Like the other Kuiper Belt Objects, the dwarf planet Pluto is so far away and so small that it is still mysterious in many ways. We do know, though, that Pluto is about 1,400 miles (2,300 kilometers) across, less than one-fifth the diameter of Earth, and smaller than the seven largest moons in the solar system. Pluto is composed mostly of ice and rock, with a surface temperature between –350 and –380 degrees Fahrenheit (–210 and –230 degrees Celsius); the bright areas observed on Pluto are most likely solid nitrogen, methane, and carbon dioxide. The dark spots may hold hydrocarbon compounds made by the chemical splitting and freezing of methane.

Pluto’s day is about six Earth days long, and its year is 248 Earth years long. Pluto travels in a highly elliptical orbit around the Sun compared to the terrestrial planets and gas giants. For twenty years out of its 248-Earth-year orbital period, it is actually closer to the Sun than is Neptune. (This phenomenon last occurred between 1979 and 1999.) When Pluto is closer to the Sun, its thin atmosphere exists in a gaseous state, and is composed primarily of nitrogen, carbon monoxide, and methane. For most of its very distant orbit, though, there is no standing atmosphere because it all freezes out and drops to the surface.

Pluto has no rings and five known moons. (Yes, dwarf planets—and even asteroids—can have moons.) The largest one, Charon, is large enough to be considered a dwarf planet in its own right.