Uncovering the true periods of the young sub-Neptunes orbiting TOI-2076^★

¹ Physikalisches Institut, University of Bern, Gesellsschaftstrasse 6, 3012 Bern, Switzerland
e-mail: hugh.osborn@unibe.ch
² Department of Physics and Kavli Institute for Astrophysics and Space Research, Massachusetts Institute of Technology, Cambridge, MA 02139, USA
³ Austrian Academy of Sciences, Schmiedlstrasse 6, 8042 Graz, Austria
⁴ Dipartimento di Fisica, Universita degli Studi di Torino, via Pietro Giuria 1, 10125 Torino, Italy
⁵ Bay Area Environmental Research Institute, PO Box 25, Moffett Field, CA 94035, USA
⁶ NASA Ames Research Center, Moffett Field, CA 94035, USA
⁷ Observatoire Astronomique de l’Université de Genève, Chemin Pegasi 51, Versoix, Switzerland
⁸ Center for Space and Habitability, University of Bern, Gesellschaftsstrasse 6, 3012 Bern, Switzerland
⁹ Aix Marseille Univ., CNRS, CNES, LAM, 38 rue Frédéric Joliot-Curie, 13388 Marseille, France
¹⁰ Division Technique INSU, CS20330, 83507 La Seyne sur Mer cedex, France
¹¹ Astrophysics Group, Keele University, Staffordshire ST5 5BG, UK
¹² INAF, Osservatorio Astronomico di Padova, Vicolo dell’Osservatorio 5, 35122 Padova, Italy
¹³ Center for Astrophysics – Harvard & Smithsonian, 60 Garden Street, Cambridge, MA 02138, USA
¹⁴ Instituto de Astrofisica e Ciencias do Espaco, Universidade do Porto, CAUP, Rua das Estrelas, 4150-762 Porto, Portugal
¹⁵ Universidad Nacional Autónoma de México, Instituto de Astronomía, AP 70-264, CDMX 04510, Mexico
¹⁶ Instituto de Astrofisica de Canarias, 38200 La Laguna, Tenerife, Spain
¹⁷ Dept. Astrofisica, Universidad de La Laguna (ULL), 38206 La Laguna, Tenerife, Spain
¹⁸ Astrophysics Group, Cavendish Laboratory, University of Cambridge, J.J. Thomson Avenue, Cambridge CB3 0HE, UK
¹⁹ Space sciences, Technologies and Astrophysics Research (STAR) Institute, Université de Liège, 19C Allée du 6 Août, 4000 Liège, Belgium
²⁰ Centre for Exoplanet Science, SUPA School of Physics and Astronomy, University of St Andrews, North Haugh, St Andrews KY16 9SS, UK
²¹ Instituto de Astrofisica de Canarias, 38200 La Laguna, Tenerife, Spain
²² Departamento de Astrofisica, Universidad de La Laguna, 38206 La Laguna, Tenerife, Spain
²³ Institut de Ciencies de l’Espai (ICE, CSIC), Campus UAB, Can Magrans s/n, 08193 Bellaterra, Spain
²⁴ Institut d’Estudis Espacials de Catalunya (IEEC), 08034 Barcelona, Spain
²⁵ ESTEC, European Space Agency, 2201AZ Noordwijk, The Netherlands
²⁶ Depto. de Astrofisica, Centro de Astrobiologia (CSIC-INTA), ESAC campus, 28692 Villanueva de la Canada (Madrid), Spain
²⁷ Space Research Institute, Austrian Academy of Sciences, Schmiedlstrasse 6, 8042 Graz, Austria
²⁸ Max Planck Institut für Extraterrestrische Physik. Gießenbachstraße 1, 85748 Garching bei München, Germany
²⁹ INAF – Osservatorio Astronomico di Padova. Vicolo Osservatorio 5, 35122 Padova, Italy
³⁰ Université Grenoble Alpes, CNRS, IPAG, 38000 Grenoble, France
³¹ Department of Astronomy, Stockholm University, AlbaNova University Center, 10691 Stockholm, Sweden
³² Admatis, 5. Kandó Kálmán Street, 3534 Miskolc, Hungary
³³ Departamento de Fisica e Astronomia, Faculdade de Ciencias, Universidade do Porto, Rua do Campo Alegre, 4169-007 Porto, Portugal
³⁴ Institute of Planetary Research, German Aerospace Center (DLR), Rutherfordstrasse 2, 12489 Berlin, Germany
³⁵ Université de Paris, Institut de physique du globe de Paris, CNRS, 75005 Paris, France
³⁶ Centre for Mathematical Sciences, Lund University, Box 118, 22100 Lund, Sweden
³⁷ Astrobiology Research Unit, Université de Liège, Allée du 6 Août 19C, 4000 Liège, Belgium
³⁸ Space sciences, Technologies and Astrophysics Research (STAR) Institute, Université de Liège, Allée du 6 Août 19C, 4000 Liège, Belgium
³⁹ Center for Space and Habitability, Gesellsschaftstrasse 6, 3012 Bern, Switzerland
⁴⁰ Leiden Observatory, University of Leiden, PO Box 9513, 2300 RA Leiden, The Netherlands
⁴¹ Department of Space, Earth and Environment, Chalmers University of Technology, Onsala Space Observatory, 43992 Onsala, Sweden
⁴² Universidad Nacional Autónoma de México, Instituto de Astronomía, AP 106, Ensenada 22800 BC, Mexico
⁴³ Department of Astrophysics, University of Vienna, Tuerkenschanzstrasse 17, 1180 Vienna, Austria
⁴⁴ Department of Physics, University of Warwick, Gibbet Hill Road, Coventry CV4 7AL, UK
⁴⁵ 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
⁴⁶ Konkoly Observatory, Research Centre for Astronomy and Earth Sciences, 1121 Budapest, Konkoly Thege Miklós út 15–17, Hungary
⁴⁷ ELTE Eötvös Loránd University, Institute of Physics, Pázmány Péter sétány 1/A, 1117 Budapest, Hungary
⁴⁸ IMCCE, UMR8028 CNRS, Observatoire de Paris, PSL Univ., Sorbonne Univ., 77 av. Denfert-Rochereau, 75014 Paris, France
⁴⁹ Institut d’astrophysique de Paris, UMR7095 CNRS, Université Pierre & Marie Curie, 98bis blvd. Arago, 75014 Paris, France
⁵⁰ Dipartimento di Fisica e Astronomia “Galileo Galilei”, Universita degli Studi di Padova, Vicolo dell’Osservatorio 3, 35122 Padova, Italy
⁵¹ INAF, Osservatorio Astrofisico di Catania, Via S. Sofia 78, 95123 Catania, Italy
⁵² Deptartamento de Astrofisica, Universidad de La Laguna (ULL), 38206 La Laguna, Tenerife, Spain
⁵³ Cavendish Laboratory, JJ Thomson Avenue, Cambridge CB3 0HE, UK
⁵⁴ Center for Astronomy and Astrophysics, Technical University Berlin, Hardenberstrasse 36, 10623 Berlin, Germany
⁵⁵ Patashnick Voorheesville Observatory, Voorheesville, NY 12186, USA
⁵⁶ ELTE Eötvös Loránd University, Gothard Astrophysical Observatory, 9700 Szombathely, Szent Imre h. u. 112, Hungary
⁵⁷ MTA-ELTE Exoplanet Research Group, 9700 Szombathely, Szent Imre h. u. 112, Hungary
⁵⁸ Institute of Optical Sensor Systems, German Aerospace Center (DLR), Rutherfordstraße 2, 12489 Berlin, Germany
⁵⁹ Institute of Astronomy, University of Cambridge, Madingley Road, Cambridge CB3 0HA, UK

