51 Pegasi b

51 Pegasi b, the first extrasolar planet confirmed to orbit a sunlike star. The planet orbits a fifth-magnitude star, 51 Pegasi, located 48 light-years away from Earth in the constellation Pegasus. 51 Pegasi has physical properties (luminosity and temperature, for example) very similar to those of the Sun. The extrasolar planet is not visible from Earth, but its presence was deduced in 1995 from the wobble that its gravity induces in the parent star’s motion in a 4.23-day cycle. Swiss astronomers Michel Mayor and Didier Queloz were the first to find a planet through such induced motions, and the discovery of 51 Pegasi b led to thousands more extrasolar planets being found. (Mayor and Queloz won the 2019 Nobel Prize for Physics for their discovery.) The planet orbits surprisingly close (7.8 million km [4.8 million miles]) to the star—much closer than Mercury orbits the Sun (at a distance of 57.9 million km [35.9 million miles]). It has a mass 46 percent that of Jupiter but has a radius 1.9 times that of Jupiter because it is inflated by its star’s heat. 51 Pegasi b was the first known “hot Jupiter,” a gas giant planet orbiting very close to its star. Such planets upended then-current ideas of planetary system formation, which were based on the solar system, in which gas giants orbit far from the Sun. The hot Jupiters likely formed far from their stars and migrated inward.