The Collaborative International Dictionary of English v.0.48:

Planet \Plan"et\, n. [OE. planete, F. plan[`e]te, L. planeta,
   fr. Gr. ?, and ? a planet; prop. wandering, fr. ? to wander,
   fr. ? a wandering.]
   1. (Astron.) A celestial body which revolves about the sun in
      an orbit of a moderate degree of eccentricity. It is
      distinguished from a comet by the absence of a coma, and
      by having a less eccentric orbit. See Solar system.
      [1913 Webster]

   Note: The term planet was first used to distinguish those
         stars which have an apparent motion through the
         constellations from the fixed stars, which retain their
         relative places unchanged. The inferior planets are
         Mercury and Venus, which are nearer to the sun than is
         the earth; the superior planets are Mars, the
         asteroids, Jupiter, Saturn, Uranus, and Neptune, which
         are farther from the sun than is the earth. Primary
         planets are those which revolve about the sun;
         secondary planets, or moons, are those which revolve
         around the primary planets as satellites, and at the
         same time revolve with them about the sun.
         [1913 Webster]

   2. A star, as influencing the fate of a men.
      [1913 Webster]

            There's some ill planet reigns.       --Shak.
      [1913 Webster]

   Planet gear. (Mach.) See Epicyclic train, under
      Epicyclic.

   Planet wheel, a gear wheel which revolves around the wheel
      with which it meshes, in an epicyclic train.
      [1913 Webster]

WordNet (r) 3.0 (2006):

planet gear
    n 1: an outer gear that revolves about a central sun gear of an
         epicyclic train [syn: planetary gear, epicyclic gear,
         planet wheel, planet gear]