Alien worlds have been pouring out of the sky in recent years. Exoplanet searches like those led by the Kepler space telescope predict that there are as many as 30 billion planets in our galaxy suitable for life. But how to tell which ones are inhabited?
Since the 1960s, strategies for hunting aliens rested on the assumption that they would use Earth-like chemistry – a huge assumption. Now, Sara Seager at the Massachusetts Institute of Technology and her colleagues are broadening the search. They propose a way of identifying the signatures of non-Earth-like life forms in alien atmospheres.
Seager (see "Rockstar planet hunter: Genius award will free my brain") and MIT colleagues William Bains and Renyu Hu suggest looking for any gas that is out of equilibrium. If, for instance, astronomers detected high concentrations of a gas that degrades naturally, that would indicate something was replenishing supplies. On Earth, oxygen, ozone and methane eliminate each other rapidly and other gases are destroyed by ultraviolet radiation from the sun. Without life, those gases would not be here at all.
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