51 Pegasi b

51 Pegasi b is the first ever exoplanet discovered and also the first discovered class of planets we now call Hot Jupiters. Although it has a much lower mass than Jupiter, it is probably bigger in size, as its atmosphere is superheated (surface temperature of ~1000°C) by its host star. It is likely that the planet was formed much farther away from its host star than from where it is today, and migrated towards its host star through one of a few possible migration mechanisms.