Entry 1 sense Webster, 1913 Interplanetary /ĭnˌ-tẽrp-lăn'-ət-ĕrˌ-ē/ · In·ter·plan·et·a·ry · IPA /ˌɪntərplˈænətˌɛriː/ 01 a. Between planets; as, interplanetary spaces; interplanetary travel. 1. Between planets; as, interplanetary spaces; interplanetary travel.