The first whiff that humans catch of life beyond Earth might be methane, but scientists will need to tread carefully before declaring a discovery based on the chemical.
If an exoplanet's atmosphere contains methane, the chemical could be a sign of life — as long as planetary conditions meet certain criteria. Researchers from the University of California, Santa Cruz, have now established a framework for those conditions to guide scientists evaluating alien worlds. The analysis is well-timed since NASA's recently launched James Webb Space Telescope should be able to detect atmospheric methane at certain alien worlds.
In the case of a small rocky planet orbiting a star like our sun, the researchers found that atmospheric methane is more likely to be a biosignature (a sign that indicates past or present life) if the planet meets three conditions: if there is also atmospheric carbon dioxide, if the atmosphere has more methane than carbon monoxide, and if the planet is not water-rich.
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"One molecule is not going to give you the answer — you have to take into account the planet's full context," lead author Maggie Thompson, a graduate student at U.C. Santa Cruz, said in a statement. "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."
While methane has long been considered a potential biosignature, there are a number of nonbiological processes that can produce the gas, from volcanic eruptions to asteroid impacts. So the U.C. Santa Cruz researchers also focused on reducing the likelihood of false positives when identifying atmospheric methane as a biosignature.
For example, outgassing from volcanoes would not only add methane to the atmosphere, but also carbon monoxide, whereas the biological creation of methane would likely consume carbon monoxide. Therefore, if an atmosphere has both large quantities of methane and carbon monoxide, the methane is not likely a biosignature.
As NASA's James Webb Space Telescope comes to life this summer, it will be able to study atmospheric methane in greater detail than other potential biosignatures, hence the scientists' interest in the new analysis. But the researchers caution that real data about real planets may still pose puzzles.
"This study is focused on the most obvious false positives for methane as a biosignature," co-author Joshua Krissansen-Totton, an astrobiologist at U.C. Santa Cruz, said in the statement. "The atmospheres of rocky exoplanets are probably going to surprise us, and we will need to be cautious in our interpretations."
The study was published Monday (March 28) in the journal Proceedings of the National Academy of Sciences.