Discovery In the 1840s,
Urbain Le Verrier used
Newtonian mechanics to predict the position of the then-undiscovered planet
Neptune after analyzing
perturbations in the orbit of
Uranus. Subsequent observations of Neptune in the late 19th century led astronomers to speculate that Uranus's orbit was being disturbed by another planet besides Neptune. In 1906,
Percival Lowell—a wealthy
Bostonian who had founded
Lowell Observatory in
Flagstaff, Arizona, in 1894—started an extensive project in search of a possible ninth planet, which he termed "
Planet X". The name was published on May 1, 1930. The name
Pluto had received some 150 nominations among the letters and telegrams sent to Lowell. The first had been from
Venetia Burney (1918–2009), an eleven-year-old schoolgirl in
Oxford, England, who was interested in
classical mythology. The choice was further helped by the fact that the first two letters of
Pluto were the initials of Percival Lowell; indeed, 'Percival' had been one of the more popular suggestions for a name for the new planet.
Symbol Once named, Pluto's
planetary symbol was then created as a
monogram of the letters "PL". The name 'Pluto' was soon embraced by wider culture. In 1930,
Walt Disney was apparently inspired by it when he introduced for
Mickey Mouse a canine companion named
Pluto, although
Disney animator
Ben Sharpsteen could not confirm why the name was given. Astronomers initially calculated its mass based on its presumed effect on Neptune and Uranus. In 1931, Pluto was calculated to be roughly the mass of
Earth, with further calculations in 1948 bringing the mass down to roughly that of
Mars.
Classification From 1992 onward, many bodies were discovered orbiting in the same volume as Pluto, showing that Pluto is part of a population of objects called the
Kuiper belt. This made its official status as a planet controversial, with many questioning whether Pluto should be considered together with or separately from its surrounding population. Museum and planetarium directors occasionally created controversy by omitting Pluto from planetary models of the
Solar System. In February 2000 the
Hayden Planetarium in New York City displayed a Solar System model of only eight planets, which made headlines almost a year later. Although the first Kuiper belt objects discovered were quite small, objects increasingly closer in size to Pluto were soon discovered, some large enough (like Pluto itself) to satisfy geological but not dynamical ideas of planethood. In 1998,
Brian G. Marsden of
Harvard University's
Minor Planet Center suggested that Pluto be given the
minor planet number 10000 while still retaining its official position as a planet. The prospect of Pluto's "demotion" created a public outcry, and in response the
International Astronomical Union clarified that it was not at that time proposing to remove Pluto from the planet list. The debate became unavoidable when, in July 2005, these astronomers announced the discovery of a new object,
Eris, which was substantially more massive than Pluto and the most massive object discovered in the Solar System since
Triton in 1846. The press initially called it the
tenth planet, although there was no official consensus at the time on whether to call it a planet.
IAU classification The debate came to a head in August 2006 during the triennial meeting of the
IAU, when
Uruguayan astronomers
Julio Ángel Fernández and
Gonzalo Tancredi first proposed the new definition for the term "planet". According to their proposal, there are three conditions for an object in the
Solar System to be considered a planet: • The object must be in orbit around the
Sun. • The object must be massive enough to be rounded by its own gravity. More specifically, its own gravity should pull it into a shape defined by
hydrostatic equilibrium. • It must have
cleared the neighborhood around its orbit. Its mass is substantially less than the combined mass of the other objects in its orbit: 0.07 times, in contrast to Earth, which is 1.7 million times the remaining mass in its orbit (excluding the moon). There has been some resistance within the astronomical community toward the reclassification, and in particular planetary scientists often continue to reject it, considering Pluto, Charon, and Eris to be planets for the same reason they do so for Ceres. In effect, this amounts to accepting only the second clause of the IAU definition. He also stated that because less than five percent of astronomers voted for it, the decision was not representative of the entire astronomical community. Researchers on both sides of the debate gathered in August 2008, at the Johns Hopkins University
Applied Physics Laboratory for a conference that included back-to-back talks on the IAU definition of a planet. Entitled "The Great Planet Debate", the conference published a post-conference press release indicating that scientists could not come to a consensus about the definition of planet. In June 2008, the IAU had announced in a press release that the term "
plutoid" would henceforth be used to refer to Pluto and other planetary-mass objects that have an orbital
semi-major axis greater than that of Neptune, though the term has not seen significant use. == Orbit ==