Some questions have arisen over the recent International Astronomical Union (IAU) resolutions that defined three categories of bodies in the solar system: planets, dwarf planets, and small solar system bodies. These concerns are not surprising, given the long and difficult history of efforts to reach agreement on just what a planet is, and the unwillingness of nature to be categorized into neat compartments.

The Division for Planetary Sciences (DPS) of the American Astronomical Society (AAS) recognizes the authority of the IAU to render a decision, and notes that it had considerable input by DPS members in the process. However, it is also mindful of the fact that future refinements of this definition will almost certainly be desired. All definitions have a degree of fuzziness that requires intelligent application: what does “round” really mean? What does it mean to “control a zone”? These are technical issues to be addressed by Division III of the IAU, currently chaired by Ted Bowell, a fellow DPS member. There is still work to be done, too, in constructing a definition that is generally applicable to extra-solar planetary systems. These and other changes, radical or moderate, presumably will be addressed at the next IAU General Assembly in Rio de Janeiro in 2009, and the DPS community will continue to be involved in all stages of this process.

Ultimately, the definition of a planet will come through common usage and scientific utility. There is no need to throw away current school texts; Pluto has not gone away. We will continue to explore Pluto and the other objects orbiting beyond Neptune with telescopic observations and spacecraft missions to obtain a fundamental understanding of their place in our solar system.

The DPS is the largest international professional organization of planetary scientists with approximately 1300 members of which about 30% are from non-US countries.

The full text of the IAU resolutions is at:


Sanjay S. Limaye, DPS Press Officer, Space Science and Engineering Center, University of Wisconsin-Madison, 1225 West Dayton Street, Madison, Wisconsin 53706, 1-608-262-9541.

Richard G. French, Chair, DPS/AAS. Dept. of Astronomy, Wellesley College, 106 Central St. Wellesley, MA 02481. Email: Work Phone 1-781-283-3747 Fax: 1-781-283-3667