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Pluto revolves about the sun once in 247.7 Earth years at an average distance of 5.9 billion km (3.67 billion mi) from the sun. The orbit is so eccentric that at certain points along its path Pluto is closer to the sun than is Neptune. No possibility of collision exists, however, because Pluto's orbit is inclined more than 17.2° to the plane of the ecliptic and never actually crosses Neptune's path.
Visible only through large telescopes, Pluto appears to have a yellowish color. For many years very little was known about the planet, but in 1978 astronomers discovered a relatively large moon orbiting Pluto at a distance of only about 19,000 km (about 12,000 mi) and named it Charon. The orbits of Pluto and Charon caused them to pass repeatedly in front of one another from 1985 through 1990, enabling astronomers to determine their sizes fairly accurately. The Hubble Space Telescope allowed astronomers to determine the sizes of Pluto and Charon even more accurately in 1994. Pluto is about 2320 km (1440 mi) in diameter, about two-thirds the size of Earth's moon. Charon is about 1270 km (790 mi) in diameter, making Pluto and Charon the planet-satellite pair closest in size in the solar system. Pluto was also found to have a thin atmosphere, probably of methane, exerting a pressure on the planet's surface that is about 100,000 times weaker than the Earth's atmospheric pressure at sea level.
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 Pluto 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.
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