Surface and Oceanic Habitability of Trappist-1 Planets under the Impact of Flares

Abstract

The discovery of potentially habitable planets around the ultracool dwarf star Trappist-1 naturally poses the question: could Trappist-1 planets be home to life? These planets orbit very close to the host star and are most susceptible to the UV radiation emitted by the intense and frequent flares of Trappist-1. Here, we calculate the UV spectra (100-450 nm) of a superflare observed on Trappist-1 with the K2 mission. We couple radiative transfer models to this spectra to estimate the UV surface flux on planets in the habitable zone of Trappist-1 (planets e, f, and g), assuming atmospheric scenarios based on a prebiotic and an oxygenic atmosphere. We quantify the impact of the UV radiation on living organisms on the surface and on a hypothetical planet ocean. Finally, we find that for non-oxygenic planets, UV-resistant life-forms would survive on the surface of planets f and g. Nevertheless, more fragile organisms (i.e., Escherichia coli) could be protected from the hazardous UV effects at ocean depths greater than 8 m. If the planets have an ozone layer, any life-forms studied here would survive in the habitable zone planets.

Keywords: Astrobiology; Exoplanets; Habitability; Ocean planet; Ozone.