protoplanet

noun

1. An 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

   “Tyrix, one of the largest objects in Thal's asteroid belt, is a metallic asteroid with an iron-nickel core under a rocky olivine mantle. It is possibly a remnant protoplanet prevented from forming into anything bigger by gravitational disturbances from Xerceo.”