Pluto, the ninth planet from the Sun, is the smallest and most remote planet known in the Solar System. The astronomer Percival Lowell, at his private observatory in Flagstaff, Ariz., instituted a search for another planet that eventually resulted in the discovery of Pluto by Clyde W. Tombaugh on Feb. 18, 1930. It was named for the Pluto of mythology.

Pluto's average distance from the Sun is 5.9 billion km (3.66 billion mi, or 39.44 astronomical units), but because of a high orbital eccentricity (0.249), it comes as near as 4.42 billion km (2.75 billion mi.) and travels as far as 7.40 billion km (4.60 billion mi.) from the Sun. This unusual orbit brings Pluto inside the orbit of planet Neptune during its close approach to the Sun--as, for example, during the current period between Jan. 23, 1979, and Mar. 15, 1999. The actual orbital paths do not cross, however. Pluto's orbit is inclined an unusually high 16 degrees to the ecliptic, or Earth-Sun orbital plane. The planet revolves around the Sun once every 248.4 years. In 1988 a computer simulation revealed that the orbit of Pluto is chaotic--that is, not completely predictable.

With a visual magnitude of 15.3, Pluto has long appeared only as a faint point of light in even the largest telescopes, and it has not been visited by any space probes. 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's axis is tilted more than 112 degrees to its orbital plane, so that the planet is "lying on its side" much as is Uranus. The planet may have a diameter of about 2,284 km (1,416 mi.). Its very thin atmosphere may lie a few kilometers deep, however, making the diameter figure uncertain. The atmosphere varies 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 re-distributions of frost on a contoured planetary surface. Pluto's core--perhaps of silicate rock--may be relatively large, with a radius of nearly 885 km (550 mi.). This would help to account for Pluto's apparent high density of about 2.1 g/cu cm (131 lb./cu ft).

Charon, 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 mi.) from the center of Pluto at an inclination of 55 degrees to the ecliptic. It may have a trace atmosphere and is about 1,160 km (721 mi.) wide. 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.