorb (or emit) particular wavelengths of light, and these absorbed wavelengths show up as spectral lines superimposed on the blackbody background. (Obviously this also allows identification of those elements, by spectroscopy!) Furthermore, an element’s spectral signature typically changes with temperature. At higher temperatures more atoms are ionized, because one or more of their electrons are knocked off by the increasingly violent collisions with other atoms. The upshot is that the spectrum of a star changes with temperature, and so the spectral type reflects temperature. From hottest to coolest, they are: O, B, A, F, G, K, M. (They’re out of alphabetical order for historical reasons.) They’re further subdivided by number; e.g., G5 is halfway between G0 and K0. The Sun is a type G2. Typical “effective temperatures” of the different spectral types are shown in table 2, column Teeff(K). (The effective temperature is the temperature of a perfect blackbody that puts out the same amount of radiation.)

Escape Velocity

Vesc = Kesc * PradiusM * sqrt(ρKgm)


  • Vesc = Escape Velocity (m/s)
  • Kesc = Escape Constant = 2.365×10-5 = sqrt( (8 * π * G) / 3)
  • PradiusM = radius of planet (m) use same value as used above
  • PρKgm = mean density of planet (kg/m2) use same value as used above

This not only tells you how much delta V a spacecraft will need to escape from the planet, it also tells you which atmospheric elements will escape into space.

Terra Example

Terra has a radius of 6,371,000 meters and a mean density of 5,500 kg/m3

  • Vesc = Kesc * PradiusM * sqrt(ρKgm)
  • Vesc = 2.365×10-5 * 6,371,000 * sqrt(5,500)
  • Vesc = 2.365×10-5 * 6,371,000 * 74.2
  • Vesc = 11,180 m/s = 11.2 km/s

Distance to Horizon

Hdist = sqrt( (PradiusM + Eheight)2 - PradiusM2 )


  • Hdist = distance to horizon (m). Terra = ~4,700 m
  • PradiusM = radius of planet (m) use same value as used above
  • Eheight = height observer's eyes are above planet's surface (m) Standing human average is 1.75 m

The distance to the horizon calculated geometrically here will not be the same as the distance as seen on a planet with an atmosphere. The pesky atmosphere refracts light so you can see a bit farther, but the actual amount changes with the current temperature gradient. As a rough rule of thumb, you can correct for this by multiplying the value for PradiusM by 1.20 in both places in the equation.

It is often mentioned that the horizon on Luna is so close that astronauts felt like they were constantly in danger of stepping off a cliff. Theoretically on a planet with a larger radius that Terra people can see farther and may start to feel like they were tiny ants or otherwise insignificant.

A World Called Cleopatra

Standing on a flat plain or sea, a man of normal height observes the horizon as being about 7 km off, compared to about 8 on Earth—not a terribly striking difference, especially in rugged topography or hazy weather.

From A WORLD CALLED CLEOPATRA by Poul Anderson (1974)

Planetary Atmosphere

Atomic hydrogen. H
Molecular oxygen, O2
Molecular hydrogen, H2
Hydrogen sulfide, H2S
Helium, He
Argon, Ar
Atomic nitrogen, N
Carbon dioxide, CO2
Atomic oxygen, O
Nitrous oxide, N2O
Methane, CH4
Nitrogen dioxide. NO2
Ammonia. NH3
Ozone, O3
Water vapor, H2O
Sulfur dioxide, SO2
Neon, Ne
Sulfur trioxide, SO3
Molecular nitrogen, N2
Krypton. Kr
Carbon monoxide, CO
Xenon, Xe
Nitric oxide, NO

The basic idea is simple. If a molecule is moving faster than the planet's escape velocity, it goes streaking into the inky depths of space. Otherwise it sticks around and helps comprise the planet's atmosphere.

You have already calculated the planet's escape velocity.

A molecule's speed depends upon two things, the molecule's weight and the molecule's temperature.

Molecular weight is easy. You can look it up in Wikipedia or something, all molecules of a given chemical compound have identical masses.Molecular temperature depends upon the the planet's average temperature. To the right is a table of molecular weights of various gasses likely to be atmospheric components.

Temperature is problematic, since my references are a bit vague on whether you should use the temperature at the planet's surface or at the planet's exosphere. We have the equation for the average temperature of the planet's surface, but not for the exosphere.

The bottom line is that for a given type of gas to be found in a planet's atmosphere, that gas average speed at the planet's temperature should be less than 1/6 of the planet's escape velocity (Jeans Escape). Otherwise the gas will escape in a few million years, much less the few billion years the planet will need to become habitable.

VescJean = Vesc / 6

Molvel = sqrt( (3 * Kmolar * Ptemp) / Molweight)


Figure out the Jeans escape velocity (VescJean). Any atmospheric gas in the table which the formula calculates a Molvel higher than VescJean is not going to be in the planet's atmosphere.

There should be a way to rearrange the equation so it yields the maximum molecular weight a planet can hang on to, but I'm getting odd results when I try.

Please note this is for a primordial planetary atmosphere. Specifically if the planet has no life (more specifically: no plants) there is not going to be any oxygen in the atmosphere. Oxygen is too darn reactive: any that shows up in the atmosphere is quickly turned into oxide minerals. The only way a planet can have O2 in the atmo is if it is continuously renewed, and that means plant life.

Whether an atmosphere is breathable for human beings depends upon percentage of oxygen and the barometric pressure. See the chart. Having said that, if the percentage of oxygen is too high, everything is constantly catching on fire.

On Terra the percentage of oxygen is about 20%. But 65 million years ago in the late Cretaceous period, it was more like 25% to 35% oxygen. Fossil Cretaceous charcoal deposits suggest that Tyrannosaurus Rex spent a lot of time fleeing forest fires and constantly getting a hotfoot. Paleontologists could not figure out how pteranodon biochemistry could possibly generate enough energy to allow the creatures to fly. But the turbocharging effect of 35% oxygen made it easy.

This also explains the curious geological layers at the K–T boundary. This was when the Dinosaur Killer asteroid wiped them out. The geological layers show an iridum layer from the asteroid strike, followed by a world-wide layer of finely divided carbon. Researchers are now of the opinion that 35% oxygen ensured that when the asteroid impacted, every forest on the entire freaking planet instantly ignited like they were made out of napalm. That's where the carbon layer came from. Any dinosaur that managed to avoid being barbecued in the continental fire-storms would have starved to death as the following two years of black clouds killed off all the plants.

Atmospheric pressure, on the other hand, depends upon how much gas the planet has managed to hold on to. Which means it could be anything, choose whatever you want. Terra and Venus are about the same size and mass, but the atmospheric pressure on Venus is about 90 times that of Terra.

Click here for an interactive gas retention plot.

A World Called Cleopatra

Despite its lesser dimensions, Cleopatra has quite a terrestroid atmosphere. In fact, the sea level pressures on the two planets are almost identical. It is thought that this is due to the hot, dense mass of the planet outgassing more than Earth did, early hi their respective histories, and to the fact that, ever since, the strong magnetic field has helped keep too many molecules from getting kicked away into space by solar and cosmic ray particles.

Air pressure drops with altitude more slowly than on Earth, because of the lower gravity. On Earth, at about 5.5 km the pressure is one-half that at sea level; but on Cleopatra, one must go up 6.35 km to find this. Not only does that moderate surface conditions, it extends life zones higher, and offers more possibilities to flyers both living and mechanical.

From A World Called Cleopatra by Poul Anderson (1974)

Axial Tilt and Planetary Day

The ecliptic is the plane that the orbits of the solar system's planet (mostly) lies in. For purposes of planetary climate, the important point is that the sun's rays that hit a planet travel more or less parallel to the ecliptic. The planet's axial tilt (AKA "Obliquity") is the angle a given planet's rotational axis makes with the ecliptic (but it is stated as how many degrees it is away from being 90° from the ecliptic).

The bottom line is that the axial tilt creates the planet's seasons. The angle that the sunlight hits the ground affects the concentration of heat. It is hottest when it hits the ground perpendicular to the ground plane.

Axial tilt also controls how many hours of daylight and night time there are per day at various parts of the year. During the winter the days are short and the nights are long, the reverse is true in summer. At the equinoxes (equal-night) the hours of daylight and night time are equal.

Terra has an axial tilt of 23.5°, which means it makes an angle of 66.5° with the ecliptic.

A World Called Cleopatra

There having been less tidal friction acting on it through most of its existence, Cleopatra spins faster than Earth: once in 17 hr 21 m 14.8 s, or about 17.3 hr or 0.72 Earth diurnal period. Its year therefore lasts 639 of its own days, give or take a little bit because of trepidation, precession, etc.

The axial tilt is 28°, somewhat more than Earth's. However, the climate of high latitudes is not necessarily more extreme on that account. Certainly winters are less cold. It is the difference in the length of seasons—a fourth again as much—which'is most important. Likewise, the seasonal variation of day and night lengths is more marked than on Earth, and the Arctic and Antarctic come nearer to the equator.

The stronger sun, which supplies more energy; the longer year, which gives more time to overcome thermal lag; 'the smaller size, which brings zones closer together; the larger axial tilt, which exaggerates the differences between them; the quicker spin, which generates more potent cyclonic forces; the lower pressures but the longer distance up to a stratosphere, which make for more extensive air masses moving at a given time under given conditions — all these create "livelier" weather than on Earth. Storms are more common and violent, though they tend to be short-lived. Huge thunderstorms in the river valley, twisters on the plains, hurricanes in the tropics, and blizzards near the poles are things which colonists must expect; they have to build stoutly and maintain an alert, meteorological service.

But this seeming drawback has its good side. With such variability, both droughts and deluges are rare; chilly fogs don't linger; inversion layers break up before they accumulate unpleasant gases; daytime cloud patterns can be gorgeous to watch, while nights are brilliantly clear more often than not, in most areas of the planet.

From A World Called Cleopatra by Poul Anderson (1974)

Temperature Latitudes

Temperature Latitudes
Latitude NameLatitudeTemperature
North Pole90° north
Arctic ZoneArctic Circle to
North Pole
Arctic Circle90-axialTilt ° north
North Temperate ZoneTropic of Cancer
to Arctic Circle
Tropic of CanceraxialTilt ° north
North Tropical ZoneEquator to
Tropic of Cancer
South Tropical ZoneEquator to
Tropic of Capricorn
Tropic of CapricornaxialTilt ° south
South Temperate ZoneTropic of Capricorn
to Antarctic Circle
Antarctic Circle90-axialTilt ° south
Antarctic ZoneAntarctic Circle
to South Pole
South Pole90° south

All planets have latitude and longitude to measure geographic positions. Latitude measures distance from the planet's equator. 0° latitude is on the equator, the north pole is at 90° north latitude, the south pole is at 90° south latitude (which pole is north? use the right hand rule).

There are special latitudes linked to planetary temperatures, and other special latitudes linked to planetary winds and precipitation. These are important because those are the two factors that determine a region's climate and biome.

This section is for temperature latitudes.

Unlike wind latitudes, the temperature latitudes depend upon the planet's axial tilt.

The planet's Arctic Circle and Antarctic Circle are at a latitude of 90° minus the axial tilt, north and south. Terra's arctic zone extends from latitude 66.6° (90 - 23.5 = 66.5) north to 90° north. The antarctic zone is from latitude 66.6° south to 90° south.

The planet's tropics are at a latitude equal to the axial tilt, north and south. Terra's tropic zone extend from 23.5° north (Tropic of Cancer) down to 0° (equator) then down to 23.5° south (Tropic of Capricorn)

The part of the planet that is not arctic zone or tropic zone is called the temperate zone.

These named latitudes help figure out the average temperature of various regions of the planet. Naturally the arctic and antarctic zone are where it is colder than average, and if the planet has any ice caps they will be here. This is also the area where you'll find the "land of the midnight sun" and "polar night". And of course the tropic zone is where the planet tends to be warmer than average. The temperate zone tends to be right at the average.

The trouble is that while we have an equation for the average planetary temperature, we do not have one for colder than average and hotter than average.

Until I find something better, the best thing I can think of is to look at Terra. Temperatures on Terra vary from 183.95K to 329.85K with an average of 289.15K (-89.2°C to 56.7°C, average 16°C). This means the temperatures are plus or minus 40 degrees from the average. Though the amount of axial tilt is probably a factor.

The precise latitude of, say, the Tropic of Cancer varies from planet to planet, depending upon the axial tilt. It controls the temperature of the region.

In a later section you will learn about the temperature latitudes like the Doldrums and the Horse Latitudes. These are always at the same latitude from planet to planet (e.g., the northern Horse Latitude is always at 30° N).

For a planet to have seasons that are reasonably close to Terra the axial tilt should be from about 15° to 32°. The greater the tilt, the more seasonal variation in temperature the planet will experience.

A planet with an axial tilt of 0° (90° to ecliptic) would have no seasons at all, all zone would have the same climate all year round. The planet would be all temperate zone, with no tropic zones nor arctic zones. Pedantically the tropic of cancer, the tropic of capricorn, and the equator would all be at 0° latitude, and the arctic would extend from 0° north to 0° north. The antarctic would the same as the arctic except in the south. Things like planetary climates and continental erosion depend upon yearly differences in regional temperature, the zero tilt planet has no yearly differences in temperature. It does have temperature difference between regions, the poles are going to get incredibly cold and the equator is going to fry.

When the axial tilt gets larger than 45° things get weird. The arctic and tropic zone overlap, and the "temperate zone" is the overlap region.

Above an axial tilt of 54°, the poles get more heat than the equator.

When the axial tilt is close to 90° seasons get extreme. Both the arctic and tropic zone cover the entire globe. There are times in the year when half of the entire planet has eternal daylight and the other have has eternal night. The glaciers form on the equator instead of the poles. Seasonal temperatures will be so extreme that human beings would probably have to do mass migration during the year to stay in the part of the planet that had survivable temperatures.

Keep in mind that a lot of the temperate zone rely upon melting snow for much of its water. If the axial tilt is too small, there will be less seasonal variation in temperature, leading to less snow melting in summer, leading to widespread deserts in the temperate zone. On the other hand, too much axial tilt means more seasonal temperature variation, leading to less snow fall, also leading to widespread deserts in the temperate zone.

In addition to controlling the seasons, axial tilt also changes the distribution of surface heat on your planet. The larger the tilt, the more the heat is evenly spread. For the heat to be perfectly evenly spread you'd need an axial tilt of around 54°.

For predicting the average temperature, I am toying with calculating insolation as a function of date and latitude. The link gives access to a spreadsheet for this, it is based on these equations. They give the energy density of sunlight at the top of the atmosphere given a latitude and a date, and the planet's inclination. So the average temperature will appear as a series of horizontal bands by lattitude.

Wind Latitudes

Wind Latitudes
Latitude NameLatitudePrecipitation
North Polar High90° northDry
North Polar Easterlies60° to 90° northDry
North Polar Front
Subpolar Low
60° northWet
North Westerlies30° to 60° northWet
North Horse Latitude
Subtropical High
30° northVery dry
NE Trade Winds0° to 30° northVery Wet
Doldrums (ITCZ)Very wet
SE Trade Winds0° to 30° southVery Wet
South Horse Latitude
Subtropical High
30° southVery dry
South Westerlies30° to 60° southWet
South Polar Front
Subpolar Low
60° southWet
South Polar Easterlies60° to 90° southDry
South Polar High90° southDry

Unlike Temperature Latitudes, the Wind Latitudes are fixed. They are the same for all planets. However, they are not straight lines, they wiggle like a snake with diarrhea from hour to hour. This means over the year their boundaries are fuzzy and broad, but centered on their "official" latitude.

Annual precipitation is partially controlled by the wind latitudes. It is also controlled by oceans, mountains, and other geographical features. Precipitation is important because it is one of the two factors that determine a region's climate.

A World Called Cleopatra

Theoretically, the mean temperature at a given latitude on Cleopatra should be some 20 °C higher than the corresponding value for Earth. In practice, the different spectral distribution and the atmosphere and hydrosphere, modify things considerably. Cleopatra is warmer, and lacks polar icecaps. But thenj this was true of Earth throughout most of its existence. Even at the equator, some regions are balmy rather than hot, while the latitudes comfortable to man reach further north and south than on present-day Earth, People simply avoid the furnace-like deserts found here and there.

