Early Excitation of Spin-Orbit Misalignments in Close-in Planetary Systems

TL;DR

This paper investigates how gravitational perturbations and magnetic interactions can cause spin-orbit misalignments in close-in planetary systems, suggesting these phenomena are compatible with disk-driven migration as the main formation process.

Contribution

It provides an analytical framework showing that spin-orbit misalignments can arise naturally from gravitational and magnetic effects, aligning with observations of close-in planets.

Findings

Spin-orbit misalignments depend on primordial disk-binary inclination.

Magnetic torques can lead to more complex dynamical evolution.

Observed misalignments are consistent with disk-driven migration.

Abstract

Continued observational characterization of transiting planets that reside in close proximity to their host stars has shown that a substantial fraction of such objects posses orbits that are inclined with respect to the spin axes of their stars. Mounting evidence for the wide-spread nature of this phenomenon has challenged the conventional notion that large-scale orbital transport occurs during the early epochs of planet formation and is accomplished via planet-disk interactions. However, recent work has shown that the excitation of spin-orbit misalignment between protoplanetary nebulae and their host stars can naturally arise from gravitational perturbations in multi-stellar systems as well as magnetic disk-star coupling. In this work, we examine these processes in tandem. We begin with a thorough exploration of the gravitationally-facilitated acquisition of spin-orbit misalignment and analytically show that the entire possible range of misalignments can be trivially reproduced. Moreover, we demonstrate that the observable spin-orbit misalignment only depends on the primordial disk-binary orbit inclination. Subsequently, we augment our treatment by accounting for magnetic torques and show that more exotic dynamical evolution is possible, provided favorable conditions for magnetic tilting. Cumulatively, our results suggest that observed spin-orbit misalignments are fully consistent with disk-driven migration as a dominant mechanism for the origin of close-in planets.