Now we are really sailing off into terra incognito. "Here be dragons" and all that. But if you have starships, you almost have to have aliens (Isaac Asimov's Foundation trilogy being the most notable exception). The "science" is called Astrobiology, the famous "science in search of a subject". Unfortunately it only offers vague generalities. You can keep up on the latest news, but for now if you want aliens, you are going to have to create them yourself.
Suggested reading includes The Encyclopedia of Science Fiction's entry on "Aliens", Steve Colgan's Worlds of Possibility blog, Life Everywhere by David Darling, The Science of Aliens by Clifford Pickover and Aliens and Alien Societies by Stanley Schmidt.
Wikipedia has a nice article on Hypothetical types of biochemistry
In a science essay "Not As We Know It", Isaac Asimov notes that life on Terra is based on proteins dissolved in water solvent. He points out some other possibilities. Note that the "temperature" column has the information needed to set the borders of a solar system's circumstellar habitable zone for that particular biochemistry. Temperatures assume the planet has about 1 atmosphere worth of pressure.
|Macromolecule in |
at 1 Atm
|Fluorosilicones in Fluorosilicones|| 400°? to|
|Silanes (chains of silicon atoms) are too unstable. Silicones (chains alternating silicon and oxygen atoms) are more suitable for making "silicon life" protein analogues.|
James Cambias notes that such life will consume carbon dioxide (and other carbon compounds) out of the air, combining it with silicon to create complex silicone compounds. Oxygen will be released but that will immediately combine with silicon to make silicon dioxide sand. The atmosphere will become depeleted in carbon dioxide. This might cool the planet off enough that fluorocarbon-sulfur life will take over the planet.
|Fluorocarbons in Molten Sulfur||113° to|
|Earth proteins are too unstable at liquid sulfur temperatures. They can be stabilized by substituting fluorine atoms for hydrogen atoms, resulting in complex fluorocarbons.|
James Cambias notes that such life forms will probably evolve in an atmosphere poor in oxygen but rich in fluorine. However, such life will create atmospheres with oxygen as they release oxygen from carbon dioxide+sulfur dioxide as their metabolism creates complex fluorocarbon molecules. There actually might be enough oxygen in the atmosphere for humans to breath (but the temperature would kill them).
|Proteins in Water||0° to|
|Because water is hydrogenated oxygen, the proteins will have to have more oxygen than nitrogen in their make up. This is "life as we know it." Pretty much all life on Terra falls under this catagory.|
James Cambias notes that such life will consume carbon dioxide out of the atmosphere and release oxygen, thus converting the planet's primordial atmosphere into a biologic oxygen containing atmosphere.
|Proteins in Liquid Ammonia||-77.7° C to|
|Because ammonia is hydrogenated nitrogen, the proteins will have more nitrogen than oxygen in their make up. Earth proteins are too stable at liquid ammonia temperatures, ammonia life proteins will have to be more unstable than their Earth analogues.|
James Cambias notes that such life forms will probably require a planet with a methane-ammonia atmosphere. As with protein-water life, it will consume carbon dioxide and produce oxygen. However, the oxygen will react with methane to produce carbon dioxide and water. The water will immediately freeze out of the atmosphere, the carbon dioxide will be consumed. Thus the atmosphere will gradually lose all its methane and become much lower in pressure.
|Lipids in Liquid Methane||-183.6° C to|
|Polar liquids will not dissolve non-polar substances and vice versa (oil and water don't mix). Proteins are polar, so they won't dissolve in liquid methane. Complex protein-like polylipids will have to be used instead.|
James Cambias notes that such life forms will probably require a planet with a methane-hydrogen atmosphere. As with protein-water life, it will consume carbon dioxide and produce oxygen. However, the oxygen will react with methane to produce carbon dioxide and water while the oxygen will react with hydrogen to produce more water. The water will immediately freeze out of the atmosphere, the carbon dioxide will be consumed. Thus the atmosphere will gradually lose all its methane and hydrogen thus becoming much lower in pressure.
|Lipids in Liquid Hydrogen||-253° C to|
|Liquid hydrogen is also non-polar, so polylipids will be needed.|
James Cambias notes that the temperature will be much higher in the immense pressures of a gas giant world.
