3,666 million miles from sun. Diameter: 1,860 miles. Moons: 1 known. Length of year: 248 Earth years.
Pluto is so far away that the Sun is not much bigger or brighter than most of the other stars in its sky. It is only about two-thirds the size of our Moon and is a cold, dark, frozen place. Its composition is presumed to be rock and ice, with a thin atmosphere of nitrogen, carbon monoxide and methane. The Hubble Space Telescope has produced only fuzzy images of Pluto.
Pluto is usually the most distant planet in our solar system. Once every 248 Earth years, Pluto swings inside the orbit of Neptune for the next twenty years. During that time, Pluto is closer to the Sun than Neptune. While it is closer to the Sun, Pluto has an atmosphere. The methane and nitrogen frozen at the poles thaw out, rise, and temporarily form an atmosphere. As it moves toward its farthest point from the Sun, Pluto's atmosphere freezes and falls back on the surface of the planet.
Pluto has only one known moon, Charon, that is nearly as big as the planet itself. Some astronomers call Pluto and Charon a double planet because they are so close in size. Other astronomers do not think Pluto is a real planet at all. They think it may be a moon which escaped from Neptune's gravitational pull .
