Alien life may reveal itself as color in the clouds of distant planets

An Earth-like world with heavy clouds might not hide life at all. It could make alien life easier to spot if those clouds carry colored microbes from the planet’s skies.

Cornell scientists report the first reflectance spectra – precise color patterns in reflected light – for airborne microbes that thrive high above Earth.

The team’s new color key includes seven microbe strains and turns cloudy exoplanets into promising targets for life detection.

Clouds as a signal of alien life

The work was led by Lígia Coelho at Cornell’s Carl Sagan Institute (CSI). Her research centers on biopigments and how they leave detectable traces on planets.

“There is a vibrant community of microorganisms in our atmosphere that produce colorful biopigments, which have fascinated biologists for years,” said Coelho.

“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 (CSI).

This challenges a long-standing assumption in the exoplanet community. Thick clouds might not only mute surface signals, they might also carry their own biological signature.

What a color key actually measures

The team measured how these microbes reflect and absorb light across visible and infrared wavelengths.

Those patterns act like fingerprints that telescopes can search for in a planet’s reflected starlight.

The key term is biosignature, a measurable trace of life that cannot be explained by nonliving processes. If microbes color large portions of a cloud deck, their combined signal can reshape a planet’s overall spectrum.

How scientists collected the samples

The microbes in this catalog came from the lower stratosphere – a dry, cold air layer above the weather filled troposphere. On Earth that is a harsh place, yet certain bacteria endure and keep their color.

Balloon missions have recovered viable cells up to about 24 miles above sea level. The Cornell team analyzed seven strains gathered between roughly 13 and 18 miles, then grew them and recorded their spectra under wet and dry conditions.

Dry samples reflected more strongly, boosting the visibility of yellow and orange pigments. Wet samples showed sharper features in the 400 to 600 nanometer range, where many of the key pigments absorb light.

Signs of alien life in the clouds

Astronomers model how cloud-covered worlds would look with and without pigmented microbes. The modeled spectra show distinct changes near 500 nanometers when microbe-laden clouds are present.

NASA’s Habitable Worlds observatory is being designed to seek life on nearby rocky planets. The new color library helps set instrument requirements and observation plans for that mission.

On the ground, ESO’s Extremely Large Telescope will push exoplanet spectroscopy with a 39-meter mirror. First light is scheduled for 2029, according to an official announcement.

Clouds usually brighten a planet compared to dark oceans, which increases detectability. If microbes tint those clouds, they may imprint a spectral twist that stands out against nonliving clouds.

Why pigments matter for survival

Microbes produce biopigments, protective molecules that color cells and help them survive stress from sunlight or dryness. These include carotenoids and other compounds that either shield or safely dissipate energy.

Microbial UV sunscreens are well documented in a comprehensive review. That background supports the idea that pigmented life could be common in irradiated cloud layers where water is available.

Carotenoids, a familiar group of pigments that absorb light between 400 and 600 nanometers, can protect cells from oxidative stress. In some bacteria, they also help capture light for metabolism.

Another important concept is albedo, the fraction of light that is reflected by a surface or cloud. Adding pigments to cloud droplets can change that reflectivity and the detailed shape of the reflected spectrum.

Study limitations and unknowns

Earth’s atmospheric microbes are thinly spread, which makes their combined signal hard to detect at long distances. Denser atmospheres or stronger vertical mixing on another world could raise concentrations to detectable levels.

Telescopes still face limits on how faint and small a world they can study in reflected light. That pushes designs toward superb starlight suppression, stable instruments, and long exposures on the best targets.

Not every color bump in a spectrum is biological. Mineral dust, photochemical hazes, or instrument artifacts can mimic broad features and must be ruled out with multiple lines of evidence.

To count as a strong biosignature, a pigment signal would likely need to cohere with a plausible atmospheric context. Water-rich clouds, consistent chemistry, and repeatable features across observations would all add weight.

The search for life continues

Traditionally, life detection strategies favor gases like oxygen paired with methane. The cloud pigment approach adds a third path, using light reflected by living communities above the surface.

The research also opens a window on planets whose surfaces are hidden by global cloud decks. That means fewer worlds get written off early, and more end up worth a careful look.

The new spectra give observers a target template for planning time on future facilities. They also offer modelers a library to test retrieval methods before the first datasets arrive.

None of this guarantees an easy discovery. But it does put cloud tops squarely among the places where life could reveal itself in unmistakable data.

The study is published in The Astrophysical Journal Letters.

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