Super-Earth of 8 Mearth in a 2.2-day orbit around the K5V star K2-216
Journal article, 2018

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. Our aim was to confirm the detection and derive the main physical characteristics of K2-216b, including the mass. We performed a series of follow-up observations: high resolution imaging with the FastCam camera at the TCS, the Infrared Camera and Spectrograph at Subaru, and high resolution spectroscopy with HARPS (ESO, La Silla), HARPS-N (TNG), and FIES (NOT). The stellar spectra were analyzed with the SpecMatch-Emp and SME codes to derive the stellar fundamental 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 floating chunk offset (FCO) technique to simultaneously model the planetary signal and correlated noise associated with stellar activity. 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 Msun and radius 0.72+/-0.03 Rsun. Planet b is found to have a radius of 1.75+0.17-0.10 Rearth and a 2.17-day orbit in agreement with previous results. We find consistent results for 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 is a super-Earth. 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.

Stars: individual: K2-216

Planets and satellites: Atmospheres

Techniques: photometric

Techniques: radial velocities

Planetary systems

Planets and satellites: composition


Carina Persson

Chalmers, Space, Earth and Environment, Astronomy and Plasmaphysics, Galactic Astrophysics

Malcolm Fridlund

Chalmers, Space, Earth and Environment, Astronomy and Plasmaphysics, Galactic Astrophysics

O. Barragan

F. Dai

D. Gandolfi

A. P. Hatzes

T. Hirano

S. Grziwa

J. Korth

J. Prieto-Arranz

L. Fossati

V. Van Eylen

A. Bo Justesen

J. Livingston

D. Kubyshkina

H. J. Deeg

E. W. Guenther

G. Nowak

J. Cabrera

Ph Eigmüller

Sz. Csizmadia

A. M. S. Smith

A. Erikson

S. Albrecht

R. Alonso

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

Astronomy and Astrophysics

0004-6361 (ISSN) 1432-0746 (eISSN)

Vol. 618 33

Exoplanets from space – CHEOPS and PLATO, ESA’s next two projects

Swedish National Space Board, 2017-01-01 -- 2022-12-31.

Subject Categories

Astronomy, Astrophysics and Cosmology


Basic sciences


Onsala Space Observatory



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