51 Pegasi B
KeywordsRadial-velocity planet Exoplanet Hot Jupiters
51 Pegasi b is an extrasolar planet orbiting the solar-type star 51 Pegasi.
51 Pegasi b is the first Jupiter-type planet, with a minimum mass slightly smaller than half of that of Jupiter, discovered around a Sunlike star. It was detected by radial-velocity observations obtained with the Elodie spectrometer on the 1.93-m telescope at the Observatoire de Haute Provence in France (Mayor and Queloz 1995). The extraordinarily short period of 4.230785 ± 0.000036 days was completely unexpected for a planet with a mass close to that of Jupiter and led to some initial skepticism that the unseen companion of 51 Pegasi could be a gas giant planet (e.g., Gray 1997). However, the subsequent announcement of several other radial-velocity planet candidates soon convinced most people of the reality of extrasolar planets.
51 Pegasi is classified as a G dwarf similar to the Sun but slightly cooler, at a distance of 15.6 parsecs (50.9 light years). A periodic variation in the radial velocity of the star indicates an unseen companion with minimum mass of 0.472 ± 0.039 Jupiter masses, if the orbit is viewed edge-on, but the actual orbital inclination is not well established. The orbit is nearly circular, with an eccentricity of 0.013 ± 0.012, presumably the result of circularization by tidal forces. The proximity of the planet to its star, 20 times closer than the Earth to the Sun, implies a temperature on the order of 1,200 K. 51 Pegasi b is the prototype for a population of hot Jupiters that probably formed much farther from their parent stars, where conditions were cool enough for a gas giant planet to form, followed by migration into a much tighter orbit around the host star.