Photosynthesis on a Planet Orbiting an M Dwarf: Enhanced Effectiveness during Flares

On planets near M dwarfs, oxygenic photosynthesis (PS) will occur with an effectiveness that depends on the supply of visible photons with wavelengths between 400 and 700 nm. In this paper, we quantify the effectiveness of PS in two contexts that are relevant for M dwarfs. First, using photons from an M dwarf in its quiescent (nonflaring) state, we find that PS on an M dwarf planet in the habitable zone (HZ) of its parent star is less effective than on Earth by a factor of 10 for a flare star with mid-M spectral type. For a flare star with late-M spectral type, PS effectiveness (PSE) is smaller than on Earth by a factor of 100 or more. Second, using photons that are incident on the HZ planet during flares, we find that PSE can increase by factors of 5-20 above the quiescent values. In the case of a flare star with mid-M spectral type, we find that the PSE during a flare can increase up to as much as 50%-60% of the values on Earth. However, for a late-M flare star, even during flares, the PSE remains almost one order of magnitude smaller than on Earth. We suggest that for biological processes on M dwarf planets, the stellar activity cycle may replace the orbital period as the “year.”