Consequences of planetary migration on the minor bodies of the early solar system

Abstract

Pebble accretion is an efficient mechanism that is able to build up the core of the giant planets within the lifetime of the protoplanetary disc gas-phase. The core grows via this process until the protoplanet reaches its pebble isolation mass and starts to accrete gas. During the growth, the protoplanet undergoes a rapid, large-scale, inward migration due to the interactions with the gaseous protoplanetary disc. In this work, we have investigated how this early migration would have affected the minor body populations in our solar system. In particular, we focus on the Jupiter Trojan asteroids (bodies in the coorbital resonance 1:1 with Jupiter, librating around the L 4 and L 5 Lagrangian points called, respectively, the leading and the trailing swarm) and the Hilda asteroids. We characterised their orbital parameter distributions after the disc dispersal and their formation location and compare them to the same populations produced in a classical in situ growth model. We find that a massive and eccentric Hilda group is captured during the migration from a region between 5 and 8 au and subsequently depleted during the late instability of the giant planets. Our simulations also show that inward migration of the giant planets always produces a Jupiter Trojans' leading swarm more populated than the trailing one, with a ratio comparable to the current observed Trojan asymmetry ratio. The in situ formation of Jupiter, on the other hand, produces symmetric swarms. The reason for the asymmetry is the relative drift between the migrating planet and the particles in the coorbital resonance. The capture happens during the growth of Jupiter's core and Trojan asteroids are afterwards carried along during the giant planet's migration to their final orbits. The asymmetry and eccentricity of the captured Trojans correspond well to observations, but their inclinations are near zero and their total mass is three to four orders of magnitude higher than the current population. Future modelling will be needed to understand whether the dynamical evolution of the Trojans over billions of years will raise the inclinations and deplete the masses to observed values.

Jupiter's Composition Suggests its Core Assembled Exterior to the N2 Snowline

“…This favors a scenario where the core alone formed at large distances and then migrated inwards. Recent work by Pirani et al (2019) also places Jupiter's core formation in the outer solar system, based on Trojan data, while most of the gas envelope is accreted closer to 5 au, consistent with our fiducial model. The next issue is whether nitrogen and other elements accumulated during core formation would remain trapped in the core, or become well-mixed throughout Jupiter's interior after the planet's gas envelope was captured.…”

On the inclinations of the Jupiter Trojans

“…The mass growth of Jupiter then shrinks these orbits into tadpole orbits, originating the asymmetry. Despite this good agreement with observations, the simulations of Pirani et al (2019) showed a final mass of the Trojans that is 3-4 orders of magnitude higher than the current one and an inclination distribution that is much flatter than the current one.…”

“…Moreover, the late instability of the giant planets has not been simulated in this paper, since the mechanism, time and time-scales of this event are still uncertain. A good fraction of the Trojan's mass is probably lost in this event as shown by Pirani et al (2019) who estimated that only roughly 20% of the Jupiter Trojans survive if the giant planet jumps suddenly from 5.4 to 5.2 au. We conclude that a pre-stirred planetesimal disc is the most likely scenario for the Trojans' capture, since this can explain simultaneously the high inclinations, the low total mass and the asymmetry ratio of the Jupiter Trojans.…”

“…The amount of depletion to be attributed to the late instability is very dependent on which version we consider and we are not going to explore it in this paper. Pirani et al (2019) showed that Trojans and their original asymmetry can survive a single jump of Jupiter of 0.2 au. In line with this, we will attribute a fictitious 80% of depletion of the Trojans to the late instability when we will analyse our results.…”

“…The aim of this follow-up paper is to address the Trojan mass and inclination issues identified in Pirani et al (2019). In order to do that, we explore three different plausible ways to incline the Trojans up to 40 • and keep track of the mass depletion in each different scenario.…”