Pluto, distant dwarf planet
Pluto is a cold dark dwarf planet in which the Sun appears as only a bright star in the sky. Pluto was discovered as the result of a telescopic search inaugurated in 1905 by the American astronomer Percival Lowell, who postulated the existence of a distant planet beyond Neptune as the cause of slight perturbations in the motions of Uranus. Continued by members of the Lowell Observatory staff, the search ended successfully in 1930, when the American astronomer Clyde William Tombaugh found Pluto near the position Lowell had predicted. It was known as the ninth and outermost planet in the Solar System for over 70 years until re-designated as a dwarf planet by the International Astronomical Union following the discovery of a new solar system body: Eris.
Pluto is smaller than the Earth's Moon and consists of rock and ice. It has a thin atmosphere that forms when it is close to the Sun but freezes as it moves away. Pluto's orbit is tilted farther from the ecliptic, 17.2°, than that of any of the eight planets and is also very elongated. One orbit lasts 248.5 years at an average distance of 5.9 billion km (3.67 billion miles) from the Sun, and for about 20 of those years, Pluto is closer to the Sun than Neptune is. These unusual features lead some astronomers to propose that it may, in fact, be a huge asteroid.
Visible only through large telescopes, Pluto appears to have a yellowish colour. Charon, Pluto's largest moon and discovered in 1978, probably once had a similar composition to its parent dwarf planet. Today, however, Charon is covered mainly by dark water ice, and Pluto mainly by bright methane ice. It may be that methane molecules have been gradually attracted away from Charon toward Pluto by the larger body's stronger gravitational field. Like any objects in an orbital system, Pluto and Charon orbit around a common centre of mass. A large moon, Charon's diameter (1270km - 790 miles) is about half that of Pluto (2320km - 1440 miles) and accounts for 12% of the system's mass. The system's centre of mass lies outside Pluto's orbit.
Pluto also has two other known moons, which are tiny in comparison with Charon. The two satellites were found in 2005, 27 years aftet Charon's discovery. The moons, named Nix and Hydra, orbit about two to three times farther away from Pluto as Charon does. The two satellites are thousands of times more dim than Pluto. More will be revealed about these worlds once Pluto is visited by New Horizons in 2015.
In 1994 the Hubble Space Telescope imaged 85 percent of Pluto's surface, revealing bright and dark areas of startling contrast. Astronomers believe that the bright areas are shifting fields of nitrogen ice and the dark areas are fields of methane ice coloured by interaction with sunlight. Some of the dark areas may also be valleys or fresh impact craters. These images support the theory that extensive ice caps form on Pluto's poles, especially when the dwarf planet is farthest from the Sun.
With a density about twice that of water, Pluto is apparently made of much rockier material than are the other planets of the outer solar system. This may be the result of the kind of chemical combinations that took place during the formation of the planet under cold temperatures and low pressure. Many astronomers think it may be a former satellite of Neptune, knocked into a separate orbit during the early days of the solar system. Charon could be an accumulation of the lighter materials resulting from the collision.
Author: Marc Delehanty; updated by Kelly Whitt