Planetary Migration

When Jupiter's Roche Lobe radius exceeded the scale height of the protoplanetary disk, Jupiter opened a gap in the disk. When the gap was wide enough, tidal torques from the disk interior and exterior to Jupiter were suppressed and migration continued on the accretion time scale. In the 'minimum solar nebula' about two Jupiter masses of gas remained between Jupiter and Saturn and about five Jupiter masses between Jupiter and Uranus. Unless all but a Jupiter mass of the outer disk was removed, Jupiter would have migrated into the Sun on the accretion time scale. So far, no mechanism has been postulated to remove the outer disk except for photo-evaporation which might work exterior to Saturn. For extra-solar Jupiters at an AU or less from the central star, photo-evaporation would not remove the outer disk. We propose a new mechanism, relying on the irradiation of the exposed edge of the gap by the Sun, to cause gas from the outer disk to cross the gap and become part of the inner disk, whence it was later accreted by the Sun.