CARMENES Search for Exoplanets Around M dwarfs: A low-mass planet in the temperate zone of the nearby K2-18

K2-18 is a nearby M2.5 dwarf, located at 34 pc and hosting a transiting planet which was first discovered by the K2 mission and later confirmed with Spitzer Space Telescope observations.

With a radius of ∼ 2 R⊕ and an orbital period of ∼ 33 days, the planet lies in the temperate zone of its host star and receives stellar irradiation similar to 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 Kb ∼ 3.5 m s−1 and a mass of Mb ∼ 9M⊕, yielding a bulk density around ρb ∼ 4gcm−3. 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.

Sarkis Paula, Henning Thomas, Kurster Martin, Trifonov Trifon, Zechmeister Mathias, Tal-Or Lev, Anglada-Escude Guillem, Hatzes Artie, Lafarga Marina, Dreizler Stefan, Ribas Ignasi, Caballero Jose, Reiners Ansgar, Mallonn Matthias, Morales Juan, Kaminski Adrian, Aceituno Jesus, Amado Pedro, Bejar Victor, Hagen Hans-Jurgen, Jeffers Sandra, Quirrenbach Andreas, Launhardt Ralf, Marvin Christopher, Montes David
(Submitted on 2 May 2018)

Comments: 25 pages, 15 figures, accepted in AJ. For a short summary, see this https URL
Subjects: Earth and Planetary Astrophysics (astro-ph.EP)
Cite as: arXiv:1805.00830 [astro-ph.EP] (or arXiv:1805.00830v1 [astro-ph.EP] for this version)
Submission history
From: Paula Sarkis [view email]
[v1] Wed, 2 May 2018 14:08:58 GMT (1673kb,D)
https://arxiv.org/abs/1805.00830
Astrobiology