From Misaligned Sub-Saturns to Aligned Brown Dwarfs: The Highest M p /M * Systems Exhibit Low Obliquities, Even around Hot Stars*

Abstract

We present a pattern emerging from stellar obliquity measurements in single-star systems: planets with high planet-to-star mass ratios (MP/M* ≥ 2 × 10−3)—such as super-Jupiters, brown dwarf companions, and M dwarfs hosting Jupiter-like planets—tend to be aligned, even around hot stars. This alignment represents a 3.7σ deviation from the obliquity distribution observed in systems with lower mass ratios (MP/M* < 2 × 10−3), which predominantly include Jupiters and sub-Saturns. The only known outlier system, XO-3, exhibits misalignment confirmed via our newly collected Rossiter–McLaughlin effect measurement (λ = ). However, the relatively large Gaia renormalized unit weight error of XO-3 suggests that it may harbor an undetected binary companion, potentially contributing to its misalignment. Given that tidal realignment mechanisms are weak for hot stars, the observed alignment in high MP/M* systems is likely primordial rather than resulting from tidal interactions. One possible explanation is that only dynamically isolated planets can continue accreting gas and evolve into super-Jupiters while maintaining their primordial alignment. Conversely, planets formed in compact configurations may be unable to grow beyond the gap-opening mass, for which our work suggests an empirical boundary MP/M* = 2 × 10−3 identified between aligned high MP/M* systems and misaligned low MP/M* systems, with dynamical instabilities contributing to the diverse spin–orbit misalignments observed in the latter.