▁the ▁terrest rial ▁plan ets ▁( Mer cur y , ▁Ven us , ▁Earth , ▁and ▁Mars ). ▁These ▁comp ounds ▁are ▁quite ▁rare ▁in ▁the ▁Un iverse , ▁compr ising ▁only ▁ 0 . 6 % ▁of ▁the ▁mass ▁of ▁the ▁ne b ula , ▁so ▁the ▁terrest rial ▁plan ets ▁could ▁not ▁grow ▁very ▁large . ▁The ▁terrest rial ▁emb ry os ▁grew ▁to ▁about ▁ 0 . 0 5 ▁Earth ▁masses ▁() ▁and ▁ceased ▁accum ulating ▁matter ▁about ▁ 1 0 0 , 0 0 0   year s ▁after ▁the ▁formation ▁of ▁the ▁Sun ; ▁subsequent ▁coll isions ▁and ▁mer gers ▁between ▁these ▁planet - s ized ▁bodies ▁allowed ▁terrest rial ▁plan ets ▁to ▁grow ▁to ▁their ▁present ▁sizes ▁( see ▁Terr est rial ▁plan ets ▁below ). ▁ ▁When ▁the ▁terrest rial ▁plan ets ▁were ▁forming , ▁they ▁remained ▁imm ers ed ▁in ▁a ▁disk ▁of ▁gas ▁and ▁dust . ▁The ▁gas ▁was ▁partially ▁supported ▁by ▁pressure ▁and ▁so ▁did ▁not ▁orbit ▁the ▁Sun ▁as ▁rapidly ▁as ▁the ▁plan ets . ▁The ▁resulting ▁drag ▁and , ▁more ▁important ly , ▁gravit ational ▁interactions ▁with ▁the ▁surrounding ▁material ▁caused ▁a ▁transfer ▁of ▁angular ▁momentum , ▁and ▁as ▁a ▁result ▁the ▁plan ets ▁gradually ▁migr ated ▁to ▁new ▁or bits . ▁Mod els ▁show ▁that ▁density ▁and ▁temperature ▁variations ▁in ▁the ▁disk ▁govern ed ▁this ▁rate ▁of ▁migration , ▁but ▁the ▁net ▁tr end ▁was ▁for ▁the ▁inner ▁plan ets ▁to ▁migr ate ▁in ward ▁as ▁the ▁disk ▁diss ip