Posted: Fri, Jul 29, 2005, 9:45 PM ET (0145 GMT)
Planetary astronomers announced late Friday that they have discovered a distant object that, based on its size, would qualify as the solar system's tenth planet. The object, designated 2003 UB313, was first seen by astronomers at Palomar Observatory in late 2003; since then observers have tracked the object to determine its orbit and brightness. They announced Friday that the object is currently 97 astronomical units from the Sun, making it the farthest known object in the solar system. Based on that distance and its size, Caltech scientists concluded that the object is at least as large as Pluto, and potentially 50% larger. Efforts by the Spitzer Space Telescope to detect 2003 UB313 failed, putting an upper limit on its diameter at around 3,000 kilometers. The object is the first object in the Kuiper Belt found to be bigger than Pluto, the smallest planet, hence the claim that 2003 UB313 is the "tenth planet" in the solar system. The discovery may raise new questions about whether objects like 2003 UB313 and even Pluto should be categorized as planets. The announcement comes after Spanish astronomers reported the discovery of another large Kuiper Belt object, 2003 EL61; the Caltech team believes that 2003 UB313 is larger but that 2003 EL61 has a small moon.

Related Links:

•  Sky and Telescope article
•  AP article
•  New York Times article
•  BBC article
•  NASA press release