Super-Earth of 8 M⊕ in a 2.2-day orbit around the K5V star K2-216
Chalmers University of Technology, Department of Space, Earth and Environment, Onsala Space Observatory,
2 Leiden Observatory, University of Leiden, PO Box 9513, 2300 RA, Leiden, The Netherlands
3 Dipartimento di Fisica, Universitá di Torino, via Pietro Giuria 1, 10125 Torino, Italy
4 Department of Physics and Kavli Institute for Astrophysics and Space Research, MIT, Cambridge, MA 02139, USA
5 Department of Astrophysical Sciences, Princeton University, 024B, Peyton Hall, 4 Ivy Lane, Princeton, NJ 08544, USA
6 Thüringer Landessternwarte Tautenburg, 07778 Tautenburg, Germany
7 Department of Earth and Planetary Sciences, Tokyo Institute of Technology, Meguro-ku, Tokyo, Japan
8 Rheinisches Institut für Umweltforschung an der Universität zu Köln, Aachener Strasse 209, 50931 Köln, Germany
9 Instituto de Astrofísica de Canarias (IAC), 38205 La Laguna, Tenerife, Spain
10 Departamento de Astrofísica, Universidad de La Laguna, 38206 La Laguna, Tenerife, Spain
11 Space Research Institute, Austrian Academy of Sciences, Schmiedlstrasse 6, 8042 Graz, Austria
12 Stellar Astrophysics Centre, Department of Physics and Astronomy, Aarhus University, Ny Munkegade 120, 8000 Aarhus C, Denmark
13 Department of Astronomy, The University of Tokyo, 7-3-1 Hongo, Bunkyo-ku, Tokyo 113-0033, Japan
14 Institute of Planetary Research, German Aerospace Center (DLR), Rutherfordstrasse 2, 12489 Berlin, Germany
15 Department of Astronomy and McDonald Observatory, University of Texas at Austin, 2515 Speedway, Stop C1400, Austin, TX 78712, USA
16 National Astronomical Observatory of Japan, NINS, 2-21-1 Osawa, Mitaka, Tokyo 181-8588, Japan
17 Landessternwarte Königstuhl, Zentrum für Astronomie der Universität Heidelberg, Königstuhl 12, 69117 Heidelberg, Germany
18 Astrobiology Center, NINS, 2-21-1 Osawa, Mitaka, Tokyo 181-8588, Japan
19 Center for Astronomy and Astrophysics, TU Berlin, Hardenbergstr. 36, 10623 Berlin, Germany
20 European Southern Observatory, Alonso de Córdova 3107, Vitacura, Casilla, 19001 Santiago de Chile, Chile
Accepted: 28 June 2018
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.
Key words: planetary systems / stars: individual: K2-216 / techniques: photometric / techniques: radial velocities / planets and satellites: atmospheres / planets and satellites: composition
© ESO 2018