51 Pegasi Solar System, solar system composed of a star similar to the sun and one known planet orbiting the star. The solar system is located 50 light-years from the earth in the constellation Pegasus. The star, 51 Pegasi, is more massive than the sun and is about 1.6 times the sun’s diameter.

Swiss astronomers Didier Queloz and Michel Mayor announced the discovery of the planet 51 Pegasi B, the first planet discovered orbiting another sunlike star, on October 6, 1995. 51 Pegasi B is too dim to be visible from the earth, so Queloz and Mayor used an indirect method to discover it. As the planet 51 Pegasi B orbits the star 51 Pegasi, its gravity tugs at the star from different directions. When 51 Pegasi B pulls 51 Pegasi away from the earth, the star’s light appears to redden slightly (see Redshift). The star’s light becomes slightly bluer when the planet pulls the star toward the earth. The color change is so small that only sensitive instruments can detect it.

51 Pegasi B is about half as massive as Jupiter and orbits 51 Pegasi at a distance of about 8 million km (about 5 million mi), about 100 times closer than Jupiter orbits the sun. The time it takes for 51 Pegasi B to orbit a star, known as the planet’s year, is only 4.2 Earth days. The planet is probably a gas giant like Jupiter, and its temperature is very high—about 1000° C (about 1800° F).

Astronomers theorize that 51 Pegasi B formed from a disk of dust around its star, the same way planets formed in our solar system. They think it formed about as far from 51 Pegasi as Jupiter is from the sun. The 51 Pegasi dust disk was more massive than the sun’s dust disk, slowing 51 Pegasi B and causing it to spiral in toward its star. The dust disk dissipated by the time 51 Pegasi B reached its present distance from the star. Had the dust disk been more massive or lasted longer, 51 Pegasi B would have fallen into 51 Pegasi.

A second suspected planet orbiting 51 Pegasi has been discovered by American astronomers Geoffrey Marcy and Paul Butler. This planet is thought to have a year 2.5 Earth years long, although this has not been confirmed.