A Nasa spacecraft has begun a search for other planets just like Earth. Previously found worlds outside our own solar system have all been giant balls of gas similar to Jupiter.

Current searches for new worlds are mainly being carried out from ground observatories, including the robotic SuperWASP observatories.

Now, by observing from beyond the atmosphere in the vacuum of space, scientists believe their Deep Impact probe will detect much smaller, rocky planets.

The unmanned spaceprobe is so named because it made a spectacular attack on a comet, called Tempel 1, in July 2005. Its 820 lb missile blew a hole the size of a soccer stadium in its surface, creating a brilliant explosion that could be seen back on Earth.

Deep Impact is now heading for another comet, Hartley 2, after its first choice for a second target was lost. But as it cruises en route, it will aim its onboard telescope at five nearby stars around which giant planets have already been spotted.

The alien worlds were detected as they passed in front of their parent stars, as seen from the ground, causing a tiny dimming of starlight. Analysis of the light showed that they were giant planets with massive atmospheres.

Now Nasa scientists will study these planets’ movements even more closely, watching for changes in their orbits. Such effects will show that there are other planets, possibly as small as the Earth, pulling on their bigger neighbours.

Deep Impact’s extended mission has been labelled Epoxi. Principal investigator Drake Deming of Nasa’s Goddard Space Flight Centre, in Maryland, said: “We’re on the hunt for planets down to the size of Earth, orbiting some of our closest neighbouring stars.”

A European satellite called Corot, launched in December 2006, is also hunting Earth-sized worlds from space.

Picture: Moment Deep Impact blasted a crater in Comet Tempel 1.

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