Dwarf planets are objects that exist in the solar system that are larger than meteors or comets but fall short of the definition of a planet. At least five dwarf planets have been identified in the solar system, including the famous former planet Pluto, though many more are suspected to exist.
According to the International Astronomical Union, a dwarf planet is an object that is not a satellite, is spherical in shape and has not cleared the neighborhood of its orbit. When an object "clears its neighborhood," that means it is no longer affected by the gravity of similar-sized objects; clearing the neighborhood is the only aspect that distinguishes dwarf planets from regular eight planets. Dwarf planets are able to have moons and other objects captured in their gravity.
Where to Find Them
Due to their small size, dwarf planets are difficult to locate. Nearly all the known dwarf planets in the solar system are located beyond the farthest planet, Neptune. The Kuiper Belt is a vast region in the outer reaches of the solar system that contains asteroids, comets and other small frozen objects. At least four dwarf planets are located in the Kuiper Belt and, because of the belt's distance from Earth and the fact no probe has yet reached it, scientists believe that there are many dwarf planets in the Kuiper Belt.
The most famous of the dwarf planets is Pluto, which prior to 2006 was classified as one of the nine planets. Pluto was discovered in 1930 by Clyde Tombaugh, and it has three known satellites: Charon, the largest one; Nix; and Hydra. Pluto is approximately 2,400 kilometers (1,500 miles) in diameter, and it is believed to be made up entirely of ice and rock. As of 2011, images of Pluto are fuzzy, though the space probe New Horizons is expected to reach the dwarf planet in 2015.
Besides Pluto, at least four other dwarf planets are known: Ceres, Eris, Haumea and Makemake. As of June 30, 2014, scientists at the California Institute of Technology were "nearly certain" of the existence of 10 dwarf planets in the Kuiper Belt. Of these, Eris is the largest and is in fact about 30 percent larger than Pluto; its discovery in 2005 caused scientists to downgrade Pluto's classification as a planet. Eris has one moon, Dysnomia. Ceres was discovered in 1801 and was classified alternately as a planet, then an asteroid, until being upgraded to a dwarf planet in 2006. Ceres is not in the Kuiper Belt; it is in the solar system's asteroid belt, between Mars and Jupiter.