Add a tenth planet to the solar system -- or possibly subtract one.

Astronomers announced yesterday that they had found a lump of rock and ice that was larger than Pluto and the farthest known object in the solar system. The discovery will probably rekindle debate over the definition of ''planet'' and whether Pluto still merits the designation.

The new object -- as yet unnamed, but temporarily known as 2003 UB313 -- is now 9 billion miles away from the Sun, or 97 times as far away as Earth and about three times Pluto's current distance from the Sun. Its 560-year elliptical orbit brings it as close as 3.3 billion miles. Pluto's orbit ranges from 2.7 billion miles to 4.6 billion.

The astronomers do not have an exact size for the new planet, but its brightness and distance tell them that it is larger than Pluto, the smallest of the nine known planets.

''It is guaranteed bigger than Pluto,'' said Michael E. Brown, a professor of planetary astronomy at the California Institute of Technology and a member of the team that made the discovery. ''Even if it were 100 percent reflective, it would be larger than Pluto. It can't be more than 100 percent reflective.''

The discovery was made Jan. 8 at Palomar Observatory in California. Dr. Brown and the other members of the team -- Chadwick A. Trujillo of the Gemini Observatory in Hawaii and David L. Rabinowitz of Yale University -- then found that they had, unknowingly, taken images of the planet, using the observatory's 48-inch telescope, as far back as 2003.

Last year, the same team announced the discovery of a distant body they named Sedna, which, until the latest discovery, had held the title of farthest known object in the solar system. But Sedna, smaller than Pluto, is on a far stranger, 10,500-year orbit that takes it as far out as 84 billion miles.