As many as half the known planetary systems outside the Solar System could harbour alien "Earths" capable of supporting life, scientists said yesterday.

Experts from the Open University came to the conclusion after simulating the environments around stars to see if they would allow Earthlike planets to survive.

The findings show that large numbers of habitable Earths are not only possible but likely elsewhere in our galaxy, the Milky Way.

Their existence could be confirmed in the next ten years as new space telescopes powerful enough to identify small worlds enter service for the first time.

Just over 130 stars are known, like our Sun, to have families of orbiting planets.

At present astronomers have to rely on indirect methods of detecting extra-solar planets, such as measuring the way their gravity causes their parent stars to "wobble".

But they can only identify Jupiter-like giant gaseous planets with these techniques.

Smaller, rocky planets like the Earth may be there, too, but are currently invisible. Trying to spot one of these planets directly is like looking for a glow-worm in the glare of a searchlight.

While not being able to see other Earths, the Open University team tested whether or not they could exist by simulating known star systems. Professor Barrie Jones said: "We were particularly interested in the possible survival of 'Earths' in the habitable zone.

"This is often called the 'Goldilocks zone', where the temperature of an 'Earth' is just right for water to be liquid at its surface. If liquid water can exist, so could life as we know it."

The scientists presented their findings at the RAS's annual meeting in Birmingham. They created mathematical models of known star systems before launching Earth-sized planets into them to see if they survived.

The study showed that giant- sized planets are accompanied by two "disaster zones", one outside the giant's orbit and one inside where the giant's gravity causes a catastrophic change in an Earthlike planet's orbit. The outcome is a collision with either the giant planet or the star, or ejection to the cold outer reaches of the system.

The team found the locations of these disaster zones depended not only on the mass of the giant planet but also on its orbit.

Having established the rules for determining the extent of disaster zones, researchers applied them to all the known star systems. They discovered about half offered a safe time period long enough for life to have developed on Earth-like planets.