Why M-dwarf flares have limited impact on the atmospheric evaporation of sub-Neptunes and Earth-sized planets

TL;DR

This study shows that M-dwarf stellar flares have a limited effect on the atmospheric evaporation of close-in Earth-sized and sub-Neptune planets, suggesting their atmospheres are resilient despite stellar activity.

Contribution

The paper introduces a detailed time-dependent hydrodynamic model to quantify the impact of stellar flares on planetary atmospheric escape, revealing minimal overall effects.

Findings

Flares cause less than a twofold increase in atmospheric mass loss.

The impact of flares is greater at larger orbital distances.

A characteristic flare energy maximizes the contribution to mass loss.

Abstract

M-type stars are prime targets for exoplanet searches within their habitable zones (HZs). These stars also exhibit significant magnetic flaring activity, particularly during their first billion years, which can potentially accelerate the evaporation of the hydrogen-helium envelopes of close-in planets. We employ the time-dependent photoionization hydrodynamics code ATES to investigate the impact of flares on atmospheric escape, focusing on an Earth-sized and a sub-Neptune-sized planet orbiting an early M-type star at distances of 0.01, 0.1, and 0.18-0.36 AU-the inner and outer edges of the HZ. Stellar flaring is modeled as a 1 Gyr-long high-activity phase followed by a 4 Gyr-long low-activity phase, each characterized by an appropriate flare frequency distribution. We find that flares have a modest impact-less than a factor of two-on the cumulative atmospheric mass loss, with the greatest absolute enhancement occurring when the planets are at their closest separation. However, the relative enhancement in mass loss between flaring and non-flaring cases is greater at larger orbital separations. This trend arises because, as stellar irradiation fluctuates between quiescent levels and peak flares, the proportion of time that a planet spends in the energy-limited versus recombination-limited mass loss regimes depends on its orbital separation. Additionally, we demonstrate the existence of a characteristic flare energy, intermediate between the minimum and maximum values, that maximizes the fractional contribution to flare-driven mass loss. Our results indicate that the flaring activity of M-dwarfs does not significantly affect the atmospheric retention of close-in planets, including those within the HZ. The potential occurrence of rare super-flares, which current observational campaigns may be biased against, does not alter our conclusions.