Confirmation of Velocity Variations of 51 Peg

Geoff Marcy & Paul Butler
U.C. Berkeley
San Francisco State Univ.

On Oct. 6, Michel Mayor and Didier Queloz at Geneva Observatory announced that 51 Pegasus has a planet orbiting it. If confirmed, this would be the first detected planetary system around a normal star. 51 Peg is a Solar-Like star, 40 light years away.

We have obtained 27 independent spectra of 51 Peg using the 3-meter telescope at Lick Observatory. The observations span 4 days. The goal was to detect the wobble of 51 Peg in response to the gravitational pull of the supposed planet. Velocities of stars are derived from the Doppler effect, as a part of our long-term search for planetary systems. Our Doppler technique yields a velocity precision of 3 m/s --- human walking speed.

We find that the velocities of 51 Peg during 4 nights are sinusoidal, with an amplitude of 106 m/s, peak to peak (53 m/s semiamplitude). The RMS residual (scatter) is 2.5 m/s from the sine wave, consistent with our errors. We find a period of 4.2 days which is presumably the orbital period of the unseen companion as it gravitationally pulls its host star around in a circle.

We therefore confirm the sinusoidal velocity variations reported by Michel Mayor and Didier Queloz of Geneva Observatory. The orbital period of 4.2 days is exactly what they reported. Our velocity amplitude of 53 m/s compares favorably with their measured amplitude of 59 m/s, within their errors.

The best, and straightforward, interpretation is that 51 Peg is orbited by an unseen companion having a mass of about 0.5 Jupiter masses, and orbiting the star at a distance of 1/20 the Earth-Sun distance.

The unknown inclination of the orbital plane leaves some uncertainty in the mass of the planet; it could be somewhat larger than 0.5 Jupiter Masses.

Many question remain unanswered. How did a Jupiter-like planet form so close to its host star? Can a planet really survive so close, since its surface temperature would be about 1000 Centigrade? Are there other planets orbiting 51 Peg?