Gap opening by planets in discs with magnetised winds

TL;DR

This study investigates how magnetised disc winds influence planet gap opening in protoplanetary discs, revealing that such winds are less effective than turbulence, leading to smaller gap-opening masses and favoring super-Earth formation over gas giants.

Contribution

The paper introduces a new gap opening criterion applicable to discs with magnetised winds, highlighting the reduced efficiency of winds compared to turbulence in planet gap formation.

Findings

Magnetised winds are less effective than turbulence in counteracting planetary tidal torques.

Gap opening masses are smaller in discs with realistic wind parameters, favoring super-Earth formation.

Planets can detach from the disc at masses below 0.1 Jupiter masses inside 10 AU.

Abstract

Planets open deep gaps in protoplanetary discs when their mass exceeds a gap opening mass, $M_{\rm gap}$. We use one- and two-dimensional simulations to study planet gap opening in discs with angular momentum transport powered by MHD disc winds. We parameterise the efficiency of the MHD disc wind angular momentum transport through a dimensionless parameter $\alpha_{\rm dw}$, which is an analogue to the turbulent viscosity $\alpha_{\rm v}$. We find that magnetised winds are much less efficient in counteracting planet tidal torques than turbulence is. For discs with astrophysically realistic values of $\alpha_{\rm dw}$, $M_{\rm gap}$ is always determined by the residual disc turbulence, and is a factor of a few to ten smaller than usually obtained for viscous discs. We introduce a gap opening criterion applicable for any values of $\alpha_{\rm v}$ and $\alpha_{\rm dw}$ that may be useful for planet formation population synthesis. We show that in discs powered by magnetised winds, growing planets detach from the disc at planet masses below $\sim 0.1M_{\rm Jup}$ inside 10 AU. This promotes formation of super-Earth planets rather than gas giants in this region, in particular precluding formation of hot jupiters in situ. On larger scales, ALMA gap opening planet candidates may be less massive than currently believed. Future high-resolution observations with instruments such as the extended ALMA, ngVLA, and SKA are likely to show abundant narrow annular features at $R < 10$ AU due to ubiquitous super-Earth planets.