Pluto, the ninth and farthest planet from the Sun, is one of the least well understood objects in the solar system. It is the smallest of the major planets and has a most unusual orbit. Pluto is only 1,428 miles (2,300 kilometers) in diameter. Since the planet is 3.66 billion miles (5.89 billion kilometers) away from the Sun, it takes almost 250 years for it to complete one revolution around the Sun. However, it takes Pluto only 6.39 Earth days to complete one rotation about its own axis.
In Greek mythology, Pluto is the god of the underworld. The planet was given its name for several reason. First, due to its great distance from the Sun, Pluto is almost always dark. The sunlight it receives is about equal in intensity to moonlight on Earth. Second, Pluto is the mythological brother of Jupiter and Neptune. And finally, the planet's name begins with "PL," the initials of Percival Lowell (1855–1916), the American astronomer who spent the final years of his life searching for the elusive planet.
Shortly after the discovery of Neptune in 1846, astronomers began looking for an even more distant planet. They believed some celestial body existed at the outer reaches of the solar system that caused disturbances in the orbit of Uranus. The gravitational field of Neptune accounted for some of its neighbor's orbital irregularities, but not all of them.
Percival Lowell used traditional mathematical calculations to guess the location of the suspected planet. He also set up a photographic search for it, but all his attempts proved unsuccessful. Pluto was finally discovered in 1930 during a painstaking search of photographic plates by American astronomer Clyde Tombaugh while he was working at the Lowell Observatory in Arizona.
Before Pluto was located, astronomers had expected it to be a large planet about the size of Jupiter, since it was able to influence the orbit of Uranus, located two planets away. At that time, the solar system appeared to fit a neat pattern: small, dense planets were closest to the Sun and giant, gaseous planets were farther away. Pluto broke this pattern: it is a small, dense planet at the farthest reaches from the Sun.
Pluto's orbit also differs from the pattern set by the other planets in the solar system. While the other eight planets orbit the Sun on the same plane, Pluto travels on an inclined orbit that crosses that plane. Its orbit—the most oval in shape of all the planets—lies mostly outside of that of its closest neighbor, Neptune. At times, however, it crosses inside Neptune's orbit, bringing it closer to the Sun than Neptune.
Pluto is so distant that no Earth-bound telescope has been able to provide a detailed picture of its surface features. The best image to date was taken by the Hubble Space Telescope (HST) in early 1996, in which the planet looks like a fuzzy soccer ball. The HST only revealed that Pluto has frozen gases, icy polar caps, and mysterious bright and dark spots.
Beyond that, astronomers can rely only on imprecise observations and what is known about the planet's density to paint a more complete picture of the planet. Pluto is probably composed of mostly rock and some ice, with surface temperatures between −350 and −380°F (−212 and −228°C). The bright areas on its surface are most likely nitrogen ice, solid methane, and carbon monoxide. The dark spots may hold some form of organic material, possible hydrocarbons from the chemical splitting and freezing of methane.
Pluto's atmosphere is probably made of nitrogen, carbon monoxide, and methane. At Pluto's perihelion (pronounced pear-a-HEE-lee-an; the point on its orbit closest to the Sun), its atmosphere exists in a gaseous state. For most of its orbit, the atmosphere is frozen.
Much of what is known about Pluto was learned following the 1978 discovery of Pluto's moon, Charon (pronounced Karen, and named for the mythological character who transported the dead to the underworld). Prior to Charon's discovery, astronomers thought Pluto and its moon together were one larger object. Charon has a diameter over half that of Pluto, making it the largest moon relative to its planet in the solar system. For this reason, some astronomers consider the two bodies to be a double planet.
Most theories regarding Pluto's origin connect the planet with Neptune's moon Triton. This is because Pluto, like Triton, rotates in a direction opposite that of most other planets and their satellites.
One theory is that Pluto used to be one of Neptune's moons. Struck by a massive object, Pluto was broken in two, creating Charon. The two were then sent into orbit around the Sun. A more popular theory, however, is that both Pluto and Triton started out in independent orbits and that Triton was captured by Neptune's gravitational field.
More questions about Pluto and Charon were to be answered early in the twenty-first century when the National Aeronautics and Space Ad ministration (NASA) planned to send the first unmanned mission to Pluto and its moon. The Pluto-Kuiper Express, which was scheduled to be launched in 2004, was to have consisted of two spacecraft. They were to arrive at Pluto by 2012. They were expected to encounter Pluto near its perihelion, before its atmosphere froze once again, a seasonal deep freeze that lasts more than 100 years. The spacecraft were to study the atmosphere, surface features, and geologic composition of Pluto and Charon, then fly by Pluto into the Kuiper Disk, a ring filled with hundreds of thousands of small, icy objects that are well-preserved remnants of the early solar system. This ring is located between Neptune and Pluto (sometimes beyond Pluto, depending on its oval orbit), some 3.6 billion miles (5.8 billion kilometers) from Earth.
In September 2000, however, NASA issued a stop-work order on the project because of spiraling costs. The project was then canceled in April 2001 when the 2002 budget issued by President George W. Bush's administration provided no money for it.
[ See also Solar system ]