How to spot life in the clouds on other worlds

Cloud cover is bad for picnics and for viewing stars through a telescope. But an exoplanet with dense or even total cloud cover could help astronomers search for signs of life beyond our planet.

Cornell 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, 51 Pegasi b Postdoctoral Fellow in astronomy in the College of Arts and Sciences (A&S) and fellow at the Carl Sagan Institute (CSI). “I thought astronomers should know about them.”

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,” which is published in The Astrophysical Journal Letters.

Clouds as new frontiers for biosignatures

“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 CSI director, 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.

Collecting and analyzing atmospheric microbes

The colorful microbes that produced Coelho’s spectra are rare in Earth’s atmosphere and took specialized work to collect. She got her seven from 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.

Coelho grew the cultures with equipment and expertise from Stephen Zinder, professor of microbiology emeritus in the College of Agriculture and Life Sciences, a CSI Fellow. She then analyzed their reflective spectra in the lab of another CSI Fellow, Bill Philpot, professor of civil and environmental engineering in Cornell Engineering.

Simply having the spectra of these bacteria opens new ways of searching for life, Coelho said. But still more information is encoded in the color signatures of these microbes. They tell about the conditions in which the microbes are living, because pigmentation protects life forms from elements such as radiation, dryness or extreme temperature. Many forms of life, including bacteria, produce pigments for protection from ultraviolet rays—which they need so high in Earth’s atmosphere, Coelho said.

Implications for future telescope missions

By modeling possible extremes, the researchers determined that a cloudy planet with widespread colorful bacteria in its clouds would look significantly different than a planet without them, giving colorful bacteria the potential to be a detectable biosignature.

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.”