CHEOPS observations of the HD 108236 planetary system: a fifth planet, improved ephemerides, and planetary radii^★

¹ Space Research Institute, Austrian Academy of Sciences, Schmiedlstrasse 6, 8042 Graz, Austria
e-mail: andrea.bonfanti@oeaw.ac.at
² Space sciences, Technologies and Astrophysics Research (STAR) Institute, Université de Liège, Allée du six Août 19C, 4000 Liège, Belgium
³ Astrobiology Research Unit, Université de Liège, Allée du six Août 19C, 4000 Liège, Belgium
⁴ Observatoire de Genève, Université de Genève, Chemin des Maillettes 51, 1290 Sauverny, Switzerland
⁵ Physikalisches Institut, University of Bern, Gesellsschaftstrasse 6, 3012 Bern, Switzerland
⁶ School of Physics and Astronomy, Physical Science Building, North Haugh, St Andrews, UK
⁷ Aix Marseille Univ, CNRS, CNES, LAM, Marseille, France
⁸ Lund Observatory, Dept. of Astronomy and Theorical Physics, Lund University, Box 43, 22100 Lund, Sweden
⁹ NCCR/PlanetS, Centre for Space & Habitability, University of Bern, Bern, Switzerland
¹⁰ Department of Physics and Kavli Institute for Astrophysics and Space Research, Massachusetts Institute of Technology, Cambridge, MA 02139, USA
¹¹ Instituto de Astrofísica e Ciências do Espaço, Universidade do Porto, CAUP, Rua das Estrelas, 4150-762 Porto, Portugal
¹² INAF, Osservatorio Astrofisico di Torino, via Osservatorio 20, 10025 Pino Torinese, Italy
¹³ Astrophysics Group, Cavendish Laboratory, J.J. Thomson Avenue, Cambridge CB3 0He, UK
¹⁴ Institute of Planetary Research, German Aerospace Center (DLR), Rutherfordstrasse 2, 12489 Berlin, Germany
¹⁵ INAF, Osservatorio Astronomico di Padova, Vicolo dell’Osservatorio 5, 35122 Padova, Italy
¹⁶ Astrophysics Group, Keele University, Staffordshire, ST5 5BG, UK
¹⁷ Departamento de Física e Astronomia, Faculdade de Ciências, Universidade do Porto, Rua do Campo Alegre, 4169-007 Porto, Portugal
¹⁸ Université Grenoble Alpes, CNRS, IPAG, 38000 Grenoble, France
¹⁹ Leiden Observatory, University of Leiden, PO Box 9513, 2300 RA Leiden, The Netherlands
²⁰ Department of Space, Earth and Environment, Chalmers University of Technology, Onsala Space Observatory, 43992 Onsala, Sweden
²¹ Dipartimento di Fisica e Astronomia “Galileo Galilei”, Universià degli Studi di Padova, Vicolo dell’Osservatorio 3, 35122 Padova, Italy
²² INAF, Osservatorio Astrofisico di Catania, Via S. Sofia 78, 95123 Catania, Italy
²³ ELTE Eötvös Loránd University, Gothard Astrophysical Observatory, 9700 Szombathely, Szent Imre h. u. 112, Hungary
²⁴ MTA-ELTE Exoplanet Research Group, 9700 Szombathely, Szent Imre h. u. 112, Hungary
²⁵ Center for Space and Habitability, Gesellsschaftstrasse 6, 3012 Bern, Switzerland
²⁶ ESTEC, European Space Agency, Keplerlaan 1, 2201 AZ Noordwijk, The Netherlands
²⁷ Instituto de Astrofísica de Canarias (IAC), 38200 La Laguna, Tenerife, Spain
²⁸ Departamento de Astrofísica, Universidad de La Laguna (ULL), 38206 La Laguna, Tenerife, Spain
²⁹ Admatis, Miskok, Hungary
³⁰ Depto. de Astrofísica, Centro de Astrobiologia (CSIC-INTA), ESAC campus, 28692 Villanueva de la Cãda (Madrid), Spain
³¹ Sub-department of Astrophysics, Department of Physics, University of Oxford, Oxford, OX1 3RH, UK
³² Department of Astronomy, Stockholm University, AlbaNova University Center, 10691 Stockholm, Sweden
³³ Institut de Physique du Globe de Paris (IPGP), 1 rue Jussieu, 75005 Paris, France
³⁴ Center for Astronomy and Astrophysics, Technical University Berlin, Hardenberstrasse 36, 10623 Berlin, Germany
³⁵ University of Vienna, Department of Astrophysics, Türkenschanzstrasse 17, 1180 Vienna, Austria
³⁶ Division Technique INSU, BP 330, 83507 La Seyne cedex, France
³⁷ Konkoly Observatory, Research Centre for Astronomy and Earth Sciences, 1121 Budapest, Konkoly Thege Miklós út 15-17, Hungary
³⁸ IMCEE, UMR8028 CNRS, Observatoire de Paris, PSL Univ., Sorbonne Univ., 77 av. Denfert-Rochereau, 75014 Paris, France
³⁹ Institut d’astrophysique de Paris, UMR7095 CNRS, Université Pierre & Marie Curie, 98bis blvd. Arago, 75014 Paris, France
⁴⁰ Institute of Optical Sensor Systems, German Aerospace Center (DLR), Rutherfordstr. 2, 12489 Berlin, Germany
⁴¹ Department of Physics, University of Warwick, Gibbet Hill Road, Coventry CV4 7AL, UK
⁴² Institut für Geologische Wissenschaften, Freie Universität Berlin, 12249 Berlin, Germany
⁴³ Institut de Ciències de l’Espai (ICE, CSIC), Campus UAB, C/CanMagrans s/n, 08193 Bellaterra, Spain
⁴⁴ Institut d’Estudis Espacials deCatalunya (IEEC), Barcelona, Spain
⁴⁵ Italian Space Agency, Via del Politecnico, 00133 Rome, Italy
⁴⁶ Mullard Space Science Laboratory, University College London, Holmbury St. Mary, Dorking, Surrey, RH5 6NT, UK

