A hot Jupiter for breakfast? --- Early stellar ingestion of planets may be common

TL;DR

This paper proposes that early ingestion of hot Jupiters by stars can explain observed stellar spin alignments and misalignments, and predicts that such ingestion is common in planetary systems, affecting stellar and planetary dynamics.

Contribution

It introduces a model linking hot Jupiter ingestion to stellar spin alignment and explains observed patterns in exoplanet system orientations.

Findings

Ingestion of hot Jupiters can realign stellar spins in solar-type stars.

Misaligned planets are more common around hot stars due to less effective realignment.

The model accounts for the upper mass limit of retrograde hot Jupiters.

Abstract

Models of planet formation and evolution predict that giant planets form efficiently in protoplanetary disks, that most of these migrate rapidly to the disk's inner edge, and that, if the arriving planet's mass is $\lesssim$ Jupiter's mass, it could remain stranded near that radius. We argue that such planets would be ingested by tidal interaction with the host star on a timescale $\lesssim1\,$Gyr, and that, in the case of a solar-type host, this would cause the stellar spin to approach the direction of the ingested planet's orbital axis even if the two were initially highly misaligned. Primordially misaligned stars whose effective temperatures are $\gtrsim6250\,$K cannot be realigned in this way because, in contrast with solar-type hosts, their angular momenta are typically higher than the orbital angular momentum of the ingested planet as a result of inefficient magnetic braking and of a comparatively large moment of inertia. Hot Jupiters located farther out from the star can contribute to this process, but their effect is weaker because the tidal interaction strength decreases rapidly with increasing semimajor axis. We demonstrate that, if $\sim50\%$ of planetary systems harbored a stranded hot Jupiter, this scenario can in principle account for (1) the good alignment exhibited by planets around cool stars irrespective of the planet's mass or orbital period, (2) the prevalence of misaligned planets around hot stars, (3) the apparent upper bound on the mass of hot Jupiters on retrograde orbits, and (4) the inverse correlation between stellar spin periods and hot-Jupiter masses.