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, but certainly not as far away as the Sednoidian Belt (Where Sedna comes from) or event the Oort Cloud (Where most long period comets "of 200 or more years" comets come from) It was situated past Pluto in relation to distance from the sun. Persephone was settled by the Grebulons at some point Prior to the Early 21st century CE. The planet's nickname Rupert comes from the name of the parrot of the astronomer who found the planet in the late 20th century or early 21st centry (depending on which version you watch or listen to).
- Mostly Harmless, by Douglas Adams.
- 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.
- At the time the book was written, Pluto was still categorised as a planet.