What is a dwarf planet?
A dwarf planet is a world that orbits the Sun and is round, but is not big enough to clear other objects out of its path. Pluto and Ceres are dwarf planets.
In 2006, astronomers from around the world agreed on rules for what counts as a planet. A planet must:
1. Travel around the Sun. 2. Be big enough for its own gravity to pull it into a round shape. 3. Be big enough to have cleared its path around the Sun of most other objects.
A dwarf planet follows rules 1 and 2, but not rule 3. It is round and orbits the Sun, but it shares its path with lots of other objects.
There are five official dwarf planets:
- Ceres, in the asteroid belt between Mars and Jupiter, about 940 kilometres across. - Pluto, beyond Neptune, about 2,377 kilometres across. It was called the ninth planet from 1930 until 2006. - Eris, a little smaller than Pluto, even farther away. - Haumea, which spins so fast that it is shaped like a stretched egg. - Makemake, another icy world beyond Neptune.
Astronomers think there could be many more dwarf planets waiting to be discovered far out in the Solar System.
Spacecraft have visited two: NASA's Dawn orbited Ceres, finding bright salty spots, and New Horizons flew past Pluto in 2015, finding icy mountains and a heart-shaped glacier.