Pluto is the ninth planet from the Sun but was recently re-categorized as a dwarf planet and plutoid.
Slimer was mistaken for a typical Earthling and brought to a facility on Pluto for a series of tests to determine his weaknesses. However, Slimer enjoyed the tests and indirectly caused the machinery to overload and destroy the facility. Half of the planet was blown up as a result. Slimer's captors, Morty Feeb and Captain Zeto, returned him back to Earth.