Seventy-five years after the discovery of the planet Pluto, U.S. astronomers say they have discovered a tenth planet far beyond Pluto in the outlying region of the solar system. The object is so distant that the scientists have not yet been able to determine its precise size and mass.

California Institute of Technology astronomer Michael Brown and colleagues first saw this planet in January with a telescope at the Palomar Observatory near San Diego. They had planned to withhold an announcement until they could figure out its exact diameter and mass. But they rushed the news out prematurely Friday when they discovered that someone had broken into their internet website and learned of the finding, possibly with the intent of breaking the information.

Mr. Brown estimates the new planet to be about 1 1/2 times Pluto's size, but nearly twice as far from the Sun at the most distant point in its oval orbit. That makes it 97 times further from the Sun than Earth is. It is so far away that it takes 280 years to go around the Sun and is the farthest solar system object ever found.

"It's a very cold, very distant place. If you were standing on the surface and you held a pin at arm's length, you could cover the Sun with the head of the pin. It's not a place you'd really want to go for a summer vacation," he said.

The light emitted from the body indicates that its surface is covered mostly with methane ice, as Pluto is. Also like Pluto, it is in the same group of celestial objects known as the Kuiper Belt, a region beyond Neptune where icy bodies are abundant. Kuiper Belt objects such as Pluto and the new planet are also much higher than the plane of the first eight planets.

Because of the location and because it is smaller than the other seven, some scientists have wanted to demote Pluto from planetary status and call it an asteroid. Many may not recognize the new object as a planet, either. Mr. Brown says he once held the same opinion of Pluto. But he finally changed his mind and therefore sees the new body as a planet, too.

"Pluto has been called a planet for so long that I think we're never going to not call Pluto a planet. If Pluto is going to be called a planet, then anything larger than Pluto is a planet. Things that are smaller I think we just call them typical members of the Kuiper Belt and they don't join this very special class of things that are planets," he said.

Mr. Brown says the new planet is bright enough for amateur astronomers to see it now that the location is known. He notes that it remained undiscovered until now simply because it is so high above the plane of the solar system.

"This object is almost at a 45 degree angle outside of this plane. Nobody looks that high up in the sky for these sorts of objects. The only reason we have been looking that high is because we have looked everywhere else so far and that is where were looking next," he said.

The scientists have submitted the information and a suggested name to the International Astronomical Union, a professional organization in Paris that foster cooperation among astronomers in member national academies. They are not revealing the name until it is approved.