Astronomers said Friday that they have discovered a planet larger than Pluto in orbit around the sun, an announcement likely to renew debate over what constitutes a planet and whether Pluto should keep its status.

The planet -- the farthest known object in the solar system -- is currently 9 billion miles from the sun, or about three times Pluto's distance.

"This is the first object to be confirmed to be larger than Pluto in the outer solar system," said Michael Brown, a planetary scientist at the California Institute of Technology.

Brown called the object a 10th planet, but some scientists dispute the classification of Pluto as such.

Astronomers do not know the new planet's exact size, but its brightness shows that it is at least as large as Pluto and could be up to 11/2 times as big. The research was funded by NASA.

Brown has submitted a name for the new planet to the International Astronomical Union, which has yet to act on the proposal. He did not release the proposed name Friday.

The briefing was arranged after Brown received word that a secure Web site containing the discovery was hacked and the hacker threatened to release the information.

Brown and colleagues Chad Trujillo of the Gemini Observatory and David Rabinowitz of Yale University first photographed the object in 2003 using a 48-inch telescope at the Palomar Observatory. But it was so far away that its motion was not detected until data was analyzed again in January. It will take at least six months for astronomers to determine its exact size.

It has taken scientists this long to find the planet because its orbit is at an angle compared with the orbits of most planets. The new planet is rocky and icy, similar to Pluto, Brown said.

Alan Stern of the Southwestern Research Institute in Boulder, Colo., said he was not surprised by the discovery because other objects around the size of Pluto have been found in a disc of icy debris beyond Neptune.