Young Solar System's Fifth Giant Planet?

TL;DR

This study uses N-body simulations to explore whether the early solar system included a fifth giant planet, finding that such a scenario better matches current planetary orbits and supports the idea of planet ejection, consistent with free-floating planet observations.

Contribution

It demonstrates that an initial five-planet configuration with an ejected ice giant provides a more plausible formation history for the solar system than four-planet models.

Findings

Four-planet resonant models often fail to match current orbits.

Five-planet models with ejected ice giant show higher success rates.

Ejection of a planet aligns with observations of free-floating planets.

Abstract

Recent studies of solar system formation suggest that the solar system's giant planets formed and migrated in the protoplanetary disk to reach resonant orbits with all planets inside 15 AU from the Sun. After the gas disk's dispersal, Uranus and Neptune were likely scattered by gas giants, and approached their current orbits while dispersing the transplanetary disk of planetesimals, whose remains survived to this time in the region known as the Kuiper belt. Here we performed N-body integrations of the scattering phase between giant planets in an attempt to determine which initial states are plausible. We found that the dynamical simulations starting with a resonant system of four giant planets have a low success rate in matching the present orbits of giant planets, and various other constraints (e.g., survival of the terrestrial planets). The dynamical evolution is typically too violent, if Jupiter and Saturn start in the 3:2 resonance, and leads to final systems with fewer than four planets. Several initial states stand out in that they show a relatively large likelihood of success in matching the constraints. Some of the statistically best results were obtained when assuming that the solar system initially had five giant planets and one ice giant, with the mass comparable to that of Uranus and Neptune, was ejected to interstellar space by Jupiter. This possibility appears to be conceivable in view of the recent discovery of a large number free-floating planets in interstellar space, which indicates that planet ejection should be common.