The darkly lit outermost world bears the name of Pluto and the first two letters, Pl, of Pluto is the planets astronomical symbol.
Pluto's astrological sign, seen above, is similar to that of Neptune, but has a circle instead of a third prong in the trident.
Pluto was the god of the underworld in both Roman and Greek mythology. His name meant riches or wealth. Pluto and Hades (the Greek name for the god of the underworld) are the same being, though they differ in character traits. While Hades is seen as violent (he abducted Persephone), Pluto is venerated and seen as a positive god who welcomes souls to the afterlife. Romans viewed Pluto as a strict ruler, but a loving husband. In Greek mythology, Hades comes to rule the underworld after the split of sovereignty; Zeus (Jupiter) ruled the Heavens, Poseidon (Neptune) ruled the Sea, and Hades (Pluto) was left to the underworld. He is oftentimes portrayed as having received a less than desirable part of the world to rule. The most famous (or infamous) story involving Hades is his kidnap of Persephone, daughter of Demeter. After Persephone's abduction, Demeter spent a very long time looking for her lost daughter. She finally finds out that Hades took her to the underworld to be his wife. She is upset, and begs Zeus to command Hades to free Persephone. However, Zeus refuses to interfere in the matter. Demeter then denies her role as goddess of the Harvest and Fertility of Crops. Famine plagues the Earth; finally, Zeus demands Hades to release Persephone. However, because Persephone consumed a pomegranite seed while in the underworld, she has to return to the underworld for 1/3 of every year. During this part of the year, Demeter does not allow the crops to grow. This story is how the Greeks explained seasons.
Other stories involving Pluto, or Hades, have never been as popular in the mythological world. Many heroes were sent to the underworld to obtain items specific for a quest or journey on which they had been sent by the gods. Possibly the most famous of the "journey to the underworld" stories is Orpheus's expedition to the underworld to retrieve the soul of his dead wife. He plays his lyre for Pluto and Persephone, hoping to impress them in order to win back his wife. In some versions of the tale, Pluto is moved to tears, and returns her soul to Orpheus.
Allowing for the influence of Neptune, the motion of Uranus still lagged a little. At the beginning of the 20th century, American astronomer Percival Lowell, among others, made calculations of where a ninth planet might be. They appropriately referred to the planet as "Planet X," because it remained unknown. The search for this planet was inspired by the fact that the discovery of the eighth planet, Neptune, did not completely account for the irregularities in the orbit of the seventh planet, Uranus. Observations of Neptune itself showed it did not orbit as expected. Between 1905 and his death in 1916, Lowell and his colleagues made telescopic sweeps of the area where the planet was thought to be, but they were unable to find it. Over a decade later, Lowells successors at the Lowell Observatory in Flagstaff, Arizona, installed a new telescope and appointed young astronomer Clyde W. Tombaugh to continue the search. He painstakingly photographed the sky and compared pictures taken several days apart, looking for moving objects. On February 18, 1930, he found something (Planet X?) that had moved against the background stars, compared with the plates he had taken on January 21, 23, and 29. It was five degrees from where Lowell had predicted it might be. The announcement of the discovery was made on March 13, 1930. Since the ninth planet moves farthest from the light of the sun and farther into the darkness of the space between the stars, it was named Pluto.
The naming of Pluto is a story by itself. Early suggestions of the name of the new planet were: Atlas, Zymal, Artemis, Perseus, Vulcan, Tantalus, Idana, Cronus. The New York Times suggested Minerva, reporters suggested Osiris, Bacchus, Apollo, Erebus. Lowells widow suggested Zeus, but later changed her mind to Constance. Many people suggested the planet be named Lowell. The staff of the Flagstaff observatory, where Pluto was discovered, suggested Cronus, Minerva, and Pluto. A few months later the planet was officially named Pluto, which was originally suggested by Venetia Burney, an 11-year-old schoolgirl in Oxford, England. Some scientists believe there may be a planet beyond Pluto, since its gravitational pull is not considered big enough to account for all the deviations of Neptune; however, no proof of another planet has been provided. The search for Planet X continued but nothing has been found, nor is it likely that it ever will be. The discrepancies vanish if the mass of Neptune determined from the Voyager 2 encounter with Neptune is used; so, there is no evidence of a tenth planet. Pluto is tiny, even compared with Earth, which is five times larger and 500 times as massive. Plutos rocky core is covered with a thick layer of water ice, overlaid with a layer of ice mixed with frozen methane. When closest to the Sun, a thin atmosphere of methane and, probably, nitrogen boils off the surface. In sunlight, its surface appears pinkish, indicating the presence of carbon and there are white polar caps, probably made of frozen methane.
In 1978, American astronomer Jim Christy found that Pluto had a satellite. The satellite, Charon, is over half Plutos diiameter and, at 12,204 miles (19,640 km) from it, twenty times closer than Earths moon is. Charon is named for the mythological figure who ferried the dead across the River Aceron into Hades (the underworld). Though officially named for the mythological figure, Charons discoverer was also naming it in honor of his wife, Charlene; thus, those in the know pronounce it with the first syllable sounding like shard (SHAHR en). Charon is much like Pluto, but too small to retain methane, so its surface is pure ice. Pluto and Charon are pitted with craters. Charon and Pluto are so close that tidal force has locked their spins and orbits. They spin in opposite directions and the same sides always face each other. Seen from Plutos surface, Charon hangs motionless in the sky, never rising nor setting.
In 2006, the IAU (International Astronomical Union) stated that a "planet" is defined by three main characteristics. First, it must orbit around the Sun. Second, it has to have enough mass to possess hydrostatic equilibruim. Third, it has to have "cleared the neighborhood" around its orbit--in other words, meaning that gravitationally, the planet is dominant. If this occurs, the planet will alone in that there are not other bodies of similar size in its orbit, aside from its satellites. Pluto met the first two conditions; however, it has not "cleared its neighborhood," so to speak. Therefore, the IAU declared that Pluto is a dwarf planet. The decision came down to choosing one of the following two options: either keep calling Pluto a planet, and induct several other bodies in the Kepler belt as planets as well; or, deny Pluto the title of planet, and give the other bodies the same title as Pluto. The IAU obviously chose the latter of these two options; however, astronomers are still debating this decision. Not everyone agrees that Pluto should not be a planet.