Nine years after leaving Earth, NASA’s New Horizons spacecraft has flown by Pluto (at an astounding 28,000 mph) on its mission to help us understand worlds at the edge of our solar system.
US astronomer Clyde Tombaugh discovered Pluto in 1930 while searching for “Planet X”—a hypothetical ninth planet thought to be responsible for irregularities in the orbits of Neptune and Uranus. Pluto was named the ninth planet, though it turned out to have too little mass to exert gravitational pull on the gas giants.
Like true planets, dwarf planets have enough mass to become spherical through their own gravity. However, they lack the gravitational force to sweep their orbits clear of other bodies.
As they form, planets clear their orbits of minor objects, such as asteroids, either by pulling them in and amalgamating with them or by flinging them elsewhere. Dwarf planets cannot do this, though they may have sufficient gravity to capture their own moons.
The most famous example of a dwarf planet is Pluto, which orbits far from the sun in the freezing Kuiper Belt at the edge of the solar system. Once referred to as the ninth planet, Pluto was demoted and assigned to the new category along with several similar bodies found in the outer solar system. Among them are Eris (the largest known dwarf planet), Haumea, and Makemake. The asteroid Ceres, located in the Asteroid Belt between Mars and Jupiter, was also given dwarf planet status in 2006.
Discover more about Pluto and the rest of the solar system in The Planets.