The Planet around 51 Pegasi^*

Doppler measurements of 51 Pegasi have been made from 1995 October through 1996 August, with a precision of 5 m s^-1. We find a period of 4.231 days, a velocity amplitude of 56 ± 1 m s^-1, and a velocity curve that is essentially sinusoidal, all in excellent agreement with Mayor & Queloz. The only viable interpretation is a companion having minimum mass, m sin i = 0.45 M_Jupiter, in a circular orbit of radius of 0.051 AU, with an eccentricity less than 0.01. Alternative explanations involving stellar surface phenomena such as pulsation or spots are ruled out. The lack of tidal spin-up of the star constrains the mass of the companion to be less than 15 M_Jupiter. If the tidal Q-value is less than ~10⁶ for the planet (close to Jupiter's presumed value), then internal dissipation is adequate to circularize the orbit and synchronize the planet's rotation. After subtracting the best-fit Keplerian velocity curve, the residuals exhibit no apparent variations at a level of 5 m s^-1 during 10 months. The absence of further reflex motion along with limits from IR speckle observations rule out additional companions in a large portion of the parameter space of mass and orbital radius, including all masses greater than 1 M_Jupiter within 2.0 AU.