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

We present the discovery and initial physical and dynamical characterization of the object 2003 UB313. The object is sufficiently bright that for all reasonable values of the albedo it is certain to be larger than Pluto. Prediscovery observations back to 1989 are used to obtain an orbit with extremely small errors. The object is currently at aphelion in what appears to be a typical orbit for a scattered Kuiper Belt object, except that it is inclined by about 44 from the ecliptic. The presence of such a large object at this extreme inclination suggests that high-inclination Kuiper Belt objects formed preferentially closer to the Sun. Observations from Gemini Observatory show that the infrared spectrum is, like that of Pluto and 2005 FY9, dominated by the presence of frozen methane, although visible photometry shows that the object is almost neutral in color compared to Pluto's extremely red color. 2003 UB313 is likely to undergo substantial seasonal change over the large range of heliocentric distances that it travels; at its current distance, Pluto is likely to prove a useful analog for better understanding the range of seasonal changes on this body.