One of the great puzzles of modern astronomy is whether the Solar System hides a distant undiscovered planet. If this planet were relatively near — orbiting close to or within the orbit of Neptune — it ought to have been discovered by now. But if it were much more distant — orbiting in the Kuiper Belt far beyond the orbit of Neptune, for example — astronomers would find it extremely hard to track down.
There is indeed evidence to support the existence of such a planet. This comes from the study of the small, icy bodies and dwarf planets, like Pluto, Haumea and Quaoar, that orbit beyond Neptune. These so-called trans-Neptunian Objects are not distributed evenly but seem to follow certain patterns of distribution, as if they were being herded be an unseen gravitational force.
