Astrobiology, and Realistic Alien Design

(This write-up primarily is modified from a classroom lesson and activity, and deals with the design of animal-like life of alien origin, rather than sapient, civilization-making aliens, and focuses on our Solar System rather than any known exoplanets. If you’re looking for stuff like that, check the further reading resources at the end of the article.)

This article includes the following content, as fit for classroom education (which is useful for teachers):

  • Recognize the major common characteristics of all planets and compare/contrast the properties of inner and outer planets.
  • Compare and contrast adaptations displayed by animals and plants that enable them to survive in different environments such as life cycles variations, animal behaviors and physical characteristics.
  • Describe how the composition and structure of the atmosphere protects life and insulates the planet.
  • Explore the scientific theory of evolution by recognizing and explaining ways in which genetic variation and environmental factors contribute to evolution by natural selection and diversity of organisms.

ice astrobiologist

Life on another planet may be very different from life familiar to us on Earth, but no matter where it lives, it must have the adaptations needed to survive in the environment it lives in. Consider the scientist in the picture here, looking into a hole in the ice at the arctic north. There is life underwater, in the cold and the dark, hanging upside down attached to frozen ice, which is quite alien and unfamiliar to most of us, and yet native to the Earth.

00 ice life

These creatures are adapted successfully to this environment, and so they thrive. So, knowledge of a planet’s environments is the foundation of any good alien design process.

For example, here are some facts about Earth:

  • Rotation: 24 hours (length of day)
  • Orbit: 365.4 days (length of year)
  • Orbital Eccentricity: 0.01 (distance from the sun is mostly consistent)
  • Axial Tilt: 23.44° (has seasons)
  • Gravity: 1 G (1 standard Earth gravity)
  • Temperature: 15°C (global average, though warming in the last century).
  • Moons: 1 moon (relatively large by moon standards compared to the planet it orbits, which keeps the Earth’s movement stable)
  • Composition: Mostly silicates (that is, rocks). Despite a small amount of carbon (and a large amount of silicon), Earth life is carbon-based, not silicon-based.
  • Primary Environment: salt water oceans (71% of the surface, mostly pH neutral/not acidic). (The environments familiar to humans, like forests, grasslands, and towns, are not the primary environment type of Earth, though, and even just looking on land most of Earth’s surface is mountains or deserts.)

Naturally, other planets have different environmental conditions. When designing or thinking about life that might have to exist on another planet, taking these conditions into consideration is key, and for one reason or another (usually temperature, lack of liquid water and atmosphere), life as we know it is considered quite unlikely.

As a mental exercise, think about the environmental challenges for life to exist on other planets in our home solar system. Below is more information about them:


  • Rotation: 58.8 days (length of day)
  • Orbit: 88 days (length of year)
  • Orbital Eccentricity: 0.2 (distance from the Sun is not consistent)
  • Axial tilt: 0.5° (has no seasons)
  • Gravity: 0.38 G (about 1/3 standard Earth gravity)
  • Temperature: 127°C (above water’s boiling point)
  • Composition: rocky
  • Primary Environment: polar ice regions, mobile “twilight zone” between day-time side and night-time side
  • Other Notes: no atmosphere


  • Rotation: 243 days (length of day; retrograde/spins “backwards” relatively)
  • Orbit: 224.65 days (length of year)
  • Orbital Eccentricity: 0.007 (distance from the sun is mostly consistent)
  • Axial Tilt: less than 3° (has no seasons)
  • Gravity: 0.9 G (nearly standard Earth gravity)
  • Temperature: 462°C (well above water’s boiling point, due to a runaway greenhouse effect).
  • Primary Environment: high-altitude clouds above the surface, away from the crushing atmosphere and high temperatures (but closer to space radiation); maybe consider the kind of aerial life that might live here off the ground?


  • Rotation: 24 hours, 39 minutes (length of day)
  • Orbit: 685 days (length of year)
  • Orbital Eccentricity: 0.09 (distance from the sun is less consistent)
  • Axial Tilt: 25.19° (has seasons)
  • Gravity: 0.37 G (about 1/3 standard Earth gravity)
  • Temperature: –63°C (varies widely from day and night, never above 35°C, usually below water’s freezing point).
  • Composition: Mostly silicates (that is, rocks).
  • Primary Environment: rocky deserts
  • Other Notes: thin CO2 atmosphere with very little air pressure; no magnetosphere to protect from space radiation


