TOI-1259Ab: A warm Jupiter orbiting a K-dwarf white-dwarf binary on a low-obliquity orbit

¹ Anton Pannekoek Institute for Astronomy, University of Amsterdam, Science Park 904, 1098 XH Amsterdam, The Netherlands
² Instituto de Astrofísica, Pontificia Universidad Católica de Chile, Av. Vicuña Mackenna 4860, 782-0436 Macul, Santiago, Chile
³ Millennium Institute for Astrophysics, Santiago, Chile
⁴ Department of Astrophysical Sciences, Princeton University, 4 Ivy Lane, Princeton, NJ 08544, USA
⁵ Department of Physics and Astronomy, Vanderbilt University, TN 37235, USA
⁶ Department of Physics and Astronomy, Tufts University, 574 Boston Avenue, Medford, MA 02155, USA
⁷ Department of Astronomy & Astrophysics, The Pennsylvania State University, 525 Davey Laboratory, University Park, PA 16802, USA
⁸ Center for Exoplanets and Habitable Worlds, The Pennsylvania State University, 525 Davey Laboratory, University Park, PA 16802, USA
⁹ Department of Physics and Astronomy, University of Pennsylvania, 209 South 33rd Street, Philadelphia, PA 19104, USA
¹⁰ Institute for Astronomy, University of Hawai’i, 2680 Woodlawn Drive, Honolulu, HI 96822, USA
¹¹ Institute for Computational and Data Sciences, The Pennsylvania State University, University Park, PA 16802, USA
¹² Center for Astrostatistics, 525 Davey Laboratory, The Pennsylvania State University, University Park, PA 16802, USA
¹³ Department of Physics and Astronomy, Amherst College, Amherst, MA 01002, USA
¹⁴ U.S. National Science Foundation National Optical-Infrared Astronomy Research Laboratory, 950 N. Cherry Ave., Tucson, AZ 85719, USA
¹⁵ Jet Propulsion Laboratory, California Institute of Technology, 4800 Oak Grove Drive, Pasadena, CA 91109, USA
¹⁶ Department of Physics & Astronomy, The University of California, Irvine, Irvine, CA 92697, USA
¹⁷ Steward Observatory, University of Arizona, 933 N. Cherry Ave, Tucson, AZ 85721, USA
¹⁸ Department of Astronomy and Astrophysics, Tata Institute of Fundamental Research, Homi Bhabha Road, Colaba, Mumbai 400005, India
¹⁹ Department of Astronomy, Indiana University, Bloomington, IN 47405, USA
²⁰ Astrophysics & Space Institute, Schmidt Sciences, New York, NY 10011, USA
²¹ School of Mathematical and Physical Sciences, Macquarie University, Balaclava Road, North Ryde, NSW 2109, Australia
²² Carleton College, One North College Street, Northfield, MN 55057, USA

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Received: 10 July 2025
Accepted: 23 December 2025

Abstract

The evolution of one member of a stellar binary into a white dwarf has been proposed as a mechanism that triggers the formation of close-in gas giant planets. The star’s asymmetric mass loss during the Asymptotic Giant Branch stage gives it a velocity recoil or “kick” that can initiate eccentric Lidov–Kozai oscillations, potentially causing a planet around the secondary star to migrate inward and perturb the eccentricity and inclination of its orbit. Here we present a measurement of the stellar obliquity of TOI-1259Ab, a gas giant in a close-in orbit around a K star with a white dwarf companion about 1650 au away. By using the NEID spectrograph to detect the Rossiter-McLaughlin effect during the planetary transit, we find the sky-projected obliquity to be λ = 7₋₂₁^+20°. When combined with estimates of the stellar rotation period, radius, and projected rotation velocity, we find the true 3D obliquity to be ψ = 24₋₁₂^+14° (ψ < 47° at 95% confidence), revealing that the orbit of TOI-1259Ab is on a low-obliquity orbit with respect to the star’s equatorial plane. Because the planet’s orbit is too wide for tidal realignment to be expected, TOI-1259Ab might have formed quiescently in this low-obliquity configuration. Alternatively, as we show with dynamical simulations, eccentric Lidov–Kozai oscillations triggered by the evolution of the binary companion expect to lead to a low-obliquity configuration with a probability of ∼14%.