Why is Pluto no longer considered a planet? 

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In 2006, the International Astronomical Union (IAU) declared that Pluto was no longer considered a planet but, instead, a dwarf planet. After Pluto was discovered in 1930, debate continued about whether it was a planet or maybe simply a small icy body lying beyond Neptune in an area that became known as the Kuiper Belt. In 1992, the first member of this area was identified. More Kuiper Belt Objects (KBOs) were discovered, including Quaoar (2002), Sedna (2003), and Eris (2005), which had roughly the same mass as Pluto.

These discoveries pushed the IAU to consider during their 2006 General Assembly in Prague what defined a planet. They produced a three-part definition. A planet should orbit around the sun. In addition, they declared that a planet has "sufficient mass for its self-gravity to overcome rigid body forces so that it assumes a hydrostatic equilibrium (nearly round) shape." The third part of the definition was that a planet has to "clear the neighborhood around its orbit." This means that it has to be dominant in terms of gravitation and have thrown other large objects out of its orbit. While Pluto met the first two criteria, it did not meet the last one, as there were other KBOs in the neighborhood of its orbit. Therefore, it was deemed not a planet. 

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Why is Pluto no longer a planet?

From 1930 to 2006, Pluto was considered to be the ninth planet of our solar system.  In 2006 it was reclassified as a dwarf planet, though many still consider it the ninth planet for historical reasons.  One reason that it lost its planetary status is because it is so small.  It has less than one percent of Earth's mass.  There are numerous other bodies in that part of the solar system (called the Kuiper belt) with similar mass so the decision was made to make the classification of dwarf planet.

Another reason that Pluto lost its planetary status is because its largest moon, Charon, is not that much smaller than Pluto itself.  As a result, many consider the Pluto-Charon system to be binary system rather than a classic planet-moon type system, though it is not technically classified as such.

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Why do you think Pluto is no longer a planet?

Pluto, formerly a planet of our solar system, was discovered in 1930. For a long time, we considered the 9 largest objects in our solar system that rotate around the Sun to be planets. However, over time, powerful telescopes helped us understand our solar system better and we have found many more objects in our solar system, apart from these 9 bodies. This caused confusion over what is planet and what is not. International Astronomical Union (IAU) came up with a definition of planet as a round body that orbits Sun and has cleared the neighborhood of its orbit. While Pluto satisfies the first two conditions (round and orbits the Sun), it is yet to clear its neighborhood. The failure to fully satisfy the criteria caused the IAU to vote out Pluto from the list of planets, on 24 August, 2006. 

Pluto constitutes only a very very small portion of the mass of bodies in its orbit. In comparison, Earth has more than a million times the mass of the other bodies in its orbit. 

Hope this helps. 

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