Pluto is a dwarf planet located in the Solar System. From 1930 to 2006, Pluto was recognized as the 9th and furthest planet from the Sun on average (between 4.4 and 7.4 billion kilometers from the Sun at any given time), but in 2006 it was reclassified as a dwarf planet. It has one satellite named Charon.
Currently, Pluto is one of the least understood objects in the Solar System due to its extreme distance from the Earth. It has a diameter of roughly 2300 km, making it smaller than the Moon by about 33%. Pluto's thin atmosphere is comprised primarily of nitrogen, methane, and carbon monoxide, the same substances that make up its icy surface. The surface temperature is approximately -230 degrees Celsius.
In the Cowboy Bebop universe, Pluto is the site of a maximum security penitentiary, which is fitting considering its desolate environment and remote distance from the other planets. A prison ship destined for Pluto Penitentiary filled with criminals, among them Udai Taxim, malfunctioned in Black Dog Serenade.