A new approach to spotting alien biology at work could increase the chances of finding life on other planets.
Scientists in the US have taken a step beyond the simple method of searching for signs of oxygen in planetary atmospheres.
The new technique involves looking for a gas mixture that includes abundant methane and carbon dioxide but lacks carbon monoxide.
Researcher Professor David Catling, from the University of Washington, said: "Our study shows that this combination would be a compelling sign of life.
"What's exciting is that our suggestion is doable, and may lead to the historic discovery of an extraterrestrial biosphere in the not-too-distant future."
Writing in the journal Science Advances, the team pointed out that it was very hard to make much oxygen without Earth-like life.
Searching for atmospheric oxygen was therefore a sensible strategy - but one that was limited.
Co-author Joshua Krissansen-Totton, a University of Washington doctoral student, said: "We don't want to put all our eggs in one basket. Even if life is common in the cosmos, we have no idea if it will be life that makes oxygen. The biochemistry of oxygen production is very complex and could be quite rare."
The scientists looked at all the ways a planet could produce methane, from asteroid impacts to out-gassing to reactions of rocks and water.
It concluded that it would be hard to produce a lot of methane on a rocky, Earth-like planet without any living organisms.
Finding methane and carbon dioxide together, without carbon monoxide, would be a strong signal of life, said the researchers.
Non-biological processes such as volcanic eruptions that generated methane and carbon dioxide also produced carbon monoxide.
In addition, carbon monoxide would be "readily eaten" by microbes.
"So if carbon monoxide were abundant, that would be a clue that perhaps you're looking at a planet that doesn't have biology," said Mr Krissansen-Totton.
Powerful new generation telescopes such as Nasa's James Webb space telescope, due to be launched next year, will be able to conduct the first searches for evidence of life in the atmospheres of planets circling distant stars.