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what was the new definition adopted by the IAU in which pluto didn' t fit?

The definition of planet set in 2006 by the International Astronomical Union (IAU) states that, in the Solar System, a planet is a celestial body which:

  1. is in orbit around the Sun,
  2. has sufficient mass to assume hydrostatic equilibrium (a nearly round shape), and
  3. has "cleared the neighbourhood" around its orbit.

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  • Planets have always been defined by their dynamics. The word "planets" comes from the Greek word for "wanderer", an object that moves across the background of the fixed stars.
  • Moons have always been defined by their dynamics. A moon is an object that moves around (orbits) a planet. There are moons that look round and there are moons that don't look round at all. Their shape is irrelevant to the classification.
  • A classification based on dynamics is far easier to implement than a classification based on physical properties. Newly discovered objects can be classified immediately, long before the details of their physical properties are known.
  • We already classify other objects in our solar system based on their dynamics, such as the near-Earth asteroids (Aten - Apollo - Amor), the Trojan asteroids, the Centaurs, the trans-Neptunian objects (resonant - scattered - classical).
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