As of September 2019, over 3800 potentially habitable exoplanets have been detected by various research organizations on Earth. These are faraway worlds that orbit with the “Goldilocks” zone, just the right distance from their host star that allows for the existence of liquid water and temperatures moderate enough to avoid feeling like a planet-wide oven or deep freezer.

Water is Life

Our only reference point for life is what we have found on Earth so far. It is a very wide variety, indeed, from extremophiles that live beside boiling undersea vents to the over 10,000 known species of birds. The one thing that all life on Earth seems to have in common is the need for water. The vast majority of anything we might consider life on our planet also uses DNA as its information storage method, but there are some viruses that only hold RNA. Water, though, seems to be a universal requirement.

Water is a great solvent; exists in liquid, solid and gaseous states across the fairly narrow band of temperatures on Earth; has surface tension that makes it work well inside of biological systems; a high specific heat which helps living creatures maintain body temperatures; and has a solid form (ice) that is actually less dense than its liquid form and therefore freezes from the top down in bodies of water, creating an insulation effect that preserves a slightly warmer, non-frozen zone underwater where life can continue on rather than freeze solid.

Water, in short, is apparently tied closely to the “miracle of life”. But again, that is how it appears from our admittedly limited point of view here on Earth.

Life may indeed need water to originate. If it does, there is a greater likelihood that alien life may in many ways resemble life as it has evolved on Earth. If water is not strictly necessary, the options may be nearly as infinite as the universe itself.

What might our cosmic cousins look like?

Speculation has ranged widely over the past century as to what forms life on other worlds might take. The directions such speculation takes generally fall into two categories: Potential lifeforms that might evolve based around the scientific data for a given alien exoplanet, and lifeforms that might evolve given a known branch on the tree of life as found on Earth, extant or extinct.

Aliens Based on Known Forms

The type of alien that some conspiracy theorists insist was found in the wreckage at Roswell falls into a category called “Grays”. These are considered an entire classification unto themselves at this point, as they have proven to be a mainstay in science fiction as well as in the public consciousness and interviews with people who claim to have been abducted by aliens.

Interestingly, the origins of the Grays’ appearance is decidedly terrestrial. First described by H. G. Wells in an article titled “Man of the Year Million”, this first mention actually describes what Wells imagined humans to appear as in roughly one million years. He further delved into human evolution with his masterwork The Time Machine in 1895. His Grays became alien, however, a few years later, first as “Selenites” in The First Men in the Moon, and then as Martians in War of the Worlds.

As such, Grays fall into the category of aliens that would evolve in a very similar environment that produced homo sapiens — what in our context we will call “humanoids”. Examples of this can be found in Close Encounters of the Third Kind and the Star Trek and Star Wars universes.

In our fictions, we tend to anthropomorphize nearly everything, from animals to inanimate objects. This tendency extends to aliens. It helps us relate to things that we don’t fully understand. Most of the aliens we’ve created and depict in writing and film exhibit humanoid forms and/or human traits and emotions. Some of our best speculation tries to represent alien life as truly different. See: the Alien films, Vernor Vinge’s A Fire Upon the Deep series, David Brin’s Uplift novels. Even the very best often apply human emotion and behaviors to their alien characters, knowing that in order to care about them we need to see part of ourselves.

As well as humanoid-type aliens, we also see other common themes: insectoid, reptilian and other-mammalian, to name a few. As our own technology has progressed and we have begun to entertain ideas of artificial intelligence, a new theme regularly explored is alien life that isn’t “living” in a traditional sense. These would be “machine-kind”, smart robots that operate free of any biological limitations and reproduce themselves endlessly as they colonize the galaxy. This race would begin its evolution as a form of self-directed space probe first imagined by physicist John von Neumann, replicating and spreading itself throughout the stars over hundreds of thousands of years.

Returning to a biological basis, C. M. Koseman, an artist and researcher with interests in evolution and paleontology, whose work was featured in the book All Yesterdays: unique and Speculative Views of Dinosaurs and Other Prehistoric Animals, developed a large sample of animals that might have evolved on a world where creatures similar to snails completely dominate the landscape:

The modus operandi in scenarios such as that described in C. M. Koseman’s projects is to project how a single group of lifeforms might adapt to fill various roles for different biomes of their world.

Aliens Based on the Unknown

And then there is the other side of the coin: Life as we know it on Earth is only one version of what life could be, and it’s inherently unique because the precise combination of elements, the qualities of our orbit, sun and moon, are unique to Earth. No other planet would be exactly the same, so what will life be like there?

Carbon-based life seems like it be the most common, as it is the fourth most common element in the universe when measured spectroscopically, at 4,600 parts per million. Ranking just above that is oxygen, at a little over 10,000 PPM. The top two elements are hydrogen and helium, the lightest, produced in the first moments after the Big Bang and making up 74% and 24% of the universe’s baryonic matter, respectively. So, given a moderately wet world that isn’t too hot or too cold, and has a protective atmosphere like Earth’s, most exobiologists agree that life would end up being carbon-based there.

One is startled towards fantastic imaginings by such a suggestion: visions of silicon-aluminium organisms — why not silicon-aluminium men at once? — wandering through an atmosphere of gaseous sulphur, let us say, by the shores of a sea of liquid iron some thousand degrees or so above the temperature of a blast furnace.

-H. G. Wells, “Another Basis for Life,” Saturday Review, p. 676 (December 22, 1894).

