Dwarf Planet

A dwarf planet is a small, round world that orbits the Sun but is not big enough to count as a regular planet. Dwarf planets are real worlds with gravity, shapes, and sometimes even moons. They just did not pass all the tests that astronomers use to decide what a planet is. The word "dwarf planet" was first used in 2006.

Astronomers use three rules to decide if something is a planet. First, it has to orbit the Sun. Second, it has to be big enough that its own gravity pulls it into a round shape. Third, it has to have "cleared its neighborhood." That means it has swept up or pushed away most of the other objects near its orbit. A dwarf planet passes the first two tests but fails the third. It shares its orbit with lots of other rocks or icy bodies.

The most famous dwarf planet is Pluto. Until 2006, Pluto was called the ninth planet. Then astronomers found other icy worlds out past Neptune that were about the same size. They had to pick between adding many new planets or making the rules stricter. They made the rules stricter, and Pluto was moved into the new dwarf planet group.

There are five official dwarf planets right now: Pluto, Eris, Haumea, Makemake, and Ceres. Four of them live in the icy Kuiper Belt beyond Neptune. Ceres is the odd one out. It sits in the asteroid belt between Mars and Jupiter, and it is the only dwarf planet in the inner solar system. Ceres is about 590 miles across, which is roughly the distance from New York City to Chicago.

Dwarf planets can be strange places. Haumea spins so fast that it is shaped like a squashed egg, with one full day lasting only four hours. Makemake is covered in frozen methane. Eris is so far from the Sun that a year there lasts 557 Earth years.

Scientists think there may be many more dwarf planets waiting to be found. Some estimate that the outer solar system could hold hundreds of them, hidden in the dark past Neptune. The 2006 decision is also still argued about today. Some astronomers, including the scientist who led the New Horizons mission to Pluto, think the rules are unfair and should be changed back. For now, though, the dwarf planets sit in their own special group, round little worlds that are not quite planets.