(42301) 2001 UR163, also written as (42301) 2001 UR163, is a likely dwarf planet that resides in the scattered disc. It was discovered on October 21, 2001 by the Deep Ecliptic Survey (DES) program at Kitt Peak. Light-curve-amplitude analysis shows only small deviations, suggesting that 2001 UR163 is a spheroid with small albedo spots.Mike Brown's website lists it as a highly likely dwarf planet, but the diameter of the object has never been measured.
2001 UR163 has the reddest color index of any object in the Solar System. On October 31, 2002, the 3.6 meter Canada-France-Hawaii telescope observed 2001 UR163 set a record red reading of B-R=2.28. This makes 2001 UR163 even redder than 5145 Pholus, (119070) 2001 KP77, 90377 Sedna, and comet C/2001 T4. Reddening of the spectrum is caused by cosmic irradiaton by ultraviolet radiation and charged particles. Becoming bluer in the spectrum is caused by impact collisions exposing the interior of an object. In the visible spectrum2001 UR163 would appear orange-brown depending on its albedo.