Discovery of a Planetary-sized Object in the Scattered Kuiper Belt

We present the discovery and initial physical and dynamical characterizationof the object 2003 UB313. The object is sufficiently bright that for allreasonable values of the albedo it is certain to be larger than Pluto.Pre-discovery observations back to 1989 are used to obtain an orbit withextremely small errors. The object is currently at aphelion in what appears tobe a typical orbit for a scattered Kuiper belt object except that it isinclined by about 44 degrees from the ecliptic. The presence of such a largeobject at this extreme inclination suggests that high inclination Kuiper beltobjects formed preferentially closer to the sun. Observations from GeminiObservatory show that the infrared spectrum is, like that of Pluto, dominatedby the presence of frozen methane, though visible photometry shows that theobject is almost neutral in color compared to Pluto's extremely red color. 2003UB313 is likely to undergo substantial seasonal change over the large range ofheliocentric distances that it travels; Pluto at its current distance is likelyto prove a useful analog for better understanding the range of seasonal changeson this body.Comment: 9 pages, 1 figur