A planet similar in size to the Earth has been found orbiting a star in the Alpha Centauri system, the Earth'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 about 1,500C (2,732F). 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. This last 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, who measured tiny wobbles in the star's motion caused by a gravitational tug of war with the orbiting planet. A high-precision instrument called Harps on the ESO's 3.6m telescope at La Silla in Chile was used to spot light wave fluctuations generated by the wobbles.
Data published in the journal Nature shows 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.
The planet hugs its parent star at a distance of just 6m kilometres, much nearer than Mercury is to the sun. It makes a complete orbit every 3.2 days.
Its surface is so hot it would probably resemble molten lava. However, previous experience tells the scientists that when they find one low-mass exoplanet of this kind, it is likely to be part of a bigger family.
The astronomers also know that Alpha Centauri B has a stable "habitable zone", also known as the "goldilocks zone". This is the orbital band where conditions would be "just right" to support surface water and, potentially, life.
Professor Greg Laughlin, from the University of California at Santa Cruz, said: "I think the prospects are excellent for finding further planets in this system.
"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 backyard, and to find out that planet formation is occurring there is just extraordinary." The scientists say they are "99.9%" sure they have not been fooled by a false result caused by background noise in the data.
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."
Not everyone is convinced though. Writing about the discovery in Nature, Prof Artie Hatzes at the Thuringian State Observatory, in Tautenburg, Germany, asked: "Is this Earth-mass planet real? Only time will tell. As the American astronomer Carl Sagan once said, 'Extraordinary claims require extraordinary evidence.' Although a planet-like signal is present in the data, the discovery does not quite provide the 'extraordinary evidence'. It is a weak signal in the presence of a larger, more complicated signal. In my opinion, the matter is still open to debate."
Further "wobble" observations will be difficult to carry out until the two A and B Alpha Centauri stars move further apart in a few years' time.
But if the astronomers are lucky they might spot a "transit" – the crossing of a planet in front of its star as seen from the earth. When this happens, the star light dims very slightly, betraying the planet's presence. Increasing numbers of planets are being detected in this way.
There have been more than 800 confirmed exoplanet finds, but none so close to home.