What Can We Predict about Alien Life on Earth-Like Exoplanets?

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At first blush, this seems like an unknowable matter. We’ve been able to glean a surprising amount of information about an ever swelling number of exoplanets based on spectrometry, speed of orbit, distance from parent star and so on. However, our only direct experience is with Earth, a sample size of one. Are we at the limits of what can be determined about Earth-like exoplanets from afar?

First of all, chirality will likely be a consideration. Earth life is all based on left handed proteins. This would just be mildly interesting trivia except that it could just as easily have been based on right handed-proteins, and the two are totally bio-incompatible; if we were to visit an exoplanet where life is based on right handed proteins for example, we’d be unable to digest any of the food. On the upside we’d also be unable to catch any indigenous illness.

What a shame it would be to send a sleeper ship of colonists on a one-way trip to an Earthlike exoplanet, only for them to discover upon arrival that they’re doomed to starvation because none of the indigenous flora is digestible. This would also impact all other potential applications of alien genetic material, such as medicine or bioweapons.

I want to add an important stipulation here: We’re discussing Earth-like exoplanets, or roughly so. I don’t feel much can be predicted with any hope of accuracy about what forms or qualities life may adopt if it evolves on planets significantly different from Earth. But given mostly Earth-like conditions, there are some attributes we should expect to see in the indigenous flora and fauna:

1. There are very good odds that photosynthesis will have evolved, because on Earth the most common photosynthetic mechanism evolved independently 62 times in 18 plant families. This does not preclude the possibility of photosynthetic animals, though on Earth that’s a rarity. The sea slug Elysia timida, the spotted salamander, the pea aphid the the oriental hornet are the only examples I know of.
2. The plants, or plant equivalents, will probably be green. Plants on Earth reflect green light and preferentially absorb red light even though our sun emits more green light than any other part of the spectrum for reasons explained here. Therefore the wavelength of light coming from the parent star may not matter; if a lifeform survives primarily by harvesting energy from sunlight to process nutrients in soil, we should expect it to be green.
3. Crabs will be present in some form due to carcinization. For those unfamiliar with this phenomenon, the morphology of crabs (and many of their characteristic internal structures) independently evolved five times. It’s evidently a very competitive body plan, we should expect Earth-like exoplanets to have something suspiciously crab-like in its oceans.
4. Eyes will have evolved, as they have evolved independently more than 50 times on Earth. Whatever lives on Earthlike exoplanets, we have even better odds of finding critters with eyeballs than crab-shaped ones.
5. Flight has independently evolved four times. This should be unaffected by atmospheric composition (except the availability of oxygen or whatever the metabolically important gas in their atmosphere is), only pressure and gravity which will affect the wingspan and maximum size/weight of flying alien fauna.
6. To expand on number 5; gravity, pressure and availability of whatever portion of the atmospheric gas mixture their metabolism uses should reliably predict certain anatomical features. Many Earth species were able to grow much larger in past epochs, when our world had a more oxygen rich atmosphere than it does today.
7. Something very conspicuously fish-shaped should be present in alien oceans. The convergent evolution of fish (particularly sharks) and cetaceans towards a very similar body plan indicates there’s a relatively narrow range of right answers to the question of how fast moving marine life should be shaped. Hydrodynamic principles apply to water on any planet, so look for streamlined marine life with flippers or fins. Other strategies for moving through water at different speeds/scales should also be represented to varying extents.
8. We should not expect humanoid life, as it has independently evolved on Earth only once. There exist a variety of compelling arguments about the necessity of certain human anatomical features to the evolution of our big, complex brains like stereoscopic vision, prehensile digits, opposable thumbs and upright posture, but we have only a sample size of one to judge by in this case. To me, this is the biggest question mark involved.
9. If intelligent life does exist there and has developed instrumentality, depending upon how far they have advanced scientifically and technologically, we should expect to see many of the same technologies that humans also developed. This is because while gravity, pressure, atmospheric mixture and so on vary from planet to planet, the laws of physics do not. Technology is just applications of scientific discoveries concerning the laws of physics, which are the same everywhere.
10. We should not expect recognizable equivalents of reptiles, mammals, avians, insects, etc. Even on Earth there are species that blur the line. The platypus is a mammal with a duck-like bill which lays eggs, has a venom gland and a stinger. Some chameleons, snakes and fish give live birth. Marsupials differ markedly from all other mammals in how they gestate and nourish their young. Specific attributes should be present because of their repeated independent evolution like eyes, scales, fur, feathers, etc. but not necessarily clustering reliably in the combinations we’re used to.

Tantalizing as all of this speculation is, it may be many human lifetimes before we’re able to get a closer look at Earth-like exoplanets. Depending on your criteria for what constitutes “Earth-like”, there’s about half a dozen planets within 50 light years with a rocky composition, sufficient gravity and within thermal tolerances for the existence of liquid water on their surfaces.

As our sample size increases, our estimations of how many habitable planets exist in our galaxy also increase. It was up to 300 million at last count. The bad news is that nearly all of these are probably forever out of reach, barring some sort of astonishing breakthrough in spacecraft propulsion. The good news is that until then, this solar system is our playground. Our far future may or may not be Star Trek, but our near future is almost certainly The Expanse.

Anyway this is all that occurs to me at the time of writing. By all means let me know in the comments if there are any glaring omissions. A big part of what I get out of writing for this platform is engagement with clever, insightful commenters. Also be sure to check out my other articles on related topics.