Effects of M dwarf magnetic fields on potentially habitable planets

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, which can be used as a first study to be followed by more sophisticated models. We show that hypothetical Earth-like planets with similar terrestrial magnetisation (∼1 G) 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 (3.4 Gyr 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. At present, we do not know how small the smallest area of the planetary surface is that could be exposed and would still not affect the potential for formation and development of life in a planet. 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, we provide an analytical expression for estimating the shortest stellar rotation period for which an Earth-analogue in the habitable zone could sustain an Earth-sized magnetosphere. We find that the required rotation rate of the early-and mid-dM stars (with periods 37-202 days) is slower than the solar one, and even slower for the late-dM stars ( 63-263 days). Planets orbiting in the habitable zone of dM stars that rotate faster than this have smaller magnetospheric sizes than that of the Earth magnetosphere. Because many late-dM stars are fast rotators, conditions for terrestrial planets to harbour Earth-sized magnetospheres are more easily achieved for planets orbiting slowly rotating early-and mid-dM stars.

The effects of stellar winds on the magnetospheres and potential habitability of exoplanets

“…It is also interesting to compare our results to those of Vidotto et al (2013), whose results we have plotted in Fig. 1 with square points.…”

“…Our results suggest that a level of protection comparable to the early Earth's should be possible for planets orbiting stars of age greater than roughly 1.5 Gyr under the Cranmer & Saar model and almost immediately after a star enters the main sequence according to the Parker model. The result is striking when compared to that of Vidotto et al (2013). Most terrestrial planet searches focus on M dwarfs because their low luminosities and masses are ideal for the transit and radial velocity techniques.…”

“…While most of the planets can maintain a Paleoarchean magnetosphere, it is still beneficial for them to have larger magnetospheres since the auroral opening, through which atmospheric leakage occurs, shrinks for larger magnetospheres (Tarduno et al 2010;Vidotto et al 2013). It would therefore be useful to identify the observational signatures of stars with weaker winds.…”

HADES RV Programme with HARPS-N at TNG

“…Moreover, from an observational point of view, the chances of finding an Earth-like planet in the habitable zone of a star increase as the stellar mass decreases. Still, habitability is not guaranteed simply by an assessment of the distance from the star: several other factors, such as stellar activity, may move and/or shrink the habitability zone of a star (see Vidotto et al 2013, and references therein). Thus, it is crucial to better understand the activity of M dwarfs and how it can affect the circumstellar environment.…”

Stellar magnetic activity and exoplanets

“…The host star magnetism can have effects on planetary mass-loss [3,4], habitability [5,6], and on the magnetospheres of exoplanets [7], to name a few. It has also been suggested that exoplanets, in particular those orbiting close to the star, can also affect stellar activity, through tidal interactions and/or magnetic reconnection events [8][9][10][11][12].…”