Pluto was discovered in 1930 and is the second-largest known dwarf planet, behind Eris. It was originally considered to be the ninth planet in the solar system, but was reclassified as a dwarf planet in 2006. It is composed mostly of rock and ice.
A single day on Pluto is roughly equivalent to 6.4 days on Earth, as it takes Pluto 153 Earth hours to rotate once on its axis. By the time Pluto orbits once around the sun, 248 years have passed on Earth. If Earth was the size of a nickel, Pluto would be the size of a head of a pin. It is two-thirds the diameter of Earth's moon.
Pluto cannot support any forms of known life, but scientists believe there may be an ocean hidden underneath its surface. Pluto is similar to a comet in that its atmosphere expands when approaching the sun and contracts when moving away from it.
Pluto was named by 11-year-old Venetia Burney of Oxford, England in 1930. She named it after the Roman god of the underworld. Her grandfather sent her suggestion to the Lowell Observatory and it was accepted. Pluto's five known moons are also named after mythological figures associated with the underworld: Charon, Nix, Hydra, Cherberos and Styx.