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Destroyers of worlds - are planets around close binary stars doomed?

posted Apr 14, 2013, 1:44 PM by Jeremy Drake   [ updated Jun 3, 2013, 12:01 PM ]
In the late 80's and early 90's, mysterious excess infrared emission was reported around a particular class of stars called RS CVn's.  These are close binary stars with orbital periods of only a few days, and orbital separations of only a few stellar radii.  The infrared excess was reminiscent of the discovery of "debris disks" around stars by the joint US, UK and Netherlands Infrared Astronomical Satellite (IRAS).  
The debris are the remains of the protoplanetary disk phase characterising the first few million years of the life of a star like the Sun, in which planetary systems are born.  The debris dust is warmed by the light from the central star to temperatures of a hundred to a thousand or so degrees - hotter for close-in dust and cooler for dust further away.   The dust glows in the infrared and produces the excess emission over that expected from the star itself.  The debris then betrays the likely presence of some kind of planetary system around a star.  

The problem with the RS CVn's having such a disk is that the debris are gradually eroded over time, both outward by radiation pressure, and inward by what is essentially drag from radiation pressure ("Poynting-Robertson drag") and the stellar wind.  By ages of few hundred million years, the debris is generally gone leaving only a very faint residual, like that seen in the zodiacal light of our own solar system.  
RS CVn-type binaries are considerably older than this though - with ages of a billion years or more - and their dust must have been formed relatively recently in stellar terms.  This raised a serious question of whether the original detections of excess infrared emission were real.  Doubts were expressed that the IRAS data were not of sufficient quality to reliably distinguish the relatively small signal of dust and debris over the much brighter stellar light.  To test this, we observed a small sample of RS CVn-type stars using the Spitzer Space Telscope, which is much more sensitive than its predecessor IRAS.  We found conclusive proof of infrared excesses in three stars, betraying the presence of surprisingly warm dust with temperatures in excess of a thousand degrees Kelvin. This dust must have been replenished or created recently.  The most promising way to do this is through collisions of rocky bodies - planetesimals or planets.  But why would this be happening in an old planetary system?  Over about a billion years or so, RS CVn stars gradually spiral in toward each other because they lose angular momentum to outflowing magnetised stellar winds.  We think the gradual change in the separation of the central binary renders inner planetary orbits unstable, leading to chaos and  planetary collisions.  The RS CVn's are generally old enough for their planets to have developed life. The implication, then, is that any life that develops in such systems is likely doomed as it is gradually thrown into chaos by the inspiral of its parent stars.  See the Astrophysical Journal paper and the press release..