Received: 7 January 2022
Accepted: 1 March 2022

Abstract

Context. TOI-2076 is a transiting three-planet system of sub-Neptunes orbiting a bright (G = 8.9 mag), young (340 ± 80 Myr) K-type star. Although a validated planetary system, the orbits of the two outer planets were unconstrained as only two non-consecutive transits were seen in TESS photometry. This left 11 and 7 possible period aliases for each.

Aims. To reveal the true orbits of these two long-period planets, precise photometry targeted on the highest-probability period aliases is required. Long-term monitoring of transits in multi-planet systems can also help constrain planetary masses through TTV measurements.

Methods. We used the MonoTools package to determine which aliases to follow, and then performed space-based and ground-based photometric follow-up of TOI-2076 c and d with CHEOPS, SAINT-EX, and LCO telescopes.

Results. CHEOPS observations revealed a clear detection for TOI-2076 c at $P={21.02538}_{-0.00074}^{+0.00084}$ d, and allowed us to rule out three of the most likely period aliases for TOI-2076 d. Ground-based photometry further enabled us to rule out remaining aliases and confirm the P = 35.12537 ± 0.00067 d alias. These observations also improved the radius precision of all three sub-Neptunes to 2.518 ± 0.036, 3.497 ± 0.043, and 3.232 ± 0.063 R_⊕. Our observations also revealed a clear anti-correlated TTV signal between planets b and c likely caused by their proximity to the 2:1 resonance, while planets c and d appear close to a 5:3 period commensurability, although model degeneracy meant we were unable to retrieve robust TTV masses. Their inflated radii, likely due to extended H-He atmospheres, combined with low insolation makes all three planets excellent candidates for future comparative transmission spectroscopy with JWST.