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Planets can affect the spin-down of their host stars

posted Apr 15, 2013, 6:28 PM by Jeremy Drake   [ updated Jun 3, 2013, 11:59 AM ]
The first planets detected were in the Jupiter class, orbiting very close to their host stars within a few stellar radii.  At such close proximity, these gas giants are heated to much higher temperatures than Jupiter itself, and have been nicknamed "hot jupiters".  Studies of the rotation period of the stars hosting these hot jupiters suggested that they rotate faster than stars without giant close-in planets.  One possibly explanation for this is that the planet can help spin up the star, or rather, prevent it from slowing down so much, through tidal interactions.
Once the star slows to a rotation period similar to the orbital period of the planet, tidal friction can tend to arrest the slowdown, with angular momentum subsequently sapped from the planetary orbit to sustain faster stellar spin. 

Another possibility illustrated here is that the expected magnetic fields of close-in planets can interact with those of the parent star, affecting the stellar wind that causes the spin-down. We investigated this using the same magnetohydrodynamic modelling techniques and code that is routinely applied to the Sun and space weather within our own solar system.  See also the post on the wound up magnetosphere of AB Doradus for more using similar modelling.  We found that the presence of a close-in planet could reduced the spin-down of the parent star by factors of up to 4.  This happens because  the mass loss rate is reduced by up to a factor of 2 due to the planetary magnetic field impeding the flow, and because the planetary and stellar magnetic fields change the global topology and "magnetic lever arm" through which the wind applies the slowing torque,  The figure shows the magnetic field structure of the star-planet system in the vertical plane joining them; the star is in the centre and the plant to the bottom right, The results were published in the 2010 October 10 edition of the Astrophysical Journal Letters.