True spin-orbit obliquity distribution: Data-driven confirmation of no clustering of misaligned planets

¹ Dipartimento di Fisica, Università degli Studi di Milano, Via Celoria 16, Milano, Italy
² INAF – Osservatorio Astronomico di Brera, Via E. Bianchi 46, 23807 Merate (LC), Italy

^★ Corresponding author: This email address is being protected from spambots. You need JavaScript enabled to view it. ; This email address is being protected from spambots. You need JavaScript enabled to view it.

Received: 15 April 2025
Accepted: 12 November 2025

Abstract

Context. True spin-orbit obliquities Ψ 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 small sample, and has brought into question whether this dichotomy would persist with a larger dataset. Simultaneously, a similar dichotomous behavior has been suggested for Neptune-sized planets.

Aims. Our aim was to investigate the distribution of true spin-orbit obliquities Ψ 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 λ, we homogeneously computed true obliquities Ψ for 116 planets using the rotation period method. We combined them with four further values gathered from the literature and we then studied their distribution, also as a function of various star-planet system parameters.

Results. Our data-driven work was based on 120 true obliquities Ψ, the largest sample to date, and 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_*, 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 the data available today, but this needs confirmation with an enlarged sample, also because an anisotropic distribution of stellar inclination may be one of the factors hindering the real distribution.

Conclusions. The future increase in the measured Ψ sample over different planet types will allow a better investigation of the relation between misalignment and system properties and will provide a more comprehensive picture of the planetary evolution processes.