K2-111: an old system with two planets in near-resonance†
Annelies Mortier, M. R. Zapatero Osorio, L. Malavolta, Y. Alibert, Ken Rice, J. Lillo-Box, Andrew Vanderburg, M. Oshagh, Lars A. Buchhave, V. Adibekyan, E. Delgado Mena, Mercedes López‐Morales, David Charbonneau, S. G. Sousa, C. Lovis, L. Affer, Carlos Allende Prieto, S. C. C. Barros, S. Benatti, A. S. Bonomo, W. Boschin, F. Bouchy, Alexandre Cabral, A. Collier Cameron, R. Cosentino, S. Cristiani, O. D. S. Demangeon, P. Di Marcantonio, V. D’Odorico, X. Dumusque, D. Ehrenreich, P. Figueira, A. F. Martínez Fiorenzano, A. Ghedina, J. I. Gónzalez Hernández, Jonas Haldemann, A. Harutyunyan, R. D. Haywood, David W. Latham, B. Lavie, G. Lo Curto, J. Maldonado, A. Manescau, C. J. A. P. Martins, M. Mayor, D. Mégevand, A. Mehner, G. Micela, P. Molaro, E. Molinari, N. J. Nunes, F. Pepe, Ε. Πάλλη, David F. Phillips, G. Piotto, M. Pinamonti, E. Poretti, Marco Riva, R. Rébolo, N. C. Santos, Dimitar Sasselov, A. Sozzetti, A. Suárez Mascareño, S. Udry, R. G. West, C. A. Watson, T. G. Wilson
Abstract
ABSTRACT This paper reports on the detailed characterization of the K2-111 planetary system with K2, WASP, and ASAS-SN photometry, as well as high-resolution spectroscopic data from HARPS-N and ESPRESSO. The host, K2-111, is confirmed to be a mildly evolved (log g = 4.17), iron-poor ([Fe/H] = −0.46), but alpha-enhanced ([α/Fe]=0.27), chromospherically quiet, very old thick disc G2 star. A global fit, performed by using PyORBIT, shows that the transiting planet, K2-111 b, orbits with a period Pb = 5.3518 ± 0.0004 d and has a planet radius of $1.82^{+0.11}_{-0.09}$ R⊕ and a mass of $5.29^{+0.76}_{-0.77}$ M⊕, resulting in a bulk density slightly lower than that of the Earth. The stellar chemical composition and the planet properties are consistent with K2-111 b being a terrestrial planet with an iron core mass fraction lower than the Earth. We announce the existence of a second signal in the radial velocity data that we attribute to a non-transiting planet, K2-111 c, with an orbital period of 15.6785 ± 0.0064 d, orbiting in near-3:1 mean motion resonance with the transiting planet, and a minimum planet mass of 11.3 ± 1.1 M⊕. Both planet signals are independently detected in the HARPS-N and ESPRESSO data when fitted separately. There are potentially more planets in this resonant system, but more well-sampled data are required to confirm their presence and physical parameters.