Methane in a planet’s atmosphere could be a sign of life

A new study led by the University of California, Santa Cruz has assessed the planetary context in which the detection of methane in an exoplanet’s atmosphere could be considered a compelling “biosignature,” or sign of life. According to the scientists, the presence of methane is one of the few potential signs of biological activity that could readily be detected by the newly developed James Webb Space Telescope (JWST), which will begin astronomical observations this year.

“Oxygen is often talked about as one of the best biosignatures, but it’s probably going to be hard to detect with JWST,” explained study lead author Maggie Thompson, a doctoral student in Astrophysics at UC Santa Cruz.

Together with her colleagues, Thompson has constructed an in-depth assessment of the planetary conditions needed for methane to be considered a reliable biosignature. “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,” she said.

Non-biological sources of methane include volcanoes, comet or asteroid impacts, and reactions in mid-ocean ridges, hydrothermal vents, or tectonic subduction zones. However, fluctuations in the concentrations of methane could point towards a biological source that would steadily replenish this gas in order to maintain high atmospheric levels.

“If you detect a lot of methane on a rocky planet, you typically need a massive source to explain that,” said study co-author Joshua Krissansen-Totton, a postdoctoral fellow at UC Santa Cruz. “We know biological activity creates large amounts of methane on Earth, and probably did on the early Earth as well because making methane is a fairly easy thing to do metabolically.”

According to the scientists, for a rocky planet orbiting a sun-like star, atmospheric methane is likely to be considered a sign of life if the atmosphere also contains carbon dioxide, methane is more abundant than carbon monoxide, and the planet is not covered by massive amounts of water.

“One molecule is not going to give you the answer – you have to take into account the planet’s full context. Methane is one piece of the puzzle, but to determine if there is life on a planet you have to consider its geochemistry, how it’s interacting with its star, and the many processes that can affect a planet’s atmosphere on geologic timescales,” concluded Thompson.

The study is published in the journal Proceedings of the National Academy of Sciences.

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By Andrei Ionescu, Earth.com Staff Writer