The Kuiper belt is an area of the solar system extending from the orbit of Neptune (at 30 AU) to 50 AU from the Sun. The objects within the Kuiper Belt together with the members of the scattered disk extending beyond are collectively referred to as trans-Neptunian objects.
The interaction with Neptune (2:1 orbital resonance) is thought to be responsible for the apparent edge at 48 AU (a sudden drop in the number of objects) but the current models have yet to explain this peculiar distribution in detail.
Over 800 Kuiper belt objects (KBOs) have been discovered in the belt, almost all of them since 1992. Among the largest are Pluto and Charon, but since the year 2000 other large objects that approached their size were identified. 50000-Quaoar, discovered in 2002, which is a KBO, is half the size of Pluto and is larger than the largest asteroid, 1-Ceres 2005-FY9 and 2003-EL61, both announced on 29 July 2005, are larger still. Other objects, such as 28978-Ixion (discovered in 2001) and 20000-Varuna (discovered in 2000) while smaller than Quaoar, are nonetheless quite sizable. Sedna, a small red planetoid with a diameter roughly half-way between Pluto and Quaoar, was first sighted on November 14th 2003. The exact classification of these objects is unclear since they are probably fairly different from the asteroids of the asteroid belt. The largest recent discovery is 2003-UB313, codenamed Xena.
It has led scientists to question the definition of the term Planet, as it is larger than Pluto and has already been called a tenth planet by some.
On July 30th 2005 the discovery of a tenth planet was announced by its discoverer Michael E. Brown. Its official scientific name is 2003 UB313 or "Eris" but its informal codename used by its discoverers is Xena. The International Astronomical Union published the definition of the term "planet" in early September 2006, which decided that 2003 UB313 is not classified as a planet, nor is Pluto.
The dwarf planet is very cold and has an almost white surface. Probably it is covered with frozen Methane. Its axis is lent for 44 degrees compared to the other planets. This is one reason why it wasn't discovered earlier. The dwarf planet is located in the Kuiper belt and is slightly bigger than Pluto.
Eris has the moon with the name Dysnomia, which has is at least one tenth of Eris' size.
- Distance from the Sun: 97 AU
- Diameter: 2390 km (0.188 times Earth)
- Orbit period: 557 years = 203,440 days