A numerical simulation of a "super-Earth" core delivery from ~ 100 AU to ~ 8 AU (1010.1489v2)

Abstract: We use SPH simulations with an approximate radiative cooling prescription to model evolution of a massive and large ($\sim 100$ AU) very young protoplanetary disc. We also model dust growth and gas-grain dynamics with a second fluid approach. It is found that the disc fragments onto a large number of $\sim 10$ Jupiter mass clumps that cool and contract slowly. Some of the clumps evolve onto eccentric orbits delivering them into the inner tens of AU, where they are disrupted by tidal forces from the star. Dust grows and sediments inside the clumps, displaying a very strong segregation, with the largest particles forming dense cores in the centres. The density of the dust cores may exceed that of the gas and is limited only by the numerical constraints, indicating that these cores should collapse into rocky planetary cores. One particular giant planet embryo migrates inward close enough to be disrupted at about 10 AU, leaving a self-bound solid core of about 7.5 $\mearth$ mass on a low eccentricity orbit at a radius of $\sim$ 8 AU. These simulations support the recent suggestions that terrestrial and giant planets may be the remnants of tidally disrupted giant planet embryos.

• The paper demonstrates that SPH simulations reveal disc fragmentation into massive clumps that migrate inward under gravitational instability.
• The paper shows that efficient dust growth and sedimentation within clumps lead to high-density core formation and eventual collapse.
• The paper supports the tidal downsizing hypothesis by linking inward migration and tidal stripping to the emergence of a solid 7.5 Earth-mass core.

Overview of Super-Earth Core Formation from Outer Protoplanetary Discs

This paper, authored by Seung-Hoon Cha and Sergei Nayakshin, presents a theoretical framework and numerical simulations exploring the formation and migration of a Super-Earth core from approximately 100 AU to around 8 AU within a protoplanetary disc. The paper employs Smoothed Particle Hydrodynamics (SPH) simulations incorporating a radiative cooling prescription to simulate the evolution of massive and large protoplanetary discs, extending the understanding of planet formation dynamics.

Key Findings and Numerical Results

1. Disc Fragmentation and Clump Formation:
   • The simulations reveal that the protoplanetary disc fragments into numerous clumps, each with masses around 10 Jupiter masses. These clumps slowly cool and contract over time. Importantly, certain clumps adopt eccentric orbits, which transport them into the inner regions of the disc where tidal forces from the host star ultimately disrupt their gaseous envelopes.
2. Dust Dynamics and Core Formation:
   • Within these clumps, dust particles exhibit substantial growth and sedimentation. A significant concentration of dust particles occurs in the center of these clumps, forming dense cores. The density of the dust cores occasionally surpasses that of the surrounding gas, leading to core collapse under high-density conditions.
3. Migration and Disruption:
   • A critical event in the simulations involves the inward migration of a giant planet embryo close enough to the star to face tidal disruption, leaving behind a self-bound solid core. This solid core, approximately 7.5 Earth masses, ultimately resides on a low-eccentricity orbit around 8 AU from the star.

Implications on Planet Formation Theories

The results of these simulations provide substantial support to the tidal downsizing hypothesis, suggesting that both terrestrial and giant planets may indeed be the remnants of initially massive gas clumps undergoing tidal disruption. The findings hold significant implications for the understanding of planet formation dynamics, particularly for systems where traditional models, such as core accretion, struggle to account for certain observations.

• Core Accretion and Gravitational Instability:

The outcomes challenge the traditional core accretion model, posing that planets, especially the terrestrial category, might predominantly form through the gravitational instability (GI) and subsequent migration and downsizing rather than growth through successive accumulation of planetesimals.

• Practical Relevance:

This mechanism might explain the presence of large solid cores observed in various exoplanetary systems, especially those possessing high metallicity or located at considerable distances from their parent stars, potentially reshaping the understanding of both the architecture and dynamics observed in exoplanetary systems.

Future Research Directions

The simulations offer a clear proof of concept; however, further exploration using enhanced computational models incorporating more refined physical details—such as refined radiative transfer models, better treatment of dust growth, and fragmentation processes—is needed. Additionally, exploring a wider parameter space and considering the long-term evolutionary dynamics beyond the simulated timeframe will be essential for fully elucidating the potential of the tidal downsizing model.

This research indicates a promising avenue toward understanding the complexities of planet formation, emphasizing the vivid interplay between gravitational dynamics, radiative processes, and dust physics. As computational capabilities advance, future studies will likely produce a more comprehensive picture of these processes, potentially bridging the gap between theory and the exoplanetary systems observed by astronomers.