The star HIP 41378 potentially misaligned with its cohort of long-period planets

¹ Aix Marseille Univ, CNRS, CNES, Institut Origines, LAM, Marseille, France
² Observatoire de Genève, Université de Genève, Chemin Pegasi 51, 1290 Versoix, Switzerland
³ Centre for Exoplanets and Habitability, University of Warwick, Coventry, CV4 7AL, UK
⁴ Department of Physics, University of Warwick, Coventry, CV4 7AL, UK
⁵ Department of Astronomy, 501 Campbell Hall #3411, University of California, Berkeley, CA 94720, USA
⁶ Astronomy & Astrophysics Division, Physical Research Laboratory, Ahmedabad, India
⁷ Indian Institute of Technology, Gandhinagar, India
⁸ Centro de Astrobiología (CAB, CSIC-INTA), Depto. de Astrofísica, ESAC campus, 28692, Villanueva de la Cañada (Madrid), Spain
⁹ Instituto de Astrofísica e Ciências do Espaço, Universidade do Porto, CAUP, Rua das Estrelas, 4150-762 Porto, Portugal
¹⁰ Departamento de Física e Astronomia, Faculdade de Ciências, Universidade do Porto, Rua do Campo Alegre, 4169-007 Porto, Portugal
¹¹ Department of Physics and Astronomy, University of Kansas, Lawrence, KS, USA
¹² Department of Astronomy, California Institute of Technology, Pasadena, CA 91125, USA
¹³ University Observatory Munich, Faculty of Physics, Ludwig-Maximilians-Universität München, Scheinerstr. 1, 81679 Munich, Germany
¹⁴ Lowell Observatory, 1400 W Mars Hill Rd. Flagstaff. AZ, USA
¹⁵ School of Physics & Astronomy, University of Birmingham, Edgbaston, Birmingham, B15 2TT, UK
¹⁶ Instituto de Astrofísica de Canarias (IAC), 38205 La Laguna, Tenerife, Spain
¹⁷ Departamento de Astrofísica, Universidad de La Laguna (ULL), 38206 La Laguna, Tenerife, Spain
¹⁸ Department of Astronomy, Yale University, New Haven, CT 06511, USA
¹⁹ European Southern Observatory, Santiago, Chile
²⁰ Laboratoire Lagrange, Observatoire de la Côte d’Azur, CNRS, Université Côte d’Azur, Nice, France
²¹ Max-Planck Institute for Extraterrestrial Physics, Giessenbachstrasse 1, 85748 Garching, Germany

^★ Corresponding author: salome.grouffal@lam.fr

Received: 12 May 2025
Accepted: 31 July 2025

Abstract

The obliquity between the stellar spin axis and the planetary orbit, detected via the Rossiter-McLaughlin (RM) effect, is a tracer of the formation history of planetary systems. While obliquity measurements have been extensively applied to hot Jupiters and short-period planets, they remain rare for cold and long-period planets due to observational challenges, particularly their long transit durations. We report the detection of the RM effect for the 19-hour transit of HIP 41378 f, a temperate giant planet on a 542-day orbit, observed through a worldwide spectroscopic campaign. We measured a slight projected obliquity of 21 ± 8° and a significant 3D spin-orbit angle of 52 ± 6°, based on the measurement of the stellar rotation period. HIP 41378 f is part a transiting system of five planets with planets close to mean motion resonances. The observed misalignment likely reflects a primordial tilt of the stellar spin axis relative to the protoplanetary disk, rather than dynamical interactions. HIP 41378 f is the first non-eccentric long-period planet (P>100 days) observed with the RM effect, opening new constraints on planetary formation theories. This observation should motivate the exploration of planetary obliquities across a longer range of orbital distances through international collaboration.