This paper reports on the detailed characterisation of the K2-111 planetary system. We used K2, WASP, and ASAS-SN photometry as well as high-resolution spectroscopic data from HARPS-N and ESPRESSO to characterise this system. A global fit of the K2 transit and radial velocity data were performed using PyORBIT, comparing several models. The host, K2-111, is confirmed to be a mildly evolved (\(\log g=4.17\)), iron-poor (\(\text{[Fe/H]}=-0.46\)), but alpha-enhanced (\([\alpha\text{/Fe}]=0.27\)), very old G2 star, likely belonging to the thick disc population of the Galaxy. The transiting planet, K2-111 b, orbiting with a period \(P_b=5.3518\pm0.0004\) d, is precisely and accurately characterised to have a planet radius of \(1.82^{+0.11}_{-0.09}\) \(R_\oplus\) and a mass of \(5.29^{+0.76}_{-0.77}\) \(M_\oplus\), resulting in a bulk density slightly lower than that of the Earth. The stellar chemical composition and the planet properties are, within two sigma, 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 sinusoidal signal in the radial velocity data, unlikely to have arisen from stellar activity in this chromospherically quiet star. We attribute this signal to a non-transiting planet, K2-111 c, with an orbital period of \(15.6785\pm 0.0064\) days, orbiting in near 3:1 mean-motion resonance with the transiting planet, and a minimum planet mass of \(11.3\pm1.1\) \(M_\oplus\). 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 in this old system. Systems like K2-111, occupying a parameter space less well-explored, allow us to improve planet formation and evolution theories.
Authors: A. Mortier, M.R. Zapatero Osorio, L. Malavolta, Y. Alibert, K. Rice, J. Lillo-Box, A. Vanderburg, M. Oshagh, L. Buchhave, V. Adibekyan, E. Delgado Mena, M. Lopez-Morales, D. Charbonneau, S.G. Sousa, C. Lovis, L. Affer, S.C.C. Barros, S. Benatti, A.S. Bonomo, W. Boschin, F. Bouchy, A. Collier Cameron, R. Cosentino, S. Cristiani, V. D’Odorico, X. Dumusque, D. Ehrenreich, P. Figueira, A. Fiorenzano, A. Ghedina, J.I. González Hernández, J. Haldemann, A. Harutyunyan, R.D. Haywood, D.W. Latham, B. Lavie, G. Lo Curto, J. Maldonado, C. J. A. P. Martins, M. Mayor, A. Mehner, G. Micela, N. J. Nunes, F.A. Pepe, E. Palle, D. Phillips, M. Pinamonti, E. Poretti, R. Rebolo, N.C. Santos, D. Sasselov, A. Sozzetti, A. Suárez Mascareño, S. Udry, R.G. West, C.A. Watson, T. G. Wilson