51 Pegasi b
 

51 Pegasi b (abbreviated 51 Peg b), sometimes though unofficially named Bellerophon, is an extrasolar planet approximately 50 light-years away in the constellation of Pegasus. 51 Pegasi b was the first planet to be discovered orbiting a main sequence star, the Sun-like 51 Pegasi (the first exoplanets at all were discovered in 1992 by Aleksander Wolszczan around pulsar PSR 1257), and marked a breakthrough in astronomical research. It is the prototype for a class of planets called hot Jupiters.

The name 51 Pegasi b is the official astronomical name of the planet. As with all extrasolar planets, the 'b' is used to indicate that this planet was the first discovered orbiting its parent star. Further undiscovered planets of 51 Pegasi would be designated c, d, e, f, and so on. All extrasolar planets have lowercase letters to differentiate from companion stars in the system (which are designated with an uppercase letter).

Though not officially recognized, 51 Pegasi b is sometimes referred to as "Bellerophon." This name comes from the Greek hero Bellerophon, who tamed Pegasus (the Winged Horse). This has direct relations with the constellation in which lies the planet (Pegasus). This name is usually used as the informal name to show the similarities to the planets of the Solar System. The same argument and association with the constellation of host star was used to suggest informal names for other extrasolar planets.

The exoplanet's discovery was announced on October 6, 1995, by Michel Mayor and Didier Queloz in Nature, using the radial velocity method at the Observatoire de Haute-Provence with the ELODIE spectrograph.

On October 12, 1995, less than a week after the announcement of the discovery, confirmation came from Geoffrey Marcy of San Francisco State University and Paul Butler of the University of California, Berkeley, using the Hamilton Spectrograph at the Lick Observatory near San Jose in California.

The planet was discovered using a sensitive spectroscope that could detect the slight and regular velocity changes in the star's spectral lines of around 70 metres per second. These changes are caused by the planet's gravitational effects from just 7 million kilometres' distance from the star.

This was the first discovery of an exoplanet orbiting a Sun-like star. 

It marked a turning point and forced astronomers to accept that giant planets could exist in short-period orbits. Once astronomers realized that it was worth looking for giant planets with the currently available technology, much more telescope time was devoted to radial velocity planet searches, and hence many more exoplanets in the Sun's neighborhood have been discovered.

After its discovery, many teams confirmed the planet's existence and obtained more observations of its properties. It was discovered that the planet orbits the star in around 4 Earth days, is much closer to it than Mercury is to our Sun, moves at an orbital speed of 136 km/s, yet has a minimum mass about half that of Jupiter (about 150 times that of the Earth). 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. However, since then, numerous other 'hot Jupiters' have been discovered (see 55 Cancri and τ Boötis, for example), and astronomers are revising their theories of planet formation to account for them by studying orbital migration.

Assuming the planet is perfectly grey with no greenhouse or tidal effects, and a Bond albedo of 0.1, the temperature would be 1265K (around 1000 degrees Celsius / 1800 degrees Fahrenheit). This is between the predicted temperatures of HD 189733 b and HD 209458 b (1180–1392K), before they were measured.

It was initially assumed that 51 Pegasi b was a terrestrial planet, but it is now believed to be a gas giant. It is sufficiently massive that its thick atmosphere is not blown away by the star's solar wind.

51 Pegasi b probably has a greater radius than that of Jupiter despite its lower mass. This is because its superheated atmosphere must be puffed up into a thick but tenuous layer surrounding it. Beneath this, the gases that make up the planet would be so hot that the planet would glow red. Clouds of silicates may exist in the atmosphere.

The planet is tidally locked to its star, always presenting the same face to it.

The planet (with Upsilon Andromedae b) is deemed a candidate for direct imaging by Planetpol. It is also a candidate for "near-infrared characterisation.... with the VLTI Spectro-Imager". The earlier, rocky-planet model was utilized as a setting by Hal Clement in the story Exchange Rate.