Colorful microbes in Earth’s clouds may reveal how to spot alien life

Carl Sagan once imagined creatures floating through Jupiter’s clouds. Back then, it was just an idea. Now, scientists have taken the first real step toward testing that vision.

A team at Cornell University’s Carl Sagan Institute has discovered that microorganisms in Earth’s upper atmosphere carry colorful, light-protective pigments.

Those pigments – tiny shields made of molecules like carotenoids and flavins – reflect light in recognizable ways. The reflections might be strong enough for future telescopes to detect on distant planets.

For the first time, scientists have measured how these airborne microbes scatter and absorb light. The results suggest that life could leave visible traces in the skies of other worlds.

Clouds teeming with microbes

The atmosphere is not empty. It moves with life – cells, spores, and bacteria lifted by winds from land and sea.

Scientists have counted tens of thousands of living cells in a single cubic meter of air. Some survive years aloft, drifting across continents.

Many of these organisms feed on trace gases such as hydrogen and methane. A few can even draw water directly from chemical reactions in the air. Others rest inside cloud droplets, where short bursts of moisture allow them to grow.

From ground level, it’s easy to forget that the sky above hosts its own ecosystem. But for life looking for a home, clouds might be as good as soil or sea.

Color as survival armor

Sunlight at high altitude can destroy cells in seconds. Pigments act like armor. Reds, yellows, and oranges absorb ultraviolet radiation and convert it into harmless heat.

That color isn’t decoration – it’s survival chemistry. When light hits these pigments, the reflection forms a pattern unique to living material.

Each pigment produces its own set of wavelengths, like a barcode. Telescopes capable of detecting those patterns could one day identify clouds rich in biological color.

Building a microbial color library

To see what that would look like, Lígia Coelho and her colleagues grew seven bacterial strains originally collected from the stratosphere – 13 to 18 miles above Earth. The microbes had already survived intense sunlight and freezing air.

In the lab, the team measured how each culture reflected light using a precision spectrometer. The same samples were tested fresh and after drying for a week to mimic the difference between wet and dry clouds.

Every strain showed a distinct reflection between 400 and 600 nanometers – the visible color range. Pink species carried pigments similar to lycopene; yellow ones showed patterns like zeaxanthin and lutein.

Those variations became the first library of what “life in the air” looks like when converted into light.

Dry clouds signal microbes

Moisture changed everything. Dry microbes reflected more light overall. Wet ones showed sharper color peaks.

That means the brightness of a planet’s clouds could hint at humidity as well as biology including microbes.

A dry, desert-like atmosphere might appear lighter; a wetter one could display clear pigment signatures. Either scenario could stand out in telescope data.

Simulating alien skies

The researchers went a step further and modeled how these pigments would appear on exoplanets. Using a simulation called ExoPrime II, they tested ocean planets, snowball worlds, and Earth-like systems with different cloud layers.

Once biopigments were added, the reflection curves changed noticeably around 500 nanometers – right where many carotenoids absorb light. Wet clouds darkened slightly; dry clouds became brighter.

The models suggest that living aerosols could alter a planet’s overall color enough for future instruments such as NASA’s Habitable Worlds Observatory to detect.

In other words, a yellow or pink tint in a planet’s reflected light might someday point to microbial life floating above its surface.

Life suspended in air

Most life-detection strategies rely on two signs: atmospheric gases like oxygen and methane, or surface colors from vegetation. Coelho’s work adds a third – life suspended in the air.

Microbial pigments could leave visible marks even if a planet’s surface stays hidden beneath thick haze.

That possibility expands the search for habitable environments far beyond oceans and soil. A world might be lifeless below but alive in its clouds.

When clouds become homes

On Earth, clouds already support active chemistry. Microbes there trigger reactions that form rain droplets and influence climate. Some can multiply within the short lifespan of a cloud.

Other planets could host similar cycles. Around red dwarf stars, for instance, ultraviolet radiation is far stronger than on Earth.

Organisms in those skies would need powerful pigments to survive – making them easier to detect from space. Clouds once seemed to block our view of life. Now, they might reveal it.

Finding microbes in clouds

Detecting those faint color shifts requires instruments with extreme precision. Future observatories, including the Habitable Worlds Observatory and the Extremely Large Telescope, are being designed for that purpose.

Their sensors will collect reflected light from distant planets. Artificial intelligence models could then compare those readings with the pigment spectra from Coelho’s experiments.

That process would help sort possible biological signatures from background noise. The approach won’t confirm life instantly, but it could tell astronomers where to look closer.

Colors that guide discovery

Each bacterium in the study has its own story. Modestobacter versicolor produces pink and black pigments to handle radiation. Roseomonas vinacea forms dark pink colonies. Micrococcus luteus shines yellow. 

Curtobacterium aetheriumand and C. oceanosedimentum resist UV-C radiation while active. Massilia niabensis and Noviherbaspirillum soli grow with yellow-orange pigments that handle dryness.

These colors are more than biology; they’re blueprints for recognizing life across the universe.

The research turns a long-standing idea into a measurable test. Instead of searching only for gases or green vegetation, scientists can now consider color in the sky as a clue.

If those same spectral fingerprints appear in light from another planet, it might mean the clouds above it are alive.

The next generation of telescopes will decide how far that idea goes. For now, one thing is clear – life doesn’t always need ground to stand on. Sometimes, it just needs air to float in.

The study is published in the preprint server arXiv.

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