According to the International Astronomical Union, which sets definitions for planetary science, a dwarf planet is a celestial body that:
- Orbits the sun.
- Has enough mass to assume a nearly round shape.
- Has not cleared the neighborhood around its orbit.
- Is not a moon.
The main distinction between a dwarf planet and a planet is that planets have cleared the path around the sun while dwarf planets tend to orbit in zones of similar objects that can cross their path around the sun, such as the asteroid and Kuiper belts. Dwarf planets also are generally smaller than the planet Mercury.
The first five recognized dwarf planets are Ceres, Pluto, Eris, Makemake and Haumea. Scientists believe there may be dozens or even more than 100 dwarf planets awaiting discovery.
The IAU recognized Pluto's special place in our solar system by designating dwarf planets that orbit the sun beyond Neptune as plutoids. Eris, which orbits far beyond Neptune, is a plutoid while Ceres, which orbits in the main asteroid belt between Mars and Jupiter is a dwarf planet.