The Collaborative International Dictionary of English v.0.48:

Interplanetary \In`ter*plan"et*a*ry\, a.
   Between planets; as, interplanetary spaces; interplanetary
   travel. --Boyle.
   [1913 Webster +PJC]

WordNet (r) 3.0 (2006):

interplanetary
    adj 1: between or among planets; "interplanetary travel"

Moby Thesaurus II by Grady Ward, 1.0:

48 Moby Thesaurus words for "interplanetary":
   Cynthian, anagalactic, asteroidal, astral, astrologic,
   astrologistic, astrologous, astronomic, astrophysical, celestial,
   circumplanetary, cislunar, empyreal, empyrean, equinoctial,
   extragalactic, galactic, heavenly, heliacal, intercosmic,
   intersidereal, interstellar, lunar, lunary, lunate, lunular,
   lunulate, meteoric, meteoritic, nebular, nebulose, nebulous,
   planetal, planetarian, planetary, planetesimal, semilunar,
   sidereal, solar, sphery, star-spangled, star-studded, starry,
   stellar, stellary, terrestrial, uranic, zodiacal