K2-110 b: a massive mini-Neptune exoplanet

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%.

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