Detection of planetary transits across a sun-like star

We report high-precision, high-cadence photometric measurements of the star HD 209458, which is known from radial velocity measurements to have a plan etary-mass companion in a close orbit. We detect two separate transit event s at times that are consistent with the radial velocity measurements. In bo th cases, the detailed shape of the transit curve due to both the limb dark ening of the star and the finite size of the planet is clearly evident. Ass uming stellar parameters of 1.1 R. and 1.1 M., we find that the data are be st interpreted as a gas giant with a radius of 1.27 +/- 0.02 R-Jup in an or bit with an inclination of 87 degrees 1 +/- 0 degrees 2. We present values for the planetary surface gravity, escape velocity, and average density and discuss the numerous observations that are warranted now that a planet is known to transit the disk of its parent star.