They also take precautions against the higher level of ultraviolet light, especially in the tropics. Again, this poses no severe problem. One can safely sunbathe in the temperate zones, and do so well into the polar regions in summer. Usually there is no undue glare of light; the more extensive atmosphere (vide infra) helps in scattering and softening illumination. Winter nights are usually ornamented by fantastically bright and beautiful auroras, down to lower latitudes than is the case on Earth—in spite of Qeopatra's strong magnetic field. To be sure, solar-atmospheric interference with radio and the like can get pretty bad, especially at a peak of the sunspot cycle (for Caesar, about 14 Earth-years long, as opposed to Sol's 11). But once installed, laser transceivers aren't bothered.

From A World Called Cleopatra by Poul Anderson (1974)

Planet Geography

Continents are ruled by plate tectonics and continental drift.

For terrestrial planets, they are more likely to have plate tectonics if they are more massive than Terra. Terra may be a borderline case, owing its tectonic activity to abundant water since silica and water form a deep eutectic. Having said that, some researchers claim to have detected plate tectonic activity on Mars, Europa, and Titan. The jury is still out on the question of super-earths, though.

The basic idea is that the surface of a planet (the lithosphere) is composed of a series of huge rigid "plates" that are floating on the more fluid asthenosphere. The oceans cover the plates, and the continents are just the bits of the plate that are higher than sea level.

The plates slide around on the asthenosphere, scraping and colliding with other plates. This form volcanoes, mountains, mid-ocean ridges, and oceanic trenches. It is also the cause of continental drift.

This is why it is of interest to worldbuilders inventing planet maps.

Plates are covered with two types of topping: oceanic crust and continental crust. The edges of a given plate covered in oceanic crust are the oceanic edges, the other edges are continental edges.

There are three kinds of plate boundaries:

  1. Divergent boundaries are where two plates are being pushed apart. The boundaries are mid-ocean ridges (rift valleys lined with mountains and volcanoes), where stuff from the interior of Terra is rising up. This is the reason why it looks like South America and Africa fit together like two jig-saw puzzle pieces, because they are two titanic jig-saw pieces. Keep in mind that South America and Africa are just the parts of the plates that stick out of the ocean. The actual plates come into contact at the Mid-Atlantic Ridge divergent boundary. Divergent boundaries caused the breakup of the supercontinent Pangea 200 million years ago.
  2. Convergent boundaries are where two plates are colliding with each other. This generally always forms a mountain range along the collision line.
    • Subduction zones happen when one plate has an oceanic edge colliding with another plate's continental edge. The oceanic edge goes underneath the continental edge. This forms an oceanic trench. Past the trench the subducted oceanic edge causes volcanoes to form on the surface of the crustal edge. It also forms a mountain range along the coast. An example is the "Ring of Fire" or circum-Pacific belt. The zone will be subject to earthquakes.
    • Obduction zones are like subduction zones, but instead the continental edge goes under the oceanic edge. This does not happen very often. Generally the oceanic ridge buckles, turning the edge into a mid-ocean ridge (underwater mountain range) and the oceanic edge starts subducting under the continental edge.
    • Orogenic belts aka suturing happen when two continental edges collide. The result is a huge mountain range, but usually not near the coast. No volcanoes, but prone to earthquakes. For example, the Himalayan mountain range formed where the Indian tectonic plate crashed into the Eurasian Plate
    • Unknown Zone I have no idea what a two oceanic plate collision is called. One of the two plate edges is subducted. There is a deeper than usual oceanic trench and a string of underwater volcanoes. These will for lines of volcanically created islands.
  3. Transform boundaries are where two plates are neither approaching nor receding, but their edges are grinding.

The type of plate boundary can evolve over time. Consider two plates Alfa and Bravo. Both are continental in the center but oceanic on the rim. Say they start to collide and become convergent boundaries.

  1. The two oceanic edge collide, forming the "unknown zone" type of convergent boundary. Say plate Alfa subducts under plate Bravo. A deep oceanic trench forms along with a string of volcanically created islands.
  2. After a few aeons, all of plate Alfa's oceanic edge has subducted under plate Bravo. Alfa's continental edge hits Bravo's oceanic edge. Suddenly the boundary transforms into an Obduction zone. The oceanic ridge buckles, Alfa's continental edge stops subducting, volcanic island formation halts.
  3. Eventually Bravo's oceanic edge starts subducting under Alfa's continental edge. Suddenly the boundary transforms into a subduction zone. Mountains start forming on Alfa's coastline, along with volcanoes.
  4. After a few aeons, Bravo runs out of oceanic edge as well. When both continental edges collide, suddenly the boundary transforms into an Orogenic belt. No more volcanoes, but the mountain range becomes more massive.

Turning back to the globe itself: Its greater mean density than Earth's is due to a higher percentage of heavy elements, especially those later hi the periodic table than iron. This leads to a particularly hot core which, combined with the rapid rotation, is the source of the magnetic field screening the atmosphere from solar wind. (Of course, the field is far weaker than in any generator—roughly twice as strong as Earth's—but it reaches way out.) Having not only more interior heat but a smaller volume, Cleopatra radiates more strongly.

This means that it is geologically, or planetologically, more active. There are more hot springs, geysers, volcanoes, quakes, and tsunamis, especially along the leading edges of continents and in midocean (vide infra). There is faster mountain-building, aided by the lower gravity which permits higher upheavals and steeper slopes. (The same is true of sand dunes.) Erosion proceeds more rapidly too; hence spectacularly sculptured uplands are quite common.

With the crustal plates more mobile than on Earth, we get an overall situation—there are many local exceptions, of course—about as follows. No continent is as big as Eurasia; the largest is comparable to North America. Their shelves drop sharply off to more profound depths than Terrestrial. They define—in the same rough way as on Earth—four major oceans, each surrounded by its "ring of fire" and marked down the middle by archipelagos of which numerous islands are volcanic. Elsewhere are smaller, shallower seas. Along with the tide patterns (vide infra), these factors tend to inhibit the generation of great ocean currents, and thus to somewhat isolate the latitudes from each other. That isn't all bad if "Norway" has no "Gulf Stream" to warm it, neither does the "Pacific Northwest" have a "Kuroshio" to chill it, and marine life is even more varied than on Earth.

The proportion of land to water surface is slightly higher than Terrestrial, mainly because of the powerful upthrust of crustal masses—though doubtless the splitting of H2O molecules by ultraviolet quanta, before there was a protective ozone layer, also has a good deal to do with this. However, there is no water shortage; in fact, the smaller size of individual land blocks and the vigorous air circulation make for better distribution of this substance and keep continental interiors reasonably temperate.

The abundance of heavy metals is a boon to industry, yet not altogether a blessing. Some of these elements and their compounds are poisonous to man. Concentrated hi certain areas, they make the soil, or organisms living there, dangerous. But again, this is by no means the universal case, and precautions are not hard to take once people have been warned. Several beautiful minerals and gemstones appear to be unique to this planet.

From A WORLD CALLED CLEOPATRA by Poul Anderson (1974)

If you’ll note, I’ve placed the equator, the tropic lines, and the arctic lines (24° from the equator and 24° from the poles respectively) based upon the tilt decided upon in step one. This isn’t important now, but it does impact on weather later on.

I’ve drawn out some hasty coastlines for two large landmasses connected by small isthmuses along with a single large island/ continent. You’ll notice the peninsula on continent B jutting out westward. I drew this because I like the shape. However, this probably means that that peninsula is mountainous since almost all such peninsulas from our base planet are mountainous. There are some exceptions (Florida), but most peninsulas are mountainous (Italian, Iberian, Malaysian, Kamchatka, Scandinavian, Korean, Baja). This also means that I’ve probably got a subduction zone offshore because most of these peninsulas do as well. I’ll keep that in mind as I go along.

I’ve also drawn dotted lines showing where my continents linked up in their Pangea stage. This will help me place my mountains in the next step.

I’ve filled in the mountains on the map. Mountain ranges A and D are formed by the collision of continents A and D. Range B results from continent A and B splitting as well as from subduction. When a rift occurs, one plate usually goes off in one direct and the other continues its path. This often creates a new subduction zone since one of the two plates isn’t moving exactly along with the rift’s movement. Mountain range C is pure subduction while mountain range E is a mixture of separation and collision. The split of continent E and D formed the north-south part, and the east-west part is from a plate that separated from E and then returned again. Range BE is subduction created.

But I’m jumping ahead of myself here. What I really did was put down the mountains where I thought they looked good, where they pleased my eye after looking at atlases for a long time. I came up with how they were formed after I already put them on the map. This is the great freedom of trusting your eye when placing mountains. If you look at atlases, find their patterns, and mimic them somewhat, the reasons for your mountains’ existence become almost self-explanatory.

I’ve determined the basic (very basic) plate structure of my world. There’s a large rift (oceanic ridge) between continents BE and CD. This follows along with how they fit in the pangea stage. There’s also a ridge between A and B for pretty much the same reason. The subduction zone off B’s peninsula is where the oceanic crust (created at the rift) is pushed under B’s continental plate. This means that mountain range A formed both by rifting as well as subduction. I’ve also added the two subduction zones that are creating mountain ranges BE and C. This could mean that at one time continents B and E were separated and the subduction zone linked them together through its mountain building, but it doesn’t have to. I’ve also included a small oceanic subduction zone offshore of E. This is where the ridge is subducted under the continental shelf of E (which extends under the water). I’ve only placed one slip plate in the ocean between continents AB and E. I’ve also indicated the suturing on continent E where its calved island is crashing back and the suturing between continents D and A where they’re coming together.

With this in mind, placing islands on the map is fairly simple. The large peninsula on B gains an archipelago (islands A), and islands form over the subduction zone offshore of E (islands B). More islands form along the slip plate (islands C). I’ve also placed some continentally connected islands (like England, islands D) and a few hotspot islands (like Hawaii, islands E). This gives you rough guidelines when you actually place your islands in creation. Namely, this lets me know where to put larger islands and why they are there. A more thorough look at plate structures would also provide more islands, but I’m only summarily addressing the plate details.

From A MAGICAL SOCIETY: GUIDE TO MAPPING by Joseph Browning (2004)


Ever since Galileo's first telescopic glimpse of the moon. scientists have known that the lunar surface has many enormous mountains. Because earth's mountains are steep until weathering erodes them, it was assumed that mountains on the moon would be like the steep-sided peaks shown here. The reasoning: there could be no weathering since the moon has no atmosphere


Continued observation and refined techniques of measurement now indicate that while the moon does indeed have enormous mountains, very few of them are steep-sided, as was once believed. Instead these lunar mountains tend to slope gradually, as shown here. with grades seldom steeper than a 10° angle. Precipitous slopes have been eroded by meteorites.

From MAN AND SPACE by Arthur C. Clarke (1964). A LIFE science library book

Planet Climate

Temperature Latitudes
Latitude NameLatitudeTemperature
North Pole90° north
Arctic ZoneArctic Circle to
North Pole
Arctic Circle90-axialTilt ° north
North Temperate ZoneTropic of Cancer
to Arctic Circle
Tropic of CanceraxialTilt ° north
North Tropical ZoneEquator to
Tropic of Cancer
South Tropical ZoneEquator to
Tropic of Capricorn
Tropic of CapricornaxialTilt ° south
South Temperate ZoneTropic of Capricorn
to Antarctic Circle
Antarctic Circle90-axialTilt ° south
Antarctic ZoneAntarctic Circle
to South Pole
South Pole90° south
Wind Latitudes
Latitude NameLatitudePrecipitation
North Polar High90° northDry
North Polar Easterlies60° to 90° northDry
North Polar Front
Subpolar Low
60° northWet
North Westerlies30° to 60° northWet
North Horse Latitude
Subtropical High
30° northVery dry
NE Trade Winds0° to 30° northVery Wet
Doldrums (ITCZ)Very wet
SE Trade Winds0° to 30° southVery Wet
South Horse Latitude
Subtropical High
30° southVery dry
South Westerlies30° to 60° southWet
South Polar Front
Subpolar Low
60° southWet
South Polar Easterlies60° to 90° southDry
South Polar High90° southDry

For a given region, its climate and biome type can be predicted by the region's average temperature and annual precipitation.

Average temperature is controlled by the temperature latitudes: Arctic Zone, North Temperate Zone, North and South Tropical Zone, South Temperate Zone, and the Antarctic Zone. These vary from planet to planet since their precise latitudes depend upon the planet's axial tilt.

Annual precipitation is partially controlled by the wind latitudes: North Polar High, North Polar Easterlies, North Polar Front, North Westerlies, North Horse Latitude, NE Trade Winds, Doldrums, SE Trade Winds, South Horse Latitude, South Westerlies, South Polar Front, South Polar Easterlies, and the South Polar High. Their precise average latitudes are fixed and are the same on all planets. However, they are not straight lines, they writhe like a worm on brown acid from hour to hour. Annual precipitation is also controlled by oceans, mountains, and other geographical features.

Given a region's mean annual temperature and the mean annual precipitation the climate and biome can be predicted by using the Whittaker Biome Diagram, Köppen climate classification, or with Holdridge life zones

Whittaker Biome Diagram

This is from R. H. Whittaker's Communities and Ecosystems (1975). The diagram has a few variations. It is intended for Terra, with temperatures ranging from 30°C to -15°C and annual precipitation from 0 cm to 450cm. You can scale these to fit the temperature and precipitation ranges on your planet. For instance, if your planet's precipitation ranges from 0 to 675 cm annual, divide each region's precip by 1.5 before looking it up on Whittaker's diagram.

Köppen climate classification

Köppen climate classification is a bit more complicated, but it is favored by real-world scientists.

Each climate is assigned a code of one to three letters. The first letter is the Climate Type. The second letter is Precipitation Pattern. The third letter is Temperature.

Köppen Symbol First Letter
ATropical climate
  • Monthly average temperature > 18°C
  • No winter season
  • Strong annual precipitations (higher than evaporation)
BDry climate / Desert
  • Annual evaporation higher than precipitations
  • No permanent rivers
CHot moderate climate
  • The 3 coldest months average a temperature between -3°C and 18°C
  • Hottest month average temperature > 10°C
  • The summer and winter seasons are well defined
DCold moderate climate
  • Coldest month average temperature of the coldest month < -3°C
  • Hottest month average temperature > 10°C
  • The seasons summer and winter seasons are well defined
EPolar climate
  • Average temperature of the hottest month > 10°C
  • The summer season is very little different from the rest of the year
Köppen Symbol Second Letter
CodeDescriptionApplies to
  • Steppe climate (semi-arid)
  • Annual precipitations range between 380 and 760 mm
  • Dry (Arid and semi-arid) climates
  • Annual precipitations < 250 mm
  • Wet climate
  • Precipitations occur every month of the year
  • No dry season
  • Dry season in winter
  • Dry season in summer
  • Monsoon climate:
  • Annual precipitations > 1500 mm
  • Precipitations of the driest month < 60 mm
  • Average temperature of the hottest month between 0 and 10°C
  • Average temperature of the hottest month < 0°C
  • Abundant precipitations
  • Mild winter
  • Af: Tropical rain forest climate. Examples : Singapore, Belém, Brazil.
  • Aw: Tropical wet and dry or savanna climate. Examples : Bangalore, India,
  • Veracruz, Mexico, Townsville, Australia.
  • Am: Tropical monsoon climate. Examples : Conakry, Guinea, Chittagong, Bangladesh.
  • BS: steppe climate
  • BW: desert climate
  • Cf: humid moderate climate without dry seasons
  • CW: humid moderate climate with dry winter
  • Cs: Mediterranean climate : humid moderate climate with dry summer
  • Df: cold continental climate without dry season
  • Dw: cold continental climate with dry winter
  • ET: Tundra climate. Examples : Iqaluit, Nunavut, Canada. Provideniya, Russia. Deception Island, Antarctica. Longyearbyen, Svalbard.
  • EF: Ice cap climate
  • EM: subarctic maritime climate
Köppen Symbol Third Letter
CodeDescriptionApplies to
a: hot summer
  • Average temperature of the hottest month > 22°C
b: moderate summer
  • Average temperature of the hottest month < 22°C
  • The 4 hottest months average temperatures > 10°C
C: short and cold summer
  • Average temperature of the hottest month < 22°C
  • Monthly average temperatures > 10°C for less than 4 months
  • Average temperature of the coldest month > -38°C
D: very cold winter
  • Average temperature of the coldest month < -38°C
H: dry and heat
  • Annual average temperature > 18°C
K: dry and cold
  • Annual average temperature < 18°C
  • BWh : Sahara
  • BWh : Yuma, Arizona
  • Cfb : France
  • Dfc : Siberia
  • Etw : Canada's Yukon Territory

Geoff Eddy's modified Köppen
TemperaturePrecipitationLocation, for
NameKöppenSummerWinterSummerWinterlatitude in degrees
Tropical rainforestAfHotHotWetWet0-10
Tropical monsoonAmHotWarmVery wetShort and dry5-15; east and south-east coasts only
SavannahAwHotWarmWetLong and dry5-15
Hot desertBWhVery hotWarmDryDry10-30, especially on west coasts with cold currents
Hot steppeBShHotWarmLow to dryLow to dry10-35; typically next to deserts
Cold desertBWkHotColdDryDryInteriors, rain shadow
Cold steppeBSkWarmColdLow to dryLow to dryInteriors, rain shadow
Maritime east coastCfaHotWarm to mildWetModerate20-40; east coasts only
Maritime west coastCfb, CfcWarm to mildCool to coldWetWet40-60; west coasts only
MediterraneanCsa, CsbHotMildDryModerate30-45, west coasts only
Temperate monsoonCwa, CwbHotMild to coldWetDry20-40; east coasts only
LaurentianDfa, DfbWarm to mildColdModerateLow40-60; not on west coasts
SubarcticDfc, DfdMild to coldVery coldModerateVery low60-80; not on west coasts
ManchurianDwa, DwbWarm to mildColdModerateDry40-50; east coasts only
Subarctic eastDwc, DwdMild to coldVery coldModerateDry45-70; east coasts only
TundraETColdVery coldLowDry60-80
IcecapEFVery coldVery coldLowDry75+

Geoff Eddy stated:

The climates given in italics are those which, generally speaking, are subject to the same influences throughout the year. The other climates may be regarded as transitions between these; for example, the Mediterranean climate is a combination of hot desert in the summer and maritime west coast in the winter.

