Magnetic Origins of the Stellar Mass-Obliquity Correlation in Planetary Systems

TL;DR

This paper explains the observed correlation between stellar mass and spin-orbit misalignment in exoplanet systems through magnetic star-disk interactions, highlighting how magnetic torques influence stellar spin orientations during the disk phase.

Contribution

It introduces a magnetic torque-based model that accounts for the mass-dependent spin-orbit misalignment trend observed in exoplanet systems.

Findings

Magnetic star-disk torques explain the mass-dependent misalignment trend.

Lower-mass stars realign faster, reducing misalignments.

Massive stars retain misalignments due to weaker magnetic fields.

Abstract

Detailed observational characterization of transiting exoplanet systems has revealed that the spin-axes of massive (M > ~1.2 solar masses) stars often exhibit substantial misalignments with respect to the orbits of the planets they host. Conversely, lower-mass stars tend to only have limited obliquities. A similar trend has recently emerged within the observational dataset of young stars' magnetic field strengths: massive T-Tauri stars tend to have dipole fields that are ~10 times weaker than their less-massive counterparts. Here we show that the associated dependence of magnetic star-disk torques upon stellar mass naturally explains the observed spin-orbit misalignment trend, provided that misalignments are obtained within the disk-hosting phase. Magnetic torques act to realign the stellar spin-axes of lower-mass stars with the disk plane on a timescale significantly shorter than the typical disk lifetime, whereas the same effect operates on a much longer timescale for massive stars. Cumulatively, our results point to a primordial excitation of extrasolar spin-orbit misalignment, signalling consistency with disk-driven migration as the dominant transport mechanism for short-period planets. Furthermore, we predict that spin-orbit misalignments in systems where close-in planets show signatures of dynamical, post-nebular emplacement will not follow the observed correlation with stellar mass.