A trans-Neptunian object (TNO) is any object in the solar system with all or most of its orbit beyond that of Neptune. The Kuiper belt and Oort cloud are names for some subdivisions of that volume of space. Pluto and its moon Charon are trans-Neptunian objects, and if Pluto had been discovered today, it might not have been called a planet.
Gravity, the attracting force between all matter, also attracts planets. Due to the changes in the orbits of the known planets in the early 1900s, it was assumed that there was one or more planets beyond Neptune, but not yet identified. (See Planet X.) This search for these led to the discovery of Pluto, and since then a few other significant objects have been found. These are still too small to explain the perturbations, though, and revised estimates of Neptune's mass showed that the problem was fictitious.
Trans-Neptunian objects of note:
The hypothetical companion star Nemesis would have fallen under the definition of trans-Neptunian object, but it appears that no such star actually exists.
A fuller list of objects is being compiled in the list of trans-Neptunian objects.
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