A Primordial Origin for Misalignments Between Stellar Spin Axes and Planetary Orbits

TL;DR

This paper proposes that misalignments between stellar spin axes and planetary orbits can naturally originate from gravitational perturbations by binary companions during the early disk phase, challenging the idea that such misalignments require post-formation dynamical processes.

Contribution

It introduces a primordial mechanism for orbital misalignments driven by binary star perturbations, linking stellar multiplicity to observed spin-orbit misalignments.

Findings

Misalignments can result from gravitational torques from binary companions.

The fraction of misaligned systems correlates with stellar multiplicity rates.

Disk perturbations can produce a wide range of orbital inclinations.

Abstract

The presence of gaseous giant planets whose orbits lie in extreme proximity to their host stars ("hot Jupiters"), can largely be accounted for by planetary migration, associated with viscous evolution of proto-planetary nebulae. Recently, observations of the Rossiter-McLaughlin effect during planetary transits have revealed that a considerable fraction of detected hot Jupiters reside on orbits that are misaligned with respect to the spin-axes of their host stars. This observational fact has cast significant doubts on the importance of disk-driven migration as a mechanism for production of hot Jupiters, thereby reestablishing the origins of close-in planetary orbits as an open question. Here we show that misaligned orbits can be a natural consequence of disk migration. Our argument rests on an enhanced abundance of binary stellar companions in star formation environments, whose orbital plane is uncorrelated with the spin axes of the individual stars. We analyze the dynamical evolution of idealized proto-planetary disks under perturbations from massive distant bodies and demonstrate that the resulting gravitational torques act to misalign the orbital planes of the disks relative to the spin poles of their host stars. As a result, we predict that in the absence of strong disk-host star angular momentum coupling or sufficient dissipation that acts to realign the stellar spin axis and the planetary orbits, the fraction of planetary systems (including systems of hot Neptunes and Super-Earths), whose angular momentum vectors are misaligned with respect to their host-stars should be commensurate with the rate of primordial stellar multiplicity.