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Originally Posted by Ken G
That really surprises me, since the disk is also orbiting and it is much easier to see how disk interactions could cause circularization than it is to see how they cause the "death spiral", and so quickly.
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The underlying physics is beyond me, but the inward movement occurs because of an asymmetry in the way angular momentum is transferred between the planet and its inner and outer Lindblad resonances in the disc. There are varying regimes of migration, depending on the mass of the planet.
Nelson et al's paper
The Migration and Growth of Protoplanets in Protostellar Discs gives the details.
Grant Hutchison
PS: Bad memory strikes again. That paper relates to a dense viscous gaseous disc, so the dynamics are rather different from a planetesimal disc. Planetesimal discs also causes inward migration for Jupiter in simulations, but evidently to a
much lesser extent.