51 Pegasi is the (Flamsteed designation) name of a Sun-like star 14.7 parsecs (47.9 light-years) from Earth in the constellation of Pegasus. It was the first Sun-like star to be found 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. The discovery was made with the radial velocity method at the Observatoire de Genève .

The star

The star itself is of apparent magnitude 5.49, and so 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 HIP 113357 in the Hipparcos Catalogue and HD 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. Its spectral type is G2.5V.

In 1996 astronomers Baliunas, Sokoloff, and Soon reported measurements of a sample of stars' Calcium II H and K spectral lines and thereby measured a rotational period of 37 days for 51 Pegasi.

The planet 51 Pegasi b

The name of the exoplanet is '51 Pegasi b' (see the article for more detailed information); the 'b' is used to indicate that it is the first discovered planetary-mass companion of its parent star. Further such companions would be designated c, d, and so on. The planet 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 theories of planet formation and resulted in discussions of orbital migration .

References

• (lists rotational periods for 112 stars)