The next two most common elements that are solid at room temperature are iron and silicon. Indeed, silicon can form four bonds just like carbon, but carbon is capable of more stable and varied structures, and is able to incorporate nitrogen and oxygen into complex molecules. Exobiologists believe that silicon-based life might be more possible on worlds more unlike Earth, and at higher temperatures. On our planet, there are some bacteria that are able to metabolize arsenic, which is poisonous to all other life. And the earliest life on Earth is believed to have metabolized sulphur. Life is constantly challenging boundaries.

Scientists also theorize that life may exist in altogether unrecognizable forms, based on other forces (gravity, strong and weak nuclear forces), and even other mediums, like plasma.

Worlds that Could Harbor Life

According to the Planetary Habitability Laboratory based out of Arecibo in Puerto Rico, there are currently 53 potentially habitable worlds that have been discovered. These are not confirmed to be habitable in every regard — in many cases they are simply too distant for us to gather the pertinent information with modern technology.

Almost 24 light-years away, or 125 trillion miles, in the Scorpius constellation, is the potentially habitable exoplanet Gliese 667 Cc, orbiting a red dwarf star. At 3.7 Earth masses and an equilibrium temperature well above freezing, if the planet holds a formidable atmosphere then it is a good candidate for harboring life. However, Gliese 667 Cc is likely tidally locked, meaning that it doesn’t spin on its axis and one side is forever facing its sun while the other is in shadow. A thick atmosphere might transfer enough heat to the shadow side, and also maintain stable-enough temperatures in the light/shadow border, that life could potentially thrive there.

Kepler 186 f is 500 light-years away, only 10 percent more massive than Earth, and roughly as cold as Mars. Since we have already confirmed water ice on Mars, and we know that temperature isn’t too cold to prevent some of the hardiest bacteria on Earth to survive there, this world may be one of our best candidates.

Another strong candidate for habitability is Kepler 442 b, over 1,100 light-years away in the Lyra constellation. This world is probably not tidally locked. However, both Kepler 442 b and Gliese 667 Cc lose points for potential habitability as continued study has shown that their stars both emit solar winds much stronger than our own Sun, and this radiation could limit any chances life may have.

Indeed, as Andrew LePage says in this highly recommended essay, “ …follow-up observations and new insights into the properties of planets larger than the Earth have cast doubts on some of these initial optimistic proclamations…”

Looking closer to home, we have our aforementioned neighbor Mars, as well as Jupiter’s moon Europa and Saturn’s moon Enceladus as potential harborers of otherworldly life in our own solar system. These present two possible cases. For mars, microscopic life may exist in the very “dirt” of the planet, among frozen water ice. The moons of the gas giants present a situation where, although the surfaces are frozen, there is potential for very deep and much warmer oceans below, in which life could evolve and glide through an eternal night.

Convergent Evolution

All life evolves by the differential survival of replicating entities.

-Richard Dawkins, The Selfish Gene

Here on Earth, “eyes” have evolved at least 40 different times independently of each instance. Wings are another case of what we call convergent evolution, or the idea that whenever there is a specific need an organism must meet in order to survive, it must adapt to meet that need in whatever way its current form allows. A bat’s wings match some of the function of those of a bird or an insect, but each variation is very different in many ways and evolved on differing paths in order to make an animal that could take to the air at will.

In the words of the Ian Malcolm character in Jurassic Park, “Life finds a way.”

Many astrobiologists believe most life we will ever find beyond Earth will be single-celled organisms, based on the harshness of even most worlds we’ve so far seen in habitable zones, and on the fact that life existed on Earth in a single-cell state for about 3 billion years before evolving multicellular forms. The galaxy could indeed be teeming with life, but most of it is probably microscopic.

But as soon as those single cells begin to join together, and especially develop some mode of sexual reproduction, the principle of convergent evolution would begin to take hold. Certain functions, and therefore the forms that go with them, would be universal (pun intended):

Eyes, again. Most living things would need to see, so on any world where some visible light is available eyes would likely appear. So too with other sensory organs for smell, hearing, touch and taste. Sharks on Earth have evolved ampullae of Lorenzini to detect tiny electrical impulses in the muscles of their prey.

Means of locomotion. Most life in the universe will probably begin in a sea, thereby requiring some types of flagella, fins and tails, or a main body that can convulsively expel fluid. As life expands its boundaries, new ways of moving about would be required: limbs, tentacles, wings. There may be forms of locomotion that we haven’t seen in any organisms on Earth, such as utilizing gas bladders or even some type of biologically-controlled magnetic levitation.

A mouth. An alien girl’s gotta eat. Unless she doesn’t need a mouth because she absorbs nutrients and water straight through her skin or some other weird organ.

An anti-mouth, of course.

A body to house innards that become increasingly complex as life on that world continues to evolve. This can be completely soft, like those of octopi, or protected by an exoskeleton, or be a framework of bone covered in flesh.

It is likely that most multicellular life takes on more plant-like forms, as that is the dominant kingdom on Earth. In this case, convergent evolution will probably be slightly less variable, as plants possess fewer functions that require many changes in form. They merely exist, attached to a substrate or free-floating in fluid, only needed enough structure to outcompete other plant life for energy, water and nutrients.

No matter what we find, whether it is a single cell or a floating space whale the size of Mount McKinley living among the clouds of a gas giant, it will be an amazing time for human discovery. We will know we are not completely alone in the universe. We will know that there are other places among the stars that could welcome us if and when the need arises.

Whether or not we can ever relate to them or communicate with them in any way is another matter entirely…

Thank you for reading and sharing!

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