On this day: the first planet around a Sun-like star

On 6 October 1995, Michel Mayor and Didier Queloz of the University of Geneva announced the discovery of a planet around the star 51 Pegasi, roughly 50 light-years from Earth in the constellation Pegasus. It is the first known planet around a main-sequence star, and the discovery became a turning point for astronomy.

The planet was found by the radial velocity method with the ELODIE spectrograph at the Haute-Provence Observatory in France. The instrument registered regular shifts in the star's spectral lines of about 70 metres per second, caused by the pull of an invisible companion. Just a week after the announcement, another group working at Lick Observatory in California confirmed the planet's existence.

51 Pegasi b turned out to be a hot gas giant. Its mass is about 0.6 times Jupiter's, and it completes one full orbit around its star in about four days. Its semi-major axis is only 0.052 astronomical units, meaning the planet is noticeably closer to its star than Mercury is to the Sun, and its orbital speed reaches 136 km/s. The temperature of its scorched side is estimated at about 1250 K, and the planet is most likely always turned toward the star with the same side.

In 1995, the close orbit of a gas giant did not fit theories of planet formation and looked like an anomaly. Over time, similar objects were set apart as a separate class, the hot Jupiters, and a mechanism of orbital migration was added to the theory. 51 Pegasi b became the prototype of this class.

In 2017, traces of water were found in the planet's atmosphere. In December 2015, the International Astronomical Union, following a public vote, gave it the name Dimidium, which in Latin means "half" and refers to its mass of about half that of Jupiter. For the discovery itself, Mayor and Queloz received the 2019 Nobel Prize in Physics.

Image: NASA/JPL-Caltech Public domain

Translation editor: Ловушкин А.