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Paper: Are Short-Period Extrasolar Planets Unipolar Inductors?
Volume: 323, Star Formation in the Interstellar Medium: In Honor of David Hollenbach, Chris McKee, and Frank Shu
Page: 393
Authors: Dong, S.; Lin, D.N.C.
Abstract: We address the possibility of short-period extrasolar planets as unipolar inductors. At main-sequence phase, the host stars may possess magnetic fields with strength comparable to that of the sun. By "cutting the field lines", the orbit motion of a short-period "hot Jupiter" around the star can induce an emf of order 107 volts across its radial diameter in the coordinate frame fixed to the host star. This voltage difference drives a current along the magnetic flux tube which passes through the planet. The ohmic heating of the induction current within the planets thus provides a significant energy source, which can alter the planet's internal structure and inflate its size. The torque associated with this induced current derives from the differential rotation between the stellar spin and the planet's orbit, and it drives the planet toward co-rotation. Most planet- search target stars are slow rotators and this process causes the orbit of their planets to decay, which provides a natural explanation of the absence of planets with less than 3 days orbital period. However, at the T Tauri phase, young host stars normally spin much faster and possess a much stronger magnetic field. The magnetic interaction due to unipolar induction causes an outward torque. This torque can effectively stop the orbital migration of the planet. A natural consequence of the unipolar induction model is the periodic radio bursts due to coherent cyclotron radiation modulated by the planet. It could be employed as another signature of short-period extrasolar planets.
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