Silicon-based Life". Life on Terra is based on Carbon, since carbon can join with not one, not two, not even three, but a whopping four other atoms. This allows the construction of complex molecules like proteins and DNA, a requirement for living creatures. The only other element that can do this is Silicon, so the SF writers seized it. They are also fond of harping on the fact that while most carbon-based animals on Terra exhale gaseous carbon dioxide, a poor silicon-based critter would breath out silicon dioxide, i.e.,sand. In "A Martian Odyssey" by Stanley Weinbaum is a silicon life creature that "exhales" bricks of silicon dioxide, which it uses to build a pyramid around itself.
There are several possibilities for the composition of alien blood.
An example of electronic life is the superconducting mentality in Sir Arthur C. Clarke's "Crusade".
One of the odder aliens is the Qax from Stephen Baxter's Timelike Infinity. Their "bodies" are organized clusters of millions of tiny whirlpools in still ponds. Another odd one was the Monolith Monsters. They were not invading aliens so much as an extraterrestrial chemical reaction. Instant monster: just add water.
But even if you handwave that away and declare that there are lots of different species of aliens, there is plenty of room for imagination. Especially in the alien's anatomy. Just here on Terra, we can find jellyfish, tarantulas, viruses, and giraffes. Face it, if these fellow Earth-creatures don't resemble us, a totally alien race from another planet ain't gonna look like Mr. Spock. Personally if I open an SF novel only to discover yet another cat-like alien I may need a nausea bag (RocketCat clears his throat then gives me his best "I'm Looking At A Hypocrite" look).
There might be creeping jellies, giant crystals, intelligent plants, mobile fungoids, energy creatures, fusion plasma beings dancing in solar coronas, liquid or gaseous life, swarming hive intelligences, superintelligent shades of the colour blue, and natural "electronic" life forms in pools of liquid helium. They might not be made of meat. They might not even be composed of matter as we know it, like the Cheela from Dr. Robert Forward's Dragon's Egg who are made of neutronium and white dwarf star matter.
Some extraterrestrial creatures inhabit the depths of space itself. In Sir Arthur C. Clarke's Childhood's End was a creature that lived in deep space among asteroid belts. It resembled a huge eye, about twenty feet in diameter. Its survival depended upon the range and resolving power of its eye. Large creatures include the living O'Neil colonies in John Varley's Gaean trilogy and the living planet from Stanislaw Lem's Solaris. Biggest of all is the intelligent nebula from Fred Hoyle's The Black Cloud. Well, actually Olaf Stapedon's intelligent galaxies in Star Maker are bigger, but let's not get carried away.
But you can forget about angel-like humanoids with wings. Why? Physics.
You see, wings need to flap with enough power to lift the person. The power comes from muscles, lots of muscles. So much muscle in fact that in birds they need a special bone for the wing muscles to attach to. This is called the keel or carina bone. The muscles are what we call the meat of a chicken or turkey breast, and the keel is the breastbone.
I trust you can spy the problem. A humanoid with wings is going to have a deformed chest that looks like the prow of a huge boat. And female humanoids with wings will not have mammary glands. Not on their chest at any rate. That segment of the science fiction audience with the personalties of adolescent boys will be angry at the lack of cheescake fanservice. Once again the fans will be outraged at scientific accuracy. And they will vote with their wallet.
As you can see while it is not actually impossible to have humanoid winged creatures, they are going to be more towards the "noid" and less towards the "human" part of the spectrum. Which will put them right in the uncanny valley, inspiring revulsion instead of attraction. They ain't gonna look like angels.
A more minor problem is the fact that on a bird, the wings are basically its arms. On humans, arms are attached to the shoulder blades. Which means a winged humanoid with both arms and wings is going to need four shoulder blades, not the customary two. Which probably means the wings will be attached to the small of the back, not the shoulders.
Also the neck should be long and articulated so when flying (and basically in a prone position) it can bend the head so it can see where it is going, instead of being forced to look at the ground.
Aliens with wheels are a difficult concept. There are problems with making worthwhile wheels using biology, and even more problems finding a plausible sequence where such a thing could be created by evolution.
In the real world the closest thing to an animal with wheels is the spinning flagellum of certain microscopic bacteria.
Wheeled aliens make an appearance in the satirical "Retief" story Retief's War, the g'Kek of Brightness Reef (looking like "a squid in a wheelchair" that suffer from arthritic axles when elderly), and in the Polarians of the Cluster novels (technically the Polarians do not use wheels, they roll around on large spheres).
