What Would Extraterrestrial Life Look Like?

There are at least one sextillion stars in the observable universe, so it only seems rational to think about extraterrestrial life, what it might look like and how we’ll find it. We might have yet to see any direct evidence that life on other planets exists, but then we hadn’t even discovered the first planet beyond our solar system until the confirmation of the existence of one orbiting a distant pulsar back in 1992. Today, we’re getting closer and closer to discovering the life beyond the borders of our world, and there’s a pretty good chance that we’ll discover it in our own solar system before we have definitive evidence of the existence of life amongst the stars.

Kepler 62fNASA

Dubbed a super Earth, Kepler-62f is one of the more likely candidates for extraterrestrial life. However, being significantly bigger than Earth, and home to a very different environment, any life there will likely be very unfamiliar to us.

Complete Fantasy, Pure Speculation or an Educated Guess?

Almost all popular representations of extraterrestrial life, particularly of the intelligent kind, fall into the realms of complete fantasy. Filmmakers, for example, typically want to create characters that people can relate to, and they also have budgetary constraints to think about when it comes to special effects and the like. As such, most aliens depicted in film are humanoid and, in the case of Star Trek, seem to breathe oxygen and, even more incredibly, speak English. In other cases, alien lifeforms are designed to look as terrifying as possible to compliment the theme of the story, such as the infamous xenomorph in the Alien series.

Pure speculation takes a rather fantastical approach as well, albeit not for the same reasons and, to an extent, it’s usually rooted in some sort of science. For example, just as a person would swat a fly, there may be aliens out there that are so much more advanced than ourselves that we couldn’t possibly relate to them and vice versa. As such, they might swat us aside as we would swat aside the fly. Similarly, there may entirely different biochemistries which, instead of being carbon-based, may be based on other elements that can form the complex molecules required for life, such as silicon, boron or even sulphur. Ultimately, however, we simply don’t have a clue, hence the pure speculation.

Now we get to the interesting part, interesting because it is tangible, something we can relate to and something we can actually find. In the search for life on other planets, we need to look for what we’re familiar with, and that means that science must lead the way. As such, it’s perhaps better to redefine our search for extraterrestrial life into the search for Earth-like extraterrestrial life. And no, that doesn’t mean looking for space hippies or tribbles; it means looking for that which we can define as life, whether it’s microbial or something much, much bigger. In other words, that means looking for the building blocks of all life as we know it:

  • The necessary elements for biological chemical processes to take place
  • DNA and RNA, or something equivalent, for encoding genetic information
  • A solvent, such as water, to allow the chemicals of life to develop
  • An energy source, such as the Sun or deep-sea hydrothermal vents
  • An environment that’s stable for long enough for life to develop

Fortunately, the basic building blocks for life are thought to be plentiful throughout the universe and, indeed, in our own solar system. There’s also a possibility that certain elements which, while critical to all life on Earth, may be substituted by something else. For example, arsenic may take the place of phosphorous as an essential building block for life. The methane seas of Saturn’s moon Titan may take water’s place as the solvent necessary to promote the chemical reactions required to form the molecules that make up living things. On Jupiter’s moon Europa, the hydrothermal vents could sustain life independently of the Sun. Such environments have more in common with our own than you might imagine.

Based on the above, it’s easy to assume that life on other planets would look much like that here on Earth, but nothing could be further from the truth. After all, life on Earth is incredibly versatile, and there are countless millions of different species, and more are being discovered all the time. Added to that is the fact that life on Earth has been going strong for at least 3.5-billion years, seeing countless extinctions in the process. Ultimately, there’s not much in common between a prehistoric bilateral sea pen-like creature and you or I. In other words, you only need to pay a visit to your nearest zoo to see just how different our own critters look.

