is a trans-Neptunian object that orbits the Sun in the Kuiper belt beyond Neptune. This TNO briefly garnered scientific attention when it was found to have an unexpectedly low density of about 0.82 g/cm3. has an absolute magnitude of about 4.0, and Spitzer Space Telescope results estimate it to be about 681 km in diameter. The low density of this and many other mid sized TNOs implies that they have likely never compressed into fully solid bodies, let alone differentiated or collapsed into hydrostatic equilibrium, and so are highly unlikely to be dwarf planets. It was discovered on 30 October 2002, by the Spacewatch program. Numbering and naming This minor planet was numbered (55637) by the Minor Planet Center on 16 February 2003 (). , it has not been named. Classification has a perihelion of 36.7 AU, which it will next reach in 2065. As of 2020, is 40 AU from the Sun. The Minor Planet Center classifies as a cubewano while the Deep Ecliptic Survey (DES) classifies it as scattered-extended. The DES using a 10 My integration (last observation: 2009-10-22) shows it with a minimum perihelion (qmin) distance of 36.3 AU. It has been observed 212 times with precovery images dating back to 1991. Physical characteristics A variability of the visual brightness was detected which could be fit to a period of 14.38 or 16.78 h (depending on a single-peaked or double peaked curve). The light-curve amplitude is ΔM = . The analysis of combined thermal radiometry of from measurements by the Spitzer Space Telescope and Herschel Space Telescope indicates an effective diameter of and albedo of 0.107. Assuming equal albedos for the primary and secondary it leads to the size estimates of ~664 km and ~190 km, respectively. If the albedo of the secondary is half of that of the primary the estimates become ~640 and ~260 km, respectively. Using an improved thermophysical model slightly different sizes were obtained for UX25 and its satellite: 659 km and 230 km, respectively. has red featureless spectrum in the visible and near-infrared but has a negative slope in the K-band, which may indicate the presence of the methanol compounds on the surface. It is redder than Varuna, unlike its neutral-colored "twin" , in spite of similar brightness and orbital elements. Composition With a density of 0.82 g/cm3, assuming that the primary and satellite have the same density, is one of the