2017A&A...604A..19O

We report the discovery of the exoplanet K2-110 b (previously EPIC212521166b) from K2 photometry orbiting in a 13.8637d period around an old, metal-poor K3 dwarf star. With a V-band magnitude of 11.9, K2-110 is particularly amenable to RV follow-up. A joint analysis of K2 photometry and high-precision RVs from 28 HARPS and HARPS-N spectra reveal it to have a radius of 2.6±0.1R_⊕ and a mass of 16.7±3.2M_⊕, hence a density of 5.2±1.2g/cm³, making it one of the most massive planets yet to be found with a sub-Neptune radius. When accounting for compression, the resulting Earth-like density is best fitted by a 0.2M_⊕ hydrogen atmosphere over an 16.5M_⊕ Earth-like interior, although the planet could also have significant water content. At 0.1 AU, even taking into account the old stellar age of 8±3Gyr, the planet is unlikely to have been significantly affected by EUV evaporation. However the planet likelydisc-migrated to its current position making the lack of a thick H₂ atmosphere puzzling. This analysis has made K2-110 b one of the best-characterised mini-Neptunes with density constrained to less than 30%.