Twenty thousand light years away, in the constellation of Sagittarius near the centre of the Milky Way, a frigid rock is orbiting a small star. Although the distant world is probably too cold ever to support life, scientists believe it is the most Earth-like planet to be discovered beyond our solar system.

Known to astronomers as OGLE-2005-BLG-390Lb, the planet, described in Nature today, is five times as dense as Earth and orbits its sun at three times the distance Earth is from our own star, leaving surface temperatures at a frosty -220C.

The discovery was hailed by scientists not so much as a potential home from home, but as a success for a new planet-hunting technique called gravitational microlensing.

The method uses a network of telescopes to watch for changes in light from distant stars. If a planet moves between a star and a telescope on Earth, its gravity bends the light and magnifies it.

Michael Turner of the US National Science Foundation, said: "The team has discovered the most Earth-like planet yet."