Characterisation of the TOI-421 planetary system using CHEOPS, TESS, and archival radial velocity data^★,★★

¹ Space Research Institute, Austrian Academy of Sciences, Schmiedl-strasse 6, 8042 Graz, Austria
e-mail: andreas.krenn@oeaw.ac.at
² Weltraumforschung und Planetologie, Physikalisches Institut, University of Bern, Gesellschaftsstrasse 6, 3012 Bern, Switzerland
³ Observatoire Astronomique de l’Université de Genève, Chemin Pegasi 51, 1290 Versoix, Switzerland
⁴ Centre Vie dans l’Univers, Faculté des sciences, Université de Genève, Quai Ernest-Ansermet 30, 1211 Genève 4, Switzerland
⁵ Centre for Space and Habitability, University of Bern, Gesellschaftsstrasse 6, 3012 Bern, Switzerland
⁶ Institute of Planetary Research, German Aerospace Centre (DLR), Rutherfordstrasse 2, 12489 Berlin, Germany
⁷ Centre for Exoplanet Science, SUPA School of Physics and Astronomy, University of St Andrews, North Haugh, St Andrews KY16 9SS, UK
⁸ Instituto de Astrofisica e Ciencias do Espaco, Universidade do Porto, CAUP, Rua das Estrelas, 4150-762 Porto, Portugal
⁹ CFisUC, Departamento de Física, Universidade de Coimbra, 3004-516 Coimbra, Portugal
¹⁰ Astrobiology Research Unit, Université de Liège, Allée du 6 Août 19C, 4000 Liège, Belgium
¹¹ Space sciences, Technologies and Astrophysics Research (STAR) Institute, Université de Liège, Allée du 6 Août 19C, 4000 Liège, Belgium
¹² Department of Astronomy, Stockholm University, AlbaNova University Centre, 10691 Stockholm, Sweden
¹³ Instituto de Astrofisica de Canarias, Via Lactea s/n, 38200 La Laguna, Tenerife, Spain
¹⁴ Departamento de Astrofisica, Universidad de La Laguna, Astrofísico Francisco Sanchez s/n, 38206 La Laguna, Tenerife, Spain
¹⁵ Institut de Ciencies de l’Espai (ICE, CSIC), Campus UAB, Can Magrans s/n, 08193 Bellaterra, Spain
¹⁶ Institut d’Estudis Espacials de Catalunya (IEEC), Gran Capità 2-1-, 08034 Barcelona, Spain
¹⁷ European Space Agency (ESA), European Space Research and Technology Centre (ESTEC), Keplerlaan 1, 2201 AZ Noordwijk, The Netherlands
¹⁸ Admatis, 5. Kandó Kálmán Street, 3534 Miskolc, Hungary
¹⁹ Depto. de Astrofisica, Centro de Astrobiologia (CSIC-INTA), ESAC campus, 28692 Villanueva de la Cañada, Madrid, Spain
²⁰ Departamento de Fisica e Astronomia, Faculdade de Ciencias, Universidade do Porto, Ruado Campo Alegre, 4169-007 Porto, Portugal
²¹ Université Grenoble Alpes, CNRS, IPAG, 38000 Grenoble, France
²² INAF, Osservatorio Astronomico di Padova, Vicolo dell’Osservatorio 5, 35122 Padova, Italy
²³ Université de Paris Cité, Institut de physique du globe de Paris, CNRS, 1 Rue Jussieu, 75005 Paris, France
²⁴ INAF, Osservatorio Astrofisico di Torino, Via Osservatorio, 20, 10025 Pino Torinese To, Italy
²⁵ Centre for Mathematical Sciences, Lund University, Box 118, 221 00 Lund, Sweden
²⁶ Aix Marseille Univ, CNRS, CNES, LAM, 38 rue Frédéric Joliot-Curie, 13388 Marseille, France
²⁷ Leiden Observatory, University of Leiden, PO Box 9513, 2300 RA Leiden, The Netherlands
²⁸ Department of Space, Earth and Environment, Chalmers University of Technology, Onsala Space Observatory, 439 92 Onsala, Sweden
²⁹ Dipartimento di Fisica, Università degli Studi di Torino, via Pietro Giuria 1, 10125, Torino, Italy
³⁰ Department of Astrophysics, University of Vienna, Türkenschanzstrasse 17, 1180 Vienna, Austria
³¹ Institute for Theoretical Physics and Computational Physics, Graz University of Technology, Petersgasse 16, 8010 Graz, Austria
³² Konkoly Observatory, Research Centre for Astronomy and Earth Sciences, 1121 Budapest, Konkoly Thege Miklós út 15–17, Hungary
³³ ELTE Eötvös Loránd University, Institute of Physics, Pázmány Péter sétány 1/A, 1117 Budapest, Hungary
³⁴ IMCCE, UMR8028 CNRS, Observatoire de Paris, PSL Univ., Sorbonne Univ., 77 av. Denfert-Rochereau, 75014 Paris, France
³⁵ Institut d’astrophysique de Paris, UMR7095 CNRS, Université Pierre & Marie Curie, 98bis blvd. Arago, 75014 Paris, France
³⁶ Astrophysics Group, Lennard Jones Building, Keele University, Staffordshire ST5 5BG, UK
³⁷ INAF, Osservatorio Astrofisico di Catania, Via S. Sofia 78, 95123 Catania, Italy
³⁸ Institute of Optical Sensor Systems, German Aerospace Centre (DLR), Rutherfordstrasse 2, 12489 Berlin, Germany
³⁹ Dipartimento di Fisica e Astronomia “Galileo Galilei”, Università degli Studi di Padova, Vicolo dell’Osservatorio 3, 35122 Padova, Italy
⁴⁰ Department of Physics, University of Warwick, Gibbet Hill Road, Coventry CV4 7AL, UK
⁴¹ ETH Zurich, Department of Physics, Wolfgang-Pauli-Strasse 2, 8093 Zurich, Switzerland
⁴² Cavendish Laboratory, JJ Thomson Avenue, Cambridge CB3 0HE, UK
⁴³ Zentrum für Astronomie und Astrophysik, Technische Universität Berlin, Hardenbergstr. 36, 10623 Berlin, Germany
⁴⁴ Institut fuer Geologische Wissenschaften, Freie Universitaet Berlin, Maltheserstrasse 74–100, 12249 Berlin, Germany
⁴⁵ Weltraumforschung und Planetologie, Physikalisches Institut, University of Bern, Sidlerstrasse 5, 3012 Bern, Switzerland
⁴⁶ Université de Liège, Allée du 6 Août 19C, 4000 Liège, Belgium
⁴⁷ ELTE Eötvös Loránd University, Gothard Astrophysical Observatory, 9700 Szombathely, Szent Imre h. u. 112, Hungary
⁴⁸ MTA-ELTE Exoplanet Research Group, 9700 Szombathely, Szent Imre h. u. 112, Hungary
⁴⁹ Institute of Astronomy, University of Cambridge, Madingley Road, Cambridge CB3 0HA, UK

