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

TL;DR

This paper addresses whether M-dwarf flares significantly enhance atmospheric escape from close-in Earth-sized and sub-Neptune planets. It employs a time-dependent photoionization hydrodynamics approach (ATES) with a flare-aware XUV flux model, incorporating two stellar activity phases (≈1 Gyr high, ≈4 Gyr low) and four orbital separations spanning the inner edge to the outer habitable zone around a GJ 436-like M-dwarf, plus analytical framing of evaporation regimes. The key finding is that flares contribute only modestly to cumulative mass loss (generally less than a factor of two), with larger relative enhancements at wider orbits due to the transition from energy-limited to recombination-limited evaporation, and there exists a characteristic flare energy that maximizes the flare-driven mass loss; rare super-flares do not alter this conclusion. These results imply that M-dwarf flare activity is not a dominant driver of atmospheric loss for close-in planets within the HZ and provide a physically grounded baseline for habitability assessments, while suggesting future work to explore a broader planetary parameter space and time-varying spectral energy distributions.

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.

Figures (5)

• Figure 1: Temporal evolution of atmospheric mass loss in response to stellar flares of different energies. The ratio of the instantaneous mass loss rate to the quiescent mass loss rate is shown for input flares ranging from $10^{31}$ erg (dark blue) to $10^{36}$ erg (dark red). Values are provided at the system's Hill radius. The top and bottom panels refer to the Earth- and sub-Neptune-sized planet, respectively.
• Figure 2: Fractional mass loss enhancement per flare (as given by Eq. \ref{['eq:DMsingleflare']}) as a function of flare energy for both the low-activity (red) and high-activity (blue) cases, each evaluated at four different orbital distances; the 0.18-0.36 AU range brackets the extent of the HZ for the chosen host star. The horizontal lines at the top denote the energy range over which the FFDs were observationally constrained Loyd_2018Loyd_2023. Our simulations extend to include flare energies up to $10^{36}$ erg. The left and right panels refer to the Earth- and sub-Neptune-sized planet, respectively.
• Figure 3: Fractional contributions to the flare-driven mass loss enhancement (Eq. \ref{['eq:DMflare']}) from different flare energies, shown for the high-activity (blue histogram) and low-activity (red) phase, and for four orbital separations. The left and right panels correspond to the Earth-sized planet and the sub-Neptune-sized planet, respectively.
• Figure 4: Ratio of the average efficiency of the outflow to the value of the quiescent efficiency as a function of the flare energy for the high-activity phase (blue) and the low-activity phase (red). Efficiencies are calculated using the analytical fitting formula given in Appendix A of Caldiroli_2022. The top and bottom panels correspond to the Earth-sized planet and the sub-Neptune-sized planet, respectively.
• Figure 5: The left-hand panels display one-year light curves corresponding to the high-activity stellar phase. The right-hand panel presents the flare occurrence rate as a function of energy over the same time interval.