Lessons from early Earth: UV surface radiation should not limit the habitability of active M star systems

Abstract

The closest potentially habitable worlds outside our Solar system orbit a different kind of star than our Sun: smaller red dwarf stars. Such stars can flare frequently, bombarding their planets with biologically damaging high-energy UV radiation, placing planetary atmospheres at risk of erosion and bringing the habitability of these worlds into question. However, the surface UV flux on these worlds is unknown. Here we show the first models of the surface UV environments of the four closest potentially habitable exoplanets: Proxima-b, TRAPPIST-1e, Ross-128b, and LHS-1140b assuming different atmospheric compositions, spanning Earth-analogue to eroded and anoxic atmospheres and compare them to levels for Earth throughout its geological evolution. Even for planet models with eroded and anoxic atmospheres, surface UV radiation remains below early Earth levels, even during flares. Given that the early Earth was inhabited, we show that UV radiation should not be a limiting factor for the habitability of planets orbiting M stars. Our closest neighbouring worlds remain intriguing targets for the search for life beyond our Solar system.

This article has been accepted for publication in MNRAS, published by Oxford University Press on behalf of the Royal Astronomical Society

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• No Massive Companion to the Coherent Radio-Emitting M Dwarf GJ 1151
• Finding Signs of Life in Transit: High-resolution Transmission Spectra of Earth-like Planets around FGKM Host Stars
• High-resolution reflection spectra for Proxima b and Trappist-1e models for ELT observations
• Stratospheric Clouds Do Not Impede JWST Transit Spectroscopy for Exoplanets with Earth-Like Atmospheres
• High-Resolution spectral models of TRAPPIST-1e seen as a Pale Blue Dot for ELT and JWST observations
• Observability of ultraviolet N I lines in the atmosphere of transiting Earth-like planets