  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.
