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Published July 2025 | Version Published
Journal Article Open

A Hot Jupiter with a Retrograde Orbit Around A Sun-like Star and a Toy Model of Hot Jupiters in Wide Binary Star Systems

Creators

  • 1. ROR icon California Institute of Technology
  • 2. ROR icon Flatiron Institute
  • 3. ROR icon University of Hawaii at Manoa
  • 4. ROR icon University of California, Berkeley
  • 5. ROR icon Jet Propulsion Lab
  • 6. ROR icon W.M. Keck Observatory
  • 7. ROR icon University of California, Santa Cruz
  • 8. ROR icon Massachusetts Institute of Technology
  • 9. ROR icon University of California, Los Angeles
  • 10. Astrophysics & Space Institute, Schmidt Sciences, New York, NY 10011, USA
  • 11. ROR icon Macquarie University

Abstract

We report an observation of a transit of the hot Jupiter (HJ) KELT-23A b with the Keck Planet Finder spectrograph and a measurement of the sky-projected obliquity (λ) of its Sun-like (T eff ≈ 5900 K) host star. We measured a projected stellar obliquity of λ ≈ 180°, indicating that the orbit of the HJ is retrograde relative to the direction of the stellar spin. Due to the slow sky-projected rotational velocity of the host star ( v sin i ⋆ ≈ 0.5 km s⁻¹), the true orbit of the HJ could be closer to polar. HJs around stars with effective temperatures below the Kraft break—such as KELT-23A—are generally found to have prograde orbits that are well-aligned with the equatorial planes of their host stars (i.e., λ ∼ 0°), most likely due to spin–orbit realignment driven by stellar tidal dissipation. This system is therefore a unique outlier that strains migration and tidal theories. The fact that the HJ has a highly misaligned orbit may suggest that the planet arrived at its close-in orbit relatively recently, possibly via interactions with the wide-separation (570 au) M-dwarf companion in the system, or that it has stalled near an antialigned or polar orientation while realigning. Using Gaia DR3, we determined the orbit of the stellar companion to be moderately face-on (γ = 60° ± 4°). We show that the distribution of observed systems in the γ–λ plane can be broadly reproduced using a toy model in which the orbits of the planetary and stellar companions begin aligned with the equatorial plane of the primary star and, upon migrating inwards, the planet preferentially obtains either an aligned or polar orbit.

Copyright and License

© 2025. The Author(s). Published by IOP Publishing Ltd on behalf of the Astronomical Society of the Pacific (ASP). Original content from this work may be used under the terms of the Creative Commons Attribution 4.0 licence. Any further distribution of this work must maintain attribution to the author(s) and the title of the work, journal citation and DOI.

Acknowledgement

We thank Heather A. Knutson, Joshua N. Winn, Jessie L. Christiansen, and Kevin Hardegree-Ullman for their helpful discussions involving this paper.

The data presented herein were obtained at the W. M. Keck Observatory, which is operated as a scientific partnership among the California Institute of Technology, the University of California, and the National Aeronautics and Space Administration. The Observatory was made possible by the generous financial support of the W. M. Keck Foundation. Keck Observatory occupies the summit of Maunakea, a place of significant ecological, cultural, and spiritual importance within the indigenous Hawaiian community. We understand and embrace our accountability to Maunakea and the indigenous Hawaiian community, and commit to our role in long-term mutual stewardship. We are most fortunate to have the opportunity to conduct observations from Maunakea.

We gratefully acknowledge the efforts and dedication of the Keck Observatory staff for support of KPF and remote observing.

S.G. is supported by an NSF Astronomy and Astrophysics Postdoctoral Fellowship under award AST-2303922. A.W.H. acknowledges funding support from NASA grant No. 80NSSC24K0161.

The research was carried out, in part, at the Jet Propulsion Laboratory, California Institute of Technology, under a contract with the National Aeronautics and Space Administration (80NM0018D0004)

Facilities

TESS - , Keck:I (KPF) - .

Software References

BATMAN (Kreidberg 2015), DYNESTY (Speagle 2020).

Files

Giacalone_2025_PASP_137_074401.pdf

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Additional details

Related works

Is new version of
Discussion Paper: arXiv:2507.02667 (arXiv)
Is supplemented by
Dataset: https://iopscience.iop.org/article/10.1088/1538-3873/adecc2/data (URL)

Funding

National Science Foundation
AST-2303922
National Aeronautics and Space Administration
80NSSC24K0161
National Aeronautics and Space Administration
80NM0018D0004

Dates

Submitted
2025-05-30
Accepted
2025-07-07
Available
2025-07-28
Published

Caltech Custom Metadata

Caltech groups
Astronomy Department , Infrared Processing and Analysis Center (IPAC) , Division of Physics, Mathematics and Astronomy (PMA)
Publication Status
Published