Planets similar to Earth are far more common than previously thought and may be found in a third of all the galaxy's solar systems, say scientists.

Astronomers hunting for planets have found one with the mass of Neptune 9,000 light years away from our Sun. They believe it is similar in composition to Earth.

Initial analysis suggests that, like Earth, it has a rocky core, though at minus 184 degrees Centigrade (minus 300 Fahrenheit) its surface is covered with ice rather than water.

The discovery, using a new technique called gravitational microlensing, has forced scientists to revise their estimates of how many Earth-type planets exist.

Microlensing is also opening up the prospects of finding another planet of the same type and mass as Earth, regarded as the holy grail of astronomers, because it is such a sensitive method.

Andrew Gould, professor of astronomy at Ohio State University, said that the discovery, reported in Astrophysical Journal Letters, was exciting.

"It appears to be a terrestrial planet of rock and ice," he said. "The implication is that these icy super-Earths are pretty common."

It is the first time that such a planet has been found occupying the same region of a solar system as the gas giants Jupiter and Saturn do in ours.

Astronomers were able to find the planet, one of the coldest yet discovered outside our solar system, by taking readings during gravitational microlensing in the galaxy.

Gravitational microlensing takes place when a massive object, such as a star, passes in front of another object. The passing object's gravity bends the light from the background star and magnifies it like a lens so that it appears brighter.

Although invisible to the astronomers, the object's gravity is sufficient to betray its presence through "tiny warping" of the starlight and radiation.

More than 1,000 measurements of the event had to be taken and special software was written to eliminate all other possible causes of the warping.

Professor Gould said it was unclear why the Earth-type planet orbiting the star, half the size of our Sun, had failed to develop into a gas giant, but he speculated that all the gas in the region had dispersed by the time the planet had formed.

Far out

— The Earth-type planet dominates the same region of its solar system as the gas giants Jupiter and Saturn do in ours

— Thirteen times the size of Earth with twice the gravitational pull, it is thought to be a "failed Jupiter" in that it coalesced in the same way but never developed into a gas giant

— In the past 10 years, more than 170 planets have been found outside our solar system. Only six others are Earth-types, two of them being in cold regions and four orbiting so close to stars that they are too hot for life