Originally regarded as the smallest and most distant of the nine ‘traditional’ planets in the Solar System, Pluto was reclassified in 2006 by the International Astronomical Union as a dwarf planet and is now assigned the minor planet number (134340). This reclassification recognizes the fact that Pluto has many characteristics that distinguish it from the eight planets from Mercury to Neptune. It is far smaller than any of those, with a diameter of only 2377 km, less than that of our Moon. Its orbit has a greater inclination than any of the major planets, 17°.1 to the ecliptic, and its orbit is also the most elliptical (eccentricity 0.25). At aphelion Pluto lies 7375 million km from the Sun, but only 4425 million km at perihelion, inside the orbit of Neptune; it last reached perihelion in 1989. Its mean opposition magnitude is +15. Pluto was discovered in 1930 by C. W. Tombaugh. Its rotation axis is tilted at 122°.5 to its orbital plane, so that its rotation is retrograde, and it presents its poles and its equator alternately towards the Sun and the Earth as it moves around its orbit. Its axial rotation period, 6.387 days, is the same as the orbital period of its largest satellite, Charon, so that Pluto always keeps the same face towards Charon. Two much smaller and more distant moons, Hydra and Nix, were discovered by the Hubble Space Telescope in 2005, a fourth, Kerberos, in 2011, and a fifth, Styx, in 2012.

Pluto has an extremely thin atmosphere with a surface pressure of about 10 μ‎bar, composed of methane, with some nitrogen and carbon monoxide. This atmosphere may be seasonal, forming when the planet heats up and releases surface volatiles around the time of perihelion. Methane may escape from the atmosphere near perihelion, so that Pluto behaves somewhat like a comet. Its mean surface temperature is estimated at −220°C. Pluto is thought to have a large rocky core, probably surrounded by a layer of frozen water and other icy materials, and a surface layer of methane. Pluto is now regarded as simply the largest of the sub-group of trans-Neptunian objects known as Plutinos. The New Horizons probe flew past Pluto in 2015 July finding apparent cryovolcanoes as well as various formations resulting from surface–atmospheric interactions, glacial flows, and impacts. Cthulhu Regio is an elongated dark area along the equator thought to be composed of complex hydrocarbons (so-called tholins). Tombaugh Regio is a large, bright, heart-shaped area, probably an old impact feature. Its western lobe, Sputnik Planitia, contains frozen nitrogen and other ices which flow like glaciers. These smooth regions are estimated to be less than 10 million years old, indicating that Pluto is geologically active.

https://photojournal.jpl.nasa.gov/catalog/PIA19952