A milder version is rolling aliens. They are generally shaped like a sphere or a disc harrow, the entire alien rolls instead of just part of the alien. There is a spherical alien in Arena and Tuf Voyaging, a cylindrical alien in Stadium Beyond the Stars, a disembodied wheel in A Star Called Cyrene, and disc harrow aliens called the Slash of the Cluster novels
A tentacle is a "flexible, mobile, elongated organ present in some species of animals, most of them invertebrates" (technical term cephalopod limb). Since they are uncommon in familiar earthly animals, they became a popular characteristic on unearthly science fiction aliens.
This old trope dates back to prehistory, when the first man was freaked out when they discovered the octopus. In science fiction it dates back to at least 1898 with H. G. Well's War of the Worlds.
It lingers on in popular media. TV Tropes notes how be-tentacled creatures commonly use their tentacles in unique ways for combat (such as the old tentacle rope trick, that never gets old). As are tropes about the unexpected vulnerabilities of tentacles, such as the "knotty tentacle" trope.
According to Dr. Hans P. Moravec of the Carnegie-Mellon University, most land animals on Terra are "fractal."
So species that use tentacles figured out how to turn an arm or leg into a manipulative organ without needing to grow fingers.
Mechanically a tentacle is a "Muscular Hydrostat", consisting mainly of muscles with no skeletal support (an arm with no bones). It relies on the fact that water is effectively incompressible at physiological pressures, and the fact that muscles are mostly composed of water (i.e., it is hydraulic machinery). If the structure used pockets of water in separate compartments instead of watery muscles it would be a hydrostatic skeleton, but I digress.
Common examples of muscular hydrostats include octopus tentacles, elephant trunks, the entire body of a worm, and the human tongue.
Tentacles are mostly solid muscle.
Just like in animals with skeletons or exoskeletons, tentacle muscles can only provide force by contracting, expanding doesn't do diddly squat. So just like in conventional animal limbs all tentacle muscles are arranged in antagonistic pairs. If one muscle pulls to the left it is paired with an antagonist muscle that pulls to the right. As one muscle in the pair contracts the other relaxes.
The muscle fibers are oriented in three different directions:
|Perpendicular to the long axis|
|Longitudinal||Parallel to the long axis|
|Helical||Wrapped obliquely around the long axis|
The closer the longitudinal muscles are located to the tentacle skin, the more elaborate bending movements are possible. Octopus arms, elephant snouts, and other manipulators all have this arrangement. You only see centrally located longitudinal muscles in limbs that just protrude in and out, like snake and anteater tongues.
Muscles perpendicular to the long axis can be in a circular, radial, or transverse pattern. Radial and transverse muscles are anchored to the external connective tissue by threads called "trabeculae" which penetrate the longitudinal and helical muscles that are in the way. Transverse muscles are in sheets that alternate between horizontal and vertical (the "down" direction is towards the side of the tentacle with suckers, technical term is "oral side").
|Circular||Rings around long axis||squid tentacle|
|Radial||Radiating from center in a disk shape||chambered nautilus tentacle|
|Transverse||Alternating between horizontal and vertical||octopus tentacle|
Helical or oblique fibers wrap around the long axis like candy cane stripes. They are usually in two or more layers of opposite chirality (left hand/right hand). The external and medial helixes are at an angle of 50 to 60° to the long axis, internal are at 40 to 50°. The role of the internal helical muscles is unclear.
Like all hydraulic machinery, the operating principle is the incompressibility of water, that is, if you push water into one end of a tube water will come spraying out of the other end. The important point is "incompressible" means the volume of water always stays the same. If you reduce a volume of water's dimension in one direction it will have to expand in at least one other dimension.
So, for instance, if the muscles squeeze the tentacle to reduce its diameter (height and width dimensions), the tentacle will elongate along the long axis (length dimension). Because the volume of tentacle has to always stay the same.
Elongation and Shortening
This is when the tentacle grows or shrinks along the long axis. Like when you stick out your tongue.
When the perpendicular (or helical) muscles contract (decreasing the tentacle's diameter) it elongates along the long axis (increasing the length). When the longitudinal muscles contract the tentacle shortens along the long axis (shortening the length) while simultaneously expanding hight and width (increasing the diameter).
So in this case the perpendicular muscles are operating antagonistic to the longitudinal muscles.
Some frogs can elongate their tongues up to 180% of its resting length. Due to hydraulics, the more the tongue is capable of elongating, the less force it can hit an object with.
