Purple Photosynthesis: Experts Suggest Aliens May Not Be Found on Green Earth-Like Planets

Green is the color we most associate with surface life on Earth, from house plants and gardens, to fields and forests. The conditions in our planet favored the evolution of organisms which carry out oxygen-producing photosynthesis using the green pigment chlorophyll a.

If a person were asked to imagine an alien planet, it would be reasonable to imagine a green Earth-like rock that orbits somewhere in a distant solar system. However, a new study suggests that we might be getting this color palette all wrong.

Not-So-Ordinary Inhabitants

A team of researchers suggests that an Earth-like planet orbiting another star might look very different, as it might be covered by bacteria that receives little or no visible light or oxygen, as in some environments on Earth. Instead, these microorganisms may be using invisible infrared radiation as a source of energy for photosynthesis.

Here on Earth, many such bacteria contain purple pigments instead of green. The purple worlds on which they are dominant would create a distinctive "light fingerprint" which can be detected by next-generation ground- and space-based telescopes.

Purple bacteria can thrive under a wide range of environmental conditions, making it one of the major candidates for life that could dominate a variety of worlds. They may be collectively called purple bacteria, but they actually have a range of colors, like red, brown, orange, and yellow. These microorganisms thrive on low-energy red or infrared light using simpler photosynthesis mechanisms that utilize forms of chlorophyll which absorb infrared and do not make oxygen.

Purple Is the New Green

To date, scientists have confirmed over 5,500 exoplanets, including over 30 potentially Earth-like planets. Planned observatories like the Habitable Worlds Observatory and Extremely Large Telescope aim to explore the chemical composition of these planets in their stars' habitable zones, or the region where the conditions are conducive to the existence of liquid water on the surface. Researchers realized the need to create a database for signs of life to make sure that their telescopes do not miss life if it happens not to appear exactly like what we encounter everyday here on Earth.

Using life on Earth as a guide, a multidisciplinary team of scientists from Carl Sagan Institute (CSI) are cataloging the chemical signatures and colors that a diverse range of minerals and organisms would present in the reflected light of an exoplanet. Their study is described in the paper "Purple is the new green: biopigments and spectra of Earth-like purple worlds".

Led by Lígia Fonseca Coelho, the scientists gathered and grew samples of over 20 purple sulfur and purple non-sulfur bacteria which can be found in different environments, such as coasts, marshes, shallow water, and deep-sea hydrothermal vents. After measuring the biopigments and light fingerprints of purple bacteria, the research team created models of Earth-like planets with different conditions and cloud cover. Across various simulated environments, both wet and dry purple bacteria created intensely colored biosignatures.

According to Coelho, if purple bacteria thrive on the surface of an ocean world, a snowball Earth, a frozen Earth, or a modern Earth that orbits a cooler star, then scientists now have the tools to search for them. Finding a "pale purple dot" in another star system would trigger intensive observations of the planet to try to rule out other color sources.