## Unseen planet orbits the star 51 Pegasi

Fig. 16.8 . Discovery data for the first planet found orbiting a normal star other than the Sun. The giant, unseen planet is a revolving around the solar-type star 51 Pegasi, located 50 light-years away. The radial velocity of the star, in units of meters per second, has been measured from the Doppler shift of the star’s spectral lines. The velocity exhibits a sinusoidal variation with a 4.23-day period, caused by the invisible planetary companion that orbits 51 Pegasi with this period. The observational data (solid dots) are fit with the solid line, whose amplitude implies that the mass of the companion is roughly 0.46 times the mass of Jupiter. The 4.23-day period indicates that the unseen planet is orbiting 51 Pegasi at a distance of 0.05 AU, where 1.00 AU is the mean distance between the Earth and the Sun. [Adapted from Michael Mayor and Didier Queloz, "A Jupiter-mass companion to a solar-type star", Nature 378, 355-359 (1995).]

Copyright 2010, Professor Kenneth R. Lang, Tufts University