Pluto and Charon (Courtesy to Microsoft Encarta for this photograph.)

Pluto is the smallest planet in the solar system. It is about two-thirds the size of Earth's moon. Pluto is the only planet out of all the outer planets that solid. Pluto is only visible through big telescopes. It appears to have a yelloish color. In 1978, astronomers discovered a large moon orbiting Pluto at a distance of 19,000 km (12,000 mi). They named it Charon. Charon is Pluto's only moon. Pluto also has a thin atmosphere, probably of methane, exerting a pressure on the planet's surface that is about 100,000 times weaker than Earth's atmospheric pressure at sea level.

Pluto is made out of much rockier material than 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 might have once been a satellite of Neptune, and then later on it was knocked into another orbit. This all took place during the early days of the solar system.

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