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Probe finds 'planetary missing link'

An artist's conception shows the rocky planet Kepler-10b

NASA's Kepler spacecraft has detected a rocky planet that's one of the closest analogs to Earth — except for the fact that it's way too close to its sun.

Rocky worlds have been detected around alien stars before, but Kepler-10b is the first of what's expected to be hundreds of Earth-scale planets found by the Kepler mission. It's too hot to sustain life as we know it, but it buoys hopes for finding other Earths and "super-Earths" that may be more habitable.

Natalie Batalha, an astronomer from San Jose State University who is part of the discovery team, said Kepler-10b is a "scorched world." The temperatures on the planet's sun-facing side would be 2,500 degrees Fahrenheit (1,370 degrees Celsius), almost hot enough to melt iron. Temperatures on the dark side would be too chilly for life, and Batalha said there was no chance that the planet could hold onto an atmosphere.

Kepler-10b's diameter is 1.4 times that of Earth, and its mass is 4.6 times Earth's, Batalha said. That makes it one of the smallest worlds ever found in a distant planetary system like our own. But if Kepler-10b were in our own solar system, it would orbit more than 20 times closer to the sun than Mercury — so close that it makes a complete orbit in just a little more than 20 hours.

Geoffrey Marcy, an astronomer at the University of California at Berkeley who is one of the pioneers in the effort to detect planets beyond our solar system, said the discovery "will be marked as among the most profound scientific discoveries in human history." Marcy explained that Kepler-10b served as a "planetary missing link" between the giant planets that dominate the list of more than 500 distant worlds found to date, and the Earth-size worlds that scientists hope to find in the future.

The find was reported today at the American Astronomical Socyty's winter meeting in Seattle, and a research paper on the discovery has been accepted for publication in the Astrophysical Journal.

How the world was found
The $600 million Kepler mission, launched in March 2009, looks for distant planets by staring at a patch of sky between the constellations Cygnus and Lyra. What it's looking for is the faint dimming of starlight that's produced regularly when a planet passes over the bright disk of the star it orbits. Kepler is monitoring 150,000 stars for those telltale signals, and in principle, it should be able to find Earthlike planets in Earthlike orbits around sunlike stars.

Kepler-10b, circling a star 560 light-years from Earth, was one of the mission's first good candidates for an Earth-scale planet. Its signature showed up in data collected while the spacecraft was being commissioned for science operations, just a couple of months after launch. Scientists collected eight months' worth of readings pointing to Kepler-10b's existence, but they needed to confirm that the planet was really there and get a better estimate of its mass and size.

For the planet's mass, they turned to the W.M. Keck Observatory's 10-meter telescope in Hawaii. The Keck telescope detected the right pattern of tiny wobbles in the movement of the parent star — which is similar to the mass and size of the sun but is more than 8 billion years old (as opposed to the 4.6 billion-year age of the sun).

To confirm the planet's size, astronomers had to figure out the width of the parent star. They resorted to analyzing high-frequency oscillations in the star's brightness that are caused by "starquakes." The oscillations can serve as an indication of how big the star is, just as the pitch of a pipe organ's musical note could be used to estimate the size of the pipe making that tone.

Batalha said the size of the star could be determined to an accuracy of 4 to 6 percent. The researchers combined that measurement with the others to confirm the planet's mass and size as well as its density.

"All of our very best capabilities have converged on this one result," she said.

Where Kepler-10b fits
The Kepler team reported that the exoplanet's density is 8.8 grams per cubic centimeter, which is far denser than Earth's 5.5 grams per cubic centimeter. Batalha said the best explanation for that density is that Kepler-10b is a rocky planet like Earth, only bigger. However, Kepler-10b's proximity to its star means that it would look nothing like Earth. The heat might well be blasting away rock, sending flurries of debris into space. She said mountains wouldn't have much chance to rise up, but canyons could be carved into the planet's surface by flowing lava.

Batalha recalled that a century ago, astronomers were looking for a planet that might orbit our own sun within Mercury's orbit, known as Vulcan. "The thing that came to me is, wow, this is our planet Vulcan," she said. 

A couple of years ago, a European planet-hunting probe called CoRoT detected a similar "lava planet," which has been designated CoRoT-7b. That planet is thought to be a little larger than Kepler-10b (1.8 times as wide as Earth, vs. 1.4 for Kepler-10b), and Batalha said there were still some uncertainties surrounding CoRoT-7b's size estimate. Uncertainties also surround the reported discovery of an Earth-scale planet known as Gliese 581g.

Berkeley's Marcy said the Kepler find would likely "go into textbooks" around the world, due in part to the innovations that were used to nail down the planet's vital statistics. But there's more to come: Batalha said that there might be yet another planet in the Kepler-10 system with an orbital period of 45 days. Those observations still had to be confirmed, she emphasized.

She also noted that the Kepler mission has turned up more than 300 other yet-to-be-confirmed planet candidate, with most of them thought to be smaller than Neptune. More revelations are likely to come to light in February, when the next big batch of Kepler data is due to be released. Although Kepler-10b may be a milestone, it's by no means the end of the road for planet-hunters.

"The discovery of Kepler 10-b is a significant milestone in the search for planets similar to our own," Douglas Hudgins, Kepler program scientist at NASA Headquarters in Washington, said today in a news release. "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."

More about the planet quest:

Correction: Ugh, did I really say two hours per orbit? I meant 20 hours, or 0.84 Earth days. These conversions always get me in trouble. Sorry about that.

Update for 3:35 p.m. ET Jan. 11: Kepler researcher Natalie Batalha told me that the research paper on this discovery lists the Kepler-10 star's age as 11.9 billion years, plus or minus 4 billion years. Some reports have gone with the 11.9 billion-year figure, but Batalha prefers to say that the star is "more than 8 billion years old," and so that's what I'm going with.

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