Dynamical Architectures of S-type Transiting Planets in Binaries II: A Dichotomy in Orbital Alignment of Small Planets in Close Binary Systems

TL;DR

This study leverages Hipparcos-Gaia astrometric accelerations to assemble a sample of 54 TOIs within 300 pc and identify 35 with stellar companions (10–200 AU) and 12 with substellar companions. Through adaptive optics imaging, RVs, and Gaia DR3 data, the authors derive 3D orbits for 12 binaries hosting planets, constraining the line-of-sight mutual inclinations ΔI_{los}. Hierarchical Bayesian analysis across 26 S-type planet binaries reveals a dichotomy: a tightly aligned population with a small mutual inclination (σ_1 ≈ 2.4°) and a broader or isotropic misaligned population (σ_2 ≈ 23.6° or isotropic), with misalignment emerging for binaries with periastron > ~40 AU. The results point to distinct dynamical histories—primordial alignment or recent realignment in close binaries versus inclined companions driving misalignment in wider systems—and have implications for planet formation and survival in binary environments. Future Gaia DR4 time-series data will enable larger-sample tests of these trends and a deeper understanding of planet formation in multiple-star systems.

Abstract

Stellar multiplicity plays a crucial role in shaping planet formation and dynamical evolution. We present a survey of 54 TESS Objects of Interest (TOIs) within 300 pc that exhibit significant Hipparcos-Gaia astrometric accelerations. We identified 35 TOIs with stellar companions at projected separations between $0.1^{\prime\prime}$ to $2^{\prime\prime}$ (or $10-200$ AU). We also identified 12 TOIs that could host planetary-mass or brown dwarf companions, including 6 that are newly discovered. Furthermore, we perform three-dimensional orbital characterization for 12 binaries hosting confirmed planets or planet candidates, allowing us to constrain the line-of-sight mutual inclination, $ΔI_{\mathrm{los}}$, between the planetary and binary orbits. Combining our sample with previous measurements, we apply Bayesian hierarchical analysis to a total of 26 binary systems with S-type transiting planets ($r_p<5R_{\oplus}$). Specifically, we fit the $ΔI_{\mathrm{los}}$ distribution with both single (Rayleigh) and mixture models (two-component Rayleigh and Rayleigh-isotropic mixture). We find the mixture models are strongly favored ($\log Z\gtrsim13.9$, or $\approx$5$σ$), indicating the observed planet-binary $ΔI_{\mathrm{los}}$ values likely originate from two underlying populations: one nearly aligned ($σ_1 = 2^{\circ}.4^{+0.7}_{-0.9}$) and one with more scattered mutual inclinations ($σ_2 = 23^{\circ}.6^{+8.8}_{-7.1}$). Alternatively, the misaligned systems can be equally well described by an isotropic distribution of inclinations. This observed dichotomy likely reflects different dynamical histories. Notably, the misaligned population only emerges in systems with stellar periastron distances $>40$ AU while systems with close-in or eccentric stellar companions (periastron distances $<40$ AU) preserve planet-binary alignment.

Figures (30)

• Figure 1: Gaia DR3 RUWE versus astrometric acceleration for stars in our sample. Orange stars represent systems with detected stellar companions. Gray stars indicate unresolved stellar companions ($<0.1^{\hbox{$^{\prime\prime}$}}$) inferred from astrometric acceleration. Blue circles mark systems with confirmed planetary or brown dwarf companions. Gray circles indicate newly identified planetary or brown dwarf companion candidates suggested by astrometric acceleration and AO imaging detection limit.
• Figure 1: Astrometric acceleration from Hipparcos-Gaia reveals the presence of unresolved stellar companions within $0.1^{\hbox{$^{\prime\prime}$}}$ of 8 TOIs. This plot shows the inferred constraints on companion mass and semi-major axis.
• Figure 1: Joint posterior distributions for selected orbital parameters of TOI-402 B. These are the host star's mass ($\rm M_{pri}$), the companion mass ($\rm M_{sec}$), the semi-major axis $a$, the orbital eccentricity $e$, and the orbital inclination $I$. The values and histogram distributions of the posteriors of selected parameters are shown, along with 1 $\sigma$ uncertainties. The complete set of joint posterior distributions (12 figures) is available as a figure set in the online Journal
• Figure 2: Keck/NIRC2 AO images of TOIs with resolved stellar companions detected from our survey. Images are present in linear scale. For TOI-1684, we present the AO image after PSF subtraction to enhance visibility. The orientation is the same in all images, with North pointed up and East to the left.
• Figure 2: Orbital characterization of TOI-128 B around A using relative astrometry and absolute astrometry from Hipparcos and Gaia. Similar to figure \ref{['fig:figure402']}