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

A. Psaridi, H. P. Osborn, F. Bouchy, M. Lendl, L. Parc, N. Billot, C. Broeg, S. G. Sousa, V. Zh. Adibekyan, O. Attia, A. Bonfanti, H. Chakraborty, K. A. Collins, J. Davoult, E. Delgado-Mena, N. Grieves, T. Guillot, A. Heitzmann, R. Helled, C. Hellier, J. M. Jenkins, H. Knierim, A. Krenn, J. J. Lissauer, R. Luque, D. Rapetti, N. C. Santos, O. Suárez, J. Venturini, F. P. Wilkin, T. G. Wilson, J. N. Winn, C. Ziegler, T. Zingales, Y. Alibert, A. Brandeker, J. A. Egger, D. Gandolfi, M. J. Hooton, A. Tuson, S. Ulmer-Moll, L. Abe, R. Allart, R. Alonso, D. R. Anderson, G. Anglada-Escudé, T. Bárczy, D. Barrado, S. C. C. Barros, W. Baumjohann, M. Beck, T. Beck, W. Benz, X. Bonfils, L. Borsato, V. Bourrier, D. R. Ciardi, A. C. Cameron, S. Charnoz, M. Cointepas, S. Csizmadia, P. E. Cubillos, G. Lo Curto, M. B. Davies, T. Daylan, M. Deleuil, A. Deline, L. Delrez, O. Demangeon, B.-O. Demory, C. Dorn, X. Dumusque, D. Ehrenreich, A. Erikson, A. Lecavelier Des Etangs, D. de Miguel, A. Fortier, L. Fossati, Y. G. C. Frensch, M. Fridlund, M. Gillon, M. Güdel, M. N. Günther, J. Hagelberg, C. Helling, S. Hoyer, K. G. Isaak, L. L. Kiss, K. W. F. Lam, J. Laskar, B. Lavie, C. Lovis, D. Magrin, L. Marafatto, P. F. L. Maxted, S. McDermott, D. Mékarnia, C. Mordasini, F. Murgas, V. Nascimbeni, L. D. Nielsen, G. Olofsson, R. Ottensamer, I. Pagano, E. Pallé, G. Peter, G. Piotto, D. Pollacco, D. Queloz, R. Ragazzoni, D. Ramos, N. Rando, H. Rauer, C. Reimers, I. Ribas, S. Seager, D. Ségransan, G. Scandariato, A. E. Simon, A. M. S. Smith, M. Stalport, M. Steller, G. M. Szabó, N. L. Thomas, T. A. Pritchard, S. Udry, C. Corral Van Damme, V. Van Grootel, E. Villaver, I. Walter, N. A. Walton, C. N. Watkins, R. G. West

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.

techniques: photometric / techniques: radial velocities / planets and satellites: composition / planets and satellites: detection / planets and satellites: formation / stars: individual: TOI-815

- The photometric and radial velocity data are available at the CDS via anonymous ftp to ( or via
- This study uses TESS data, CHEOPS data observed as part of guaranteed time observation programs CH_PR10031 and CH_PR10048, and ESPRESSO data collected with the ESO 3.6 m telescope under programs 105.20P7.001, 109.23DX.001, and 110.2481.001 (PI: Bouchy).

Astronomy & Astrophysics
Volume 685, Article Number A5, Number of pages 24
2024 May

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Faculdade de Ciências da Universidade de Lisboa Universidade do Porto Faculdade de Ciências e Tecnologia da Universidade de Coimbra
Fundação para a Ciência e a Tecnologia COMPETE 2020 PORTUGAL 2020 União Europeia