  • Rotation: 10 hours (length of day)
  • Orbit: 12 years (length of year)
  • Orbital Eccentricity: 0.05 (distance from the sun is somewhat consistent)
  • Axial Tilt: 3.13° (has no seasons)
  • Gravity: 2.5 G (2 ½ times standard Earth gravity)
  • Temperature: –108°C (well below water’s freezing point)
  • Moons: many, including Europa, Callisto, and Ganymede
  • Composition: Gaseous hydrogen and helium
  • Primary Environment: cloudy atmospheres

There is debate as to whether the moons of gas giant planets like Jupiter or Saturn could harbor some form of life, with ice melted from tidal flexing rather than the Sun’s distant heat. So, some of these moons are detailed:

Ganymede, largest moon of Jupiter

  • Gravity: 0.15 G (around 1/10th standard Earth gravity)
  • Temperature: –163°C (well below water’s freezing point)
  • Composition: rocky, with an icy surface and subsurface ocean.
  • Primary Environment: sub-ice oceans, made from tidal flexing

Callisto, second largest moon of Jupiter

  • Gravity: 0.13 G (around 1/10th standard Earth gravity)
  • Temperature: –139°C (well below water’s freezing point)
  • Composition: rocky, with an icy surface and subsurface ocean.
  • Primary Environment: sub-ice oceans, made from tidal flexing

Europa, sixth-closest moon of Jupiter

  • Gravity: 0.13 G (around 1/10th standard Earth gravity)
  • Temperature: –171°C (well below water’s freezing point)
  • Composition: rocky, with an icy surface and subsurface ocean.
  • Primary Environment: sub-ice oceans, made from tidal flexing


  • Rotation: 10.65 hours (length of day)
  • Orbit: 29.5 years (length of year)
  • Orbital Eccentricity: 0.05 (distance from the sun is somewhat consistent)
  • Axial Tilt: 26.73° (has seasons)
  • Gravity: 1.06 G (nearly standard Earth gravity)
  • Temperature: –139°C (well below water’s freezing point)
  • Moons: many, including Titan
  • Composition: Gaseous hydrogen and helium
  • Primary Environment: cloudy atmospheres

Titan, largest moon of Saturn

  • Gravity: 0.14 G (around 1/10th standard Earth gravity)
  • Temperature: –179°C (well below water’s freezing point)
  • Composition: Hydrous silicate core, with an icy surface and subsurface ocean.
  • Primary Environment: mountains of ice (instead of rock), bodies (oceans/rivers/lakes) of liquid ethane/methane (instead of water)
  • Other Notes: dense atmosphere

Enceladus, 6th-largest moon of Saturn:

  • Gravity: 0.1 G (1/10th standard Earth gravity)
  • Temperature: –198°C (well below water’s freezing point)
  • Composition: Mostly silicates (that is, rocks), with an icy surface and subsurface ocean 10 km deep.
  • Primary Environment: sub-ice oceans, made from tidal flexing


  • Rotation: 17 hours (length of day)
  • Orbit: 84 years (length of year)
  • Orbital Eccentricity: 0.04 (distance from the Sun is somewhat consistent)
  • Axial Tilt: 97.92° (42-year long “seasons” of light and darkness)
  • Gravity: 0.9 G (nearly standard Earth gravity)
  • Temperature: –209°C (well below water’s freezing point)
  • Composition: gaseous methane, hydrogen and helium; deep icy mantle, rocky core
  • Primary Environment: frigid atmosphere, ice mantle


  • Orbit: 165 years (length of year)
  • Rotation: 16 hours (length of day)
  • Orbital Eccentricity: 0.01 (distance from the Sun is consistent)
  • Axial Tilt: 28.8° (has seasons)
  • Gravity: 1.137 G (nearly Standard Earth gravity)
  • Composition: gaseous methane ice, hydrogen and helium; deep icy mantle, rocky core
  • Primary Environment: frigid atmosphere, ice mantle
  • Temperature: –222°C (well below water’s freezing point)
  • Other Notes: storms in the atmosphere

In addition to considering the environment, basic facts about biology are also important. If considering how your alien design might live, know that your alien needs to meet some criteria to be alive:

  • Typically, other planets are inhospitable to humans; if a human tries to walk around on Mars, they would die. Your native creature has to have adaptations to survive the atmospheric and temperature conditions of its home world for at least some time, hopefully long enough to grow, thrive and reproduce.

  • It has to be able to sustain itself and keep itself alive, usually by getting the nutrients such as water, food, and possibly sunlight.

  • It has to be able to respond to things in its environment, such as changes in seasons, the presence of food and predators, or similar issues.

In short, the Rules are: It can’t die out, it mustn’t starve, and it should be able to do things. How it goes about this is up to you.

