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Who Run the World? (Flares): Ozone production in steam atmospheres during M-star superflares

Presentation #625.04 in the session Planetary Atmospheres - Terrestrial Planets and Mini-Neptunes.

Published onApr 03, 2024
Who Run the World? (Flares): Ozone production in steam atmospheres during M-star superflares

The majority of planets that we have observed within a system’s habitable zone have an M-type host star, which are long-lived, smaller, and cooler stars compared to our Sun. Most terrestrial planets suitable for atmospheric characterization orbit M stars due to their high signal to noise ratios, short orbital periods and high occurrence rates. However, M-type stars are notorious for their frequent stellar flares, during which the star can brighten by more than 10,000 times its usual total brightness, with even greater enhancements in the FUV wavelengths that cause heating in planetary atmospheres. In addition to such flaring events, terrestrial planets residing within the habitable zones of these stars are also likely to experience an early runaway greenhouse phase, which can lead to loss of hydrogen from water vapor due to heightened XUV fluxes during the star’s youth before it reaches its main sequence. While the detection of atmospheric O2 is a key item of interest in the search for life, these steam atmospheres could lead to O2 atmospheres containing up to hundreds bars, resulting in high O3 atmospheres. Previous models have considered the effects of M type flares on Earth-like atmospheres. However, the effects of these flares on early steam atmospheres have not yet been considered. We consider the effects of both pre-main sequence high UV activity and one main sequence superflare on three known habitable zone terrestrial planets around M-type stars: TRAPPIST 1-e, 1-f, and Proxima Centauri b. For each planet, we consider three different initial water abundances: 0.1, 1, and 10 Earth oceans. Across each test case, we find that each planet produces orders of magnitude more ozone than the modern Earth, with slight exceptions for the 10 ocean cases at the bottom of those atmospheres. As a result, there is negligible XUV and FUV and very little NUV reaching the surface of these planets. Because the synthesis of genetic material requires UV to reach the surface, our model results show that terrestrial planets around M-type stars are not conducive to life at 1 Ga, though may allow UV to reach the surface through continued flaring events

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