Michel Mayor and Didier Queloz told an astronomy conference in Florence on October 6, 1995 that they had found a planet orbiting 51 Pegasi, a star very much like the sun and about 50 light years away. The planet had at least half the mass of Jupiter. What made it strange was its orbit: it went round its star every 4.2 days, which put it roughly eight times closer than Mercury is to the sun. No theory allowed it.
They never saw it. No telescope of the time could photograph one, and the star is far too bright for that. As the planet pulls on its star, the star moves a little toward earth and a little away, and that motion stretches and squeezes the wavelengths of the light it sends out, which a spectrograph of the right sensitivity can measure. The ELODIE spectrograph at the Haute-Provence Observatory in southern France could pick up a shift of about 70 metres per second, and the pattern Mayor and Queloz recorded repeated every 4.23 days.
Skepticism was immediate and reasonable. Astronomers had assumed giant planets could only form far out, where ice and rock survive, and two earlier claims of planets around ordinary stars had been withdrawn when nobody could reproduce them. Mayor and Queloz spent months checking their own data before saying anything. Geoff Marcy and Paul Butler, running a similar search in California, confirmed the finding within weeks, and two more hot Jupiters turned up before the year ended.
More than 5,000 planets around other stars have been confirmed since, most of them by the same method, and the existence of hot Jupiters forced a rewrite of how planetary systems form and then move. Mayor and Queloz shared the Nobel Prize in Physics in 2019. The discovery is usually taken as the moment when other worlds stopped being a subject for argument and became an ordinary branch of astronomy with a catalogue to keep.