Should life develop on a planet that shares very similar characteristics to life on Earth, which would be quite a coincidence if it happened in a solar system anywhere near our own, it stands to reason that many among its lifeforms would look similar to our own, at least in terms of fundamental characteristics. We have yet to find any evidence of a truly Earth-like planet, but that certainly doesn’t mean that life can only develop on an Earth analogue. Because all potentially habitable planets we’ve seen are not really very Earth-like at all, any possible life on them will be very different to what we’re used to, since it will be an evolutionary product of its unique environment.

The sensationalist media loves to talk about so-called habitable zones around stars, where the surface temperature of a planet should be about the right temperature to accommodate life as we know it. However, this hypothesis fails to consider just about every other possible variable. For example, Europa is far beyond the habitable zone of our own star, yet its subsurface ocean, kept liquid by tidal heating from Jupiter, presents by far the most promising environment for life in our solar system. On Mars, which is just outside the habitable zone, certain microbes from Earth can live and may even be able to reproduce.

Evolution on Alien Worlds

Timeline of Evolution

Life on Earth is purely the product of evolution, a fundamental characteristic of which is the ability to adapt to the conditions provided.

Absolutely every characteristic of every lifeform that ever existed is a product of evolution, natural selection due to the survival of the fittest being a key element. Humans walk on two legs so they can keep their hands free to use tools. Certain dinosaur species grew so enormous, probably as a self-defence mechanism or an excess of vegetation to feed on. Early whales had legs that evolved into flippers as they adapted to an amphibious and, finally, a fully aquatic lifestyle. Giraffes grew long necks so they could reach foliage that no other herbivore could get to. All life evolves to adapt to its environment, and there’s no reason that rule shouldn’t apply to any alien world.

So, what would alien life look like? That depends on three main factors:

  • Whether that life came from somewhere else, a hypothesis known as panspermia, and a favourite, but unrealistic, explanation for humanoid aliens.
  • The environmental qualities influencing the evolutionary adaptations of the extraterrestrial lifeforms.
  • The biochemical properties. However, alternative biochemistries are currently purely speculative.

We’ll disregard the third point for now, since this article is supposed to give an in-depth answer into what extraterrestrial life would look like based on what we know, and not what we might wilfully imagine. The first point is also rather a moot one, since life would still either adapt to any alien conditions or die. Additionally, panspermia really only concerns extremely basic forms of life rather than humans and other animals. The second point provides by far the best approach to determining the qualities of alien life, since there are relatively few things in evolution that happen purely by mistake. In the following sections, we’ll explore the fascinating effects of evolution and how they determine what our celestial neighbours might look like.

Subsurface Oceans

Europa's SurfaceNASA

Deep beneath this icy crust, which envelopes the moon of Europa, lies a subsurface water ocean up to 62 miles (100 km) deep, making it one of the most likely places in the Solar System to support extraterrestrial life.

By far the most promising place to look for extraterrestrial life in our own solar system is Europa, owing to its subsurface ocean. Other possibilities include Saturn’s moon Enceladus and the minor planet of Ceres in the asteroid belt. Though all these worlds are far beyond the traditional habitable zone of our sun, subsurface oceans existing beneath several miles of icy crust could present a perfect biosphere for life. After all, hydrothermal vents at the bottom of the Earth’s oceans are home to biomes that function completely independently from the Sun and instead draw their energy purely from volcanic activity.

Europa is particularly interesting, since its subsurface ocean is kept in liquid form due to tidal heating from Jupiter. In fact, sometimes the tidal heating is so intense that it causes the icy crust to crack in some areas and water vapour to spew out in some of the most spectacular geysers in the solar system. The ocean on Europa is estimated to be up to 62 miles (100 km) deep and contain at least three times more water than all the oceans on Earth. If this is the case, the lowest areas of the Europan ocean would be under extreme, crushing pressures several times greater than the deepest points on Earth, making them less likely to sustain life.

Any life on Europa would likely be similar to that found around Earth’s deep-sea hydrothermal vents, simply because the environments are likely very similar. Some of the bizarre creatures that thrive around these volcanic formations include giant tube worms, mussels, clams, crabs and shrimp. There is, however, a strong possibility that Europa’s ocean is either extremely cold or very high in salt concentration, which could preclude the possibility of forms of life more advanced than certain bacteria and archaea from evolving. Similarly, the complete impossibility of photosynthesis developing prevents the existence of any life like that nearer Earth’s surface.

