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In search for Earth-size world, a historic first: Kepler finds rocky planet

For the first time, scientists have confirmed an exoplanet that is rocky like Earth – not a gas giant. It's also 2,500 degrees F and denser than iron, so it's no candidate for life, say members of the team working with the planet-hunting Kepler spacecraft.

Scientists working with the Kepler spacecraft have discovered a rocky planet orbiting a star 560 light-years away. The planet, Kepler 10b (imagined here in an artist's rendering) is 20 times closer to its star than Mercury is to the sun and is the smallest exoplanet yet found, according to the researchers.

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A team of astronomers has made what it says is the first unambiguous discovery of an Earth-size rocky planet orbiting a distant star – one 560 light-years from Earth. It is the smallest planet discovered to date, the team says.

Don't look for life there. The planet is orbiting so close to its host star that the surface temperature on the star-lit portion of the planet is more than 2,500 degrees Fahrenheit, hot enough to melt iron, the team estimates.

But it's plausible that the planet could have canyons carved by rivers of molten material on its torrid surface, says one team member.

The discovery, made with NASA's Kepler planet-hunting spacecraft and announced Jan. 10, "will be marked as among the most profound scientific discoveries in human history," says Geoffrey Marcy, an accomplished planet-hunter at the University of California at Berkeley who was not part of the discovery team.

Much as anthropologists have uncovered so-called missing links along the multimillion-year history of human evolution, Dr. Marcy says, this discovery provides a missing link between gas-giant planets that have dominated extrasolar-planet discoveries so far, and the much-sought Earth-size planets capable of hosting life.

Other scientists are a bit more cautious, noting that the new planet may in fact be the rocky core of a close-in gas giant whose gas has boiled away over the star's 8-billion-year life so far.

No atmosphere and a tail of iron fragments?

Still, for a mission designed to hunt for Earth-mass planets at Earth-like distances around far-away suns, Monday's announcement represents a milestone for the Kepler team and for humanity, says Kepler team member Natalie Batahla, an astronomer at San Jose State University.

The planet, known as Kepler 10b, has 1.4 times Earth's radius and tips the scale at some 4.6 times Earth's mass. That gives it a density significantly higher than any of the inner planets in our solar system. Indeed, its density lies somewhere between that of bulk iron and copper.

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