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In 2006, the IAU (International Astronomical Union) for the first time came up with a definition for a planet. After coming up with this definition the IAU decided that Pluto is not a planet. There are many scientists who diagreed and still disagree with this decision.
Pluto is now considered the second largest dwarf planet in the solar system and is a part of the Kuiper Belt.
A dwarf planet is a celestial body that orbits the sun but is not heavy enough to be considered a planet. One of the differences between a planet and a dwarf planet is that a planet has cleared it's area of orbit from debris and a dwarf planet has not.
Some other dwarf planets are Ceres and Eris.
Pluto, is not only smaller and less massive than all the planets, but having less than 0.2 of Moon's mass is also smaller and less massive than even the first seven satellites in the solar system: Ganymede, Titan, Callisto , Io, Earth's Moon, Europa and Triton. But Pluto is about two times larger in diameter and 12 times more massive in the mass than Ceres, the largest dwarf planet in the asteroid belt.
Ceres was higher than any other known object in the Kuiper Belt - until, in 2005, was discovered dwarf planet 2003 UB313.
Pluto has received the status of dwarf planet, because it has not "cleaned up" the Space from the vicinity of its orbit.
Also, in 2006, it was discovered that Pluto is not a planet but an asteroid. Still it is not known exactly if this information is true.
International Astronomical Union officially identified 5 heavenly bodies that have received the status of dwarf planets: Ceres, Pluto, Haumea, Makemake and Eris.
As defined by International Astronomers Union (IAU) a planet is a natural object orbiting the Sun and meeting some other conditions related to its size. As per this definition Pluto is not a planet but a dwarf planet.
Pluto is definitely a fairly large natural object orbiting the sun, but not large enough to qualify as a planet. Existence of Pluto was discovered in 1930, and from that time to 2006 scientists regarded it as a planet. However later it was reclassified as a dwarf planet.
After discovery of Pluto many objects orbiting the sun similar to Pluto were discovered. This included some objects like Eris which has a mass 27% higher than that of Pluto. With this discovery a need was felt to differentiate between larger bodies orbiting the sun which could be classified planets and the smaller ones as dwarf planets. On August 24, 2006 an IAU, adopted definitions that made distinction between planets and dwarf planets. Following this the name of Pluto was struck off from scientifically recognized list of planets.
To be classified as a planet as per the new definition, an object must orbit the sun, should have become large enough to become round under the force of its own gravity, and should dominate the neighborhood around its orbit. Pluto fails to meet the criteria of dominating the neighborhood around its orbit.
Further details on this topic are available on the site referred below.
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