Radiation Hydrodynamics of Self-gravitating Protoplanetary Disks: I. Direct Formation of Gas Giants via Disk Fragmentation

TL;DR

This study uses advanced radiation hydrodynamics simulations to show that gravitational instability in protoplanetary disks can directly form gas giant planets, challenging previous limitations that favored brown dwarf formation.

Contribution

The paper demonstrates that realistic 3D radiation hydrodynamics simulations reveal conditions under which disk fragmentation produces planet-mass objects, supporting GI as a viable gas giant formation mechanism.

Findings

Disk mass and opacity influence fragmentation likelihood.

Fragment masses range from 0.3 to 10 Jupiter masses.

Gravitational instability can produce gas giants under realistic conditions.

Abstract

Gravitational instability (GI) has long been considered a viable pathway for giant planet formation in protoplanetary disks (PPDs), especially at wide orbital separations or around low-mass stars where core accretion faces significant challenges. However, a primary drawback is that disk fragmentation from GI was generally found to produce over-massive clumps, typically in the mass range of brown dwarfs, although most numerical studies adopted simplified cooling prescriptions or with limited numerical resolution. We conduct a suite of global three-dimensional radiation hydrodynamics (RHD) simulations of self-gravitating PPDs using the meshless finite-mass (MFM) method. By implementing radiation transport via the M1 closure and systematically varying disk mass and opacity, we show that increasing disk mass and lowering opacity promote fragmentation by enhancing radiative cooling. Non-fragmenting disks settle into a gravito-turbulent state with low-order spiral structures and effective angular momentum transport characterized by $\alpha \sim \beta_\mathrm{cool}^{-1}$. In fragmenting disks, a subset of gravitationally bound clumps survives as long-lived fragments. Their initial masses form a consistent distribution around $\Sigma \cdot\lambda_\mathrm{T} \cdot 2 (c_s/\Omega_\mathrm{K})$ (with $\lambda_T$ the Toomre wavelength), corresponding to $\sim 0.3 - 10\,M_\mathrm{J}$ in our simulations, consistent with being gas giants. These results demonstrate that GI can produce planet-mass fragments under more realistic conditions, reinforcing it as a viable gas giant formation pathway and motivating further studies of fragment evolution and observational signatures.