W ith a visual magnitude of 15.3, Pluto has long appeared only as a faint point of light. It has not been visited by any space probes, that is why our knowledge of Pluto is very limited. In the mid-1990s, however, the Hubble Space Telescope (HST) was able to provide images showing about 12 major zones where Pluto's surface is either bright or dark. Pluto is titled such that the planet appears to be "lying on its side" much as is Uranus.

T he planet may have a diameter estimated to be 1/6 of that of Earth. Its very thin atmosphere may lie only a few kilometers deep. The atmosphere appears to vary seasonally in thickness according to the planet's distance from the Sun. The HST images show that Pluto has ice caps, probably of frozen nitrogen. The darker areas are most likely methane frost colored by exposure to the Sun. The patterns of light and dark may represent seasonal redistribution of frost on a contoured planetary surface. Pluto's core, thought to be composed of silicate rock, may be relatively large, with a radius of nearly 885 km (550 miles). This would help to account for Pluto's apparent high density of about 2.1 g/cm³).

C haron, Pluto's grayish satellite, was discovered by American astrophysicist James W. Christy on June 22, 1978. Its average orbital path lies 19,000 km (11,800 miles) from the center of Pluto. Charon completes one revolution in 6.39 days, the same as Pluto's rotation period. The two objects may be relics of the early days of the solar system.