Solar System -

Trans-Neptunian Objects

While no longer classified as a planet, Pluto remains the ninth largest, and tenth heaviest, object that orbits the sun directly.  It is 2,372 km (1,483 miles) across, compared, say, to the Moon which is 3,474 km (2,159) in diameter, so it is relatively small.  In fact, the SDO Eris is more than 25% heavier, though a few kilometers less in diameter at 2,326 km (1,454 miles) across.  Pluto has five moons: Charon , Styx , Nix, Kerberos, and Hydra in order outwards from Pluto. 

Pluto orbits the sun every 248 Earth years. While the other planets orbit more or less in the plane of the ecliptic, Pluo's orbit is inclined over 17.  Its orbit is also rather an eccentric elipse taking it closer to the Sun than Neptune during part of its orbit for a period of about 20 Earth years.  Pluto's orbit is in a 2:3 resonance with Neptune's meaning that for every three orbits Neptune makes around the Sun, Pluto makes just two, repeating every 496 years (approximately). 

Pluto System

Name Diameter/Size (km) Diameter/Size (Miles) Orbits Pluto every (Days) Discovered
Charon 1,212 758 6.39 1978
Styx 16 x 9 x 8 10 x 5 x 5 20.16 2012
Nix 50 x 35 x 33 31 x 22 x 21 24.85 2005
Kerberos 19 x 10 x 9 12 x 6 x 6 32.17 2011
Hydra 65 x 45 x 25 41 x 28 x 16 38.2 2005
Click here to see a series of Images of the Pluto system.