- sequence ▁star ▁today . ▁ ▁Form ation ▁of ▁the ▁plan ets ▁ ▁The ▁various ▁plan ets ▁are ▁thought ▁to ▁have ▁formed ▁from ▁the ▁solar ▁ne b ula , ▁the ▁disc - sh aped ▁cloud ▁of ▁gas ▁and ▁dust ▁left ▁over ▁from ▁the ▁Sun ' s ▁formation . ▁The ▁currently ▁accepted ▁method ▁by ▁which ▁the ▁plan ets ▁formed ▁is ▁acc ret ion , ▁in ▁which ▁the ▁plan ets ▁began ▁as ▁dust ▁gra ins ▁in ▁orbit ▁around ▁the ▁central ▁proto star . ▁Through ▁direct ▁contact ▁and ▁self - organ ization , ▁these ▁gra ins ▁formed ▁into ▁cl umps ▁up ▁to ▁ 2 0 0 ▁metres ▁in ▁diameter , ▁which ▁in ▁turn ▁coll ided ▁to ▁form ▁larger ▁bodies ▁( plan etes im als ) ▁of ▁~ 1 0   kil omet res ▁( km ) ▁in ▁size . ▁These ▁gradually ▁increased ▁through ▁further ▁coll isions , ▁growing ▁at ▁the ▁rate ▁of ▁cent imet res ▁per ▁year ▁over ▁the ▁course ▁of ▁the ▁next ▁few ▁million ▁years . ▁ ▁The ▁inner ▁Sol ar ▁System , ▁the ▁region ▁of ▁the ▁Sol ar ▁System ▁inside ▁ 4   AU , ▁was ▁too ▁warm ▁for ▁vol atile ▁mole cules ▁like ▁water ▁and ▁m eth ane ▁to ▁con den se , ▁so ▁the ▁plan etes im als ▁that ▁formed ▁there ▁could ▁only ▁form ▁from ▁comp ounds ▁with ▁high ▁mel ting ▁points , ▁such ▁as ▁met als ▁( like ▁iron , ▁nick el , ▁and ▁al umin ium ) ▁and ▁rock y ▁sil ic ates . ▁These ▁rock y ▁bodies ▁would ▁become