Lurking on the edge (and beyond) of our Solar System are the strange set of distant objects that astronomers describe as "Trans-Neptunian Objects". The closest of these are the objects of "Kuiper Belt".
According to Wikipedia:
The Kuiper belt, sometimes called the Edgeworth-Kuiper belt, is a region of the Solar System beyond the planets extending from the orbit of Neptune (at 30 AU) to approximately 55 AU from the Sun. It is similar to the asteroid belt, although it is far larger; 20 times as wide and 20-200 times as massive. Like the asteroid belt, it consists mainly of small bodies (remnants from the Solar System's formation) and at least one dwarf planet -- Pluto. But while the asteroid belt is composed primarily of rock and metal, the Kuiper belt objects are composed largely of frozen volatiles (dubbed "ices"), such as methane, ammonia and water.
Since the first was discovered in 1992, the number of known Kuiper belt objects (KBOs) has increased to over a thousand, and more than 70,000 KBOs over 100 km in diameter are believed to reside there.
Surprisingly, the Kuiper Belt is not the furthest component of our Solar System. Beyond the Kuiper Belt is the Scattered Disk -- which is best thought of as a very sparse and scattered collection of the kinds of objects that make up the Kuiper Belt. And beyond the Scattered Disk is a cloud of comets that stretches out into interstellar space. That region is called the Oort Cloud. Taken together, the objects that make up the Kuiper Belt, the Scattered Disk and the Oort Cloud are described as "Trans-Neptunian Objects".
The possibility that Pluto was a very large Kuiper Belt object -- rather than a true planet -- was suspected from the moment that the Kuiper Belt was discovered. The argument was pretty much theoretical until 2003, when an object was discovered that was larger than Pluto. That object was actually located beyond the Kuiper Belt, in the Scattered Disk. It was eventually named "Eris".
Back to Wikipedia:
Eris, formally designated 136199 Eris and formerly 2003 UB313, is the largest known dwarf planet in the Solar System and the ninth largest body known to orbit the Sun directly. It is approximately 2,500 kilometres in diameter and 27% more massive than Pluto.
Eris was first spotted in 2003 by a Palomar Observatory-based team led by Mike Brown but not identified until 2005. It is a trans-Neptunian object (TNO) native to a region of space beyond the Kuiper belt known as the scattered disc. Eris has one moon, Dysnomia; recent observations have found no evidence of further satellites. The current distance from the Sun is 96.7 AU, roughly three times that of Pluto. With the exception of some comets, the pair are the most distant known bodies in the Solar System.
Because Eris is larger than Pluto, its discoverers and NASA called it the Solar system's tenth planet. This, along with the prospect of other similarly sized objects being discovered in the future, motivated the International Astronomical Union (IAU) to define the term "planet" for the first time. Under a new definition approved on August 24, 2006, Eris was designated a "dwarf planet" along with Pluto and Ceres.
If you look at the image that accompanies this post. You'll notice that 2003 UB313 is described as "Xena" rather than "Eris". "Xena" also has a moon named (naturally) "Gabrielle". But Eris' moon is actually named "Dysnomia". What's up with that?
Well, "Xena" was the nickname that the team of astronomers gave to 2003 UB313 when they discovered it. They claim that they never intended it to be the actual final name of the object, but I have my doubts about that. In any case, there was no way that the squares at the International Astronomical Union would have allowed them to get away with naming their discovery after a television show character. So instead, 2003 UB313 was named "Eris", after the Greek goddess of discord and strife.
Which brings us to the name of Xena's... uhm, Eris' moon. In Greek mythology, "Dysnomia" was the name of Eris' daughter. However, that name roughly translates as "Lawlessness"...