A low accretion efficiency of planetesimals formed at planetary gap edges

Abstract

Observations and models of giant planets indicate that such objects are enriched in heavy elements compared to solar abundances. The prevailing view is that giant planets accreted multiple Earth masses of heavy elements after the end of core formation. Such late solid enrichment is commonly explained by the accretion of planetesimals. Planetesimals are expected to form at the edges of planetary gaps, and here we address the question of whether these planetesimals can be accreted in large enough amounts to explain the inferred high heavy element contents of giant planets. We performed a series of N-body simulations of the dynamics of planetesimals and planets during the planetary growth phase, taking gas drag into account as well as the enhanced collision cross section caused by the extended envelopes. We considered the growth of Jupiter and Saturn via gas accretion after reaching the pebble isolation mass and we included their migration in an evolving disk. We find that the accretion efficiency of planetesimals formed at planetary gap edges is very low: less than 10% of the formed planetesimals are accreted even in the most favorable cases, which in our model corresponds to a few Earth masses. When planetesimals are assumed to form beyond the feeding zone of the planets, extending to a few Hill radii from a planet, accretion becomes negligible. Furthermore, we find that the accretion efficiency increases when the planetary migration distance is increased and that the efficiency does not increase when the planetesimal radii are decreased. Based on these results, we conclude that it is difficult to explain the large heavy element content of giant planets with planetesimal accretion during the gas accretion phase. Alternative processes most likely are required, such as accretion of vapor deposited by drifting pebbles.

Breaking Degeneracies in Formation Histories by Measuring Refractory Content in Gas Giants

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Growth after the streaming instability: The radial distance dependence of the planetary growth

“…However, we note that a fraction of pebble fragments at the abovementioned pressure bump can still pass the gap and replenish the inner disk region (Liu et al 2022). In addition, the pressure bump can also be the formation site of the next generation planetesimals, since the inward drifting pebbles are continuously accumulated (Eriksson et al 2021(Eriksson et al , 2022. Such effects are not considered here.…”

The Influence of Cold Jupiters in the Formation of Close-in Planets. I. Planetesimal Transport

“…However, there could also be a traveling secular resonance and an accumulation of material in the outer region that could create a second giant planet external to the first one, as explored in Guo & Kokubo (2021). This could work together with the dust traps that are known to be created at the edges of giant planets (Eriksson et al 2022) to form planetesimals and subsequently a second planet outside. We have neglected the effect of higher-order terms in eccentricity ( e 4…”