As the most distant planet, Pluto seemed to be at the very edge of the Solar System. Now, as 134340 Pluto, the largest Kuiper Belt object, it's the gateway to the outer Solar System.
Pluto takes 248 years to orbit the Sun at a distance varying from 30 to 49 times the distance from the Earth to the Sun. This variation shows that Pluto is not only a long way away, but that its orbit is quite eccentric. Eccentricity is a measure of how far off a perfect circle an orbit is.
The eight planets orbit in about the same plane - rather as if they were traveling around the Sun on a disk. This makes sense because astronomers think that planetary systems evolve from the dusty disks spinning around young stars. However Pluto orbits at an angle of 17 degrees to this plane. It also has an elongated orbit that sometimes brings it closer to the Sun than Neptune is.
When Clyde Tombaugh discovered Pluto in 1930, people were expecting to find Planet X, a new planet large enough to affect the orbits of Neptune and Uranus. The new planet was named Pluto after the Roman god of the underworld, following the suggestion of Venetia Burney, an 11-year-old English schoolgirl.
Pluto was only coincidentally in the right region. It's too small even to affect Neptune, being about two-thirds the diameter of our own moon and with a fifth its mass. In addition, new calculations of Neptune's orbit after Voyager's visit in 1989 showed that the orbit is as expected.
Pluto's rotation on its axis is slow, one day being equal to more than six Earth days. Like Uranus, it orbits on its side, so its seasons are extreme. At the solstices one entire hemisphere is dark and the other completely light. We can detect that the surface of Pluto changes, and it's probably due to these seasonal variations.
It's likely that Pluto has a dense rocky core surrounded by a layer of water ice. At -230 degrees Celsius, its atmosphere is also frozen, except at its nearest approach to the Sun. The atmosphere is primarily nitrogen, but with some carbon dioxide and methane. Both its structure and atmosphere are similar to Neptune's moon Triton, which may well be a captured Kuiper Belt Object.
Pluto has five known satellites. The largest of them is Charon, named for the ferryman who took the souls of the dead over the Styx to Pluto's realm. It is so big compared to Pluto that the pair is often considered to be a binary system. For now Charon remains officially a moon, since there is no definition for a dwarf binary.
Two smaller satellites were discovered with the Hubble Space Telescope in 2005. Nix was named for Charon's mother, goddess of night. Hydra was the nine-headed serpent that guarded Pluto's kingdom. A fourth moon, discovered in 2011, is Kerberos who was a monstrous three-headed dog that ensured the dead didn't escape from Hades. A fifth tiny moon was discovered in 2012, also by the Hubble Space Telescope. It's called Styx.
I said at the start that Pluto is the largest Kuiper Belt Object. Yet it's not the largest body discovered beyond Neptune. That honor goes to Eris, discovered in 2005 and named for the Greek goddess of discord. Eris is in the scattered disk, which is beyond the Kuiper Belt. In 2006 the International Astronomical Union (IAU) formally define what a planet is. The definition meant that both Pluto and Eris were designated as dwarf planets.
Pluto is so far away that even the Hubble Space Telescope can't discern more than large surface features, but NASA's New Horizons mission is expected to arrive there in 2015. Besides its scientific instruments it is also carrying an ounce of Clyde Tombaugh's ashes.
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