There are circumstances in which the presence of methane in the atmosphere of a planet beyond the Solar System would be a strong marker for biological processes, a new study claims. If so, this would be an extremely important step in the prospects for finding life, and one the JWST may well be capable of making.

Unless we get a “hello” transmission from an alien civilization, the most likely way to find evidence for life beyond the Solar System is to spot molecules made by life in the atmosphere of a planet orbiting another star. Unfortunately, molecules only made by life are likely to be scarce and hard to spot. Other molecules, widely produced by life on Earth, can also have geologic origins. Chief among these is methane, produced by cow stomachs and rotting vegetation, but also released by volcanoes and even comets.

Consequently, University of California Santa Cruz graduate student Maggie Thompson set out to find ways to distinguish the source of methane on a world we can't visit. In Proceedings of the National Academy of Sciences, she and her co-authors claim to have found them.

“We wanted to provide a framework for interpreting observations, so if we see a rocky planet with methane, we know what other observations are needed for it to be a persuasive biosignature,” Thompson said in a statement. The biggest clue is probably the quantity of methane an atmosphere holds. Despite what climate change deniers claim, volcanic eruptions add much less methane than human activities, let alone natural biological sources like methanogenic bacteria in ruminants and swamps.

If methane was stable over long periods of time, even slow release from volcanoes might create enough to resemble biological production. However, CH₄ reacts in the presence of sunlight to produce carbon dioxide and hydrogen, or turns to aerosols that eventually fall to the ground.

The other test is to look for other gasses. When volcanoes release methane they also put out a lot of carbon monoxide, so a planet with both might be more interesting to volcanologists than biologists. On Earth, however, life forms are net consumers of carbon monoxide. We can't be sure that would be true on every planet, but if we spot methane without the CO it's a pretty big hint.

“One molecule is not going to give you the answer—you have to take into account the planet’s full context,” Thompson said. Unfortunately, we won't always have as much context as we might like.

“Oxygen is often talked about as one of the best biosignatures, but it’s probably going to be hard to detect with JWST,” Thompson noted. The same is true for phosphine, which had a moment of attention when it was reported, possibly erroneously, on Venus in 2020. So we need to think about the molecules with strong signals in the parts of the spectrum the JWST can detect.

An atmosphere rich in carbon dioxide with more methane than carbon monoxide represents the best combination we are likely to be able to see in the near future. Planets orbiting young stars or made up of more than 10 percent water by weight might also have methane-rich atmospheres without hosting life.

For a long time, discussions like this were fairly theoretical because the only planets whose atmospheres we could study were too large or too close to their stars to support life anyway. That's about to change in a matter of months, provided astrobiologists can wrest the JWST from those wishing to study the dawn of time or black hole environments. Moreover, within a few years, a new class of giant ground-based telescopes may be capable of similar observations.