Pluto

Pluto is a dwarf planet located in the Kuiper Belt, a region of icy bodies beyond the orbit of Neptune. Discovered on February 18, 1930 by Clyde Tombaugh at Lowell Observatory, Pluto was considered the ninth planet for 76 years before being reclassified as a dwarf planet by the International Astronomical Union in 2006. With a diameter of only 2,377 km, Pluto is smaller than Earth's Moon and even smaller than several other moons in the solar system. Pluto's highly eccentric orbit (eccentricity 0.2488) takes it from 4.4 billion km to 7.4 billion km from the Sun, and its orbital plane is tilted 17.16 degrees relative to the ecliptic -- far more than any of the eight major planets. At times during its 248-year orbit, Pluto actually comes closer to the Sun than Neptune, though the two objects never collide because of an orbital resonance. Pluto rotates on its side with an axial tilt of about 122.5 degrees and has a retrograde rotation, completing one rotation every 6.39 Earth days. NASA's New Horizons spacecraft flew past Pluto on July 14, 2015, providing the first close-up images of this distant world. The flyby revealed a surprisingly complex and geologically active surface, including Sputnik Planitia -- a vast heart-shaped plain of nitrogen ice over 1,000 km across. Mountains of water ice reaching 3-5 km in height border this plain, and evidence suggests Pluto may have a subsurface ocean of liquid water. Pluto has five known moons: Charon (so large relative to Pluto that the two are sometimes considered a binary system), Nix, Hydra, Kerberos, and Styx.