The X-ray evidence that the 51 Peg companion is a planet

Abstract

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° 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 × 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. Steven Beckwith