Discovery of Pluto reaches Harvard College Observatory, March 13, 1930
Astronomers began pondering the existence of Planet X in the late 19th century when they began to speculate that Uranus' orbit was being disturbed by another planet besides Neptune.
In 1906, Percival Lowell, a wealthy Bostonian who had founded the Lowell Observatory in Flagstaff, Ariz, in 1894, started an extensive project in search of a possible ninth planet, which he termed "Planet X." By 1909, Lowell and William H Pickering had suggested several possible celestial coordinates for such a planet.
Lowell and his observatory conducted his search until his death in 1916, but without any solid findings, or so he thought. After his death, the observatory announced that on March 19, 1915, surveys had captured faint images of what would become known as Pluto, but they were not recognized for what they were at the time. Complicating matters, there were 15 other known pre-discoveries of Pluto by 1915, the first made in 1909 by the Yerkes Observatory.
After Lowell died, his widow entered a massive legal battle over the million-dollar observatory, which delayed the search for Planet X until 1929. On February 18, 1930, the observatory obtained photographs of Planet X and, after producing confirmatory photographs, the news was telegraphed to the Harvard College Observatory on March 13. (Coincidentally, Uranus, for which the orbit was originally disturbed by Planet X spurring scientific investigation, was discovered on March 13, 1781, by William Herschel.)
The discovery made headlines across the globe. The Lowell Observatory, which had the right to name the new object, received over 1,000 suggestions from all over the world, including one from Lowell’s widow who suggested naming the Planet X after herself.
The name Pluto was proposed by Venetia Burney, an 11-year-old girl in England who was interested in classical mythology and considered “Pluto,” a name for the god of the underworld, appropriate for such a presumably dark and cold world.
From its naming in 1930 until 2006, Pluto was classified as a planet. But in the 1970s astronomers began to question whether Pluto, with its relatively low mass, could be classified as a planet. Around this time, many other objects similar to Pluto were discovered.
In 2006, the International Astronomical Union (IAU) defined what it means to be a "planet" within the Solar System, a definition that excluded Pluto as a planet and added it as a member of the new category "dwarf planet" along with Eris and Ceres. After the reclassification, Pluto was added to the list of minor planets and given the number 134340.
The definitions of a planet continue to be disputed among astronomers and scientists, with some arguing that Pluto should continue to be classified as a planet and that other dwarf planets should also be added to the list of planets.
The public also disputed whether Pluto should be classified as a planet. In reaction to the IAU decision, several “Pluto Protests” were held. Indeed, some people were so angered that they pushed for competing legislation declaring Pluto a planet.
As example, a resolution introduced by some members of the California State Assembly light-heartedly denounces the IAU for "scientific heresy” after it announced its reclassification. New Mexico's House of Representatives passed a resolution in honor of Clyde Tombaugh, the astronomer credited with Pluto’s final discovery and a longtime resident of the state, which declared that Pluto will always be considered a planet while in New Mexican skies and that March 13, 2007, was Pluto Planet Day. And the Illinois State Senate passed a similar resolution in 2009 that asserted Pluto was "unfairly downgraded to a 'dwarf' planet" by the IAU. It did so on the basis that Tombaugh was born in Illinois.
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Editor's note: This article was originally posted on March 13, 2013 and edited on March 13, 2017.