Most life on Earth, specifically all aerobic, oxygen-breathing life, wants nothing to do with phosphine, neither producing it nor relying on it for survival.

Now MIT researchers have found that phosphine is produced by another, less abundant life form: anaerobic organisms, such as bacteria and microbes, that don't require oxygen to thrive. The team found that phosphine cannot be produced in any other way except by these extreme, oxygen-averse organisms, making phosphine a pure biosignature—a sign of life (at least of a certain kind).

In a paper recently published in the journal Astrobiology, the researchers report that if phosphine were produced in quantities similar to methane on Earth, the gas would generate a signature pattern of light in a planet's atmosphere. This pattern would be clear enough to detect from as far as 16 light years away by a telescope such as the planned James Webb Space Telescope. If phosphine is detected from a rocky planet, it would be an unmistakable sign of extraterrestrial life.

"Here on Earth, oxygen is a really impressive sign of life," says lead author Clara Sousa-Silva, a research scientist in MIT's Department of Earth, Atmospheric and Planetary Sciences. "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."