Orbiting in the dim and frozen outlands of our solar system, Pluto was discovered in 1930. An early guess at Pluto’s size made it about the same as Earth, but estimates have been revised steadily downward, and now Pluto is classed as a dwarf planet only 1,430 miles (2,302 kilometers) in diameter.
Pluto and its largest satellite Charon are tidally locked to one another. That is, each presents the same face to the other, and neither body moves in the other’s sky. Similarly, our moon is tidally locked to the Earth and always presents its “near side” face to us.
Pluto and Charon both orbit around a point called the barycenter. In the only known such case in the solar system, the Pluto-Charon barycenter lies outside the body of either object. However, the International Astronomical Union has stopped short of classifying the system as a binary planet, instead calling Charon a satellite of Pluto.
Pluto is so distant that telescopes have been able to make only crude maps of it since the 1980s. This Hubble Space Telescope map (below) shows red areas indicating methane ice. The dark areas may be dirty water ice, and lighter areas indicate nitrogen frost. The bright spot near the center of the map could be carbon monoxide. Pluto’s surface has one of the widest contrast ranges in the solar system, comparable to Saturn’s moon Iapetus.
Pluto is surrounded by a thin atmosphere of nitrogen, methane and carbon monoxide. The atmosphere’s thickness varies as Pluto’s distance from the sun changes. As Pluto moves farther away, the gases freeze and fall to Pluto’s surface.
Pluto’s path around the sun is off-center and and highly inclined compared with the major planets. This means that some of the time Pluto is closer to the sun than giant planet Neptune. At its farthest, Pluto is 4.5 billion miles (7.3 billion kilometers) from the sun. Pluto takes 248 Earth years to orbit the sun.