Refined parameters of the HD 22946 planetary system and the true orbital period of planet d^★,★★

¹ MTA-ELTE Exoplanet Research Group, 9700 Szombathely, Szent Imre h. u. 112, Hungary
e-mail: zgarai@ta3.sk
² ELTE Gothard Astrophysical Observatory, 9700 Szombathely, Szent Imre h. u. 112, Hungary
³ Astronomical Institute, Slovak Academy of Sciences, 05960 Tatranská Lomnica, Slovakia
⁴ Physikalisches Institut, University of Bern, Gesellsschaftstrasse 6, 3012 Bern, Switzerland
⁵ Department of Physics and Kavli Institute for Astrophysics and Space Research, Massachusetts Institute of Technology, Cambridge, MA, 02139 USA
⁶ Dipartimento di Fisica, Universita degli Studi di Torino, via Pietro Giuria 1, 10125 Torino, Italy
⁷ Department of Astronomy, Stockholm University, AlbaNova University Center, 10691 Stockholm, Sweden
⁸ Instituto de Astrofísica e Ciencias do Espaco, Universidade do Porto, CAUP, Rua das Estrelas, 4150-762 Porto, Portugal
⁹ Astrophysics Group, Cavendish Laboratory, University of Cambridge, J.J. Thomson Avenue, Cambridge CB3 0HE, UK
¹⁰ Astrobiology Research Unit, Université de Liége, Allée du Six-Août 19C, 4000 Liége, Belgium
¹¹ Space Research Institute, Austrian Academy of Sciences, Schmiedlstrasse 6, 8042 Graz, Austria
¹² Observatoire Astronomique de 1’Université de Genève, Chemin Pegasi 51, Versoix, Switzerland
¹³ Centre for Exoplanet Science, SUPA School of Physics and Astronomy, University of St Andrews, North Haugh, St Andrews KY16 9SS, UK
¹⁴ Space Research Institute, Austrian Academy of Sciences, Schmiedlstrasse 6, 8042 Graz, Austria
¹⁵ INAF, Osservatorio Astronomico di Padova, Vicolo dell’Osservatorio 5, 35122 Padova, Italy
¹⁶ Department of Astrophysics, University of Vienna, Tuerkenschanzs-trasse 17, 1180 Vienna, Austria
¹⁷ Institute of Planetary Research, German Aerospace Center (DLR), Rutherfordstrasse 2, 12489 Berlin, Germany
¹⁸ European Space Agency (ESA), European Space Research and Technology Centre (ESTEC), Keplerlaan 1, 2201 AZ Noordwijk, The Netherlands
¹⁹ Instituto de Astrofísica de Canarias, 38200 La Laguna, Tenerife, Spain
²⁰ Institut d’Estudis Espacials de Catalunya (IEEC), 08034 Barcelona, Spain
²¹ Depto. de Astrofísica, Centro de Astrobiologia (CSIC-INTA), ESAC campus, 28692 Villanueva de la Cañada (Madrid), Spain
²² Université Grenoble Alpes, CNRS, IPAG, 38000 Grenoble, France
²³ Université de Paris, Institut de physique du globe de Paris, CNRS, 75005 Paris, France
²⁴ Departamento de Fisica e Astronomia, Faculdade de Ciencias, Universidade do Porto, Rua do Campo Alegre, 4169-007 Porto, Portugal
²⁵ Center for Space and Habitability, University of Bern, Gesellschaftsstrasse 6, 3012 Bern, Switzerland
²⁶ Leiden Observatory, University of Leiden, PO Box 9513, 2300 RA Leiden, The Netherlands
²⁷ Department of Space, Earth and Environment, Chalmers University of Technology, Onsala Space Observatory, 43992 Onsala, Sweden
²⁸ Space sciences, Technologies and Astrophysics Research (STAR) Institute, Université de Liége, 19C Allée du Six-Août, 4000 Liége, Belgium
²⁹ Departamento de Astrofísica, Universidad de La Laguna, 38206 La Laguna, Tenerife, Spain
³⁰ Institut de Ciencies de l'Espai (ICE, CSIC), Campus UAB, Can Magrans s/n, 08193 Bellaterra, Spain
³¹ Aix-Marseille Univ., CNRS, CNES, LAM, 38 rue Frédéric Joliot-Curie, 13388 Marseille, France
³² IMCCE, UMR8028 CNRS, Observatoire de Paris, PSL Univ., Sorbonne Univ., 77 av. Denfert-Rochereau, 75014 Paris, France
³³ Max Planck Institute for Extraterrestrial Physics, Giessenbachstrasse 1, 85748 Garching bei München, Germany
³⁴ Dipartimento di Fisica e Astronomia “Galileo Galilei”, Universita degli Studi di Padova, Vicolo dell’Osservatorio 3, 35122 Padova, Italy
³⁵ INAF, Osservatorio Astrofísico di Catania, Via S. Sofia 78, 95123 Catania, Italy
³⁶ Astrophysics Group, Keele University, Staffordshire ST5 5BG, UK
³⁷ Admatis, 5. Kandó Kálmán Street, 3534 Miskolc, Hungary
³⁸ Konkoly Observatory, Research Centre for Astronomy and Earth Sciences, 1121 Budapest, Konkoly Thege Miklós út 15–17, Hungary
³⁹ Institut d’astrophysique de Paris, UMR7095 CNRS, Université Pierre-et-Marie-Curie, 98bis blvd. Arago, 75014 Paris, France
⁴⁰ Centre for Mathematical Sciences, Lund University, Box 118, 22100 Lund, Sweden
⁴¹ Department of Physics, University of Warwick, Gibbet Hill Road, Coventry CV4 7AL, UK
⁴² Center for Astronomy and Astrophysics, Technical University Berlin, Hardenberstrasse 36, 10623 Berlin, Germany
⁴³ Institute of Astronomy, University of Cambridge, Madingley Road, Cambridge CB3 0HA, UK
⁴⁴ INAF, Osservatorio Astrofísico di Torino, Via Osservatorio 20, 10025 Pino Torinese, TO, Italy
⁴⁵ Department of Space and Climate Physics, Mullard Space Science Laboratory, Holmbury St Mary RH5 6NT, UK
⁴⁶ Brorfelde Observatory, Observator Gyldenkernes Vej 7, 4340 Tøl-løse, Denmark

