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The KESPRINT consortium identified K2-216 as a planetary candidate host starin the K2 space mission Campaign 8 field with a transiting super-Earth. Theplanet has recently been validated as well. Our aim was to confirm thedetection and derive the main physical characteristics of K2-216b, includingthe mass. We performed a series of follow-up observations: high resolutionimaging with the FastCam camera at the TCS, the Infrared Camera andSpectrograph at Subaru, and high resolution spectroscopy with HARPS (ESO, LaSilla), HARPS-N (TNG), and FIES (NOT). The stellar spectra were analyzed withthe SpecMatch-Emp and SME codes to derive the stellar fundamental properties.We analyzed the K2 light curve with the Pyaneti software. The radial-velocitymeasurements were modelled with both a Gaussian process (GP) regression and thefloating chunk offset (FCO) technique to simultaneously model the planetarysignal and correlated noise associated with stellar activity. Imaging confirmsthat K2-216 is a single star. Our analysis discloses that the star is amoderately active K5V star of mass 0.70+/-0.03 Msun and radius 0.72+/-0.03Rsun. Planet b is found to have a radius of 1.75+0.17-0.10 Rearth and a2.17-day orbit in agreement with previous results. We find consistent resultsfor the planet mass from both models: 7.4+/-2.2 Mearth from the GP regression,and 8.0+/-1.6 Mearth from the FCO technique, which implies that this planet isa super-Earth. The planet parameters put planet b in the middle of, or justbelow, the gap of the radius distribution of small planets. The density isconsistent with a rocky composition of primarily iron and magnesium silicate.In agreement with theoretical predictions, we find that the planet is a remnantcore, stripped of its atmosphere, and is one of the largest planets found thathas lost its atmosphere.

Super-Earth of 8 Mearth in a 2.2-day orbit around the K5V star K2-216