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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. Zh. Adibekyan, J.-M. Almenara, D. Barrado, D. Bayliss, I. Boisse, F. Bouchy, D. Brown, A. C. Cameron, D. Charbonneau, M. Deleuil, R. F. DÝaz, E. HÚbrard, J. Kirk, G. King, K. W. F. Lam, D. W. Latham, J. Lillo Box, T. Louden, C. Lovis, M. Marmier, J. McCormac, E. Molinari, F. Pepe, D. Pollacco, S. G. Sousa, S. Udry, R. Walter

Abstract
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±1.2 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 0.2M 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±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 H2 atmosphere puzzling. This analysis has made K2-110 b one of the best-characterised mini-Neptunes with density constrained to less than 30%.

Keywords
planets and satellites: detection

Astronomy and Astrophysics
Volume 604, Article Number A19, Number of pages 8
2017 July

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Instituto de Astrof├şsica e Ci├¬ncias do Espa├žo Universidade do Porto Faculdade de Ciências da Universidade de Lisboa Funda├ž├úo para a Ci├¬ncia e a Tecnologia
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