▁bel t ▁by ▁Jup iter . ▁A ▁population ▁of ▁main - b elt ▁com ets ▁discovered ▁in ▁ 2 0 0 6 ▁has ▁been ▁also ▁suggested ▁as ▁a ▁possible ▁source ▁for ▁Earth ' s ▁water . ▁In ▁contrast , ▁com ets ▁from ▁the ▁K ui per ▁bel t ▁or ▁farther ▁regions ▁delivered ▁not ▁more ▁than ▁about ▁ 6 % ▁of ▁Earth ' s ▁water . ▁The ▁p ans perm ia ▁hypothesis ▁holds ▁that ▁life ▁itself ▁may ▁have ▁been ▁depos ited ▁on ▁Earth ▁in ▁this ▁way , ▁although ▁this ▁idea ▁is ▁not ▁widely ▁accepted . ▁ ▁Planet ary ▁migration ▁ ▁According ▁to ▁the ▁ne b ular ▁hypothesis , ▁the ▁outer ▁two ▁plan ets ▁may ▁be ▁in ▁the ▁" wr ong ▁place ". ▁U ran us ▁and ▁Ne pt une ▁( known ▁as ▁the ▁" ice ▁gi ants ") ▁exist ▁in ▁a ▁region ▁where ▁the ▁reduced ▁density ▁of ▁the ▁solar ▁ne b ula ▁and ▁longer ▁orb ital ▁times ▁render ▁their ▁formation ▁highly ▁imp laus ible . ▁The ▁two ▁are ▁instead ▁thought ▁to ▁have ▁formed ▁in ▁or bits ▁near ▁Jup iter ▁and ▁Sat urn ▁( known ▁as ▁the ▁" gas ▁gi ants "), ▁where ▁more ▁material ▁was ▁available , ▁and ▁to ▁have ▁migr ated ▁out ward ▁to ▁their ▁current ▁positions ▁over ▁hundreds ▁of ▁millions ▁of ▁years . ▁ ▁The ▁migration ▁of ▁the ▁outer ▁plan ets ▁is ▁also ▁necessary ▁to ▁account ▁for ▁the ▁existence ▁and ▁properties ▁of ▁the ▁Sol ar ▁System ' s ▁outer most ▁regions . ▁Bey ond ▁Ne pt une , ▁the ▁Sol ar ▁System