51 Pegasi b was the exoplanet whose discovery earned Michel Mayor and Didier Queloz a share of the 2019 Nobel Prize in Physics.
Mayor and Queloz announced the planet in 1995 after detecting a periodic wobble in the motion of the Sun-like star 51 Pegasi. They used the radial-velocity method, measuring tiny shifts in the star’s spectral lines caused by the planet’s gravitational pull. This was the first confirmed discovery of an exoplanet orbiting a main-sequence star similar to the Sun.
51 Pegasi b is a hot Jupiter, a gas giant orbiting extremely close to its star. It completes an orbit in roughly four Earth days, far faster than Jupiter’s approximately 12-year orbit around the Sun. Its unexpected location challenged earlier assumptions that gas giants formed only far from their stars.
The 2019 Physics Nobel recognized the discovery of an exoplanet orbiting a solar-type star, along with cosmological discoveries by James Peebles.