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

TL;DR

This study characterizes the old, metal-poor K2-111 star system, revealing a terrestrial planet near Earth size and mass, and a second non-transiting planet in near-resonance, indicating a complex planetary system.

Contribution

First detailed characterization of the K2-111 system, discovering a terrestrial planet and a second non-transiting planet in near-resonance using combined photometry and high-resolution spectroscopy.

Findings

K2-111b is a terrestrial planet with a radius of 1.82 R⊕ and mass of 5.29 M⊕.

A second planet, K2-111c, has a period of 15.68 days and a minimum mass of 11.3 M⊕.

Both planets are in near 3:1 mean-motion resonance, suggesting a dynamically interesting system.

Abstract

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 ($\log g=4.17$), iron-poor ([Fe/H]$=-0.46$), but alpha-enhanced ([$\alpha$/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 $P_b=5.3518\pm0.0004$ d, and has 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 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 $15.6785\pm 0.0064$ 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.