Pluto

Until 2006, Pluto was counted as the ninth planet in the solar system. It is much smaller than any of the other planets, and has a highly elliptical and inclined orbit unlike the others'. Today, Pluto is no longer considered to be a planet, but is rather the first of a new category of objects called dwarf planets.

Discovery and Declassification

Pluto was discovered in 1930 by the American astronomer Clyde Tombaugh, working at Lowell Observatory. This observatory, founded by Percival Lowell of Mars fame, was dedicated to the task of finding a "Planet X" after observations of Uranus and Neptune in the late 19th century caused astronomers to speculate that Uranus's orbit was being disturbed by another planet in addition to Neptune. Tombaugh found Pluto near Planet X's predicted position after a year of searching.

In 1978, astronomer James Christy discovered that Pluto has a moon of its own, named Charon. Observations of Charon allowed the mass of Pluto to be computed directly for the first time, and it was found to be less than 0.2% the mass of the Earth. Pluto's small size and mass cast doubt on the idea that Pluto could be Planet X. With a revised estimate of the mass of Neptune after the 1989 Voyager 2 flyby, the discrepancies in the position of Uranus - and with them, the need for Planet X - vanished. Today the consensus among astronomers is that Planet X does not exist, and that Tombaugh found Pluto by coincidence.

As astronomers began to discover other objects in the outer solar system beyond Neptune - some larger than Pluto - it became clear that Pluto did not deserve major planet status. In 2006, the International Astronomical Union officially de-classified Pluto as a planet, and gave it the minor planet designation, (134340) Pluto.

Orbit

Pluto orbits the Sun once every 248 years. Pluto's orbit is quite eccentric, and takes Pluto from about 30 A.U. to about 48 A.U. from the Sun. For a few years during every orbit, Pluto is actually closer to the Sun than Neptune. Pluto's orbit also highly inclined to the Ecliptic (by about 17 degrees). This high inclination helps Pluto avoid a collision with Neptune, since when their orbits cross, Pluto is far above the plane of the solar system.

Pluto orbits in a zone known as the Kuiper Belt. This region lies beyond the orbit of Neptune, and is believed to contain hundreds, if not thousands, of bodies similar to Pluto. Pluto is also the prototype and namesake of a group of objects called Plutinos. The Plutinos orbit in the same region of space and, like Pluto, have a 2:3 orbital resonance with Neptune (completing 2 orbits for every 3 orbits of Neptune). In addition to Pluto, the other largest Plutinos include (90482) Orcus, (28978) Ixion, (38083) Rhadamanthus, and (38628) Huya.

Size and Composition

Pluto's diameter is about 1430 miles (2300 km) - only about two-thirds the diameter of the Earth's Moon. Pluto's density is also low (about 2 grams per cubic centimeter) so it contains only about one-sixth the mass of the Moon. If you weighed 100 pounds on Earth, you would only weigh 8 pounds on Pluto. Pluto's rotational axis is also tilted about 120 degrees to the Ecliptic, so it has a retrograde rotation, like Uranus and Venus.

Between 1985 and 1990, Pluto and its companion, Charon, were situated in such a way that many mutual eclipses and transits occurred. Observing these events led to a wealth of data, including a surface albedo map showing that Pluto has polar caps, dark spots, and light areas. Pluto's surface is covered with frozen nitrogen and methane. Pluto has a very thin atmosphere of nitrogen and methane which freezes and falls to the surface when Pluto is farther from the Sun. Pluto's average surface temperature is -370°F (-220°C).

The Moons of Pluto

Pluto has three known moons, Charon (discovered in 1978), and two smaller bodies, Nix and Hydra (discovered in 2005). Charon is about half the size of Pluto and even less dense, leading to speculation that they formed at different times or in different locations. Charon orbits Pluto closely, with a period of 6.39 days. Both objects are tidally locked, always showing the same face to each other. The two new, smaller moons, Nix and Hydra, orbit outside the Pluto-Charon pair.

No spacecraft have yet visited Pluto, but NASA's New Horizons mission is currently en route, and will fly by Pluto and its moons in 2015.