Note the following:

  • Steppe and desert climates experience large diurnal variations in temperature, which means cold nights.
  • In the subarctic and tundra climates, winters are long, dark, and cold, and the other seasons are short.
  • Some sources mention Köppen climate types As and Ds, which are like Aw and Dw but with the dry season in summer rather than winter. I don’t know what causes these particular climates; they are very rare anyway and can probably be safely ignored.

Questworld Zones

In an article called "Questworld" (Different Worlds magazine of adventure role-playing games December 1981) they make a simplified Köppen system.

low+30° to -50° C
med+40° to -30°C
high+50° to +4°C
low0 to 38 cm
med38 to 89 cm
high89 cm and up
TempRainVegetation Type
lowlowGlacial, tundra: lichens, moss, some grasses, few if any trees.
medlowChapparal, wasteland: broken grasses, scrub, dry mountains.
highlowDesert: succulents.
lowmedForests: pines, firs, river brush: short-season blooming plants.
medmedGrassy plains, light deciduous forests.
highmedGrassy plains, light forests: highly variable seasonal rains.
lowhighAlpine: short growing season, deep snows; limited trees.
medhighMixed or all-evergreen forests, especially in wet mountains.
highhighTropical forests: vines, creep­ers, broad-leaf evergreens.

Holdridge life zones

Holdridge life zones are a bit new, but interesting. The system has been shown to fit tropical vegetation zones, Mediterranean zones, and boreal zones, but is less applicable to cold oceanic or cold arid climates where moisture becomes the determining factor.

A Magical society: Guide to Mapping

Although the sun is weather’s primary driver, the oceans provide the lifegiving water that the sun’s heat moves through the planet. The movement of water on a planet is fairly complex, but it can be easily simplified for mapping purposes. Water evaporates under the sun’s heat and collects in the air forming clouds. When the moisture level of the air becomes greater than it can hold (usually because a temperature change) rain falls back on the surface of the planet. The movement of air carries this water vapor off the oceans and onto the land (most of the rain on a planet comes from oceanic evaporation) and into the life on the land.

Water is generally subject to the air currents and their subsequent rain patterns, but it also influences them. The oceans heat and cool slower and to a lesser degree than land. This difference is very important. Water can store about five times the heat energy that land can store, which means water can absorb about five times more energy without its temperature increasing. The sun’s rays are also diffused over a much greater area of water (since light can penetrate water), which further reduces the maximum temperature water reaches in comparison with land. Water is also mobile allowing convection to distribute uneven heating easier than land and the unlimited amount of moisture in water means it can evaporate (and hence cool) unlimitedly when compared to land. All of this means that because water retains more heat, it cools slower during winter than land; conversely it takes longer to heat up once summer arrives again.

All of water’s unique properties have significant effects upon weather, and over time, climate because it changes temperatures. The hottest and coldest places on a planet will be on the interior of continents, far away from the influence of the oceans. The oceans act as a great heat sink; absorbing heat in summer and releasing it in winter. You should look at the amount of water in the northern and southern hemispheres of your new planet. The hemisphere with the most water will have less variance in annual temperature ranges for each latitude. On Earth, the Northern Hemisphere is 39% land while the Southern Hemisphere is 19% land. This causes the more extreme temperatures typical of the Northern Hemisphere.

Land has just as great an impact upon weather and climate as water. Unlike water, land quickly gains and loses heat. This leads to generally more erratic winds over landmasses than over oceans as the land cools quickly and in different proportions depending on its vegetative cover (the more plants, the slower it gains and loses heat; the fewer plants the quicker the process). This difference in cooling is noticeable in mountainous areas as mountains have more surface area per square mile than most other terrain types and particularly noticeable in deserts, which lose their day’s heat very quickly. Another important difference in weather over the land and over water is humidity. More evaporation occurs over water, so most humid air (the air that brings rain) comes from evaporation over oceans or other large bodies of water, like the Great Lakes. Most of the rain falling on the continents comes from evaporation off the oceans. Thus, if a continent is large, the centers will be very dry because most of the moisture has already dropped out of the wind. Central Asia (Gobi Desert) is a good example of this.

Terrain types and their respective vegetation levels influence weather through their respective heat absorption and release levels, but mountains are the only geographical features capable of affecting weather patterns outside of the sun’s influence. Mountains are physical barriers to wind and the cause massive disturbances in weather patterns, particularly rainfall. Air rises as it goes up a mountain, cooling it. This cooling reduces the amount of moisture the air can hold and often results in rain. This means, that in general, a mountain range will have a wet and a dry side. If the range is a large one and winds are fairly consistent in their direction, the mountain can create a rain shadow, effectively creating a desert. This can even happen on a smaller scale, like the island of Hawaii, where the eastern side receives the trade winds and an annual rainfall of 150 inches while the other side of the island only receives 9 inches of rain a year. A few (or a pair in the case of Hawaii) mountains can dramatically change weather.

Mapping all of the complexities of weather is something simply beyond the need of most new worlds. The general principles discussed above should provide you with enough raw information to look at your maps and make some decisions.

First, you should basically mimic the air patterns as influenced by the Coriolis effect. This provides a baseline that is agreeable to every other assumption about the working of weather. Adopting the basic ocean currents to the new world is the next step. Continental placement will affect this more than air patterns, but as long as the same general patterns of movement (gyres, areas of lows and highs) are maintained, the currents should closely mimic the Earth’s because they’re also influenced by heat and rotation. Our goal is to make a map that takes into account the natural functions of the universe. Before we can put down an ancient jungle kingdom, we’d best make sure it’s where the planet is going to naturally create a jungle. We can use magic to do it, but pre-planning avoids a lot of post-creation headaches.

Around the equator and around 60° N and S there are wet zones. Around 30° N and S (and more exactly the tropics) will be dry areas. This is a gross simplification, but it’ll get us where we need to go for right now. Mountains will affect the degree of rain, so be certain to indicate rain shadows based upon wind movement.

To map the weather, I used the equator, the tropics, arctic circles, and latitudes 30º and 60º. Since my tilt is very Earth-like, I don’t have to worry about weather patterns drastically diverging from Earth norms. I drew in the wind patterns based upon the Coriolis effect. After the air, I mapped the water currents, showing the typical gyre patterns. This is fairly straightforward, even though it’s a very complex physical process. I then mapped in the wet and dry latitudinal zones, again based upon the Coriolis effect.

Throughout this process, I’ve made a lot of arbitrary, but plausible, decisions. The movement of the wind is more complex than I’ve shown, but again, the pattern generally follows what I’ve put down. The same is true of ocean currents. They almost all follow the gyres according to their hemisphere (clockwise in the north, counterclockwise in the south), but there are some exceptions. I have a few currents that split and head in differing directions, but even these currents eventually follow the overall pattern. For example, the current off the east coast of continent B splits and flows up the coastline while the other part gyres up to continent C. The coastal flow up continent B eventually gyres back and rejoins at continent C. A good example of split currents is the Atlantic Equatorial Current. It travels from Africa to South American and splits. One flow goes south along the east coast of South America, and the other flows along the northern coast of South America. The southern split maintains the traditional counterclockwise gyre, but the northern current crosses the equator and eventually gyres clockwise as part of the Florida Current and the Gulf Stream. Generally, cold currents are moving from high latitudes to low latitudes, while warm currents are moving from low latitudes to high latitudes. On my world, a strong cold current flows from the south to the north along the west coast of continent E while a warm current moves north along the east coast of continent B.

Climate is where rain and temperature mix, therefore latitude, altitude, and wind pattern all shape climates. An idealized world has the pattern shown in the List of Climate zones.

List of Climate Zones

  • Arctic/Polar Region-North of Arctic Circle
  • Wet Zone- Mostly south of 60 N
  • Transition from Dry to Wet Zone
  • Transition to Desert Zone
  • Desert Zone-Tropic Circle and 30N
  • Transition to Desert Zone
  • Transition from Wet to Dry Zone
  • Very Wet Zone- Equator
  • Transition from Wet to Dry Zone
  • Transition to Desert Zone
  • Desert Zone-Tropic Circle and 30S
  • Transition to Desert Zone
  • Transition from Dry to Wet Zone
  • Wet Zone-Mostly north of 60 S
  • Arctic/Polar Region-South of Arctic Circle

Place each type of climate on your map in roughly the same manner. Again, pay attention to where the wind blows and where the mountains are. General elevation may play a role depending on how vast an area you’ve elevated. The Tibetan plateau is a good example of an elevated area changing the expected climate. More than elevated areas, ocean currents play an important role in determining climate. Warm currents heat the air around them, making Europe very habitable for example, while cold currents can temper a warm climate. Cold currents sometimes reduce rainfall along coastlines because they cool the air above them, restricting the amount of water the air can carry.

Rivers are easy to place at this scope; we’re just looking to place a few major rivers on each continent. Remember that water flows downhill and wet areas have more water than dry areas. Rivers are the easiest part of this step, so have fun and pay attention to where they’re going, because they’ll be the cradles of your forthcoming civilizations. Overall, this step is the most complex of all the mapping steps. The vast diversity of climate and the intricacies that make up each climate can’t be modeled without extraordinary effort. But even this very basic climate map of the world will help when discussing cultural development.

Following the general wind patterns, I first placed equatorial areas with heavy rain. The mountain range BE is packed with water because it’s not only on the equator, but the mountains catch the water and send it downstream in torrents. The northern part of continent D is very wet since there’s nothing interfering with winds, as is the northern part of continent A. These areas are probably rainforests because they have a lot of rain and plenty of sun.

The next step is to place the transitional areas that are more wet than dry. These were placed north and south of the very wet areas. Most are probably deciduous forests mixed with the remnants of rainforests, grasslands mixed with deciduous forests, and the beginning of the dryer lands. They could be simply grassland as well. Notice that these two zones are mostly within the tropic bands. Their placement also reflects what the wind is doing. On continent A, this zone loops around because the wind is coming from a particular direction while on continent D, the zone remains more horizontal for generally the same reason. You could change these zones based upon what you wish to happen. As long as they’re relatively in the same location, such change can easily be supported.

The next step moves into dryer lands by placing the transitional dry zone. These are mostly grasslands/scrublands, and they generally abut the transitional wet zones. Such zones are plentiful throughout the dry latitudinal zone and often abut a dessert zone. I didn’t place these zones next on the map, however. It’s easier if you go right to the deserts, and then look to see where these zones fit best.

Deserts are almost always along 30° N, 30° S, or the tropic lines. I placed my deserts along these areas and paid particular attention to wind direction. The desert on continent A is in a dry zone, but it also has a range of mountains that interfere with rain, so it stretches farther north into the wetter latitudinal zones. A similar thing happens with the desert on continent D. The desert on A exists because it’s in the dry zone, but notice that I placed a dry transition zone along the southern coast. The ocean air is relatively dry in this zone, but what little moisture it holds drops along this curve. All things considered, the desert on A is probably fairly wet for a desert until you go in deeper. The great desert on E was the hardest to place because there are many factors to weigh. It is a mixture of dry zone, wind patterns, mountain range and large landmass. For these reasons, I decided that this was the Sahara of my world: the big, sandy, unfriendly desert.

I then placed the dry transitional zones between the deserts and the wet transitional zone. It’s much easier to do it this way, even though you have to consider the next wet transitional zone leading to the wet band around 60° north and south latitude. Again, I found it easier to just jump zones and place the midlatitude wet zones before placing both the dry transitional zones and the wet transitional zones.

Placing the next wet areas was very easy. I just followed the wind patterns and land as I did with the equatorial wet zones. These wet zones aren’t as wet as the equator, so if you find yourself faced by a large continent, the water won’t travel as far inland as it would at the equator. I mapped my midlatitude wet zones, then placed my wet transitionals, and finally the dry transitionals. These dry transitional zones are mostly grassland/scrublands while the wet transitional zones are mostly forest/grassland mixes. The midlatitude wet zones can be temperate rain forests if there are mountains to catch enough rain, like along the Pacific Northwest coast. With all that done, I capped of my world with the cold zones north and south of the arctic circles. These zones can be grasslands, boreal forest, or tundra depending on their rainfall and how close they are to the poles. I haven’t differentiated between the polar climates for this map, but by now, you should be familiar enough to place your taiga and tundra without guidance.

Next, I placed rivers on each continent, keeping in mind wind patterns and general elevation. Most of them are straightforward and not worth mentioning except for the major river on continent B that runs through the desert. This river provides water in the otherwise dry expanse and will no doubt play a role in intelligent creatures’ interactions. It could also mimic the Nile as its headwaters are in wetlands. If I wanted, I could make these headlands have a particularly rainy period that would mimic the yearly floods of the great river on Earth. I think I will.

From A Magical society: Guide to Mapping by Joseph Browning (2004)

Planetary Maps

Planets are more or less spheres, while maps are generally flat pieces of paper. It is impossible to smush a three dimensional surface onto a two dimensional map without distorting it in some fashion. Try peeling the skin off an orange and getting it to lie perfectly flat to get an idea of the problems. The best you can do is decide what part gets distorted, because something has to be. Cartographers have been pulling their hair out by the roots over this problem ever since man discovered that Terra was not flat. Flat paper maps are so convenient and easy to produce, globes are clunky and very expensive to create.

Smashing a globe onto a flap map is the art and science of Map Projection. There are a million-six different kinds, with new projections being invented from time to time.

Back in the 1980's the innovative role playing game Traveller was invented. The "game masters" of a Traveller game faced much the same problem as a science fiction author worldbuilding a science fictional planet. That is, they have far too much work to do in worldbuilding, there is no time to futz around with complicated mathematical projections just to make the freaking map. There are more important things to do. They need something quick-n-dirty that is both easy to make yet accurate enough for some simplistic travel-time estimates.

The Traveller solution was the icosahedral world map.

A regular icosahedron was chosen as the basis for the map (familiar to gamers as a "20 sided die"). Of all the Platonic solids it has the highest number of faces, and thus is the Platonic closest to being a sphere.

When it comes to determining the distance between two points, wargames and role playing games use maps ruled off into zillions of hexagons in a hex grid. You jump from hex to adjacent hex proceeding along shortest path from start to destination then multiply number of jumps by the distance between adjacent hexagon centers in kilometers, this gives the distance. This will not work if the map is ruled off into squares or triangles (the only other alternatives), since jumping diagonally is a longer distance than jumping orthogonally. In hex grids, all six neighbor hexagon centers are the same distance.

