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 (see orbital elements). 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.
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� 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 redistributions of frost on a contoured planetary surface. Pluto's core�perhaps of silicate minerals�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/cm7 (131 lb/ft7).
