The impact of stellar winds and tidal locking effects on the habitability of Earth-like exoplanets around M dwarf stars

TL;DR

The study investigates how magnetic and ram pressures from M-dwarf stellar winds shape the magnetospheric shielding of Earth-like exoplanets in the HZ, and how tidal locking and departures from the Parker spiral affect habitability. It combines a 30-star M-dwarf sample with Kopparapu-based HZ boundaries and a magnetopause-pressure balance framework, exploring three cases: magnetic pressure only, ram pressure only, and both, across slow and fast winds. Key findings show ram pressure dominates for the lowest-mass stars, while magnetopause currents can boost shielding in some higher-mass cases; even then, Earth-sized magnetospheres are rare, especially under Case 3, and most planets are likely tidally locked, reducing dynamo effectiveness depending on rotational evolution. The results highlight significant implications for atmospheric retention and habitability around M dwarfs, underscoring the importance of wind physics, tidal effects, and planetary magnetic field strength in assessing exoplanet habitability near low-mass stars.

Abstract

We present an assessment of the effects of stellar wind magnetic and mechanical components on the habitability of Earth-like exoplanets orbiting the inner and outer radii of the habitable zone (HZ) of M dwarfs. We consider stars with masses in the range of $0.09 - 0.75 M_\odot$ and planets with a surface dipolar magnetic field of 0.5 G. We estimate the size of the magnetospheres of such exoplanets using the pressure balance equation including the contribution of magnetic and ram pressures from stellar winds. We explore different scenarios, including fast and slow stellar winds, to assess the relevance of kinetic contribution. Furthermore, the effect of tidal locking and potential deviations from the Parker spiral, typically used to describe the interplanetary magnetic field, are analyzed. We show that for low mass stars ($M < 0.15 M_\odot$), the ram pressure exerted by stellar winds affects the size of the magnetosphere more than the stellar wind magnetic pressure. Interestingly, when the ram pressure is not much stronger than the magnetic pressure, typically for higher mass stars, the inclusion of ram pressure can be beneficial to the magnetosphere due to the magnetopause currents. A magnetosphere with the size of that of modern Earth is difficult to achieve with the current assumptions. However, an early Earth magnetosphere is achieved by roughly half of our hypothetical planets orbiting the outer radius of the HZ in most of the considered cases. We find that deviations from the Parker spiral can affect the results significantly, reducing the magnetosphere by $56\%$ in extreme cases. Most of the hypothetical planets are most likely (or might be) tidally locked, with the notable exception of those orbiting the outer HZ of GJ 846 and V1005 Ori.

Figures (5)

• Figure 1: The magnetopause standoff distance of an Earth-like planet ($B_\mathrm{p,0} = 1$ G) orbiting the 30 M dwarfs from our sample, estimated in Case 1 (left panels), Case 2 (middle panels) and Case 3 (right panels), for slow ($v_\mathrm{w} = v_\mathrm{esc}$) and fast ($v_\mathrm{w} = 3v_\mathrm{esc}$) winds. The results of Vidotto2013 were also added for comparison. In the upper (bottom) panels the planet is orbiting the outer (inner) radius of the HZ. The dot marker indicates that the star is in the sample of Vidotto2013, and the x marker that it was not included in their work.
• Figure 2: Differences between the three considered cases, $\Delta_\mathrm{(1-3)} r_\mathrm{M}(r_\mathrm{orb})$, $\Delta_\mathrm{(1-2)} r_\mathrm{M}(r_\mathrm{orb})$ and $\Delta_\mathrm{(2-3)} r_\mathrm{M}(r_\mathrm{orb})$ following Eqs (\ref{['delta1']})-(\ref{['delta3']}). The blue (orange) dots represent planets orbiting the inner (outer) radii of the HZ. The results for the slow wind are given on the panels on the left, the results for the fast wind on the right.
• Figure 3: The magnetopause standoff distance of a planet orbiting our stellar sample. The theoretical decay of the Parker model (blue) and that measured in the solar system (red) are used in the estimations for Case 1 (left panels) and for Case 3, fast wind (right panels). In the upper (bottom) panels the planet is orbiting the outer (inner) radius of the HZ. The gray dashed line represents Earth's early magnetosphere. The results of Vidotto2013 were added for comparison.
• Figure 4: The timescale for tidal locking estimated for an Earth-like planet orbiting the inner and outer radii of the HZ around our stellar sample, following Eq. (\ref{['TL1']}). Due to the uncertainty of the tidal dissipation parameter $Q$, we use the current value (left panel), and an estimation based on the historical average (right panel). The vertical dashed lines represent the different considered regimes.
• Figure 5: Magnetopause distance as a function of the stellar radius of representative stars from our sample. The blue and orange lines represent Case 1 (only stellar magnetic pressure is considered) and Case 3 (stellar magnetic and ram pressures are considered) with slow wind, respectively. The red line is Case 1 with the alternative magnetic field decay $B_r \propto r^{-5/3}$. The solid green area is the HZ of the star. The gray horizontal dashed lines indicate the size of the magnetosphere of early ($5 r_\mathrm{p}$) and modern Earth ($11.7 r_\mathrm{p}$). The black vertical dotted lines represent the different tidal locking regimes assuming $Q=100$ (see Section \ref{['TL']}).