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Grand Tack Hypothesis
In planetary astronomy, the Grand Tack Hypothesis proposes that after its formation at 3.5 AU Jupiter migrated inward to 1.5 AU, before reversing course after capturing Saturn in a resonance, eventually halting near its current orbit at 5.2 AU. The reversal of Jupiter's migration is likened to the path of a sail boat changing directions (tacking) as it travels against the wind.
The planetesimal disk is truncated at 1.0 AU by Jupiter's migration, limiting the material available to form Mars. Jupiter twice crosses the asteroid belt, scattering asteroids outward then inward. The resulting asteroid belt has a small mass, a wide range of inclinations and eccentricities, and a population originating from both inside and outside Jupiter's original orbit. Debris produced by collisions among planetesimals swept ahead of Jupiter may have driven an early generation of planets into the sun.
==Description==
In the Grand Tack model Jupiter undergoes a two-phase migration after its formation, migrating inward to 1.5 AU before reversing course and migrating outward. Jupiter's formation takes place near the ice line, at roughly 3.5 AU. After clearing a gap in the gas disk Jupiter undergoes type II migration, moving slowly toward the Sun with the gas disk. If uninterrupted, this migration would have left Jupiter in a close orbit around the sun like recently discovered hot-Jupiters in other planetary systems. Saturn also migrates toward the Sun, but being smaller undergoes type I migration, moving faster through the gas disk.〔 Saturn is captured in a 2:3 mean-motion resonance with Jupiter during this migration. An overlapping gap in the gas disk then forms between Jupiter and Saturn,〔(【引用サイトリンク】url=http://www.astrobio.net/topic/solar-system/new-research-suggests-solar-system-may-have-once-harbored-super-earths/ )〕 altering the balance of forces on these planets which are now migrating together. Jupiter having a greater mass is pushed outward more by the inner disk than Saturn is inward by the outer disk. This, along with a transfer of angular momentum from gas streaming through the gap, reverses their migration when Jupiter is at 1.5 AU. Their outward migration continues until the gas disk dissipates, ending with Jupiter near its current orbit.
抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』
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