interplanetary (adj.)

word-forming element used freely in English, "between, among, during," from Latin inter (prep., adv.) "among, between, betwixt, in the midst of" (also used extensively as a prefix), from PIE *enter "between, among" (source also of Sanskrit antar, Old Persian antar "among, between," Greek entera (plural) "intestines," Old Irish eter, Old Welsh ithr "among, between," Gothic undar, Old English under "under"), a comparative of root *en "in."

A living prefix in English from 15c. and used with Germanic as well as Latinate words. Spelled entre- in French; most words borrowed into English in that form were re-spelled 16c. to conform with Latin except entertain, enterprise. In Latin, spelling shifted to intel- before -l-, hence intelligence, etc.

late Old English planete, in old astronomy, "star other than a fixed star; star revolving in an orbit," from Old French planete (Modern French planète) and directly from Late Latin planeta, from Greek planētēs, from (asteres) planētai "wandering (stars)," from planasthai "to wander," a word of uncertain etymology.

Perhaps it is from a nasalized form of PIE root *pele- (2) "flat; to spread," on the notion of "spread out," "but the semantics are highly problematic," according to Beekes, who notes the similarity of meaning to Greek plazein "to make devious, repel, dissuade from the right path, bewilder," but adds, "it is hard to think of a formal connection."

So called because they have apparent motion, unlike the "fixed" stars. Originally including also the moon and sun but not the Earth; modern scientific sense of "world that orbits a star" is from 1630s in English. The Greek word is an enlarged form of planes, planetos "who wanders around, wanderer," also "wandering star, planet," in medicine "unstable temperature."

adjective and noun word-forming element, in most cases from Latin -arius, -aria, -arium "connected with, pertaining to; the man engaged in," from PIE relational adjective suffix *-yo- "of or belonging to." The neuter of the adjectives in Latin also were often used as nouns (solarium "sundial," vivarium, honorarium, etc.). It appears in words borrowed from Latin in Middle English. In later borrowings from Latin to French, it became -aire and passed into Middle English as -arie, subsequently -ary.