So, you want to design a space alien? Well, popular culture gives many people poor ideas about what an alien would be like.

Scientifically, this is NOT a realistic space alien:

01 superman

Superman (as seen in Young Justice)

nor is this:

02 navi

Na’vi (as seen in James Cameron’s Avatar)

or this:

03 yoda

Yoda (as seen in Star Wars)

The main problem is that each of these “aliens” are essentially only Weird Men: different skin color, different facial structure, different heights, or some other feature that’s alien, but definitely not too alien.

(Particularly egregious is of course the classic and beloved Superman and his oft-copied superpowers: “How can this white dude fly and shoot laser-beams from his eyes?” one might ask. “Why, he’s actually a space alien!” the reply comes, “he’s different from us humans, so he just do that.” Well, excuse me: I’m not from Kansas, but that doesn’t mean I can walk through walls or throw cars around: and just because Superman (or any alien you might come up with) isn’t from planet Earth or count as a member of homo sapiens doesn’t mean (if we’re trying to be realistic) that he should be able to break the laws of nature either.)

Ultimately, when designing creatures that are alien, remember that it is not only scientifically unrealistic, but also lazy design, to copy the kinds of examples you’ve seen above.

By contrast, these creatures, which look alien, are actually from Earth, your home planet:

04 tardigrade

A tardigrade, water bear/moss piglet

05 vampire squid

Vampyroteuthis infernalis, vampire squid

06 star-nosed mole

Condylura cristata, star-nosed mole

And these are some of the more tame examples. If you sought them out in their native habitats, you could find these real-like creatures on your own home planet, alien as they are. Weirdness is very real, and it thrives.

To get you thinking, here are some artist’s impressions of possible alien designs, which are scientifically plausible, but fictional:

07 Nemo Ramjet

seismopus kurash, by Snaiad artist Nemo Ramjet

08 Wayne Barlowe

daggerwrist, by Expedition/Alien Planet artist Wayne Barlowe

09 Abiogenesis

birrin, by Birrin project artist Abiogenisis

So, What Can I Use in My Designs? Convergent Evolution

Sometimes in guides on science fiction it can feel like the science is always telling authors and creative types what we cannot use, such as faster than light speed travel being impossible – this section is to provide some things you definitely can use!

When considering what an alien from another planet might look like, there are many factors to consider, ranging from atmospheric composition, star life cycles, geologic processes and evolutionary trends. One of these latter elements includes a phenomenon observed many times in nature, that of convergent evolution. This means that, in a given environment, different and unrelated living things will develop similar adaptations to survive in similar situations. Classic examples are the similar streamlined body shapes of dolphins and sharks and the evolution of bird and bat wings to achieve powered flight. Even though each of these animals are unrelated to each other, they developed adaptations that perform the same function because that biological design simply works in that situation or environment.

For any realistic alien you might design, these components listed below may be thought of like the Lego pieces scientists know you’d probably wind up working with at least once in a while. So here are some of the building blocks we might see in alien life, since we see these designs repeatedly on Earth:

  • beaks as mouth parts, like in birds and squids

  • mouths in general

  • eyes, which have evolved many times (though they are less useful on planets or in environments with limited sunlight (such as far from their star or in deep oceans)

  • a colorful tail on prey animals to distract predators and protect the head from being targeted, as in some lizards and fish

  • a tail or similarly shaped arm that can detach and grow back, and wriggles to distract predators while the prey animal escapes, as in lizards and octopus

  • spines that defend the life-form from predators, such as hedgehogs or cacti

  • body/flesh that is poisonous or foul-tasting, and whose colorful body is a warning (aposematism) to predators, as in monarch butterflies, poison arrow frogs and some sea slugs; or the animal is venomous, as in coral snakes

  • an appearance that closely mimics a known dangerous creature (Batesian mimicry), though it is itself harmless, as in milk snakes and some butterflies (Pretty sneaky!)

  • able to curl into a ball to defend itself (though it cannot roll anywhere), as in pill bugs, pangolins and three-banded armadillos

  • camouflage of various kinds, usually to blend into a specific environment, as in jaguars and stick insects; others will resemble rocks or wood patterns on trees

  • countershading, a type of camouflage where the top half of the animal’s body is darker than the underside, as in antelope and sharks

  • vemon, as in scorpions, vipers, and platypus (note: for some reason venom is more common in tropical/warm-climate animals than temperate/cold-climate animals)

  • produces silk, either for webs or cocoons, as in spiders and silk worms

  • pincers, as in scorpions or crabs

  • grows its own food, as leaf cutter ants and damselfish (and also humans)

