A limit on the presence of Earth-mass planets around a Sun-like star

We present a combined analysis of all publicly available visible Hubble Space Telescope observations of transits of the planet HD 209458b. We derive the times of transit, planet radius, inclination, period, and ephemeris. We then use the times of transit to place constraints on the existence of secondary planets in the system. We show that planets near an Earth-mass can be ruled out in a low-order mean-motion resonance, while planets less than an Earth-mass are ruled out in an interior, 2:1 resonance. We also present a combined analysis of the transit times and 68 high-precision radial velocity measurements of the system. These results are compared to theoretical predictions for the constraints that can be placed on secondary planets.