Determining distance is useful for so many things when you are writing the plot for your science fiction story. Can army Alfa get to the choke point before enemy army Bravo passes through, thus heading them off at the pass? Where are the land and sea trade routes on this planet? If Flash Fearless crashes his starship at point X in the Inferno Desert, how many kilometers is it to the nearest city?

Travel time is simply the distance divided by the average speed. If your hero travels 3 hexes and the distance between hex centers is 1,000 kilometers, the distance is 3,000 kilometers. If your hero is in a groundcar with an average speed of 97 kilometers per hour, it will take them 3,000 / 97 = 31 hours.

Note that when figuring the average speed you should factor in the time the hero spends resting and sleeping (that is, not traveling). If the hero is resting for 8 hours out of every 24, they are only moving for 16 hours out of every 24. 16 / 24 = 0.666... So if the ground car travels at 97 km/hr, its average speed is actually 97 * 0.66 = 65 km/hr.

In wargames they get fancy. Groundcars and people walking on foot have their speed modified by how rugged the terrain is. Their speed will be faster on a road as compared to slogging through a swamp or a mountain range. Wargames commonly color code the hexagons by "terrain type", and have a little table showing the relative ease of travel. Often there will be special terrain on specific hex edges, where the movement is impeded only if you cross that edge when jumping into the next hex. This is usually used for rivers and mountain ranges.

Naturally anybody traveling by air can more or less ignore the terrain (unless it is infested with enemy anti-aircraft weapons). And somebody using a low-flying hovercraft or rocket belt will be partially affected by the terrain: there is some effect but much reduced compared to actually walking on the ground.

The icosahedron is cut apart along the edges and unfolded so it lies flat. In the map, the horizontal lines show the connections between split hexagons, they are considered a single hex for distance counting. In addition, the hexes split along the left and right edges of the map are also considered a single hex.

Yes, if you wanted an actual globe you could print such a map on card stock, cut it out with tabs on the edges, and glue it together.

The polygons at the vertexes are actually pentagons, not hexagons, but don't worry about that. It is impossible to tile a convex object totally in hexagons, you need a few pentagons to get things connected (as you can tell by looking at a Telstar-style international foot ball, what folks in the US call a "soccer ball." The black polygons are pentagons).

The original classic Traveller world maps were seven hexagons wide on each triangle edge (count from hexagon center to next hexagon center, not counting the start hex). There is nothing special about seven, they just picked one that made a pleasing map. A map maker can use any number of hexagons along triangle edges that they want.

Since all classic Traveller maps use the same template, the distance between adjacent hexagon centers varies according to the diameter of the planet in question. If a map maker had several planets to map all with different diameters, and for some reason they wanted the hex distance to be the same for all the maps, they could vary the number of hexagons per triangle edge to accommodate this. (Quite a few Traveller supplements suggested this as an optional rule, but it didn't become a core rule until Traveller 5th edition.)

Since the hexagons will probably have separation distances on the order of several hundred kilometers, you might find this a bit coarse resolution if you are using them to figure foot travel and/or coding them by terrain type. That is, if the hex is 1,000 kilometers wide, this is about one-third the width of the United States. One US hex could contain a bit of a mountain range, some desert, and prairie flat land. Averaging all this into a single terrain type loses quite a bit of detail.

You might have to subdivide the hex into lots of smaller sub-hexes, to get the detail necessary to plot your protagonist's personal journey. The large hexes will be good enough for plotting details on a planetary scale, like cities and continents.

Given the radius of your planet and the number of hexagons per triangle side, the distance from hex center to adjacent hex center is:

Hw = (2 * π * Pr) / (Te * 5)


Hw = adjacent hex center distance (kilometers)
Pr = Planet radius (kilometers)
Te = Triangle side length (in hexagons, hex center to hex center, not counting starting center)
5 = number of triangle sides along the equator
π = 3.14159...
(2 * π * Pr) = equation for circumference of circle, i.e., how many kilometers in the equator
(Te * 5) = number of hexagons along the equator

Using the GURPS template (9 hexes per triangle side) for a planet with a radius of 6,371 kilometers (Terra), what is the adjacent hex center distance?

Hw = (2 * π * Pr) / (Te * 5)
Hw = (2 * 3.14159 * 6,371) / (9 * 5)
Hw = 40,030 / 45
Hw = 890 kilometers

If for some odd reason you want the adjacent hex center distance on a huge icosahedron instead of a spherical planet, the equation is:

Hw = Pr / (0.9510565163 * Te)


Hw = width of a hexagon from hex edge to hex edge (kilometers)
Pr = Planet radius (kilometers)
0.9510565163 = sine[ ( 2 * π ) / 5]
Te = Triangle side length (hexagons, hex center to hex center, not counting starting center)

This is a smaller distance because the circumference of a circle is larger than the circumference of an icosahedron.

Hw = Pr / (0.9510565163 * Te)
Hw = 6,371 / (0.9510565163 * 9)
Hw = 6,371 / 8.5595086467
Hw = 744 kilometers

In classic Traveller, they had one single map template (with 7 hexagons per triangle side) used for mapping all the planets. Since planets vary in radius, the distance between hex centers had to vary as well. The map template had a box to record the hex size.

In Traveller 5th edition the distance between hex centers was fixed at 1,000 kilometers. This means the number of hexagons per triangle size varied with the planetary radius. The drawback was you needed multiple map templates, one for each range of planetary radii. The advantage was that you didn't have to do a lot of math based on the variable hex size when trying to figure distances.

Traveller Map Templates
TypeHex SizeHexagons per
triangle side
Number of
Map Templates
Classic TravellerVariableFixed (7)1
Traveller 5thFixed (1,000 km)VariableLots

In Traveller: 2300 the distance between hex centers was also fixed at 1,000 kilometers.

The game master would take a "geodesic map" (an icosahedral map template WITHOUT any hexagons) and roughly sketch the continents, rivers, mountains, cities, and other broad features the players could see from orbit. The game master would give this map to the players.

The game master would then secretly create detailed maps of any map triangles where the players could encounter adventures. The players would only see the details if they landed and explored the hard way.

On the geodesic map, the triangles are numbered. To map a particular triangle:

  1. Print out a fresh blank triangle mapping template.
  2. Write the triangle number (from the geodesic map) in the box at the bottom.
  3. Consult Geodesic Map Triangle Sizes table. "World Size" is diameter of planet in thousands of kilometers (Terra = 12). Triangle Side is number of hexes per triangle side.
  4. On the triangle map, along both Side A and Side B, mark a dot at the Triangle Side number from table (Terra = 8). Draw a line between the two dots. Ignore the hexes further out (i.e., steps 3 & 4 are so the game company can avoid having to publish nineteen different templates).
  5. Using the rough sketch on the geodesic map triangle as a model, draw in the details on the triangle map. Then add monster lairs, fabulous locations, treasure sites, traps, and other items to surprise the players. Remember that each hexagon is 1,000 kilometers wide hex center to hex center.

Any hexagon on the triangle map that needs more detail can be mapped using the Region Map template (this is what Traveller 5 calls a "World Hex"). Hexagons there are 100 kilometers wide. At this scale it is worthwhile to define the terrain types and write them on the terrain key. Be sure to fill in the entries for World Name, Geodesic Map Triangle number, and Triangle hex number.

Any hexagon on the triangle map that needs more detail can be mapped using the Region Map template (this is a map for a Traveller 5 "World Hex"). Hexagons there are 100 kilometers wide. At this scale it is worthwhile to define the terrain types and write them on the terrain key. Be sure to fill in the entries for World Name, Geodesic Map Triangle number, and Triangle hex number.

There is a nice selection of free downloadable icosahedron maps of various planets and moons in our solar system available here and here. Traveller artist Ian Stead has a blank world sheet here with an example of use here. Artist Shawn Driscoll has made some nice Traveller maps here. The Icosahedral WorldMap Generator will randomly create worldmaps per specification (requires Java to run). A plug-in for Adobe Photoshop (or other graphic software that handles such plug-ins) is Flexify 2, it is a commercial product (costs money) which is which can import conventional equirectangular (or other format maps) and convert them into icosahedron maps. Commercial software Cosmographer 3 has a mode specifically designed to create Traveller style icosahedron maps.

Satellites and Rings

Planet Moth and its Wings

But the most beautiful thing about Moth was not Drallar, with its jewelled towers and chromatic citizenry, nor the innumerable lakes and forests, nor the splendid and variegated things that dwelt therein. It was the planet itself. It was that which had given to it a name and made it unique in the Arm. That which had first attracted men to the system. Ringed planets were rare enough.

Moth was a winged planet.

The 'wings' of Moth doubtless at one time had been a perfect broad ring of the Saturn type. But at some time in the far past it had been broken in two places — possibly the result of a gravitational stress, or a change in the magnetic poles. No one could be certain. The result was an incomplete ring consisting of two great crescents of pulverized stone and gas which encircled the planet with two great gaps separating them. The crescents were narrower near the planet, but out in space they spread out to a natural fan shape due to the decreasing gravity, this forming the famed 'wing' effect. They were also a good deal thicker than the ancient Saturnian rings, and contained a higher proportion of fluorescent gases, The result was two gigantic triangular shapes of a lambent butter-yellow springing out from either side of the planet.

Inevitably, perhaps, the single moon of Moth was designated Flame. Some thought it a trite appelation, but none could deny its aptness. It was about a third again smaller than Terra's Luna, and nearly twice as far away, It had one peculiar characteristic. It didn't 'burn' as the name would seem to suggest, although it was bright enough. In fact, some felt the label 'moon' to be altogether inappropriate, as Flame didn't revolve around its parent planet at all but instead preceded it around the sun in approximately the same orbit. So the two names stuck. The carrot leading a bejewelled ass, with eternity forever preventing satisfaction to the latter. Fortunately the system's discoverers had resisted the impulse to name the two spheres after the latter saying. As were so many of nature's freaks, the two were too uncommonly gorgeous to be so ridiculed.

(ed note: I do not believe Flame's orbit is stable)

From The Tar Aiym Krang by Alan Dean Foster (1972)
A World Called Cleopatra

Cleopatra has no moon in the usual sense. Perhaps it once did, or perhaps an asteroid was captured. In any event, at some point in the fairly recent past (estimated 10 million years ago), this body (estimated mass, 0.001 that of Luna) came within the Roche limit and was pulled asunder by tidal forces.

Numerous fragments fell. The biggest left traces in the form of huge circular lakes, bays and valleys. Meteorites are still coming down as perturbation maneuvers them out of orbit. So there are many pitted rocks, many craters great and small, on Cleopatra, the newest sharply denned, the oldest blurred by erosion. On any clear night, shooting stars may be seen delightfully often.

But most of the disrupted mass formed a ring, at a mean distance of some 7500 km from the surface, which is still around and will probably last for millions of years to come. It is not like the,ring(s) of Saturn, the latter consisting of tiny ice particles. Cleopatra is surrounded by a belt of stony and metallic fragments, ranging in size down to gravel and fine dust. There is considerable space between the average pair of rocks, though of course this varies.

Except for Charmian and Iras (vide infra), the satellites are too small to be seen by day against sun glare. Moreover, being nearly in the equatorial plane, the ring shows best in the tropics. In high latitudes one sees it low in the sky, often obscured by mountains, woods, or haze; and one cannot see it at all in the polar regions (above latitude 66°) aside from a few isolated, far-out particles.

The ring is at its most spectacular at equatorial midnight around the time of solstice. Then a band of hundreds of glittering, twinkling fireflies streams across the sky from west to east, the faster (nearer) overtaking the slower (further out) though all move swiftly. Irregular in shape, scoured and scored by dust, many sparkle in prismatic hues as well as white. The dust itself forms a dimly glowing background, through which stars can be seen. Though the band has no constant or definite boundaries, it averages about 10° wide, brightest at the middle, fading out toward the edges.

The mean synodic period of a particle, i.e., the time for a complete cycle from rising to rising as observed on the ground, is 7.5 hr or about 0.43 Cleopatran day. This is 48° per hr, or rather more than three times as fast as Sol or Luna crosses the Terrestrial sky. However, the ring is too close in for the entire half arc to be visible anywhere on the planet, so the maximum time observed (at the equator) is 1 hr 22 m.

That time is really only interesting as concerns the two members of the ring which are so big that they may be called tiny moons. They have, indeed, been given names, Charmian and Iras. (At the nomenclature conference, one faction wanted a Ftaatateeta but was voted down). Charmian is the larger and slightly closer. In fact, it seems just about the same size as Luna does on Earth, though its actual mean diameter is not quite 70 km. Iras has about half the linear cross section and moves a little slower. (The respective synodic periods are 7.6 and 8.2 hr, which means that Charmian overtakes Iras every 102 hr or 5.9 Cleopatran days. These figures are subject to some oscillation because of assorted gravitational influences.) The two orbits are so skewed that, while they come near, the moonlets seldom overlap.

In other words, they move along the ring approximately four times in a Cleopatran day and night, going through approximately 5.6 phase-change cycles as they do; but most of this cannot be seen from any single place on the ground.

Neither looks much like Luna. Charmian is only roughly spheroidal, Iras still less so. They show angles, facets, promontories and markings as they orbit the planet while spinning in a wobbly fashion. They both resemble Luna in being large and reflective enough to remain visible during an eclipse.

This eclipse is due to the fact that Cleopatra's shadow crosses the rings. There is sufficient axial tilt that at a solstice, only a small "bite" is taken out of the lower edge of the band at its lowest point—and the band is irregular, fluctuating, and vaguely denned enough for this not to be particularly noticeable. But as the planet moves on around its sun, the geometry changes. About 23 Cleopatran days after solstice, the shadow arc entirely bisects the ring. By equinox, ca. 160 days after solstice (ca. 115 Earth days), the eclipse is at a maximum.

At this season, when watched from the equator, the ring—including the two moons—streams upward from the west as before. But at an azimuth of about 52°, not quite 60% of the way up to the zenith, the particles blank out. They do not reappear until they are correspondingly near the eastern horizon and descending. Charmian, Iras, and a few of the largest meteoroids remain visible but turn dull coppery red from atmosphere-refracted light, as they transit the dark gap.

This cycle of eclipse and full illumination is repeated twice in the course of a year. The precise appearance of the ring, as well as its position in the heavens, depends on time and location of the observer.

But at any season—what with auroras, background skyglow, stars, ring, and the frequently seen changeable moonlets—Cleopatran nights are not unduly dark. In clear weather, a human can make his way around pretty well without artificial light.

The tidal pull of Caesar is small, about one-third that of Sol on Earth or less than one-fifth the total of what Earth gets. Were the ring particles concentrated in one mass, the total heave would be enormous, about 18 times what Luna gives to Earth. Scattered as they are, they produce only minor effects individually. But the resultants are complex and variable. The seas do not get stagnant, and crosscurrents often make them choppy.

From A World Called Cleopatra by Poul Anderson (1974)

Natives Flora and Fauna

Keep in mind that there are alien chemistries that could be the basis of the biochemistry of the planet's flora and fauna, given the proper temperature and planetary make-up.

Also see GURPS: Uplift.


