51 Pegasi b

On 6 October 1995, at a conference in Florence, two Swiss astronomers, Michel Mayor and Didier Queloz, announced a planet around 51 Pegasi, a Sun-like star about 50 light-years away. It was the first planet ever found around an ordinary star like the Sun. Twenty-four years later it won them the Nobel Prize in Physics.

A wobble in a star

The planet itself has never been seen. Mayor and Queloz detected it through its pull on the star. As a planet orbits, the star swings in a small circle in response, moving slightly towards us and then away, and that motion shifts the colour of its light back and forth. In spring 1994 they had started using a new spectrograph at the Haute-Provence Observatory in France that could measure stellar speeds to 10 or 15 metres per second. 51 Pegasi was swinging by about 59 metres per second, regularly, every 4.23 days.

Nobody expected it

The result made little sense at the time. Jupiter-sized planets were thought to form far from their stars, where it is cold enough for ice, and to take years to complete an orbit. This one had at least half Jupiter’s mass and circled its star in four days, at 0.052 AU, about a seventh of Mercury’s distance from the Sun. Almost no one believed it at first. Then Geoffrey Marcy and Paul Butler in the United States confirmed the wobble, and a new kind of planet, the hot Jupiter, had to be explained. The answer that emerged was migration: giant planets can form far out and then move inward.

First, but not quite the very first

51 Pegasi b was not the first planet found beyond the Solar System. In 1992 Aleksander Wolszczan and Dale Frail had found planets around a pulsar, the remains of an exploded star. But it was the first around a star like the Sun, and the Nobel committee gave Mayor and Queloz half of the 2019 prize “for the discovery of an exoplanet orbiting a solar-type star”.

In 2015 the planet and its star were given names proposed from Switzerland, in a public vote run by the International Astronomical Union: the star became Helvetios and the planet Dimidium, Latin for half, because it has about half Jupiter’s mass.

Half a Jupiter, more or less

Because the planet does not pass in front of its star, its size has never been measured, and the wobble method alone gives only a minimum mass, about 0.47 Jupiters. In 2017 astronomers detected water vapour in its atmosphere by its signature in the starlight. In 2026 measurements with the CRIRES+ instrument on ESO’s Very Large Telescope confirmed the water, worked out how the orbit is tilted, and put the planet’s true mass at about 0.61 Jupiters, higher than earlier estimates. A 2015 claim to have seen light reflected from the planet has not held up: later studies found it reflects very little light at all.