FactSentinel

On August 24, 2006, the International Astronomical Union (IAU), the body responsible for naming and classifying celestial objects, voted at its General Assembly in Prague to adopt a formal definition of "planet." Under Resolution 5A, a planet must orbit the Sun, be massive enough for its own gravity to pull it into a roughly round shape, and have cleared the neighborhood around its orbit of other objects of comparable size. Pluto meets the first two criteria but not the third, since it shares its orbital region with many other icy bodies in the Kuiper Belt. The IAU therefore placed Pluto in a new category called "dwarf planet."

The decision followed years of debate sparked by the discovery of Eris in 2005, an object in the outer solar system about as massive as Pluto, and by the finding of many other large Kuiper Belt objects. Astronomers had to decide whether to add several new planets or to define the term more precisely. The IAU chose the latter. Pluto was later designated the prototype of a subgroup called plutoids, and it holds minor planet number 134340.

A common misconception is that Pluto was "demoted" because new data showed it was smaller than expected. In fact, the change reflected a new definition rather than a new measurement. Another misunderstanding is that the vote was unanimous among all astronomers. Only a few hundred of the IAU's members attended the final vote, and some scientists still dispute the definition, though it remains the official classification.