A TRANSITING '5l PEG--LIKE' PLANET

摘要

Doppler measurements from Keck exhibit a sinusoidal periodicity in the velocities of the G0 dwarf HD 209458, having a semiamplitude of 8l m/s and a period of 3.5239 days, which is indicative of a "5l Peg--like" planet with a minimum mass (M sin i) of 0.62 M_jup.. and a semimajor axis of 0.046 AU. Follow-up photometry reveals a drop of 0.0l7 mag at the predicted time (within the errors) of transit by the companion based on the velocities. This is the first extrasolar planet observed to transit its star The radius of the planet derived from the magnitude of the dimming is l .42 R_jup, which is consistent with models of irradiated Jupitermass planets. The transit implies that sin i > 0.993, leading to a true mass of 0.62 M_jup, for the planet. The resulting mean density of 0.27 g cm 3 implies that the companion is a gas giant.