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.