We report the discovery and characterisation of a pair of sub-Neptunes transiting the bright K-dwarf TOI-1064 (TIC 79748331), initially detected in TESS photometry. To characterise the system, we performed and retrieved CHEOPS, TESS, and ground-based photometry, HARPS high-resolution spectroscopy, and Gemini speckle imaging. We characterise the host star and determine \(T_\mathrm{eff,$\ast$}=4734\pm67\) K, \(R_\ast=0.726\pm0.007\) \(R_\odot\), and \(M_\ast=0.748\pm 0.032\) \(M_\odot\). We present a novel detrending method based on PSF shape-change modelling and demonstrate its suitability to correct flux variations in CHEOPS data. We confirm the planetary nature of both bodies and find that TOI-1064 b has an orbital period of \(P_b=6.44387\pm 0.00003\) d, a radius of \(R_b=2.59\pm0.04\) \(R_\oplus\), and a mass of \(M_b=13.5^{+1.7}_{-1.8}\) \(M_\oplus\), whilst TOI-1064 c has an orbital period of \(P_c=12.22657^{+0.00005}_{-0.00004}\) d, a radius of \(R_c=2.65\pm 0.04\) \(R_\oplus\), and a \(3\sigma\) upper mass limit of 8.5 \(M_\oplus\). From the high-precision photometry we obtain radius uncertainties of ∼1.6%, allowing us to conduct internal structure and atmospheric escape modelling. TOI-1064 b is one of the densest, well-characterised sub-Neptunes, with a tenuous atmosphere that can be explained by the loss of a primordial envelope following migration through the protoplanetary disc. It is likely that TOI-1064 c has an extended atmosphere due to the tentative low density, however further RVs are needed to confirm this scenario and the similar radii, different masses nature of this system. The high-precision data and modelling of TOI-1064 b are important for planets in this region of mass-radius space, and it allows us to identify a trend in bulk density-stellar metallicity for massive sub-Neptunes that may hint at the formation of this population of planets.
Authors: Thomas G. Wilson, Elisa Goffo, Yann Alibert, Davide Gandolfi, Andrea Bonfanti, Carina M. Persson, Andrew Collier Cameron, Malcolm Fridlund, Luca Fossati, Judith Korth, Willy Benz, Adrien Deline, Hans-Gustav Florén, Pascal Guterman, Vardan Adibekyan, Matthew J. Hooton, Sergio Hoyer, Adrien Leleu, Alexander James Mustill, Sébastien Salmon, Sérgio G. Sousa, Olga Suarez, Lyu Abe, Abdelkrim Agabi, Roi Alonso, Guillem Anglada, Joel Asquier, Tamas Bárczy, David Barrado y Navascues, Susana C. C. Barros, Wolfgang Baumjohann, Mathias Beck, Thomas Beck, Nicolas Billot, Xavier Bonfils, Alexis Brandeker, Christopher Broeg, Edward M. Bryant, Matthew R. Burleigh, Marco Buttu, Juan Cabrera, Sébastien Charnoz, David R. Ciardi, Ryan Cloutier, William D. Cochran, Karen A. Collins, Knicole D. Colón, Nicolas Crouzet, Szilard Csizmadia, Melvyn B. Davies, Magali Deleuil, Laetitia Delrez, Olivier Demangeon, Brice-Olivier Demory, Diana Dragomir, Georgina Dransfield, David Ehrenreich, Anders Erikson, Andrea Fortier, Tianjun Gan, Samuel Gill, Michaël Gillon, Crystal L. Gnilka, Nolan Grieves, Sascha Grziwa, Manuel Güdel, Tristan Guillot, Jonas Haldemann, Kevin Heng, Keith Horne, Steve B. Howell, Kate G. Isaak, Jon M. Jenkins, Eric L. N. Jensen, Laszlo Kiss, Gaia Lacedelli, Kristine Lam, Jacques Laskar, David W. Latham, Alain Lecavelier des Etangs, Monika Lendl, Kathryn V. Lester, Alan M. Levine, John Livingston, Christophe Lovis, Rafael Luque, Demetrio Magrin, Wenceslas Marie-Sainte, Pierre F. L. Maxted, Andrew W. Mayo, Brian McLean, Marko Mecina, Djamel Mékarnia, Valerio Nascimbeni, Louise D. Nielsen, Göran Olofsson, Hugh P. Osborn, Hannah L. M. Osborne, Roland Ottensamer, Isabella Pagano , Enric Pallé, Gisbert Peter, Giampaolo Piotto, Don Pollacco, Didier Queloz, Roberto Ragazzoni, Nicola Rando, Heike Rauer, Seth Redfield, Ignasi Ribas, George R. Ricker, Martin Rieder, Nuno C. Santos, Gaetano Scandariato, François-Xavier Schmider, Richard P. Schwarz, Nicholas J. Scott, Sara Seager, Damien Ségransan, Luisa Maria Serrano, Attila E. Simon, Alexis M. S. Smith, Manfred Steller, Chris Stockdale, Gyula Szabó, Nicolas Thomas, Eric B. Ting, Amaury H. M. J. Triaud, Stéphane Udry, Vincent Van Eylen, Valérie Van Grootel, Roland K. Vanderspek, Valentina Viotto, Nicholas Walton, Joshua N. Winn