Here’s a new way to search for life on alien worlds: Look for the light it reflects into space. To prepare for such a search, scientists have made a library of life’s colors, cataloging the spectra, or wavelengths of light, reflected by 137 types of microorganisms, they report online today in the Proceedings of the National Academy of Sciences. In the visible wavelengths of light, the microbes’ spectra depended on their pigments—what we think of as their color—which varied from species to species. In the infrared, however, the microbes all looked similar, absorbing light at particular wavelengths due to the water inside of them. If scientists detected a pattern of wavelengths that matched the types of pigmentation seen in earthly species, and displayed the universal absorption features in the infrared, it would be evidence for life. The database could help scientists design searches for life on watery, habitable exoplanets like the artist’s conception above, using the new, highly sensitive telescopes on the horizon. But such a search would be a technical challenge, because the light reflected by a planet is swamped by that of its star. And, the technique isn’t foolproof—it works only if the planet has a transparent atmosphere with few clouds, so that light reflected from microorganisms can escape, and is probably most useful for life that is likewise carbon-based and therefore has the same chemistry constraining its spectrum. Still, remote-sensing techniques like these may be our only hope for finding our kin in the far-flung corners of universe.