In other words, aliens shouldn’t have to breathe oxygen. Although humans need oxygen to survive, our planet’s atmospheric layers are composed of various gases and not just oxygen. If we take a look at the volume of gasses, dry air contains 78.09% nitrogen, 20.95% oxygen, 0.93% argon, 0.04% carbon dioxide, and small amounts of other gases. Our atmosphere, together with other essential factors, allows for life on Earth to exist.
But on a distant alien planet, this may not be the case. In fact, for all we know, an alien species on a potentially habitable world could generate carbon dioxide and produce oxygen in the process. Humans breathe oxygen and produce carbon dioxide.
For all we know, aliens can survive in the vacuum. We know that microbes can, and even organisms on Earth called Tardigrades–also known as Water Bears–are able to survive in space.
Tardigrades are teeny tiny micro-animals known to exist nearly everywhere from mountaintops to volcanos, from tropical rainforests to the freezing climate in Antarctica. These little creatures are nearly indestructible. Tardigrades are able to resist the extremely low pressure of a vacuum and also very high pressures–more than 1,200 times atmospheric pressure. If animals on Earth such as water bears can survive in unimaginable places where humans can’t, doesn’t that tell us–at least in theory–that alien life on distant worlds could survive just as Tardigrades do in extreme environments?
Currently, when astronomers search for exoplanets in the Galaxy, we look for certain similarities that those plants might have with Earth. For example, experts believe that life is more likely to exist on exoplanets that orbit their host stars in the so-called Goldilocks Zone, or habitable zone. This is an area around the star that’s perfect for liquid water to exist on the surface. This is important because we believe that where there is water, there could be life.
This is because so far, the only life we know is hurt carbon-based life, and without water, it would be possible. That’s why it is entirely reasonable to assume that exoplanets with water on their surface may have developed life as well.
As of writing (May 17, 2020), there are 4,154 confirmed exoplanets and 5,142 candidates. Many of these worlds orbit their stars in the habitable zone, and a green number of exoplanets are similar to Earth, at least in size and mass. This doesn’t necessarily mean there could be life on those worlds, but the more similar an exoplanet is to Earth, the more likely it is to contain life, as wear as we are concerned.
Although this is logical thinking, maybe we should look for what’s not entirely logical and start looking for exoplanets that are, in fact, the entire opposite of Earth. There are no rules as far as we are concerned when it comes to alien life, and until we do confirm the exists of extraterrestrial organisms on a planet in our solar system, exoplanet or satellite, we can assume that life can come in all sizes and different colors. By that, I mean that there could be alien life forms out there that are so different from anything we are accustomed to seeing here on Earth, that we could practically not even notice or process their existence.
William Borucki, the principal investigator for NASA’s Kepler mission, explained that “If we find lots of planets like ours… we’ll know it’s likely that we aren’t alone, and that someday we might be able to join other intelligent life in the universe.”
However, if we even don’t find a bunch of exoplanets that are similar to Earth, we probably should know that we are not the only planet with life and that we are not alone out there. The universe is a massive place. Our Galaxy is enormous, and our solar system has barely been explored.
The idea that aliens don’t necessarily need oxygen or even the conditions that carbon-based life needs are backed up by a study published in Nature Astronomy. The authors essentially say that we should start looking for planets that are not necessarily similar to Earth in terms of size and mass, and whose atmospheres are different from Earth, based, for example, on hydrogen and no oxygen. For all we know, alien life could need hydrogen, and not oxygen to survive.
For a rocky planet to have gravity strong enough to have a hydrogen atmosphere, it must be a “super-Earth” with a mass about two to ten times that of planet Earth. Hydrogen could also be released by a chemical reaction between water and iron.
The idea that alien life could need hydrogen to survive was even demonstrated by the above-mentioned study.
Through a series of observations, the researchers discovered that the E. coli bacteria could survive and multiply under a hydrogen atmosphere in the complete absence of oxygen. According to studies, there is a “diversity” of dozens of gases produced by these bacteria, which would increase the chances of recognizing signs of life on an exoplanet, which is dissimilar to Earth.
However, there are plenty of other microbes on Earth that live within our plant’s crust that are able to survive by metabolizing hydrogen. Furthermore, teachers have even discovered multicellular organisms that can survive in an oxygen-free zone on the floor of the Mediterranean.
Although the above is not evidence that alien life must exist in such worlds, it is a good argument that tells us that we might need to change our approach when looking for distant worlds that could harbor life.