2016ApJ...830...43E

We report the discovery of K2-56b, a high-density sub-Neptune exoplanet, made using photometry from Campaign 4 of the two-wheeled Kepler (K2) mission, ground-based radial velocity (RV) follow-up from HARPS and high-resolution lucky and adaptive optics imaging obtained using AstraLux and MagAO, respectively. The host star is a bright (V=11.04, K_s=9.37), slightly metal-poor ([Fe/H]=-0.15±0.05 dex) solar analogue located at 152.1_–7.4^+9.7 pc from Earth, for which we find a radius of R_*=0.928_–0.040^+0.055R_☉ and a mass of M_*=0.961_–0.029^+0.032M_☉. A joint analysis of the K2 photometry and HARPS RVs reveal that the planet is in a ≃42 day orbit around its host star, has a radius of 2.23_–0.11^+0.14R_⊕, and a mass of 16.3_–6.1^+6.0M_⊕. Although the data at hand put the planet in the region of the mass-radius diagram where we could expect planets with a pure rock (i.e., magnesium silicate) composition using two-layer models (i.e., between rock/iron and rock/ice compositions), we discuss more realistic three-layer composition models which can explain the high density of the discovered exoplanet. The fact that the planet lies in the boundary between "possibly rocky" and "non-rocky" exoplanets makes it an interesting planet for future RV follow-up.