Pluto is distinguished by properties other than its size, and its representation in “Nine planets, or eight?” as just another gray ball was misleading. It has the most contrasting surface known in the solar system (bright nitrogen ice caps and dark carbonaceous equatorial areas). To understand the processes ongoing on Pluto’s surface and within its atmosphere and interior, we look primarily (though not exclusively) to planetary analogs-not asteroids or comets or other Kuiper Belt objects, for that matter. Whether Pluto retains its planetary status in the future will require consideration of all of its known properties and their comparison with the extrinsic properties of other objects: asteroids, comets, and planets. Mark V. Sykes
University of Arizona
Tucson, Ariz.