Persephone, commonly called Rupert, was a planet somewhere in Earth's outer solar system, either in the Kuplier Belt or even as far away as the Scattered disk, It was situated past Pluto in relation to distance from the sun. Persephone was settled by the Grebulons. The planet's nickname Rupert comes from the name of the parrot of the astronomer who found the planet.
In January 2005, an actual object larger than Pluto, later designated as a dwarf planet and named 'Eris', was discovered by a Palomar Observatory–based team led by Mike Brown, and its identity was verified later that year. Its still unclear if Eris really is meant to be Rupert/Persephone. It is also presently unknown whether any Grebulons are currently residing there or not.
Persephone appeared in the Douglas Adams novel Mostly Harmless.
At the time the book was written, Pluto was still categorised as a planet. However by the time the movie was made, it was as Pluto was getting a change in status.