  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)
