Hubble Space TelescopeTime‐Series Photometry of the Transiting Planet of HD 209458

We have observed 4 transits of the planet of HD 209458 using the STISspectrograph on HST. Summing the recorded counts over wavelength between 582 nmand 638 nm yields a photometric time series with 80 s time sampling andrelative precision of about 1.1E-4 per sample. The folded light curve can befit within observational errors using a model consisting of an opaque circularplanet transiting a limb-darkened stellar disk. In this way we estimate theplanetary radius R_p = 1.347 +/- 0.060 R_Jup, the orbital inclination i = 86.68+/- 0.14 degrees, the stellar radius R_* = 1.146 +/- 0.050 R_solar, and oneparameter describing the stellar limb darkening. Our estimated radius issmaller than those from earlier studies, but is consistent within measurementerrors, and is also consistent with theoretical estimates of the radii ofirradiated Jupiter-like planets. Satellites or rings orbiting the planet would,if large enough, be apparent from distortions of the light curve or fromirregularities in the transit timings. We find no evidence for eithersatellites or rings, with upper limits on satellite radius and mass of 1.2Earth radii and 3 Earth masses, respectively. Opaque rings, if present, must besmaller than 1.8 planetary radii in radial extent. The high level ofphotometric precision attained in this experiment confirms the feasibility ofphotometric detection of Earth-sized planets circling Sun-like stars.Comment: 28 pages, 9 figures. To be published in the Astrophysical Journa