K2-110 b: a massive mini-Neptune exoplanet

¹ Department of Physics, University of Warwick, Gibbet Hill Road, Coventry, CV4 7AL, UK
e-mail: h.p.osborn@warwick.ac.uk
² Instituto de Astrofísica e Ciências do Espaço, Universidade do Porto, CAUP, rua das Estrelas, 4150-762 Porto, Portugal
³ Aix Marseille Université, CNRS, Laboratoire d’Astrophysique de Marseille UMR 7326, 13388 Marseille, France
⁴ Departamento de Física e Astronomia, Universidade do Porto, rua Campo Alegre, 4169-007 Porto, Portugal
⁵ Observatoire Astronomique de l’Université de Genève, 51 Chemin des Maillettes, 1290 Versoix, Switzerland
⁶ Dipartimento di Fisica e Astronomia “Galileo Galilei”, Universita’ di Padova, Vicolo dell’Osservatorio 3, 35122 Padova, Italy
⁷ INAF–Osservatorio Astronomico di Padova, Vicolo dell’Osservatorio 5, 35122 Padova, Italy
⁸ ARC, School of Mathematics & Physics, Queen’s University Belfast, University Road, Belfast BT7 1NN, UK
⁹ Department of Physics, University of Zanjan, University Blvd, 45371-38791 Zanjan, Iran
¹⁰ Université Grenoble Alpes, IPAG, 38000 Grenoble, France
¹¹ CNRS, IPAG, 38000 Grenoble, France Depto. de Astrofísica, Centro de Astrobiología (CSIC-INTA), ESAC campus 28692 Villanueva de la Cañada ( Madrid), Spain
¹² School of Physics & Astronomy, University of St. Andrews, North Haugh, St. Andrews Fife, KY16 9SS, UK
¹³ Harvard-Smithsonian Center for Astrophysics, 60 Garden Street, Cambridge, Massachusetts 02138, USA
¹⁴ Universidad de Buenos Aires, Facultad de Ciencias Exactas y Naturales, Buenos Aires, Argentina
¹⁵ CONICET – Universidad de Buenos Aires, Instituto de Astronomía y Física del Espacio (IAFE) Buenos Aires, Argentina
¹⁶ Institut d’Astrophysique de Paris, UMR 7095 CNRS, Université Pierre & Marie Curie, 98bis boulevard Arago, 75014 Paris, France
¹⁷ Observatoire de Haute-Provence, Université d’Aix-Marseille & CNRS, 04870 Saint Michel l’ Observatoire, France
¹⁸ European Southern Observatory (ESO), Alonso de Cordova 3107, Vitacura, Casilla 19001, Santiago de Chile, Chile
¹⁹ INAF – Fundación Galileo Galilei, Rambla José Ana Fernandez Pérez 7, 38712 Berña Baja, Spain
²⁰ INAF – IASF Milano, via Bassini 15, 20133 Milano, Italy

Received: 13 May 2016
Accepted: 25 April 2017

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%.