Astronomers at the California Institute of Technology have discovered what they believe is the 10th and most distant planet in our solar system, a ball of rock about twice as big as Pluto and about three times as far out.
The new object, temporarily called 2003 UB313, is currently at its farthest distance from the sun, about 97 times the distance between the sun and Earth.
It lies in the fringes of the Kuiper Belt, a conglomeration of asteroids, comets, and other materials circling the sun well beyond the orbit of Pluto.
When it gets halfway through its orbit in 280 years, 2003 UB313 will be about 36 times the Earth-sun distance -- or nearly as close as Neptune.
''If Pluto is a planet, then anything larger than Pluto is a planet, and this is definitely larger than Pluto," said Caltech astronomer Michael A. Brown, who announced the discovery at a press conference yesterday.
Brown's claim of planethood for 2003 UB313 is certain to be controversial. Astronomers have long debated whether Pluto is a planet because of its small size and odd orbit, although many scientists are content to retain its current designation.
The surface of the new object is similar to that of Pluto, a mixture of about 70 percent rock and 30 percent water ice. It is probably about minus 400 degrees Fahrenheit. ''It's not a very pleasant place to live," Brown said.
Brown said 2003 UB313 has probably not been discovered before because its orbit lies at a 45-degree angle from the plane, known as the ecliptic, in which the nine known planets circle the sun.
''Nobody looks that high up in the sky," he said. He only looked there, he said, because he could not find any more objects in the ecliptic.
Brown first saw the new planet Jan. 8 using the 48-inch Samuel Oschin Telescope at Palomar Observatory, along with his colleagues Chad Trujillo of the Gemini Observatory and David Rabinowitz of Yale University.
It had actually been photographed first in 2003 -- hence the name -- but nobody realized what it was until its motion became apparent.
The planet 2003 UB313 has since been observed with other telescopes. Researchers have attempted to measure its heat output with the Spitzer Space Telescope, but the orbiting observatory could not find it, putting an upper limit on its size of twice Pluto's diameter.