Evryscope And K2 Constraints On TRAPPIST-1 Superflare Occurrence and Planetary Habitability


Updated flare frequency distribution of TRAPPIST-1. Yellow points are flares observed by K2. No flares were observed by the Evryscope.

The nearby ultracool dwarf TRAPPIST-1 possesses several Earth-sized terrestrial planets, three of which have equilibrium temperatures that may support liquid surface water, making it a compelling target for exoplanet characterization. TRAPPIST-1 is an active star with frequent flaring, with implications for the habitability of its planets.

Superflares (stellar flares whose energy exceeds 10^33 erg) can completely destroy the atmospheres of a cool star's planets, allowing ultraviolet radiation and high-energy particles to bombard their surfaces. However, ultracool dwarfs emit little ultraviolet flux when quiescent, raising the possibility of frequent flares being necessary for prebiotic chemistry that requires ultraviolet light.

We combine Evryscope and Kepler observations to characterize the high-energy flare rate of TRAPPIST-1. The Evryscope is an array of 22 small telescopes imaging the entire Southern sky in g' every two minutes. Evryscope observations, spanning 170 nights over 2 years, complement the 80-day continuous short-cadence K2 observations by sampling TRAPPIST-1's long-term flare activity.

We update TRAPPIST-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 in the habitable-zone planets' atmospheres, and find that TRAPPIST-1's flare rate is insufficient to deplete ozone if present on its planets.

In addition, we calculate the flare rate needed to provide enough ultraviolet flux to power prebiotic chemistry. We find TRAPPIST-1's flare rate is likely insufficient to catalyze some of the Earthlike chemical pathways thought to lead to RNA synthesis, and flux due to flares in the biologically relevant UV-B band is orders of magnitude less for any TRAPPIST-1 planet than has been experienced by Earth at any time in its history.

Amy L. Glazier, Ward S. Howard, Hank Corbett, Nicholas M. Law, Jeffrey K. Ratzloff, Octavi Fors, Daniel del Ser
Comments: 12 pages, 9 figures. Accepted in The Astrophysical Journal subject to revisions
Subjects: Earth and Planetary Astrophysics (astro-ph.EP); Solar and Stellar Astrophysics (astro-ph.SR)
Cite as: arXiv:2006.14712 [astro-ph.EP] (or arXiv:2006.14712v1 [astro-ph.EP] for this version)
Submission history
From: Amy Glazier
[v1] Thu, 25 Jun 2020 21:31:28 UTC (900 KB)
https://arxiv.org/abs/2006.14712
Astrobiology

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