Implantation of icy planetesimals into the main asteroid belt due to the growth and outward migration of giant planets
Tomoki Takeichi, Sei-ichiro Watanabe
Icarus, In Press, Journal Pre-proof, Available online 9 July 2026
“Highlights
- Simulate implantation of icy planetesimals into the main asteroid belt
- Introduce a growth-and-outward-migration scenario for the four giant planets
Implantation is controlled by Saturn’s growth and gas-disk dispersal timescales - Uniform gas depletion in the MMSN overdelivers mass (~2× the primordial limit)
Faster gas loss inside Jupiter reproduces main-belt mass and source fractions”
“Samples returned from the C-type asteroid Ryugu suggest that its parent body formed beyond proto-Saturn and was implanted into the main asteroid belt within a few Myr. Previous studies of icy-planetesimal delivery considered Jupiter–Saturn U-turn migration or inward migration of the ice giants. However, no definitive evidence for such inward migrations is preserved in the present Solar System, whereas outward migration of the giant planets is supported by Kuiper Belt observations.
Here we perform orbital simulations of icy planetesimals in a scenario in which Saturn and the ice giants form interior to their current orbits and subsequently grow and migrate outward. We vary the migration/growth timescales of the four giant planets and gas-disk dissipation, modeled as exponential decay followed by photoevaporation, and evaluate the implantation fractions into the main belt (fMB) and the terrestrial-planet region for planetesimals originating beyond proto-Saturn. We find fMB≃max(5.15μg-2.49, 0.85)%, where μg is the ratio of the gas-dissipation timescale to the gas giants’ growth timescale. This dependence reflects eccentricity excitation by Saturn’s growth and the weakening of gas drag during disk dispersal. For a solid inventory based on the minimum-mass solar nebula model, post-Jupiter-growth implantation in the uniform-dissipation cases delivers about twice the estimated maximum primordial belt mass. In contrast, faster gas depletion inside Jupiter’s orbit reduces implantation into both the main belt and the terrestrial-planet region, bringing the implanted masses within the estimated ranges. These results support a relatively low initial mass budget for the main belt, consistent with other recent independent studies.”