Permanent Twilight Zones

Red Dwarf PlanetNASA

Planets orbiting red dwarf stars, such as this one depicted by an artist’s impression, are likely to be tidally locked, leading to one side of the planet being in permanent daylight and the other being perpetually dark.

Red dwarf stars are by far the most plentiful in the universe, so if any planets accompanying them are proven to support life, it would likely mean that life is common throughout the cosmos. However, red dwarfs are much cooler, dimmer and smaller than our own star, which is a yellow dwarf. Consequently, their habitable zones are much closer and narrower. Because any planet in the estimated habitable zone would be very close to its host star, it’s likely that it would also be tidally locked, meaning that one side of the planet always faces the star, just like the moon only ever shows one side of itself to us.

A tidally locked planet whereby one half of the world always faces the sun and the other is in permanent darkness would, for obvious reasons, have a very different environment to Earth, thus giving rise to some extraordinary lifeforms if, indeed, life as we know it were even possible. For a start, the sun-facing side would be bathed in extreme heat, while the other side would be completely frigid, leaving only a relatively narrow strip of land in the middle where’s it’s permanent twilight. In theory, life could perhaps evolve in this twilight zone and perhaps some way into the night zone, since wind would generally blow from the day to the night side, thus warming the latter up. A sort of atmospheric ‘air conditioning’ system would also be essential for keeping the planet’s climate from descending into complete chaos. At the same time, however, the wind would be both perpetual and very strong, leading to the possibility of most lifeforms being permanently or semi-permanently airborne.

The day and night cycle is an essential evolutionary factor to most life on the surface of Earth, so any life existing in an environment where it’s permanently one or the other is likely to be very different. On a red dwarf planet, there probably wouldn’t be oceans either, since they would boil away on the day side or freeze on the night side, unless an extreme, and probably life-destroying, greenhouse effect such as that on Venus made the global atmosphere more consistent. As such, it’s likely that only primitive and extremely hardy organisms could evolve in the twilight zone, and the difference between any day- and night-side life would be profound indeed.

Colouring the World

On a planet orbiting a red dwarf star, such as Proxima b depicted here, any kind of photosynthetic life would likely look very different due to the colour of the light reaching the planet’s surface.

Photosynthesis first appeared on Earth as long as 3.5-billion years ago, and it has since become one of the most important processes for almost all life on the planet. Almost all life on Earth relies, directly or indirectly, on photosynthetic processes, and it is a fundamental requirement for most life as we know it. As such, if photosynthetic bacteria evolved on other planets, as they did on Earth, it would inevitably give rise to an oxygenated atmosphere (since photosynthesis produces oxygen) and what would, in many ways, be a similar environment to that of Earth. However, photosynthesis is also responsible for colourising life as we know it.

Photosynthetic organisms on Earth are primarily green, since they contain chlorophyll, an essential ingredient for the photosynthetic process. Chlorophyll absorbs all but the green part of the visible spectrum, basically reflecting shades of red and blue and resulting in things like green leaves. However, while most of the Sun’s visible light belongs to the greener area of the spectrum, that is not necessarily true of the light that hits the surface of other planets. In other words, the reason why plants on Earth are green is partly because of the colour of the Sun’s light once it reaches the surface. On other planets, this could be quite different.

The planet Gliese 667 Cc orbits right in the habitable zone of a red dwarf star just under 24 light-years from our own. Assuming the planet had an Earth-like atmosphere, any photosynthetic organisms, such as plants, would range from dark red to black, depending on the intensity (or lack thereof) of the star. On a planet orbiting a bluer star such as Procyon, however, plants would more likely be shades of yellow or orange. However, while a star’s light is a major influencing factor on the colour of photosynthetic life, the elemental composition of an alien atmosphere also has a major impact on the light that reaches the surface due to a phenomenon called Rayleigh scattering.

