Discovery of two warm mini-Neptunes with contrasting densities orbiting the young K3V star TOI-815

Author(s): Angelica Psaridi, Hugh Osborn, François Bouchy, Monika Lendl, Léna Parc, Nicolas Billot, Christopher Broeg, Sérgio G. Sousa, Vardan Adibekyan, Omar Attia, Andrea Bonfanti, Hritam Chakraborty, Karen A. Collins, Jeanne Davoult, Elisa Delgado-Mena, Nolan Grieves, Tristan Guillot, Alexis Heitzmann, Ravit Helled, Coel Hellier, Jon M. Jenkins, Henrik Knierim, Andreas Krenn, Jack J. Lissauer, Rafael Luque, David Rapetti, Nuno C. Santos, Olga Suárez, Julia Venturini, Francis P. Wilkin, Thomas G. Wilson, Joshua N. Winn, Carl Ziegler, Tiziano Zingales, Yann Alibert, Alexis Brandeker, Jo Ann Egger, Davide Gandolfi, Matthew J. Hooton, Amy Tuson, Solène Ulmer-Moll, Lyu Abe, Romain Allart, Roi Alonso, David R. Anderson, Guillem Anglada Escudé, Tamas Bárczy, David Barrado, Susana C.C. Barros, Wolfgang Baumjohann, Mathias Beck, Thomas Beck, Willy Benz, Xavier Bonfils, Luca Borsato, Vincent Bourrier, David R. Ciardi, Andrew Collier Cameron, Sébastien Charnoz, Marion Cointepas, Szilárd Csizmadia, Patricio Cubillos, Gaspare Lo Curto, Melvyn B. Davies, Tansu Daylan, Magali Deleuil, Adrien Deline, Laetitia Delrez, Olivier D.S. Demangeon, Brice Olivier Demory, Caroline Dorn, Xavier Dumusque, David Ehrenreich, Anders Erikson, Alain Lecavelier des Etangs, Diana de Miguel, Andrea Fortier, Luca Fossati, Yolanda G.C. Frensch, Malcolm Fridlund, Michaël Gillon, Manuel Güdel, Maximilian N. Günther, Janis Hagelberg, Christiane Helling, Sergio Hoyer, Kate G. Isaak, Laszlo L. Kiss, Kristine W.F. Lam, Jacques Laskar, Baptiste Lavie, Christophe Lovis, Demetrio Magrin, Luca Marafatto, Pierre Maxted, Scott McDermott, Djamel Mékarnia, Christoph Mordasini, Felipe Murgas, Valerio Nascimbeni, Louise D. Nielsen, Göran Olofsson, Roland Ottensamer, Isabella Pagano, Enric Pallé, Gisbert Peter, Giampaolo Piotto, Don Pollacco, Didier Queloz, Roberto Ragazzoni, Devin Ramos, Nicola Rando, Heike Rauer, Christian Reimers, Ignasi Ribas, Sara Seager, Damien Ségransan, Gaetano Scandariato, Attila E. Simon, Alexis M.S. Smith, Manu Stalport, Manfred Steller, Gyula Szabó, Nicolas Thomas, Tyler A. Pritchard, Stéphane Udry, Carlos Corral Van Damme, Valérie Van Grootel, Eva Villaver, Ingo Walter, Nicholas Walton, Cristilyn N. Watkins, Richard G. West
Abstract: We present the discovery and characterization of two warm mini-Neptunes transiting the K3V star TOI-815 in a K–M binary system. Analysis of its spectra and rotation period reveal the star to be young, with an age of 200⁺₋⁴⁰⁰₂₀₀ Myr. TOI-815b has a 11.2-day period and a radius of 2.94 ± 0.05 R₊ with transits observed by TESS, CHEOPS, ASTEP, and LCOGT. The outer planet, TOI-815c, has a radius of 2.62 ± 0.10 R₊, based on observations of three nonconsecutive transits with TESS; targeted CHEOPS photometry and radial velocity follow-up with ESPRESSO were required to confirm the 35-day period. ESPRESSO confirmed the planetary nature of both planets and measured masses of 7.6 ± 1.5 M₊ (ρ_P = 1.64⁺₋⁰₀³³₃₁ g cm⁻³) and 23.5 ± 2.4 M₊ (ρ_P = 7.2⁺₋¹₁¹₀ g cm⁻³), respectively. Thus, the planets have very different masses, which is unusual for compact multi-planet systems. Moreover, our statistical analysis of mini-Neptunes orbiting FGK stars suggests that weakly irradiated planets tend to have higher bulk densities compared to those undergoing strong irradiation. This could be ascribed to their cooler atmospheres, which are more compressed and denser. Internal structure modeling of TOI-815b suggests it likely has a H-He atmosphere that constitutes a few percent of the total planet mass, or higher if the planet is assumed to have no water. In contrast, the measured mass and radius of TOI-815c can be explained without invoking any atmosphere, challenging planetary formation theories. Finally, we infer from our measurements that the star is viewed close to pole-on, which implies a spin-orbit misalignment at the 3σ level. This emphasizes the peculiarity of the system’s orbital architecture, and probably hints at an eventful dynamical history.
Organisation(s): Department of Astrophysics
External organisation(s): Université de Genève, Universität Bern, Massachusetts Institute of Technology, Universidade do Porto, Österreichische Akademie der Wissenschaften (ÖAW), Harvard-Smithsonian Center for Astrophysics, Université Côte d'Azur, Universität Zürich (UZH), Keele University, National Aeronautics & Space Administration (NASA), University of Chicago, Universities Space Research Association, Union College, University of Warwick, Princeton University, Stephen F. Austin State University, University of Padova, Osservatorio Astronomico, Stockholm University, University of Turin, University of Cambridge, University of Montreal, Institute of Astrophysics of the Canary Islands, University of La Laguna, Spanish National Research Council (CSIC), Institute of Space Studies of Catalonia (IEEC), Admatis Ltd., European Space Astronomy Centre (ESA), University of Grenoble Alpes, California Institute of Technology (Caltech), University of St. Andrews, Université de Paris, Deutsches Zentrum für Luft- und Raumfahrt e.V. (DLR), European Southern Observatory (Germany), Lund University, McDonnell Center for the Space Sciences, Aix-Marseille Université, Université de Liège, Eidgenössische Technische Hochschule Zürich, Université Paris VI - Pierre-et-Marie-Curie, Airbus Defence and Space, Leiden University, Chalmers University of Technology, Science and Operations Department - Science Division (SCI-SC), Technische Universität Graz, Eötvös Loránd Research Network, Eötvös Loránd University Budapest, Université de recherche Paris Sciences et Lettres, Proto-Logic LLC, INAF-Osservatorio Astrofisico di Catania, Technische Universität Berlin, Freie Universität Berlin (FU), HUN-REN Hungarian Research Network
Journal: Astronomy and Astrophysics
Volume: 685
No. of pages: 24
ISSN: 0004-6361
DOI: https://doi.org/10.1051/0004-6361/202348678
Publication date: 05-2024
Peer reviewed: Yes
Austrian Fields of Science 2012: 103003 Astronomy, 103004 Astrophysics, 103038 Space exploration
Keywords
ASJC Scopus subject areas: Astronomy and Astrophysics, Space and Planetary Science
Portal url: https://ucrisportal.univie.ac.at/en/publications/e9416ed6-2779-4e6b-a4f6-fa8b06c6b45e

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