When astronomers imagine a planet covered in life, green landscapes may come to mind. But a study published in April 2024 suggests that some inhabited worlds could display very different colors, including shades associated with purple bacteria found on Earth.

Researchers at Cornell University investigated how these microorganisms reflect light and whether similar organisms, if widespread on another planet, might leave recognizable signatures in telescope observations. Their findings expand the possibilities scientists can consider when searching for life beyond Earth.

Why Purple Matters in the Search

Earth is the main reference point for scientists studying possible signs of extraterrestrial life. Much of our planet's vegetation appears green because chlorophyll absorbs certain wavelengths of sunlight while reflecting others. Astronomers can also investigate the distinctive reflectance patterns of vegetation, including the sharp increase in reflectance near the boundary between visible red and near-infrared light.

However, life on another planet would not necessarily resemble Earth's forests or grasslands. Different atmospheric conditions, available nutrients, and types of starlight could favor organisms with alternative pigments and energy-acquisition strategies.

Purple bacteria provide one example. These microorganisms include groups capable of performing anoxygenic photosynthesis, which uses light without producing oxygen as a byproduct. Many contain bacteriochlorophylls and other pigments that allow them to absorb red or near-infrared light.

Despite their name, purple bacteria are not always visibly purple. Depending on their pigments, they can appear reddish, brown, orange, or yellow. Their scientific importance lies less in one particular color than in the distinctive wavelengths they absorb and reflect.

Studying Purple Bacteria

The research was led by astrobiologist Lígia Fonseca Coelho at Cornell University's Carl Sagan Institute, with collaborators from Cornell and the University of Minnesota. The findings were published in Monthly Notices of the Royal Astronomical Society: Letters in April 2024.

The team studied a collection of purple sulfur and purple non-sulfur bacteria, including laboratory cultures and samples obtained from aquatic environments. Some came from locations in New York and Massachusetts, while others were provided by collaborating researchers.

Rather than examining exactly 40 individual species, as sometimes reported in summaries of the study, the researchers analyzed a collection that included enrichment cultures and pure cultures. An enrichment culture can contain multiple species, so it should not be counted as a single identified species.

The scientists measured how the samples reflected light across wavelengths from 400 to 2,500 nanometers, spanning visible light and parts of the infrared spectrum. They then used these measurements to simulate the light reflected by hypothetical Earth-like planets with extensive coverage of similar microorganisms.

The resulting spectral database enables researchers to compare potential biological surface signatures with future observations. It does not establish that such organisms exist anywhere beyond Earth.

Different Stars Could Mean Different Life

Not every planet receives the same mixture of light that reaches Earth. Cooler stars, including red dwarfs, emit a greater proportion of their radiation at longer wavelengths than the Sun does. This could influence which light-harvesting pigments would be useful to organisms living on nearby planets.

Some purple bacteria on Earth already use red and near-infrared light for photosynthesis. Their existence shows that oxygen-producing photosynthesis, familiar from plants and algae, is not the only biological way to capture energy from light.

The researchers modeled several hypothetical environments, including ocean-covered planets, frozen worlds, and planets resembling modern Earth. They explored how extensive populations of purple bacteria might influence the reflected light from these surfaces.

Their simulations suggest that biological pigments could leave distinctive spectral features under certain conditions. However, the strength of those features depends on how much of the surface is covered by organisms, the properties of the surrounding environment, and the planet's atmosphere.

Clouds can obscure surface features, while atmospheric gases can absorb light at wavelengths that overlap with possible pigment signatures. These complications mean that identifying a biological signal would require much more than detecting an unusual color.

Future Telescopes May Help

The study could help astronomers prepare for future observations of potentially habitable exoplanets. Researchers need to know which wavelengths might contain useful information before deciding how to analyze the extremely faint light reflected by distant worlds.

Two important observatories discussed in this research are the European Southern Observatory's Extremely Large Telescope, under construction in Chile, and NASA's proposed Habitable Worlds Observatory. Both are relevant to the longer-term effort to study planets beyond our solar system, although their capabilities and observing methods will differ.

Detecting surface pigments on a distant planet would be extremely challenging. A planet's light is faint compared with its host star, and clouds, atmospheric absorption, and surface minerals can complicate interpretation.

A possible purple-bacteria signature would therefore warrant further investigation, not proof of life. Scientists would need to examine the planet's atmosphere, surface conditions, and other possible explanations before reaching a biological conclusion.

By measuring a wider variety of Earth organisms, researchers can improve the reference data used to interpret future telescope observations.

Earth Still Provides the Starting Point

The study does not demonstrate that purple bacteria exist on another planet, nor has any extraterrestrial life been confirmed. Its contribution is to show how familiar organisms on Earth can broaden the range of possible biological signatures that astronomers investigate.

Searching only for signals associated with green vegetation or oxygen-rich environments could overlook other possibilities. Earth's microbial diversity provides examples of organisms that use different pigments, energy sources, and metabolic processes.

At the same time, finding a similar spectral pattern on another world would not necessarily mean that the same bacteria were responsible. Nonbiological materials can also produce unusual reflectance features, making careful comparisons essential.

The researchers' models offer a starting point for those comparisons. They show how hypothetical planets dominated by purple bacteria might appear in reflected light and provide data that future observing strategies can build upon.

The broader lesson is that the search for life should remain open to environments and biological processes that differ from those most visible on Earth today.

A distant planet with an unusual purple or reddish spectral signature would not necessarily be a living world, but it could still be worth investigating for astronomers. By studying the pigments of bacteria already found on Earth, scientists are developing a broader picture of how biological activity might affect the light reflected by other planets. The 2024 research provides a useful foundation for future observations while leaving the central question unanswered: whether life has developed anywhere beyond Earth.