Persephone, commonly called Rupert, was a planet somewhere in Sol's outer solar system, either in the Kuiper 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.
- At the time the book was written, Pluto was still categorised as a planet. However, by the time the movie was made, Pluto was getting a change in status.
- 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. It's possible that Eris is the real-life counterpart of Rupert/Persephone; both names were popular choices with the public when the planet's name was yet to be decided. It is, however, presently unknown whether any Grebulons are currently residing there or not.