Extract from Lucky Planet
The first widely accepted planet orbiting a normal star other than our own Sun was discovered less than twenty years ago by Michel Mayor and Didier Queloz. These Swiss-based astronomers detected the very slight changes in starlight caused by a planet orbiting the star 51-Peg, 45 light years from Earth. On a dark winter’s night, 51-Peg, which sits half way down the west side of the square of Pegasus, is just visible to the naked eye, but nothing would make it stand out to a casual observer. More detailed study has shown that this star is remarkably similar to the Sun although it is slightly heavier, slightly brighter and probably just a little younger than our own star. More remarkably still, Mayor and Queloz were able to show convincingly that a planet half the size of Jupiter revolves around 51-Peg every four days in an orbit so tight that temperatures on the planet’s sun-scorched surface must be close to 2,000°C. For comparison, Jupiter takes twelve years to orbit our Sun, and even Mercury, the closest planet to our star, takes nearly three months to complete each orbit…
It’s probably worth saying a few words at this point about planet-naming conventions. The world discovered by Mayor and Queloz is known as 51-Peg b rather than, as you might expect, 51-Peg a. This results from a long-standing convention for naming multiple stars, stars that come in pairs or bigger groups and orbit around each other. The nearest such star system to the Earth is Alpha Centauri, consisting of the Sun-like star Alpha Centauri A and the slightly dimmer Alpha Centauri B, which take 80 years to orbit around each other. There is a third, very faint, member of this system that should be called Alpha Centauri C but more often goes by the name Proxima Centauri since it happens to be the closest star to Earth. Planets are named using this same convention except that lower-case letters are used to indicate that the companion is a planet rather than a star. Hence 51-Peg b because 51-Peg A is the star itself.