Received: 7 October 2020
Accepted: 29 December 2020

Abstract

Context. The detection of a super-Earth and three mini-Neptunes transiting the bright (V = 9.2 mag) star HD 108236 (also known as TOI-1233) was recently reported on the basis of TESS and ground-based light curves.

Aims. We perform a first characterisation of the HD 108236 planetary system through high-precision CHEOPS photometry and improve the transit ephemerides and system parameters.

Methods. We characterise the host star through spectroscopic analysis and derive the radius with the infrared flux method. We constrain the stellar mass and age by combining the results obtained from two sets of stellar evolutionary tracks. We analyse the available TESS light curves and one CHEOPS transit light curve for each known planet in the system.

Results. We find that HD 108236 is a Sun-like star with R_⋆ = 0.877 ± 0.008 R_⊙, M_⋆ = 0.869_−0.048^+0.050 M_⊙, and an age of 6.7_−5.1^+4.0 Gyr. We report the serendipitous detection of an additional planet, HD 108236 f, in one of the CHEOPS light curves. For this planet, the combined analysis of the TESS and CHEOPS light curves leads to a tentative orbital period of about 29.5 days. From the light curve analysis, we obtain radii of 1.615 ± 0.051, 2.071 ± 0.052, 2.539_−0.065^+0.062, 3.083 ± 0.052, and 2.017_−0.057^+0.052 R_⊕ for planets HD 108236 b to HD 108236 f, respectively. These values are in agreement with previous TESS-based estimates, but with an improved precision of about a factor of two. We perform a stability analysis of the system, concluding that the planetary orbits most likely have eccentricities smaller than 0.1. We also employ a planetary atmospheric evolution framework to constrain the masses of the five planets, concluding that HD 108236 b and HD 108236 c should have an Earth-like density, while the outer planets should host a low mean molecular weight envelope.

Conclusions. The detection of the fifth planet makes HD 108236 the third system brighter than V = 10 mag to host more than four transiting planets. The longer time span enables us to significantly improve the orbital ephemerides such that the uncertainty on the transit times will be of the order of minutes for the years to come. A comparison of the results obtained from the TESS and CHEOPS light curves indicates that for a V ~ 9 mag solar-like star and a transit signal of ~500 ppm, one CHEOPS transit light curve ensures the same level of photometric precision as eight TESS transits combined, although this conclusion depends on the length and position of the gaps in the light curve.