On the Radii of Extrasolar Giant Planets

We have computed evolutionary models for extrasolar planets which range inmass from 0.1 to 3.0 Jovian Masses, and which range in equilibrium temperaturefrom 113 K to 2000 K. We present four sequences of models, designed to show thestructural effects of a solid core and of internal heating due to theconversion of kinetic to thermal energy at pressures of tens of bars. The modelplanetary radii are intended for comparisons with radii derived fromobservations of transiting extrasolar planets. To provide such comparisons, weexpect that of order 10 transiting planets with orbital periods less than 200days can be detected around bright (V<10) main-sequence stars for whichaccurate, well-sampled radial velocity measurements can be readily accumulated.Through these observations, structural properties of the planets will bederivable, particularly for low-mass, high-temperature planets. Implicationsregarding the transiting companion to OGLE-TR-56 recently announced by Konackiet al. are discussed. With regard to the confirmed transiting planet, HD 209458b, we find, inaccordance with other recent calculations, that models without internal heatingpredict a radius that is 35 percent smaller than the observed radius. Weexplore the possibility that HD 209458b owes its large size to dissipation ofenergy arising from ongoing tidal circularization of the orbit. We show thatresidual scatter in the current radial velocity data set for HD 209458b isconsistent with the presence of an as-of-yet undetected second companion, andthat further radial velocity monitoring of the star is indicated.Comment: 23 pages, 3 figures, accepted by Astrophysical Journa