The Collaborative International Dictionary of English v.0.48:

Planetary \Plan"et*a*ry\, a. [Cf. L. planetarius an astrologer,
   F. plan['e]taire planetary. See Planet.]
   1. Of or pertaining to the planets; as, planetary
      inhabitants; planetary motions; planetary year.
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   2. Consisting of planets; as, a planetary system.
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   3. (Astrol.) Under the dominion or influence of a planet.
      "Skilled in the planetary hours." --Drayton.
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   4. Caused by planets. "A planetary plague." --Shak.
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   5. Having the nature of a planet; erratic; revolving;
      wandering. "Erratical and planetary life." --Fuller.
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   Planetary days, the days of the week as shared among the
      planets known to the ancients, each having its day.
      --Hutton.

   Planetary nebula, a nebula exhibiting a uniform disk, like
      that of a planet.
      [1913 Webster]

WordNet (r) 3.0 (2006):

planetary nebula
    n 1: a nebula that was once thought to be a star with its
         planets but is now thought to be a very hot star surrounded
         by an expanding envelope of ionized gases that emit a
         fluorescent glow because of intense radiation from the star