Discovery of a multi-planet system orbiting the aged Sun-like star HD 224018^★

¹ INAF – Osservatorio Astrofisico di Torino, Via Osservatorio 20, 10025 Pino Torinese, Italy
² School of Physics & Astronomy, University of Birmingham, Edgbaston, Birmingham B15 2TT, UK
³ SUPA, School of Physics & Astronomy, University of St Andrews, North Haugh, St Andrews, KY169SS, UK
⁴ Centre for Exoplanet Science, University of St Andrews, North Haugh, St Andrews, KY169SS, UK
⁵ Space Research and Planetary Sciences, Physics Institute, University of Bern, Gesellschaftsstrasse 6, 3012 Bern, Switzerland
⁶ Sub-department of Astrophysics, University of Oxford, Keble Rd, Oxford OX13RH, UK
⁷ Observatoire Astronomique de l’Université de Genève, Chemin Pegasi 51, 1290 Versoix, Switzerland
⁸ Department of Physics, University of Warwick, Gibbet Hill Road, Coventry CV4 7AL, UK
⁹ DTU Space, Technical University of Denmark, Elektrovej 328, 2800 Kgs. Lyngby, Denmark
¹⁰ University of Southern Queensland, Centre for Astrophysics, West Street, Toowoomba, QLD 4350 Australia
¹¹ Space sciences, Technologies and Astrophysics Research (STAR) Institute, Université de Liège, Allée du 6 Août 19C, 4000 Liège, Belgium
¹² Astrobiology Research Unit, Université de Liège, Allée du 6 Août 19C, 4000 Liège, Belgium
¹³ Fundación Galileo Galilei - INAF, Rambla José Ana Fernandez Pérez 7, 38712 Breña Baja, Tenerife, Spain
¹⁴ Center for Astrophysics I Harvard & Smithsonian, 60 Garden Street, Cambridge, MA 02138, USA
¹⁵ Astrobiology Center, 2-21-1 Osawa, Mitaka, Tokyo 181-8588, Japan
¹⁶ National Astronomical Observatory of Japan, 2-21-1 Osawa, Mitaka, Tokyo 181-8588, Japan
¹⁷ Department of Astronomy, The Graduate University for Advanced Studies (SOKENDAI), 2-21-1 Osawa, Mitaka, Tokyo, Japan
¹⁸ INAF – Osservatorio Astronomico di Padova, Vicolo dell’Osservatorio 5, 35122 Padova, Italy
¹⁹ Dipartimento di Fisica e Astronomia ‘Galileo Galilei’, Università degli Studi di Padova, Vicolo dell’Osservatorio 3, 35122 Padova, Italy
²⁰ NASA Ames Research Center, Moffett Field, CA 94035, USA
²¹ SUPA, Institute for Astronomy, University of Edinburgh, Blackford Hill, Edinburgh, EH9 3HJ, UK
²² Centre for Exoplanet Science, University of Edinburgh, Edinburgh, EH9 3HJ, UK
²³ DTU Space, National Space Institute, Technical University of Denmark, Elektrovej 328, 2800 Kgs. Lyngby, Denmark
²⁴ Affiliate member, Cavendish Laboratory, University of Cambridge, J J Thomson Avenue, Cambridge, CB3 0HE, UK
²⁵ Astronomy Department of the University of Geneva, Chemin Pegasi 51, 1290 Versoix, Switzerland
²⁶ Astrophysics Research Centre, School of Mathematics and Physics, Queen’s University Belfast, Belfast, BT7 1NN, UK

^★★ Corresponding author: mario.damasso@inaf.it

Received: 2 June 2025
Accepted: 18 August 2025

Abstract

Context. Exoplanetary systems show a large diversity of architectures and planet types. Among the increasing number of exodemographics studies, those exploring correlations between the presence of close-in small planets and cold Jupiters are the object of particular attention.

Aims. In 2016, Kepler/K2 detected a system of two sub-Neptunes transiting the star HD 224018, one of them showing a mono-transit event. In 2017, we began a spectroscopic follow-up with HARPS-N to measure the dynamical masses of the planets using radial velocities, and collected additional transit observations using CHEOPS.

Methods. We measured the fundamental physical parameters of the host star, which is an ‘old Sun’ analogue. We analysed radial velocities and photometric time series, also including data by TESS, to provide precise ephemerides, radii, masses, and bulk densities of the two planets, and possibly modelling their internal structure and composition.

Results. The system turned out to be more crowded than was shown by Kepler/K2. Radial velocities revealed the presence of two additional bodies: a candidate cold companion on an eccentric orbit with a minimum mass nearly half that of Jupiter (eccentricity 0.60_−0.08^+0.07; semi-major axis 8.6_−1.6^+1.5 au), and an innermost super-Earth (orbital period 10.6413±0.0028 d; mass 4.1±0.8 M_⊕) for which we discovered previously undetected transit events in Kepler/K2 photometry. TESS data revealed a second transit of one of the two companions originally observed by Kepler/K2. This allowed us to constrain its orbital period to a grid of values, the most likely being ~138 days, which would imply a mass less than 9 M_⊕ at a 3σ significance level. Given the level of precision of our measurements, we were able to constrain the internal structure and composition of the second-most distant planet from the host star, a warm sub-Neptune with a bulk density of 3.9±0.5 g cm⁻³.

Conclusions. HD 224018 hosts three close-in transiting planets in the super-Earth-to-sub-Neptune regime, and a candidate cold and eccentric massive companion. Additional follow-up is needed to better characterise the physical properties of the planets and their architecture, and to study the evolutionary history of the system.