▁O ort ▁cloud ; ▁those ▁objects ▁scattered ▁to ▁a ▁less er ▁degree ▁by ▁the ▁migr ating ▁Ne pt une ▁formed ▁the ▁current ▁K ui per ▁bel t ▁and ▁scattered ▁disc . ▁This ▁scenario ▁explains ▁the ▁K ui per ▁bel t ' s ▁and ▁scattered ▁disc ' s ▁present ▁low ▁mass . ▁Some ▁of ▁the ▁scattered ▁objects , ▁including ▁Pl uto , ▁became ▁grav itation ally ▁tied ▁to ▁Ne pt une ' s ▁orbit , ▁forcing ▁them ▁into ▁mean - m otion ▁reson ances . ▁Event ually , ▁fr iction ▁within ▁the ▁plan etes imal ▁disc ▁made ▁the ▁or bits ▁of ▁U ran us ▁and ▁Ne pt une ▁circular ▁again . ▁ ▁In ▁contrast ▁to ▁the ▁outer ▁plan ets , ▁the ▁inner ▁plan ets ▁are ▁not ▁thought ▁to ▁have ▁migr ated ▁significantly ▁over ▁the ▁age ▁of ▁the ▁Sol ar ▁System , ▁because ▁their ▁or bits ▁have ▁remained ▁stable ▁following ▁the ▁period ▁of ▁giant ▁impact s . ▁ ▁Another ▁question ▁is ▁why ▁Mars ▁came ▁out ▁so ▁small ▁compared ▁with ▁Earth . ▁A ▁study ▁by ▁South west ▁Research ▁Institute , ▁San ▁Antonio , ▁Texas , ▁published ▁June ▁ 6 , ▁ 2 0 1 1 ▁( called ▁the ▁Grand ▁tack ▁hypothesis ), ▁propos es ▁that ▁Jup iter ▁had ▁migr ated ▁in ward ▁to ▁ 1 . 5   AU . ▁After ▁Sat urn ▁formed , ▁migr ated ▁in ward , ▁and ▁established ▁the ▁ 2 : 3 ▁mean ▁motion ▁reson ance ▁with ▁Jup iter , ▁the ▁study ▁assumes ▁that ▁both ▁plan ets ▁migr ated ▁back ▁to