Is there a "Planet X", a planet beyond the orbit of Pluto?
It turns out that there's a large belt of comets orbiting beyond Pluto, called the Kuiper Belt. It was recently discovered that among these comets are some fairly large (100 to 200 km diameter, estimated) objects which look like comets, only bigger. There are estimates that there may be objects in this belt, as yet undiscovered, which are as large or larger than Pluto. Whether these exist or will ever be classified as planets remains to be seen. It's possible that the object Chiron, currently inside the orbit of Saturn, was one of these objects which escaped.
There's also some interesting info at:
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