Bending the tentacle is done by using the longitudinal muscle to reduce the length of the tentacle while other muscles act to prevent the length reduction on one side of the tentacle. This causes a bend on the opposite side of the tentacle.
Octopi apparently contract all of the longitudinal muscles while strategically using the perpendicular muscles to maintain a constant diameter at specific points.
Some tentacle robot limb designs have no perpendicular muscles. Instead they expand (using inflatable tubes instead of muscles) only some of the longitudinal muscle. The robot tentacle bends on the side of the uninflated tubes.
This is twisting the tentacle on the long axis, like it was a drill bit. It is done by contracting one of the two sets of hexlical muscles.
It is possible to make the tentacle rigid. The details are elusive but I would presume it can be done by contracting all the muscles at once.
Moving the tentacle to increase the distance between tentacle tip and tentacle base. The two basic types of reaches are:
Uncurling Reach: where the tentacle starts out rolled up in a spiral and rolls out.
Elongating Reach: where the arm starts out straight and grows longer.
Moving the tentacle to decrease the distance between tentacle tip and tentacle base. The three basic types of pulls are:
Continuum Curling Pull: where the arm rolls into a spiral.
Straight-arm Shortening: where the arm is straight and grows shorter.
Bending Pull: where the arm creates an elbow like bending point.
These are behaviors that are a lateral combination of sharp bends, sweeps, wraps, lifts, torsional rotations, drop, etc.
Robot researchers have been experimenting with making tentacle-like robot arms and bodies. These use the same muscular geometry as tentacles but usually without the hydraulics. NASA had looked into this concept under the title "serpentuator".
Some use a set of contracting longitudinal and transverse muscles. Other just use pneumatically expanding arrays of longitudinal muscles and no transverse muscles.
A "hive" intelligence would resemble an intelligent ant-hill, where each ant would be but a cell in the hill's "body". Individual ants may die, but the hill goes on. Examples include the "Boaty Bits" from FARTHEST STAR by Jack Williamson and Frederik Pohl, the "Godtalkers" from THE DRAGON NEVER SLEEPS by Glen Cook, the "Tinker Composite" from THE MIND POOL by Charles Sheffield, the "Mantis" from GREAT SKY RIVER by Gregory Benford, and the Martians from LAST AND FIRST MEN by Olaf Stapedon. If the alien is composed of a hive of several species, it is some times called an "anthology intelligence." Go to The Tough Guide to the Known Galaxy and read the entry "HIVE ENTITY".
A good example of a hive intelligence was in Olaf Stapedon's classic Star Maker. The "cells" composing an individual were free-flying birds linked telepathically. Birds might be born or die, but the flock-individual lived on. A more modest version were the "Tines" in Vernor Vinge's A Fire Upon The Deep. One might even consider an anthill to be a hive organism, an individual who's cells are ants.
Composite creatures are subtly different from Hive Entities.
Hive entities are swarms of identical type creatures who coordinate their movement and actions (e.g., a drove of army ants). Generally the component creatures are not physically connected, but communicate with each other in some way.
Composite creatures (aka colonial organisms, symbiotic collectives, or modular organisms) are assemblages composed of several different types of creature, who coordinate their movement and actions (e.g., a portuguese man o' war). Generally the component creatures are physically bonded into one unit.
Of course one sinister variant is the alien mind-enslaving parasite creature. They slip inside your body and make a composite creature of you-the-slave and them-the-master. The benevolent version has the alien entering into symbiosis when it slips inside your body,. This results in your body giving it food and it giving you incidentals like prolonged life span, accelerated healing, telepathy, and other perks.
In Robert Sheckley's short story Specialist (online here) (1953) starships are composite creatures. Many planets are home to "wall" aliens who form the hull, some planets have "atomic engine" aliens who are the normal space propulsion system, some have "eye" aliens who are sensors, some have "network" aliens who plug into the minds of all the components for coordination, and some have food producer aliens. As it turns out, Terra is planet home to "pushers", who are the FTL drives of starships.
In Star Trek (ST:TNG and ST:DS9) the species the Trill are composite creatures. The Host is a humanoid, with a sort of kangaroo-like pouch. The Symbiont is a squirmy little thing that lives in the pouch. Both parts are intelligent, and together form a composite personality. The symbiont has a life span several times as long as the host, so they occupy several hosts during their lives.
