ject ed ▁by ▁Jup iter , ▁while ▁others ▁may ▁have ▁migr ated ▁to ▁the ▁inner ▁Sol ar ▁System ▁and ▁played ▁a ▁role ▁in ▁the ▁final ▁acc ret ion ▁of ▁the ▁terrest rial ▁plan ets . ▁During ▁this ▁primary ▁de ple tion ▁period , ▁the ▁effects ▁of ▁the ▁giant ▁plan ets ▁and ▁planet ary ▁emb ry os ▁left ▁the ▁astero id ▁bel t ▁with ▁a ▁total ▁mass ▁equivalent ▁to ▁less ▁than ▁ 1 % ▁that ▁of ▁the ▁Earth , ▁composed ▁mainly ▁of ▁small ▁plan etes im als . ▁This ▁is ▁still ▁ 1 0 – 2 0 ▁times ▁more ▁than ▁the ▁current ▁mass ▁in ▁the ▁main ▁bel t , ▁which ▁is ▁now ▁about ▁. ▁A ▁secondary ▁de ple tion ▁period ▁that ▁brought ▁the ▁astero id ▁bel t ▁down ▁close ▁to ▁its ▁present ▁mass ▁is ▁thought ▁to ▁have ▁followed ▁when ▁Jup iter ▁and ▁Sat urn ▁entered ▁a ▁temporary ▁ 2 : 1 ▁orb ital ▁reson ance ▁( see ▁below ). ▁ ▁The ▁inner ▁Sol ar ▁System ' s ▁period ▁of ▁giant ▁impact s ▁probably ▁played ▁a ▁role ▁in ▁the ▁Earth ▁acqu iring ▁its ▁current ▁water ▁content ▁( ~ 6   kg ) ▁from ▁the ▁early ▁astero id ▁bel t . ▁Water ▁is ▁too ▁vol atile ▁to ▁have ▁been ▁present ▁at ▁Earth ' s ▁formation ▁and ▁must ▁have ▁been ▁subsequently ▁delivered ▁from ▁outer , ▁col der ▁parts ▁of ▁the ▁Sol ar ▁System . ▁The ▁water ▁was ▁probably ▁delivered ▁by ▁planet ary ▁emb ry os ▁and ▁small ▁plan etes im als ▁thrown ▁out ▁of ▁the ▁astero id