In the Traveller role playing game, it broke down animal types into four broad classes: Herbivore, Omnivore, Carnivore, and Scavenger. They were further broken down into sub-types:

  • Herbivore: Animals that eat unresisting food. Plant-eaters, but also whales eating krill and anteaters eating ants.
    • Grazers: Herbivores that devote most of their time to eating. They may be solitary or grouped in herds. Their primary defense is running away very fast. Examples: antelope, moose, whale.
    • Intermittents: Herbivores that do not devote most of their time to eating. They tend to be solitary. They tend to freeze when encountering another animal but will flee if attacked by something larger. Examples: chipmunk and elephant.
    • Filters: Herbivores that pass the environment through their bodies. Grazers move towards food, filters move a flow of water or air through their body in order to gain food. They generally suck, trip, push or pull anything at close range into their digestive sack. They are solitary and tend to be slow-moving. Examples: barnacle.
  • Omnivore: Animals that eat food regardless of its resistance. For instance: bears eat berries as well as small animals.
    • Gatherers: Omnivore that display a greater tendency to herbivorous behavior. They are similar to Intermittents. Examples: raccoon and chimpanzee.
    • Hunters: Omnivore that display a greater tendency to carnivorous behavior. Similar to small or inefficient chasers. Examples: bears and humans.
    • Eaters: Omnivore that does not distinguish its food, it consumes all that it confronts. Examples: a swarm of army ants.
  • Carnivore: Animals that eat violently resisting food by attacking and killing said food.
    • Pouncers: Carnivore that kill their prey by attacking from hiding, or by stalking and springing. Generally solitary since it is hard to coordinate such attacks. If they surprise their prey they will attack, but will sometimes attack even when surprise is lost. If they themselves are surprised they will flee. Examples: cats.
    • Chasers: Carnivore that kill their prey by attacking after a chase. They tend to be pack animals. Examples: wolves.
    • Trappers: Carnivore that passively allow their prey to enter a created trap, whereupon the prey is killed and eaten. They tend to be solitary and slow, but will attack literally anything that enters the trap. Examples: spider and ant lion.
    • Sirens: Similar to Trappers, except it creates some kind of lure to draw prey into the trap. Sometimes the lure is specific to some prey animal, sometimes the lure is universal. Examples: angler fish, Venus fly trap.
    • Killers: Carnivore that devote much attention to killing, a blood lust. They have a raw killing instinct. Attacks are fierce and violent. They do not care how large their opponent is. Examples: shark.
  • Scavenger: Animals that share or steal the prey of others, or that takes the nasty unconsumed left over bits.
    • Intimidators: Scavenger that steal food from other animals by frightening or threatening. They approach another animal's kill and force it away by appearing to be a threat. Examples: coyote.
    • Hijackers: Scavenger that boldly steal food from another animal. Hijackers are stronger or larger than the victim animal, so that it cannot effectively object. Examples: lion, tyrannosaurus rex.
    • Carrion-Eaters: Scavengers that take dead meat when it becomes available, often waiting patiently for all other threats to disperse first. Examples: buzzard.
    • Reducers: Scavengers that act constantly on all available food. They eat the remains of food after all other scavengers are finished with it. They are generally microscopic. Examples: bacteria.

Note that the animal type which an intelligent alien evolved from will give clues as to that alien's psychology.

A World Called Cleopatra


Given a planet this similar to Earth, it is not surprising that here too life arose, based on proteins in water solution, and in time developed photosynthesizing plants which formed and now maintain an oxyrritrogen atmosphere. It is unusual to have so many details duplicated. (To be sure, given the vast number of worlds in the galaxy, this must happen once hi a while.) Here too life uses predominantly levoamino acids and dextro sugars. Many lipids, carbohydrates, hydrocarbons, pyrroles, etc. are the same as on Earth, chlorophyl and hemoglobin included (with some minor variations). In like manner, we find viruses, bacteria, protozoa, vegetable and animal kingdoms.

Now it would be too improbable for every detail to be the same, considering how many are the consequence of random "choice" among numerous possibilities. Much Cleopatran life can be eaten by man, is nourishing and tasty; but some of it is poisonous, and all of it lacks certain vitamins and other nutrients. Hence one can live only temporarily on an exclusive diet of it. This is not a great handicap. In fact, basically it is desirable, because it works both ways. Native germs cannot function in the human body, native viruses are not equipped to take over human genetic machinery—in short, to man this is an infection-free world.

And of course he can introduce his own plants and animals. Given a start—e.g., by eradication of deadly weeds from a range—they will flourish. Soon the problem will be to save the Cleopatran ecology. Once established, Terrestrial life will spread fast and overwhelmingly unless it is controlled. For it is further evolved.

After all, Cleopatra is younger than Earth. If anything, it is surprising how far life has developed, in so much less tune. Conceivably the energetic sun, the higher lever of actinic radiation and electrical discharge, promoted rapid development of the primitive proto-biology and later microorganisms. But afterward, perhaps, the weak tides—making for a sharper division between sea and land—delayed the conquest of the latter. At any rate, though inaccurate, it is helpful to think of this world as being in a "Mesozoic" era.


Angiosperms have not yet developed. There are primitive equivalents of the spermatophytes, including some gymnosperms. These are most common in the drier inland and upland regions. The coasts, marshes, etc. are dominated by types similar to Terrestrial bryophytes and pteridophytes, more elaborated than on present-day Earth. Because of certain root-like structures, they are known as dactylophytes.

Nothing like grass or flowers exists. Moist areas are carpeted with low, dense, intensely green vegetation resembling moss. Species of this phylum have developed protection against drying out and are therefore found elsewhere as ground cover in paler and stiffer versions. Many trees and shrubs (if one may call them that) have colorful pseudoblooms, analogous to those of our poinsettias, to lure pollinators.

Among the more picturesque plants are: The misnamed dinobryons, huge dactylophytes in wet regions which suggest spongy green many-branched coral growths; the aquatic weirplant and its land relatives, the dichtophytes, carnivorous species which grow in the form of great nets to trap sizeable prey; the Venus mirror, a bush named for its highly reflective leaves, which attract glitterwings, the chameleon plant, which exhibits changes of shade and even to some degree color, according to lighting conditions—a camouflage against eaters; the sarissa, resembling sharp-pointed bamboo but growing in clusters which bristle almost horizontally outward, supported by roots along the stalks; the grenade, a bush whose round pods explode spectacularly, though harmlessly, to scatter seeds; the Christmas memory, a primitive evergreen whose roughly shaped but brilliant red cones are like ornaments; and the delicious sugarroot.

No one region has all kinds. Some genera are circum-polar, others not. This is likewise true in the zoological field.


A biologist would vehemently deny that Cleopatra has insects, fish, amphibians, reptiles, birds, mammals, or anything else Terrestrial, other than what man may import. There are too many differences of detail, some quite fundamental. Nevertheless, resemblances are close enough—when similar environments have selected for similar characteristics—that pioneers are not inclined to split every semantic hair.

The colonists do use scientific names for the broad classes. But "worm" has so wide a meaning even on Earth that it can reasonably be applied to numerous legless invertebrates on Cleopatra. One interesting family is that of the arthroscholes, whose segments carry articulated, chitinoidal blue armor. Thus protected, they may grow to lengths of more than a meter.

"Insectoid" soon became shortened in daily language to "secto," and is as loosely applied as ever "insect" and "bug" were on Earth. There are countless kinds of secto. Among the famous are the glitterwing, like a moth whose wings are almost mirrorlike because of tiny metallic particles; a long, many-legged, bulge-eyed scuttier called the I-spy; and the smidgin, which travels in swarms that darken the air, accompanied by flyers that leisurely feast on them.

Marine invertebrates include the drifting gorgon with its mesh of lethal streamers. The big polypus has no definite number of tentacles, for injury causes more than one new one to sprout. When it has grown inconveniently many, the animal develops a second head and set of interior organs, and fissions into two—an alternative to its usual sexual reproduction. Biologists are fascinated by the problem of how this is possible in something of that size and complexity.

Besides male-female sexuality and paired eyes, parallel evolution has produced Cleopatra vertebrates which, like the Terrestrial, have just four true limbs.

Piscoids include the great, sleek, swift, carnivorous pirate and the miter-headed, grotesquely ululating sea preacher. Among marine sauroids is the macrotrach, remarkably similar in appearance to the ancient plesiosaur.

The land is dominated by sauroids. Many of them are more highly developed than any Terrestrial reptile, having efficient hearts, giving live birth and caring for their young, even showing an almost mammalian capacity to learn by experience. This is probably due to the fact that, on generally warmer Cleopatra, being homeo-thermic ("warm-blooded") confers less relative advantage than on Earth; there do not seem ever to have been any glacial periods. Thus poikilothermic ("coldblooded") animals have had more chance to flourish and evolve new capabilities.

The best-known ones include: the hipposaur, a hoofed grazer of plains and mountains, as big as a big horse; the king gator, a dry-land carnivore with long legs but otherwise rather crocodilian; the hoplite, a two-meter-wide walking dome of bony armor and spiky tail; the faber, eerily caricaturing humanity in appearance and certain behavior patterns; and the huge-winged flying deltasoar.

The homeothermic beasts remain primitive. They have hair, whose possible colors include a bright green, but no mammary glands. Most young are born with full sets of teeth, immediately able to eat the same as the parents. Where this is not the case, feeding is by regurgi-tation. Thus even some ground-dwelling animals have beaks rather than snouts, and none have lips.

They are furthest developed in the aerial forms, the ptenoids or,pseudobirds. Though none of these quite compare to Terrestrial avians in capabilities, they number some handsome species, like the colorfully plumed jackadandy. The rich-furred (not feathered) flier and diver known as the cinnamon bat is, however, a theroid.

No theroid is very large. A common forest dweller is the tree spook, suggestive of a parrot-billed lemur. On one continent, the carnivorous hootinanny runs in packs which make hideous loud noises in their throat pouches to stampede the prolific herbivorous jumping Toms; both species are rabbit-sized. In arctic regions, the snow snake has shed legs and belly fur in order to go more effectively after its own burrowing prey; with its white pelt everywhere else on the body and its affectionate ways, it makes an excellent pet. Of course, this is only a partial list.

In fact, all these remarks are quite superficial and incomplete. Any planet is a world, and therefore inexhaustible.

From A World Called Cleopatra by Poul Anderson (1974)
Planetary Purple Prose

(ed note: Lord Flitmore has discovered "centrifugal power", a species of antigravity, and has built a "world-ship"; so called-because "air-ship" is a poor term for a vessel that travels into airless space. He departs on a trip accompanied by his wife, his manservant, and a couple of friends. After touring the solar system, they are thrown off course by an errant comet, and wind up at a planet orbiting Alpha Centauri A. The author then proceed to give an extravagantly ornate description of the rainbow-lit landscape. Apparently the planet should be named Lisa Frank.)

Here was the meadow on whose soft carpet they had pitched camp.

"A soft carpet" was no mere phrase here: the finely feathered grasses with their transparent green were, in fact, as soft as down.

And the flowers of this exquisite meadow! They shone in every color; but what gave them an especial charm was that unequalled delicacy, which put to shame even the spring blossoms of earth.

So transparent were these blossoms that the background could be plainly seen glimmering through them as through the finest colored glass. But as the light fell upon them, it was thrown back in the most delicate shades, so that a colorful cone of radiance seemed to spread out from them.

This curious, yet so remarkably fascinating, transparency seemed to invest the whole plant-world of the blissful planet with its singularity. There were bushes with splendid large flowers like hanging bells, swaying like plates and saucers, like small balloons or soap-bubbles stretching upwards in round, oval, cylindrical, or composite forms; in the background rose forests of fruit-laden trees, some with slim, others with gnarled trunks, their graceful limbs swinging low with leaves in all imaginable patterns; and all things blinked and glittered where they threw back the light, and appeared thoroughly transparent where the beams of light shot through.

These transparent forms were often like crystals and prisms, and broke up the light into all the colors of the rain­bow. From the color of the object, together with the color of the rays shining through it, came the most wondrous tints and the most delicate combinations, so that even the deepest shadow gladdened the eye with the myriad richness of its colors.

And then the lake, smiling up at the sky! A blue of a tint not to be seen on earth, an aura of sapphire. Hard by the shore, it was so transparent that each of the many-colored grains of sand at the bottom could be discerned. But where the color-beams, which crossed and fused in the air, were reflected in its waters, you would find areas of the most variegated shading till the eye was bewildered and knew not which was loveliest, only to be riveted again by the golden sheen, the silver shimmer, the rosy aura here and there, unable to turn itself away from the fairy-like beauty of the scene.

But turn itself away it must: the islands and islets, the marvellous curve of the shore, the coves and headlands, the banks on the other side;, the hill-ledges, and the imposing declivities with their jagged combs and unusual forms—all these commanded its attention and drew forth ever new exclamations of wonderment.

Every moment somebody in the company believed he had discovered something new which surpassed everything seen before. They called each other's attention to many things, and eye and heart gloried in an uninterrupted holiday of rapturous enjoyment.

"Eden, Eden!" exclaimed Flitmore, won away from his customary reserved calm. "What other name could we find to give this paradise? And if the whole planet were other­wise a cheerless, terrifying wasteland, this one spot would justify us in giving it the name of the region that held the garden of paradise."

From Wunderwelten (Distant Worlds) by Friedrich Mader (1932)


When it comes to the psychology and politics of the planetary culture(s), you can make choices from various axis charts.

There are various forms of government they can use.

You might even try writing some of the culture's history, either creating it for yourself, or generating it with computer programs.

Perhaps even designing a language.

Extreme Examples


Mesklin is a fictional supergiant planet created by Hal Clement for his Hugo-award wining novel Mission of Gravity. The main interesting feature is that its gravity varies from 3g at the equator to about 700g at the poles, due to the rapid rotation.


In Mission of Gravity I’ve been playing this game as fairly as I could.

The basic idea for the story came nearly ten years ago. In 1943 Dr. K. Aa. Strand published the results of some incredibly — to anyone but an astronomer — painstaking work on the orbit of the binary star 61 Cygni, a star otherwise moderately famous for being the first to have its parallax, and hence its distance, measured. In solving such a problem, the data normally consist of long series of measurements of the apparent direction and distance of one star from the other; if the stars are actually moving around each other, and the observations cover a sufficient fraction of a revolution, it is ordinarily possible if not easy to compute the actual relative orbit of the system — that is, the path of one assuming that the other is stationary. Dr. Strand’s work differed from the more usual exercises of this type in that his measures were made from photographs. This eliminated some of the difficulties usually encountered in visual observation, and supplied a number of others; but there was a net gain in overall accuracy, to the extent that he was not only able to publish a more accurate set of orbital elements than had previously been available, but to show that the orbital motion was not regular.

The fainter star, it seemed, did not move around the brighter in a smooth ellipse at a rate predictable by the straightforward application of Kepler’s laws. It did, however, move in a Keplerian path about an invisible point which was in turn traveling in normal fashion about the other sun.

There was nothing intrinsically surprising about this discovery; the implication was plain. One of the two stars — it was not possible to tell which, since measures had been made assuming the brighter to be stationary — was actually accompanied by another, invisible object; the invisible point which obeys the normal planetary and stellar laws was the center of gravity of the star-unknown object system. Such cases are by no means unusual.

To learn which of the two suns is actually attended by this dark body, we would have to have more observations of the system, made in relation to one or more stars not actually part thereof. Some stars exist near enough to the line of sight for such observations to be made, but if they have been reduced and published the fact has not come to my attention. I chose to assume that the object actually circles the brighter star. That may cost me a point in the game when the facts come out, but I won’t be too disheartened if it does.

There was still the question of just what this object was. In other such cases where an invisible object betrayed its presence by gravity or eclipse, as in the system of Algol, we had little difficulty in showing that the companion was a star of some more or less normal type — in the case of Algol, for example, the “dark” body causing the principal eclipse is a sun larger, hotter, and brighter than our own; we can tell its size, mass, luminosity, and temperature with very considerable precision and reliability.

In the case of the 61 Cygni system, the normal methods were put to work; and they came up immediately with a disconcerting fact. The period and size of the orbit, coupled with the fairly well-known mass of the visible stars, indicated that the dark body has a mass only about sixteen thousandths that of the sun — many times smaller than any star previously known. It was still about sixteen times the mass of Jupiter, the largest planet we knew. Which was it — star or planet? Before deciding on the classification of an object plainly very close to the borderline, we must obviously decide just where the borderline lies.

For general purposes, our old grade-school distinction will serve: a star shines by its own light, while a planet is not hot enough for that and can be seen only by reflected light from some other source. If we restrict the word “light” to mean radiation we can see, there should be little argument, at least about definitions. (If anyone brings up nontypical stars of the VV2 Cephei or Epsilon2 Aurigae class I shall be annoyed.) The trouble still remaining is that we may have some trouble deciding whether this Cygnus object shines by intrinsic or reflected light, when we can’t see it shine at all. Some educated guessing seems in order.

There is an empirical relation between the mass of a star, at least a main-sequence star, and its actual brightness. Whether we would be justified in extending this relation to cover an object like 61 Cygni C — that is, third brightest body in the 61 Cygni system — is more than doubtful, but may be at least suggestive. If we do, we find that its magnitude as a star should be about twenty or a little brighter. That is within the range of modern equipment, provided that the object is not too close to the glare of another, brighter star and provided it is sought photographically with a long enough exposure. Unfortunately, 61 C will never be more than about one and a half seconds of arc away from its primary, and an exposure sufficient to reveal the twentieth magnitude would burn the image of 61 A or B over considerably more than one and a half seconds’ worth of photographic plate. A rotating sector or similar device to cut down selectively on the light of the brighter star might do the trick, but a job of extraordinary delicacy would be demanded. If anyone has attempted such a task, I have not seen his published results.

