An early dynamical instability among the Solar System’s giant planets triggered by the gas disk’s dispersal

The Solar System’s orbital structure is thought to have been sculpted by a dynamical instability among the giant planets[1–4]. Yet the instability trigger and exact timing have proved hard to pin down[5–9]. The giant planets formed within a gas-dominated disk around the young Sun. Motivated by giant exoplanet systems found in mean motion resonance[10], hydrodynamical modeling has shown that while the disk was present the giant planets migrated into a compact orbital configuration, in a chain of resonances[2,11]. Here we use a suite of dynamical simulations to show that the giant planets’ instability was likely triggered by the dispersal of the Sun’s gaseous disk. As the disk evaporated from the inside-out, its inner edge swept successively across and dynamically perturbed each planet’s orbit in turn. Saturn and each ice giants’ orbits were torqued strongly enough to migrate outward. As a given planet migrated outward with the disk’s inner edge the orbital configuration of the exterior system was compressed, triggering dynamical instability. The final orbits of our simulated systems match those of the Solar System for a viable range of astrophysical parameters. Our results demonstrate that the giant planet instability happened as the gaseous disk dissipated, constrained by astronomical observations to be a few to ten million years after the birth of the Solar System [12]. Late-stage terrestrial planet formation would occur mostly after such an early giant planet instability [13,14], thereby avoiding the possibility of de-stabilizing the terrestrial planets [15] and naturally accounting for the small mass of Mars relative to Earth and the mass depletion of the main asteroid belt [16].

Revisiting long-standing puzzles of the Milky Way: the Sun and its vicinity as typical outer disk chemical evolution

We present a scenario of the chemical enrichment of the solar neighborhood that solves the G-dwarf problem by taking into account constraints on a larger scale. We argue that the Milky Way disk within 10 kpc has been enriched to solar metallicity by a massive stellar population: the thick disk, which itself formed from a massive turbulent gaseous disk. While the inner disk, R ≲ 6 kpc, continued this enrichment after a quenching phase (7−10 Gyr), at larger distances radial flows of gas diluted the metals left by the thick disk formation at a time we estimate to be 7−8 Gyr ago, thus partitioning the disk into an inner and outer region characterized by different chemical evolutions. The key new consideration is that the pre-enrichment provided by the thick disk is not related to the mass fraction of this stellar population at the solar radius, as is classically assumed in inside-out scenarios, but is actually related to the formation of the entire massive thick disk, due to the vigorous gas phase mixing that occurred during its formation. Hence, the fact that this population represents only 15−25% of the local stellar surface density today, or 5−10% of the local volume density, is irrelevant for “solving” the G-dwarf problem. The only condition for this scenario to work is that the thick disk was formed from a turbulent gaseous disk that permitted a homogeneous – not radially dependent – distribution of metals, allowing the solar ring to be enriched to solar metallicity. At the solar radius, the gas flowing from the outer disk combined with the solar metallicity gas left over from thick disk formation, providing the fuel necessary to form the thin disk at the correct metallicity to solve the G-dwarf problem. Chemical evolution at R > 6 kpc, and in particular beyond the solar radius, can be reproduced with the same scheme. We suggest that the dilution, occurring at the fringe of the thick disk, was possibly triggered by the formation of the bar and the establishment of the outer Lindblad resonance (OLR), enabling the inflow of metal poorer gas from the outer disk to R ∼ 6 kpc, presumably the position of the OLR at this epoch, and at the same time isolating the inner disk from external influence. These results imply that the local metallicity distribution is not connected to the gas accretion history of the Milky Way. Finally, we argue that the Sun is the result of the evolution typical of stars in the disk beyond ∼6 kpc (i.e., also undergoing dilution), and has none of the characteristics of inner disk stars.