Red dwarfs may be too dim to drive complex life

New research estimates an Earth-analog at TRAPPIST-1e would receive about 0.9% of Earth's photosynthetically active radiation, and a simple scaling gives roughly 63 billion years for a Great Oxygenation Event; accounting for spectral and physiological factors reduces that estimate to about 1–5 billion years.

The rise of free oxygen on Earth during the Great Oxygenation Event enabled aerobic metabolism and, much later, complex animals. Scientists asked whether similar oxygenation could occur on planets orbiting dim red dwarfs, which are common in the galaxy. A new study by Joseph Soliz and William Welsh (San Diego State University), submitted to Astrobiology and posted on arXiv and presented at the AAS meeting, models photosynthetically active radiation (PAR) on a TRAPPIST-1e Earth-analog. The authors compare photon fluxes and simple scaling arguments to assess how long an oxygenation event might take around a late M dwarf. Key findings: - The paper models TRAPPIST-1e and finds an Earth-analog there would receive about 0.9% of the standard PAR (400–700 nm) that Earth receives from the Sun. - Under a straightforward scaling that ties oxygen accumulation to photon flux, the authors report a timescale near 63 billion years for a GOE-like event on such a planet. - When the authors include photoinhibition, extended PAR bands, and other physiological factors, the estimated timescale falls to roughly 1–5 billion years. - The study notes non-oxygenic (anoxygenic) photosynthetic bacteria can use near-infrared light out to about 1100 nm and could access roughly 22 times more photons than oxygenic systems in that environment. - The authors highlight key assumptions, including that oxygenic photosynthesis is required for complex, animal-like life and that timescales on the exoplanet are comparable to Earth’s. Summary: If the model and assumptions hold, late M-star Earth-analogs would receive far less PAR and could take very long periods to develop atmospheric oxygen, making ecosystems dominated by low-light, non-oxygenic microbes more likely. Undetermined at this time.