The CARMENES Search for Exoplanets around M Dwarfs: A Low-mass Planet in the Temperate Zone of the Nearby K2-18

DOI: 
10.3847/1538-3881/aac108
Publication date: 
01/06/2018
Main author: 
Sarkis, Paula
IAA authors: 
Anglada-Escudé, Guillem;Amado, Pedro J.
Authors: 
Sarkis, Paula;Henning, Thomas;Kürster, Martin;Trifonov, Trifon;Zechmeister, Mathias;Tal-Or, Lev;Anglada-Escudé, Guillem;Hatzes, Artie P.;Lafarga, Marina;Dreizler, Stefan;Ribas, Ignasi;Caballero, José A.;Reiners, Ansgar;Mallonn, Matthias;Morales, Juan C.;Kaminski, Adrian;Aceituno, Jesús;Amado, Pedro J.;Béjar, Victor J. S.;Hagen, Hans-Jürgen;Jeffers, Sandra;Quirrenbach, Andreas;Launhardt, Ralf;Marvin, Christopher;Montes, David
Journal: 
The Astronomical Journal
Refereed: 
Yes
Publication type: 
Article
Volume: 
155
Pages: 
257
Abstract: 
K2-18 is a nearby M2.5 dwarf, located at 34 pc and hosting a transiting planet that was first discovered by the K2 mission and later confirmed with Spitzer Space Telescope observations. With a radius of ∼2 R <SUB>⊕</SUB> and an orbital period of ∼33 days, the planet lies in the temperate zone of its host star and receives stellar irradiation similar to that of Earth. Here we perform radial velocity follow-up observations with the visual channel of CARMENES with the goal of determining the mass and density of the planet. We measure a planetary semi-amplitude of K <SUB> b </SUB> ∼ 3.5 {{m}} {{{s}}}<SUP>-1</SUP> and a mass of M <SUB> b </SUB> ∼ 9 M <SUB>⊕</SUB>, yielding a bulk density around {ρ }<SUB>b</SUB>∼ 4 {{g}} {cm}}<SUP>-3</SUP>. This indicates a low-mass planet with a composition consistent with a solid core and a volatile-rich envelope. A signal at 9 days was recently reported using radial velocity measurements taken with the HARPS spectrograph. This was interpreted as being due to a second planet. We see a weaker, time- and wavelength-dependent signal in the CARMENES data set and thus favor stellar activity for its origin. K2-18 b joins the growing group of low-mass planets detected in the temperate zone of M dwarfs. The brightness of the host star in the near-infrared makes the system a good target for detailed atmospheric studies with the James Webb Space Telescope.
Database: 
ADS
SCOPUS
URL: 
https://ui.adsabs.harvard.edu/#abs/2018AJ....155..257S/abstract
ADS Bibcode: 
2018AJ....155..257S
Keywords: 
stars: activity;stars: individual: K2-18;stars: low-mass;Astrophysics - Earth and Planetary Astrophysics