Influences of Gravity

Titan's surfaceNASA

Saturn’s moon Titan’s gravity is only 14% that of Earth’s, so humans could probably fly like birds there wearing only wingsuits. In other words, it’s much easier for life to develop the ability to fly on a low-gravity world.

Gravity would also have a major influence on the evolution of life, perhaps one of the most profound of all. Indeed, life on Earth could likely have evolved would our planet’s gravity be significantly higher or lower, but it would undoubtedly have taken a very different course. Although a planet needs to exert enough gravitational force for it to hold onto an atmosphere long enough for advanced life to evolve, unlike Mars for example, environments with as little as half the gravity of Earth could still be conducive to advanced forms of life.

Gravitational pull has a particularly profound influence on the physiological characteristics of an organism. For example, enormous dinosaurs needed equally enormous legs to carry their weighty bodies. However, on a low-gravity world, such creatures would be able to get by with much smaller legs. Similarly, there’s no chance a giant insectoid creature, such as those so passionately portrayed in films, could live on a world with the same or higher gravity than Earth, since they would crush their own spindly little legs under the wright of the rest of their bodies. In fact, our own bipedal selves would barely be able to function in a world where the gravity was just twice as strong as our own.

Low-gravity, life-bearing planets could be home to some particularly interesting lifeforms, likely predominantly of the avian kind. However, since it would be easier to fly in such worlds, organisms wouldn’t need to have wings as big as terrestrial birds, and some creatures could fly simply by jumping and gliding. On high-gravity worlds, however, one would expect to find a predominance of crawling and slithering creatures, such as giant worm-like animals, since legs are more likely to cause problems due to the additional physical stresses brought about by gravity.

Generally, creatures on high-gravity worlds would be smaller, flatter and less likely to have legs, while those on low-gravity worlds could grow much bigger while also diversifying more. However, there are of course plenty of other variables that might influence the size and shape of an organism, such as the composition of the atmosphere. For example, insects on Earth have an upper limit to their body size due to the way they process oxygen, although higher oxygen levels, such as those during the Carboniferous period 300-million years ago, also make those limits a lot higher for some pretty horrifying results.

What About Intelligent Aliens?


Perhaps the Engineers in Ridley Scott’s Prometheus weren’t so unrealistic after all.

I’m going to close this article with the best answer I can give to the question you’re probably all asking. However, this concept is somewhere between the realms of pure speculation and complete fantasy. After all, human beings are the only species, out of countless billions that have existed in the last 3.5-billion years of life on Earth, that are even able to use tools. No other form of life that exists on Earth, or ever has existed, has even come close to developing a civilization or even a language, let alone been able to discuss the possibility of intelligent aliens on other planets.

Perhaps surprisingly, it’s not unreasonable to suggest that intelligent aliens, at least those that are carbon-based and evolved in environments that are at least vaguely like our own, would actually look somewhat like us. Consider, for example, the fact that a shark, despite being a fish, boasts a similar morphology to that of a dolphin, a mammal that shares a common ancestor with a cow rather than an anchovy. This phenomenon is known as convergent evolution, which refers to the observation of completely unrelated organisms developing down a similar path due to their environmental adaptations.

It also makes sense that aliens sharing a human-level of intelligence would either be bipedal or, if not, keep some limbs free for using tools. A large brain capacity and sophisticated central nervous system also seem essential, as does an endoskeleton for any remotely human-sized aliens. Owing to this fact, it doesn’t seem at all likely that intelligent aliens would be giant insectoids as portrayed in the films.

Final Words

Dr. Ellen Stofan, NASA’s chief scientist, claimed in 2015 that we’ll likely find definitive evidence of life beyond Earth within the next 20 to 30 years, whether it’s in our own solar system or on another planet orbiting a star beyond our own. One thing is for certain though, when we discover life on another world, it will be the most profound and truly epic revelation that humanity has ever made. And, in my opinion, it’s less about if and more about when.

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