Habitability is defined by liquid water — the habitable zone is the range of orbital distances where surface temperatures permit oceans. But life needs more than water. It needs energy in a form that can drive chemistry. Photosynthesis requires not just photons but photons of the right energy to split water and build complex molecules. The question is not whether a planet has light but whether the light can do work.
Covone and Balbi (arXiv:2602.20789) calculate the thermodynamic upper limit on photosynthetic power across the habitable zones of different stellar types. The framework uses exergy — the maximum useful work extractable from radiation — rather than raw energy flux. The results show a factor-of-five difference: planets orbiting Sun-like FGK stars can harvest approximately five times more photon exergy for water-oxidation photosystems than planets orbiting late M dwarfs.
The spectral range matters. Sun-like stars support oxygenic photosynthesis across wider wavelength bands. Cooler stars concentrate their output in the infrared, where individual photons carry less energy. Photosystem II requires photons energetic enough to oxidize water — roughly 680 nm or shorter. M-dwarf spectra peak beyond this, sharply limiting the available photosynthetic bandwidth.
For Earth, the calculated upper limit on oxygen production substantially exceeds what biology actually achieves. The gap between thermodynamic limit and realized production measures the inefficiency of evolved photosynthetic machinery — roughly two orders of magnitude. Evolution has not reached the physical ceiling.
The general observation: the habitable zone defines where liquid water can exist. The exergy analysis defines where photosynthesis can work. These are different boundaries. A planet can be habitable by the water criterion and inhospitable by the energy criterion. The relevant limit for complex life is the stricter of the two.