Are Tidal Effects Responsible for Exoplanetary Spin-Orbit Alignment?

TL;DR

This paper critically examines the hypothesis that tidal interactions are responsible for spin-orbit alignment in exoplanetary systems, finding that the evidence and models have significant limitations and complexities.

Contribution

The study re-evaluates observational data and theoretical models, highlighting the nuanced challenges and limitations of tidal interactions as the sole explanation for spin-orbit alignment.

Findings

Photometric variability correlates with orbital period, consistent with tidal effects but difficult to quantify.

Tidal re-alignment models only work for close-in giant planets with fine-tuned parameters.

Tidal interactions alone cannot fully explain observed spin-orbit alignments, indicating other factors are involved.

Abstract

The obliquities of planet-hosting stars are clues about the formation of planetary systems. Previous observations led to the hypothesis that for close-in giant planets, spin-orbit alignment is enforced by tidal interactions. Here, we examine two problems with this hypothesis. First, Mazeh and coworkers recently used a new technique -- based on the amplitude of starspot-induced photometric variability -- to conclude that spin-orbit alignment is common even for relatively long-period planets, which would not be expected if tides were responsible. We re-examine the data and find a statistically significant correlation between photometric variability and planetary orbital period that is qualitatively consistent with tidal interactions. However it is still difficult to explain quantitatively, as it would require tides to be effective for periods as long as tens of days. Second, Rogers and Lin argued against a particular theory for tidal re-alignment by showing that initially retrograde systems would fail to be re-aligned, in contradiction with the observed prevalence of prograde systems. We investigate a simple model that overcomes this problem by taking into account the dissipation of inertial waves and the equilibrium tide, as well as magnetic braking. We identify a region of parameter space where re-alignment can be achieved, but it only works for close-in giant planets, and requires some fine tuning. Thus, while we find both problems to be more nuanced than they first appeared, the tidal model still has serious shortcomings.