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

1° Italian Exoplanet Science Meeting True spin-orbit obliquities distribution: no clustering of misaligned planets A. M. Rossi1, M. Rainer2, F. Borsa2, S. Facchini1 1Dipartimento di Fisica, UniMi; 2INAF –OAB Contact info: Alessandro Matteo Rossi [email protected] Recent literature has studied the distribution of true spin-orbit obliquities Ψ of the exoplanet orbit, to explore the mis/alignment frequencies and dependence on the system parameters. A study on 57 planets suggested a clustering into two groups: aligned (0°-41°) and perpendicular (misaligned, but strictly in the 80°-125° range) (Albrecht+2021). More recent literature, however, propose a single cluster followed by a uniform distribution of misaligned orbits (Dong+2023; Siegel+2023). Rossiter-McLaughlin (RML) effect provides information about λ , the angle between the stellar rotational axis and the planet orbital axis, projected onto the sky plane. A robust approximation for Ψ can be derived (Fabrycky+2009) based on system geometry and the fact that RML effect occurs during a planetary transit where Prot , v sin i and R are the stellar rotation period, projected rotational velocity and radius. Eq. 1 provides information on the stellar inclination i and the true obliquity Ψ . We performed a MCMC simulation assuming uniform priors on the quantities involved and an isotropic stellar inclination (Masuda+2020; Bowler+2023), following the same methodology as previous studies, and derived 111 true obliquities. Combined with 16 literature values, this resulted in a sample of 127 planets. Figure 1 provides different visualizations of the obliquities, showing a single peak on aligned planets and no secondary peak. This discrepancy with past findings can be explained by our larger sample size (see fig. 2). Additionally, a more uniform distribution of i may also contribute (upper histograms of fig. 2). We are investigating possible correlations between misaligned planets and stellar/system parameters, such as the stellar age (paper in preparation). References: Albrecht, S. H. et al. 2021, ApJL, 916, L1 Fabrycky, D. C. & Winn, J. N. 2009, AJ, 696, 1230 Bowler, B. P. et al. 2023, AJ, 165, 164 Masuda, K. & Winn, J. N. 2020, AJ, 159, 81 Dong, J. & Foreman-Mackey, D. 2023, AJ, 166, 112 Siegel, J. C. et al. 2023, ApJ, 950, L2 Figure 2: This plot shows Ψ values in respect to stellar inclination i . The marginal histogram in the left panel suggests a dichotomy in 3D obliquities (61-planet sample) whereas our sample (right sample, 121 planets) instead shows for a single peak for aligned obliquities. This may result from a larger and more uniform sample of i. (1) Figure 1: Different visualizations of the 127 Ψ sample. The top panel shows values drawn from Eq. 1 posteriors, fitted with a KDE (Gaussian kernel, Silverman's bandwidth). The middle panel presents a histogram of the sample, while the bottom panel applies bin perturbation resampling. These visualizations suggest a peak for aligned orbits, followed by a nearly uniform distribution for misaligned planets.