Recent discoveries of objects in the outer reaches of our Solar System have forced scientists to reconsider what it means to be a planet. The International Astronomical Union (IAU) has proposed a new definition of a planet as a celestial body whose gravity is strong enough for it to be nearly round in shape, and which is in orbit around a star but is itself neither a star nor a satellite of a planet. According to this definition, the nine traditional planets in our Solar System would be joined by Ceres (the largest of the asteroids), by Charon (Pluto’s largest moon), and by 2003 UB313 (the provisional name for a recently discovered object larger and more distant from the Sun than Pluto). Pluto and Charon would be regarded as a double planet, rather than as a planet and satellite, because their center of gravity lies outside of Pluto itself (the only such case known in our Solar System). There is a candidate list of additional objects that may be large enough to qualify as planets, subject to confirmation by the IAU.

The IAU resolution also recognizes Pluto as the prototype of a new class of planetary objects to be known as “plutons.” In contrast to the classical planets, plutons typically have quite non-circular orbits and take more than 200 years to orbit the Sun. With increasingly sensitive and broad searches of the outer solar system well underway, it is quite likely that additional Pluto-like planets will be discovered.

The Division for Planetary Sciences (DPS) of the American Astronomical Society is the world’s largest international professional society of planetary scientists. The DPS Committee, elected by our membership, strongly supports the IAU resolution. It was proposed after two years of careful review by an international panel of expert planetary scientists, followed by a broadly representative international group of historians, writers, and scientists. The new definition is clear and compact, it is firmly based on the physical properties of celestial objects themselves, and it is applicable to planets found around other stars. It opens the possibility for many new Pluto-like planets to be discovered in our Solar System.

The proposed definition will be brought to the IAU General Assembly for a vote on August 24, 2006. As representatives of an international community of planetary scientists, we urge that the resolution be approved.

Dr. Richard G. French

Chair, DPS Committee

Contact information for comments or questions:

Dr. Richard French, Wellesley College, Wellesley, MA. (781 283-3747, rfrench@wellesley.edu)

Br. Guy Consolmagno, Vatican Observatory, Tucson, AZ (520 795-9866, brother_guy@mac.com)

Dr. Linda French, Illinois Wesleyen University, Bloomington, IL (309 556-3580, Lfrench@iwu.edu)

Dr. Nadine Barlow, Northern Arizona University, Flagstaff, AZ (928 523-5452, nadine.barlow@nau.edu)

Dr. Sanjay S. Limaye, University of Wisconsin, Madison, WI (608 262-9541, SanjayL@ssec.wisc.edu)

Dr. Larry A. Lebofsky, University of Arizona, Tucson, AZ (520 621-6947, Lebofsky@lpl.arizona.edu)

Dr. Fran. Bagenal, University of Colorado, Boulder, CO (303 492-2598, bagenal.colorado.edu)

Dr. Bill McKinnon, Washington University, St. Louis, MO (314 935-5604, mckinnon@wustl.edu)

Dr. Don Davis, Planetary Science Institute, Tucson, AZ (520 622-6300, drd@psi.edu)

Dr. Bill Bottke, Southwest Research Institute, Boudler, CO ((303) 546-6066, bottke@boulder.swri.edu

Dr. Amy Simon-Miller, Goddard Space Flight Center/NASA, Goddard, MD ((301) 286-6738, amy.simon@nasa.gov

Dr. Roger Yelle, University of Arizona, Tucson, AZ (520) 621-6243, yelle@lpl.arizona.edu)