1. It orbits the Sun. (Pluto does this)
2. It has sufficient mass for its self-gravity to overcome rigid body forces so that it assumes a hydrostatic equilibrium (nearly round) shape. (Pluto does this)
3. It has cleared the neighborhood around its orbit. (Pluto does not do this)
Pluto hasn't cleared its orbital neighborhood because it shares its space with other objects in the Kuiper Belt. It's not big enough to gravitationally dominate its area, meaning there are other icy bodies of comparable size that share its orbital space.
This decision wasn't made lightly. It stemmed from the discovery of other large icy bodies in the Kuiper Belt, like Eris, which is actually slightly larger than Pluto. This raised the question: if Pluto is a planet, then what about these other large bodies?
The International Astronomical Union (IAU) ultimately decided to establish a new category, "dwarf planets", to accommodate these objects and ensure a consistent definition of "planet" for the solar system.
It's important to note that Pluto is still a very interesting and complex body. It has moons, a diverse surface, and even a thin atmosphere. While it might not be a planet anymore, its discovery and study have significantly expanded our understanding of the outer solar system.
