In the hunt for life on other worlds, astronomers scour over planets that are light-years away. They need ways to identify life from afar -- but what counts as good evidence?
Our own planet provides some inspiration. Microbes fill the air with methane; photosynthesizing plants expel oxygen. Perhaps these gases might be found wherever life has taken hold.
But on worlds very different from our own, putative signs of life can be stirred up by non-biological processes. To know a true sign when you see it, astronomer Kevin France at the University of Colorado, Boulder, says, you must look beyond the planet itself, all the way to the gleaming star it orbits.
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