As scientists look to the stars in the search for life, one Cornell researcher is identifying the colors that may be telltale signs of alien life on far-off exoplanets.
Astronomers are especially focused on worlds orbiting in a star's habitable zone, the region where temperatures could allow liquid water to exist. Out of roughly 6,000 confirmed exoplanets, only a few dozen fall into this Earth-like category, making them prime targets in the search for life beyond our planet.
With Earth as the only confirmed example of a life-bearing world, researchers often use it as a template for identifying biosignatures elsewhere. In a new preprint study, a multidisciplinary team from Cornell University, the University of Florida, and Brigham and Women's Hospital cataloged the colors and chemical fingerprints lifeforms might imprint on an exoplanet's reflected light.
Lead author Lígia Fonseca Coelho has a deep background in this field. In earlier work, she assembled a diverse collection of purple bacteria that use low-energy red and infrared light for photosynthesis. Because red dwarf stars, which emit more of this lower-energy light, are the most common in the galaxy, such microbes could thrive on many alien worlds.
"They already thrive here in certain niches," Coelho said. "Just imagine if they were not competing with green plants, algae, and bacteria: A red sun could give them the most favorable conditions for photosynthesis."
From that collection, Coelho's team measured biopigments and reflected light signatures, then modeled Earth-like planets with varying conditions and levels of cloud cover. Across a range of simulated environments that included wet or dry ones, as well as clear skies or cloudy conditions, purple bacteria produced strong, recognizable biosignatures.
Now the researchers have turned their attention skyward, focusing on life that might drift in planetary atmospheres. On Earth, microorganisms have been found from cloud layers up to the stratosphere, yet atmospheric life has rarely been considered in exoplanet searches. Historically, clouds have been seen as barriers to detecting surface biosignatures.
No previous study has attempted to model biosignature detection based on Earth's atmospheric microbes, even though atmospheric data is often the only observational window astronomers have for distant exoplanets.
To fill that gap, Coelho and her colleagues cultured seven airborne microorganisms and studied them with an ASD FieldSpec 4 spectrometer. They then fed these measurements into a simulated Earth-like atmosphere to determine how an inhabited sky would appear through a telescope.
Thanks to Coelho and her team, scientists now may be equipped with effective new tools in the search for life in exoplanetary atmospheres. The microorganisms her team studied exhibited distinct spectral fingerprints, which distinguishes them from similar materials that could lead to false positives.
And contrary to long-standing concerns, the team found that clouds can actually enhance detectability in many scenarios by boosting reflectivity, especially on ocean-covered worlds. Even so, clear-sky, snow-covered planets offered the strongest biosignature signals overall.
The spectral catalog generated by Coelho's team provides astronomers with a new set of tools for identifying atmospheric life on distant planets -- expanding the search beyond surface-based analogs and traditional greenery.
The paper, "Colors of Life in the Clouds: Biopigments of Atmospheric Microorganisms as a New Signature to Detect Life on Planets Like Earth," appeared on arXiv on September 30, 2025.