Exoplanets & Exomoons

Characterization Of The K2-18 Multi-planetary System With HARPS

By Keith Cowing
Press Release
August 2, 2017
Filed under
Characterization Of The K2-18 Multi-planetary System With HARPS

The bright M dwarf K2-18 at 34 pc is known to host a transiting super-Earth-sized planet orbiting within the star’s habitable zone; K2-18b.

Given the superlative nature of this system for studying an exoplanetary atmosphere receiving similar levels of insolation as the Earth, we aim to characterize the planet’s mass which is required to interpret atmospheric properties and infer the planet’s bulk composition. We obtain precision radial velocity measurements with the HARPS spectrograph and couple those measurements with the K2 photometry to jointly model the observed radial velocity variation with planetary signals and a radial velocity jitter model based on Gaussian process regression. We measure the mass of K2-18b to be 8.0±1.9 M⊕ with a bulk density of 3.7±0.9 g/cm3 which may correspond to a predominantly rocky planet with a significant gaseous envelope or an ocean planet with a water mass fraction ≳50%.

We also find strong evidence for a second, warm super-Earth K2-18c at ∼9 days with a semi-major axis 2.4 times smaller than the transiting K2-18b. After re-analyzing the available light curves of K2-18 we conclude that K2-18c is not detected in transit and therefore likely has an orbit that is non-coplanar with K2-18b. A suite of dynamical integrations with varying simulated orbital eccentricities of the two planets are used to further constrain each planet’s eccentricity posterior from which we measure eb<0.43 and ec<0.47 at 99% confidence. The discovery of the inner planet K2-18c further emphasizes the prevalence of multi-planet systems around M dwarfs. The characterization of the density of K2-18b reveals that the planet likely has a thick gaseous envelope which along with its proximity to the Solar system makes the K2-18 planetary system an interesting target for the atmospheric study of an exoplanet receiving Earth-like insolation. Characterization of the K2-18 multi-planetary system with HARPS: A habitable zone super-Earth and discovery of a second, warm super-Earth on a non-coplanar orbit R. Cloutier, N. Astudillo-Defru, R. Doyon, X. Bonfils, J. M. Almenara, B. Benneke, F. Bouchy, X. Delfosse, D. Ehrenreich, T. Forveille, C. Lovis, M. Mayor, K. Menou, F. Murgas, F. Pepe, J. Rowe, N. C. Santos, S. Udry, A. Wünsche
(Submitted on 13 Jul 2017)

Comments: 13 pages, 8 figures including 4 interactive figures best viewed in Adobe Acrobat. Submitted to Astronomy & Astrophysics. Comments welcome
Subjects: Earth and Planetary Astrophysics (astro-ph.EP)
Cite as: arXiv:1707.04292 [astro-ph.EP] (or arXiv:1707.04292v1 [astro-ph.EP] for this version)
Submission history
From: Ryan Cloutier
[v1] Thu, 13 Jul 2017 19:59:08 GMT (5730kb,D)

Explorers Club Fellow, ex-NASA Space Station Payload manager/space biologist, Away Teams, Journalist, Lapsed climber, Synaesthete, Na’Vi-Jedi-Freman-Buddhist-mix, ASL, Devon Island and Everest Base Camp veteran, (he/him) 🖖🏻