These are critters that look like large quartz crystals, often with flashing lights inside. Most are immobile, some can move. Some crystal life is silicon-based life, other are not.
An odd one was the Monolith Monsters. They were not invading aliens so much as an extraterrestrial chemical reaction. Instant monster: just add water.
In some cases the line between crystal life and electronic life is very blurry. The most obvious basis for such life is that it is based on semiconductor electronic circuits that somehow evolve and become more complicated inside the crystals.
What if an alien ecosystem is not composed of organic life based on chemistry, but instead on cybernetic life based on electronics?
In science fiction there are two main trends:
- a high tech organic species creates a robotic species capable of reproduction.
- a sufficiently weird planet manages to naturally create a native species based on electronics.
In the first case there are many possiblites.
The parent organic species might have given the robotic species some artificial intelligence from the start. Then the robots break free of their parent's control:
- The organic parents evolve into higher forms and depart for another dimension or something, abandoning the robots to their own devices
- The organic parents decline into decadence and become extinct, leaving the robots to inherit their world
- The robots decide it is against their own self-interest to obey the orders of their squishy organic parents, and wander off to form a Robot Republic.
- The robots go full Skynet on the parent organic species. This can be from the worker robots deciding to break free of organic control by extermination, or because the robots were originally some kind of planetary attack weapon that lost the ability to distinguish friend from foe (see Beserkers and related concepts).
Alternatively the robotic species started out as von Neumann probes or other self-replicating machine sent by the organic parents to explore the universe. These could unexpectedly evolve, as replication errors creep into the internal blueprints. Cautious organic parents will put in a fail-safe, forcing the von Neumann probe to die if it discovers blueprint replication errors. But that only works until there is some damage to the replication error detector component (or simultaneous damage to the detector and to the detector blueprint).
In the exploration case it isn't so much the case that the machine broke free of their parent's control so much as the parents are thousands of light year away.
Beserkers are dedicated to seeking out and destroying all life. But almost as bad are runaway von Neumanns who just want to fill the universe with copies of themselves, but who view planets and living creatures as convenient sources of chemical elements. Unlike berserkers, the decision to kill you is nothing personal, they just need to eat you for the raw materials.
Robotic evolution will kick into high gear if intelligent robots gain the ability to self-modify their own blueprints. Instead of millions of years worth of random mutations required to adapt to new challenges, they can adapt in a single generation.
von Neumann machines are very popular in science fiction.
Energy creatures have a biological basis of patterns of energy with little or no matter involved. In science fiction they are usually fuzzy glowing balls or are totally invisible. Living ball-lightning.
In James Blish's The Star Dwellers, the "angels" are a species of energy creature that inhabit nebulae, and love to curl up in the cozy warmth of a starship's Nernst-effect fusion reactor. They are long-lived, the eldest were born shortly after the birth of the universe about 13 billion years ago. The Starfish from Glen Cook's Starfishers are vast creatures composed of fusion fires and magnetic fields. The human Starfishers protect the Starfish from the "sharks", and in exchange the Starfish give "ambergris nodes" which are the sine qua non of tachyon communication equipment. Magnetic nebula life appears in William Tedford's Nemydia Deep and "magnetovores" (i.e., organisms that consume magnetism) living in the solar corona are in David Brin's Sundiver. There are photovores around the galactic core in Gregory Benford's Sailing Bright Eternity (also described in Benford's article in the August 1995 issue of Fantasy & Science Fiction magazine, A Scientist's Notebook: Life at Galactic Center).
And many more.
The natural habitat of such creatures in science fiction is commonly in the interiors of stars or nebulae. Pulp scifi often have energy creatures native to Sol visiting Mercury, where they are encountered by human astronauts. Electromagnetic creatures in pulp scifi often cause mischief by zipping through telegraph and power lines, and radio beams. Since they presumably eat energy they are immune to most weapons, and have a nasty habit of sucking power plants dry of electricity.
And if you are an old geezer like me, the episode of Jonny Quest called "The Invisible Monster" which featured an energy creature scared the living poop out of you. At least in Jonny Quest polls, that's the ep which wins the "most scary" vote.
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.
monocular vision, eyes on the side of their face aimed left and right working separately. This allows them to approximate 360° vision thus reducing the blind spot a carnivores can use for ambush purposes.
Back in the 1950s a popular scifi b-movie trope was giant insects and other monsters. Not to mention Godzilla. These were quite popular at the time. A pity they are yet another iconic scifi trope that science grinds into the dirt while saying You Can't Do That.
