Washington  - Scientists are one step closer to answering the question of whether we are alone in the universe.

The Kepler space telescope launched earlier this year is working and is finely tuned enough to detect Earth-sized planets orbiting distant stars, NASA scientists said Thursday.

Kepler was able to pick out the exoplanet HAT-P-7b within its first days of collecting data, and - for the first time - to measure the amount of light being emitted by the planet itself.

The planet, which had previously been pinpointed by Earth-bound telescopes, is about the size of Jupiter and orbiting extremely close to its star, but the level of detail returned by Kepler is promising, the scientists said. It indicates that Kepler is capable of taking extremely delicate measurements and should be able to pick out much smaller Earth-sized planets.

The findings are to be published Friday in the journal Science.

"Sometime in the not too distant future, we can stand outside with our children and point up at the sky and say, 'See that star? That one has an earth,'" astrophysicist Alan Boss said.

It will likely take several more years before astronomers can use the data to be gathered by Kepler to estimate the frequency of Earth- like planets.

The 590-million-dollar telescope programme is to spend at least the next three-and-a-half years pointed at a large swath of the Milky Way galaxy containing some 4.5 million stars.

The most advanced cameras ever used in space will focus on some 100,000 to 150,000 stars deemed most likely to have orbiting planets, scientists said at a prelaunch press briefing. Data from the cameras will be used to find planets by looking for distortions in the light being emitted as an orbiting planet crosses in front of the star.

An Earth-sized planet will be particularly hard to spot because of its size and the fact that it will only rarely cross in front of its sun.

Since 1995, more than 340 planets have been found outside our solar system, but these so-called exoplanets have been large, gaseous planets, like Jupiter, which tend to be closer to their stars and easier to spot because of their size. Smaller, rocky planets like Earth, which could be capable of supporting life, can exist only within a small range of orbits in the "habitable zone" - close enough to be warm but far enough to avoid being baked. (dpa)