The most Earth-like planet yet discovered beyond the solar system has been detected orbiting a distant star, boosting the chances that life exists elsewhere in the galaxy.

The icy, rocky world is just five times larger than our own, making it the smallest and most similar to Earth of all the 160 "exoplanets" around other stars that astronomers have found so far.

The planet, which has been given the unglamorous name OGLE-2005-BLG-390Lb, is considered an unlikely candidate for life, as it has a surface temperature of -220C (-364F) that precludes the presence of the liquid water thought necessary to sustain organisms.

Nevertheless, the discovery is an important breakthrough in the search for extra-terrestrial life, as it suggests that small, rocky planets like the Earth are relatively common around other stars.

If similar worlds are found to exist orbiting in the "habitable zones" of solar systems with suitably moderate temperatures, they would be prime candidates for supporting extra-terrestrial life.

Most of the exoplanets discovered to date are gas giants, similar in size or larger than Jupiter.

"The new planet confirms that we can now find small cool planets down to the mass of the Earth," said Professor Keith Horne of the University of St Andrews, a member of the international team that identified the planet.

"Our next goal is to find more of them, with lower masses, in order to measure the abundance of cool Earths and determine if habitable planets like Earth are abundant or rare. If the abundance is high, the next step is to search for life on those planets."

Michael Turner of the US National Science Foundation, which supported the research, said: "This is an important breakthrough in the quest to answer the question, ‘Are we alone?’.

"The team has discovered the most Earth-like planet yet, and more importantly, has demonstrated the power of a new technique that is sensitive to detecting habitable planets. It can probe a much greater portion of our galaxy."

OGLE-2005-BLG-390Lb was discovered using a technique called gravitational microlensing, in which the gravity of a large star acts as a natural telescope, bending light to magnify dim objects that lie behind it. Planets can then be detected as they blot out some of the light from a star as they orbit in front of it, causing a blip effect that astronomers can see.

It takes the first part of its name from the Optical Gravitational Lensing Experiment (OGLE), one of three international groups that is using the technique to search for distant planets. Details of the discovery are published tomorrow in the journal Nature, in a paper signed by 73 scientists from 32 institutions.

The planet lies close to the centre of the Milky Way, between 20,000 and 25,000 light years from the Sun. Its parent star is of a type known as a red dwarf, which is about five times less massive than the Sun.

As the star is so small, and because the planet orbits about three times further out than the Earth, it is much colder than our own world. It may have a thin atmosphere, and its rocky surface may be buried beneath frozen oceans.

Astronomers said it is probably more similar to an overgrown version of Pluto to the rocky planets of the inner solar system such as the Earth, Venus and Mars.

Though these characteristics make it a poor bet for life, its existence there remains possible if the planet is geologically active. Volcanic activity beneath frozen oceans may create pockets of liquid water that sustain life, as some scientists have suggested might be the case on one of Jupiter’s moons, Europa.

The planet was first detected last summer. "We first saw the usual brightening reaching a peak magnification on 31 July 2005, after which the event started to fade back symmetrically," said Martin Dominik of the University of St Andrews, another member of the research team.

"On August 10, however, there was a small ‘flash’ lasting about half a day. By succeeding in catching this anomaly with two of the telescopes of our network and with careful monitoring, we were able to conclude that the lens star is accompanied by a low-mass planet."

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