Direct Formation of Planetary Embryos in Self-Gravitating Disks (2204.13310v1)

Abstract: Giant planets have been discovered at large separations from the central star. Moreover, a striking number of young circumstellar disks have gas and/or dust gaps at large orbital separations, potentially driven by embedded planetary objects. To form massive planets at large orbital separations through core accretion within disk lifetime, however, an early solid body to seed pebble and gas accretion is desirable. Young protoplanetary disks are likely self-gravitating, and these gravitoturbulent disks may efficiently concentrate solid material at the midplane driven by spiral waves. We run 3D local hydrodynamical simulations of gravitoturbulent disks with Lagrangian dust particles to determine whether particle and gas self-gravity can lead to the formation of dense solid bodies, seeding later planet formation. When self-gravity between dust particles is included, solids of size $\mathrm{St} = 0.1$ to $1$ concentrate within the gravitoturbulent spiral features and collapse under their own self-gravity into dense clumps up to several $M_{\oplus}$ in mass at wide orbits. Simulations with dust that drift most efficiently, $\mathrm{St}=1$, form the most massive clouds of particles, while simulations with smaller dust particles, $\mathrm{St}=0.1$, have clumps with masses an order of magnitude lower. When the effect of dust backreaction onto the gas is included, dust clumps become smaller by a factor of a few but more numerous. The existence of large solid bodies at an early stage of the disk can accelerate the planet formation process, particularly at wide orbital separations, and potentially explain planets distant from the central stars and young protoplanetary disks with substructures.

• The paper uses 3D hydrodynamical simulations to show that rocky particles can collapse into dense planetary embryos in self-gravitating disks at wide orbital separations (~50 au).
• Particles with Stokes numbers 0.1-1 concentrate in spiral arms, forming stable clumps up to several Earth masses despite high particle velocities.
• This direct formation process provides a mechanism for rapidly building massive planets and substructures observed in young disks, supporting early planet formation scenarios.

Insights into Planetary Embryo Formation in Self-Gravitating Disks

The paper "Direct Formation of Planetary Embryos in Self-Gravitating Disks" by Baehr et al. provides a comprehensive analysis of the mechanisms leading to the formation of planetary embryos in young, self-gravitating protoplanetary disks. This work applies 3D local hydrodynamical simulations to investigate how solid particles within these disks can collapse into dense planetary embryos, particularly emphasizing the interplay between disk turbulence and particle dynamics.

Key Findings and Contributions

The authors explore the direct formation of planetary embryos within self-gravitating disks, addressing the requirements for solid material concentration sufficient for embryo formation. Their simulations are significant in demonstrating that particles with sizes corresponding to Stokes numbers between 0.1 and 1 can gravitationally collapse into planetary embryos at wide orbital separations (around 50 au). The particles concentrate within the spiral arms generated by gravitoturbulence and form dense clumps, with masses up to several Earth masses when the appropriate conditions are met.

The paper highlights the crucial role of dust backreaction on gas dynamics, revealing that this interaction influences both gas and particle velocities, occasionally driving them to supersonic speeds. Despite these high velocities, the internal particle velocity dispersion within the clumps remains low, indicating their stability and the potential for the embryos to persist, resist disruption and continue on a path towards planet formation.

Implications and Future Directions

Numerical results emphasize that self-gravitating disks can efficiently create massive solid bodies at substantial orbital distances early in their evolution. This process potentially accelerates planet formation through the subsequent accretion of pebbles and gas, providing an explanation for the existence of massive planets and substructures in young disks. These findings also support observational evidence of forming planets in disks before they reach the Class II evolutionary stage.

In terms of theoretical impact, this paper contributes to the understanding of planetesimal formation mechanisms beyond the core-accretion and streaming instability frameworks. It posits that gravitational instability in itself, without necessarily leading to disk fragmentation, can concentrate solids and provide the initial conditions favorable for planet growth. This perspective invites further investigation into the interaction between self-gravity-driven dust collapse and other instabilities, such as the streaming and Kelvin-Helmholtz instabilities, especially over a diverse range of disk conditions and particle sizes.

The results prompt detailed exploration of these phenomena in a more realistic setup, incorporating variable particle sizes, radially varying pressure gradients, and better thermalization models to account for more complex interactions in these early-forming disks.

Conclusion

Baehr and colleagues' research offers a robust numerical framework and initial insight into the early stages of planet formation within gravitoturbulent disks. Their work suggests that planetary embryos can form far from the central protostar under conditions natural to young, self-gravitating environments. Prospective research building on these findings could significantly enhance our understanding of planetary system architectures and their origins, informing our interpretation of the diverse exoplanetary systems observed today. While this paper makes strong contributions to planetary formation theory, understanding the broader context of these interactions will require future studies focusing on varied disk environments and more nuanced simulations to bridge the gap between theoretical predictions and observable features in protoplanetary disks.