51 Pegasi b

Lore & Background

The exoplanet's discovery was announced at a conference on October 6, 1995, and published in the journal Nature on November 23, 1995, by Michel Mayor and Didier Queloz of the University of Geneva. They used the radial velocity method with the ELODIE spectrograph on the Observatoire de Haute-Provence telescope in France, detecting slight velocity changes in the star's spectral lines of 59 ± 3 m/s. The planet was later confirmed by another team using the Lick Observatory in California, though not within a week of the original announcement. After discovery, many teams confirmed the planet's existence and obtained more observations. It was discovered that the planet orbits its star in around four days, much closer than Mercury is to the Sun, with a minimum mass about half that of Jupiter. At the time, the presence of a huge world so close to its star was not compatible with theories of planet formation and was considered an anomaly. Since then, numerous other hot Jupiters have been discovered, leading astronomers to revise their theories by studying orbital migration. The planet was originally designated 51 Pegasi b by Mayor and Queloz. In December 2015, the International Astronomical Union announced the winning name Dimidium, submitted by the Astronomische Gesellschaft Luzern, Switzerland; 'Dimidium' is Latin for 'half', referring to the planet's minimum mass of approximately half the mass of Jupiter.

Reader's Guide

51 Pegasi b holds immense significance as the first exoplanet discovered orbiting a main-sequence star, fundamentally changing astronomy and planetary science. Its discovery by Michel Mayor and Didier Queloz, awarded the 2019 Nobel Prize in Physics, opened the door to the study of exoplanets and revealed a new class of worlds: hot Jupiters. These gas giants orbiting extremely close to their stars challenged existing theories of planet formation, prompting the development of orbital migration models. The planet's physical characteristics—such as its likely inflated radius due to intense heating, tidal locking, and potential silicate clouds—have made it a key target for atmospheric studies, including the 2017 detection of water traces. Its legacy endures as a benchmark for understanding planetary systems beyond our own.

Did You Know?

• 51 Pegasi b was the first planet discovered orbiting a main-sequence star.
• The planet orbits its star in around four days, much closer than Mercury is to the Sun.
• In 2019, the Nobel Prize in Physics was awarded in part for the discovery of 51 Pegasi b.
• The official name Dimidium is Latin for 'half', referring to the planet's minimum mass of approximately half that of Jupiter.

The Discovery That Rewrote Astronomy

On October 6, 1995, Michel Mayor and Didier Queloz of the University of Geneva published a landmark announcement in the journal Nature, revealing a planet orbiting 51 Pegasi — a Sun-like main-sequence star roughly 50 light-years away in the constellation Pegasus. Their detection relied on the radial velocity technique: the ELODIE spectrograph mounted on the Observatoire de Haute-Provence telescope in France measured tiny, periodic shifts of about 70 metres per second in the star's spectral lines, produced by the gravitational tug of a world sitting only seven million kilometres from its host. The result made world headlines almost immediately, and within a single week an independent team at California's Lick Observatory confirmed the signal. The finding was extraordinary because no planet had previously been identified around a star of the Sun's type. Twenty-four years later, in 2019, the Nobel Prize in Physics was awarded in part to Mayor and Queloz for this breakthrough, cementing the discovery's place as a watershed moment in the history of astronomy.

A World That Defied Expectation

When the discovery first broke, such a configuration was flatly incompatible with prevailing models of how planets form, and the object was widely regarded as an anomaly. Over the years, however, astronomers have identified numerous similar worlds, including those around 55 Cancri and τ Boötis, and the field has shifted toward explaining their presence through orbital migration. The planet is now understood to be a gas giant rather than the stripped brown-dwarf core once hypothesised. The interior gases are so hot the planet would glow red, and silicate clouds may drift through the upper atmosphere. It is tidally locked, forever showing the same face to its star, and in 2017 traces of water were detected in its atmosphere.

From Designation to Dimidium

When Michel Mayor and Didier Queloz first reported the planet in 1995, it carried the straightforward catalogue label 51 Pegasi b, derived from the Flamsteed designation of its host star. The following year, astronomer Geoffrey Marcy gave it an informal mythological nickname — Bellerophon, the Greek hero who tamed the winged horse Pegasus — following a tradition of drawing on classical figures for planetary names. For nearly two decades that was as far as the naming went. In July 2014 the International Astronomical Union launched NameExoWorlds, a public campaign inviting nominations and votes for official names of select exoplanets and their stars. The Astronomische Gesellschaft Luzern, the Astronomical Society of Lucerne in Switzerland, submitted the entry that ultimately won. In December 2015 the IAU announced the result: Dimidium. The word is Latin for 'half,' a direct nod to the planet's mass of approximately half a Jupiter. The name thus encodes both a cultural choice and a physical fact, linking the object's identity to one of its most defining characteristics.

The Albedo Puzzle and the Search for Reflected Light

Since its discovery, 51 Pegasi b has been a target for direct-detection efforts that have produced conflicting results. A 2015 study using the HARPS instrument at the European Southern Observatory's La Silla site in Chile claimed to detect the planet in visible light, which would have implied a true mass of 0.46 Jupiter masses, a high albedo, and a radius as large as 1.9 ± 0.3 Jupiter radii — painting the picture of a dramatically inflated hot Jupiter. By 2021, however, that optical detection could not be replicated, pointing instead to an albedo below 0.15. The same year, a marginally detected polarised reflected-light signal appeared, though without firm assumptions about scattering mechanisms it could not constrain the albedo. The current consensus favours a low-albedo world with a radius around 1.2 ± 0.1 Jupiter radii. The planet also remains a candidate for aperture polarimetry with Planetpol and for near-infrared characterisation using the VLTI Spectro-Imager, keeping it in the queue for future observational campaigns.