A new study frames the pace of biological complexity on exoplanets as tied to cumulative photosynthetic energy over geological time. Many nearby worlds, especially those around red dwarfs, may have enough age but too little plant-driven energy to push life past microbial stages.
The study compares lifetime photosynthetic carbon fixation to Earth’s record and finds most nearby potentially habitable exoplanets are unlikely to have progressed beyond microbial life.
Planets around red dwarf stars can be older yet produce far less total photosynthesis because of lower light, less surface area for photosynthesis (tidal locking), and cooler, drier climates.
On Earth, more plant growth creates more ecological energy and niche space for animals and larger ecosystems. The researchers treat the total carbon fixed by photosynthesis over billions of years as an "evolutionary clock": if a planet has fixed little carbon, it has had less energy available to build ecological complexity.
The study combined three elements: stellar properties (age, brightness, spectrum), orbital and rotation states (including tidal locking), and simulated surface climate maps. Denis Sergeev produced modeled maps of temperature, available light, and precipitation for candidate exoplanets—variables commonly used to predict plant productivity on Earth. The authors then used those climate maps to estimate potential lifetime carbon fixation and compared those values to Earth's benchmarks.
Among the 29 worlds considered, most fell well below Earth's cumulative photosynthesis. Two planets, GJ 1061c and K2-3d, stood out as possibly having accumulated enough photosynthetic energy to support life beyond Earth's evolutionary stage. Three additional planets could be comparable to Earth during the Mesozoic era. Many others—especially planets around red dwarf (M) stars—appear more likely to remain at microbial levels even if they are billions of years older than Earth.
A concrete example: TRAPPIST-1e, despite being several billion years older than Earth, was estimated to have achieved only about 21% of Earth's lifetime carbon fixation. The authors infer that TRAPPIST-1e may be limited to microbial ecosystems under their assumptions.
Red dwarfs provide less total light per unit area and many close-in planets are tidally locked, which reduces the surface area effectively available for photosynthesis. Cooler temperatures and lower precipitation on those surfaces further suppress plant productivity in the models, producing low lifetime carbon fixation despite long stellar system ages.
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