protoplanetWiktionary 原文英美构词拆解protoproto-前缀planetplanet词干proto-An early, primitive stage of development. · Original, older.词源说明From proto- + planet.释义名词noun1An astronomical object, approximately the size of the Moon, formed from the mutual gravitational attraction of planetesimals; they are thought to collide with each other and slowly form planets同族词派生词 1protoplanetary