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We investigate the effect of the magnetic fields of M dwarf (dM) stars onpotentially habitable Earth-like planets. These fields can reduce the size ofplanetary magnetospheres to such an extent that a significant fraction of theplanet's atmosphere may be exposed to erosion by the stellar wind. We used asample of 15 active dM stars, for which surface magnetic-field maps werereconstructed, to determine the magnetic pressure at the planet orbit and hencethe largest size of its magnetosphere, which would only be decreased byconsidering the stellar wind. Our method provides a fast means to assess whichplanets are most affected by the stellar magnetic field. We show thathypothetical Earth-like planets with similar terrestrial magnetisation (1G)orbiting at the inner (outer) edge of the habitable zone of these stars wouldpresent magnetospheres that extend at most up to 6 (11.7) planetary radii. Tobe able to sustain an Earth-sized magnetosphere, with the exception of only afew cases, the terrestrial planet would either (1) need to orbit significantlyfarther out than the traditional limits of the habitable zone; or else, (2) ifit were orbiting within the habitable zone, it would require at least amagnetic field ranging from a few G to up to a few thousand G. By assuming amagnetospheric 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 unprotectedfrom transport of particles to/from interplanetary space, is twice as large. Asthe star becomes older and, therefore, its rotation rate and magnetic fieldreduce, the interplanetary magnetic pressure decreases and the magnetosphere ofplanets probably expands. Using an empirically derivedrotation-activity/magnetism relation,... (continues)

Effects of M dwarf magnetic fields on potentially habitable planets