Ceres is not considered to be a planet because in 2006 the International Astronomical Union (IAU) introduced a new definition of a planet which included three criteria:
It must orbit the sun.
It must be massive enough to have a roughly spherical shape.
It must have "cleared the neighborhood" of its orbit of other debris, meaning that it has become gravitationally dominant in its orbit.
Ceres, which is the largest object in the asteroid belt between Mars and Jupiter, satisfies only the first two criteria. It has not cleared the neighborhood around its orbit, as it is still surrounded by a significant number of other objects in the asteroid belt. Therefore, it is classified as a "dwarf planet" rather than a full-fledged planet.
This reclassification caused some controversy among scientists and the general public, but the IAU's decision was based on the need for a clear and consistent definition of what constitutes a planet in our solar system.
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