Discovery of solar system's 10th planet


Sunday, July 31, 2005; Posted: 11:31 a.m. EDT (15:31 GMT)

PASADENA, California (CNN) -- Astronomers announced Friday that an object they discovered in the distant reaches of the solar system is large enough to be the 10th planet -- a claim likely to reignite a debate over just how many objects should be called planets.

The object -- 96 times as far from the Earth as the Earth is from the sun, or nearly 9 billion miles away -- was first photographed in October 2003 by astronomers at the California Institute of Technology's Palomar Observatory, north of San Diego.

While researchers say they aren't yet sure of its actual size, they have determined the object is bigger than Pluto, the smallest planet and the one most distant from the sun.

"If Pluto is a planet, it seems reasonable that something that's bigger than Pluto, and further away than Pluto, should be called a planet, too," said Mike Brown, a Cal Tech planetary scientist who made the discovery with colleagues Chad Trujillo of the Gemini Observatory and David Rabinowitz of Yale University.

However, a number of astronomers dispute whether Pluto, discovered in 1930, should really be classified as a planet, because it is so dissimilar from the other eight. Instead, they believe it should be classified only as a Kuiper Belt object, part of an array of icy debris in the outer reaches of the solar system.

Thousands of Kuiper objects have been discovered, and more are being found all the time.

Brown concedes that both Pluto and his new planet are Kuiper objects -- but he argues they are also both big enough to be classified as planets.

The International Astronomical Union, the official arbiter of such disputes, has classified Pluto as a planet and recently declined to demote it. Brown said resolving the argument over whether the object his team found is a planet will take years.

Brown's team has submitted a name for its proposed planet to the IAU, which won't be announced until the astronomy group hands down its ruling.

While the object was first photographed in 2003, its motion was not detected until January because it was so far away. Since then, astronomers have been studying the object to estimate its size and motion.

Brown said the planet-sized object probably wasn't discovered earlier because it was in a location where planets aren't expected.

"All of the planets are in a disc around the sun, and this object is 45 degrees out of that disc," he said.