Received: 13 November 2023
Accepted: 27 March 2024

Abstract

Context. The TOI-421 planetary system contains two sub-Neptune-type planets (P_b ~ 5.2 days, T_eq,_b ~ 900 K, and P_c ~ 16.1 days, T_eq,c ~ 650 K) and is a prime target to study the formation and evolution of planets and their atmospheres. The inner planet is especially interesting as the existence of a hydrogen-dominated atmosphere at its orbital separation cannot be explained by current formation models without previous orbital migration.

Aims. We aim to improve the system parameters to further use them to model the interior structure and simulate the atmospheric evolution of both planets, to finally gain insights into their formation and evolution. We also investigate the possibility of detecting transit timing variations (TTVs).

Methods. We jointly analysed photometric data of three TESS sectors and six CHEOPS visits as well as 156 radial velocity data points to retrieve improved planetary parameters. We also searched for TTVs and modelled the interior structure of the planets. Finally, we simulated the evolution of the primordial H-He atmospheres of the planets using two different modelling frameworks.

Results. We determine the planetary radii and masses of TOI-421 b and c to be R_b = 2.64 ± 0.08 R_⊕, M_b = 6.7 ± 0.6 M_⊕, R_c = 5.09 ± 0.07 R_⊕, and M_c = 14.1 ± 1.4 M_⊕. Using these results we retrieved average planetary densities of ρ_b = 0.37 ± 0.05ρ_⊕ and ρ_c = 0.107 ± 0.012 ρ_⊕. We do not detect any statistically significant TTV signals. Assuming the presence of a hydrogen-dominated atmosphere, the interior structure modelling results in both planets having extensive envelopes. While the modelling of the atmospheric evolution predicts for TOI-421 b to have lost any primordial atmosphere that it could have accreted at its current orbital position, TOI-421 c could have started out with an initial atmospheric mass fraction somewhere between 10 and 35%.

Conclusions. We conclude that the low observed mean density of TOI-421 b can only be explained by either a bias in the measured planetary parameters (e.g. driven by high-altitude clouds) and/or in the context of orbital migration. We also find that the results of atmospheric evolution models are strongly dependent on the employed planetary structure model.