Pluto
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Pluto is a dwarf planet in the Kuiper belt, a ring of bodies beyond the orbit of Neptune. It is the ninth-largest and tenth-most-massive known object to directly orbit the Sun. It is the largest known trans-Neptunian object by volume, by a small margin, but is less massive than Eris. Like other Kuiper belt objects, Pluto is made primarily of ice and rock and is much smaller than the inner planets. Pluto has roughly one-sixth the mass of Earth's moon, and one-third its volume.
Pluto, imaged by the New Horizons spacecraft, July 2015. The most prominent feature in the image, the bright, youthful plains of Tombaugh Regio and Sputnik Planitia, can be seen at right. It contrasts the darker, cratered terrain of Belton Regio at lower left
Clyde Tombaugh, in Kansas
Sputnik Planitia is covered with churning nitrogen ice "cells" that are geologically young and turning over due to convection.
Multispectral Visual Imaging Camera image of Pluto in enhanced color to bring out differences in surface composition.
Moons of Pluto
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The dwarf planet Pluto has five natural satellites. In order of distance from Pluto, they are Charon, Styx, Nix, Kerberos, and Hydra. Charon, the largest, is mutually tidally locked with Pluto, and is massive enough that Pluto and Charon are sometimes considered a binary dwarf planet.
Charon and Pluto, to scale. Photo taken by New Horizons on approach.
The Hubble discovery image of Nix and Hydra
Discovery image of Styx, overlaid with orbits of the satellite system
Formation of Pluto's moons. 1: a Kuiper belt object approaches Pluto; 2: it collides with Pluto; 3: a dust ring forms around Pluto; 4: the debris aggregates to form Charon; 5: Pluto and Charon relax into spherical bodies.