Pluto: Ninth planet from the Sun (usually)

Average distance from the Sun: 39.53 AU (6,000 million km)
Orbital period (Length of Year): 247.7 Earth years
Rotational period (Length of Day): 6.39 Earth days (retrograde)
Orbital inclination: 17.15 deg
Average orbital speed around the Sun: 4.75 km/sec
Diameter: 2,300 km
Axial inclination: 57.5 deg
Mass: 1.29 x 10 ^ 22 kg (0.0022 Earth mass)
Relative surface gravity (Earth = 1): 0.0675
Average temperature: -236 deg C (-393 deg F)
Atmosphere: Perhaps methane and nitrogen
Albedo: 0.4
Number of moons: 1 (Charon, pronounced "KARE-on")

The farthest known planet from the Sun is usually Pluto, named for the Greek god of the Underworld. This planet has the most eccentric orbit of all the known planets, so that every time Pluto makes its 248 Earth-year orbit around the Sun, 20 Earth-years of that orbit are spent inside the orbit of Neptune. We are currently living during one of these special 20-Earth-year periods, from January 21, 1979 to March 14, 1999.

The discovery of Pluto was the result of a search for another object, besides Neptune, which was influencing Uranus's orbit. The presence of Neptune alone was not enough to explain the irregularities in Uranus's path around the Sun. Clyde Tombaugh finally located Pluto by examining photographic plates from the Lowell Observatory. In 1930, he found a tiny dot moving among the stars--the planet Pluto. Further analysis showed that the observations of Neptune's orbit that had been used to make the predictions concerning Pluto were unreliable and could not have produced a trustworthy prediction. Besides, Pluto was not nearly large enough to influence Neptune's orbit by much. Thus, the original prediction of Pluto's position in the sky, which was only 6 degrees from the discovery position, was a coincidence; it was only by chance and persistence that it was located.

In 1978 James W. Christy of the U.S. Naval Observatory discovered that Pluto had a moon, now named Charon, after the mythical ferryman who transported souls across the River Styx into the Underworld. Charon is about half the diameter of Pluto, making these two bodies very close to being a double planet. Most moons of the solar system are tidally locked to their parent planets, so that the moons each keep the same side facing their parent planets all the time, just like Earth's moon does. But in the case of Pluto, not only is Charon locked to Pluto; but Pluto is also tidally locked to Charon. Both bodies rotate and revolve around each other with a period of 6.39 Earth-days, so that they both keep the same side facing each other.

Pluto is so peculiar that some astronomers believe it may not be a true planet, but a captured ice-world from what is known as the Kuiper Belt. Unlike the rest of the planets, Pluto is composed primarily of ice. Astronomers have in the past few years discovered a zone of ice-bodies the size of asteroids out beyond Pluto in a zone now called the Kuiper Belt. It is now theorized that this zone is the source for short-period comets. (Long-period comets originate from the Oort Cloud, much further out from the sun.) In fact, if Pluto were to be pulled in too close to the Sun, its ices would sublimate into gases, and it would become a giant, oversized "comet" itself. The moons Charon, Phoebe, and Nereid are icy worlds with unusual orbits that may have originated from the Kuiper Belt as well.

No planetary probe from Earth has ever visited Pluto, so nobody knows exactly what it looks like. However, several plans for a probe have been proposed. If a probe is not launched within the next several years, though, there will be a long wait before atmospheric data can be gathered. Pluto is currently close enough to the Sun that it has a very thin atmosphere. Not very long from now, its highly elliptical orbit will take it further from the Sun, where its atmosphere will freeze and collapse on the surface, not to be seen again for over 200 years.

Information Source:  http://www.moreheadplanetarium.org/index.cfm