From Misaligned Sub-Saturns to Aligned Brown Dwarfs: The Highest <i>M</i> _<i>p</i>/<i>M</i> _* 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 * &lt; 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 = <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:mn>40</mml:mn> <mml:mover accent="true"> <mml:mrow> <mml:mi>.</mml:mi> </mml:mrow> <mml:mrow> <mml:mi>°</mml:mi> </mml:mrow> </mml:mover> <mml:msubsup> <mml:mn>2</mml:mn> <mml:mrow> <mml:mo>−</mml:mo> <mml:mn>1.8</mml:mn> </mml:mrow> <mml:mrow> <mml:mo>+</mml:mo> <mml:mn>1.9</mml:mn> </mml:mrow> </mml:msubsup> </mml:math> ). 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.