Effects of M dwarf magnetic fields on potentially habitable planets

TL;DR

This study assesses how magnetic fields of M dwarf stars impact the atmospheres of orbiting Earth-like planets, revealing that strong stellar magnetism can significantly compress planetary magnetospheres and influence habitability.

Contribution

It introduces a fast method to evaluate the effect of stellar magnetic fields on planetary magnetospheres using magnetic maps of M dwarfs, and quantifies the magnetic field requirements for habitability.

Findings

Earth-like planets near M dwarfs have magnetospheres up to 6-11.7 planetary radii.

Planets need stronger magnetic fields or farther orbits to maintain Earth-sized magnetospheres.

Older stars with weaker magnetic fields allow larger planetary magnetospheres.

Abstract

We investigate the effect of the magnetic fields of M dwarf (dM) stars on potentially habitable Earth-like planets. These fields can reduce the size of planetary magnetospheres to such an extent that a significant fraction of the planet's atmosphere may be exposed to erosion by the stellar wind. We used a sample of 15 active dM stars, for which surface magnetic-field maps were reconstructed, to determine the magnetic pressure at the planet orbit and hence the largest size of its magnetosphere, which would only be decreased by considering the stellar wind. Our method provides a fast means to assess which planets are most affected by the stellar magnetic field. We show that hypothetical Earth-like planets with similar terrestrial magnetisation (1G) orbiting at the inner (outer) edge of the habitable zone of these stars would present magnetospheres that extend at most up to 6 (11.7) planetary radii. To be able to sustain an Earth-sized magnetosphere, with the exception of only a few cases, the terrestrial planet would either (1) need to orbit significantly farther out than the traditional limits of the habitable zone; or else, (2) if it were orbiting within the habitable zone, it would require at least a magnetic field ranging from a few G to up to a few thousand G. By assuming a magnetospheric size that is more appropriate for the young-Earth (3.4Gyr ago), the required planetary magnetic fields are one order of magnitude weaker. However, in this case, the polar-cap area of the planet, which is unprotected from transport of particles to/from interplanetary space, is twice as large. As the star becomes older and, therefore, its rotation rate and magnetic field reduce, the interplanetary magnetic pressure decreases and the magnetosphere of planets probably expands. Using an empirically derived rotation-activity/magnetism relation,... (continues)