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. Animbalance between inner and outer torques causes bodies of mass $\ga 0.1$ Earthmasses to lose angular momentum and migrate inward rapidly relative to thedisk; this is known as ``Type I'' migration. However, protoplanets that grow togas giant mass, O($10^2) M_\oplus$, open a gap in the disk and are subsequentlyconstrained to migrate more slowly, locked into the disk's viscous evolution inwhat is called "Type II" migration. In a young planetary system, both Type Iand Type II bodies likely coexist; if so, differential migration ought toresult in close encounters when the former originate on orbits exterior to thelatter. We investigate the resulting dynamics, using two different numericalapproaches: an N-body code with dissipative forces added to simulate the effectof the gas disk, and a hybrid code which combines an N-body component with a1-dimensional viscous disk model, treating planet-disk interactions in a moreself-consistent manner. In both cases, we find that sub-gap-opening bodies havea high likelihood of being resonantly captured when they encounter agap-opening body. A giant planet thus tends to act as a barrier in aprotoplanetary disk, collecting smaller protoplanets outside of its orbit. Suchbehavior has two important implications for giant planet formation: First, forcaptured protoplanets it mitigates the problem of the migration timescalebecoming shorter than the growth timescale. Secondly, it suggests one path toforming systems with multiple giant planets: Once the first has formed, ittraps/accretes the future solid core of the second in an exterior mean-motionresonance, and so on. The most critical step in giant planet formation may thusbe the formation of the very first one.Comment: Accepted for publication in Ap