- 1 Greek Mythology the god of the underworld. Also called Hades.
Pluto usually orbits beyond Neptune at an average distance of 5,900 million km from the sun, though its orbit is so eccentric that at perihelion it is closer to the sun than Neptune (as in 1979–99). Pluto is smaller than earth’s moon (diameter about 2,250 km), but it has its own large satellite (Charon), and three other small moons. From the time of its discovery it was regarded as the ninth (outermost) planet of the solar system, but in the 1990s its unusual characteristics led astronomers to question its planetary nature. In August 2006 the International Astronomical Union formally declared Pluto to be a dwarf planet rather than a planet proper
via Latin from Ploutōn, the Greek name of the god of the underworld