Scientists say a stinky, flammable swamp gas could be the surest sign of alien life on another planet.
Phosphine is highly toxic and hails from the bowels of penguins, badgers and fish, among other places that you'd never want to visit. In general, most life-forms that need oxygen like we do stay far away from phosphine.
But now scientists at MIT say that phosphine can only be produced in one way: by anaerobic organisms such as bacteria that can thrive without oxygen. This means that if astronomers were able to spot phosphine in the atmosphere of another rocky planet, "it would be an unmistakable sign of extraterrestrial life," according to a release from MIT.
"Here on Earth, oxygen is a really impressive sign of life," explained MIT research scientist Clara Sousa-Silva. "But other things besides life make oxygen too. It's important to consider stranger molecules that might not be made as often, but if you do find them on another planet, there's only one explanation."
Phosphine has been spotted in space, in the atmospheres of gas giants like Jupiter and Saturn, as well as in gas jets coming off the comet 67/P visited by the Rosetta spacecraft. But if it were spotted around a more Earth-like planet, it would be a sign of some sort of action below.
Sousa-Silva led a team that spent several years ruling out all possibilities that phosphine could be created by anything else but anaerobic organisms. Their conclusions were published in a paper in the journal Astrobiology in November.
"At some point we were looking at increasingly less-plausible mechanisms, like if tectonic plates were rubbing against each other, could you get a plasma spark that generated phosphine? Or if lightning hit somewhere that had phosphorous, or a meteor had a phosphorous content, could it generate an impact to make phosphine? And we went through several years of this process to figure out that nothing else but life makes detectable amounts of phosphine."
They also found that if phosphine were produced on another world in small amounts similar to how much methane is produced on Earth, it could be detected by the upcoming James Webb Space Telescope from up to 16 light years away.
Researchers are working their way through assembling a database of "fingerprints" for over 16,000 other molecules to see if they could also be sure-fire signs of life.
"Even if some of these molecules are really dim beacons, if we can determine that only life can send out that signal, then I feel like that is a goldmine," says Sousa-Silva.
I know I'll be rooting for the molecule ethyl heptanoate, which smells like sweet grapes, to wind up being a trusted biosignature that could lead us to a more delightfully fragrant alien civilization over the planet ruled by toxic penguin dung bacteria.