Discovery of two warm mini-Neptunes with contrasting densities orbiting the young K3V star TOI-815
Authors: Psaridi Angelica , Osborn Hugh , Bouchy François , Lendl Monika , Parc Léna , Billot Nicolas , Broeg Christopher , Sousa Sérgio G. , Adibekyan Vardan , Attia Mara , Bonfanti Andrea , Chakraborty Hritam , Collins Karen A. , Davoult Jeanne , Delgado-Mena Elisa , Grieves Nolan , Guillot Tristan , Heitzmann Alexis , Helled Ravit , Hellier Coel , Jenkins Jon M. , Knierim Henrik , Krenn Andreas , Lissauer Jack J. , Luque Rafael , Rapetti David , Santos Nuno C. , Suárez Olga , Venturini Julia , Wilkin Francis P. , Wilson Thomas G. , Winn Joshua N. , Ziegler Carl , Zingales Tiziano , Alibert Yann , Brandeker Alexis , Egger Jo Ann , Gandolfi Davide , Hooton Matthew J. , Tuson Amy , Ulmer-Moll Solène , Abe Lyu , Allart Romain , Alonso Roi , Anderson David R. , Escudé Guillem Anglada , Bárczy Tamas , Barrado David , Barros Susana C. C. , Baumjohann Wolfgang , Beck Mathias , Beck Thomas , Benz Willy , Bonfils Xavier , Borsato Luca , Bourrier Vincent , Ciardi David R. , Cameron Andrew Collier , Charnoz Sébastien , Cointepas Marion , Csizmadia Szilárd , Cubillos Patricio , Lo Curto Gaspare , Davies Melvyn B. , Daylan Tansu , Deleuil Magali , Deline Adrien , Delrez Laetitia , Demangeon Olivier D. S. , Demory Brice-Olivier , Dorn Caroline , Dumusque Xavier , Ehrenreich David , Erikson Anders , Lecavelier des Etangs Alain , de Miguel Diana , Fortier Andrea , Fossati Luca , Frensch Yolanda G. C. , Fridlund Malcolm , Gillon Michaël , Güdel Manuel , Günther Maximilian N. , Hagelberg Janis , Helling Christiane , Hoyer Sergio , Isaak Kate G. , Kiss Laszlo L. , Lam Kristine W. F. , Laskar Jacques , Lavie Baptiste , Lovis Christophe , Magrin Demetrio , Marafatto Luca , Maxted Pierre , McDermott Scott , Mékarnia Djamel , Mordasini Christoph , Murgas Felipe , Nascimbeni Valerio , Nielsen Louise D. , Olofsson Göran , Ottensamer Roland , Pagano Isabella , Pallé Enric , Peter Gisbert , Piotto Giampaolo , Pollacco Don , Queloz Didier , Ragazzoni Roberto , Ramos Devin , Rando Nicola , Rauer Heike , Reimers Christian , Ribas Ignasi , Seager Sara , Ségransan Damien , Scandariato Gaetano , Simon Attila E. , Smith Alexis M. S. , Stalport Manu , Steller Manfred , Szabó Gyula , Thomas Nicolas , Pritchard Tyler A. , Udry Stéphane , Corral Van Damme Carlos , Van Grootel Valérie , Villaver Eva , Walter Ingo , Walton Nicholas , Watkins Cristilyn N. , West Richard G. 
Published: February 2024

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 200−200+400 Myr. TOI-8l5b 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-8l5c, 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⊕ (ρP = 1.64−0.31+0.33 g cm−3) and 23.5 ± 2.4 M⊕ (ρP = 7.2−1.0+1.1 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.