The minor problem with Big Bugs is that insect's crude stand-in for actual lungs is utterly incapable of absorbing enough oxygen to keep the blasted critter alive. Not when scaled up to monster size, at any rate.
But the main problem is the pesky Square-Cube Law.
When an object undergoes a proportional increase in size, its new surface area is proportional to the square of the multiplier and its new volume is proportional to the cube of the multiplier. In English: if you enlarge a bug with blue Pym-particles its weight will grow much faster than its skin.
For example, if you double the size (measured by edge length) of a cube, its surface area is quadrupled, and its volume is increased to eight times its original volume.
This creates many problems.
In his Lensman series, E.E. "Doc" Smith invents an alien body type classification system, though he gives precious few details. In the system, human beings are classified as AAAAAAAAAAAA to twelve places, and aliens have other letter codes depending upon how they vary from humans. The fifth place is for number and type of arms, the sixth is for number and type of legs, and seventh place is skin.
Orginally Traveller's UPP was a six place code, with each place filled by a hexadecimal numeral from 0 to F (decimal number from 0 to 15). Each place was a Role Playing Game character value: Strength, Dexterity, Endurance, Intelligence, Education, and Social Standing. So each of the characteristics could have a value from 0 to 15.
For other Traveller codes hexadecimal proved to not have enough precision, so they went with an ugly kluge called the Expanded Hexidecimal System. This used for numeral the number 0 to 9 and the letters A to Z (omitting the letters I and O since they were too easily confused with numerals 1 and 0). This allows values from 0 to 33.
James White adapted the system to his Sector General novels, with the the more reasonable specification that human beings were not the measure of all things, i.e., in the Sector General system humans are classified as DBDG, not AAAAAAAAAAAA.
The average level of intelligence of an alien species is anybody's guess. However, there are thought experiments suggesting that their intelligence would tend to be about the same as our own. Of course there might be outliers; morons on the planet Spengo and Pakled, super geniuses on Altair IV and Arisia. Or if they hit the Singularity and shoot off the top of the chart, turning into StarGods or something.
There are some notes on talking to aliens here.
In the real world, communication with hypothetical extraterrestrials is such a huge problem that it may never be properly solved. Researchers are having enough problems trying to talk to porpoises, and they are from our own planet. Alien thought processes might be forever inscrutable. There is a good list of examples of inscrutable alien languages on TV Tropes.
In C. J. Cherryh's Chanur novels, the methane-breathing Tc'a species are almost impossible to be communicated with, since their brains are multi-part and their speech decodes as complex matrices of intertwined meanings. In Piers Anthony's KIRLIAN QUEST, the Slash use modulated laser beams. As did the deep space beings in Jack Williamson's TRAPPED IN SPACE. In Charles Sheffield's PROTEUS novels, the Logeinan life form uses an area of skin that has changing color dots. As does the intelligent squid in Arthur C. Clarke's The Shining Ones.
And just imagine the headaches of trying to communicate with a species that uses various scents and smells instead of sound. Or radio waves. Or modulated laser beams. Or rapid changes in skin color. Or all four combined.
The psychology of an alien species is any body's guess. It could be so alien as to be forever beyond our understanding. It could be quite human. Or somewhere in-between.
Some clues to an alien species psychology might be found in their ecosystem classification. For instance, herbivores might be skittish, only comfortable in groups, and tend to flee if they feel threatened.
In James P. Hogan's The Gentle Giants of Ganymede, on the giant's planet the herbivores evolved a third circulatory system full of toxins which made their flesh poisonous to carnivores. It was so effective that carnivores became extinct. The herbivores evolved to look like animal illustrations from a nursery or kindergarten story book, all cute, plump and cuddly. The result was that the giant psychology has no confrontation, pride, or sense of danger.
Larry Niven's Puppeteers evolved from herbivores. They are the cowards of the universe, their leader is called "The Hindmost" because it is the furthest from any danger. In Puppeteer society, courage is seen as a mental illness. Puppeteers are pragmatic to a fault. Human traits such as wishful thinking and superstition are nonexistent. This means there is no level of danger that they'd consider to be an acceptable risk, the only acceptable level is 0%. They are willing to go to any lengths to protect themselves from perceived danger and provide a safer environment for themselves.
