Comets are classified as small solar system bodies that show some sort of activity, usually dust, at or after discovery with orbital motions with respect to the Sun, much like the ancients did in their time. Is activity a sufficient criterion for comet classification? This has led to dual classifications, e.g., the Centaurs, 95P/Chiron, 174P/Echeclus, and 166P/NEAT, are classified as comets, in addition to their minor planet designations. This traditional definition of a comet needs to be revisited in light of detailed knowledge gathered by spacecraft missions and modern telescopes. There now exists a diverse inventory of small solar system bodies that challenges our traditional notions of comets including, main belt comets/active asteroids, Centaurs, Trojan asteroids, Kuiper Belt objects (e.g., Arrokoth), interstellar comets (e.g., 2I/Borisov), sun-grazers, and others. A new definition should take into account physico-chemical properties, dynamics, time and place of origin, activity mechanisms, and other important aspects, distinguishing between a comet (appearance in the sky) and the comet nucleus (solid-body source of activity). These issues need to be addressed to place comets in the proper context with other small solar system bodies. As new information is gained, our understanding of comets must be revised.
This work was partially supported by NSF Planetary Astronomy Program Grant No. 0908529.