51 Pegasi b: The First
In 1995 a giant planet was found skimming a Sun-like star every four days — the discovery that opened the exoplanet era and won a Nobel Prize.
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For centuries other solar systems were pure speculation. That ended in October 1995, when Michel Mayor and Didier Queloz announced a planet around 51 Pegasi, an ordinary Sun-like star 50 light-years away.
The planet — 51 Pegasi b, informally Dimidium — was a shock: a Jupiter-mass world orbiting its star in just 4.2 days, far closer than any giant was thought able to form. The first exoplanet around a normal star was nothing anyone predicted.
It opened the floodgates. Within years, dozens more 'hot Jupiters' turned up, planet-formation theory grew a chapter on migration, and in 2019 Mayor and Queloz shared the Nobel Prize in Physics.
51 Pegasi b was found by radial velocity: a clear ~4.2-day Doppler wobble revealing a planet of at least half Jupiter's mass orbiting perilously close to its star. It was the first confirmed planet around a main-sequence star other than the Sun.
The find was doubly important — it proved the radial-velocity technique worked, and it revealed a planet type (the hot Jupiter) that demanded orbital migration and rewrote formation theory. Its informal name, Dimidium, means 'half', for its minimum mass.
The 2019 Nobel Prize in Physics recognised Mayor and Queloz for the discovery — the moment the study of other worlds turned from philosophy into observational science.
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