Pluto

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 as well as earlier discoveries showing Pluto was much smaller than originally presumed. Characteristics:

• 0.00218 Earth masses (0.177 Moon masses).
• 1188 km radius (0.186 Earth radii or 0.68 Moon radii).
• 122 degrees - axial tilt: its rotation is retrograde to its orbit, like that of Venus.
• 6 days 9 hours - solar day.
• 248 years - orbital period.
• 39.5 AU - orbital radius (semi-major axis).
• 0.2488 - orbital eccentricity.

Pluto has five known moons:

• Charon (discovered in 1978)
• Nix (2005)
• Hydra (2005)
• Kerberos (2011)
• Styx (2012)

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. All five moons' have retrograde orbits in comparison to Pluto's own orbit and all five come close to sharing a single orbital plane, which is highly tilted compared to Pluto's.

The New Horizons mission observed Pluto during a 2015 flyby.