The search for biosignatures of exoplanets is guided by whether the planetary environment can sustain photosynthesis.
Therefore, the photosynthetic habitable zone (PHZ) has recently been proposed as an overlap between the standard habitable zone and the orbital range where the stellar irradiance is sufficient to promote photosynthesis. Existing PHZ estimates rely on empirical light response curves from terrestrial phytoplankton and therefore contain an implicit geocentric bias.
We present an agnostic PHZ derived from a generalized model of photosynthesis based on thermodynamics and redox chemistry, without reference to model organisms. This model is built on the general photochemical reaction in which photon capture couples the oxidation of a donor molecule and the reduction of CO.2.
Optical properties and CO2 The reduction rate is optimized for the irradiance spectrum of an exoplanet orbiting a main-sequence star using a genetic algorithm that mimics evolution by natural selection. Our simulations predict that photosynthetic organisms compensate for the reduction in light flux by evolving larger light-harvesting structures.
As a result, photosynthetic viability decreases only linearly with orbital distance, even though the stellar flux decreases quadratically. Therefore, the agnostic PHZ extends far beyond previous Earth-based estimates. Earth-like (visible light) oxygenic photosynthesis is limited in luminous flux outside the habitable zone of cool M dwarfs. However, both anoxic photosynthesis and the hypothetical near-infrared light-driven oxygenic photosynthesis are viable throughout the habitable zones of M, K, and G stars.
This means that M-dwarf exoplanets can sustain strong oxygenic photosynthesis, but they exhibit reflectance biosignatures in the NIR band rather than visible light, unlike those seen on Earth.
Callum Gray, Cassandra Hall, Stefano Santabarbara, Klaus Schmidt-Lohr, Andrew Lingham, Edward Gillen, Thomas J. Howarth, Christopher DP Duffy
Comments: 17 pages of text, 5 figures. Submitted to MNRAS
Subject: Earth and Planetary Astrophysics (astro-ph.EP). Astrophysical Instruments and Methods (astro-ph.IM); Machine Learning (cs.LG)
Quote: arXiv:2606.24458 [astro-ph.EP] (or arXiv:2606.24458v1 [astro-ph.EP] for this version)
https://doi.org/10.48550/arXiv.2606.24458
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Birthplace: Callum Gray
[v1] Tuesday, June 23, 2026 11:45:54 UTC (3,925 KB)
https://arxiv.org/abs/2606.24458
Astrobiology, astronomy, exoplanets, biosignatures,
