Thermal instability and rocky planetesimal formation in the inner regions of protoplanetary disks

TL;DR

This study proposes a new model where thermal instability in the inner protoplanetary disk causes cyclic MRI activity, leading to dust self-accumulation and planetesimal formation near 1 au, potentially forming super-Earths.

Contribution

It introduces a self-consistent simulation framework showing how thermal instability drives cyclic MRI activation and dust accumulation, resulting in planetesimal formation in the inner disk.

Findings

Thermal instability causes cyclic MRI activation and deactivation.

Dust self-accumulation occurs during MRI inactive phases.

Planetesimal belts form near 1 au, capable of producing super-Earths.

Abstract

The inner regions of protoplanetary disks are promising formation sites of rocky planetesimals. Theoretical studies have proposed a scenario in which thermal ionization activates the magnetorotational instability (MRI) in the hot inner disk, and the resulting pressure maximum at the MRI activation boundary accumulates dust and promotes planetesimal formation. However, the inner disk may be thermally unstable, and the activation boundary can vary in time, potentially preventing the maintenance of a dust trap sustained by a steady pressure maximum. We propose an alternative scenario in which planetesimals form in a thermally unstable inner disk through dust self-accumulation driven by the coevolution of dust and disk temperature. To this end, we perform simulations that simultaneously calculate the non-equilibrium thermal evolution, the gas and dust surface density evolution, dust growth, and planetesimal formation. Our results show that thermal instability triggers cyclic MRI activation and deactivation, during which planetesimals are formed. The MRI is activated in the inner disk, and driven by thermal instability, the active region expands outward and then reverts to an inactive state. Triggered by a local enhancement in the dust surface density, dust undergoes self-accumulation while migrating inward in the MRI-inactive phase, causing planetesimal formation. Once the MRI is reactivated at a smaller radius, the cycle restarts. For a typical accretion rate of $10^{-8}M_{\odot}~{\rm yr^{-1}}$, a planetesimal belt forms near 1 au. This mechanism can produce sufficient planetesimal mass to form multiple super-Earths. This work provides a framework for a self-consistent model of planetesimal formation based on the coevolution of dust and disk temperature, serving as an initial condition for subsequent planet formation simulations.