  A planet is an end product of disk accretion around a primary star or
substar. I quantify this definition by the degree to which a body dominates the
other masses that share its orbital zone. Theoretical and observational
measures of dynamical dominance reveal a gap of four to five orders of
magnitude separating the eight planets of our solar system from the populations
of asteroids and comets. The proposed definition dispenses with upper and lower
mass limits for a planet. It reflects the tendency of disk evolution in a
mature system to produce a small number of relatively large bodies (planets) in
non-intersecting or resonant orbits, which prevent collisions between them.
