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The nearby ultracool dwarf TRAPPIST-1 possesses several Earth-sizedterrestrial planets, three of which have equilibrium temperatures that maysupport liquid surface water, making it a compelling target for exoplanetcharacterization. TRAPPIST-1 is an active star with frequent flaring, withimplications for the habitability of its planets. Superflares (stellar flareswhose energy exceeds 10^33 erg) can completely destroy the atmospheres of acool star's planets, allowing ultraviolet radiation and high-energy particlesto bombard their surfaces. However, ultracool dwarfs emit little ultravioletflux when quiescent, raising the possibility of frequent flares being necessaryfor prebiotic chemistry that requires ultraviolet light. We combine Evryscopeand Kepler observations to characterize the high-energy flare rate ofTRAPPIST-1. The Evryscope is an array of 22 small telescopes imaging the entireSouthern sky in g' every two minutes. Evryscope observations, spanning 170nights over 2 years, complement the 80-day continuous short-cadence K2observations by sampling TRAPPIST-1's long-term flare activity. We updateTRAPPIST-1's superflare rate, finding a cumulative rate of 4.2 (+1.9 -0.2)superflares per year. We calculate the flare rate necessary to deplete ozone inthe habitable-zone planets' atmospheres, and find that TRAPPIST-1's flare rateis insufficient to deplete ozone if present on its planets. In addition, wecalculate the flare rate needed to provide enough ultraviolet flux to powerprebiotic chemistry. We find TRAPPIST-1's flare rate is likely insufficient tocatalyze some of the Earthlike chemical pathways thought to lead to RNAsynthesis, and flux due to flares in the biologically relevant UV-B band isorders of magnitude less for any TRAPPIST-1 planet than has been experienced byEarth at any time in its history.

Evryscope and K2 Constraints on TRAPPIST-1 Superflare Occurrence and Planetary Habitability