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Call it the cosmic version of fool’s gold. What was once considered a sure-fire sign of life on distant planets may not be so sure-fire after all, a new study suggests. Instead, it may simply be the artifact of a lifeless world and its equally lifeless moon.

“I like this paper. It’s a great concept,” says Sara Seager, a planetary scientist at the Massachusetts Institute of Technology in Cambridge. “I think the [scientific] community is gradually realizing that it might be impossible to be absolutely certain, based on its atmosphere, that an exoplanet hosts life.”

Life on planets orbiting other stars doesn’t have to literally broadcast its existence: Radio signals are just one way earthbound scientists might detect biological activity elsewhere in the universe, says Hanno Rein, a planetary scientist at the University of Toronto, Scarborough, in Canada. Signs of life are much more likely to be subtle, especially if the organisms are simple. One way to look for such clues is to search for chemical evidence, particularly in light passing through the atmospheres of the planets, Rein says. By comparing the spectrum of light passing through an exoplanet’s atmosphere with that of the unfiltered light emitted by its parent star, astronomers can identify substances present in the exoplanet’s air.

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