The difference comes down to one criterion: whether the object has cleared its orbital neighborhood. This definition was adopted by the International Astronomical Union (IAU) in 2006, and it is why Pluto is no longer counted among the planets.
The IAU definition of a planet
A planet in our solar system is a body that:
- Orbits the Sun, not another planet.
- Has enough mass to be nearly round, because its own gravity pulls it into a sphere (hydrostatic equilibrium).
- Has cleared the neighborhood around its orbit, meaning it dominates its orbital zone gravitationally.
A dwarf planet meets the first two criteria but not the third. It is also not a moon.
Why Pluto was reclassified
Pluto was discovered in 1930 by Clyde Tombaugh and counted as the ninth planet. From the 1990s, astronomers found many similar icy bodies in the Kuiper Belt beyond Neptune. In 2005, the discovery of Eris, about as large as Pluto, forced a decision: either add many new planets or define the term. The IAU chose the second option in August 2006. Pluto shares its orbital zone with many Kuiper Belt objects, so it became a dwarf planet.
NASA’s New Horizons flyby in 2015 showed Pluto to be a complex world with mountains of water ice, nitrogen glaciers and a thin atmosphere.
The recognized dwarf planets
The IAU currently recognizes five:
- Ceres: the largest object in the asteroid belt, visited by NASA’s Dawn mission, see comets and asteroids.
- Pluto: with its large moon Charon.
- Eris: in the scattered disk, slightly smaller than Pluto but more massive.
- Haumea: elongated and spinning fast, with rings.
- Makemake: in the Kuiper Belt.
Several other bodies, such as Gonggong, Quaoar and Sedna, are likely dwarf planets but not officially classified.
A continuing debate
Some planetary scientists criticize the definition. They argue that “clearing the neighborhood” depends on the distance from the Sun and that a geophysical definition (any round body) would be more useful. Under that view, Pluto and even large moons would count as planets. For now, the IAU definition is the official one: the solar system has eight planets.
The criteria apply to the solar system. For worlds around other stars, see how exoplanets are discovered.
