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Chart of relative sizes of the dwarf planets
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C-group S-type X-group
C-group asteroids: 253 Mathilde, 2 Pallas, 324 Bamberga
S-type asteroids: 243 Ida, 15 Eunomia, 3 Juno, 532 Herculina
X-group asteroids: 16 Psyche, 76 Freia, 153 Hilda, 64 Angelina

Dwarf planets are objects that have enough mass to have their shape defined by gravity, are not satellites orbiting other planets, and have not cleared the neighborhood around their orbits of other objects. Currently only five objects in the Solar System are classified as dwarf planets.

Discovered in 1930 by astronomer Clyde Tombaugh, Pluto was considered the ninth planet in the Solar System until its reclassification in 2006 by the International Astronomical Union.
Pluto has four moons, Charon, Nix, Hydra and the yet-unnamed S/2011 P1 discovered in 2011 by a team led by the SETI Institute's Mark Showalter.
Ceres was the first dwarf planet discovered and is the only dwarf planet located inside the asteroid belt, which lies between the orbits of Mars and Jupiter. It was discovered in 1801 by Guiseppe Piazzi and is believed to have a surface made up of large amounts of water ice.
To learn more about Ceres, in 2007 NASA launched the robotic spacecraft, Dawn, which is expected to arrive at Ceres in 2015.
Though more or less the same size as Pluto, Haumea's fast four-hour rotation around its axis has given it an elongated shape which is unique among the dwarf planets. Haumea is located in the Kuiper Belt region and has two satellites orbiting around it: Namaka and Hi'iaka.
Also located in the Kuiper Belt, and discovered in 2010, not much is yet known about this dwarf planet. It takes Makemake a full 310 Earth years to orbit the Sun and its surface has been detected to contain frozen nitrogen, ethane and methane.
The farthest of the known Dwarf Planets, Eris was discovered in 2003 and is believed to be about the same size as Pluto. Its elliptical orbit takes it outside of the Kuiper belt, and so far away from the Sun, that its atmosphere often freezes and collapses on its surface.
Eris has one known moon, named Dysnomia.
Ida and Dactyl

NASA launched the spacecraft New Horizons in 2006 and expect it to arrive at Pluto and the Kuiper Belt in 2015.

meteor

Since Eris is too far away to resolve, astronomers used its moon, Dysnomia, to get an estimate of its size.

mass

The amound of matter in an object, measured in kilograms. Mass is different from weight, which is measured in pounds and can only be measured under acceleration such as a gravity field. A 1-kilogram mass weighs about 2.2 pounds on the surface of Earth, but weighs only 0.8 pounds on the surface of Mars.

gravity

One of the fundamental forces of physics; gravity is responsible for the attraction of bodies on the surface of a planet to the mass of the planet.

asteroid belt

The area between the orbits of Mars and Jupiter which contains a large number of asteroids, including Ceres, the largest asteroid in the Solar System.

Giuseppe Piazzi

Italian astronomer and priest, born in 1746 and best known for his discovery of Ceres, the largest object in the asteroid belt.

Illustration: Gabriel Alvarado-Marin (SETI Institute)

axis

A straight line around which an object rotates.

Kuiper belt

A region in the Solar System that extends beyond the orbit of Neptune and is the home of remnants from the formation of the Solar System, as well as the dwarf planet Pluto.

Nitrogen

A colorless and odorless gas, and one of the most abundant elements in Earth's atmosphere, and the seventh most abundant element in the Universe.
Nitrogen is a byproduct of the fusion process that occurs in stars.

ethane

C2H6
A colorless and odorless gas found in trace amounts in Earth's atmosphere. It has also been detected in small amounts in the atmospheres of all four gas giant planets as well as Saturn's moon, Titan.

methane

CH4
A colorless, odorless gas and the main component of natural gas.

elliptical orbit

An orbit that's elliptical, deviating from circular. The orange planet has an elliptical orbit, while the blue planet's is circular.
The same as eccentric orbit.

atmosphere

The gaseous material surrounding planets; the air surrounding Earth.