Exterior Companions to Hot Jupiters Orbiting Cool Stars are Coplanar

TL;DR

This study finds that wide companions to hot Jupiters orbiting cool stars are generally coplanar, suggesting Kozai-Lidov migration is unlikely to be the primary formation mechanism for these planets.

Contribution

It demonstrates that the orbital inclinations of wide companions are tightly aligned with hot Jupiters, challenging the role of Kozai-Lidov migration in their formation.

Findings

Wide companions are well-aligned with hot Jupiters.

Inclination distribution width must be less than ~20 degrees.

Kozai-Lidov migration is unlikely for these systems.

Abstract

The existence of hot Jupiters has challenged theories of planetary formation since the first extrasolar planets were detected. Giant planets are generally believed to form far from their host stars, where volatile materials like water exist in their solid phase, making it easier for giant planet cores to accumulate. Several mechanisms have been proposed to explain how giant planets can migrate inward from their birth sites to short-period orbits. One such mechanism, called Kozai-Lidov migration, requires the presence of distant companions in orbits inclined by more than $\sim40$ degrees with respect to the plane of the hot Jupiter's orbit. The high occurrence rate of wide companions in hot Jupiter systems lends support to this theory for migration. However, the exact orbital inclinations of these detected planetary and stellar companions is not known, so it is not clear whether the mutual inclination of these companions is large enough for the Kozai-Lidov process to operate. This paper shows that in systems orbiting cool stars with convective outer layers, the orbits of most wide planetary companions to hot Jupiters must be well aligned with the orbits of the hot Jupiters and the spins of the host stars. For a variety of possible distributions for the inclination of the companion, the width of the distribution must be less than $\sim20$ degrees to recreate the observations with good fidelity. As a result, the companion orbits are likely well-aligned with those of the hot Jupiters, and the Kozai-Lidov mechanism does not enforce migration in these systems.