Young Solar System's Fifth Giant Planet? (1109.2949v1)

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.

• The paper uses N-body simulations to investigate if a fifth primordial giant planet existed and influenced the early solar system's dynamical evolution.
• Simulations showed that starting with five giant planets resulted in a significantly higher success rate (up to 37%) in reproducing the current solar system's orbital configuration compared to starting with four.
• The hypothesis of an ejected fifth giant planet helps explain the current orbital spacings, Kuiper Belt structure, aligns with migration models, and has implications for exoplanet systems.

Overview of "Young Solar System's Fifth Giant Planet?"

This paper, authored by David Nesvorný, explores the dynamical history of the early solar system with a focus on the potential existence of a fifth primordial giant planet. The paper employs $N$-body simulations to analyze the scattering phases of giant planets and assess the plausibility of different initial orbital configurations. The objective is to reconcile the current orbital architecture of the solar system giants—Jupiter, Saturn, Uranus, and Neptune—with theoretical models of planetary migration and scattering processes during the solar system's formative years.

Methodology

The investigations leveraged both hydrodynamic and $N$-body simulations to model the early configurations of the solar system's giant planets. The Fargo software was utilized for hydrodynamic simulations to guide the subsequent $N$-body analyses executed using the SyMBA integrator. These simulations considered scenarios with both four and five initial giant planets. In the five-planet simulations, a hypothetical additional ice giant with a mass similar to Uranus and Neptune was included. This extra planet was theorized to be ejected from the solar system due to gravitational interactions, a concept supported by the observation of free-floating planets in interstellar space.

Key parameters varied in the simulations included the initial resonant relationships between planets, the mass and scope of the planetesimal disk, and the semimajor axis spacing. Simulations were subjected to rigorous criteria concerning planetary survival, final orbital characteristics, and compliance with secular resonance structures.

Findings

The simulations offered significant insights into the solar system's evolution. Notably, scenarios incorporating a fifth planet demonstrated a higher likelihood of producing a final planetary system analogous to our current solar system. The most favorable outcomes were obtained from systems with five initial planets in the (3:2, 3:2, 4:3, 5:4) resonances and a disk mass around 50 Earth masses. These configurations showed considerable success rates, with 37% achieving a stable four-planet system that matched the current solar planetary arrangement.

By contrast, simulations based on a four-planet initial configuration exhibited a substantially lower success rate, suggesting that an additional giant planet may have played a crucial role in shaping the current system.

Implications

The hypothesis of a fifth primordial planet has notable ramifications for our understanding of solar system dynamics and structure. The ejection of a fifth giant planet provides a compelling explanation for the current orbital distributions and spacings of the solar system's planets, as well as the characteristics of the Kuiper Belt. Additionally, the model aligns with the "Nice model" of solar system evolution, which involves planetary migrations facilitated by interactions with a planetesimal disk.

The enhanced dynamical understanding also lends insight into extrasolar planetary systems, where similar ejection events could be common.

Future Directions

The paper suggests future research should further refine the dynamical models, perhaps integrating additional observational constraints and exploring higher-resolution simulations. Investigations into the potential remnants of the hypothetical fifth planet, or the absence thereof, could also be pivotal. Future models may look to integrate collisional processes within the planetesimal disk, which could affect the mass distribution and dynamical damping essential for these simulations. Additional exploration into the formation sites and migration histories of gas and ice giants within our solar system further elucidates the complexity of planet formation and ejection dynamics.