Astronomers have found a new planet, the closest yet outside our solar system and just an astronomical stone's throw away at four light years.

Researchers say it is too close to its sun to support life, with a surface temperature estimated at 1,200 degrees Celsius (2,192 Fahrenheit). But previous studies suggest that when one planet is discovered orbiting a sun there are usually others in the same system.

The new Earth-sized planet, announced in science journal Nature by Stephane Udry and Xavier Dumusque at the Geneva Observatory, orbits one of the suns in Alpa Centauri, roughly 25 trillion miles away.

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

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 over 800 have been found but this is the closest to Earth.