(M. Brown (Caltech), C. Trujillo (Gemini), D. Rabinowitz (Yale), NSF, NASA, apod050731)
The largest known KBO (Kuiper Belt Object), 2003 UB313, as imaged at three different times on October 21 of 2003, by the Palomar 48-inch Schmidt telescope. Curently about 19th magnitude, the object is near aphelion, 97 AUs (about 9 billion miles) or about twice as far as Pluto from the Sun. Original estimates of its size based on its brightness and distance suggested that it might be as much as 2000 miles in diameter, making it substantially larger than Pluto, which is only about 1420 miles in diameter; but recent observations with the Hubble Space Telescope (see image below) indicate that an unusually high albedo (in the range of 85%) makes it brighter than would be expected for an object of its size, and that it is at most 5% larger than Pluto, or around 1490 miles in diameter, with an uncertainty about equal to the difference in size between Pluto and 2003 UB313. In September 2006 the object was officially named Eris (prior to that anyone could call it anything they wanted; its discoverer nicknamed it Xena, while I called it Dubi-Ubi, because of its numerical appellation and uncertain status).
Initial reports in the news media suggested that since Eris was larger than Pluto it should be called a tenth planet; but it has been uncertain for years whether even Pluto should be called a planet, and as a result of Eris' discovery the International Astronomical Union made a controversial decision to demote Pluto to "dwarf planet" status, and to simultaneously elevate Eris and Ceres (the largest asteroid, or minor planet) to the same status.
2003 UB313 (Eris) in December 2005 (M. Brown, Cal Tech, ESA, HST, NASA)
A point nearly indistinguishable from a star, implying a high albedo and small size.
Like many other objects, whether large or small, made of rocks, gases or ices, Eris has a moon, which is seen to the right of the dwarf planet in this image taken with the 10-meter Keck Telescope. The distance and motion of Dysnomia (named after the daughter of Eris in Greek mythology) are very uncertain, but as better observations are obtained, the application of Kepler's Third Law to the orbital motion should yield accurate estimates of the mass and density of Eris. Pending those currently unknown results, it is presumed, based on their distance from the Sun, that both Eris and its moon are made of frozen compounds of carbon, nitrogen and oxygen with each other and hydrogen -- that is, water ice and other ices. (W. M. Keck Observatory, apod060918)
Relative sizes of the largest known trans-Neptunian objects (Pluto and KBOs) (relabeled NASA illustration)