Astronomers Reclassify Pluto as a Dwarf Planet

On August 24, 2006, members of the International Astronomical Union assembled in Prague to decide on a formal scientific definition for the term planet. Since its discovery by Clyde Tombaugh in 1930, Pluto had been widely listed as the ninth planet in the solar system. Over the preceding decades, however, fresh observational data gradually undermined that standing. Observations aided by the discovery of its large moon Charon showed that Pluto's mass is only about one-sixth that of Earth's Moon. Beginning in the 1990s, astronomers began detecting numerous additional bodies in the Kuiper Belt with similar orbital characteristics, culminating in the July 2005 announcement of a distant Kuiper Belt object then thought to be larger than Pluto, provisionally designated 2003 UB313 and later named Eris.

These outer solar system discoveries forced the scientific community to choose between continuously expanding the catalog of recognized planets or introducing strict physical and dynamical boundaries. The union's planet definition committee initially proposed defining a planet strictly by hydrostatic equilibrium, requiring sufficient mass for self-gravity to produce a round shape while orbiting a star. Had that proposal passed, Ceres, Charon, and Eris would have immediately gained recognition as planets, with at least a dozen more Kuiper Belt objects expected to qualify. That initial draft met widespread resistance during the Prague assembly. During the debate, astronomer Julio Fernández of the University of the Republic in Montevideo, Uruguay, suggested an alternative that defined a planet as by far the largest body in its local population, emphasizing an object's dynamical dominance over its orbital zone.