51 Pegasi b

Michel Mayor and Didier Queloz spotted 51 Pegasi b in 1995, a giant planet circling a star much like our Sun. They shared half of the 2019 Nobel Prize in Physics for the discovery.

Earlier planets had been found around a dead star, but this was the first around a Sun-like one.

Context

51 Pegasi b orbits a star about 50 light-years away in the constellation Pegasus. The planet is roughly half the mass of Jupiter, yet it circles its star in only about four days, far closer than Mercury is to our Sun. This made it the prototype for a class later called hot Jupiters, which surprised astronomers because no such planet exists in our own solar system.

Mayor and Queloz found it using the radial velocity method. They measured tiny, regular shifts in the star's light caused by the planet's gravity tugging it back and forth. They used a spectrograph at the Haute-Provence Observatory in France and announced the result in October 1995, and other teams quickly confirmed it.

The discovery changed ideas about how planets form and move. It suggested that planets can migrate inward after forming, and it triggered a rush of searches that has since turned up thousands of worlds.

The 2019 Nobel Prize in Physics was split: Mayor and Queloz received one half for the exoplanet discovery, and James Peebles received the other half for theoretical work in cosmology.