What is the official IAU name of 51 Pegasi b?

The official IAU name of 51 Pegasi b is Dimidium. The International Astronomical Union approved the name in December 2015 through its NameExoWorlds public naming process.

The name is Latin for “half,” referring to the planet’s approximate mass of half that of Jupiter. Before the official name was adopted, the object was generally known by its scientific designation, 51 Pegasi b, based on the Flamsteed designation of its host star and the lowercase letter used for the first known planet in that system.

51 Pegasi b became famous after its discovery was announced in 1995 by Michel Mayor and Didier Queloz. It was the first exoplanet found orbiting a Sun-like main-sequence star and helped establish the existence of hot Jupiters—large planets orbiting extremely close to their stars.

The names Saffar, Dagon and Aegir belong to other exoplanet naming contexts, so they are not alternate official names for this planet. “Bellerophon” was also used informally for a time, but it was never the IAU-approved name.