What is the contentious third requirement for a celestial body to be defined as a planet by the IAU?
Answer
It must have "cleared the neighborhood around its orbit".
The current scientific consensus established by the International Astronomical Union (IAU) in 2006 outlines three necessary and sufficient conditions for planetary classification. The first two relate to orbiting the Sun and achieving hydrostatic equilibrium (a nearly round shape). The third condition, which proved most controversial during the reclassification discussions, mandates that the object must have achieved orbital hegemony by gravitationally dominating its orbital zone, meaning it has effectively swept up or ejected the majority of other sizable objects sharing its pathway over solar system history.
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