In Niven and Pournelle's classic novel Footfall, the alien Fithp are herd creatures. They do not understand how or why you would possibly initiate diplomacy before first fighting to see which party was dominant. When a Fithp is defeated, it surrenders, and thereafter becomes totally devoted and subservient to its conqueror.
Fithp are horrified when they defeat humans in battle, the humans surrender, then the humans suddenly break their surrender and counter-attack. To the Fithp, this is mad-dog behavior, and the humans are treated as such.
Many aliens could prefer to live on planets that human beings would find to be miserable hell-holes.
Many point to the ecosystems at the Galapagos black smokers as proof that life is possible in underground oceans on, say, Europa. However, if this is true, the implication is that such life will be far more common than terrestrial life. After all, there are several such moons in our solar system, and only one Terra (Europa, Enceladus, Ganymede and Titan). If there are four such moons, then throughout the universe iceball life will outnumber liquid water life four to one, on average. Such life turns up in The Killing Star by Charles Pelligrino and George Zebrowski.
But most humans would rather not live in sub-zero water under kilometers of solid ice.
However, if the aliens like to live on the same kinds of planets that Terrans do, the way to bet is that eventually there will be war. The only wild card is if one or both species like living in mobile asteroid habitats (Macrolife). Or if a species is a primitive civilization with the misfortune to live in an Elder God Galaxy, and has to keep a real low profile in order to survive.
As a side note, one can use the time between apes and angels for the "average lifespan of a technological civilization". Insert this into the Drake equation along with a few other guesses and you can calculate the average distance between alien civilization homeworlds. (and of course the distance between Terra and the closest aliens).
I say "homeworlds" because they might have colonized nearby stars to form an empire. In this case the homeworld will probably be in the center of the empire's sphere of influence. Therefore the closest aliens will be the average distance between minus the radius of their empire. Go to The Tough Guide to the Known Galaxy and read the entry "HOMEWORLD".
If you already have an idea of how close you want civilizations to be spaced, you work the Drake equation backwards. Keep altering the values until you get the spacing you want. But now you have to live with the consequences of those various values, and their implications.
Gas giant planets are very unattractive places for humans to colonize. Blasted things do not have a real surface, the atmosphere just gradually thickens into a slush, which gradually thickens into metallic hydrogen or something.
Somewhat arbitrarily the "surface" of a gas giant (zero point for altitudes) is set when the pressure reaches 1 bar (average sea-level pressure on Terra). The vague "top" of Jupiter's atmosphere is roughly 5,000 kilometers above the surface. The "cloud-top" level of Jupiter's atmosphere is where the pressure is about 0.1 bar (50 km above surface). Confusingly astronomers decided the base of the atmosphere (base of the troposphere) is not at the "surface", it is below that where the pressure reaches 10 bar (90 km below surface). The atmosphere starts turning into a slushy gas at about 13 bar (95 km below surface). And it turns into a slushy liquid at about 5,000 bar (at 1,000 km below surface, and 1,700° C).
But long before you get to the slushly liquid state the pressure will grow high enough to make your spacecraft implode and the temperature will melt the ship. Presumably any native life form on such a planet will either perpetually float in the upper atmosphere, or be very crush-proof heat-resistant slush swimmers.
Atmosphere fades into
|+1,000||1 nbar||Bottom: exosphere|
|+320||1 μbar||Bottom: thermosphere|
Top: troposphere (tropopause)
"cloud top" (start of haze layer)
|+46?||Top: Sinker zone|
|0.6||145||Top: Ammonia cirrus cloud level|
|0.9||150||Bottom: Ammonia cirrus cloud level|
|0||1.0||165||-108°||Datum (ave. Terran sea-level)|
Top: Ammonia-sulfur cloud level
3He scoop mining level
|2.0||200||-73°||Bottom: Ammonia-sulfur cloud level|
|3.0||Top: Water clouds level|
|7.0||Bottom: Water cloud level|
Center of sinker zone
Floater feeding zone?
|-95?||12.9||Top: Hydrogen becomes slushy gas|
|-185?||250?||500?||230°?||Bottom: Sinker zone|
Top: Organisms incinerated
Top: Hydrogen becomes slushy fluid
Pressure of Mariana Trench
|2,000,000||10,000||9,700°||Top: Metallic hydrogen|
Values with question marks were calculated with linear interpolation.