  • electroreception, the ability to sense electricity and find prey, as in electric eels and hammerhead sharks

  • electrogenesis, the ability to generate electricity to sense their surroundings and communicate (and also, yes, for defense and hunting), though the creature has poor senses otherwise (no hearing/poor sight), as in electric eels or electric rays

  • glows in the dark (bioluminescent) to attract prey or mates, as in fireflies and deep-sea anglers

  • natural antifreeze in organs, to protect against the cold of winter hibernation, as in weta, certain ice fish, and some northern frogs

  • filter feeding at it swims, usually seen in large animals that eat lots of very small animals, as in basking sharks or humpback whales

  • changing colors to communicate moods (which requires others to be able to see it), as in chameleons and cuttlefish

  • echolocation/sonar, when eyesight is not the best strategy (in murky water/dense fog/darkness), as in bats and dolphins

  • elastic tongue to snag prey, as in frogs and chameleons

  • wings for powered flight, as in bats and pteranodons (also birds, though feathers are unique to them)

  • males have colorful displays to attract mates, usually found in tropical places with lots of food, as in peacocks and guppies, while females are camouflaged

  • pursuit diving, when an animal dives down into a school of sea life (fish) to catch prey, as in pelicans and penguins

  • chemical projectile weapon, usually acid or venom, as a defense mechanism or to kill prey, as in bombardier beetles or spitting cobras (though ranged attack options are very rare in Earth’s biosphere, on your hypothetical alien world it may be more common, if there is a reason for it)

  • surviving by eating carrion, including rotten carcasses of dead animals, as in vultures and flies

  • has a long, sticky tongue and front claws for exposing and lapping up ants from mounds, as in giant anteaters and pangolins

  • hops around on long back legs, and lives in open terrain such as deserts where food is scarce, as in kangaroo rats and wallabies (in open desolate terrain this is apparently an energy-efficient means of getting around)

  • elongated canine teeth, for hunting larger prey, as in smilodon, gorgonops, and several others

  • tree-dwelling (arboreal) creature that glides between trees with skin stretched between its limbs (patagium), as in sugar gliders, flying squirrels, or colugos

  • flippers for swimming, as in sea turtles and penguins

  • ambush hunting, as in praying mantis and tigers

  • sail of skin on its back, likely for regulating body temperature, as in dimetrodon and spinosaurus

  • swims in loose sand, as in sandfish skinks (a kind of lizard) and desert-dwelling golden moles

  • tall with a long neck to browse leaves on tall trees, as in brachiosaurus and giraffes

  • wide, flat duckbills or mouths for grabbing aquatic plants or crustaceans, as in ducks, hadrosaurs, and platypus

Some Example Creature Builds:

  • a semi-aquatic streamlined predator with narrow, long, toothy heads and short limbs, as crocodiles and prehistoric salamanders

  • pollinates flowers, and though large it is agile with a long face and sticky tongue to reach nectar, as hummingbirds and certain bats

  • a flying animal with long, thin, narrow wings for high-speed flight and maneuverability, with a streamlined body, short legs for roosting rather than walking, and a large gaping mouth to catch prey in flight, as swifts or swallows

  • a generally herd-living herbivore that has horns for defense and competitions for females, as in deer and rhinoceros

  • a serpent-shaped animal that has no legs, which it lost over many generations, as snakes and caecilians

  • a herbivore whose back is covered in body plates of armor and whose tail is a club-/mace-like weapon, as ankylosaurus or glyptodons

  • a streamlined swimming predator with pectoral fins, a dorsal fin, and a finned tail, as in sharks, dolphins and ichthyosaurs

Organism designs so basic, they are fairly likely to be found on another planet:

  • anemone

  • clam, brachipod or other bivalve

  • jellyfish

  • sponge

  • worm

  • phytoplankton and zooplankton

Of course all of this relates to animal or non-civilization life, rather than human-like intelligent life, which I may detail in a separate write-up in the future.

Would You Like to Know More? Here Are Some Resources:

Nathalie Cabrol: How Mars might hold the secret to the origin of life (Especially from 0:50 to 3:15)

And, of course, the exhaustive write-up available at Atomic RocketsAliens page.

I would like to hear from you! Do you have a favorite alien design? Do you know of other artists or resources that you like or use? Did I present something wrong? I would love to read your feedback and criticisms – just enter your comment in the Leave a Reply section, and I’ll gladly consider them for future posts.

One thought on “Astrobiology, and Realistic Alien Design

  1. Pingback: Plausible Science Fiction: Cyber-telepathy – Hundreds of Worlds

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