Probing for waterworlds in the depths of space

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 completely covered in water have long been imagined by science fiction writers and movie-makers.

Kevin Costner’s Hollywood box-office flop Waterworld was set on a future Earth flooded by melting polar icecaps. But scientists believe such planets 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 ESA’s Eddington space telescope, which is now under 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 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.” Whether waterworlds would harbour life is an open question. A leading theory says life on Earth started around “black smokers”, or nutrient-rich volcanic vents on the ocean floor.

These could not occur on the thick ice beneath a waterworld’s oceans. But the possibility that life could originate on the ocean surface has not been ruled out.

A more ambitious ESA space telescope mission to be launched in about 2014 may provide the answer. Darwin, consisting of a flotilla of spacecraft, will not only be able to spot small planets but also to detect signs of life in their atmospheres.

Scientists believe Jupiter’s moon, Europa, might be a waterworld of sorts. There is evidence that beneath a thick outer layer of ice the moon is covered in liquid or slushy water.