2026/02/24 by Giovanni Covone, Amedeo Balbi · 1 voice
Physics and Astronomy · #Astronomy and Astrophysical Research #Astrophysics and Star Formation Studies #Stellar, planetary, and galactic studies #astro-ph.EP
paper · pdf · doi:10.1093/mnras/stag414
openalex created_date 2026/03/01 · openalex publication_date 2026/03/09 · openalex updated_date 2026/07/28
ABSTRACT Photosynthesis is central to Earth’s biosphere and a prime candidate for sustaining complex life on habitable exoplanets, yet a thermodynamically consistent treatment of the work potential of stellar radiation at planetary surfaces is still lacking. We develop a radiative-thermodynamic framework that quantifies the maximum useful work (exergy) extractable for a given star–planet configuration and yields exergy-based bounds on photosynthetic power and long-wavelength absorption cutoffs. From these we derive kinetically constrained red limits for high-Δ G photochemistry and apply them to Earth-like planets receiving the same bolometric flux from FGK and M blackbody hosts, computing thresholded photon supplies and truncated exergy fluxes below a photosystem II red limit. For such planets the constraints confine single-photon oxygenic photosynthesis to near-infrared bands around Solar-type stars and to somewhat bluer wavelengths around late M dwarfs. Integrated over the stellar spectrum, the thresholded photon supply and truncated exergy available to drive a photosystem water-oxidation step are larger by factors ∼ 5 around FGK hosts than around T_⋆ ≈ 3000 K M dwarfs. For the Solar–Earth system, the exergy-based upper bound on O2 production exceeds the observed O2 throughput by several orders of magnitude, consistent with Earth’s photosynthetic efficiencies. Cool M dwarfs suffer a double penalty: fewer photons above threshold and a lower shortwave exergy fraction, yielding systematically tighter ceilings on high-Δ G photosynthesis than around FGK stars. Our framework provides upper limits on photosynthetically harvestable power on habitable-zone planets and enables comparisons of photosynthetic potential across exoplanetary systems, and can be extended to multiband photosystems.