Artist’s conception of what life could look like on the surface of an exoplanet. NASA
How do you search for life on other worlds? While the gases which we typically associate with life, such as oxygen or methane, are commonly found on other planets, that doesn’t necessarily indicate that living things are present there. Take the puzzle of fluctuating methane levels on Mars, for example. The Curiosity rover has detected high levels of methane on occasion, but scientists think this is likely caused by geological forces, not biological ones. So the presence of these gases doesn’t give a positive indication of the presence of life.
“Here on Earth, oxygen is a really impressive sign of life,” Dr. Clara Sousa-Silva, a research scientist in MIT’s Department of Earth, Atmospheric and Planetary Sciences, explained in a statement. “But other things besides life make oxygen too.” That’s why research is focused on trying to identify markers that can only come from life. “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 is one such unusual molecule. It is found in Earth’s atmosphere, though only at very low concentrations. That’s probably just as well for us, as it has a particularly foul odor which is usually compared to garlic or rotting fish. It is also highly toxic, attacking the respiratory system, and it has been known to cause deaths from accidental exposure.
Despite its toxic nature, however, phosphine could be an invaluable biomarker for life. A research team from MIT has found that phosphine can only be produced by anaerobic (non-oxygen-reliant) life. The team looked at as many possible sources of phosphine as they could think of, including chemical reactions and geological anomalies. But no mechanism they could come up with was able to turn phosphorous into phosphine without the presence of living organisms.
“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?” the authors said. “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.”
Future examinations of exoplanets using tools like the upcoming James Webb Telescope should be able to detect the presence of phosphine, even in small amounts, from up to 16 light-years away. If such a signal were to be detected, that would be the strongest indication yet of the presence of life on another world.
The research is published in the journal Astrobiology.