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.
Persephone appeared in the Douglas Adams novel Mostly Harmless.
At the time the book was written, Pluto was still categorised as a planet.