2025ApJ...983L..42R

We present a pattern emerging from stellar obliquity measurements in single-star systems: planets with high planet-to-star mass ratios (M_P/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 (M_P/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 = $40\mathop{.}\limits^{^\circ }{2}_{-1.8}^{+1.9}$). 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 M_P/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 M_P/M_* = 2 × 10^–3 identified between aligned high M_P/M_* systems and misaligned low M_P/M_* systems, with dynamical instabilities contributing to the diverse spin-orbit misalignments observed in the latter.