▁massive ▁as ▁Mercur y ▁were ▁present ▁in ▁the ▁inner ▁Sol ar ▁System , ▁the ▁outer ▁Sol ar ▁System ▁was ▁much ▁more ▁compact ▁than ▁it ▁is ▁now , ▁and ▁the ▁K ui per ▁bel t ▁was ▁much ▁closer ▁to ▁the ▁Sun . ▁ ▁Terr est rial ▁plan ets ▁At ▁the ▁end ▁of ▁the ▁planet ary ▁formation ▁epo ch ▁the ▁inner ▁Sol ar ▁System ▁was ▁populated ▁by ▁ 5 0 – 1 0 0 ▁Moon - ▁to ▁Mars - s ized ▁planet ary ▁emb ry os . ▁Further ▁growth ▁was ▁possible ▁only ▁because ▁these ▁bodies ▁coll ided ▁and ▁merged , ▁which ▁took ▁less ▁than ▁ 1 0 0   mill ion ▁years . ▁These ▁objects ▁would ▁have ▁grav itation ally ▁interact ed ▁with ▁one ▁another , ▁t ug ging ▁at ▁each ▁other ' s ▁or bits ▁until ▁they ▁coll ided , ▁growing ▁larger ▁until ▁the ▁four ▁terrest rial ▁plan ets ▁we ▁know ▁today ▁took ▁shape . ▁One ▁such ▁giant ▁collision ▁is ▁thought ▁to ▁have ▁formed ▁the ▁Moon ▁( see ▁Mo ons ▁below ), ▁while ▁another ▁removed ▁the ▁outer ▁en velope ▁of ▁the ▁young ▁Mercur y . ▁ ▁One ▁un resol ved ▁issue ▁with ▁this ▁model ▁is ▁that ▁it ▁cannot ▁explain ▁how ▁the ▁initial ▁or bits ▁of ▁the ▁proto - ter rest rial ▁plan ets , ▁which ▁would ▁have ▁needed ▁to ▁be ▁highly ▁ecc ent ric ▁to ▁col lide , ▁produced ▁the ▁remark ably ▁stable ▁and ▁nearly ▁circular ▁or bits ▁they ▁have ▁today . ▁One ▁hypothesis ▁for ▁this ▁" e cc ent ric ity ▁dump ing