In 2006, Pluto was famously demoted for failing a single cosmic test. Dwarf planets might be perfectly round, but they lack the one defining trait of a true planet: gravitational dominance.
To be officially classified as a "normal" planet by the International Astronomical Union, a celestial body must meet three specific criteria.
First, it must orbit the sun. Second, it must be massive enough that its own gravity pulls it into a nearly round shape, a state known as hydrostatic equilibrium.
The third rule is where normal planets and dwarf planets part ways. A normal planet must have "cleared the neighborhood" around its orbit. Over billions of years, a true planet exerts that dominance by sweeping up, capturing, or flinging away most of the other debris in its path. Earth, Jupiter, and the other major planets are the undisputed heavyweights of their respective zones. A dwarf planet meets the first two criteria but fails the third, sharing its orbital space with a multitude of other celestial bodies.
Pluto was reclassified because it resides in the Kuiper Belt, a vast ring of icy bodies and space rock beyond Neptune. Even though Pluto is round, its mass is only a tiny fraction of the total mass of the objects in its orbit, meaning it has not cleared its neighborhood.
The solar system currently has five officially recognized dwarf planets. Alongside Pluto, there are Eris, Haumea, and Makemake in the distant Kuiper Belt. Much closer to the sun is Ceres, the largest object in the main asteroid belt between Mars and Jupiter. While Ceres accounts for about a third of the asteroid belt's total mass, it is still surrounded by millions of other asteroids, keeping it classified as a dwarf planet rather than a true planetary heavyweight.