Dwarf Planets

In 2006, the International Astronomical Union (IAU) approved dwarf planets as a new classification category for "in-between sized" objects in our Solar System. Currently, there are three dwarf planets: Eris, Ceres, and Pluto (which was demoted from its previous status as a planet). In 2008, The IAU decided on the term plutoid as a name for dwarf planets like Pluto.

(Image Right: The Dwarf Planets (Used with permission, Walter Myers.))

Dwarf planets look like planets, but they are not exactly the same. They orbit the Sun just like planets do. They also are large enough that their own gravity pulls them into the shape of a sphere, or ball-like shape. Dwarf planets are not satellites (moons of other planets). The fact that dwarf planets are unable to clear out their orbits by pulling smaller bodies into themselves or flinging them out of orbit is what distinguishes them from planets.

Features

: Composition of Pluto (Original Image Courtesy of NASA).

The dwarf planets are small, cold and dark worlds. They have solid rocky surfaces. Pluto is likely made of rock, water ice and frozen gases. Frozen nitrogen, carbon dioxide and methane probably make up Pluto's atmosphere. Ceres is composed of a rocky inner core surrounded by a mantle of water ice.

Recently discovered, Eris is an icy body on the edge of our Solar System. In the future, it is likely that there will be many more dwarf planets discovered as the Kuiper Belt, the vast region beyond Neptune, is explored.

Learn more about the dwarf planets' features from Nine Planets.

Missions

: Artist's concept of New Horizons headed towards Pluto (Courtesy of JHUAPL/SwRI)

Launched in January 2006, New Horizons swung past Jupiter for a gravity boost while collecting data for scientists. The spacecraft will reach Pluto and its moons, Charon, Nix, and Hydra, in 2015. Heading deeper into the Kuiper Belt, the spacecraft will help us answer questions about worlds at the edge of our Solar System.

In space since September 2007, a probe called Dawn is flying toward the asteroid belt and will observe the asteroid Vesta before it reaches Ceres in 2015. This mission will help us learn about the evolution of the Solar System.

Discover more about New Horizons from NASA's website.

Myths, Stories, and More

Named after the Roman goddess of the harvest, Ceres is also the most massive body in the asteroid belt. Eris, the largest dwarf planet, is named for the Roman goddess of strife. She stirred up trouble, including the Trojan War.

Pluto rules the underworld. He stole Persephone, the daughter of Demeter the grain goddess, to be his wife. Because the goddess was angry, all the crops failed. Pluto compromised with Demeter, and her daughter spent only half of each year in the underworld. But no crops would grow at that time, thus explaining winters here on Earth.

Learn more about Dwarf Planets myths and stories at Windows to the Universe.

Earth Matters

: Comparing the dwarf planets (Original images NASA. Redesign by A.Rickhoff, Adler Planetarium)

Ceres is thought to have an abundance of fresh water like Earth. Its interior is also separated into a crust, mantle and core like our planet. Ceres may turn out to be one of the gems of our Solar System. It could become a future destination for manned space missions.

Compared to Earth, Pluto is almost 40 times farther from the Sun. Pluto is about one-fifth our planet's size. If Pluto and its moon Charon were placed side-by-side, they would be about the length of the United States.

Find out more about Ceres at Space.com