Pluto (dwarf planet)
Pluto, officially designated 134340 Pluto is a dwarf planet in the Solar System. It was called the ninth planet until 2006, when the International Astronomical Union decided that it was too small to be a planet, and is only a dwarf planet. 
Pluto was originally discovered in 1930 and its moon Charon was discovered in 1978. Now, it is known that it has four other moons beside Charon: Hydra and Nix were found in 2005, and two more moons, Kerberos and Styx were found in 2012. All the moons are named after characters from Greek mythology.