From Misaligned Sub-Saturns to Aligned Brown Dwarfs: The Highest Mp/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 ( 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 3.7 σ 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.

Keywords

• Exoplanet dynamics
• Star-planet interactions
• Exoplanets
• Planetary theory
• Exoplanet systems
• Exoplanet formation