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Can close-in planets survive coronal mass ejections?

posted May 5, 2013, 6:40 AM by Jeremy Drake   [ updated Jun 3, 2013, 11:58 AM ]
The first exoplanets discovered were Jupiter-like gas giants, in close orbits with their parent stars.  These planets live in a harsh environment, not only heated and evaporated by their star's light, from infrared to X-ray wavelengths, but also buffeted by the million degree plasma wind driven by the star's magnetic activity.  This wind, just like the solar wind but orders of magnitude stronger because of close proximity, can scour away the planet's outer atmosphere, leading to significant mass loss over billions of years.  The survival, or not, of close-in planets is also central to understanding if cool, red M dwarf stars can host habitable planets.  For planets to be warm enough to sustain liquid water on their surface they must orbit much closer to faint M dwarfs than we do to the Sun.  The problem is that the magnetic activity of M dwarfs is not so faint - close in planets will be lashed by the intense wind and explosive flaring of the parent star.

We carried out a supercomputer numerical simulation of the effects of a coronal mass ejection (CME) on a close-in giant planet in an extrasolar system. The CME was driven in a similar manner to state-of-the-art simulations of solar coronal mass ejections aimed at understanding their influence on the Earth and its magnetosphere.  The first image shows the CME, like an immense magnetic plasma fist, just prior to impact on the planet, whose magnetosphere, distended and drawn out by the stellar wind in the direction away from the star, is shown in cyan.  We gave the planet a magnetic field strength a bit smaller than that of the present day Earth and were fairly surprised to find that the CME did not do too much damage to the planetary atmosphere.  The second image shows the gas density distribution in the orbital plane just after the CME front has passed the planet. A magnetic field strength even 10 times lower than that of the Earth still provides, at face value, adequate protection for the particular event simulated. However the physics of atmospheric escape is very complicated and it will take some sophisticated future studies to determine if potentially habitable planets around M dwarfs can hang on to their atmospheres.  This study was lead by postdoc Ofer Cohen and was published in the 2010 September 11 edition of the Astrophysical Journal.