Pluto is no longer considered a planet because, in 2006, the International Astronomical Union (IAU) redefined the criteria for what constitutes a planet. According to the IAU, to be classified as a full-sized planet, a celestial body must meet three criteria:
- It must orbit the Sun.
- It must be massive enough to assume a nearly round shape (hydrostatic equilibrium).
- It must have cleared the neighborhood around its orbit.
Pluto meets the first two criteria but fails the third. It shares its orbital neighborhood with other objects in the Kuiper Belt and has not become gravitationally dominant in that region. Because of this, Pluto was reclassified as a "dwarf planet" rather than a full planet. The reclassification means that there are now eight recognized planets in the Solar System, with Pluto being part of a group of similar-sized objects beyond Neptune's orbit.
