Every living thing, from the grass to your roommate, releases gas into the atmosphere. When it's a human releasing gas in an unventilated car, it's disgusting. But in the deepest depths of the galaxy, it could be how we discover extraterrestrial life — and a team at the Massachusetts Institute of Technology just gave humanity a cheat sheet.
Sara Seager, professor of planetary science and physics at MIT and lead on a corresponding paper in the journal Astrobiology, spent three years building a list of pretty much any known molecule that could be a gas.
Using a light-measurement technique called spectroscopy, astronomers can see the atmospheric gases of planets light-years away. Some of those gases indicate signs of current or past life on a planet; they're called biosignatures.
If aliens looked at our planet from light-years away, they could analyze the light passing through our atmosphere, tell that it contains oxygen and infer there's a biosphere — a place occupied by living organisms.
But there is an incredible number of gas molecules that could indicate signs of life. That's where the cheat sheet comes in.
"What we've done is [create] a list to work through all the scenarios," Seager said in a phone call on Monday.
Basically, a planetary scientist could offer a scenario that creates gases, like a volcano. Astronomers would check the byproducts of a volcano against what they see on an alien surface. If the byproducts line up with a volcano, it's just geochemistry at work.
If they don't, it might be something else.
Think of our own planet: Methane is released from vents in the ocean floor, and the gas winds up in the atmosphere. To anyone off-planet looking at Earth, that could be mistaken for life, since methane in the atmosphere could also come from cows or burps. And that's where a giant gray area exists.
"It's like forensic analysis," Seager said. "We won't be 100% sure. The unknown is if the gas is indeed produced by life."
The list includes 14,000 molecules, and about a quarter of them are known to be produced on Earth. A couple thousand of them are biogenic, or created by life on Earth. But this all assumes life on other planets is carbon-based, like our own. That's another unknown.
"Still, you can hedge your bets about what elements you start working on," Seager said.
According to Seager, this is still another 10-odd years of work, identifying molecules that might be false positives. Even then the research probably won't lead to a Close Encounters of the Third Kind scenario. But it would push humanity light-years toward understanding the biology that makes living things possible.
"Finding evidence of an exo-biosphere is so important — that's alien grass and plants," Adam Frank, a professor of physics and astronomy at the University of Rochester who wasn't involved in the paper, said in a phone interview. "To find a planet with a biosphere would be one of the greatest scientific finds in the history of human beings."