Towards a global model for planet formation in layered MHD wind-driven discs

A population synthesis approach to investigate the impact of low viscosity and accretion layer thickness

¹ Division of Space Research and Planetary Sciences, Physics Institute, University of Bern, Gesellschaftsstrasse 6, 3012 Bern, Switzerland
² Center for Space and Habitability, University of Bern, Gesellschaftsstrasse 6, 3012 Bern, Switzerland

^★ Corresponding author: This email address is being protected from spambots. You need JavaScript enabled to view it.

Received: 16 September 2025
Accepted: 19 December 2025

Abstract

Context. Planet formation is inherently linked to the evolution of the protoplanetary disc. Recent developments point towards the possibility that disc evolution results from magnetised winds, rather than turbulent viscosity. This has fundamental implications for planet formation.

Aims. We investigate planet formation in the context of magnetohydrodynamic (MHD) wind-driven disc evolution under the assumption of accretion being driven in a laminar accretion layer at the disc surface above a disc midplane with low turbulent viscosity. Our study is aimed at testing the global consequences of recent findings from 2D and 3D hydrodynamical simulations regarding inefficient midplane heating and the existence of two sub-regimes of type II migration; namely, slow viscosity-dominated and fast wind-driven migration.

Methods. To study the global, potentially observable imprints of the physical processes governing planet formation in layered MHD-wind-driven discs, we ran single-embryo planetary population syntheses with varying initial disc conditions (i.e. disc mass, size, and angular momentum transport) and varying embryo starting location. We tested different parametrisations for the accretion layer thickness, Σ_active.

Results. The extent of type II migration in layered discs depends sensitively on the considered accretion layer thickness. For thin (Σ_active≲0.01 g/cm²) or fast (≳12% sonic velocity) accretion layers, giant planets migrate in the slow viscosity-dominated regime, which strongly limits the extent of type II migration. The fast wind-driven sub-regime nearly never occurs. For thick (Σ_active ≳ 1 g/cm²) or slow (≲ 3% sonic velocity) accretion layers, fast wind-driven type II occurs in contrast frequently, leading to long-range inward migration that sets in once planets reach masses that are sufficiently high to block the accreting layer (typically several 100 M_⊕). Disc-limited gas accretion is also strongly affected by deep and early gap opening, limiting maximum giant planet masses.

Conclusions. The existence of two subtypes of type II migration, low type I to type II transition masses and limited runaway gas accretion in layered MHD wind-driven discs strongly influence the final mass–distance diagrams of planets. For thin layers, giant planets form nearly in situ once they have passed into type II migration, which happens already at a few Earth masses. This leads to a bifurcation of the formation tracks where low-mass planets (super-Earths and sub-Neptunes) form closer in while giant planets remain farther out in the disc ≳1 au. For thick layers, fast wind-driven migration leads in contrast to numerous migrated hot Jupiters. Overall, we find that while the global properties of the emerging planet population are strongly modified relative to classical viscous discs, the key properties of the observed population can be reproduced within this new paradigm.