K2-110 b: a massive mini-Neptune exoplanet

We report the discovery of the exoplanetK2-110 b (previously EPIC212521166b) from K2 photometry orbiting in a 13.8637d period around an old, metal-poor K3 dwarf star. With a V- bandmagnitude of 11.9, K2-110 is particularly amenable to RV follow-up. A joint analysis of K2 photometry and high-precision RVs from 28 HARPSand HARPS-N spectra reveal it to have a radius of 2.6 ± 0.1 R ⊕ and a mass of 16.7 ± 3.2 M ⊕ , hence a density of 5.2 ± 1.2 g cm -3 , making it one of the most massive planets yet to be found with a sub- Neptuneradius. When accounting for compression, the resulting Earth-like density is best fitted by a 0.2 M ⊕ hydrogen atmosphere over an 16.5 M ⊕ 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 ± 3 Gyr, 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 H 2 atmosphere puzzling. This analysis has made K2-110 b one of the best-characterised mini-Neptuneswith density constrained to less than 30%.