Received: 19 January 2023
Accepted: 5 April 2023

Abstract

Context. Multi-planet systems are important sources of information regarding the evolution of planets. However, the long-period planets in these systems often escape detection. These objects in particular may retain more of their primordial characteristics compared to close-in counterparts because of their increased distance from the host star. HD 22946 is a bright (G = 8.13 mag) late F-type star around which three transiting planets were identified via Transiting Exoplanet Survey Satellite (TESS) photometry, but the true orbital period of the outermost planet d was unknown until now.

Aims. We aim to use the Characterising Exoplanet Satellite (CHEOPS) space telescope to uncover the true orbital period of HD 22946d and to refine the orbital and planetary properties of the system, especially the radii of the planets.

Methods. We used the available TESS photometry of HD 22946 and observed several transits of the planets b, c, and d using CHEOPS. We identified two transits of planet d in the TESS photometry, calculated the most probable period aliases based on these data, and then scheduled CHEOPS observations. The photometric data were supplemented with ESPRESSO (Echelle SPectrograph for Rocky Exoplanets and Stable Spectroscopic Observations) radial velocity data. Finally, a combined model was fitted to the entire dataset in order to obtain final planetary and system parameters.

Results. Based on the combined TESS and CHEOPS observations, we successfully determined the true orbital period of the planet d to be 47.42489 ± 0.00011 days, and derived precise radii of the planets in the system, namely 1.362 ± 0.040 R_⊕, 2.328 ± 0.039 R_⊕, and 2.607 ± 0.060 R_⊕ for planets b, c, and d, respectively. Due to the low number of radial velocities, we were only able to determine 3σ upper limits for these respective planet masses, which are 13.71 M_⊕, 9.72 M_⊕, and 26.57 M_⊕. We estimated that another 48 ESPRESSO radial velocities are needed to measure the predicted masses of all planets in HD 22946. We also derived stellar parameters for the host star.

Conclusions. Planet c around HD 22946 appears to be a promising target for future atmospheric characterisation via transmission spectroscopy. We can also conclude that planet d, as a warm sub-Neptune, is very interesting because there are only a few similar confirmed exoplanets to date. Such objects are worth investigating in the near future, for example in terms of their composition and internal structure.