Scientists have measured the colors of microbes living high in Earth's atmosphere, revealing pigments that protect against UV light. These findings suggest that similar biosignatures in exoplanet clouds could indicate alien life. The research provides reference spectra for future astronomical observations.
Microorganisms thrive in Earth's stratosphere at concentrations of up to 100,000 per cubic metre, contributing to cloud formation. These microbes produce pigments such as carotenoids, which create yellow, orange, and pink hues to shield against intense ultraviolet radiation at altitudes between 3 and 38 kilometres.
Ligia Coelho at Cornell University in New York state led a team that analyzed microbes collected by Brent Christner at the University of Florida using helium balloons and sticky rods. By culturing these samples and measuring their reflectance spectra, the researchers generated data on how these pigments interact with light. They also modeled spectral variations under different planetary conditions, such as wetter or drier environments.
"For the first time, we now have real reflectance spectra of pigmented microorganisms from the atmosphere that can be used as reference data to model and detect life in clouds," says Coelho. She emphasizes that biopigments serve as a universal biosignature, given UV's role as a stressor for life on any starlit planet. "Biopigments are a powerful and surprisingly universal biosignature," Coelho adds. "Since UV is a universal stressor for life on any planet with a star, it’s plausible that reflective pigments serving the same function could evolve elsewhere, too."
Astronomers currently detect potential biosignatures on exoplanets by analyzing reflected light for gases like oxygen and methane, or surface markers like chlorophyll. Clouds have traditionally obscured these signals, but the study shows that high concentrations of airborne microbes could alter a planet's spectra detectably. Simulations indicate that densities akin to ocean algae blooms would be necessary for observation with instruments like NASA's proposed Habitable Worlds Observatory.
"Our planetary simulations show that if a planet’s clouds had high concentrations of these microorganisms, their spectra would potentially change in a detectable way," Coelho notes. However, challenges remain, as Earth's atmospheric microbe levels fall below current detection thresholds.
Clare Fletcher at the University of New South Wales welcomes combining carotenoid searches with chlorophyll detection but cautions that it assumes Earth-like life on exoplanets. Peter Tuthill at the University of Sydney expresses skepticism about distinguishing these faint signals from noise at distances like 20 parsecs.
The findings are detailed in a preprint on arXiv (DOI: 10.48550/arXiv.2509.25173).