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A bad habitability?

posted Jul 29, 2014, 3:33 AM by Jeremy Drake
The search for habitable environments beyond our solar system is largely focused on locating Earth-like, rocky exoplanets.  The easiest such planets to find are those around M dwarf stars - the relatively faint but most numerous type of star in the Universe, with masses of a few tenths of that of the Sun and "cool" surface temperatures of 3000-4000 K or so.  Based on the most simple definition of habitability, in which the planet's temperature is just right for sustaining liquid water on its the surface, habitable planets around M dwarfs have to orbit 5-10 times closer to their parent star than the Earth does to the Sun in order to get warmed enough for water to be in liquid form.  These close-in orbits have short periods - a year comprising only a few days to a week or so - making planetary transits easier to detect and verify.  Close-in planets also induce a larger "wobble" of the central star through their gravitational interaction than planets further out.  This wobble can be detected by careful monitoring of the line-of-sight velocity of the star. 

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.