Fifty light-years away in the constellation Pegasus sits a sun-like star with a planet that, according to every physics textbook written before 1995, had no right to exist: a Jupiter-class gas giant so close to its star that a full year lasts just 4.23 days. This deep dive tells the story of 51 Pegasi b, now officially named Dimidium, from the October 1995 announcement by Michel Mayor and Didier Queloz to the Nobel Prize in Physics that recognized their discovery in 2019.

We explain how the radial velocity method and the ELODIE spectrograph detected a 70-meter-per-second stellar wobble across fifty light-years, why a gas giant beyond the frost line should never sit closer to its star than Mercury, and how the discovery forced the rapid development of orbital migration theory and the entire hot Jupiter class. We also follow decades of shifti ...