Born eccentric: constraints on Jupiter and Saturn's pre-instability orbits

TL;DR

This paper investigates the early orbital configurations of Jupiter and Saturn, demonstrating that initial resonant states with non-zero eccentricities can reproduce the current orbital characteristics, including Jupiter's fifth eccentric mode, through dynamical instability simulations.

Contribution

It introduces a new scenario where Jupiter and Saturn originated in 2:1 resonance with non-zero eccentricities, aligning simulation outcomes with observed orbital features.

Findings

Jupiter and Saturn's current orbits can result from primordial resonant configurations.

The amplitude M55 of Jupiter's eccentric mode is often matched in simulations with initial resonance.

Uranus and Neptune's final orbits depend on primordial Kuiper belt mass and an ejected ice giant.

Abstract

An episode of dynamical instability is thought to have sculpted the orbital structure of the outer solar system. When modeling this instability, a key constraint comes from Jupiter's fifth eccentric mode (quantified by its amplitude M55), which is an important driver of the solar system's secular evolution. Starting from commonly-assumed near-circular orbits, the present-day giant planets' architecture lies at the limit of numerically generated systems, and M55 is rarely excited to its true value. Here we perform a dynamical analysis of a large batch of artificially triggered instabilities, and test a variety of configurations for the giant planets' primordial orbits. In addition to more standard setups, and motivated by the results of modern hydrodynamical simulations of the giant planets' evolution within the primordial gaseous disk, we consider the possibility that Jupiter and Saturn emerged from the nebular gas locked in 2:1 resonance with non-zero eccentricities. We show that, in such a scenario, the modern Jupiter-Saturn system represents a typical simulation outcome, and M55 is commonly matched. Furthermore, we show that Uranus and Neptune's final orbits are determined by a combination of the mass in the primordial Kuiper belt and that of an ejected ice giant.