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Complex life on planets orbiting the galaxy's most common stars may be unlikely - Astronomy & Space Astrobiology - Planetary Sciences - UNIVERSE

Artist’s impression of a M dwarf star surrounded by planets. Credit: NASA/JPL-Caltech/MSSS

In a blow to anyone dreaming that complex life may exist elsewhere in the universe, a new study suggests we're unlikely to find it around many of the most common stars in the galaxy.

Earth-like planets orbiting small, red stars known as M-dwarfs are often considered the right size and at the right distance from their sun to harbor life. However, according to researchers from San Diego State University, these worlds may not have the right kind of light to support multicellular organisms.

Photosynthesis is key

Here on Earth, plants and bacteria turn sunlight into energy through photosynthesis, releasing oxygen as a byproduct. During the Great Oxidation Event around 2.3 billion years ago, significant quantities of oxygen began to accumulate in our atmosphere, eventually reaching levels capable of supporting multicellular life. According to our understanding, a similar process would have to occur on other planets for complex life to start evolving.

Photosynthesis requires a specific kind of light known as Photosynthetically Active Radiation (PAR). This is the specific range of sunlight (400 to 700 nanometers) that plants, algae and cyanobacteria need to thrive. Although it was known that light from M-dwarf stars like TRAPPIST-1 is mostly infrared, which falls outside this range, what was unknown was how this would slow down the evolutionary clock.

The incident photon flux density for the modern-day Earth (black), Archean Earth at 2.65 Ga (blue), and TRAPPIST-1e (red). The spectral resolution has been reduced for clarity. The shaded regions represent three relevant bandpasses for photosynthesis: standard PAR (0.40-0.70 𝜇 m), extended PAR (0.40-0.75 𝜇 m), and anoxic PAR (0.40-1.1 𝜇 m). Credit: arXiv (2026). DOI: 10.48550/arxiv.2601.02548

By comparing light from these red stars to our own sun and modeling the oxygen production of various bacteria, the team calculated that because these stars produce so little usable energy, the accumulation of oxygen would be far too slow. Potentially, on a planet like TRAPPIST-1e, it would take 63 billion years in a worst-case scenario to reach the oxygen levels seen on Earth through photosynthesis.

Even when they ran more optimistic calculations that assumed alien bacteria could adapt to the light conditions or thrive in the dark, the timeline for a Cambrian Explosion (an evolutionary event marked by the appearance of a variety of complex animals) would still exceed ten billion years.

"We conclude that on such a hypothetical planet [a theoretical Earth-sized world orbiting an M-dwarf star used for the study's calculations], oxygen would never reach significant levels in the atmosphere, let alone a Cambrian Explosion," commented the researchers in their paper published on the arXiv preprint server. "Thus complex animal life on such planets is very unlikely."

There could still be something out there

Because most of the stars in our galaxy are M-dwarfs, this study suggests that the conditions required for complex biology may be rarer than previously thought. But of course, the dream of finding life elsewhere is not over.

While the math suggests these red dwarf systems may be limited to simple microbial life, complex organisms could still exist on other types of worlds. This research could help scientists focus their search on systems around the suns that produce high-energy light needed to spark an evolutionary explosion. 

by Paul Arnold, Phys.org

edited by Gaby Clark, reviewed by Robert Egan 

Source: Complex life on planets orbiting the galaxy's most common stars may be unlikely