Dwarf Planets Overview

Pluto and other dwarf planets are a lot like regular planets. So what’s the big difference? The International Astronomical Union (IAU), a world organization of astronomers, came up with the definition of a planet in 2006. According to the IAU, a planet must do three things:

1. Orbit its host star (In our solar system that’s the Sun).
2. Be mostly round.
3. Be big enough that its gravity cleared away any other objects of similar size near its orbit around the Sun.

Dwarf planets like Pluto were defined as objects that orbit the Sun, and are nearly round, but have not been able to clear their orbit of debris.

So far, the IAU has only recognized five dwarf planets. In order of distance from the Sun they are: Ceres, Pluto, Haumea, Makemake, and Eris. The IAU says there may be many more dwarf planets — perhaps more than a hundred — waiting to be discovered.

Pluto: The Star of Dwarf Planets

Pluto is by far the most famous dwarf planet. Discovered by Clyde Tombaugh in 1930, Pluto was long considered our solar system's ninth planet. But after other astronomers found similar intriguing worlds deeper in the distant Kuiper Belt — the IAU reclassified Pluto as a dwarf planet in 2006. 

There was widespread outrage on behalf of the demoted planet. Textbooks were updated, and the internet spawned memes with Pluto going through a range of emotions, from anger to loneliness.

On July 14, 2015, NASA’s New Horizons spacecraft made its historic flight through the Pluto system, and gave us the first close-up images of Pluto and its moons. New Horizons collected that has transformed our understanding of these mysterious worlds on the solar system’s outer frontier.

Ceres

Dwarf planet Ceres is not as well known as Pluto, but it's much closer to Earth. Ceres is the only dwarf planet in the inner solar system. It's located in the main asteroid belt between Mars and Jupiter. Like Pluto, Ceres also was once classified as a planet. It was even considered an asteroid for many years. Ceres was the first dwarf planet to be visited by a spacecraft, NASA’s Dawn mission. 

Haumea

Dwarf planet Haumea is in the Kuiper Belt, a doughnut-shaped region of icy bodies beyond Neptune. It orbits at an average distance of 4 billion miles (6.5 billion kilometers) from the Sun. Haumea has an equatorial diameter of about 1,080 miles (about 1,740 kilometers), making it about 1/7 the width of Earth. Haumea is one of the fastest rotating large objects in our solar system. The fast spin distorts Haumea's shape, making this dwarf planet look like a football. Two teams claim credit for discovering Haumea citing evidence from observations made in 2003 and 2004.

Makemake

Dwarf planet Makemake is in the Kuiper Belt. It orbits at an average distance of 4.3 billion miles (6.8 billion kilometers) from the Sun. Makemake has a diameter of about 888 miles (1,430 kilometers), making it about 1/9 the width of Earth. It is the second-brightest object in the Kuiper Belt as seen from Earth, after Pluto, and has one known moon, nicknamed MK 2. Makemake was discovered in March 2005 by a team led by Mike Brown at Palomar Observatory in California.

Eris

Dwarf planet Eris orbits the Sun far beyond Neptune, on a path that extends well beyond the Kuiper Belt. Its average distance from the Sun is 6.3 billion miles (10 billion kilometers). Eris has an equatorial diameter of about 1,500 miles (2,400 kilometers), making it about 1/5 the width of Earth and nearly the same size as Pluto. It has one known moon, Dysnomia. Eris was discovered on Jan. 5, 2005, by Mike Brown, Chad Trujillo, and David Rabinowitz using data gathered at the Palomar Observatory in 2003. The discovery of Eris helped trigger the debate that led to Pluto’s reclassification as a dwarf planet in 2006.