Refining the properties of the TOI-178 system with CHEOPS and TESS^★,★★

¹ Astrobiology Research Unit, Université de Liège, 19C Allée du Six-Août, 4000 Liège, Belgium
e-mail: ldelrez@uliege.be
² Space sciences, Technologies and Astrophysics Research (STAR) Institute, Université de Liège, 19C Allée du Six-Août, 4000 Liège, Belgium
³ Observatoire Astronomique de l’Université de Genève, Chemin Pegasi 51, 1290 Versoix, Switzerland
⁴ Physikalisches Institut, University of Bern, Sidlerstrasse 5, 3012 Bern, Switzerland
⁵ Department of Astronomy, Stockholm University, AlbaNova University Center, 10691 Stockholm, Sweden
⁶ Cavendish Laboratory, JJ Thomson Avenue, Cambridge CB3 0HE, UK
⁷ Centre for Exoplanet Science, SUPA School of Physics and Astronomy, University of St Andrews, North Haugh, St Andrews KY16 9SS, UK
⁸ Center for Space and Habitability, University of Bern, Gesellschaftsstrasse 6, 3012 Bern, Switzerland
⁹ ETH Zurich, Department of Physics, Wolfgang-Pauli-Strasse 2, 8093 Zurich, Switzerland
¹⁰ Space Research Institute, Austrian Academy of Sciences, Schmiedlstrasse 6, 8042 Graz, Austria
¹¹ Instituto de Astrofisica de Canarias, 38200 La Laguna, Tenerife, Spain
¹² Departamento de Astrofisica, Universidad de La Laguna, 38206 La Laguna, Tenerife, Spain
¹³ Institut de Ciencies de l’Espai (ICE, CSIC), Campus UAB, Can Magrans s/n, 08193 Bellaterra, Spain
¹⁴ Institut d’Estudis Espacials de Catalunya (IEEC), 08034 Barcelona, Spain
¹⁵ ESTEC, European Space Agency, 2201AZ, Noordwijk, The Netherlands
¹⁶ Admatis, 5. Kandó Kálmán Street, 3534 Miskolc, Hungary
¹⁷ Depto. de Astrofisica, Centro de Astrobiologia (CSIC-INTA), ESAC campus, 28692 Villanueva de la Cañada (Madrid), Spain
¹⁸ Instituto de Astrofisica e Ciencias do Espaco, Universidade do Porto, CAUP, Rua das Estrelas, 4150-762 Porto, Portugal
¹⁹ Departamento de Fisica e Astronomia, Faculdade de Ciencias, Universidade do Porto, Rua do Campo Alegre, 4169-007 Porto, Portugal
²⁰ Université Grenoble Alpes, CNRS, IPAG, 38000 Grenoble, France
²¹ INAF, Osservatorio Astronomico di Padova, Vicolo dell’Osservatorio 5, 35122 Padova, Italy
²² Institute of Optical Sensor Systems, German Aerospace Center (DLR), Rutherfordstrasse 2, 12489 Berlin, Germany
²³ Institute of Planetary Research, German Aerospace Center (DLR), Rutherfordstrasse 2, 12489 Berlin, Germany
²⁴ Université de Paris, Institut de physique du globe de Paris, CNRS, 75005 Paris, France
²⁵ INAF, Osservatorio Astrofisico di Torino, Via Osservatorio, 20, 10025 Pino Torinese To, Italy
²⁶ Centre for Mathematical Sciences, Lund University, Box 118, 221 00 Lund, Sweden
²⁷ Aix Marseille Univ., CNRS, CNES, LAM, 38 rue Frédéric Joliot- Curie, 13388 Marseille, France
²⁸ Centre Vie dans l’Univers, Faculté des sciences, Université de Genève, Quai Ernest-Ansermet 30, 1211 Genève 4, Switzerland
²⁹ Leiden Observatory, University of Leiden, PO Box 9513, 2300 RA Leiden, The Netherlands
³⁰ Department of Space, Earth and Environment, Chalmers University of Technology, Onsala Space Observatory, 439 92 Onsala, Sweden
³¹ Dipartimento di Fisica, Universita degli Studi di Torino, via Pietro Giuria 1, 10125, Torino, Italy
³² University of Vienna, Department of Astrophysics, Türkenschanzstrasse 17, 1180 Vienna, Austria
³³ Science and Operations Department - Science Division (SCI-SC), Directorate of Science, European Space Agency (ESA), European Space Research and Technology Centre (ESTEC), Keplerlaan 1, 2201-AZ Noordwijk, The Netherlands
³⁴ NASA Ames Research Center, Moffett Field, CA 94035, USA
³⁵ Konkoly Observatory, Research Centre for Astronomy and Earth Sciences, 1121 Budapest, Konkoly Thege Miklós út 15-17, Hungary
³⁶ ELTE Eötvös Loránd University, Institute of Physics, Pázmány Péter sétány 1/A, 1117 Budapest, Hungary
³⁷ IMCCE, UMR8028 CNRS, Observatoire de Paris, PSL Univ., Sorbonne Univ., 77 av. Denfert-Rochereau, 75014 Paris, France
³⁸ Center for Astrophysics, Harvard & Smithsonian, 60 Garden Street, Cambridge, MA, 02138, USA
³⁹ Institut d’astrophysique de Paris, UMR7095 CNRS, Université Pierre & Marie Curie, 98bis Bvd. Arago, 75014 Paris, France
⁴⁰ Department of Astronomy and Astrophysics, University of Chicago, Chicago, IL 60637, USA
⁴¹ Astrophysics Group, Keele University, Staffordshire, ST5 5BG, UK
⁴² Department of Astrophysics, University of Vienna, Tuerkenschanzstrasse 17, 1180 Vienna, Austria
⁴³ INAF, Osservatorio Astrofisico di Catania, Via S. Sofia 78, 95123 Catania, Italy
⁴⁴ Dipartimento di Fisica e Astronomia “Galileo Galilei”, Universita degli Studi di Padova, Vicolo dell’Osservatorio 3, 35122 Padova, Italy
⁴⁵ Department of Physics, University of Warwick, Gibbet Hill Road, Coventry CV4 7AL, UK
⁴⁶ Zentrum für Astronomie und Astrophysik, Technische Universität Berlin, Hardenbergstr. 36, 10623 Berlin, Germany
⁴⁷ Institut für Geologische Wissenschaften, Freie Universität Berlin, 12249 Berlin, Germany
⁴⁸ Kavli Institute for Astrophysics and Space Research, Massachusetts Institute of Technology, Cambridge, MA 02139, USA
⁴⁹ Department of Physics, Massachusetts Institute of Technology, Cambridge, MA 02139, USA
⁵⁰ Department of Earth, Atmospheric and Planetary Science, Massachusetts Institute of Technology, 77 Massachusetts Avenue, Cambridge, MA 02139, USA
⁵¹ Department of Aeronautics and Astronautics, MIT, 77 Massachusetts Avenue, Cambridge, MA 02139, USA
⁵² ELTE Eötvös Loránd University, Gothard Astrophysical Observatory, 9700 Szombathely, Szent Imre h. u. 112, Hungary
⁵³ MTA-ELTE Exoplanet Research Group, 9700 Szombathely, Szent Imre h. u. 112, Hungary
⁵⁴ Institute of Astronomy, University of Cambridge, Madingley Road, Cambridge, CB3 0HA, UK
⁵⁵ Department of Astrophysical Sciences, Princeton University, 4 Ivy Lane, Princeton, NJ 08544, USA

