Cornell University researchers have created the first reflectance spectra - a color-coded key - of diverse, colorful microorganisms that live in the clouds floating above Earth's surface. Astronomers don't know if these bacteria exist elsewhere in the universe and in enough abundance to be detected by telescopes; on Earth they are not. But now astronomers can use the color key in the search for life outside our world - making an exoplanet's clouds, in addition to its surface and air, a promising realm for finding signs of life.
"There is a vibrant community of microorganisms in our atmosphere that produce colorful biopigments, which have fascinated biologists for years," said astrobiologist Ligia Coelho, fellow at the Carl Sagan Institute.
Coelho led the study of "Colors of Life in the Clouds: Biopigments of Atmospheric Microorganisms as a New Signature to Detect Life on Planets Like Earth," published in Astrophysical Journal Letters on November 11.
"Finding colorful life in Earth's atmosphere has opened a completely new possibility for finding life on other planets," said Lisa Kaltenegger, professor of astronomy and director of the Carl Sagan Institute, who is second author of the study. "Now, we have a chance to uncover life even if the sky is filled with clouds on exoplanets. We thought clouds would hide life from us, but surprisingly they could help us find life."
With the spectra, she said, astronomers will be able to look for biosignatures on exoplanets that have dense or even 100% cloud cover.
The colorful microbes that produced Coelho's spectra are rare in Earth's atmosphere and took specialized work to collect. She worked with collaborators at the University of Florida, who used a latex sounding balloon to gather biota from lower altitudes in the stratosphere, between 21 and 29 kilometers above the ground.
To flourish at a high-enough density that observers could find them, the microbes would need to live in planets with humid conditions. And telescope technology will also have to catch up. Knowing that we can search for life on cloudy worlds is informing the design of future telescopes, including NASA's space-based Habitable Worlds Observatory, which is in development, and observation strategies for the European Southern Observatory's Extremely Large Telescope, which is under construction in Chile and scheduled to start science observations in the 2030s.
"Biopigments have a universal character on our planet. They give us tools to fight stresses like radiation, dryness and lack of resources. We produce them, and so do bacteria, archaea, algae, plants, other animals," Coelho said. "They are powerful biosignatures and we've discovered a new way to look for them - through the clouds of distant worlds. And if life looks like this, we finally have the tools to recognize it."
Research Report:Colors of Life in the Clouds: Biopigments of Atmospheric Microorganisms as a New Signature to Detect Life on Planets Like Earth
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