Nicola Twilley, for The New Yorker:
Until recently, astronomers had focussed on analyzing a planet’s reflected light for evidence that its atmosphere contained oxygen or other gases that are considered to be positive indicators for the presence of life. In 2002, however, they proved for the first time that a pigment—chlorophyll, the molecule that makes plants and certain algae appear green to our eyes—could serve as a biosignature. By observing Earthshine, the sunlight that is reflected from Earth onto the surface of the moon, they were able to detect a “vegetation red edge,” a distinctive spike in the near-infrared region that is caused by chlorophyll. (To an alien equipped with infrared goggles, the faint glowing pixel that is Earth would actually have a hot-pink tinge.)
It seems quite a feat to look at a single point of light to assess its capacity for life — like looking at a novel’s word cloud and guessing its genre based on the largest noun. Still, it’s a start.