By Helen Briggs
BBC News science reporter
Planet hunters have discovered an icy "super-Earth" circling a distant star.
New detection techniques help astronomers search for new planets
International astronomers suspect it is a bare, icy, rocky world, much colder than the Earth and 13 times its mass.
The planet was spotted last April but details have only just been revealed in a paper submitted to Astrophysical Journal Letters.
The extra-solar planet is one of a mere handful detected using a novel technique called microlensing.
The planet orbits a star about half as big as our Sun, positioned some 9,000 light-years away. At -201C, it is one of the coldest extra-solar planets to be discovered.
Andrew Gould, professor of astronomy at Ohio State University, US, was one of the first people to discover it.
He said the find has two main implications.
"First, this icy 'super-Earth' dominates the region around its star that in our Solar System is populated by the gas-giant planets, Jupiter and Saturn," he said.
"We've never seen a system like this before because we've never had the means to find them.
"And second, these icy 'super-Earths' are pretty common. Roughly, 35% of all stars have them."
Professor Gould is leader of the Microlensing Follow-up Network (MicroFUN) collaboration.
It is one of several international groups looking for Earth-like planets in planetary systems other than our own using the phenomenon called gravitational microlensing.
The technique is an indirect way of obtaining information about large celestial objects that are too dim to see.
When a massive object such as a star crosses the path of a background star, it acts like a powerful lens, gravitationally bending and magnifying the light rays from the more distant star.
The object's gravity amplifies the starlight, causing it to brighten as the body passes in front of the star.
This can be observed by telescopes on Earth as a brightening and fading effect, as the lens star floats across the face of the background star.
Clues to the presence of the planet were first seen last April by a Polish astronomy project led by Professor Andrzej Udalski from Warsaw University.
When Gould and Udalski realised the star was brightening extremely quickly one night, they alerted the duty astronomer at the MDM Observatory in Arizona.
"It was four in the morning," Gould recalled, "I was very excited and frantic to get someone to observe that star."
Astronomers in Arizona took more than 1,000 measurements of the event, which, coupled with software models, confirmed the presence of a Neptune-mass planet, 13 times heavier than Earth.
Gould suspects the planet is a bare, icy Earth-like one, a sort of cold "super-Earth", although he cannot be certain.
"We can't really tell for sure," he said. "If we start getting more statistics on this type of planet, we could piece together a better story."
Since the 1990s, astronomers have discovered some 170 extra-solar, or exoplanets, a planet which orbits a star other than the Sun.
There is great interest in finding extrasolar planets that are like the Earth, since these could, in theory, have the right conditions for supporting life.
In January, a new planet 5.5 times the mass of the Earth - the smallest yet - became the third exoplanet to be detected by the microlensing technique.
Tim Naylor, professor of astrophysics at Exeter University, UK, said microlensing had great promise for the future.
"It holds out the promise that we will discover many Earth-sized planets with this technique," he told the BBC News website.