Pluto (134340 Pluto) is a dwarf planet (roughly 1180 km radius) in an eccentric orbit ranging roughly from 30 AU (Neptune's orbital radius) to 50 AU. It has a steeper orbital inclination than the eight planets: 17° from the ecliptic. It and Neptune share an orbital resonance with a ratio of 3:2. Pluto was discovered in 1930 and was considered the ninth planet until the discovery of similar objects. Five moons are known:
Charon's mass is 1/6 of Pluto's and its radius about 1/2 Pluto's, by far the largest moon in the solar system compared its host planet/dwarf planet. In comparison, the Moon has 1/81 the mass and 1/4 the radius of Earth, and the Moon/Earth has been referred to as a double planet, since the size ratios of the other known moons to their host planets (Mars, Jupiter, etc.) are far greater.
The New Horizons mission observed Pluto during a 2015 flyby.