Alien Life Hunt: Oxygen Isn't the Only Possible Sign of Life

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Alien-life hunters should keep an open mind when scanning the atmospheres of exoplanets, a new study stresses. The time-honored strategy of looking for oxygen is indeed a good one, study team members said; after all, it's tough for this gas to build up in a planet's atmosphere if life isn't there churning it out. "But we don't want to put all our eggs in one basket," study lead author Joshua Krissansen-Totton, a doctoral student in Earth and space sciences at the University of Washington in Seattle, said in a statement. "Even if life is common in the cosmos, we have no idea if it will be life that makes oxygen," Krissansen-Totton added. "The biochemistry of oxygen production is very complex and could be quite rare." So, he and his colleagues took a broader view, studying Earth's history to identify combinations of gases that, if observed together by future instruments such as NASA's James Webb Space Telescope, would be strong evidence of life. They came up with what they think is a good candidate: Methane (CH4) and carbon dioxide (CO2), without any appreciable carbon monoxide (CO). As their chemical formulas show, methane and carbon dioxide are very different molecules. Their co-occurrence is indicative of an "atmospheric disequilibrium" — a term that gets astrobiologists pretty excited.

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