Dwarf planet is the name used to classify some objects in the solar system. This definition was made on August 24, 2006 by the International Astronomical Union (IAU), and can be described as; a dwarf planet is a body orbiting the Sun that is big enough to round itself by its own gravity, but has not cleared its orbital path of other rival bodies. At the same meeting the IAU also defined the term planet for the first time. Some astronomers think that the term "dwarf planet" is too confusing and needs to be changed.
The five dwarf planets, in order from the Sun are
The dwarf planets, unlike the terrestrial and gas giant planets, are in more than one region of the solar system. Ceres is in the asteroid belt, Pluto's orbit is mostly outside that of Neptune, but partly inside: it is very eccentric in shape. The others are in the trans-Neptune region.
No space probes have visited any of the dwarf planets. This will change if NASA's Dawn and New Horizons missions reach Ceres and Pluto, respectively, as planned in 2015. Dawn also orbited and observed another not sphere-like but still potential dwarf planet, Vesta, in 2011.
- Ceres was found by Guiseppe Piazzi on January 1, 1801. Classified as a dwarf planet on September 13, 2006.
- Pluto was found by Clyde W. Tombaugh on February 18, 1930. Reclassified as a dwarf planet on August 24, 2006
- Haumea was found by a team led by Mike Brown on December 28, 2004. Accepted as a dwarf planet on September 17, 2008.
- Makemake was found by a team led by Mike Brown on March 31, 2005. Accepted as a dwarf planet on July 11, 2008.
- Eris was found by Mike Brown and his science team on January 5, 2005. Accepted as a dwarf planet on September 13, 2006.
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