Though discovered early in the century - before the space age of Sputnik, Apollo, and Voyager - the planet Pluto remains as dim and remote as its mythological namesake, the Roman god of the underworld.
Super-Comet or Ice Dwarf?
Pluto is so unique it almost defies classification. Though it orbits the Sun, Pluto neither qualifies as a terrestrial nor as a gas giant planet. Though it behaves like a comet by periodically warming and losing its atmosphere into space, Pluto is far too large for that category. Pluto may be the last survivor of a lost population of objects called ice dwarfs that inhabited the primeval solar system. Neptune's moon Triton might be a distant cousin, and other relatives may dwell in the Kuiper Belt, a disk of ice debris left behind from the solar system's birth. Pluto and Triton survive because they have found gravitational niches in the solar system where they remain in stable orbits. Pluto is in a resonance orbit with Neptune (Neptune circles the Sun three times for every two orbits of Pluto), which means that Pluto never gets close enough to Neptune to be thrown out the solar system. Triton was gravitationally captured by Neptune and was therefore prevented from being ejected from the planetary region. It is believed that all of the other ice dwarfs formed inside 50 AU were ejected by gravitational interactions due to the giant planets in the ancient past.
The Double Planet
Pluto and Charon is the best example of a double planet, which occurs when two bodies are reasonably close in mass and so orbit around a common center of gravity - or barycenter (analogous to two children balancing on a teeter-totter). Charon may have been born through a head-on collision between Pluto and another large ice body, in much the same way as the Earth-Moon system is believed to have formed. According to computer models, some of the debris from this giant impact on Pluto went into orbit around Pluto and coalesced to form Charon.
A Dynamic, Unique Atmosphere
Despite its small size and remote location in the solar system, Pluto undergoes dramatic seasonal changes driven largely by Pluto's highly elliptical orbit, which carries it as close as 2.8 billion miles to the Sun (inside Neptune's orbit) and as far as 4.6 billion miles from the Sun. As Pluto recedes from the Sun, much of its atmosphere is believed to freeze out onto the surface. This explains the abundance of fresh white ice on the surface. Pluto essentially "launders" its surface by evaporating dirty, old ice in the summer, and depositing a fresh new layer of ice each 248-year orbit.
Pluto passed its closest point to the Sun in late 1989. As a result, it presently enjoys a relatively "balmy" surface temperature near -350 degrees Fahrenheit in the dark areas, and a cooler -380 degrees in the ice areas. It is likely that this sets up tremendous pressure differences at the surface, creating high wind speeds in Pluto's tenuous atmosphere. For astronomers it's a rare and ideal time for viewing Pluto and studying these changes.
The last time Pluto was this close to the Sun (and Earth!), George Washington was a boy!