Pluto, the outermost planet in our solar system, appears here in two images: one taken with a ground- based telescope and the other with the Hubble Space Telescope. Named after the Roman god of the underworld, Pluto lies on average 5.9 billion kilometers from the Sun (about 39 times the distance from Earth to the Sun). Indeed, the planet is so remote and faint that it was not discovered until 1930, when American astronomer Clyde Tombaugh found it recorded on photographic plates taken at the Lowell Observatory in Arizona.
Pluto is peculiar in many respects. Because its mass is only one 25th that of the next smallest planet, Mercury, and only about two times that of the largest