Received: 16 November 2022
Accepted: 21 August 2023

Abstract

Context. The TOI-178 system consists of a nearby late K-dwarf transited by six planets in the super-Earth to mini-Neptune regime, with radii ranging from ~1.1 to 2.9 R_⊕ and orbital periods between 1.9 and 20.7 days. All planets but the innermost one form a chain of Laplace resonances. Mass estimates derived from a preliminary radial velocity (RV) dataset suggest that the planetary densities do not decrease in a monotonic way with the orbital distance to the star, contrary to what one would expect based on simple formation and evolution models.

Aims. To improve the characterisation of this key system and prepare for future studies (in particular with JWST), we performed a detailed photometric study based on 40 new CHEOPS visits, one new TESS sector, and previously published CHEOPS, TESS, and NGTS data.

Methods. First we updated the parameters of the host star using the new parallax from Gaia EDR3. We then performed a global analysis of the 100 transits contained in our data to refine the physical and orbital parameters of the six planets and study their transit timing variations (TTVs). We also used our extensive dataset to place constraints on the radii and orbital periods of potential additional transiting planets in the system.

Results. Our analysis significantly refines the transit parameters of the six planets, most notably their radii, for which we now obtain relative precisions of ≲3%, with the exception of the smallest planet, b, for which the precision is 5.1%. Combined with the RV mass estimates, the measured TTVs allow us to constrain the eccentricities of planets c to g, which are found to be all below 0.02, as expected from stability requirements. Taken alone, the TTVs also suggest a higher mass for planet d than that estimated from the RVs, which had been found to yield a surprisingly low density for this planet. However, the masses derived from the current TTV dataset are very prior-dependent, and further observations, over a longer temporal baseline, are needed to deepen our understanding of this iconic planetary system.