By Dr David Whitehouse BBC News Online science editor

Astronomers have discovered the most distant known planet, a hot Jupiter-class world closely orbiting a star about 5,000 light-years away.

The discovery extends the range for planet detection further than the roughly 180 light-years inside which other planets have been detected.

The world orbits its parent star every 29 hours, closer than Mercury is to our Sun. It was seen because it crosses in front of the star, as seen from Earth.

It was found as a result of one of several surveys of stars looking for those dimmed by the light of a transiting planetary companion.

Ogle-ing stars

The planet, dubbed Ogle-TR-56b, is only the second planet that has been seen to pass in front of its host star.

The first one, HD 209458b, was detected in 1999. Detailed observations of it moving in front of its star enabled astronomers to determine, for the first time, broad details about the atmosphere of a so-called exoplanet.

Astronomers from the Harvard-Smithsonian Center for Astrophysics announced the discovery of Ogle-TR-56b at a meeting of the American Astronomical Society in Seattle.

A planetary transit is a rare thing, requiring a precise alignment. But when it does occur it enables many details about the planet to be discovered than is possible by other means.

The Optical Gravitational Lensing Experiment (Ogle) uses a telescope in Chile to monitor a crowded starfield in the same direction as the centre of our galaxy.

Other planets

Follow-up observations of candidate stars that appeared to dim as a result of a planet were carried out from the Keck Observatory on Mauna Kea, Hawaii.

According to Dr Dimitar Sasselov, who used the Ogle data to confirm the presence of a planet around Ogle-TR-56, astronomers stand on the threshold of a new era of exploration and discovery having developed a new way to detect distant worlds.

Astronomers say the planet is about 2.6 times the size of Jupiter, yet weighs slightly less, giving it a density similar to Saturn. It might have formed farther from the star before moving to its current tight orbit.

The total number of known planets outside our Solar System now exceeds 100. Two more Ogle candidates are also probably planets.