When you think of alien life, what image comes to mind? Does your mind conjure up glimmers of little green men, or monsters with square heads, 12 legs and 3 eyes? Ultimately, in a universe so vast, and with limitless potential, both may exist somewhere. Unfortunately, however, real alien lifeforms could wind up being so simple or so complex, we might not recognize them for what they are if we laid eyes on them.
While many astrobiologists believe alien life forms would most likely be derived from carbon (or perhaps silicon), some like to point out that, as a whole, there's more we don't know than do know.
To start, I refer you to the "weird life" report that was put out in 2007 by the National Research Council. It discusses the different ways biology elsewhere could differ from life on planet Earth, even if it wound up being carbon-based.
Even here on Earth, take organisms like archaea, prokaryotes, tubeworms, protists, and fungi for "proof of concept"—they demonstrate how certain forms of life can take detours from the evolutionary chain, ultimately ending up being radically different from our particular flavor of life. Some even thrive in dangerous and inexplicable places, like hydrothermal vents deep on the ocean floor, or in the geysers at Yellowstone National Park.
Bey0nd that, we've even stumbled across life-forms that don't rely on the Sun to survive. Instead, these organisms "chemosynthesize"—whereby they derive energy from chemical processes rather than through photosynthesis. In such a system, chemosynthesis can produce sulfur as a by-product instead of oxygen. Amazingly, these creatures can withstand temperatures close to the boiling point of water (212°F/100°C) and can consume toxic chemical without consequence.
From the "weird life" report: "Besides the possibility of alternative biosolvents, studies show that variations on some of the other basic tenets for life also might be able to support weird life. DNA on Earth works through the pairing of four chemical compounds called nucleotides, but experiments in synthetic biology have created structures with six or more nucleotides that can also encode genetic information and, potentially, support Darwinian evolution. Additionally, studies in chemistry show that an organism could utilize energy from alternative sources, such as through a reaction of sodium hydroxide and hydrochloric acid, meaning that such an organism could have an entirely non-carbon-based metabolism."
As we've learned over the years, evolution is a perpetual game of adaption... and adaption means understanding your environment and your place in it. Without basic knowledge of an alien environment, it's all but impossible to envision how life would arise, how it would look, and what may drive biological processes. Naturally, understanding is much more complex than determining things like temperature, so here are a few other factors at work:
Location is Everything:
- Above all else, for Earth-like life to arise, an alien planet must be located in the so-called Goldilocks Zone—defined as the region where water could remain on the surface in liquid form—but perhaps not for the reason you might think. Water is single-handedly the most important requisite for our kind of life, but astrobiologists have suggested a few water-adjacent replacements for it, which include biosolvent concoctions like water-ammonia and formamide. Of course, the Goldilocks zone may differ a bit depending on the solution, and what its boiling and freezing points are.The bottom line is still the same: for life to have a fighting chance, an alien planet must have temperatures conducive for stable bodies of liquid.
Don't Forget to Use Protection:
- Even if a planet meets all of the requirements for habitability, it's all for naught if the planet can't protect itself from the dangers of space. In this case, I'm referring to the often volatile relationship between a planet and its star. The closer the planet is, the more susceptible it is to the damaging effects of solar winds, which degrade everything they touch. The first line of defense is a magnetic field, which deflects and traps charged particles that comprise solar winds, ultimately preventing the planet from becoming sterilized.
Of course, radiation isn't the only thing planets need to ward off. Asteroids, comets and other forms of rock could bring about the destruction of an entire species (just ask the dinosaurs), which is why an appreciable atmosphere is an absolute must—It shields the surface from small to medium-sized space rocks, whilst also playing an important role in distributing the heat and energy a planet retains from its Sun.
Moreover, biologists and geologists note that one size doesn't really fit all when it comes to an exoplanetary atmospheres. Take Earth, for example. Long ago, its atmosphere was dramatically different than the one we have now; it was forever changed once primordial bacteria emerged, and began splitting water molecules into their constituent parts (hydrogen and oxygen). Ultimately, oxygen was introduced into our atmosphere (good for us, but very bad for most organisms living on Earth at the time).
A Lesson in Tug-o-War:
- Of the four fundamental forces of nature, we are most aware of gravity. It keeps us tethered to the ground, and it keeps Earth in orbit around the Sun. Without it, the universe as we know it wouldn't exist. What many people don't talk about, however, is how different things would be if Earth's gravitational pull suddenly increased, or if it was much stronger to begin with.
Believe it or not, gravity played an important role in sizing-up. Say Earth was actually a super-Earth exoplanet (weighing between 3 and 6 times more than it does now), lifeforms would likely appear much shorter and kind of square. Conversely, for planets with lesser surface gravity (say Mars), biologists believe that alien lifeforms would be taller and thinner.
Whereas, planets with small moons orbiting from close distances may produce small, thin creatures.
Per Seth Shostak—a senior astrobiologist from SETI (Search for Extraterrestrial Intelligence)—Earth-like worlds are unlikely to spawn creatures that are larger than the Pentagon regardless of surface gravity. I'm not sure whether to find that comforting, or terrifying.
Life needs energy. Humans need food. Food must be grown. To grow, they require the Sun for both light and heat, which aid in the photosynthesis process. The chain of consumption looks like this:
Sun → plants → herbivores → omnivores → carnivores
Everything on Earth is interconnected. If something affects one species, it's similarly going to affect other species further down the food chain. It's a safe assumption that a similar cycle would take hold on habitable planets beyond Earth. As I mentioned earlier regarding the extreme life found in the hydrothermal vents on the ocean floor, sunlight isn't the only option where energy consumption is concerned. Alternatives to sunlight run the gamut, ranging from theoretical thermal or pressure gradients to electrical or chemical fields.
Does Life Really Always Find a Way?
"Which came first: the chicken or the egg?" (Sorry, but I had to!)
Halting all reproductive practices would be a surefire way to go extinct. Therefore, when we think in terms of alien life, we must ask ourselves how any species would further itself genetically.
On Earth, we see a wide-variety of fascinating reproductive methods. In some instances, a species can reproduce both sexually and asexually. Some examples of this are certain types of plankton. They literally have a built-in sense of population control. What's more is that their reproductive practices change when their numbers grow out of control.
More specifically, they reproduce asexually when the competition is low, but they revert back to sexual reproduction once their population is large enough and thriving in its environment.
As a stunning side-note, There's actually a lizard—known as the three-toed skink—that's developing a placenta! As in, creatures that once hatched from eggs may soon emerge from wombs.
It's believed that the skinks, which mainly reside in Australia, originally started storing their eggs inside their bodies to ward off predators; Not to mention, it also helps them protect their offspring from exposure in harsh environments.
What Would Our Aliens Look Like?
And to that I say, "Your guess is just as good as mine." Judging from some of the unbelievably alien-looking life that already exists on Earth, it's hard to imagine that anything much stranger exists.
As I said, physical characteristics can't be deduced without a starting point. Even then, it's hard to say. Evolution tells us that we are, quite literally, a product of our environment. Millions of mutations, each one stemming from a different trial or tribulation our ancestors once faced, made us what we are. Without them, the reflection you see in a mirror would be drastically different.