Theory of extrasolar giant planet transits

We present a synthesis of physical effcts influencing the observed light cu rve of an extrasolar giant planet (EGP) transiting its host star. The synth esis includes a treatment of Rayleigh scattering, cloud scattering, refract ion, 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 mo del for the atmosphere of EGP HD 209458b, we perform a fit to the best avai lable transit light-curve data and infer that this planet has a radius at a pressure of 1 bar, R-1, equal to 94,430 km, with an uncertainty of similar to 500 km arising from plausible uncertainties in the atmospheric temperat ure profile. We predict that R will be a function of wavelength of observat ion, with a robust prediction of at least +/-1% variations at infrared wave lengths where opacity in the high H2O EGP atmosphere dominates.