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Ceres Designated a 'Dwarf Planet'

high res image of Ceres
ASA's Hubble Space Telescope color image of Ceres, the largest Main Belt asteroid. Astronomers optimized spatial resolution to about 18 km per pixel, enhancing the contrast in this image to bring out features on Ceres' surface, that are both brighter and darker than the average which absorbs 91% of sunlight falling on it. - Learn More

2008 - Members of the International Astronomical Union (IAU) gathered at the second General Assembly on August 24, 2006 and voted on an official definition of planet. A "planet" is defined as a celestial body that (a) is in orbit around the Sun, (b) has sufficient mass for its self-gravity to overcome rigid body forces so that it assumes a hydrostatic equilibrium and (nearly round) shape, and (c) has cleared the neighborhood around its orbit.  It is the last criterion (C) that eliminates Ceres from full planet status. The IAU defined a new category of planets designated as “dwarf planets,” which have the above properties but reside in a region of the Solar System populated by smaller objects. The current status of planets in the Solar System is the eight classical planets (Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, and Neptune). Pluto, Charon (its moon), and Ceres are dwarf planets.

A dwarf planet is a celestial body that (a) is in orbit around the Sun, (b) has sufficient mass for its self-gravity to overcome rigid body forces so that it assumes a hydrostatic equilibrium (nearly round) shape, (c) has not cleared the neighborhood around its orbit, and (d) is not a satellite."

- Learn about the IAU Resolutions
- Read an opinion of IAU's decision
- What are the educational implications of the new definition?
- Read an historical perspective by James Hilton, US Naval Observatory