Worlds of purple: Reimagining photosynthesis on distant planets

With rapidly improving space-based telescopes, the search for extraterrestrial life is becoming more feasible. As of 2025, NASA has confirmed the existence of over 5800 exoplanets, with over 30 considered to be potentially Earth-like. Yet, despite advances in data collection capabilities, one question remains hotly contested: What characteristic features should a planet with life have?

The first idea that comes to mind for many is the color green. This thought process is intuitive, as Earth — the only known planet to host life — is dominated by green plants and algae. In fact, many of the earliest searches for life using space-based telescopes looked specifically for the characteristic absorption spectra of green photosynthetic pigments. Astronomers assumed that if photosynthesis was the foundation of ecosystems on life-containing planets, they should be covered with green pigments. However, many biologists disagreed, pointing to one of the field’s longest-running enigmas: no one knows why plants specifically evolved to be green. Many argue it was actually an evolutionary anomaly. A growing number of biologists now propose it would be likely for extraterrestrial photosynthesizers to adopt other colors, with purple being a strong candidate on planets orbiting a Sun-like star.

Chemically, the reason why plants on Earth are green is straightforward. Most plants use a pigment called chlorophyll to capture solar energy. The most abundant form of this pigment, chlorophyll a, predominantly absorbs red and blue wavelengths and strongly reflects green wavelengths, making plant cells appear green. However, from an evolutionary perspective, the adoption of chlorophyll is more puzzling. The Sun — a fairly average representative of stars in the galaxy — emits its greatest intensity in green wavelengths. If green light is the most abundant wavelength reaching Earth’s surface, it seems like a fitness disadvantage that plants would evolve to reflect nearly all of it. Few hypotheses have attempted to explain this seemingly paradoxical observation, leaving it a mystery.      

However, molecular biologists Shiladitya DasSarma from the University of Maryland and Edward Schwieterman from the University of California Riverside have proposed a theory that is gaining popularity. They suggest chlorophyll-containing cells were not the first photosynthesizers on Earth. Rather, they evolved to utilize light that other phototrophic organisms were not absorbing. This stems from fossil evidence indicating that the earliest phototrophs did not produce oxygen, the major byproduct of chlorophyll-based photosynthesis. DasSarma and Schwieterman posited they instead used a simpler purple pigment called retinal, which is present in modern phototrophic Archaea like Halobacterium salinarum.

 Studies have shown that modern phototrophic cells containing retinal rely on a primitive anoxic process to produce energy that is less efficient than chlorophyll photosynthesis but utilizes the Sun’s peak green emissions. Interestingly, retinal also reflects both red and blue light at the ideal wavelengths for chlorophyll absorption. DasSarma and Schwieterman theorize that purple phototrophs using retinal once dominated early Earth’s oceans, having been the first organisms to harness solar radiation. However, species below these dense mats of purple organisms gradually learned to exploit their reflected red and blue light. They acquired chlorophyll and evolved to maximize their efficiency, as the Sun’s red and blue emissions are less intense. This novel form of photosynthesis also produced oxygen, which was likely deadly to most of the planet’s anaerobic life. Chlorophyll photosynthesizers were thus able to overtake the oceans and eventually colonize land. Under this hypothesis, a once-purple planet had turned completely green.
On exoplanets orbiting stars like the Sun, perhaps photosynthetic ecosystems relying on the most abundant wavelength are more common, and a “chlorophyll takeover” never happened. In that case, exclusively looking for chlorophyll-like pigments could overlook many life-containing habitats. To rectify this, Cornell researchers have recently created a database for the reflectance spectra of purple bacteria under various atmospheric conditions. Their hope is that their data will expand searches for life beyond historical bias toward common Earth colors. Possibly, Earth is a pale green dot in a universe of purple life.

“Their hope is that their data will expand searches for life beyond historical bias toward common Earth colors. Possibly, Earth is a pale green dot in a universe of purple life.”