Pluto

Pluto

Pluto, the dwarf planet beyond Neptune, was mentioned briefly in Mostly Harmless. At the time of writing, Pluto was still thought to be the ninth planet as opposed to a dwarf planet.

In Mostly Harmless, there was a tenth planet beyond Pluto, called Persephone and nicknamed Rupert after an astronomer's pet parrot, which was the planet where the Grebulons settled when their ship lost almost all memory.