The Time-Dependent Effect of a Stellar Flare on Terrestrial Planet Habitability and Biosignatures

Astronomy and Astrophysics – Astronomy

Scientific paper

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Scientific paper

Due to their low stellar luminosities, M dwarf habitable zones (as defined by Kasting et al. 1993) lie very near the star ( 0.2 AU or less), making planets in the habitable zone especially vulnerable to the effects of stellar activity. Although M dwarfs emit the bulk of their flux in the optical and near infrared, activity on these stars produces energetic radiation, from X rays to ultraviolet (UV), that may be dangerous for life on a planet in the habitable zone (HZ) of the star. In particular, stellar activity is a concern for the continuity of habitability on the planetary surface, as starspots or flares may cause the stellar irradiance to vary strongly with time.
Using a convective/radiative model coupled to a photochemical model, we simulated the atmosphere of an Earth-like planet located in the habitable zone of the active M dwarf AD Leo over the course of a large flare. We present the time-dependent atmospheric temperature and composition profiles for water, methane and ozone, finding that while water and ozone are photolyzed in the stratosphere, the temperature profile and methane column depth are relatively unperturbed. Ozone number density decreases in the stratosphere during the impulsive phase of the flare, but quickly recovers to preflare levels thereafter. While the UV flux at the planetary surface changes with the ozone concentration during the flare, we find that the planetary surface UV flux is less than that received on Earth's surface except during the very peak of the flare. We conclude that even large flares may not be detrimental to life on planets with Earth-like atmospheres.

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