Using Stellar Limb-Darkening to Refine the Properties of HD 209458b

We use multi-band photometry to refine estimates for the planetary radius and orbital inclination of the transiting planet system HD 209458. We gathered 1066 spectra over four distinct transits with the STIS spectrometer on the Hubble Space Telescope using two gratings with a resolution R = 1500 and a combined wavelength range of 290− 1030 nm. We divide the spectra into ten spectrophotometric bandpasses, five for each grating, of equal wavelength span within each grating, and fit a transit curve over all bandpasses simultaneously. In our fit we use theoretical values for the stellar limb-darkening to further constrain the planetary radius. We find that the radius of HD 209458b is 1.320± 0.025 RJup, which is a factor of two more precise than current estimates. We also obtain improved estimates for the orbital period P and time of center of transit TC . Although in principle the photon-limited precision of the STIS data should allow Alfred P. Sloan Research Fellow