What class of close-orbiting gas giant is 51 Pegasi b the prototype for?

51 Pegasi b is the prototype for the class of close-orbiting gas giants called hot Jupiters.

Michel Mayor and Didier Queloz announced its discovery on 6 October 1995 after detecting the star’s periodic motion with the radial-velocity method. The planet’s gravity makes its Sun-like host star wobble, producing shifts in the star’s spectral lines. The signal revealed a Jupiter-mass companion completing an orbit in only about 4.23 days.

That combination was startling: 51 Pegasi b is roughly half Jupiter’s mass but circles its star at about 0.053 astronomical units, far closer than Mercury’s orbit around the Sun. Such a large planet so near its star did not fit comfortably with the prevailing picture of planet formation, helping trigger new work on orbital migration.

The planet is officially named Dimidium, but “51 Pegasi b” remains the familiar scientific designation. It was the first exoplanet found orbiting a Sun-like main-sequence star and helped establish hot Jupiters as a major planetary category.