​The Microbial Yearbook That Will Help Us Find Aliens

If you were an alien astronomer, and you happened to pinpoint Earth in the night sky, our atmosphere's blueish glow might clue you in to the presence of life. But pale blue dots aren't the only potentially habitable worlds out there. Depending on what sorts of life forms are dominant, a whole spectrum of colorful, life-bearing worlds might be scattered throughout the cosmic hinterlands.

That's why a research team led by Siddharth Hegde of the Max Planck Institute for Astronomy has just built the world's first alien yearbook, cataloging the myriad colors our interstellar neighbors might exhibit. By documenting the reflection signatures for diverse microbial life forms, scientists will be better equipped to look at a distant world and discern whether its pale glow hides life's telltale fingerprints. The 137 microbes profiled so far are reported today in the Proceedings of the National Academies of Sciences.

"This database gives us the first glimpse at what diverse worlds out there could look like," said Lisa Kaltenegger, a co-author on the study and director of Cornell's new Institute for Pale Blue Dots in a statement. "We looked at a broad set of life forms, including some from the most extreme parts of Earth."

Astronomers have spent the last few years gearing up for a new phase of exoplanet exploration. Very soon, bigger and more powerful telescopes such as the James Webb Space Telescope will give us our first opportunity to peer into the atmospheres of distant planets, and use reflected starlight to decode their chemical compositions. When this happens, astronomers will, for the first time, be able to search exoplanets for biosignatures—chemical fingerprints that may indicate life.

Part of the challenge that'll face our new alien hunters will be determining what combinations of atmospheric chemicals—things like oxygen, CO2, and methane—are robust indicators of life's metabolism. To that end, Kaltenegger is leading an effort to build a "fingerprint database" documenting hundreds of hypothetical Earth-like atmospheres. In the future, researchers will use this database to categorize exoplanets and hone in on the most promising Earth-like candidates.

However, it's not just atmospheric chemicals that contribute to a planet's spectral fingerprint—life itself can add color. Here on Earth, sunlight reflected from vegetation gives our planet a greenish tinge. Likewise, an alien organism covering large swathes of an exoplanet's surface may be detected by its pigmentation. But most life-bearing worlds out there are are probably microbial worlds, and until now, little thought has been given to the colors these alien bugs might display.

"Much of the history of life on Earth has been dominated by microbial life," the researchers write in their paper. "It is likely that life on exoplanets evolves through single-celled stages prior to multicellular creatures." What's more, many habitable worlds out there may be "dying Earths," or planets that have been returned to the microbes after a period of multicellular life.

To begin filling in their alien yearbook, the researchers collected cultures of 137 microbial life forms found across a range of environments. A key selection criteria was finding species that display diverse pigmentations. The collection includes residents of the Atacama desert in Chile, seawater in Hawaii, and an oyster pond at Martha's Vineyard.

The team shone light on each critter, measured its glow in visible and near-infrared wavelengths, and assembled their findings into an online catalog. The result is the very first color wheel dedicated to alien detection. It'll be hosted by the Institute for Pale Blue Dots and freely available to researchers throughout the world.

"Our results show the amazing diversity of life that one can detect remotely on exoplanets," said Hegde, an incoming Cornell research associate and former doctoral student of Kaltenegger's.

Which is to say, pale blue dots may be cosmically rare, but when you throw in every other hue under the sun, our galaxy is probably brimming with color.