A decade of monitoring the HIP 41378 planetary system

Masses and orbital periods of six planets and a planet candidate

¹ Aix Marseille Univ, CNRS, CNES, Institut Origines, LAM, Marseille, France
² Univ. Grenoble Alpes, CNRS, IPAG, 38000 Grenoble, France
³ Department of Astronomy, University of Geneva, Geneva, Switzerland
⁴ Université Côte d’Azur, Observatoire de la Côte d’Azur, CNRS, Laboratoire Lagrange, CS 34229, 06304 Nice Cedex 4, France
⁵ Dipartimento di Fisica e Astronomia, Università degli Studi di Padova, Vicolo dell’Osservatorio 3, 35122 Padova, Italy
⁶ LTE, Observatoire de Paris, Université PSL, Sorbonne Université, Université de Lille, LNE, CNRS, 75014 Paris, France
⁷ INAF, Osservatorio Astronomico di Padova, Vicolo dell’Osservatorio 5, 35122 Padova, Italy
⁸ Department of Physics, University of Warwick, Gibbet Hill Road, Coventry CV4 7AL, UK
⁹ Sub-department of Astrophysics, Department of Physics, University of Oxford, Oxford OX1 3RH, UK
¹⁰ Department of Physics and Astronomy, University of Kansas, Lawrence, KS, USA
¹¹ Centre for Exoplanets and Habitability, University of Warwick, Gibbet Hill Road, Coventry CV4 7AL, UK
¹² INAF – Osservatorio Astronomico di Brera, Italy
¹³ University Observatory, Faculty of Physics, Ludwig-Maximilians-Universität München, Scheinerstr. 1, 81679 Munich, Germany
¹⁴ Center for Space & Habitability, Physikalisches Institut, Universität Bern, Gesellschaftsstrasse 6, 3012 Bern, Switzerland
¹⁵ Inst. f. Teilchen- und Astrophysik, ETH Zürich, Wolfgang-Pauli-Strasse 27, 8093 Zürich, Switzerland
¹⁶ Space Telescope Science Institute, 3700 San Martin Drive, Baltimore, MD 20218, USA
¹⁷ Instituto de Astrofísica e Ciências do Espaço, Universidade do Porto, CAUP, Rua das Estrelas, 4150-762 Porto, Portugal
¹⁸ Departamento de Física e Astronomia, Faculdade de Ciências, Universidade do Porto, Rua do Campo Alegre, 4169-007 Porto, Portugal
¹⁹ Institute for Astronomy, University of Hawai‘i, 2680 Woodlawn Drive, Honolulu, HI 96822, USA
²⁰ SUPA, Institute for Astronomy, University of Edinburgh, The Royal Observatory, Blackford Hill, Edinburgh EH9 3HJ, UK
²¹ Centre for Exoplanet Science, University of Edinburgh, Edinburgh EH9 3HJ, UK
²² INAF – Osservatorio Astrofisico di Torino, via Osservatorio 20, 10025 Pino Torinese, Italy
²³ SUPA School of Physics & Astronomy, University of St Andrews, North Haugh, St Andrews, UK
²⁴ Department of Physics & Astronomy, University of California Los Angeles, Los Angeles, CA 90095, USA
²⁵ School of Physics & Astronomy, University of Birmingham, Edgbaston, Birmingham B15 2TT, UK
²⁶ Instituto de Astrofísica de Andalucía-CSIC, Glorieta de la Astronomía s/n, 18008 Granada, Spain
²⁷ Fundación Galileo Galilei – INAF, Rambla José Ana Fernandez Pérez 7, 38712 Breña Baja, Tenerife, Spain
²⁸ Astrophysics Research Centre, Queen’s University Belfast, Belfast BT7 1NN, UK

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Received: 2 March 2026
Accepted: 1 April 2026

Abstract

Multi-planetary systems provide key constraints on planet formation and evolution, as their architecture encodes the dynamical history of planets formed within a common protoplanetary disc. However, the current population remains strongly biased towards compact, short-period systems, and only a limited number of such systems with measured masses and radii are known. HIP 41378 is an exceptional system hosting five transiting planets with orbital periods of up to 1.5 years, including an ultra-low-density planet, HIP 41378 f. The outer transiting planets d and e remained poorly constrained with unknown periods and masses, leaving the system architecture only partially characterised. We present long-term monitoring of this target with high-precision radial-velocity (RV) instruments (HARPS, HARPS-N, HIRES, and ESPRESSO) and space-based photometry spanning 2015–2024. We detected RV signals for all the planets, confirming their orbital periods and constraining their masses. In particular, the RV data strongly favour an orbital period of P_d = 278 days for planet d and refine the orbital period of planet e to P_e = 393₋₅⁺³ days. We measured a new mass of M_f = 25 ± 5 M_⊕ for HIP 41378 f, confirming its super-puff nature with a bulk density of ρ_f = 0.166_−0.036^+0.033 g cm⁻³. We also confirm the planetary nature of HIP 41378 g, a non-transiting planet with a 63-day period, and determine its minimum mass. In addition, the RVs reveal a long-period signal, with P = 2602₋₄₃₃⁺⁴⁶⁸ days, which we attribute to the candidate planet HIP 41378 h, although a stellar magnetic cycle cannot be excluded. Finally, we investigated the system’s dynamical architecture and resonant structure, assessed its completeness by constraining additional undetected planets, and looked into the implications for the origin and internal structure of the remarkable planet HIP 41378 f.