Persephone, commonly called Rupert, was a planet in Earth's solar system, situated past Pluto. 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 2005 January, 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. It is 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.