Introduction
Stellar multiplicity has long been recognized as a key factor shaping how planets form and evolve. Building on a survey of nearby systems, this study focuses on transiting planets found in binary hosts and leverages significant Hipparcos-Gaia astrometric accelerations to identify and characterize companions. In particular, we examine 54 TESS Objects of Interest (TOIs) within 300 parsecs that show notable astrometric signals, revealing a population of close stellar companions and their potential planetary partners.
Among these TOIs, 35 exhibit stellar companions with projected separations from roughly 0.1 to 2 arcseconds (about 10–200 AU at typical distances). We also identify 12 TOIs that could host planetary-mass or brown dwarf companions, including six new discoveries. With three-dimensional orbital characterizations for 12 binaries hosting confirmed planets or planet candidates, we can constrain the line-of-sight mutual inclination, ΔIlos, between planetary orbits and their stellar companions.
Data, Methods, and Key Metrics
To place these objects on a robust statistical footing, we combine our measurements with previous work in a Bayesian hierarchical framework. Across a sample of 26 binary systems hosting short-period, small-radius planets (rp < 5 R⊕), we compare different models for the distribution of ΔIlos. The models include a single Rayleigh distribution and mixture models that allow for a second, distinct population: a two-component Rayleigh and a Rayleigh–isotropic mixture. Model comparison via log evidence strongly favors the mixture scenarios (logZ ≳ 13.9, about 5σ), indicating that the observed ΔIlos values likely arise from two underlying populations rather than a single, continuous process.
Quantitatively, the aligned component exhibits a small dispersion (σ1 = 2.4° with uncertainties roughly +0.7/−0.9°), suggesting a population of planets whose orbits are closely aligned with their binaries. The second, more broadly distributed component shows a much larger dispersion (σ2 ≈ 23.6° with +8.8/−7.1°), consistent with substantially misaligned configurations. An isotropic interpretation of the misaligned population yields a comparable description, underscoring that the dichotomy may reflect distinct dynamical histories in the planet–binary container.
A Dichotomy in Orbital Alignment
The two-population result reveals a striking dichotomy in orbital architecture. In practice, about half of the transiting planets in binaries appear closely aligned with the binary plane, while a significant fraction exhibit substantial misalignment. This split cannot be easily explained by a single formation pathway and points toward multiple dynamical channels, including gentle disk remnants preserving alignment and later gravitational perturbations that tilt planetary orbits relative to the binary plane.
Interestingly, the misalignment signal is not universal across all systems. When examining physical scales, the misaligned population emerges predominantly in binaries with stellar periastron distances greater than roughly 40 AU. In contrast, systems with closer-in or more eccentric stellar companions (periastra < 40 AU) tend to preserve planet–binary alignment, suggesting that proximity and orbital eccentricity of the companion can either damp or promote dynamical torques that reorient planetary orbits.
Implications for Planet Formation and Evolution
The observed dichotomy offers a window into how planets form in complex, multi-star environments. The presence of a tight, aligned population implies that disk-fed formation can proceed in a quasi-coplanar fashion relative to the binary, at least for certain orbital configurations. The misaligned cohort points to episodes of dynamical stirring—possibly through secular interactions or planet–planet scattering—that tilt planetary orbits after formation. The dependence on periastron distance hints that the dynamical history of each system is shaped by the gravitational reach of the binary, affecting disk evolution, planetesimal accretion, and long-term stability.
By focusing on small, transiting planets with radii below 5 R⊕, the study hones in on a regime where dynamical perturbations are particularly revealing. The results complement earlier findings on giant planets in binaries and extend the narrative to a population that is abundant in the TESS era. They also provide a framework for predicting which binary configurations are more likely to host aligned or misaligned planetary systems, guiding future observational campaigns.
Conclusions and Future Work
Transiting Planets in Binaries II uncovers a clear two-population signature in the mutual inclinations between planets and binary companions. Whether through a predominantly aligned channel or a more dynamically excited one, the architecture of these systems reflects their formation histories and the gravitational choreography dictated by their stellar partners. Ongoing and future surveys, combined with refined dynamical modeling, will sharpen the estimates of ΔIlos and reveal how common each pathway is across the galaxy.
Notes on Data and Collaboration
The analysis synthesizes a broad set of observations and includes 12 systems with 3D orbital solutions, among them 26 binaries suitable for the hierarchical modeling described above. The work is a collaborative effort across multiple institutions and draws on TESS, Hipparcos, and Gaia astrometry to map planetary systems in the cosmos.
