being or occurring between the planets or between a planet and the sun.

1685–95; inter- + planetary Unabridged
Based on the Random House Dictionary, © Random House, Inc. 2014.
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World English Dictionary
interplanetary (ˌɪntəˈplænɪtərɪ, -trɪ)
of, relating to, or existing between planets

Collins English Dictionary - Complete & Unabridged 10th Edition
2009 © William Collins Sons & Co. Ltd. 1979, 1986 © HarperCollins
Publishers 1998, 2000, 2003, 2005, 2006, 2007, 2009
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Word Origin & History

1691, "existing between planets," from inter- + planet. In ref. to travel between planets, attested from 1897.
Online Etymology Dictionary, © 2010 Douglas Harper
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Example sentences
The researchers examined the interplanetary gravitational pull found there.
Many engines would be clustered together in orbit to give enough power for
  interplanetary travel.
But this would only possible if terrestrial bugs can survive the intense vacuum
  and radiation in interplanetary space.
When you go to the moon, you are making an interplanetary trip.
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