Paper detail

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

Planet formation is inherently linked to protoplanetary disc evolution, which recent developments suggest is driven by magnetised winds rather than turbulent viscosity. We study planet formation in magnetohydrodynamic (MHD) wind-driven discs, assuming accretion occurs in a laminar surface layer above a weakly turbulent midplane. Our goal is to assess the global consequences of recent hydrodynamical results, including inefficient midplane heating and the existence of two Type II migration regimes: slow viscosity-dominated and fast wind-driven migration. We perform single-embryo planetary population syntheses with varying initial disc conditions (i.e. disc mass, size and angular momentum transport), and embryo starting locations, testing different prescriptions for the accretion layer thickness $Σ_\text{active}$. Thin ($\lesssim0.01\mathrm{g\,cm^{-2}}$) or fast ($\gtrsim12\%$ sonic velocity) accretion layers result in slow, viscosity-dominated regime which strongly limits the extent of Type II migration. For thick ($\gtrsim1\mathrm{g\,cm^{-2}}$) or slow ($\lesssim3\%$ sonic velocity) accretion layers, fast wind-driven Type II migration occurs frequently, leading to long-range inward migration that sets in once planets reach masses sufficient to block the accreting layer. Disk-limited gas accretion is also strongly affected by deep and early gap opening, limiting maximum giant planet masses. These effects strongly influence the final mass-distance distribution. For thin layers, giant planets form nearly in situ once they have entered Type II migration, which happens already at a few Earth masses, while thick layers lead 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, key properties of the observed population can be reproduced within this new paradigm.