On the Radii of Extrasolar Giant Planets

Abstract

We have computed evolutionary models for extrasolar planets which range in\nmass from 0.1 to 3.0 Jovian Masses, and which range in equilibrium temperature\nfrom 113 K to 2000 K. We present four sequences of models, designed to show the\nstructural effects of a solid core and of internal heating due to the\nconversion of kinetic to thermal energy at pressures of tens of bars. The model\nplanetary radii are intended for comparisons with radii derived from\nobservations of transiting extrasolar planets. To provide such comparisons, we\nexpect that of order 10 transiting planets with orbital periods less than 200\ndays can be detected around bright (V<10) main-sequence stars for which\naccurate, well-sampled radial velocity measurements can be readily accumulated.\nThrough these observations, structural properties of the planets will be\nderivable, particularly for low-mass, high-temperature planets. Implications\nregarding the transiting companion to OGLE-TR-56 recently announced by Konacki\net al. are discussed.\n With regard to the confirmed transiting planet, HD 209458b, we find, in\naccordance with other recent calculations, that models without internal heating\npredict a radius that is 35 percent smaller than the observed radius. We\nexplore the possibility that HD 209458b owes its large size to dissipation of\nenergy arising from ongoing tidal circularization of the orbit. We show that\nresidual scatter in the current radial velocity data set for HD 209458b is\nconsistent with the presence of an as-of-yet undetected second companion, and\nthat further radial velocity monitoring of the star is indicated.\n