More on UB313
Mike Brown and his collaborators have today posted the discovery paper for 2003 UB313 on the astro-ph archive. In the paper they describe the discovery technique used to find this and other minor planets they have previously found, as well as photometry and spectroscopy of UB313's surface.
The paper has a lot of interesting facts and figures about 2003 UB313 such as the fact that it doesn't reach perihelion (the point in the orbit where it is closest to the Sun) until 2257, and even then it is still more than 38 times further away from the Sun than the Earth is. The photometry they have done shows that it's diameter is 2250 x pr-1/2 km, where p is the albedo, through a particular filter known as R-band. This is how they know that UB313 must be bigger than Pluto. If you assume an albedo of 100% (a perfect reflector) then this relationship gives a diameter larger than Pluto's. They also looked at it's spectrum and found that, although it is similar to Pluto's, there are certain differences. Both have large amounts of methane, but other molecules seen on Pluto such as N2, CO and CO2 have not been detected so far on UB313.
As UB313 has such an eccentric orbit, the temperature extremes are greater than those on Pluto. This provides a new laboratory for planetary scientists to investigate low temperature chemistry in the outer solar system.