51 Pegasi was the first sunlike star found to have a planet orbiting it. The exoplanet's discovery was announced in 1995 by Michel Mayor and Didier Queloz. The discovery was made with the radial velocity (Doppler shift) method at the Observatoire de Haute-Provence in France.
Properties
The star itself is of apparent magnitude 5.49, and is visible with binoculars or the naked eye under dark sky conditions. 51 Pegasi is a yellow dwarf star 1.3 times more luminous than the Sun, 4-6% more massive, with 1.6 times more metal content. It is estimated to be 7.5 billion years old - somewhat older than the Sun - and running low in hydrogen. It is sometimes listed as a class G2 IV subgiant with a surface temperature of 5800 K, and sometimes as a G5 V main-sequence star that (like the Sun) fuses hydrogen into helium in its core. In 1996, astronomers measured a rotational period of 37 days for 51 Pegasi.
Planet
51 Pegasi b 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. The planet is only 0.05 AU from the star, and has a highly circular orbit (e=0.03) that takes merely 4.23 days to complete. The planet contains at least 47% the mass of Jupiter.
After 51 Peg b's discovery, many teams confirmed its existence and obtained more observations of its properties, including the fact that it orbits very close to the star, suffers estimated temperatures around 1500 K, and has a minimum mass about half that of Jupiter. Assuming a Jupiter-like composition, its radius may be about 1.2 times Jupiter's, enlarged because of the torch-like stellar radiation at its inner orbit. However, the planet may be too hot to hold onto a thick hydrogen atmosphere. At the time of discovery, its close distance was not compatible with theories of planet formation, and has resulted in discussions of planetary migration.
In 2006, the SETI Institute named 51 Pegasi as a sunlike star that is old enough to qualify as a top-five candidate for radio signals from intelligent civilizations.
[Adapted from STARS by Jim Kaler, Professor Emeritus of Astronomy, University of Illinois]