From Misaligned Sub-Saturns to Aligned Brown Dwarfs: The Highest $M_{\rm p}/M{_*}$ Systems Exhibit Low Obliquities, Even around Hot Stars

TL;DR

This study finds that high planet-to-star mass ratio systems, such as super-Jupiters and brown dwarfs, tend to be aligned with their host stars' axes, indicating primordial alignment rather than tidal realignment, especially around hot stars.

Contribution

It identifies an empirical mass ratio boundary at 2×10^{-3} that separates aligned high-mass systems from misaligned lower-mass systems, highlighting the role of formation and dynamical history.

Findings

High $M_{p}/M_{*}$ systems are predominantly aligned.

Misaligned systems are mostly below the mass ratio boundary.

XO-3 is an outlier with potential undetected binary influence.

Abstract

We present a pattern emerging from stellar obliquity measurements in single-star systems: planets with high planet-to-star mass ratios ($M_{\rm p}/M{_*}$$>$ $2\times10^{-3}$) -- such as super-Jupiters, brown dwarf companions, and M-dwarfs hosting Jupiter-like planets -- tend to be aligned, even around hot stars. This alignment represents a 3.7$\sigma$ deviation from the obliquity distribution observed in systems with lower mass ratios ($M_{\rm p}/M{_*}$$<$ $2\times10^{-3}$), 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 ($\lambda=41.8^{+2.1}_{-2.0}$ degrees). However, the relatively large $\textit{Gaia}$ Renormalized Unit Weight Error (RUWE) 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 $M_{\rm p}/M{_*}$ systems is likely $\textit{primordial}$ 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_{\rm p}/M{_*}$$=$ $2\times10^{-3}$, identified between aligned high $M_{\rm p}/M{_*}$ systems and misaligned low $M_{\rm p}/M{_*}$ systems, with dynamical instabilities contributing to the diverse spin-orbit misalignments observed in the latter.