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 $R$. 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. We predict that $R$ will be a function of wavelength of observation, with a robust prediction of variations of $\pm 1$% at infrared wavelengths where H$_2$O opacity in the high EGP atmosphere dominates.