Astronomers are celebrating after finding an Earth-sized planet bang next door to us in the closest star-system to our own. Observations with a European telescope in Chile spotted the new world right on our cosmic doorstep – just 4.3 light-years away orbiting a star called Alpha Centauri B.

Though it is of similar mass to our planet, the star zips round its own sun in just 3.2 days. The surface would be so hot that the surface could be molten lava at 1,200 C.

But its discovery emphasises that planets must be commonplace throughout the galaxy, so finding one favourable like Earth can surely only be a matter of time.

One of the discoverers of the latest planet, Stéphane Udry, of 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.”

More than 800 planets have been found orbiting other stars in the galaxy in the past 20 years. But never before has one been found so close. This one was detected using an instrument called HARPS on the 3.6-metre telescope at ESO’s La Silla Observatory in Chile. HARPS is short for High Accuracy Radial velocity Planet Searcher and finds them by detecting tiny wobbles in starlight.

Alpha Centauri is one of the brightest stars in the sky and is actually a system of three stars. Two, designated Alpha Centauri A and B, are like the Sun and orbit close to each other. But there is also a more distant and faint red star known as Proxima Centauri.

The new planet’s orbit is not in the habitable zone around the star. But its discovery increases the chance that other planets will be found, including rocky ones capable of hosting life.

Xavier Dumusque, of Geneva Observatory, Switzerland, who is lead author of the paper announcing the discovery, said: “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. It’s an extraordinary discovery and it has pushed our technique to the limit!”

He added: “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!”

Alpha Centauri B is very similar to the Sun but slightly smaller and less bright. The newly discovered planet, with a mass of a little more than that of the Earth [3], is orbiting about six million kilometres away from the star, much closer than Mercury is to the Sun in the Solar System.

Some have expressed concern that the signals detected from the new planet were so feeble that they might have been artificial created by noise in the system. But at a teleconference that Skymania attended, Udry said: “I think the trust we can put in this planet is even bigger than we have had in other planets. I am very confident that more observations will just confirm what we have seen now.”

Asked if he was surprised by the discovery, Greg Laughlin, of the University of California Santa Cruz, told the press conference: “Of course not because we know they are everywhere.”

Professor Isabelle Baraffe, of the UK’s University of Exeter said today: “This is an absolutely fantastic result. The Holy Grail is to find the twin of the Earth which means having a planet with the mass of the Earth orbiting a sun and in a habitable zone which would favour the presence of liquid water and therefore we think would favour the presence of life.

“The discovery of this planet is the first step towards this Holy Grail. Of course this is not in the habitable zone as it’s very close to its star – the same distance that Mercury sits from our sun, so the conditions are burning at the surface of the planet. It does tell us, however, that the Alpha Centauri system has other planets. As it is unthinkable that there would be one planet in isolation, this star is a gold mine for the detection of planets.

“This result opens up a fantastic opportunity to study another solar system and potential to use current and future techniques to find eventually the presence of life.”

Though Alpha Centauri is the closest star system to Earth, distances in space are vast and it would take one of our spacecraft using current technology at least 18,000 years to reach it.