The Productivity of Oxygenic Photosynthesis around Cool, M Dwarf Stars

Abstract

In the search for life around cool stars, the presence of atmospheric oxygen is a prominent biosignature, as it may indicate oxygenic photosynthesis (OP) on the planetary surface. On Earth, most oxygenic photosynthesizing organisms (OPOs) use photons between 400 and 750 nm, which have sufficient energy to drive the photosynthetic reaction that generates O2 from H2O and CO2. OPOs around cool stars may evolve similar biological machinery capable of producing oxygen from water. However, in the habitable zones (HZs) of the coolest M dwarf stars, the flux of 400–750 nm photons may be just a few percent that of Earth’s. We show that the reduced flux of 400–750 nm photons around M dwarf stars could result in Earth-like planets being growth limited by light, unlike the terrestrial biosphere, which is limited by nutrient availability. We consider stars with photospheric temperatures between 2300 and 4200 K and show that such light-limited worlds could occur at the outer edge of the HZ around TRAPPIST-1-like stars. We find that even if OP can use photons longer than 750 nm, there would still be insufficient energy to sustain the Earth’s extant biosphere throughout the HZ of the coolest stars. This is because such stars emit largely in the infrared and near-infrared, which provide sufficient energy to make the planet habitable, but limits the energy available for OP. TRAPPIST-1f and g may fall into this category. Biospheres on such planets, potentially limited by photon availability, may generate small biogenic signals, which could be difficult for future observations to detect.

Does the Evolution of Complex Life Depend on the Stellar Spectral Energy Distribution?

“…Instead, this hypothesis suggests that late-K and M-dwarf stars are unlikely to develop complex life until a more distant time in the future. Such a conclusion remains consistent with previous analyses that approach the problem from other perspectives (Loeb et al 2016;Ramjan et al 2017;Lingam and Loeb 2018a,b;Lehmer et al 2018).…”

Colloquium: Physical constraints for the evolution of life on exoplanets

“…However, it was also argued therein that oxygenic photosynthesis might not evolve on planets around M-dwarfs as this process accords minimal competitive advantage. Lehmer et al (2018) investigated whether enough photons would be available to support an Earth-like biosphere on M-dwarf exoplanets, and found that many of them are incapable of doing so; see also Lingam and Loeb (2019d). In particular, if the maximum wavelength of PAR is specified to be 750 nm, none of the TRAPPIST-1 planets in the HZ appear to have the capacity for sustaining Earth-like biospheres.…”

Prospects for Life on Temperate Planets around Brown Dwarfs

“…The real issue, however, is that the net primary productivity on these worlds will be photonlimited in addition to other constraints imposed by access to nutrients and water. 8 Drawing an analogy with M-dwarf exoplanets (Lehmer et al 2018;Lingam & Loeb 2019d), it is possible that planets around brown dwarfs could likewise possess anoxic atmospheres (with negligible ozone layers) despite the presence of biospheres and thereby give rise to "false negatives" during searches for biological O 2 and O 3 .…”