H. P. Osborn, A. Santerne, S. C. C. Barros, N. C. Santos, X. Dumusque, L. Malavolta, D. J. Armstrong, S. Hojjatpanah, O. Demangeon, V. Adibekyan, J.M. Almenara, D. Barrado, D. Bayliss, I. Boisse, F. Bouchy, D. J. A. Brown, A.C. Cameron, D. Charbonneau, M. Deleuil, E. Delgado Mena, R.F. Díaz, G. Hébrard, J. Kirk, G. W. King, K. W. F. Lam, D. Latham, J. Lillo-Box, T. M. Louden, C. Lovis, M. Marmier, J. McCormac, E. Molinari, D. Pollacco, S. G. Sousa, S. Udry, S. R. Walker. 2017. K2-110 b: a massive mini-Neptune exoplanet. Astronomy and Astrophysics 604, DOI: 10.1051/0004-6361/201628932
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 +/- 0.1 R-circle plus and a mass of 16.7 +/- 3.2 M-circle plus, 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-Neptune radius. When accounting for compression, the resulting Earth-like density is best fitted by a 0.2 M-circle plus hydrogen atmosphere over an 16.5 M-circle plus 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 likely disc-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-Neptunes with density constrained to less than 30%.