The Spin–Orbit Misalignment of TOI-1842b: The First Measurement of the Rossiter–McLaughlin Effect for a Warm Sub-Saturn around a Massive Star

The mechanisms responsible for generating spin–orbit misalignments in exoplanetary systems are still not fully understood. It is unclear whether these misalignments are related to the migration of hot Jupiters or are a consequence of general star and planet formation processes. One promising method to address this question is to constrain the distribution of spin–orbit angle measurements for a broader range of planets beyond hot Jupiters. In this work, we present the sky-projected obliquity ( λ=−68.°1−14.7+21.2 ) for the warm sub-Saturn TOI-1842b, obtained through a measurement of the Rossiter–McLaughlin effect using WIYN/NEID. From this, we determine the resulting 3D obliquity (ψ) to be ψ=73.°3−12.9+16.3 . As the first spin–orbit angle determination made for a sub-Saturn-mass planet around a massive ( M * = 1.45 M ☉) star, our result presents an opportunity to examine the orbital geometries for new regimes of planetary systems. When combined with archival measurements, our observations of TOI-1842b support the hypothesis that the previously established prevalence of misaligned systems around hot, massive stars may be driven by planet–planet dynamical interactions. In massive stellar systems, multiple gas giants are more likely to form and can then dynamically interact with each other to excite spin–orbit misalignments.