Disc Substructures Trigger Sequential Giant Planet Formation

Introduction

Planet formation is a complex, multi-step process that has been the focus of extensive research in recent years. Scientists have made significant progress in understanding this process, driven by the discovery of exoplanets and observations of

around young stars. However, key challenges remain, such as the formation of

planetesimals

Small, rocky objects that form the building blocks of planets in a protoplanetary disc.

 

(the building blocks of planets) and the growth of giant planet cores.

Current planet formation models often struggle to meet both physical and chemical constraints required to explain the formation of our own Solar System. One major challenge is the "

," where planetary cores between 1 and 10 times the mass of Earth rapidly migrate inward, leading to the formation of

super-Earths

Planets that are larger than Earth but smaller than Neptune, often found orbiting very close to their host stars.

 

and

hot Jupiters

Massive gas giant planets that orbit very close to their host stars, much closer than the planet Jupiter in our solar system.

 

rather than the gas giants we see in our Solar System.

Motivated by these challenges and recent observations, researchers have identified a promising solution: the presence of substructures, or localized features, within the protoplanetary disc. These substructures can provide an ideal environment for rapid planet formation, as they can concentrate dust and create a migration trap that helps retain the growing planetary cores.

Method

To investigate this scenario, the researchers developed a comprehensive modeling framework that integrates various processes involved in planet formation, including dust coagulation, planetesimal formation, gravitational interactions,

, planet migration, and planetary gas accretion.

The researchers used two specialized computer codes to simulate the evolution of the protoplanetary disc and the formation of planets:

1. DustPy
   A computer program that simulates the evolution of gas and dust in a protoplanetary disc.

    
   : This code models the evolution of gas and dust in the disc, including the viscous evolution of the gas, the transport and growth of dust particles, and the formation of planetesimals.

2. SyMBAp
   A computer program that simulates the gravitational interactions and evolution of planets and other bodies in a protoplanetary disc.

    
   : This code simulates the gravitational interactions between the planetesimals and growing planets, as well as the effects of gas drag, pebble accretion, gas accretion, and planetary gap formation.

The researchers set up the initial conditions of the disc, including an axisymmetric gap, and then let the simulation run for 2 million years, which is the typical timescale before internal

(the process of disc dissipation) becomes significant.

Results

The researchers explored two different values for the disc viscosity parameter, α, which controls the rate of gas accretion onto the central star. They found that the initial

in the disc led to the rapid growth of planetary cores through pebble accretion, forming a pair of gas giants.

In the case with a higher viscosity (α = 5×10^-4), the formation of the first gas giants created a new pressure bump at the outer edge of their gap. This triggered the formation of a second pair of planets, this time ice giants, resulting in a compact chain of four giant planets.

The inner pair of gas giants ended up in a near 2:1

, while the outer pair of ice giants were in a near 4:3 resonance. Remarkably, the majority of the solid material (about 85% of the initial dust mass beyond 5 astronomical units) was eventually incorporated into these massive bodies, demonstrating a highly efficient planet formation process.

In the case with a lower viscosity (α = 3×10^-4), the results were more varied, with some simulations forming two or three gas giants, while others formed only one. However, in all cases, a prominent dust ring remained external to the outermost planet, and the timing of planetesimal formation was slightly delayed compared to the higher viscosity case.

Discussion

Sequential Planet Formation

The key finding of this study is the sequential nature of the planet formation process. The formation of the first gas giant creates a gap in the disc, and the outer edge of this gap becomes a favorable environment for the next generation of planet formation.

This is because the dust leakage from the pressure bump at the outer edge of the gap is less significant, and the perturbation from the existing planet is less likely to prevent the growth of new planetesimals. As a result, the formation of the ice giants is delayed compared to the gas giants, providing the necessary time delay required by models to explain their masses.

The final planetary system consists of a compact chain of giant planets, with the majority of the solid material (about 85% of the initial dust mass beyond 5 astronomical units) incorporated into these massive bodies. This suggests a highly efficient planet formation process, with a robust chemical division caused by the first gas giant.

Comparison with the Solar System

The researchers note that their model produced a pair of Jupiter-mass giants, rather than one Jupiter-mass and one Saturn-mass giant, as seen in our Solar System. This discrepancy is likely due to the absence of disc dissipation in the simulation, which would have led to a higher gas surface density throughout the disc.

To better match the formation of the outer Solar System, the researchers acknowledge the need for further developments of the model, such as incorporating the effects of photoevaporation (the process of disc dissipation).

Other Recent Works

The researchers discuss recent theoretical work on planet formation at pressure bumps in protoplanetary discs. These studies have predicted different pathways for planetary system architecture, including slow core formation, fast core formation with slow gas accretion, and fast core formation with rapid gas accretion. The results presented in this study align most closely with the fast core formation and gas accretion pathway.

Additionally, the researchers mention a "

" model, where planets can form from dust trapped between two massive planets creating pressure maxima. However, this model did not account for the full complexity of planet formation and dust fragmentation processes.

Caveats

The researchers acknowledge several caveats in their study. First, the parameter space of disc substructures, including their location, amplitude, width, and lifetime, has not been fully explored and requires further investigation.

Second, the treatment of planetary gap opening and migration as independent processes, while common, is not fully consistent with the underlying physics, which couples these phenomena. The researchers suggest that future work should incorporate a more comprehensive and consistent treatment of these disc-planet interactions.

Finally, the adoption of a fixed temperature profile in the model neglects the potential effects of

, which may be more significant in the outer regions of the disc. Incorporating a more realistic temperature profile could provide additional insights into the formation of the outer Solar System planets.

Conclusion

This study presents a comprehensive model of sequential giant planet formation triggered by an initial disc substructure. The results demonstrate that multiple planetary cores can form and grow into giant planets in successive generations, with the pressure bump at the outer edge of the planetary gap serving as the site for the next phase of planet formation.

While the specific mechanisms behind disc substructure formation are not addressed, the researchers highlight the need for further investigations to understand the characteristics of such substructures and their role in planet formation. Additionally, the study emphasizes the importance of modeling the diversity of planetary systems and the formation timeline of the Solar System's giant planets, which requires continued research on planetary gas accretion processes.