As we leave behind us the system of Neptune and its moons, we leave the world of the gas giants and approach the extreme outer limits of our solar system. This region is inhabited by a small, rocky world called Pluto, and its single moon, Charon. The darkness and isolation of these two worlds has likened them to the underworld. In fact, Pluto was named after the Roman god of the underworld. Charon was named after the mythological figure who ferried the dead across the river Styx to Hades. Pluto was discovered in 1930 by Clyde W. Tombaugh at Lowell Observatory in Arizona. It was much too small to be seen by most telescopes of the time. Astronomers first theorized that a ninth planet might exist when they observed that something seemed to be exerting a gravitational pull on the planet Neptune. An in-depth sky survey finally turned up the tiny planet. Pluto is so far away from the Sun that it takes 248 Earth years to complete one orbit. The temperature of this inhospitable world averages -382° F (-230° C).
The Twin Planets
For many decades, Pluto was thought to be alone in its dark corner of our solar system. But in 1978, Jim Christy discovered that Pluto had a companion. This moon, Charon, is more than half the size of its parent planet. In fact, many astronomers have referred to Pluto and Charon as "twin planets" because they are so close to each other in size. Because Charon is so large, it does not actually orbit around Pluto. Rather, the two bodies actually orbit around a common center of gravity somewhere between them. Pluto and Charon are also tidally locked in a synchronous orbit. Pluto's rotational period is 6.3 days. It takes 6.3 days for Charon to make one revolution around Pluto. Thus, the two bodies continuously face each other. But these are not the only strange characteristics of this remote planet. Pluto's orbit is so highly eccentric that is actually crosses inside the orbit of Neptune. Because of this, Pluto is actually closer to the Sun than Neptune for 20 years of its 248-year orbit. The 17-degree inclination of Pluto's orbit means that it there is no chance that it will eventually collide with Neptune.
Features of Pluto and Charon
Pluto is the only planet that has not been visited by a spacecraft. It is so tiny, and so far away that even the Hubble space telescope has trouble resolving any surface features on the planet. With a diameter of only 1412 miles (2274 km, Pluto is the smallest planet in the Solar System. Because of its small size, many astronomers believe that it should be classified as an asteroid instead of a planet. In fact, Pluto represents one of the great mysteries of our solar system. No one really knows why this small, rocky planet can be found among the giant gas planets of the outer solar system. Between 1985 and 1990, the Earth was aligned with the orbits of Pluto and Charon in such a way that a series of eclipses would take place. Astronomers were able to take advantage of this situation to accurately determine the size of both Pluto and Charon, and to create the first image of Pluto showing some of its larger surface features. This revealed that Pluto is the second most contrasty body in the Solar System. Astronomers are hoping that a future space probe may one day be able to show us the surface of this strange and distant world. But until then, every new discovery will bring with it a long list of new questions.
The Lost Planet
For years, astronomers believed that Pluto and Charon may be asteroids or comets that somehow got locked into an orbit around the Sun. Soon after the discovery of the Kuiper belt, most astronomers began to think that these two bodies were actually rogue Kuiper objects and should not be classified as planets. The debate raged on for decades, supported by the public's fondness for their favorite planet. Finally, on August 24, 2006, the International Astronomical Union (IAU) decided to redefine the definition of a planet. This decision officially reclassified Pluto and Charon as dwarf planets. Only 76 years after its discovery, Pluto was quietly added to the list of minor planets and given the number 134340. It was the end of an era as the Solar System's ninth planet surrendered its title.
