We now know that planets are common around M dwarfs, and the 100 billion or so M dwarfs in our own Galaxy mean that the chance of there being "habitable" worlds is extremely high. But liquid water is not the only consideration for habitability. The magnetic fields of M dwarfs and their associated energetic radiation - both in the form of a plasma "wind" and UV to X-ray emission - are generally much stronger in comparison to their visible light than for solar-like stars. As a consequence, close-in planets around M dwarfs endure a distinctly more hostile space environment than the Earth does (see the posting of May 5, 2013 on the effect of a coronal mass ejection on a close-in planet). The plasma wind can erode and strip away planetary atmospheres - and with it any hope of habitable conditions.
We investigated this effect by simulating the wind of a magnetically-active M dwarf using a sophisticated magnetohydrodynamic numerical model. We examined the plasma conditions at the orbital locations of three recently discovered close-in exoplanets, and modelled what happened to the planetary magnetospheres. The planets transition in and out of regions where the wind is "super-Alfvenic" and "sub-Alfvenic" - magnetized plasma analogies of supersonic and subsonic. The super-Alfvenic conditions give rise to a bow shock directing the wind around the planet, much like in the Earth-Sun case. But in the sub-Alfvenic region, the wind is more pernicious and stealthy, penetrating deeper into the planetary magnetosphere and atmosphere. Under these conditions the atmosphere is vulnerable and liable to be eroded, spelling danger for habitability. We also found that heating by electric currents in the planetary magnetosphere is orders of magnitude higher than the terrestrial case, and is likely to be a significant modifier of upper atmospheric structure. Are these planets still habitable? Perhaps, pending more detailed atmospheric erosion calculations, but you might not want to live there. This work was lead by SAO scientist Ofer Cohen and was published in the 2014 July 20 edition of the Astrophysical Journal. See also the press release from the 2014 summer American Astronomical Society Meeting.
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