1996NewA....1..171P

<P> 51 Pegasi is a nearby G2–3 V star which is similar to the Sun. Mayor & Queloz (1995) [Natur, 378, 355] have recently found that 51 Peg has sinusoidal radial velocity variations with a period of 4.2 days and amplitude of 59 m/s. The radial velocity, if due to orbital motion around the system center-of-mass, implies a minimum companion mass of 0.47 Jupiter masses, which results from the assumption that the inclination angle of the system is 90 deg from the line of sight. However, because the inclination angle is not directly measured there remains an uncertainty in the companion mass. In the limit of a near-zero inclination angle the companion could even be a late-type dwarf star. We argue that the low measured X-ray luminosity of the 51 Peg system supports the conclusion that the companion is a planet, independent of any assumption about the inclination angle. If 51 Peg were a binary stellar system with a 4-day orbital period its X-ray emission would be in marked contrast with ALL known binary stellar systems with similar orbital periods. When compared to the distribution of binary star X-ray luminosities, a system with the 51 Peg X-ray luminosity have an occurrence probability of only 1.7 times 10^-6. The low X-ray luminosity also confirms that 51 Peg is a slow rotator based upon the correlation between X-ray emission and rotational velocity. We discuss the lack of synchronization between the 51 Peg orbital and rotational periods and calculate model-dependent upper limits on the companion mass which also indicate that it is a planet.</P>