GJ 1214 reviewed

Abstract

Context. GJ 1214 is orbited by a transiting super-Earth-mass planet. It is a primary target for ongoing efforts to understand the emerging population of super-Earth-mass planets around M dwarfs, some of which are detected within the liquid water (habitable) zone of their host stars. Aims: We present new precision astrometric measurements, a re-analysis of HARPS radial velocity measurements, and new medium-resolution infrared spectroscopy of GJ 1214. We combine these measurements with recent transit follow-up observations and new catalog photometry to provide a comprehensive update of the star-planet properties. Methods: The distance is obtained with 0.6% relative uncertainty using CAPScam astrometry. The new value increases the nominal distance to the star by ~10% and is significantly more precise than previous measurements. Improved radial velocity measurements have been obtained analyzing public HARPS spectra of GJ 1214 using the HARPS-TERRA software and are 25% more precise than the original ones. The Doppler measurements combined with recently published transit observations significantly refine the constraints on the orbital solution, especially on the planet's eccentricity. The analysis of the infrared spectrum and photometry confirm that the star is enriched in metals compared to the Sun. Results: Using all this new fundamental information, combined with empirical mass-luminosity relations for low mass stars, we derive updated values for the bulk properties of the star-planet system. We also use infrared absolute fluxes to estimate the stellar radius and to re-derive the star-planet properties. Both approaches provide very consistent values for the system. Our analysis shows that the updated expected value for the planet mean density is 1.6 ± 0.6 g cm-3, and that a density comparable to the Earth (~5.5 g cm-3) is now ruled out with very high confidence. Conclusions: This study illustrates how the fundamental properties of M dwarfs are of paramount importance in the proper characterization of the low mass planetary candidates orbiting them. Given that the distance is now known to better than 1%, interferometric measurements of the stellar radius, additional high precision Doppler observations, and/or or detection of the secondary transit (occultation), are necessary to further improve the constraints on the GJ 1214 star-planet properties.