The Pluto debate:
In August 2006 3,000 astronomers and scientists of the International Astronomical Union (IAU) gathered in Prague, Czech Republic to deliberate. The organization planned to publish an official definition of “planet”, ruling on whether to call Pluto a planet, dwarf planet or a Kuiper Belt Object (KBO). The draft proposal classified Pluto along with 2003 UB313 and any other spherical TNOs that may be discovered, as planets, although they would have been informally labelled ‘plutons’. The asteroid 1 Ceres and Pluto’s moon, Charon, would also have been considered planets.
On August 24, 2006, however, the previous draft was reversed, according to the newly passed rule, Pluto was demoted from planetary status to a dwarf planet. There are three main conditions for an object to be called a ‘planet’, according to the IAU resolution. The first is that the object must be in orbit around a star, but not be a star itself. Secondly, the object must be massive enough to be a sphere by its own gravitational force. Thirdly, it must have cleared the neighborhood around its orbit. Pluto fails to meet these conditions but will act as a prototype for a yet to be named category of Trans-Neptunian objects.
In an attempt to reconcile Pluto’s small apparent size with its identification as “Planet X”, the theory of specular reflection was proposed. This held that observers were measuring only the diameter of a bright spot on the highly reflective surface of a much larger planet which could thereby be massive without having an exceptionally high density.The uncertainty was conclusively resolved by the discovery of Pluto’s satellite Charon in 1978. This made it possible to determine the combined mass of the Pluto-Charon system which turned out to be lower even than that anticipated by skeptics of the specular reflection theory, which was then rendered completely untenable. The accepted figure for Pluto’s diameter today makes it considerably smaller than the Moon, with only a fraction of the Moon’s mass on account of its being largely composed of ice. More recently, measurements of the path of Voyager 2 have shown that Neptune has a greater mass than previously believed and that when the updated mass is taken into account there is no anomalous movement of Uranus or Neptune.
Thus Pluto’s discovery and Lowell’s 1915 prediction were largely coincidental as Pluto actually has no role in what were believed to be anomalies in Neptune and Uranus’ motion. Pluto’s discovery was mostly due to the diligence of Tombaugh’s search.
Information about planet Pluto:
Pluto is the ninth and smallest of the traditional planets of the Solar system, though its status as a planet has been disputed in recent years. It qualifies as a planet under the draft definition, to be submitted to vote by the General Assembly of the International Astronomical Union in August 2006.
It has an eccentric orbit that is highly inclined with respect to the other planets and takes it closer to the Sun than Neptune during a portion of its orbit. It is also the smallest planet and indeed is smaller than several moons of other planets (see the list of solar system objects by radius). Pluto itself has a large moon named Charon (but see below); two small moons named Nix and Hydra were discovered in 2005. The New Horizons spacecraft, which lifted off from Cape Canaveral, Florida on January 19, 2006, is expected to become the first spacecraft to fly by Pluto on July 14, 2015.
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, who had searched extensively for a ninth planet and who had lent his name to Lowell Observatory, the place from where, after initiating several earlier searches, Pluto was eventually discovered. (Another symbol sometimes used for Pluto is an astrological symbol and not an astronomical one. It resembles that of Neptune, but has a circle in place of the middle spoke of the trident.)
Pluto and its satellite Charon have often been considered a binary planet because they are more nearly equal in size than any other planet/moon combination in the Solar System. Under the aforementioned planet definition proposal, since they orbit each other around a center of mass that is outside either body, they would be officially considered a binary planet system.
The story of how Pluto was discovered actually begins with the discovery of Neptune. In the 1840s, using Newtonian mechanics, Urbain Le Verrier and John Couch Adams had correctly predicted the position of the then-undiscovered planet Neptune after analysing the perturbations in the orbit of Uranus which could only have been caused by the gravitational pull of another massive planet. Thanks to their calculations, Neptune was discovered by Johann Gottfried Galle on September 23, 1846.
By the late 19th century, astronomers started speculating that Neptune’s orbit too was being disturbed by another planet. By 1909, William H. Pickering and Percival Lowell had suggested several possible celestial coordinates for such a planet. In May 1911, the Bulletin of the Astronomical Society of France published the calculations of the Indian astronomer Ketakar which predicted a location for the undiscovered planet. Although Lowell died in 1916, the search for the elusive planet continued.
Pluto was discovered after an extensive search by the astronomer Clyde Tombaugh at Lowell Observatory in Arizona on February 18, 1930 when he compared photographic plates taken on January 23 and 29. Tombaugh also referenced a lesser-quality photo taken on January 20 to confirm movement. 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. Its mean distance from Earth and its mean daily motion turned out to be 39.48 AU and 14.283″.
Ironically, Pluto is far too small to have the effect on Neptune’s orbit that initiated the search. The discrepancies in Neptune’s orbit observed by 19th century astronomers were due instead to an inaccurate estimate of Neptune’s mass. Tombaugh’s discovery is therefore even more surprising, given that the proximity of the predictions of Pickering, Lowell and Ketakar were coincidences.
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, and many more. One complication was that many of the mythological names had already been allotted to the numerous asteroids.
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 (née Burney), at the time an eleven-year-old girl from Oxford, England. 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 to suggest 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.
In the Chinese, Japanese, and Korean languages, the planet’s name was translated as star of the king of the dead 冥王星. In Vietnamese it is named after Yama (Diêm Vương Tinh), the Guardian of Hell in Buddhist traditions.