If we assume the thing to be a planet, we find that a disk of the same reflecting power as Jupiter and three times his diameter would have an apparent magnitude of twenty-five or twenty-six in 61 C’s location; there would be no point looking for it with present equipment. It seems, then, that there is no way to be sure whether it is a star or a planet; and I can call it whichever I like without too much fear of losing points in the game.

I am supposing it to be a planet, not only for story convenience but because I seriously doubt that an object so small could maintain at its center the temperatures and pressures necessary for sustained nuclear reactions; and without such reactions no object could maintain a significant radiation rate for more than a few million years. Even as a planet, though, our object has characteristics which will call for thought on any author’s part.

Although sixteen times as massive as Jupiter, it is not sixteen times as bulky. We know enough about the structure of matter now to be sure that Jupiter has about the largest volume of any possible “cold” body. When mass increases beyond this point, the central pressure becomes great enough to force some of the core matter into the extremely dense state which we first knew in white dwarf stars, where the outer electronic shells of the atoms can no longer hold up and the nuclei crowd together far more closely than is possible under ordinary — to us, that is — conditions. From the Jupiter point on up, as mass increases the radius of a body decreases — and mean density rises enormously. Without this effect — that is, if it maintained Jupiter’s density with its own mass—61 C would have a diameter of about two hundred fifteen thousand miles. Its surface gravity would be about seven times that of the Earth. However, the actual state of affairs seems to involve a diameter about equal to that of Uranus or Neptune, and a surface gravity over three hundred times what we’re used to.

Any science fiction author can get around that, of course. Simply invent a gravity screen. No one will mind little details like violation of the law of conservation of energy, or the difference of potential across the screen which will prevent the exchange of anything more concrete than visual signals; no one at all. No one but Astounding readers, that is; and there is my own conscience too. I might use gravity screens if a good story demanded them and I could see no legitimate way out; but in the present case there is a perfectly sound and correct means of reducing the effective gravity, at least for a part of a planet’s surface. As Einstein says, gravitational effects cannot be distinguished from inertial ones. The so-called centrifugal force is an inertial effect, and for a rotating planet happens to be directed outward — in effect — in the equatorial plane. I can, therefore, set my planet spinning rapidly enough to make the characters feel as light as I please, at least at the equator.

If that is done, of course, my nice new world will flatten in a way that would put Saturn to shame; and there will undoubtedly be at least one astronomer reading the story who will give me the raised eyebrow if I have it squashed too little or too much. Surely there is some relation between mass, and rate of spin, and polar flattening—

I was hung up on that problem for quite a while. Since I had other things to do, I didn't really concentrate on it; but whenever a friend whose math had not collapsed with the years crossed my path, I put it up to him. My own calculus dissolved in a cloud of rust long, long ago. I finally found the answer—or an answer—in my old freshman astronomy text, which is still in my possession. I was forcibly reminded that I must also take into account the internal distribution of the planet's mass; that is, whether it was of homogeneous density or, say, almost all packed into a central core. I chose the latter alternative, in view of the enormous density almost certainly possessed by the core of this world and the fact that the outer layers where the pressure is less are presumably of normal matter.

I decided to leave an effective gravity of three times our own at the equator, which fixed one value in the formula. I had the fairly well known value for the mass, and a rough estimate of the volume. That was enough. A little slide-rule work gave me a set of characteristics which will furnish story material for years to come. I probably won’t use it again myself — though that’s no promise—and I hereby give official permission to anyone who so desires to lay scenes there. I ask only that he maintain reasonable scientific standards, and that’s certainly an elastic requirement in the field of science fiction.

The world itself is rather surprising in several ways. Its equatorial diameter is forty-eight thousand miles. From pole to pole along the axis it measures nineteen thousand seven hundred and forty, carried to more significant figures than I have any right to. It rotates on its axis at a trifle better than twenty degrees a minute, making the day some seventeen and three quarter minutes long. At the equator I would weigh about four hundred eighty pounds, since I hand-picked the net gravity there; at the poles, I’d be carrying something like sixty tons. To be perfectly frank, I don’t know the exact value of the polar gravity; the planet is so oblate that the usual rule for spheres, to the effect that one may consider all the mass concentrated at the center for purposes of computing surface gravity, would not even be a good approximation if this world were of uniform density. Having it so greatly concentrated helps a great deal, and I don’t think the rough figure of a little under seven hundred Earth gravities that I used in the story is too far out; but anyone who objects is welcome if he can back it up. (Some formulae brought to my attention rather too late to be useful suggest that I’m too high by a factor of two; but whose formulae are the rougher approximations I couldn’t guess — as I have said, my math has long since gone to a place where I can’t use it for such things. In any case, I’d still stagger a bit under a mere thirty tons.)

I can even justify such a planet, after a fashion, by the current(?) theories of planetary system formation. Using these, I assume that the nucleus forming the original protoplanet had an orbit of cometary eccentricity, which was not completely rounded out by collisions during the process of sweeping up nearly all the raw material in the vicinity of its sun. During the stage when its “atmosphere” extended across perhaps several million miles of space, the capture of material from orbits which were in general more circular than its own would tend to give a spin to the forming world, since objects from outside its position at any instant would have a lower velocity than those from farther in. The rotation thus produced, and increased by conservation of angular momentum as the mass shrank, would be in the opposite direction to the world’s orbital motion. That does not bother me, though; I didn’t even mention it in the story, as nearly as I can now recall.

The rate of spin might be expected to increase to the point where matter was actually shed from the equator, so I gave the planet a set of rings and a couple of fairly massive moons. I checked the sizes of the rings against the satellite orbits, and found that the inner moon I had invented would produce two gaps in the ring similar to those in Saturn’s decoration. The point never became important in the story, but it was valuable to me as atmosphere; I had to have the picture clearly in mind to make all possible events and conversations consistent. The inner moon was ninety thousand miles from the planet’s center, giving it a period of two hours and a trifle under eight minutes. The quarter-period and third-period ring gaps come about twelve and nineteen thousand miles respectively from the world’s surface. The half-period gap would fall about thirty-three thousand miles out, which is roughly where Roché’s Limit would put the edge of the ring anyway (I say roughly, because that limit depends on density distribution too.)

On the whole, I have a rather weird-looking object. The model I have of it is six inches in diameter and not quite two and a half thick; if I added the ring, it would consist of a paper disk about fourteen inches in diameter cut to fit rather closely around the plastic wood spheroid. (The model was made to furnish something to draw a map on; I like to be consistent. The map was drawn at random before the story was written; then I bound myself to stick to the geographic limitations it showed.) I was tempted, after looking at it for a while, to call the book Pancake in the Sky, but Isaac Asimov threatened violence. Anyway, it looks rather more like a fried egg.

There are a lot of characteristics other than size, though, which must be settled before a story can be written. Since I want a native life form, I must figure out just what conditions that form must be able to stand. Some of these conditions, like the temperature and gravity, are forced on me; others, perhaps, I can juggle to suit myself. Let’s see.

Temperature depends, almost entirely, on how much heat a planet receives and retains from its sun. 61 Cygni is a binary system, but the two stars are so far apart that I needn't consider the other one as an influence on this planet's temperature; and the one which it actually circles is quite easy to allow for. Several years ago I computed, partly for fun and partly for cases like this, a table containing some interesting information for all the stars within five parsecs for which I could secure data. The information consists of items such as the distance at which an Earth-type planet would have to revolve from the star in question to have the present temperatures of Earth, Venus, and Mars, and how long it would take a planet to circle the sun in question in each such orbit. For 61 Cygni A, the three distances are about twenty-eight, thirty-nine, and sixty-nine million miles, respectively. As we have seen, 61 C's orbit is reasonably well known; and it is well outside any of those three distances. At its closest—and assuming that the primary star is 61 A—it gets almost near enough to be warmed to be about fifty below zero, Centigrade. At the other end of its rather eccentric orbit, Earth at least would cool to about minus one hundred eighty, and it’s rather unlikely that this world we are discussing gets too much more out of the incoming radiation. That is a rather wide temperature fluctuation.

The eccentricity of the orbit is slightly helpful, though. As Kepler’s laws demand, the world spends relatively little time close to its sun; about four fifths of its year it is outside the minus one hundred fifty degree isotherm, and it is close enough to be heated above minus one hundred for only about one hundred thirty days of its eighteen-hundred-day year — Earth days, of course. Its year uses up around one hundred forty-five thousand of its own days, the way we’ve set it spinning. For practical purposes, then, the temperature will be around minus one hundred seventy Centigrade most of the time. We’ll dispose of the rest of the year a little later.

Presumably any lifeform at all analogous to our own will have to consist largely of some substance which will remain liquid in its home planet’s temperature range. In all probability, the substance in question would be common enough on the planet to form its major liquid phase. If that is granted, what substance will meet our requirements?

Isaac Asimov and I spent a pleasant evening trying to find something that would qualify. We wanted it not only liquid within our temperature limits, but a good solvent and reasonably capable of causing ionic dissociation of polar molecules dissolved in it. Water, of course, was out; on this world it is strictly a mineral. Ammonia is almost as bad, melting only on the very hottest days. We played with ammonia’s analogues from further along the periodic table — phosphine, arsine, and stibine — with carbon disulfide and phosgene, with carbon suboxide and hydrogen fluoride, with saturated and unsaturated hydrocarbons both straight and with varying degrees of chlorine and fluorine substitution, and even with a silicone or two. A few of these met the requirements as to melting and boiling points; some may even have caused dissociation of their solutes, though we had no data on that point for most. However, we finally fell back on a very simple compound.

It boils, unfortunately, at an inconveniently low temperature, even though we assume a most unlikely atmospheric pressure. It cannot be expected to be fruitful in ions, though as a hydrocarbon it will probably dissolve a good many organic substances. It has one great advantage, though, from my viewpoint; it would almost certainly be present on the planet in vast quantities. The substance is methane — CH4.

Like Jupiter, this world must have started formation with practically the “cosmic” composition, involving from our viewpoint a vast excess of hydrogen. The oxygen present would have combined with it to form water; the nitrogen, to form ammonia; the carbon to form methane and perhaps higher hydrocarbons. There would be enough hydrogen for all, and plenty to spare — light as it is, even hydrogen would have a hard time escaping from a body having five thousand times the mass of Earth once it had cooled below red heat — at first, that is. Later, when the rotational velocity increased almost to the point of real instability, it would be a different story; but we’ll consider that in a moment. However, we have what seems to be a good reason to expect oceans of methane on this world; and with such oceans, it would be reasonable to expect the appearance and evolution of life forms using that liquid in their tissues.

But just a moment. I admitted a little while ago that methane boils at a rather lower temperature than I wanted for this story. Is it too low? Can I raise it sufficiently by increasing the atmospheric pressure, perhaps? Let’s see. The handbook lists methane’s critical temperature as about minus eighty-two degrees Centigrade. Above that temperature it will always be a gas, regardless of pressure; and to bring its boiling point up nearly to that value, a pressure about forty-six times that of our own atmosphere at sea level will be needed. Well, we have a big planet, which should have held on to a lot of its original gases; it ought to have a pressure of hundreds or even thousands of atmospheres — whoops! we forgot something.

At the equator, effective gravity — gravity minus centrifugal effect — is three times Earth normal. That, plus our specification of temperature and composition of the atmosphere, lets us compute the rate at which atmospheric density will decrease with altitude. It turns out that with nearly pure hydrogen, three g’s, and a temperature of minus one hundred fifty for convenience, there is still a significant amount of atmosphere at six-hundred-miles altitude if we start at forty-odd bars for surface pressure—and at six hundred miles above the equator of this planet the centrifugal force due to its rotation balances the gravity! If there had ever been a significant amount of atmosphere at that height, it would long since have been slung away into space; evidently we cannot possibly have a surface pressure anywhere near forty-six atmospheres. Some rough slide-rule work suggests eight atmospheres as an upper limit—I used summer temperatures rather than the annual mean.

At that pressure methane boils at about minus one hundred forty-three degrees, and for some three hundred Earth days, or one-sixth of each year, the planet will be in a position where its sun could reasonably be expected to boil its oceans. What to do?

Well, Earth’s mean temperature is above the melting point of water, but considerable areas of our planet are permanently frozen. There is no reason why I can’t use the same effects for 61 C; it is an observed fact that the axis of rotation of a planet can be oriented so that the equatorial and orbital planes do not coincide. I chose for story purposes to incline them at an angle of twenty-eight degrees, in such a direction that the northern hemisphere’s midsummer occurs when the world is closest to its sun. This means that a large part of the northern hemisphere will receive no sunlight for fully three quarters of the year, and should in consequence develop a very respectable cap of frozen methane at the expense of the oceans in the other hemisphere. As the world approaches its sun the livable southern hemisphere is protected by the bulk of the planet from its deadly heat output; the star’s energy is expended in boiling off the north polar “ice” cap. Tremendous storms rage across the equator carrying air and methane vapor at a temperature little if any above the boiling point of the latter; and while the southern regions will warm up during their winter, they should not become unendurable for creatures with liquid methane in their tissues.

Precession should be quite rapid, of course, because of the tremendous equatorial bulge, which will give the sun’s gravity a respectable grip even though most of the world’s mass is near its center. I have not attempted to compute the precessional period, but if anyone likes to assume that a switch in habitable hemispheres occurring every few thousand years has kept the natives from building a high civilization I won’t argue. Of course, I will also refrain from disagreement with anyone who wants to credit the periodic climate change with responsibility for the development of intelligence on the planet, as our own ice ages have sometimes been given credit for the present mental stature of the human race. Take your pick. For story purposes, I’m satisfied with the fact that either possibility can be defended.

The conditions of the planet, basically, are pretty well defined. There is still a lot of detail work. I must design a life form able to stand those conditions — more accurately, to regard them as ideal — which is not too difficult since I don’t have to describe the life processes in rigorous detail. Anyone who wants me to will have to wait until someone can do the same with our own life form. Vegetation using solar energy to build up higher, unsaturated hydrocarbons and animal life getting its energy by reducing those compounds back to the saturated form with atmospheric hydrogen seemed logical enough to me. In the story, I hinted indirectly at the existence of enzymes aiding the reduction, by mentioning that plant tissues would burn in the hydrogen atmosphere if a scrap or two of meat were tossed onto the fuel.

The rest of the detail work consists of all my remaining moves in the game — finding things that are taken for granted on our own world and would not be true on this one. Such things as the impossibility of throwing, jumping, or flying, at least in the higher latitudes; the tremendously rapid decrease of air density with height in the same regions, producing a mirage effect that makes the horizon seem above an observer all around; the terrific Coriolis force that splits any developing storm into a series of relatively tiny cells — and would make artillery an interesting science if we could have any artillery; the fact that methane vapor is denser than hydrogen, removing a prime Terrestrial cause of thunderstorm and hurricane formation; the rate of pressure increase below the ocean surface, and what that does to the art of navigation; the fact that icebergs won’t float, so that much of the ocean bottoms may be covered with frozen methane; the natural preference of methane for dissolving organic materials such as fats rather than mineral salts, and what that will do to ocean composition — maybe icebergs would float after all. You get the idea.

The trouble was, I couldn’t possibly think of all these things in advance; time and again a section of the story had to be rewritten because I suddenly realized things couldn’t happen that way. I must have missed details, of course; that’s where your chance to win the game comes in. I had an advantage; the months during which, in my spare hours, my imagination roamed over Mesklin’s vast areas in search of inconsistencies. Now the advantage is yours; I can make no more moves in the game, and you have all the time you want to look for the things I’ve said which reveal slips on the part of my imagination.

Well, good luck — and a good time, whether you beat me or not.

From WHIRLIGIG WORLD by Hal Clement (1953)


Rocheworld is an exceptionally fine example of extreme worldbuilding by Robert L. Forward. The twin planets are so close that their atmosphere commingles. You can actually travel from one planet to the other by airplane!




