[Three rules are used to categorize a planet.](https://en.m.wikipedia.org/wiki/IAU_definition_of_planet)
1). Does it orbit the sun
2). Is it large enough that it has become nearly round over time
3). Has it “cleared the neighborhood” around its orbit.
Planets can say yes to all three, but dwarf planets have not “cleared the neighborhood”
The definition of clearing the neighborhood is a bit tricky to nail down exactly but it essentially means that it’s gravitationally dominant, or that nothing has a strong gravitational force effecting it other than the sun and any of its moons. In Pluto’s case, it crosses Neptune’s orbit, proving that Neptune has some pull on Pluto’s orbit.
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