A $595 million space probe is rocketing through space on a mission to find planets like Earth where ET could live. NASA’s Kepler telescope will keep constant watch on more than 100,000 stars in one small region of the Milky Way for signs of small rocky worlds.

It is the first telescope capable of detecting planets in so-called Goldilocks zones which are “not too hot and not too cold” and so liquid water may be found.

Astronomers are optimistic that they will discover hundreds of Earth-sized planets in the three and a half to six years planned for the mission.

Kepler, named after a 17th century German astronomer, blasted off on top of a Delta 2 rocket at Cape Canaveral, Florida, on 6 March. It will eventually trail the Earth in orbit around the Sun.

“It was a stunning launch,” said Kepler Project Manager James Fanson of NASA’s Jet Propulsion Laboratory in Pasadena, California. “Kepler will help us understand if our Earth is unique or if others like it are out there.”

The 15ft long spacecraft has a mirror – its “eye” – more than 3ft wide plus a 94-megapixel digital camera that is the most powerful ever sent into space. Astronomers say it will help solve the riddle of whether we are alone in the universe.

Since the mid nineties, astronomers have used various techniques to identify around 340 planets orbiting other stars. But these have all been much larger than Earth and mostly superhot balls of gas like Jupiter whizzing round close to their parent stars. The most Earthlike found so far was picked up by another satellite, Corot, and is thought to be about twice the mass, or size, of our own world.

Scientists will finally have the power to spot much smaller worlds. It will do so by watching to catch the moment they pass in front of the stars they are orbiting. It will be like watching for a mosquito to pass in front of a searchlight – a blip so tiny that it can only be observed from above our atmosphere.

Kepler’s telescope will stay permanently trained on one arm of our Milky Way galaxy about 20 full moons wide in the constellations of Lyra and Cygnus. It is expected first to mop up many more of the larger fast-moving planets.

But then it will start to collect the smaller worlds which, like Earth, take longer to go round their stars. The astronomers want to catch three orbits to be sure of each planet’s existence, so one like the Earth with an orbit a year long will take three years to confirm.

They will then home in with other instruments, such as Hubble’s successor, the tennis-court sized James Webb space telescope, which is due for launch in 2013, to find out more about the new planets. These should reveal whether they are rocky, have oceans and even what is in their atmospheres.

A habitable planet is expected to lie in the so-called Goldilocks zone where conditions are not too hot and not to cold, just like her porridge. (Photo: NASA).

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