K2-111: an old system with two planets in near-resonance†

Mortier, A; Zapatero Osorio, M R; Malavolta, L; Alibert, Yann; Rice, K; Lillo-Box, J; Vanderburg, A; Oshagh, M; Buchhave, L; Adibekyan, V; Delgado Mena, E; Lopez-Morales, M; Charbonneau, D; Sousa, S G; Lovis, C; Affer, L; Allende Prieto, C; Barros, S C C; Benatti, S; Bonomo, A S; ... (2020). K2-111: an old system with two planets in near-resonance†. Monthly notices of the Royal Astronomical Society, 499(4), pp. 5004-5021. Blackwell Publishing Ltd 10.1093/mnras/staa3144

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This paper reports on the detailed characterisation 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 (logg=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, K2-111b, 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 K2-111b 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, K2-111c, with an orbital period of 15.6785±0.0064 days, 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.

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