The Spin-Orbit Alignment of Eight Warm Gas Giant Systems

Essential information about the formation and evolution of planetary systems can be found in their architectures—in particular, in stellar obliquity (ψ)—as they serve as a signature of their dynamical evolution. Here we present ESPRESSO observations of the Rossiter-McLaughlin (RM) effect of eight warm gas giants, revealing that, independently of the eccentricities, all of them have relatively aligned orbits. Our five warm Jupiters (WASP-106 b, WASP-130 b, TOI-558 b, TOI-4515 b, and TOI-5027 b) have sky-projected obliquities ∣λ∣ ≃ 0-10 °, while the two less massive warm Saturns (K2-139 b and K2-329 A b) are slightly misaligned, having ∣λ∣ ≃ 15-25 °. Furthermore, for K2-139 b, K2-329 A b, and TOI-4515 b, we also measure true 3D obliquities ψ ≃ 15-30 °. We also report a nondetection of the RM effect produced by TOI-2179 b. Through hierarchical Bayesian modeling of the true 3D obliquities of hot and warm Jupiters, we find that around single stars warm Jupiters are statistically more aligned than hot Jupiters. Independent of eccentricities, 95% of the warm Jupiters have ψ ≲ 25 ° with no misaligned planets, while hot Jupiters show an almost isotropic distribution of misaligned systems. This implies that around single stars warm Jupiters form in primordially aligned protoplanetary disks and subsequently evolve in a more quiescent way than hot Jupiters. Finally, we find that Saturns may have slightly more misaligned orbits than warm Jupiters, but more obliquity measurements are necessary to be conclusive.