In 1916, the American astronomer Edward E. Barnard measured the proper motion of a dim red star cataloged as BD+04 deg 3561. He found it was moving through the sky at the amazing speed of 10.3 seconds of arc per year, or more than half the diameter of the Moon in a century. Barnard's Star (or Barnard as it is known now) is very close to the solar system, only 5.9 lightyears away, but it is so small and dim that it takes a telescope to see it.

The cold statistics for Barnard are given in the table:


     Distance: 5.9 ly
     Right Ascension: 17 hr 55 min
     Declination: 4 deg 33 min
     Coordinates: X=-0.1 ly, Y=-5.9 ly, Z=+0.5 ly
     Spectral type: M5
     Effective Temperature: 58% solar (3330 K)
     Luminosity: 0.05% solar (visual), 0.37% solar (thermal)
     Mass: 15% solar mass
     Radius: 12% solar radius
     Proper motion: 10.31 arcsec/yr
     Radial velocity: -108 kilometers/sec

Barnard Planetary System 

The planetary system around Barnard is dominated by a gigantic planet, aptly named Gargantua. A huge gas giant like Jupiter, Gargantua is four times more massive than Jupiter. Since the parent star, Barnard, has a mass of only fifteen percent that of our Sun, this means that the planet Gargantua is one-fortieth the mass of its star. If Gargantua had been more massive, it would have turned into a star, and the Barnard system would have been a binary star system.

Gargantua seems to have swept up into itself most of the original stellar nebula that was not used in making the star, for there are no other large planets in the system. Gargantua has four satellites that would be planets in our solar system, plus a multitude of smaller moons. These planets will be the subject of further exploration by the Barnard mission. Today, however, we will be concentrating on the first world (or worlds) that they landed on—Rocheworld.

As seen in Figure 1, Rocheworld is in a highly elliptical orbit around Barnard. The period of Rocheworld about Barnard is forty days, while Gargantua's orbital period is exactly three times the Rocheworld orbital period. Thus, once every three orbits, Rocheworld passes within six million kilometers of the giant planet Gargantua, not too far from the orbit of Gargantua's outer moon, Zeus. It is believed that the present orbit was established many million years ago by the encounter of a stray planetoid with what was once an outer large moon of Gargantua.

Orbits such as that of Rocheworld are usually not stable. The three to one resonance condition usually results in an oscillation of the orbit of the smaller body that builds up in amplitude until the smaller planet is thrown into a different orbit, or a collision occurs. Due to Rocheworld's close approach to Barnard, however, the tides from Barnard cause a significant amount of dissipation, which stabilizes the orbit. This also supplies a great deal of heating which keeps Rocheworld warmer than it would normally be if the heating were due to radiation alone.


Rocheworld is a dumbbell-shaped double planet. As shown in Figure 2, it consists of two moon-sized rocky bodies that whirl about each other with a rotation period of six hours. There are exactly 160 rotations of Rocheworld around its common center (a Rocheworld "day") to one rotation of Rocheworld in its elliptical orbit around Barnard (a Rocheworld "year"), while there are exactly three orbits of Rocheworld around Barnard to one rotation of Gargantua around Barnard. This locking of Rocheworld's rotation period and orbital period to the orbital period of Gargantua keeps the strange double planet in its highly elliptical orbit. The energy needed to drive the Rocheworld configuration and compensate for energy losses due to tidal dissipation comes from the gravitational tug of Gargantua on Rocheworld during their close passage every third orbit.

The two planetoids or lobes of Rocheworld are so close that they are almost touching, but their spin speed is high enough that they maintain a separation of about eighty kilometers. If each were not distorted by the other's gravity, the two planets would have been spheres about the size of our Moon. Since their gravitational tides act upon one another, the two bodies have been stretched out until they are elongated egg-shapes, 3500 kilometers in the long dimension and 3000 kilometers in cross section. Although the two planets do not touch each other, they do share a common atmosphere. The resulting figure-eight configuration is called a Roche-lobe pattern after E.A. Roche, a French mathematician of the later 1880s, who calculated the effects of gravity tides on stars, planets, and moons. The word "roche" also means "rock" in French, so the rocky lobe of the pair of planetoids was given the name Roche, while the water-covered lobe was named Eau after the French word for "water".

The average gravity at the surface of these moonlets is about ten percent of Earth gravity, slightly less than that of Earth's Moon because of their lower density. This average value varies considerably depending upon your position on the surface of the elongated lobes. The gravity at one of the outward facing poles is eight percent of Earth gravity, rising to eleven percent in a belt that includes the north and south spin poles of each lobe, increases slightly to a maximum of eleven and a half percent at a region some thirty degrees inward, then drops precipitously to a half percent at the inner-pole surface. This low gravity point is some forty kilometers below the zero gravity point between the two planetoids, where the gravity from the mass of the two lobes cancels out.

On each side of the double planet are the L-4 and L-5 points where there is a minimum in the combined gravitational and centrifugal forces of the system. A satellite placed at either of these two points will stay there, rotating synchronously with the two planets, without consumption of fuel. For the Earth-Moon system, where the Earth is much more massive than the Moon, those stable points are in the orbit of the Moon at plus and minus sixty degrees from the Moon. In the Rocheworld system, where the two bodies are the same mass, the stable points are at plus and minus ninety degrees. The exploration crew established communication satellites at these two points to give continuous coverage of each side of both lobes.

The Roche lobe is slightly less dense than the Eau lobe, thus is larger in diameter. It has a number of ancient craters upon its surface, especially in the outer-facing hemisphere. Although the Eau lobe masses almost as much as the Roche lobe, it has a core that is denser. Since its highest point is some twenty kilometers lower in the combined gravitational well, it is the "lowlands" while the Roche lobe is the "highlands." Eau gets most of the rain that falls from the common atmosphere and thus has captured nearly all of the liquids of the double planet to form one large ocean. The ocean is primarily ammonia water, with trace amounts of hydrogen sulfide and cyanide gas.

The Roche Lobe is dry and rocky, with traces of quiescent volcano vents near its pointed pole. The Eau lobe has a pointed section like the Roche lobe, but the point is not made of rock. The peak is a mountain of ammonia water a hundred and fifty kilometers high with sixty degree slopes! One would think that the water would 'seek its own level' and flow out until the surface of the ocean became spherical, but because of the unusual configuration of the gravity fields of the double planet, the basic mountain shape is stable—except at periapsis.

When Rocheworld is at its furthest distance from Barnard, everything is serene on the double-planet. The two lobes whirl about each other and the gravity from the star causes modest tides on the ocean on Eau. As Rocheworld moves around in its orbit, it experiences stronger tides as it approaches either Gargantua or Barnard. At these times, the variations in gravitational tides from one rotation to the next causes large surges in the seas. The low gravity accentuates these surges into large waves that reach kilometers in height, breaking at the low gravity pole between the two planetoids.

As Rocheworld begins to approach Barnard in its elliptical orbit, the effect of the tides from the star begins to become very large. The peak of the water mountain now begins to rise and fall a number of kilometers, with the pattern repeated each half-rotation. As is shown in Figure 3, when Barnard is on one side of Rocheworld, the two lobes separate by thirty kilometers. This causes the mountain of water to drop one hundred kilometers.

[Dr. Philipson interrupted his prepared text at this point to interject a comment.]

Dr. Philipson. By the way. This behavior is not what would be predicted by a naive model of the gravity forces. I myself would have thought that with Barnard off to the side, the gravity tidal forces from Barnard would have drawn the lobes closer together, not farther apart. I also would have expected the change in the height of the mountain of water to be about the same as the change in the separation. But recent detailed computer studies here on Earth, that take into account the coupling of the angular rotation and the orbital motion with the planetary dynamics, confirm what Captain Thomas St. Thomas calculated at the time, and they both agree with what really happened on Rocheworld six years ago when we nearly lost the first landing party.

[The record returns to the prepared text.]

Then, just a quarter-rotation later, the tidal forces go the other way. Although the decrease in spacing of the two lobes is only seven kilometers, the effects are so nonlinear that, as shown in Figure 4, the mountain of water that has built up on the Eau lobe reaches up forty kilometers to the zero-gravity point midway between the two planetoids—and beyond.

The crest of the mountain drops as a rapidly accelerating, multiply-fragmenting waterfall on the hot dry rocks of the Roche lobe forty kilometers below. For the next two half-turns of the double planet, the showers of water repeat, and the torrent from the interplanetary waterfall pours onto the volcanos on the disturbed surface in a drenching torrent. Rapidly moving streams of water form on the slopes of drowned volcanos, to merge with other streams that soon become giant raging rivers, streaking out across the dry highlands of Roche.

Mr. Ootah. Thank you very much, Dr. Philipson. That's quite a spectacular planetary system there. If your schedule permits, we will proceed with the other witnesses and then have the questions and answers.


Rocheworld Ocean 

There is an ocean covering one of the two lobes of Rocheworld. The liquid is a cold mixture of ammonia and water similar to what was found inside Jupiter's moon Europa. There are no land areas of any size, so the climate is determined by the heating patterns from Barnard as modified by the shadowing effects of the Roche lobe. There is a warm "crescent" that is centered on the outer pole and reaches around the equator. This crescent receives the most sunlight and the surface temperature reaches minus twenty degrees centigrade. The cold crescent is centered about the inner pole and reaches out to include the north and south polar regions. The temperature of the ocean surface here is minus forty degrees or colder. Because of these two regions covering Eau like the two halves of the cover of a baseball, we have quite unusual weather patterns. The ammonia boils from the surface in the hot crescents, leaving behind the heavier water, and falls on the cold crescent. We then get strong currents, with the warm heavy water flowing under the cold lighter ammonia-rich mixture. At the bottom of the ocean underneath these surface currents, it is very cold, reaching minus 100 degrees centigrade.

There are a number of mixtures of water and ammonia possible in the ocean. This is seen in Figure 12, which is a phase diagram for ammonia and water at 0.2 atmospheres. At this pressure level, pure water boils at plus 64 degrees centigrade, while pure ammonia boils at minus 61 degrees. The ocean composition varies from twenty to eighty percent ammonia, so a good portion of the phase diagram is covered.

There are four kinds of ice possible, one pure water, one pure ammonia, and two with varying ratios of water molecules to ammonia molecules. Ice floats on water, but sinks when the ammonia content of the ocean exceeds 23 percent. Since the cold inner poles are generally ammonia-rich from the ammonia rain falling on the cold crescent, the water ice that forms drops to the bottom and accumulates into glaciers. Ice-2 floats and Ice-3 sinks, leading to situations where you can have underwater snowstorms with one type of snow falling down and the other type falling up.

Rocheworld Aliens 

The aliens on Rocheworld live in the ocean. In genetic makeup and complexity level they have a number of similarities to slime-mold amoebas here on Earth, as well as analogies to a colony of ants. Each of their units can survive for a while on its own, but is not intelligent. A small collection of cells can survive as a coherent cloud with enough intelligence to hunt smaller prey and look for plants to eat. Larger collections of cells form into more complex structures. When the collection becomes large enough, it becomes an intelligent being. Yet if that being is torn into millions of pieces, each piece can survive. If the pieces can get together again, the individual is restored, only a little worse for its experience.

The aliens are large, weighing many tons. They normally stay in a formless, cloudlike shape, moving with and through the water. When they are in their mobile, cloudlike form, the clouds in the water range from ten to thirty meters in diameter and many meters thick. They often concentrate the material in their cloud into a dense rock formation a few meters in diameter. They seem to do this when they are thinking, and it is supposed that the denser form allows for faster and more concentrated cogitation.

The aliens are very intelligent, but nontechnological—like dolphins and whales here on Earth. They have a highly developed system of philosophy, and extremely advanced abstract mathematical capability. There is no question that they are centuries ahead of us in mathematics, and further communication with them could lead to great strides in human capabilities in this area. However, because of their physical makeup and their environment, the aliens are not yet aware of the potential of technology—again, the similarity to the cetaceans is striking.

The alien use chemical senses for short-range information and sonar for long range. They have some sensitivity to light, but cannot see like humans. In general, sight is a secondary sense, about as important to them as taste is to humans. One of the aliens is known, however, to deliberately form an imaging lens that it used to study the stars and planets in their stellar system. Called White Whistler by the humans, this individual was one of the more technologically knowledgeable of the aliens.

There are fauna on Rocheworld, all in the ocean and similar in chemistry, genetics, and structure to the intelligent aliens. One type are huge grey rocks that stay quiescent for long periods of time, only to suddenly explode, stunning all within a hundred meters and capturing them in their sticky thread nets. After absorbing their prey, they reform into multiple rocks that slowly convert the captured food into copies of itself.

Another type are bird-like creatures that don't do much except float around, perfume the water, and make twittering sonic vibrations. The aliens seem to tolerate them as pets.

The major flora are grey and brown plants which look like sedentary rocks with controlled thick clouds about them. They send out streamers and form new bud rocks at the ends. The plants do not use photosynthesis, since the red light from Barnard is too weak. Instead the whole food chain is based on the energy and minerals emitted by volcanic vents. We have similar isolated colonies of plants and animals around underwater vents in our own ocean depths. All life on the planet is concentrated at these few oases and the rest of the ocean is barren, without significant numbers of bacteria or other microscopic life forms. Because of this, the exploration crew was unaware there was anything living on the planet until one of the aliens made contact with them.

Reproduction for the aliens is a multiple-individual experience. The aliens to not seem to have sexes, and it seems that any number from two aliens on up can produce a new individual. The usual grouping for reproduction is thought to be three or four. The creating of a new alien seems to be more of a lark or a creative exercise like music or theater than a physically driven emotional experience. The explorers witnessed one such coupling put on for their benefit. In this case it involved four aliens, Loud Red, White Whistler, Green Fizzer, and Yellow Hummer. They each extended a long tendril that contained a substantial portion of their mass, estimated to be one-tenth of the mass of each parent. These tendrils, each a different color, met at the middle and intertwined with a swirling motion like colored paints being stirred together. There was a long pause as each tendril began to lose its distinctive color. We don't know exactly what happened, but obviously some chemical change was taking place that removed the strong host-origin identity from the units in the tendrils. Then finally the tendrils were snapped off, leaving the pale cloud floating in the center by itself, about forty percent of the size of the adults that created it. After a few minutes, the mass of cells formed themselves into a new individual, who took on a color that was different than any of its progenitors. The humans called the new baby Blue Warbler, because of its color and the distinctive acoustic note that it used for sonar sensing. The adults then take it upon themselves to train the new youngster. The adults and youngsters stay together for hunting and protection, the group again being very much like a pod of whales or porpoises.

The aliens have a complex art-form similar to acting, which involves carrying out simulations of real or imaginary happenings by forming a replica of the scene with their bodies. You can see this activity on a short segment of videotape that was transmitted back by the crew. I apologize that we have only a flatview version of the scene. The technology to produce holoprojection tapes had not yet been developed when the crew left the solar system.

[The prepared testimony was interrupted by the showing of a flatview projection tape. Copies may be viewed in the holoprojection rooms at the Library of Congress or purchased from the G.U.S. Government Printing Office, Washington, DC 20402.]

More than one actor takes part. The alien Yellow Hummer seemed to be most proficient in this art-form, and used it as one method of communicating with the humans. The aliens warned the explorers of the danger of the ocean transfer by simulating the Rocheworld with its seas. Two of the aliens, lighter in color, formed the rocky worlds. Another, blue in color, acted out the part of the seas. They showed how the rocky worlds whirled about each other, and as the year passes, and the elliptical orbit of the dual planet approaches periapsis, the tidal forces become stronger, and the sea on the smaller Eau lobe sloshes back and forth, gaining momentum. Then as the tidal forces become great enough, the aliens showed the humans how the seas cross the gap between the planets in a huge interplanetary waterfall that nearly engulfs the larger Roche lobe. Warned by the aliens, the humans made their dramatic escape off the Eau lobe by riding a huge wave, then gliding back through tornadoes to their rocket, which took them off the planet before the tidal wave struck.

Dr. Winners. That's all the information that we have at the present time on the aliens, since the crew had to leave the planet. However, they have informed us that they will go back on a prolonged visit, this time landing their rocket in a safe place in one of the larger craters of the dry lobe, Roche, so they can stay there through a number of tidal cycles while they get to know the aliens better. They plan to leave some interstellar laser communicators behind and teach the aliens how to use them to communicate directly with Earth, while the exploration crew goes off to visit the other worlds and moons about Barnard.

