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A high mutual inclination system around KOI-134 revealed by transit timing variations

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Abstract

Few planetary systems have measured mutual inclinations, and even fewer are found to be non-coplanar. Observing the gravitational interactions between exoplanets is an effective tool to detect non-transiting companions to transiting planets. Evidence of these interactions can manifest in the light curve through transit timing variations (TTVs) and transit duration variations (TDVs). Here, through analysis of Kepler photometry and joint TTV–TDV modelling, we confirm the detection of KOI-134 b, a transiting planet with mass and size similar to Jupiter on a period of ~67 days, and find that it exhibits high TTVs (20-h amplitude) and significant TDVs. We explain these signals with the presence of an innermost non-transiting planet in 2:1 resonance with KOI-134 b. KOI-134 c has a mass \(M=0.22{0}_{-0.011}^{+0.010}{M}_{{\rm{Jup}}}\) and a moderately high mutual inclination with KOI-134 b of \({i}_{{\rm{mut}}}=15.{4}_{-2.5}^{+2.{8}^{\circ }}\). Moreover, the inclination variations of KOI-134 b are so large that the planet is predicted to stop transiting in about 100 years. This system architecture cannot be easily explained by any one formation mechanism, with other dynamical effects needed to excite the planets’ mutual inclination while still preserving their resonance.

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Fig. 1: Per-epoch Kepler light curves of KOI-134 b.
Fig. 2: TTVs and TDVs of KOI-134 b.
Fig. 3: Visualization of high versus low mutual inclination systems.
Fig. 4: Evolution of the KOI-134 system architecture.

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Data availability

The Kepler data used in this analysis can be accessed via the Kepler Data Search & Retrieval Tool (https://archive.stsci.edu/kepler/data_search/search.php). Any other datasets that were generated can be obtained from E.N. upon reasonable request.

Code availability

This study makes use of the following publicly available packages: astroARIADNE, batman, numpy, matplotlib, RadVel, REBOUND and scipy. The scripts used for this analysis are available via GitHub at https://github.com/enabbie/KOI134.

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Acknowledgements

We thank R. Dawson for helpful discussions throughout our analysis. E.N. acknowledges the PhD scholarship provided by the ARC Discovery Project DP220100365. C.X.H. and A.V. thank the support of the ARC DECRA project DE200101840. A.V. thanks M. Omohundro, K. Deck, A. Vanderburg and J. Becker for helpful comments during the inception of this study. G.Z. thanks the support of the Australian Research Council project FT230100517. We acknowledge support from the Swiss NCCR PlanetS and the Swiss National Science Foundation. This work has been carried out within the framework of the NCCR PlanetS supported by the Swiss National Science Foundation under grant numbers 51NF40182901 and 51NF40205606. J.K. acknowledges support from the Swedish Research Council (project grants 2017-04945 and 2022-04043) and of the Swiss National Science Foundation under grant number TMSGI2_211697. H.P. acknowledges support by the Spanish Ministry of Science and Innovation with the Ramon y Cajal fellowship number RYC2021-031798-I. Funding from the University of La Laguna and the Spanish Ministry of Universities is acknowledged.

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Contributions

E.N. led the light curve and N-body analyses, interpretation of the results and preparation of the paper. C.X.H. supervised the project, conducted N-body simulations, facilitated analysis of the system’s TDVs and contributed to the writing of the paper. J.K. and H.P. performed the photodynamical analyses. S.W. conducted disk migration simulations to investigate the system’s formation and evolution. A.V. identified the system and contributed to the analysis of the Kepler observations and mid-transit times. R.W. supervised the project and contributed to the analysis. G.L. contributed to the theoretical dynamical analyses. D.N.C.L. contributed to analysis and interpretation of disk migration simulations. A.B., D.W.L. and G.Z. were responsible for the TRES radial velocity observations and data reduction.

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Correspondence to Emma Nabbie.

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Extended data

Extended Data Fig. 1 Direct imaging observations of KOI-134.

Panel (a): 5σ sensitivity curve depicting the difference in magnitude versus orbital separation. Panel (b): High resolution i-band direct imaging of KOI-134 observed with the P60 telescope’s Robo-AO instrument. The 8" square image cutout is centered on the star, with a pixel scale of 0.021" per pixel.

Extended Data Fig. 2 KOI-134 b in context with other Kepler TTV systems.

The histogram shows the distribution of TTV amplitudes from Kepler systems with significant TTVs, taken from Table 5 of the Kepler TTV catalog by74. The dashed line shows the position of KOI-134 b in comparison to this population.

Extended Data Fig. 3 The proximity of KOI-134 to an exact 2:1 resonance.

The position of KOI-134 within the 2:1 resonant ___domain, re-parametrized to a 1-degree of freedom model30. Randomly-sampled points from the planet parameter posterior distribution are shown in red circles. X measures the system’s separatrices and stable/unstable fixed points, and γ quantifies how close the system is to an exact resonance. The shaded region denotes the formally-defined resonant ___domain.

Extended Data Fig. 4 Trajectories of the resonant angles of KOI-134 b and c.

Trajectories of the resonant angles (Θe,b and Θe,c) and the secular apsidal angle Δω, as they evolve over a time scale of 1 million years. Panel (a) shows these trajectories for KOI-134 b, while panel (b) shows those for KOI-134 c. The color scheme is shared with Fig. 4.

Extended Data Fig. 5 The population of near-MMR multi-planet systems with giant planets.

Confirmed multi-planet systems with a giant planet, where at least one pair of orbital periods are near a first-order resonance. The empty circles represent non-transiting planets. Marker size corresponds to relative planet radius.

Extended Data Table 1 Stellar and Planet Parameters for KOI-134
Extended Data Table 2 High-Precision System Parameters for N-body Integration

Supplementary information

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Supplementary text, Figs. 1–4, Tables 1–3 and references.

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Nabbie, E., Huang, C.X., Korth, J. et al. A high mutual inclination system around KOI-134 revealed by transit timing variations. Nat Astron (2025). https://doi.org/10.1038/s41550-025-02594-8

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