When scientists search for life beyond Earth, they do not necessarily expect to find landscapes covered in green plants. Life elsewhere may have developed under completely different conditions, using different materials and adapting to forms of light that are less common on our planet.

One intriguing possibility is that some living worlds could appear purple, offering astronomers a potential clue as they examine distant planets.

Why Life Might Not Be Green

Green is a familiar color in Earth's natural environment because chlorophyll-a is the main photosynthetic pigment found in many plants. It is also present in cyanobacteria, microorganisms that played an important role in the evolutionary history of plant photosynthesis. However, photosynthesis does not always depend on the same pigments or light conditions. Some bacteria use different biological pigments to collect energy, allowing them to survive in environments where sunlight is limited or has a different composition.

Purple bacteria are particularly interesting because they can use infrared radiation and thrive in a variety of conditions. Their existence suggests that photosynthetic life need not resemble the green vegetation that dominates many parts of Earth. Astrobiologist Lígia Fonseca Coelho of Cornell University's Carl Sagan Institute has identified purple bacteria as promising candidates for what might dominate the surfaces of other worlds.

Red Dwarf Stars Offer a Different Setting

The type of star a planet orbits can influence the conditions available to life. Most stars in the Milky Way are not similar to the Sun. Red dwarf stars are smaller, cooler, and emit less heat and visible light. They account for up to 75 percent of the galaxy's stars. This raises an interesting possibility for astrobiology: could life develop on planets orbiting red dwarfs, and would it look different from the life found on Earth?

A red dwarf provides a different light environment from the one surrounding our planet. Under such conditions, organisms capable of using red and infrared wavelengths could have an advantage. Purple bacteria on Earth offer researchers a useful example of how life might adapt to a different energy source.

How Researchers Studied Purple Bacteria

To investigate this possibility, researchers at the Carl Sagan Institute examined more than 20 bacterial species that use pigments called carotenoids to capture light energy. The organisms studied can make use of red and infrared light. Their light-harvesting systems differ from those commonly associated with plants and cyanobacteria. They also use bacteriochlorophylls, which absorb wavelengths not typically used by familiar photosynthetic organisms.

An important difference is that this type of photosynthesis does not produce oxygen. That detail matters because scientists cannot assume that all photosynthetic life beyond Earth would operate in the same way as the organisms most familiar to us. The team measured the bacteria's pigments and used the results to build models of hypothetical planets. The models included various surface and atmospheric conditions to examine how the organisms might affect the appearance of a world as seen from a distance.

What Planetary Colors Could Tell Us

Purple is one possible outcome, but it is not the only color that could indicate the presence of similar organisms. Depending on the bacterial species and its pigments, the resulting appearance could lean toward orange, red, or brown. This variation is important because scientists should not expect a single color to represent every form of photosynthetic life.

A world covered by widespread microbial life might therefore display a distinctive color pattern that differs from Earth's familiar green landscapes. If astronomers could detect such a signal while studying an exoplanet, it might prompt closer investigation of that planet.

By studying Earth's microbial diversity and modeling how those organisms might appear on other planets, researchers can expand the range of signals worth looking for. A purple world would not necessarily mean that life is present, but it could be an intriguing clue in the ongoing search for habitable environments beyond our solar system.