Red-edge position of habitable exoplanets around M-dwarfs

TL;DR

This paper explores how the red-edge reflectivity feature of habitable exoplanets around M-dwarfs might shift to longer wavelengths over time due to the evolution of photosynthetic organisms adapting to different light environments.

Contribution

It presents a model of the evolution of photosynthesis on M-dwarf exoplanets, predicting a potential shift in the red-edge position from Earth-like to longer wavelengths.

Findings

Initial red-edge similar to Earth

Evolution of NIR photosynthesis on land

Potential shift to longer wavelength red-edge

Abstract

One of the possible signs of life on distant habitable exoplanets is the red-edge, which is a rise in the reflectivity of planets between visible and near-infrared (NIR) wavelengths. Previous studies suggested the possibility that the red-edge position for habitable exoplanets around M-dwarfs may be shifted to a longer wavelength than that for Earth. We investigated plausible red-edge position in terms of the light environment during the course of the evolution of phototrophs. We show that phototrophs on M-dwarf habitable exoplanets may use visible light when they first evolve in the ocean and when they first colonize the land. The adaptive evolution of oxygenic photosynthesis may eventually also use NIR radiation, by one of two photochemical reaction centers, with the other center continuing to use visible light. These "two-color" reaction centers can absorb more photons, but they will encounter difficulty in adapting to drastically changing light conditions at the boundary between land and water. NIR photosynthesis can be more productive on land, though its evolution would be preceded by the Earth-type vegetation. Thus, the red-edge position caused by photosynthetic organisms on habitable M-dwarf exoplanets could initially be similar to that on Earth and later move to a longer wavelength.