New Earth-sized planet discovered next door

Researchers say the newly discovered planet is too close to its sun to support known forms of life - it has a surface temperature estimated at 1200C. But previous studies suggest that when one planet is discovered orbiting a sun there are usually others in the same system.

The Earth-sized planet, the discovery of which was announced by Stephane Udry and Xavier Dumusque, of the Geneva Observatory, orbits one of the suns in the three-star Centauri system, roughly 40 trillion kilometres away.

"It's a landmark discovery because it has a very low mass and it's our closest neighbour," said Udry. "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."

Commenting on the find, University of California astronomer Greg Laughlin said: "This is our back yard so to find out that planet formation occurred there is just extraordinary."

Since the discovery of the first exoplanets - those outside our solar system - in the early 1990s, more than 800 have been found but this one is the closest to Earth.

Getting there is unlikely in the foreseeable future. Laughlin estimates it would take about 40000 years to travel to the planet with current technology.

It was detected using the HARPS instrument on a telescope at the European Southern Observatory's La Silla site, in Chile.

The device is able to pick up tiny changes in the colour of the light coming from a host star as it wobbles under the gravitational influence of orbiting planets.

The gravitational effect in this case is minute, causing it to move back and forth by no more than 51cm a second.

Two of the Centauri stars are similar to our sun; the third is a faint red star called Proxima Centauri. The planet orbits Alpha Centauri B.

"Our observations extended over more than four years using the HARPS instrument," said Dumusque.

"I still have my doubts," astronomer Artie Hatzes, of Thuringian State Observatory, in Tautenburg, Germany, said.

Hatzes said the wobble detected in the star's trajectory could be caused by other factors, including sun spots, so the data will have to be tested by other researchers.