Waterworlds twice the size of Earth may be among the first planets found by a new European space telescope due to be launched in 2008, scientists believe.

Planets covered in water have long been imagined by science fiction writers and movie-makers. Kevin Costner starred in the Hollywood box-office flop Waterworld, set on a future Earth flooded by melting polar icecaps.

But scientists believe real waterworlds may exist among the stars, and have calculated what they might be like.

The unique nature of waterworlds makes them ideal candidates for discovery by the European Space Agency's Eddington space telescope, which is being development.

Up to now, astronomers have only been able to detect vast gassy planets the size of Jupiter or larger by measuring the way their gravity tugs on their parent stars and makes them ''wobble''. More than 100 extra-solar planets have been discovered this way. But Eddington aims to find small rocky planets by looking out for tiny dips of light as they drift across the faces of stars.

The telescope is designed to detect planets down to half the size of the Earth. However, a waterworld would be much easier to find.

Fabio Favata, ESA's Eddington project scientist, said: ''A waterworld passing in front of a star, somewhat cooler than the Sun, will cause a dimming in the stellar light by almost one part in a thousand. That's almost 10 times larger than the smallest variation Eddington is designed to detect.

''So waterworlds - if they exist - will be a very easy catch for Eddington.''

Planetary scientists discussed waterworlds at a recent Towards Other Earths conference co-sponsored by ESA.

Experts believe that, at the centre of a waterworld, there would be a metallic core with a radius of about 2500 miles. This would be surrounded by a rocky mantle extending out a further 2000 miles, followed by a layer of ice 3000 miles thick.

Finally, an ocean would blanket the world to a depth of 65 miles.