Ceres and Pluto have been considered planets at time in the past. The discovery of other bodies in the asteroid belt led to the reclassification of Ceres in the mid-19th cent. as an asteroid. The discovery of other bodies beyond the orbit of Neptune that were similar in size to Pluto led to the declassification of Pluto as a planet in 2006, when the International Astronomical Union adopted the classification of dwarf planet.
The Columbia Electronic Encyclopedia, 6th ed. Copyright 2012, Columbia University Press. All rights reserved.
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