Characterization of the K218 multiplanetary system with HARPS. A habitable zone superEarth and discovery of a second, warm superEarth on a noncoplanar orbit
Abstract
Aims: The bright M2.5 dwarf K218 (M_{s} = 0.36 M_{⊙}, R_{s} = 0.41 R_{⊙}) at 34 pc is known to host a transiting superEarthsized planet orbiting within the star's habitable zone; K218b. 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 K218b to be 8.0 ± 1.9M_{⊕} with a bulk density of 3.3 ± 1.2 g/cm^{3} 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 superEarth K218c (m_{p,c}sini_{c} = 7.5 ± 1.3 M_{⊕}) at approximately nine days with a semimajor axis 2.4 times smaller than the transiting K218b. After reanalyzing the available light curves of K218 we conclude that K218c is not detected in transit and therefore likely has an orbit that is noncoplanar with the orbit of K218b although only a small mutual inclination is required for K218c to miss a transiting configuration;  Δi 12°. 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 e_{b} < 0.43 and e_{c} < 0.47 at the level of 99% confidence.
Conclusions: The discovery of the inner planet K218c further emphasizes the prevalence of multiplanet systems around M dwarfs. The characterization of the density of K218b reveals that the planet likely has a thick gaseous envelope which, along with its proximity to the solar system, makes the K218 planetary system an interesting target for the atmospheric study of an exoplanet receiving Earthlike insolation.
Table A.2 is also available at the CDS via anonymous ftp to http://cdsarc.ustrasbg.fr (http://130.79.128.5) or via http://cdsarc.ustrasbg.fr/vizbin/qcat?J/A+A/608/A35
 Publication:

Astronomy and Astrophysics
 Pub Date:
 December 2017
 DOI:
 10.1051/00046361/201731558
 arXiv:
 arXiv:1707.04292
 Bibcode:
 2017A&A...608A..35C
 Keywords:

 techniques: radial velocities;
 methods: statistical;
 planets and satellites: detection;
 planets and satellites: fundamental parameters;
 planets and satellites: individual: K218;
 Astrophysics  Earth and Planetary Astrophysics
 EPrint:
 13 pages, 8 figures including 4 interactive figures best viewed in Adobe Acrobat. Submitted to Astronomy &