Similar-mass versus diverse-mass planetary systems in wind-driven accretion discs

TL;DR

This study explores how wind-driven accretion disc dynamics influence the formation and diversity of planetary systems, revealing that wind efficiency impacts whether systems are similar-mass or diverse-mass, aligning with observed exoplanet trends.

Contribution

It introduces a model linking wind-driven disc evolution with planetary system diversity, highlighting the role of magnetic wind efficiency in planet formation outcomes.

Findings

Higher magnetic lever arm parameter leads to faster planet formation and similar-mass systems.

Lower parameter results in slower formation and more diverse planetary masses.

Metal-rich discs favor super-Earth and cold Jupiter formation, matching observations.

Abstract

Many close-in multiple-planet systems show a peas-in-a-pod trend, where neighbouring planets have similar sizes, masses, and orbital spacing. Others, including the Solar System, have a more diverse size and mass distribution. Classical planet formation models tend to produce the former rather than the latter, and the origin of this difference remains unclear. Recent studies suggest disc evolution is largely driven by magnetic winds rather than viscosity alone. In such wind-driven accretion discs, the mass accretion rate varies radially instead of being constant, as in classical viscous discs. We investigate how the wind's efficiency in removing disc mass affects planet formation and migration. We performed single-core planet formation simulations via pebble accretion in wind-driven accretion discs. We varied wind efficiency via the magnetic lever arm parameter $ \lambda $ and studied outcomes by considering a range of initial disc masses and accretion timescales. Our simulations show that higher $ \lambda $ discs, with less wind mass loss, lead to faster formation and migration, generating similar-mass planetary systems. Lower $ \lambda $ discs lead to slower formation and migration and more diverse-mass planetary systems. A mass jump occurs for all $ \lambda $ cases when planet formation and disc accretion timescales are comparable, but is larger for lower $ \lambda $ discs. Super-Earth systems with cold Jupiters form more frequently in metal-rich discs, consistent with observations. Our simulations suggest similar-mass and diverse-mass systems are approximately separated at $ \lambda \sim 2\text{--}3 $.