Orbiting in the dim and frozen outlands of the solar system, Pluto remained undiscovered until 1930. An early guess at Pluto’s size had it about the same as Earth’s. Now Pluto is known to be a dwarf planet only 1,473 miles (2,370 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.