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51 Pegasi b (informal name Bellerophon) is the first planet discovered around a sun-like star outside of the solar system. It is the prototypical hot Jupiter. It orbits the star 51 Pegasi in the Pegasus constellation.

The name

The official name of the exoplanet is 51 Pegasi b (51 Peg b for short); the 'b' is used to indicate that it is the first companion of its parent star. Further companions would be designated c, d, and so on. It has been informally named 'Bellerophon', after Bellerophon, the Greek hero who tamed Pegasus (the Winged Horse), referring to the constellation of Pegasus in which the planet is located.

Characteristics

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, and is much closer to it than Mercury is to our Sun, suffering temperatures of around 1000 degrees Celsius (1800 degrees Fahrenheit), yet is about half the mass 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.

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

Bellerephon probably has a greater radius than 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.

Discovery process

The exoplanet's discovery was announced on October 6 1995 by Michael Mayor and Didier Queloz in Nature, volume 378, page 355, using the radial velocity method at the Observatoire de Genève.

After the announcement, on October 12, 1995, confirmation came from Dr. Geoffrey Marcy from San Francisco State University and Dr. Paul Butler from the University of California, Berkeley using the Hamilton Spectrograph at the Lick Observatory near San Jose in California.

Discovery method

The planet was discovered using a sensitive spectroscope that could detect the slight, 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 discovery of this first exoplanet established a milestone in astronomical research, as it forced astronomers to realize 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 neighbourhood have been discovered.

See also

• Extrasolar planet
• List of stars with confirmed extrasolar planets
• PSR 1257+12, the first star discovered with exoplanets.

External links

• Current Notes from the Extrasolar Planet Encyclopedia
• Extrasolar Visions: Information and pictures
• Velocity curve and information from the Geneva based Planet Search Team
• Velocity curves and information from the California & Carnegie Planet Search Team
• solstation entry

• Extrasolar planets
• Pegasus constellation