Friday, June 21, 2013

1306.4789 (A. A. Vidotto et al.)

The effects of M dwarf magnetic fields on potentially habitable planets    [PDF]

A. A. Vidotto, M. Jardine, J. Morin, J. -F. Donati, P. Lang, A. J. B. Russell
We investigate the effect on potentially-habitable Earth-like planets of the magnetic fields of M dwarf (dM) stars. Such 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 use the sample of 15 active dM stars, for which surface magnetic field maps have been reconstructed, to determine the magnetic pressure at the planet's orbit and hence the minimum size of its magnetosphere, which would only be increased 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. With the exception of a couple of cases, to be able to sustain an Earth-sized magnetosphere, 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 a minimum 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 smaller. However, in this case, the polar cap area of the planet that is unprotected from transport of particles to/from interplanetary space is twice larger. As the star ages and its rotation rate and magnetic field reduce, the interplanetary magnetic pressure decreases and the magnetosphere of planets should become larger. Using an empirically-derived rotation-activity/magnetism relation,... (continues)
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