Kepler spacecraft finds hot, distant planet

The spacecraft Kepler, searching for planets far beyond our solar system, has found a rocky one that's not much larger than Earth, but it zips around its parent star so closely that it's furiously hot - more than three times hotter than molten iron, astronomers say.

Peering out into space from its orbit around the Earth, Kepler has discovered seven "exoplanets" so far in its three-year quest to detect distant planets where life might be possible among the 156,000 stars on its target list.

"This one is the first unquestionably rocky exoplanet anyone has ever found, and it's one more link in the chain of finding Earth-like ones," said Natalie Batalha, a professor of astronomy at San Jose State University and deputy leader of the Kepler team at NASA's Ames Research Center in Mountain View.

But the new-found planet is by no means inside what planet hunters term its star's "habitable zone," where liquid water - and life - might possibly exist. In our own solar system, both Earth and Mars are well within that zone.

Batalha reported the planet's discovery Monday at the annual meeting of the American Astronomical Society in Seattle.

The spacecraft's highly sensitive instruments, she noted, detected the extremely slight dimming of the planet's star as the planet passed between the star and the spacecraft - an event called transiting.

To confirm that a planet caused the dimming, Batalha said, the Kepler team gathered transiting data from the spacecraft for eight months, starting in May 2008, and sent the information to astronomers at the immensely powerful Keck telescope in Hawaii. The astronomers there spotted the planet using their telescope and confirmed its properties by measuring the star's infinitely small movements back and forth as the passing planet tugged at it - in other words, they measured its radial velocity.

The new planet is called Kepler 10b. It is 1.4 times the size of Earth but 4.6 times more massive, and it speeds around its star in only 84 days. And it is hot: nearly 10,000 degrees Fahrenheit - more than three times the temperature of Mercury.

Steven S. Vogt, an astronomer at UC Santa Cruz who is not connected with the Kepler team, noted that the new exoplanet must be "the equivalent of a well-lit charcoal briquet," but is "not the first such hot rock planet that we know of."

He recalled that the European Space Agency's CoRoT spacecraft detected a similar exoplanet two years ago, called COROT-7b that has almost the same mass as this one and orbits its star at the same speed.

In September, Vogt and Paul Butler of the Carnegie Institution in Washington, after 11 years of searching with the Keck telescope in Hawaii, reported the discovery of an Earth-sized but extremely dense exoplanet orbiting the nearby star named Gliese and located well within that star's "habitable zone."

E-mail David Perlman at dperlman@sfchronicle.com.

This article appeared on page A - 6 of the San Francisco Chronicle