An Earth-mass planet has been found orbiting a star in the Alpha Centauri system, the Sun's next door neighbour in space.
The mystery world circling Alpha Centauri B is thought to be much too hot to support life, with surface temperatures of around 1,500C.
But scientists say it is likely to be part of a more extensive solar system containing other planets, one or more of which might be habitable.
At just 4.3 light years from the Sun, Alpha Centauri B is only a step away in astronomical terms. It is part of a triple system consisting of a pair of Sun-like stars, Alpha Centauri A and B, and a more distant and faint red companion, Proxima Centauri. The latter is the closest star to the Sun by a small margin.
Astronomers described the discovery as "extraordinary".
Despite its closeness, it would still take thousands of years to reach Alpha Centauri using current rocket technology. But scientists do not rule out the possibility of sending an unmanned space mission there in the not-too-distant future.
Xavier Dumusque, a member of the European team from Geneva Observatory in Switzerland, said: "This result represents a major step towards the detection of a twin Earth in the immediate vicinity of the Sun. We live in exciting times."
The planet was detected by European Southern Observatory (ESO) astronomers. Data published in the journal Nature show that the as-yet unnamed planet is unusually light, containing only a little more material than the Earth. It is the lightest exoplanet ever found orbiting a Sun-like star.
Leading US planet hunter and astronomer Professor Greg Laughlin, from the University of California at Santa Cruz, said: "Everything we know indicates that when you find one planet like this you're very likely to find additional planets further out, so it's very exciting in terms of looking forward to further detection. Alpha Centauri is our closest neighbour. This is our back yard, and to find out that planet formation is occurring there is just extraordinary."
Co-author Dr Stephane Udry, also from Geneva Observatory, 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, but it may well be just one planet in a system of several."