Researchers recently said methane may be the first detectable sign of life outside of our planet.
But the gas would only suggest the presence of life if discovered in the atmosphere of a rocky planet orbiting in the "habitable zone."
This area is neither too hot nor too cold for liquid water to exist.
Scientists are working to understand the signs of life, known as biosignatures, that might be present in observations of planets in other solar systems.
The researchers, in a study published in Proceedings of the National Academy of Sciences, showed that atmospheric methane in the correct amount could be a sign of life if other conditions are met.
Methane is an important trace gas in Earth's atmosphere, at less than 2 parts per million by volume.
Unlike other possible biosignatures such as atmospheric oxygen, methane is one of the few gases that could be detected by using the James Webb Space Telescope.
The telescope was launched by NASA in December and is due to become operational within months.
Study lead author Maggie Thompson, a graduate student at the University of California, Santa Cruz, said "On Earth, the ... majority of methane is produced by life."
Most of the methane is created directly by life: methane-producing microbes in wetlands, rice fields or in the digestive system of large animals.
Methane is also produced through human activities such as burning fossil fuels including coal and oil, which are the remains of dead organisms.
The researchers made a three-part case for methane as a possible biosignature.
"First, it would be unsurprising for life elsewhere to produce methane," study co-writer Joshua Krissansen-Totton said.
Secondly, methane would not last for long in atmospheres of habitable rocky planets without constant replenishment, possibly by living organisms, the researchers said.
Thirdly, they added, it would be difficult for other processes, such as chemical reactions, to maintain replenishment without leaving behind some kind of evidence.
This evidence would suggest the methane was not biologically produced.
Gas-spewing volcanoes, for instance, would release carbon monoxide alongside methane.
But biological activity generally reduces carbon monoxide's atmospheric concentrations.
Thus, the researchers explained, nonbiological processes cannot easily produce rocky planet atmospheres rich in both methane and carbon dioxide, as on Earth, but with little or no carbon monoxide.
Scientists are expecting greater insight into exoplanet atmospheres using Webb and other new telescopes.
Oxygen, more plentiful in Earth's atmosphere than methane, is another possible biosignature.
It is fed into Earth's atmosphere by biological processes--in this case photosynthesis by plants and microbes.
But oxygen could be difficult for Webb to detect.
Krissansen-Totton explained that "it's important to note that the diversity of planetary environments elsewhere is probably" very large.
He added, "There could be other... methane-making .... processes that no one has yet considered."
I'm John Russell.