What Is a Planet Today?
According to the new definition, a full-fledged planet is an object that orbits the sun and is large enough to have become round due to the force of its own gravity. In addition, a planet has to dominate the neighborhood around its orbit.
Pluto has been demoted because it does not dominate its neighborhood. Charon, its large “moon,” is only about half the size of Pluto, while all the true planets are far larger than their moons.
In addition, bodies that dominate their neighborhoods, “sweep up” asteroids, comets, and other debris, clearing a path along their orbits. By contrast, Pluto’s orbit is somewhat untidy.