In the beginning of the 20th century, astronomers were under the impression that something was wrong with the orbits of Neptune and Uranus. Because their orbits did not revolve around the sun as circularly as they should, the scientists assumed that a ninth planet must be causing the disruption. In 1905, Lowell Observatory started an extensive project in search of a possible ninth planet. Twenty-five years later, on January 30, 1930, an astronomer by the name of Clyde Tombaugh discovered Pluto by taking pictures of the celestial sky, one to two weeks apart, and then looking for objects that had moved between images.
Seventy-five years later, the scientific community began to debate as to whether Pluto was actually a planet at all. In August 2006, the International Astronomical Union (IAU), the organization charged by the scientific community with classifying astronomical objects, voted to remove Pluto from the list of planets and to give it the new classification of dwarf planet. According to the IAU resolution, there are three main conditions for an object to be called a planet:
- The object must orbit around the Sun.
- The object must be massive enough to be a sphere by its own gravitational force.
- It must have cleared the neighborhood around its orbit.
Pluto fails to meet the third condition. The change in classification reflects astronomers' realization that Pluto is a large member of the Kuiper belt, a collection of debris of ice and rock left over from the formation of the solar system and now revolving around the Sun beyond Neptune's orbit.
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