The new planet is a little less than twice the mass of Jupiter, the largest planet in our solar system.

It was found by a team of American astronomers at San Francisco State University undertaking a survey of 400 nearby stars to see if any of them have planets around them.

The new planet circles a faint red dwarf star called Gliese 876. It is only about 15 light years from our sun. The discovery has been confirmed by European astronomers.

Astronomers believe that the new world would be very similar to Jupiter which is a gas-giant planet. Orbiting Gliese 876 in a tight orbit every 61 days it would be very cold.

But it may have warm cloud layers in its atmosphere and it could possibly have small rocky moons. There is no evidence of life but astronomers point out that finding planets increases the chances of it being on one of the newly-discovered worlds.

Runnaway planet found in May

The discovery is part of a revolution in understanding how many planets there are in space.

Only last month Nasa astronomers announced that they had found a runaway planet in space.

Scientists and philosophers have wondered for hundreds of years if there are any other worlds circling the stars we can see. Until recently there was scant evidence for this.

But now, for the first time in history, we know of more worlds circling other stars than we do circling our own sun. Our view changed October 1995 with the discovery of a planet orbiting the star 51 Pegasi. In the short time since then many more new worlds have been discovered.

Astronomers attribute this sudden burst of discovery to advances in spectrometers, instruments that break up starlight into its component colours for detailed analysis and in computer software for detecting fluctuations in starlight that reveal the telltale motions of so-called exoplanets.

A planet-forming ring around a young star

Now we know of 12 new worlds circling other stars and astronomers will shortly announce that they have found five more.

Some of them are smaller than Jupiter, but most are larger. Some orbit their star in months if not days, others take years. No one knows if they are solid bodies, like Earth and Mars, or gaseous spheres, like Jupiter and Saturn. No one can say if they have moons.