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

University of Dundee, School of Science and Engineering, Dundee DD1 4HN, UK

^★ Corresponding authors; 2440931@dundee.ac.uk; s.matsumura@dundee.ac.uk

Received: 1 October 2024
Accepted: 9 December 2024

Abstract

Context. Many close-in multiple-planet systems show a peas-in-a-pod trend, where sizes, masses, and orbital spacing of neighbouring planets are comparable to each other. On the other hand, some planetary systems have a more diverse size and mass distribution, including the Solar System. Classical planet formation models tend to produce the former type of planetary systems rather than the latter, and the origin of their difference is not well understood.

Aims. Recent observational and numerical studies support the disc evolution that is largely driven by magnetic winds rather than by the traditional disc’s viscosity alone. In such a wind-driven accretion disc, the disc mass accretion rate varies radially, instead of being constant in radius as in the classical viscously accreting disc. We investigate how the wind’s efficiency in removing the disc mass affects the outcome of planet formation and migration.

Methods. We performed single-core planet formation simulations via pebble accretion in wind-driven accretion discs. We varied the wind’s efficiency via the magnetic lever arm parameter λ and studied the outcome of planet formation and migration by considering a range of initial disc masses and disc accretion timescales.

Results. Our simulations show that higher λ discs with less wind mass loss lead to faster formation and migration of planets and tend to generate similar-mass planetary systems, while lower λ discs lead to slower formation and migration as well as more diverse- mass planetary systems. Furthermore, we find that a planetary system with a mass jump happens for all λ cases as long as the planet formation timescale is comparable to the disc accretion timescale, but the jump is larger for lower λ discs. The super-Earth systems accompanied by cold Jupiters can be generated in such systems, and we find their frequencies are higher in metal-rich discs, which agrees with the observational trend. Our simulations indicate that similar-mass and diverse-mass systems are approximately separated at λ ∼ 2–3.