True spin-orbit obliquities distribution: data-driven confirmation of no clustering of misaligned planets

TL;DR

This study analyzes the distribution of true spin-orbit obliquities in exoplanets using a large, homogeneous dataset, confirming a single aligned cluster and an isotropic distribution of misaligned planets, with implications for planetary system evolution.

Contribution

It provides the largest homogeneous dataset of true obliquities and confirms the absence of a dichotomous distribution, challenging previous biased interpretations.

Findings

Confirmed a single cluster of aligned planets

Found an isotropic distribution of misaligned planets

Tentatively supported a dichotomy for Neptunian planets

Abstract

Context. True spin-orbit obliquities {\Psi} offer valuable insights into the evolutionary history of exoplanetary systems. Previous studies have suggested that exoplanets tend to occupy either aligned or perpendicular orbits. However, recent research has indicated potential biases caused by the low sample, questioning whether this dichotomy would persist with a larger dataset. Simultaneously, a similar dichotomous behavior has been suggested for Neptune-sized planets. Aims. We aim to investigate the distribution of true spin-orbit obliquities {\Psi} with an enlarged sample, looking for confirmation of the disputed dichotomy previously found, with a focus also on the obliquities of Neptunes. Methods. Starting from a sample of 264 projected obliquities {\lambda}, we homogeneously compute true obliquities {\Psi} for 116 planets using the rotation period method. We combine them with 4 further values gathered from literature and we then study their distribution, also as a function of various star-planet system parameters. Results. Our data-driven work based on 120 true obliquities {\Psi} - the largest sample to date - strongly confirms the presence of a single cluster of aligned planets, followed by an isotropic distribution of misaligned planets with no preferred misalignment. This result is based on a uniform distribution of stellar inclinations {i_\star} , for which non-uniformity could have biased previous interpretations of the arrangement of true obliquities. We confirm that Neptunians show a tentative dichotomous distribution with data available today, but its veracity needs confirmation with an enlarged sample, also because an anisotropic distribution of stellar inclination may be one of the factors hindering the real distribution.