51 Pegasi
51 Pegasi is the name of a Sun-like star in the constellation of Pegasus 14.7 parsecs (47.9 light-years) from Earth. This was the first Sun-like star that was shown to have a planet orbiting it, a discovery that was announced in 1995.
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.
The star
The star itself is visible from the Earth with binoculars, or with the naked eye by those with good vision in very dark sky conditions. 51 Pegasi, numbered 113357 in the Hipparcos Catalogue and 217014 in the Henry Draper Catalogue, is a yellow dwarf star estimated to be 7.5 billion years old, somewhat older than the Sun, 4% more massive, with more metal content and running low in hydrogen.
The planet 51 Pegasi b
The name of the exoplanet is "51 Peg b", the b used to indicate 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 its discovery many teams confirmed its existence and obtained more observations of its properties, including the fact it orbits very close to the star, suffers temperatures around 1000 Celsius, and is about half the mass of Jupiter. At the time this close distance was not compatible with theory and resulted in discussions of orbital migration.
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.
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.
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 avaliable 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.