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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. F. Rowe, N. C. Santos, S. Udry, A. Wunsche

Abstract
Aims. The bright M2.5 dwarf K2-18 (MS = 0.36 MSun, RS = 0.41 RSun) 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.

Methods. We have obtained precision radial velocity measurements with the HARPS spectrograph. We then coupled those measurements with the K2 photometry to jointly model the observed radial velocity variation with planetary signals and a correlated stellar activity model based on Gaussian process regression.

Results. We measured the mass of K2-18b to be 8.0±1.9 MEarth with a bulk density of 3.3±1.2 g/cm3 which may correspond to a predominantly rocky planet with a significant gaseous envelope or an ocean planet with a water mass fraction greater than or similar to 50%. We also find strong evidence for a second, warm super-Earth K2-18c (mp,c sin i c = 7.5±1.3 MEarth) at approximately nine days with a semi-majoraxis similar to 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 the orbit of K2-18b although only a small mutual inclination is required for K2-18c to miss a transiting configuration; |Δi| similar to 1-2 degrees. A suite of dynamical integrations are performed to numerically confirm the system's dynamical stability. By varying the simulated orbital eccentricities of the two planets, dynamical stability constraints are used as an additional prior on each planet's eccentricity posterior from which we constrain eb < 0.43 and ec < 0.47 at the level of 99% confidence.

Conclusions. 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.

Keywords
techniques: radial velocities, methods: statistical, planets and satellites: detection, planets and satellites: fundamental parameters, planets and satellites: individual: K2-18

Astronomy and Astrophysics
Volume 608, Article Number A35, Number of pages 13
2017 December

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Instituto de Astrofísica e Ciências do Espaço Universidade do Porto Faculdade de Ciências da Universidade de Lisboa
Fundação para a Ciência e a Tecnologia COMPETE 2020 PORTUGAL 2020 União Europeia