DREAM

II. The spin–orbit angle distribution of close-in exoplanets under the lens of tides

Observatoire Astronomique de l’Université de Genève, Chemin Pegasi 51b, 1290 Versoix, Switzerland
e-mail: omar.attia@unige.ch

Received: 17 October 2022
Accepted: 18 April 2023

Abstract

The spin–orbit angle, or obliquity, is a powerful observational marker that allows us to access the dynamical history of exoplanetary systems. For this study, we have examined the distribution of spin–orbit angles for close-in exoplanets and put it in a statistical context of tidal interactions between planets and their host stars. We confirm the previously observed trends between the obliquity and physical quantities directly connected to tides, namely the stellar effective temperature, the planet-to-star mass ratio, and the scaled orbital distance. We further devised a tidal efficiency factor τ combining critical parameters that control the strength of tidal effects and used it to corroborate the strong link between the spin–orbit angle distribution and tidal interactions. In particular, we developed a readily usable formula θ (τ) to estimate the probability that a system is misaligned, which will prove useful in global population studies. By building a robust statistical framework, we reconstructed the distribution of the three-dimensional spin–orbit angles, allowing for a sample of nearly 200 true obliquities to be analyzed for the first time. This realistic distribution maintains the sky-projected trends, and additionally hints toward a striking pileup of truly aligned systems. In fact, we show that the fraction of aligned orbits could be underestimated in classical analyses of sky-projected obliquities due to an observational bias toward misaligned systems. The comparison between the full population and a pristine subsample unaffected by tidal interactions suggests that perpendicular architectures are resilient toward tidal realignment, providing evidence that orbital misalignments are sculpted by disruptive dynamical processes that preferentially lead to polar orbits. On the other hand, star–planet interactions seem to efficiently realign or quench the formation of any tilted configuration other than for polar orbits, and in particular for antialigned orbits. Observational and theoretical efforts focused on these pristine systems are encouraged in order to study primordial mechanisms shaping orbital architectures, which are unaltered by tidal effects.