By Dr David Whitehouse BBC News Online science editor

An artist's impression of the new planet

The find was made through measurements of the effect the combined mass of the planet and its parent star had on the light of another, more distant star.

This "gravitational lensing" technique was first predicted by Albert Einstein in his General Theory of Relativity.

It only works when all the objects are aligned precisely as viewed from Earth.

The gravitational fields of the star-planet pair bend and focus the light of the distant star, like a lens in a telescope.

By analysing changes in the detail of the background star's light, astronomers can reveal information about the foreground objects.

Working together

The newly discovered planet is deep in the constellation of Sagittarius and orbits a dim so-called red-dwarf star.

The planet is probably about one-and-a-half times the mass of Jupiter and orbits its star about three times further out than the Earth orbits the Sun.

The far-distant star, whose alignment made this discovery possible, is about 24,000 light-years away, near the Milky Way's centre.

The discovery was achieved through the cooperation of two teams of astronomers: Microlensing Observations in Astrophysics (Moa) and the Optical Gravitational Lensing Experiment (Ogle).

Future discoveries

"The real strength of microlensing is its ability to detect low-mass planets," said Dr Ian Bond, of the Institute for Astronomy in Edinburgh, UK. He is the lead author of a paper detailing the planet's discovery that will appear in a forthcoming edition of Astrophysical Journal Letters.

A gravitational lens in action

Dr Bohdan Paczynski, of Princeton University, US, who first proposed using the gravitational lensing technique to search for planets around other stars is delighted by the discovery.

"I'm thrilled to see the prediction come true with this first planet detection through microlensing," he said.

He added that he and his colleagues believed that over the next few years the method might lead to the discovery of Neptune-sized and even Earth-sized planets around distant stars.