Pluto is the ninth planet in the solar system. Discovered in 1930 and immediately classified as a planet, its status is currently under dispute [6]. Pluto has an eccentric orbit that is highly inclined in respect to the other planets and takes it inside the orbit of Neptune. Its largest moon is Charon, discovered in 1978; two smaller moons were discovered in 2005. Pluto's astronomical symbol is a P-L monogram, ♇. This represents both the first two letters of the name Pluto and the initials of Percival Lowell, the man who lent his name to the observatory that was used to find Pluto. An alternate symbol resembles that of Neptune, but has a circle in place of the middle spoke in the top center.
Due to its size and unusual orbit, there has been debate regarding Pluto's classification as a major or a minor planet, and there is increasing momentum for recognizing "dual status." Pluto is classified as a trans-Neptunian object. As of July 31, 2005, one other trans-Neptunian object, 2003 UB313, had been found that is larger than Pluto.
Pluto was discovered by the astronomer Clyde Tombaugh at the Lowell Observatory in Arizona on February 18, 1930 after an extensive search when he compared photographic plates taken on January 23 and 29. After the observatory obtained confirming photographs, the news of the discovery was telegraphed to the Harvard College Observatory on March 13, 1930. The planet was later found on photographs dating back to March 19, 1915. Tombaugh was searching for a "Planet X" to explain discrepancies in the predicted orbit of Neptune. It is now known these discrepancies were an artifact of the slightly incorrect value then assumed for the mass of Neptune.
In the matter of Pluto, the discretion of naming the new object belonged to the Lowell Observatory and its director, Vesto Melvin Slipher, who, in the words of Tombaugh, was "urged to suggest a name for the new planet before someone else did". Soon suggestions began to pour in from all over the world. Constance Lowell, Percival's widow who had delayed the search through her lawsuit, proposed Zeus, then Lowell, and finally her own first name, none of which met with any enthusiasm. One young couple even wrote to ask that the planet be named after their newborn child. Mythological names were much to the fore: Cronus and Minerva (proposed by the New York Times, unaware that it had been proposed for Uranus some 150 years earlier) were high on the list. Also there were Artemis, Athene, Atlas, Cosmos, Hera, Hercules, Icarus, Idana, Odin, Pax, Persephone, Perseus, Prometheus, Tantalus, Vulcan, Zymal, and many more. One complication was that many of the mythological names had already been allotted to the numerous asteroids. Virtually all the female names had been used up, and male names were usually reserved for objects with unusual orbits.
The name retained for the planet is that of the Roman god Pluto, and it is also intended to evoke the initials of the astronomer Percival Lowell, who predicted that a planet would be found beyond Neptune. The name was first suggested by Venetia Phair (n¨¦e Burney), at the time an eleven-year-old girl from Oxford, England [7]. Over the breakfast table, one morning her grandfather, who worked at Oxford University's Bodleian Library, was reading about the discovery of the new planet in the Times newspaper. He asked his granddaughter what she thought would be a good name for it. Venetia, who was quite interested in Greek and Roman myths and legends, suggested the name of the Roman God of the underworld. Professor Herbert Hall Turner cabled his colleagues in America with this suggestion, and after favourable consideration which was almost unanimous, the name Pluto was officially adopted and an announcement made by Slipher on May 1, 1930.
Pluto's orbit is unlike those of the other planets. It is highly inclined above the plane of the ecliptic, and highly eccentric (non-circular). The eccentricity of its orbit is such that it crosses the orbit of Neptune, and making Pluto only the eighth-most distant planet from the Sun for part of each orbit; the most recent occurrence of this phenomenon lasted from February 7, 1979 through February 11, 1999. Mathematical calculations indicate that the previous occurrence only lasted fourteen years from July 11, 1735 to September 15, 1749. However, the same calculations indicate that Pluto was the eighth-most distant planet between April 30, 1483 and July 23, 1503, which is almost exactly the same length as the 1979 to 1999 period. Recent studies suggest each crossing of Pluto to inside Neptune's orbit lasts alternately for approximately thirteen and twenty years with minor variations.
Pluto orbits in a 3:2 orbital resonance with Neptune. When Neptune approaches Pluto from behind their gravity start to pull on each other slightly, resulting in an interaction between their positions in orbit of the same sort that produces Trojan points. Since the orbits are eccentric, the 3:2 periodic ratio is favoured because this means Neptune always passes Pluto when they're almost farthest apart. Half a Pluto orbit later, when Pluto is nearing its closest approach, it initially seems as if Neptune is about to catch up to Pluto. But Pluto speeds up due to the gravitational acceleration from the Sun, stays ahead of Neptune, and pulls ahead until they meet again on the other side of Pluto's orbit.
Beginning in the 1990s, other trans-Neptunian objects have been discovered, and a certain number of these also have a 3:2 orbital resonance with Neptune. TNOs with this orbital resonance are named "plutinos", after Pluto.
Pluto is not only smaller and much less massive than every other planet, but at less than 0.2 lunar masses it is also smaller and less massive than seven moons: Ganymede, Titan, Callisto, Io, Earth's Moon, Europa and Triton. However, Pluto is more than twice the diameter, and a dozen times the mass, of Ceres, the largest minor planet in the asteroid belt, and it was larger than any other object known in the trans-Neptunian Kuiper belt until 2003 UB313 was announced in 2005. See List of solar system objects by mass and List of solar system objects by radius.
Pluto's mass and diameter could only be estimated for many decades after its discovery. The discovery of its satellite Charon in 1978 enabled a determination of the mass of the Pluto-Charon system by simple application of Newton's formulation of Kepler's third law. Later Pluto's diameter was measured when it was occulted by Charon, and its disk can now be resolved by telescopes using adaptive optics.
Pluto's thin atmosphere is most likely nitrogen and carbon monoxide, in equilibrium with solid nitrogen and carbon monoxide ices on the surface. As Pluto moves away from its perihelion and farther from the Sun, more of its atmosphere freezes. When it returns to a closer proximity to the sun, the temperature of Pluto's solid surface will increase, causing the nitrogen ice to sublimate into gas - creating an anti-greenhouse effect. Much as sweat evaporating from the surface of human skin, this sublimation has a cooling effect on the planet and scientists have recently discovered [9], by use of the Submillimeter Array, that Pluto's temperature is 10 kelvins less than they expected.
Pluto was found to have an atmosphere from an occultation observation in 1988. When an object with no atmosphere occults a star, the star abruptly disappears; in the case of Pluto, the star dimmed out gradually. From the rate of dimming, the atmosphere was determined to have a pressure of 0.15 Pa, roughly 1/700,000 that of earth.
In 2002, another occultation of a star by Pluto was observed and analyzed by teams led by Bruno Sicardy of the Paris Observatory [10] and by Jim Elliot of MIT [11] and Jay Pasachoff of Williams College [12]. Surprisingly, the atmosphere was estimated to have a pressure of 0.3 Pa, even though Pluto was further from the Sun than in 1988, and hence should be colder and have a less dense atmosphere. The current best hypothesis is that the south pole of Pluto came out of shadow for the first time in 120 years in 1987, and extra nitrogen sublimated from a polar cap. It will take decades for the excess nitrogen to condense out of the atmosphere.
