UV Surface Habitability of the TRAPPIST-1 System

With the discovery of rocky planets in the temperate habitable zone (HZ) of the close-by cool star TRAPPIST-1 the question of whether such planets could harbour life arises. Habitable planets around red dwarf stars can orbit in radiation environments that can be life-sterilizing. UV flares from these stars are more frequent and intense than solar flares. Additionally, their temperate HZs are closer to the star. Here we present UV surface environment models for TRAPPIST-1's HZ planets and explore the implications for life. TRAPPIST-1 has high X-ray/EUV activity, placing planetary atmospheres at risk from erosion. If a dense Earth-like atmosphere with a protective ozone layer exists on planets in the HZ of TRAPPIST-1, UV surface environments would be similar to present-day Earth. However an eroded or an anoxic atmosphere, would allow more UV to reach the surface, making surface environments hostile even to highly UV-tolerant terrestrial extremophiles. If future observations detect ozone in the atmospheres of any of the planets in the HZ of TRAPPIST-1, these would be interesting targets for the search for surface life. We anticipate our assay to be a starting point for in-depth exploration of stellar and atmospheric observations of the TRAPPIST-1 planets to constrain their UV-surface-habitability.