Astronomers say they’ve found the 10th planet

Way out there
The new world is about 97 astronomical units from the sun. An astronomical unit is the distance between the sun and Earth. 2003 UB313 becomes the farthest-known object in the solar system, and the third brightest of the Kuiper Belt objects.

It is colder than Pluto and "not a very pleasant place to be," Brown said.

It was found using the Samuel Oschin Telescope at Palomar Observatory.

Backyard astronomers with large telescopes, some experience and a map may be able to spot 2003 UB313.

"It will be visible over the next six months and is currently almost directly overhead in the early-morning eastern sky, in the constellation Cetus," said Brown, who confirmed the status of the object on Jan. 8 with colleagues Chad Trujillo of the Gemini Observatory and David Rabinowitz of Yale University.

The team had hoped to analyze the data further before announcing the planet, but were forced to do so Friday evening because word had leaked out, Brown said.

"Someone hacked our Web site," he said, and "they were planning to make [the data] public."

Brown and Trujillo first photographed the distant world with the 48-inch Samuel Oschin Telescope on Oct. 31, 2003. However, the object was so far away that its motion was not detected until they reanalyzed the data in January of this year. In the last seven months, the scientists have been studying the planet to better estimate its size and its motions.

Estimating size
Scientists infer the size of a solar-system object by its brightness and distance. The reflectiveness of the new planet is not known, however, which is why the estimate of its diameter ranges from one to two times the size of Pluto. But those constraints are well supported by the data, Brown said.

"Even if it reflected 100 percent of the light reaching it, it would still be as big as Pluto," said Brown. "I'd say it's probably one and a half times the size of Pluto, but we're not sure yet of the final size. But we are 100 percent confident that this is the first object bigger than Pluto ever found in the outer solar system."

The upper size limit is constrained by results from the Spitzer Space Telescope, which records heat in the form of infrared light. Because Spitzer can't detect the new planet, the overall diameter must be less than twice Pluto's size, Brown said.

Brown has had a running bet for five years with a friend that an object larger than Pluto would be found by Jan. 1 this year. 2003 UB313 was identified as such an object on Jan. 8.

"My first reaction was, 'Aw, I lost the bet by seven days,'" he said.

Brown's team has submitted a name proposal to the International Astronomical Union and has chosen not to divulge it until that body makes a decision.

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