Lessons from Earth: Our guide to characterising irradiated habitable worlds

We have now found Earth-sized planets in the habitable zones of nearby stars; notably the TRAPPIST-1 system, Proxima Centauri-b and LHS-1140b. These are all in M star systems, which could present a challenge for habitability in the form of strong, frequent flares. Given their proximity, these M star systems are likely to be the first we characterise in detail. It is therefore important to explore how M stars shape the evolution of planetary atmospheres, how their flare activity brackets the surface UV environments on their planets, and how this affects planetary habitability and observable biosignatures.

We used a coupled 1D radiative-convective atmosphere code for rocky exoplanets (EXO-Prime) to estimate the UV surface environments of the TRAPPIST-1 planets and Proxima Centauri-b for flaring and non-flaring stellar models. We considered both oxygen-containing and anoxic atmospheres, accounting for erosion by exploring a range of different atmospheric densities and resulting surface pressures. If a planet can maintain atmospheric ozone, its UV surface environment can be milder than present-day Earth’s. However, if atmospheres are eroded and ozone is destroyed by frequent flaring, the biologically damaging high-energy UV flux can become orders of magnitude larger than on present-day Earth.

Yet even high surface UV environments may not preclude the habitability of planets. By exploring the methods employed by life to survive under harsh UV conditions we propose novel biosignatures that could be associated with habitable worlds orbiting active M stars, such as UV-protective biofluorescence.

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