Of course, since it takes six years for messages to reach us from Barnard, that next visit has already taken place, and the radio message to us is somewhere in transit in the empty space between there and here. But, in a few years, we will be back with more news and information about what the aliens can teach us in the way of abstract thought and mathematics. We also expect that the crew will have a much better idea of the chemical and genetic makeup of this new race of beings after a year or so of study. This could have a profound effect on our understanding of the life process itself, and will produce great advances in medicine, perhaps even a life-prolonging drug without the side effects of No-Die.

Mr. Ootah. Thank you very much for your fascinating testimony, Dr. Winners. We also would like to commend the brave exploration crew who are out there gathering this information for us. They certainly will deserve a heroic welcome when they return.

Dr. Winners. The Chairman forgets. This is an interstellar mission. They will not return—ever.

Mr. Ootah. Oh... Yes. I forgot. There was a great outcry prior to the start of this mission that we were sending these brave people on a one-way "suicide mission". Yet, as one of them said, "We all are on a one-way mission through life." These people are fortunate enough to be doing something really significant for mankind with their lives, and probably having fun doing it.

Dr. Winners. If it were possible, I would trade positions with any one of them instantly.

From THE FLIGHT OF THE DRAGONFLY by Robert L. Forward (1984)

Uller and Niflheim

Back in 1953 a fellow by the name of John Ciardi (author of my personal favorite translation of Dante's Inferno) came up with a concept for a series of science fiction collections. The idea was to have a board of highly qualified scientists and consultants to create a hypothetical but scientifically possible planet quite different from Terra. The data was to be given to three different established science fiction authors, each of which would write their own story set in that hypothetical world. The result would be published as a hardcover containing the three stories.

Ciardi called his little company "Twayne Publishing", so the series was called "Twayne Triplets."

Alas, it flopped badly.

One volume was issued, a second partial volume came out before the company folded.

The first triplet (The Petrified Planet) featured two worlds (Uller and Niflheim) created by Dr. John D. Clark. The three stories were The Long View by Fletcher Pratt, Daughters of Earth by Judith Merril, and Uller Uprising by H. Beam Piper.

Isaac Asimov's story Sucker Bait was written for a Twayne Triplet which never came out, so Asimov gave it to Astounding magazine. Another triplet that died aborning would have included Get Out of My Sky by James Blish, Second Landing by Murray Leinster, and First Cycle started by H. Beam Piper and completed by Michael Kurland.

Anyway the point of all this is the wonderful worldbuilding done by Dr. Clark:



Dr. John D. Clark



The planet is named Uller (it seems that when interstellar travel was developed, the names of Greek Gods had been used up, so those of Norse gods were used). It is the second planet of the star Beta Hydri, right angle 0:23, declension -77:32, G-0 (solar) type star, of approximately the same size as Sol; distance from Earth, 21 light years.

Uller revolves around it in a nearly circular orbit, at a distance of 100,000,000 miles, making it a little colder than Earth. A year is of the approximate length of that on Earth. A day lasts 26 hours.

The axis of Uller is in the same plane as the orbit, so that at a certain time of the year the north pole is pointed directly at the sun, while at the opposite end of the orbit it points directly away. The result is highly exaggerated seasons. At the poles the temperature runs from 120°C to a low of -80°C. At the equator it remains not far from 10°C all year round. Strong winds blow during the summer and winter, from the hot to the cold pole; few winds during the spring and fall. The appearance of the poles varies during the year from baked deserts to glaciers covered with solid CO2. Free water exists in the equatorial regions all year round.


As seen from the north pole—no sun is visible on Jan. 1. On April 1, it bisects the horizon all day, swinging completely around. April 1 to July 1, it continues swinging around, gradually rising in the sky, the spiral converging to its center at the zenith, which it reaches July 1. From July 1 to October 1 the spiral starts again, spreading out from the center until on October 1 it bisects the horizon again. On October 1 night arrives to stay until April 1.

At the equator, the sun is visible bisecting the southern horizon for all 26 hours of the day on January 1. From January 1 to April 1, the sun starts to dip below the horizon at night, to rise higher above it during the day. During all this time it rises and sets at the same hours, but rises in the southeast and sets in the southwest. At noon it is higher each day in the southern sky until April 1, when it rises due east, passes through the zenith and sets due west. From April 1 to July 1, its noon position drops down to the north, until on July 1, it is visible all day, bisected by the northern horizon.


Calcium and chlorine are rarer than on earth, sodium is somewhat commoner. As a result of the shortage of calcium there is a higher ration of silicates to carbonates than exists on earth. The water is slightly alkaline and resembles a very dilute solution of sodium silicate (water glass). It would have a pH of 8.5 and tastes slightly soapy. Also, when it dries out it leaves a sticky, and then a glassy, crackly film. Rocks look fairly earthlike, but the absence or scarcity of anything like limestone is noticeable. Practically all the sedimentary rocks are of the sandstone type.

All rivers are seasonal, running from the polar regions to the central seas in the spring only, or until the polar cap is completely dried out.


As on Earth life arose in the primitive waters and with a carbon base, but because of the abundance of silicone, there was a strong tendency for the microscopic organisms to develop silicate exoskeletons, like diatoms. The present invertebrate animal life of the planet is of this type and is confined to the equatorial seas. They run from amoeba-like objects to things like crayfish, with silicate skeletons. Later, some species of them started taking silicone into their soft tissues, and eventually their carbon-chain compounds were converted to silicone type chains, from

with organic radicals on the side links. These organisms were a transitional type, with silicone tissues and water body fluids, resembling the earthly amphibians, and are now practically extinct. There are a few species, something like segmented worms, still to be seen in the backwaters of the central seas.

A further development occurred when the silicone chain animals began to get short-chain silicones into their circulatory systems, held in solution by OH or NH2 groups on the ends and branches of the chains. The proportion of these compounds gradually increased until the water was a minor and then a missing constituent. The larger mobile species were, then, practically anhydrous. Their blood consists of short-chain silicones, with quartz reinforcing for the soft parts and their armor, teeth, etc., of pure amorphous quartz (opal). Most of these parts are of the milky variety, variously tinted with metallic impurities, as are the varieties of sapphires.

These pure silicone animals, due to their practical indestructibility, annihilated all but the smaller of the carbon animals, and drove the compromise types into odd corners as relics. They developed into a fish-like animal with a very large swim-bladder to compensate for the rather higher density of the silicone tissues, and from these fish the land animals developed. Due to their high density and resulting high weight, they tend to be low on the ground, rather reptilian in look. Three pairs of legs are usual in order to distribute the heavy load. There is no sharp dividing line between the quartz armor and the silicone tissue. One merges into the other.

The dominant pure silicone animals only could become mobile and venture far from the temperate equatorial regions of Uller, since they neither froze nor stiffened with cold, nor became incapacitated by heat. Note that all animal life is cold-blooded, with a negligible difference between body and ambient temperatures. Since the animals are silicones, they don't get sluggish like cold snakes.


The plants are of the carbon-metabolism, silicate-shell type, like the primitive animals. They spread out from the equator as far as they could go before the baking polar summers killed them. They have normal seasonal growth in the temperate zones and remain dormant and frozen in the winter. At the poles there is no vegetation, not because of the cold winter, but because of the hot summer. The winter winds frequently blow over dead trees and roll them as far as the equatorial seas. Other dead vegetation, because of the highly silicious water, always gets petrified unless it is eaten first. What with the quartz-speckled hides of the living vegetation and the solid quartz of the dead, a forest is spectacular.

The silicone animals live on the plants. They chew them up, dehydrate them, and convert their silicious outer bark and carbonaceous interiors into silicones for themselves. When silicone tissue is metabolized, the carbon and hydrogen go to CO2 and H2O, which are breathed out, while the silicone goes into SiO2, which is deposited as more teeth and armor. (Compare the terrestrial octopus, which makes armor-plating out of calcium urate instead of excreting urea or uric acid.) The animals can, of course, eat each other too, or make a meal of the small carbonaceous animals of the equatorial seas.

Further note that the animals cannot digest plants when they are cold. They can eat them and store them, but the disposal of the solid water and CO2 is too difficult a problem. When they warm up, the water in the plants melts and can be disposed of, and things are simpler.




The planet named Niflheim is the fourth planet of Nu Puppis, right angle 6:36, declension -43:09; B8 type star, blue-white and hot, 148 light years distant from Earth, which will require a speed in excess of light to reach it.

Niflheim is 462,000,000 miles from its primary, a little less than the distance of Jupiter from our sun. It thus does not receive too great a total amount of energy, but what it does receive is of high potential, a large fraction of it being in the ultra-violet and higher frequencies. (Watch out for really super-special sunburn, etc., on unwarned personnel.)

The gravity of Niflheim is approximately 1 g, the atmospheric pressure approximately 1 atmosphere, and the average ambient temperature about -60°C; -76°F.


The oxidizer in the atmosphere is free fluorine (F2) in a rather low concentration, about 4 or 5 percent. With it appears a mad collection of gases. There are a few inert diluents, such as N2 (nitrogen), argon, helium, neon, etc., but the major fraction consists of CF4 (carbon tetrafluoride), BF3 (boron trifluoride), SiF4 (silicon tetrafluoride), PF5 (phosphorous pentafluoride), SF6 (sulphur hexafluoride) and probably others. In other words, the fluorides of all the non-metals that can form fluorides. The phosphorous pentafluoride rains out when the weather gets cold. There is also free oxygen, but no chlorine. That would be liquid except in very hot weather. It sometimes appears combined with fluorine in chlorine trifluoride. The atmosphere has a slight yellowish tinge.


Above the metallic core of the planet, the lithosphere consists exclusively of fluorides of the metals. There are no oxides, sulfides, silicates or chlorides. There are small deposits of such things as bromine trifluoride, but these have no great importance. Since fluorides are weak mechanically, the terrain is flattish. Nothing tough like granite to build mountains out of. Since the fluoride ion is colorless, the color of the soil depends upon the predominant metal in the region. As most of the light metals also have colorless ions, the colored rocks are rather rare.


They consist of liquid hydrofluoric acid (HF). It melts at -83°C and boils at 19.4°C. In it are dissolved varying quantities of metallic and non-metallic fluorides, such as boron trifluoride, sodium fluoride, etc. When the oceans and lakes freeze, they do so from the bottom up, so there is no layer of ice over free liquid.


The plants function by photosynthesis, taking HF as water from the soil, and carbon tetrafluoride as the equivalent of carbon dioxide from the air to produce chain compounds, such as:

and at the same time liberating free fluorine. This reaction could only take place on a planet receiving lots of ultra-violet because so much energy is needed to break up carbon tetrafluoride and hydrofluoric acid. The plant catalyst (doubling for the magnesium in chlorophyll) is nickel. The plants are colored in various ways. They get their metals from the soil.


Animals depend upon two main reactions for their energy, and for the construction of their harder tissues. The soft tissues are about the same as the plant molecules, but the hard tissues are produced by the reaction:

resulting in a teflon boned and shelled organism. He's going to be tough to do much with. Diatoms leave strata of powdered teflon. The main energy reaction is:

The blood catalyst metal is titanium, which results in colorless arterial blood and violet veinous, as the titanium flips back and forth between tri and tetra-valent states.


Water decomposes into oxygen and hydrofluoric acid. All organic matter (earth type) converts into oxygen, carbon tetrafluoride, hydrofluoric acid, etc., with more or less speed. A rubber gas mask lasts about an hour. Glass first frosts and then disappears. Plastics act like rubber, only a little slower. The heavy metals, iron, nickel, copper, monel, etc., stand up well, forming an insoluble coat of fluorides at first and then doing nothing else.


Large natural crystals of fluorides, such as calcium difluoride, titanium tetrafluoride, zirconium tetrafluoride, are extremely useful in optical instruments of various forms. Uranium appears as uranium hexafluoride, all ready for the diffusion process. Compounds of such non-metals as boron are obtainable from the atmosphere in high purity with very little trouble. All metallurgy must be electrical. There are considerable deposits of beryllium, and they occur in high concentration in its ores.

From Introduction to ULLER UPRISING by Dr. John D. Clark (1952)

Suggested Reading

How to Build a Planet
Poul Anderson, SFWA HANDBOOK 1991. Poul Anderson and Stephen Gillett did an update of "How to Build a Planet". It's #19 in the "Writer's Chapbook Series".
A World Named Cleopatra
Poul Anderson 1974. An essay describing a fascinating extraterrestrial planet, which was carefully worldbuilt by the author.
The Creation of Imaginary Worlds: The World Builder's Handbook and Pocket Companion
Poul Anderson 1974.
How To Build A Habitable Planet
Wallace Broecker, Eldigio Press, 1987, This book is an outgrowth of an undergraduate course taught by the author at Columbia College and Barnard College.
A Magical society: Guide to Mapping
Joseph Browning, Expeditious Retreat Press, 2004, Contains a system to make realistic continents, weather patterns, and climates.
Star Hero 6th Edition
James Cambias and Steven S. Long, Hero Games, 2002, Contains a scientifically accurate system to generate entire solar systems.
World Building Tips Volume 1 & Volume 2
Deborah Teramis Christian, Volume 1 covers things ranging from structural meta-considerations, to a hands-on geography development process, to things like illness, games, and global weather. Volume 2 of this series delves into transportation, communication, travel, and a variety of miscellanea such as omens and dialects.
Mission of Gravity
Hal Clement 1953. Science fiction adventure set on a rapidly rotating planet where the surface gravity varies fro 3 g to 700 g. A stunning example of worldbuilding that set the bar for all who folow.
Close to Critical
Hal Clement 1958. Science fiction adventure set on a planet where the atmosphere is close to the critical point of water. At night the temperatures are lowered to the point where the atmosphere starts to condense into 15 meter wide "raindrops".
Habitable Planets for Man
Stephen H. Dole, 1964. The standard text on what factors allow a human-habitable planet. It is a little dated but still valuable.
Medea: Harlan's World
Edited by Harland Ellison, 1985. A magnificent example of collaborative worldbuilding. Hal Clement created the Astrophysics and Geology then passed it on. Poul Anderson created the Geology, Meteorology, Oceanography, Geography, Nomenclature, and Biology then passed it on. Larry Niven created the Biology, Ecology, and Xenology then passed it on. Frederik Pohl created the Sociology, Politics, Theology, Mathematics and more Xenology. It them went to some seminars. Eleven story plots were hashed out, and eleven authors wrote the stories.
A Planet Dweller's Dreams
Martyn J. Fogg, ANALOG magazine October 1992. Martyn Fogg has spent almost a decade investigating planetary systems, concentrating his recent efforts on the study of planetary habitability and terraforming. He is internationally recognized as Britain's principal researcher on terraforming, publishing numerous scientific articles and papers on the topic. Fogg has also been a consultant to Time-Life and BBC television and radio, and has given numerous lectures and presentations worldwide on terraforming.
World Tamers Handbook
Game Designers Workshop : GDW 0311 ( Out of Print ) ISBN 1-55878-168-4. A supplement to the Traveller role playing game, this book contains lots of information for worldbuilders. Thanks to Daniel Cleyne for this reference!
On Building an Earth-Like Planet
Stephen L. Gillett, ANALOG magazine July 1989.
World Building: A writer's guide to constructing star systems and life-supporting planets
Stephen L. Gillett, Writer's Digest Books, ISBN # 0-89879-707-1. This should be on the book shelf of every world builder. I believe that it is the textbook for World Building Class at Cal State. Mr. Gillett has a Ph.D in geology, is a frequent contributor of science fact articles to Analog magazine, and has conducted worldbuilding seminars at Contact. The book has all the equations and facts you need to get started.
World Generation: Generic System & Planet Building Resources
Tyge Sjöstrand. It can be downloaded here. A generalized method of creating entire solar systems. It tries very hard to be scientifically accurate, though the author does not guarantee it. It covers gravity, planetography, plate tectonics, weather patterns, tons of stuff. A pity that the latter chapters have yet to be written.
GURPS: Uplift
This game contains a sophisticated alien psychology generation system.
GURPS Space, 4th Edition
Jon Zeigler, and James Cambias, Steve Jackson Games, 2006. Contains a scientifically accurate system to generate entire solar systems.
GURPS Traveller: First In
Jon Zeigler, Steve Jackson Games, 2006. Contains a scientifically accurate system to generate entire solar systems.

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