Dynamical evidence for an early giant planet instability

TL;DR

This study models the early dynamical evolution of the Solar System to determine the most probable timing of the giant planet instability, finding it likely occurred within 100 million years of formation, with implications for planetary migration history.

Contribution

The paper provides the first self-consistent simulations linking planetesimal disk sculpting during planet formation to the timing of the giant planet instability, favoring an early occurrence within 100 Myr.

Findings

Instability likely occurred within 100 Myr of Solar System formation.

Jupiter's inward migration of 10 AU or more delays instability beyond 100 Myr.

Long-range inward migration of Jupiter conflicts with current Kuiper belt observations.

Abstract

The dynamical structure of the Solar System can be explained by a period of orbital instability experienced by the giant planets. While a late instability was originally proposed to explain the Late Heavy Bombardment, recent work favors an early instability. We model the early dynamical evolution of the outer Solar System to self-consistently constrain the most likely timing of the instability. We first simulate the dynamical sculpting of the primordial outer planetesimal disk during the accretion of Uranus and Neptune from migrating planetary embryos during the gas disk phase, and determine the separation between Neptune and the inner edge of the planetesimal disk. We performed simulations with a range of migration histories for Jupiter. We find that, unless Jupiter migrated inwards by 10 AU or more, the instability almost certainly happened within 100 Myr of the start of Solar System formation. There are two distinct possible instability triggers. The first is an instability that is triggered by the planets themselves, with no appreciable influence from the planetesimal disk. Of those, the median instability time is $\sim4$Myr. Among self-stable systems -- where the planets are locked in a resonant chain that remains stable in the absence of a planetesimal's disk-- our self-consistently sculpted planetesimal disks nonetheless trigger a giant planet instability with a median instability time of 37-62 Myr for a reasonable range of migration histories of Jupiter. The simulations that give the latest instability times are those that invoked long-range inward migration of Jupiter from 15 AU or beyond; however these simulations over-excited the inclinations of Kuiper belt objects and are inconsistent with the present-day Solar System. We conclude on dynamical grounds that the giant planet instability is likely to have occurred early in Solar System history.