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

C. M. Persson, M. Fridlund, O. Barragán, F. Dai, D. Gandolfi, A. P. Hatzes, T. Hirano, S. Grziwa, J. Korth, J. Prieto-Arranz, L. Fossati, V. Van Eylen, A. B. Justesen, J. Livingston, D. Kubyshkina, H. J. Deeg, E. W. Guenther, G. Nowak, J. Cabrera, Ph EigmöllerSz Csizmadia, A. M.S. Smith, A. Erikson, S. Albrecht, Alonso Sobrino, W. D. Cochran, M. Endl, M. Esposito, A. Fukui, P. Heeren, D. Hidalgo, M. Hjorth, M. Kuzuhara, N. Narita, D. Nespral, E. Palle, M. Pätzold, H. Rauer, F. Rodler, J. N. Winn

Research output: Contribution to journalArticlepeer-review

29 Scopus citations


Context. Although thousands of exoplanets have been discovered to date, far fewer have been fully characterised, in particular super-Earths. The KESPRINT consortium identified K2-216 as a planetary candidate host star in the K2 space mission Campaign 8 field with a transiting super-Earth. The planet has recently been validated as well. Aims. Our aim was to confirm the detection and derive the main physical characteristics of K2-216 b, including the mass. Methods. We performed a series of follow-up observations: high-resolution imaging with the FastCam camera at the TCS and the Infrared Camera and Spectrograph at Subaru, and high-resolution spectroscopy with HARPS (La Silla), HARPS-N (TNG), and FIES (NOT). The stellar spectra were analyzed with the SpecMatch-Emp and SME codes to derive the fundamental stellar properties. We analyzed the K2 light curve with the pyaneti software. The radial velocity measurements were modelled with both a Gaussian process (GP) regression and the so-called floating chunk offset (FCO) technique to simultaneously model the planetary signal and correlated noise associated with stellar activity. Results. Imaging confirms that K2-216 is a single star. Our analysis discloses that the star is a moderately active K5V star of mass 0.70 ± 0.03 M and radius 0.72 ± 0.03 R. Planet b is found to have a radius of 1.75-0.10 +0.17 R and a 2.17-day orbit in agreement with previous results. We find consistent results for the planet mass from both models: Mp ≈ 7.4 ± 2.2 M from the GP regression and Mp ≈ 8.0 ± 1.6 M from the FCO technique, which implies that this planet is a super-Earth. The incident stellar flux is 2.48-48 +220 F. Conclusions. The planet parameters put planet b in the middle of, or just below, the gap of the radius distribution of small planets. The density is consistent with a rocky composition of primarily iron and magnesium silicate. In agreement with theoretical predictions, we find that the planet is a remnant core, stripped of its atmosphere, and is one of the largest planets found that has lost its atmosphere.

Original languageEnglish (US)
Article numberA33
JournalAstronomy and Astrophysics
StatePublished - Oct 1 2018

All Science Journal Classification (ASJC) codes

  • Astronomy and Astrophysics
  • Space and Planetary Science


  • Planetary systems
  • Planets and satellites: Atmospheres
  • Planets and satellites: composition
  • Stars: individual: K2-216
  • Techniques: photometric
  • Techniques: radial velocities


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