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)
Highlights
M dwarf stars have been the prime targets for terrestrialplanet searches for two reasons: (1) because they are inherently low-luminosity objects, they provide a good contrast to detect smaller-radius planets in transit searches and (2) because they are low-mass objects, the reflex motion induced by a terrestrial planet is within reach of current spectrograph sensitivities in radial velocity searches
Another interesting aspect of dM stars is that the region where life could potentially develop is located significantly closer to dM stars than it is for solar-type stars (Kasting et al 1993; Selsis et al 2007)
We quantitatively evaluated the extent of planetary magnetospheres as a result of the pressure of intense stellar magnetic fields found around M dwarf stars
Summary
M dwarf (dM) stars have been the prime targets for terrestrialplanet searches for two reasons: (1) because they are inherently low-luminosity objects, they provide a good contrast to detect smaller-radius planets in transit searches and (2) because they are low-mass objects, the reflex motion induced by a terrestrial planet is within reach of current spectrograph sensitivities in radial velocity searches. The extent of the magnetosphere of planets orbiting in the HZ of dM stars have been investigated by other authors (e.g. Grießmeier et al 2005, 2009; Khodachenko et al 2007; Lammer et al 2007; Vidotto et al 2011b,c), but, to the best of our knowledge, a detailed analysis of the particular contribution of the stellar magnetic field remains to be made Part of this limitation is justified by the fact that it was only recently that the large-scale magnetic field of dM stars was reconstructed for the first time (Donati et al 2006).
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