Right now, there are more than 4,000 known exoplanets and plenty of other possible planets that need to be assessed. While we're not going to put human boots on the ground on all these worlds to search for alien life, there are a variety of remote methods to look for current or ancient life.
Scientists aren't on the lookout for little green men. What astrobiologists and planetary scientists are searching for is akin to the life that developed and flourished for the billions of years before humans even evolved. They're looking for evidence of basic life, like single- or multi-celled organisms on the order of bacteria, viruses, or algae.
That other life could be found on planets that have a very different atmosphere than our own. After all, even here, life developed under conditions than would seem strange to us. The younger Earth had less-intense sunlight and a lot more methane in the atmosphere compared to our current oxygen-rich air. Understanding that may be key to finding life elsewhere.
"[Our research] is not looking for another Earth per se," Timothy Lyons, a professor in the Department of Earth and Planetary Sciences at the University of California, Riverside told Astrobiology Magazine. Lyons heads up NASA's Alternative Earths team, which gathers information on what we can learn from the early days on this planet to better understand what might support life elsewhere.
"It's more about looking for the different pieces of what it is to be a planet that can sustain life. Once you know what those processes do on a planet like Earth, you can assemble them into countless other planetary scenarios that may or may not be able to do the same thing."
Why hydrogen-based life is possible
Looking around our galaxy, most accessible rocky planets are hydrogen-based, but could life develop and survive there? Until we find life on one of these planets, we can't know the answer for sure. But we do know it's possible, according to new research via the Massachusetts Institute of Technology.
Perhaps surprisingly, the scientists found that some tough earth-based organisms could survive in a hydrogen-based atmosphere: E. coli (the same as the kind that lives in our gut) and yeast "can survive and grow in a 100% H2 atmosphere," according to the paper in Nature Astronomy.
Hydrogen is just one element life could be based on — nitrogen or silicon are other possibilities. (See videos above and below to learn more.)
They also found an "astonishing diversity of dozens of different gases produced by E. coli, including many already proposed as potential biosignature gases (for example, nitrous oxide, ammonia, methanethiol, dimethylsulfide, carbonyl sulfide and isoprene)," the paper's authors write.
How atmospheres can reveal possible life
Knowing which gases could be indicators of hydrogen-based life, or biosignatures, is the key.
Scientists can do this from Earth by looking at the light that passes through that atmosphere when the planet passes in front of its star. How the light is broken down as it passes through the atmosphere can give details about what's in that atmosphere. Of course, this takes a very powerful telescope, but it's possible.
So if researchers find a hydrogen-based planet, and find the biosignature gases, that could indicate that life exists there. Of course, it's possible that life that evolved on an exoplanet might not give off those specific gases, but it would be a useful clue about where to search if they did.
All of this information isn't a guarantee of where to go and what we might find when we get there, but with over 4,000 planets to consider, it's useful to have a way to narrow down where to start the search for alien life.