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US scientists said they were searching for signs of planets with atmospheres rich in methane and carbon dioxide and lacking in 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 alien 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.
Mr Krissansen-Totton said:”So if carbon monoxide were abundant, that would be a clue that perhaps you’re looking at a planet that doesn’t have biology.”
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.
Scientists had previously just looked for signs of oxygen in planetary atmospheres.