Alpha Centauri, only 4.3 light-years away, is actually a triple star -- a system consisting of two stars similar to the Sun orbiting close to each other, designated Alpha Centauri A and B, and a more distant and faint red component known as Proxima Centauri.
The planet was detected using the HARPS instrument on the 12-foot telescope at the European Southern Observatory La Silla Observatory in Chile.
Since the nineteenth century astronomers have speculated about planets orbiting these close cosmic neighbors, but searches of increasing precision had revealed nothing -- until now.
"Our observations extended over more than four years using the HARPS instrument and have revealed a tiny, but real, signal from a planet orbiting Alpha Centauri B every 3.2 days," lead study author Xavier Dumusque of the Geneva Observatory in Switzerland said in a release from ESO headquarters in Garching, Germany.
Alpha Centauri B is very similar to our sun but slightly smaller and less bright, astronomers said.
"This is the first planet with a mass similar to Earth ever found around a star like the Sun. Its orbit is very close to its star and it must be much too hot for life as we know it," co-author Stephane Udry of the Geneva Observatory said, "but it may well be just one planet in a system of several."