The smallest-ever planet outside our solar system has been spotted by NASA’s Kepler space telescope — a rocky planet similar in size to the Earth, the US space agency said on Monday.

The planet, named Kepler-10b, is about 1.4 times the size of Earth and spins around its star more than once a day, an orbit much too close for life to survive, NASA said in a statement.

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“Since it orbits once every 0.84 days, Kepler-10b is more than 20 times closer to its star than Mercury is to our sun and not in the habitable zone,” NASA said.

According to Douglas Hudgins, Kepler program scientist at NASA, the discovery is promising even though it appears that no life could exist there.

“The discovery of Kepler 10-b is a significant milestone in the search for planets similar to our own,” said Hudgins.

“Although this planet is not in the habitable zone, the exciting find showcases the kinds of discoveries made possible by the mission and the promise of many more to come,” he said.

The new planet has a mass 4.6 times that of the Earth, and an average density of 8.8 grams per cubic centimeter, similar to an iron dumbbell, NASA said.

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The findings are described in the Astrophysical Journal.

Kepler is NASA’s first mission in search of Earth-like planets orbiting suns similar to ours.

It launched in 2009, equipped with the largest camera ever sent into space — a 95-megapixel array of charge-coupled devices — and is expected to continue sending information back to Earth until at least November 2012.

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The space telescope is searching for planets as small as Earth, including those orbiting stars in a warm, habitable zone where liquid water could exist on the surface of the planet.