Trans Neptune ObjectsDownload GOOGLE EARTH for free here:
There are several Objects outside the orbit of Neptune, called trans-Neptunian objects. Until now there are three areas of these Objects outside the Neptune Orbit: The Kuiper belt, the Scattered disk, and the Oort cloud.
Pluto and its moons are trans-Neptunian objects also. If Pluto had been discovered today, it most likely would not have been designated as a planet, but a trans-Neptunian object.
In the early 1900s the scientists found differences between the observed and the expected orbits of the planets known until then. They assumed that there are probably more planets outside Neptune. Searching for the so called "Planet X" led to the discovery of Pluto - but Pluto was way too small to explain the differences. Later revised estimates of the mass of the planet Neptune proved that this was unjustified.
After the discovery of Pluto 1930 it took until the 1990s to find another trans-Neptunian objects (except Charon, Plutos Moon). But in the last several years more than thousand trans-Neptunian objects have been found.
The largest trans-Neptunian objects are 2003 UB313 with a diameter of approx. 3000 ±400 km, which is even larger than Pluto, then Pluto with a diameter of approx. 2300 km, 2005 FY9 with a diameter of approx. 1800 ±200 km, 2003 EL61 with a diameter of approx. 1500 km, then Plutos moon Charon with a diameter of 1200 km.