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 shifting science, from a brown dwarf hypothesis to the 2017 detection of water vapor and a reversal on the planet’s reflectivity and size.

• How the Doppler shift in starlight reveals a planet you can never photograph directly
• The planet’s extreme stats: 136 kilometers per second orbital speed, about half of Jupiter’s mass, and a puffed-up, glowing atmosphere
• Tidal locking and hydrostatic equilibrium: why one face is scorched and why the atmosphere is not stripped away
• The albedo controversy, from a 2015 claim of a highly reflective 1.9 Jupiter-radius world to later studies pointing to a dark planet around 1.2 Jupiter radii
• From Bellerophon to Dimidium: how the IAU NameExoWorlds vote gave the planet its Latin name meaning half