From 0 to 5,000 Planets in Exactly 20 Years

Illustration of 51 Pegasi b circling its star. Twenty years ago it was the first exoplanet detected; this year, it was the first seen directly by its reflected light. (Credit: ESO/M. Kornmesser/Nick Risinger) Twenty years ago today, an invisible object circling an obscure star in the constellation Pegasus overturned everything astronomers knew about planets around other stars. No, the fallout was even bigger than that. The indirect detection of 51 Pegasi b—the first planet ever found around a star similar to the sun—revealed that they had never really known anything to begin with. At the time, even the most adventurous minds blithely assumed that our solar system was more or less typical, a template for all the others. 51 Peg b threw a big splash of reality in their faces. The newfound world was bizarre, a Jupiter-size world skimming the surface of its star in a blistering-fast “year” that lasted just 4.2 days. Its existence ran counter to the standard theories of how planets form and evolve. It answered one big question: Yes, other planetary systems really do exist. But it raised a thousand others. Michel Mayor still sounds giddy as he recalls the profound confusion sparked by his discovery. In the fall of 1994, he and Didier Queloz, his graduate student at the University of Geneva, were trying out a brand-new spectrograph they had built, called ELODIE. It was designed to split up light in a way that can reveal the very precise motions of stars. Mayor and Queloz used ELODIE to search for subtle back-and-forth stellar dance steps caused by the gravitational tug of a planet or brown dwarf (bigger than a planet but not big enough to shine) orbiting around it. They succeeded almost right away, but the signal they detected from the star 51 Peg was like nothing they anticipated.