But now there is evidence there could be a much larger planet ten times further away.
The evidence is indirect and comes from the announcement this week of the discovery of another dwarf planet dubbed 2012 VP113. Analysis of the orbit of 2012 VP113 is interesting enough—at its closest approach it is about twice as far away as Pluto, but its highly eccentric orbit means that it gets as far away as ten times as far as Pluto, and it takes roughly 4000 years to orbit the Sun. It is only the second example ever found of a class of objects orbiting the Sun far beyond the Kuiper Belt, the other being a dwarf planet named Sedna found about ten years ago.
The really interesting part comes from an analysis of the orbits of Sedna and 2012 VP113. Models for the formation of the solar system show that it is highly unlikely that either 2012 VP113 or Sedna could have formed in their present locations, due to the eccentricity of their orbits. One explanation would be that they were perturbed into their current orbits by an as-yet-unseen planet about ten times the mass of Earth orbiting the Sun about thirty times further out than Pluto.
Tantalizing as it is it is only one possible scenario though, and there are other explanations—with so little data to go on it's just impossible to narrow it down at this point. But ... if it were to exist wouldn't it truly be a prince of the outer darkness?