Suppression of the water ice and snow albedo feedback on planets orbiting red dwarf stars and the subsequent widening of the habitable zone

TL;DR

This study models how the reduced albedo of ice and snow at longer wavelengths around M-dwarf stars weakens the ice-albedo feedback, potentially extending the habitable zone for planets orbiting these stars.

Contribution

It provides the first detailed spectral modeling of cryospheric albedo effects on planets orbiting M-dwarfs, showing a weaker ice-albedo feedback and a wider habitable zone.

Findings

Ice and snow albedos are lower at wavelengths >1 μm, reducing feedback.

The habitable zone around M-stars may be 10-30% farther than previously estimated.

Surface temperatures are higher for icy planets orbiting M-stars at the same stellar flux.

Abstract

M-stars comprise 80% of main-sequence stars, and so their planetary systems provide the best chance for finding habitable planets, i.e.: those with surface liquid water. We have modelled the broadband albedo or reflectivity of water ice and snow for simulated planetary surfaces orbiting two observed red dwarf stars (or M-stars) using spectrally resolved data of the Earth's cryosphere. The gradual reduction of the albedos of snow and ice at wavelengths greater than 1 ?m, combined with M-stars emitting a significant fraction of their radiation at these same longer wavelengths, mean that the albedos of ice and snow on planets orbiting M-stars are much lower than their values on Earth. Our results imply that the ice/snow albedo climate feedback is significantly weaker for planets orbiting M-stars than for planets orbiting G-type stars such as the Sun. In addition, planets with significant ice and snow cover will have significantly higher surface temperatures for a given stellar flux if the spectral variation of cryospheric albedo is considered, which in turn implies that the outer edge of the habitable zone around M-stars may be 10-30% further away from the parent star than previously thought.