American scientists find new way to detect alien life

WASHINGTON, Jan. 24 (Xinhua) -- An American study has found a simple approach to look for evidence of life on other planets that might be more promising than just looking for oxygen.

The study, published Wednesday on Science Advances, offers a new recipe for providing evidence that a distant planet harbors life.

The new study looks at the history of life on Earth to find times when the planet's atmosphere contained a mixture of gases that are out of equilibrium and could exist only in the presence of living organisms.

The study shows that life's ability to make large amounts of oxygen has only occurred in the past one-eighth of Earth's history.

By taking a longer view, the researchers identified a new combination of gases that would provide evidence of life: methane plus carbon dioxide, minus carbon monoxide.

"We need to look for fairly abundant methane and carbon dioxide on a world that has liquid water at its surface, and find an absence of carbon monoxide," said co-author David Catling. "Our study shows that this combination would be a compelling sign of life...and may lead to the historic discovery of an extraterrestrial biosphere in the not-too-distant future."

The paper looks at all the ways that a planet could produce methane and finds that it would be hard to produce a lot of methane on a rocky, Earth-like planet without any living organisms.

If methane and carbon dioxide are detected together, especially without carbon monoxide, that's a chemical imbalance that signals life, it shows.

The carbon monoxide tends not to build up in the atmosphere of a planet that harbors life, it shows.

"Carbon monoxide is a gas that would be readily eaten by microbes," said corresponding author Joshua Krissansen-Totton. "So if carbon monoxide were abundant, that would be a clue that perhaps you're looking at a planet that doesn't have biology."

"Life that makes methane uses a simple metabolism, is ubiquitous, and has been around through much of Earth's history," Krissansen-Totton said. "It's an easy thing to do so it's potentially more common than oxygen-producing life." Enditem