A Safety Net for Fast Migrators: Interactions between Gap-opening and Sub-Gap-opening Bodies in a Protoplanetary Disk

Young planets interact with their parent gas disks through tidal torques. An imbalance between inner and outer torques causes bodies of mass ≳0.1 Earth masses (M⊕) to lose angular momentum and migrate inward rapidly relative to the disk; this is known as "Type I" migration. However, protoplanets that grow to gas-giant mass, O(102) M⊕, open a gap in the disk and are subsequently constrained to migrate more slowly, locked into the disk's viscous evolution in what is called "Type II" migration. In a young planetary system, both Type I and Type II bodies likely coexist; if so, differential migration ought to result in close encounters when the former originate on orbits exterior to the latter. We investigate the resulting dynamics, using two different numerical approaches: an N-body code with dissipative forces added to simulate the effect of the gas disk, and a hybrid code that combines an N-body component with a one-dimensional viscous disk model, treating planet-disk interactions in a more self-consistent manner. In both cases, we find that sub-gap-opening bodies have a high likelihood of being resonantly captured when they encounter a gap-opening body. A giant planet thus tends to act as a barrier in a protoplanetary disk, collecting smaller protoplanets outside of its orbit. Such behavior has two important implications for giant planet formation. First, for captured protoplanets it mitigates the problem of the migration timescale becoming shorter than the growth timescale when Mproto ≳ 1 M⊕. Second, it suggests one path to forming systems with multiple giant planets: once the first has formed, it traps (or indeed accretes) the future solid core of the second in an exterior mean-motion resonance, and so on. The most critical step in giant planet formation may thus be the formation of the very first one.