Could Jupiter or Saturn Have Ejected a Fifth Giant Planet?

TL;DR

This study assesses the likelihood that Jupiter or Saturn ejected a fifth ice giant planet by analyzing the orbital perturbations on their satellites, finding a plausible ejection scenario for Jupiter but not for Saturn.

Contribution

It introduces a method to constrain giant planet ejection events based on the current orbits of their regular satellites, providing quantitative likelihood estimates.

Findings

Approximately 42% likelihood for Jupiter's ejection scenario to match Callisto's orbit.

Only about 1% likelihood for Saturn's ejection scenario to match Iapetus's orbit.

Uncertainties in Iapetus's formation complicate the interpretation of Saturn's ejection likelihood.

Abstract

Models of the dynamical evolution of the early solar system following the dispersal of the gaseous protoplanetary disk have been widely successful in reconstructing the current orbital configuration of the giant planets. Statistically, some of the most successful dynamical evolution simulations have initially included a hypothetical fifth giant planet, of ice giant mass, which gets ejected by a gas giant during the early solar system's proposed instability phase. We investigate the likelihood of an ice giant ejection event by either Jupiter or Saturn through constraints imposed by the current orbits of their wide-separation regular satellites Callisto and Iapetus respectively. We show that planetary encounters that are sufficient to eject an ice giant, often provide excessive perturbations to the orbits of Callisto and Iapetus making it difficult to reconcile a planet ejection event with the current orbit of either satellite. Quantitatively, we compute the likelihood of reconciling a regular Jovian satellite orbit with the current orbit of Callisto following an ice giant ejection by Jupiter of ~ 42% and conclude that such a large likelihood supports the hypothesis of a fifth giant planet's existence. A similar calculation for Iapetus reveals that it is much more difficult for Saturn to have ejected an ice giant and reconcile a Kronian satellite orbit with that of Iapetus (likelihood ~ 1%), although uncertainties regarding the formation of Iapetus, on its unusual orbit, complicates the interpretation of this result.