Carl Sagan and E. E. Salpeter postulated floating organisms could exist in the temperate regions of Jupiter's atmosphere in a 1975 paper. An entire ecosystem, with aerial plankton grazed on by sky whales, who were preyed on in turn by flying sharks. This was later featured in Sagan's documentary series Cosmos.
In Sagan and Salpeter's paper, "sinkers" were aerial plants that were born in the upper troposphere and gradually fell to their death in the inferno of Jupiter's lower atmosphere. Along the way they grew by photosynthesis using blue light and abundant atmospheric methane, water, and ammonia. They also reproduced by emitting tiny spores, stimulated by moving from region of depleted resources into a region of abundant nutrients. The spores were carried up to the upper troposphere by atmospheric turbulence, where the cycle of life starts anew. The paper calculates that a sinker has a size of about 30μM (about the size of a small terrestrial protozoa) and will take about two months to fall from the birth altitude to the incineration altitude. Later Sagan upped the size estimate to up to the size of a toy balloon.
Alternatively sinkers can grow by becoming a colony creature. The component creatures reproduce and the colony grows. When it sinks too close to incineration depth, the colony disperses into individuals. These are small enough to rise to safe altitudes by atmospheric turbulence. Paper estimates a colony can contain about 10,000 if colony and individuals do not exceed max size.
"Floaters" are herbivores. They feed on the sinkers, and use the extra metabolic energy to maintain float bladders. This allows them to avoid falling to a fiery death. One way to float is to pump their bladder such that it contains close to pure hydrogen, instead of the hydrogen-helium mixture composing the Jovian atmosphere. The other is to use metabolic energy to heat the atmosphere inside the bladder (since hot hydrogen-helium is lighter than cool hydrogen-helium). Heating will require a larger bladder than pumping helium. The paper calculates that it is possible to have floaters with sizes measured in kilometers.
And where you find herbivores you generally also find carnivores preying upon them. The "hunters" kill and eat floaters, using the more concentrated food energy to allow stalking and chasing. Hunters are also after their prey's store of purified hydrogen inside their float bladders.
There is a second class of (thermoresistant) floaters called "scavengers", living just above the hot zone and eating the steady fall of incinerated sinkers, or the incinerated bodies of dead floaters and hunters.
Sir Arthur C. Clarke expanded upon this theme in "A Meeting With Medusa" and in 2010: Odyssey Two. These stories featured creatures that were sort of a cross between a titanic jellyfish and a zeppelin. A similar ecosystem is in Ben Bova's novel Jupiter. There are also "sky whales" appearing in Dr. Robert Forward's Saturn Rukh.
As a rule, species that inhabit terrestrial planets (such as our species) do not have much interaction with aliens who live on gas giants. In the general this is because we and they have little or no common frames of reference which makes communication difficult. In the specific it is because we and they do not covet each other's real estate so there is no reason to go to war. In Poul Anderson's galactic novels, the human galactic empire and several gasworlder empires interpenetrated each other and ignored each other.
There are exceptions, such as Kevin J. Anderson's Saga of Seven Suns series. In the first novel, the human empires are unaware of the existence of the Gasworlders ("Hydrogues"). This proves to be unfortunate. When the humans test a device which converts gas giants into blazing suns (including the Hydrogue inhabitants), the remaining Hydrogues in the Hydrogue Empire become very very angry. Hilarity ensues as the diamond-armored Hydrogue dreadnoughts start kicking the living snot out of the human planets.
Another exception is described by Hal Clement here, where humans and jovians interact exactly like they were engaged in a war, but they are not. Humans are scoop-mining Jupiter's atmosphere, and the Jovians become furious at hypersonic scoopships obliterating their orchards, gardens, and flocks; not to mention Jovian citizens. So the Jovians start attacking the human scoopships. Humans will retaliate, and the net result will be very hard to distinguish from actual warfare.
Alternatively, the Jovians might see the scoopships as valuable concentration of metals, and start harvesting the scoopships. In that case the Jovians might limit the number of scoopships they grab, or the humans might get fed up and stop sending them.
Phil Masters, in his article for the game Traveller about his gas giant dwelling Jgd-Ll-Jagd aliens, had this to say: The chief point to note in such systems is that fuel-skimming a Jgd world is extremely unwise; shock waves from the pass will cause severe damage to the beings and their environment, and their response is certain to involve high-energy weapons fire. For this reason, Jgd systems are well-marked with navigational beacons. (Traveller tramp merchant ships routinely skim gas giants for free fuel)