ar X iv : a st ro - p h / 01 01 02 4 v 2 9 J un 2 00 1 Theory of Extrasolar Giant Planet Transits

We present a synthesis of physical effects influencing the observed lightcurve of an extrasolar giant planet (EGP) transiting its host star. The synthesis includes a treatment of Rayleigh scattering , cloud scattering, refraction, and molecular absorption of starlight in the EGP atmosphere. Of these effects, molecular absorption dominates in determining the transit-derived radius R, for planetary orbital radii less than a few AU. Using a generic model for the atmosphere of EGP HD209458b, we perform a fit to the best available transit lightcurve data, and infer that this planet has a radius at a pressure of 1 bar, R 1 , equal to 94430 km, with an uncertainty of ∼ 500 km arising from plausible uncertainties in the atmospheric temperature profile. We predict that R will be a function of wavelength of observation, with a robust prediction of variations of at least ±1% at infrared wavelengths where H 2 O opacity in the high EGP atmosphere dominates.