Pluto is the most famous dwarf planet and for 76 years the ninth planet in our Solar System. Its 2006 reclassification as a dwarf planet triggered worldwide controversy that continues to the present. Scientifically, Pluto is the largest known Kuiper Belt Object by volume and the prototype of the “plutinos” — KBOs locked in 2:3 mean-motion resonance with Neptune.

NASA’s New Horizons mission flew past Pluto on 14 July 2015, transforming it from a dim telescopic dot into a stunningly varied world. Sputnik Planitia — the western “heart” of Pluto’s famous heart-shaped region Tombaugh Regio — is a 1,050 km wide basin filled with slowly convecting nitrogen ice. Towering water-ice mountains rise around the basin; Pluto’s thin nitrogen atmosphere produces blue-hued photochemical haze; and complex tholins stain the equatorial Cthulhu Macula red-brown.

Pluto and its largest moon Charon are mutually tidally locked, forming a true binary system with a barycenter outside Pluto’s surface. Five known moons — Charon, Styx, Nix, Kerberos and Hydra — make the Pluto system a miniature planetary system of its own.