From the star to the transiting exoplanets: Characterisation of the HD 219134 system

Exoplanets' properties are directly linked to that of their host star. This is even more true in the case of transiting exoplanets, where the planetary radius cannot be derived if the stellar radius is unknown. Interferometry seems the best technique in this context, as it provides in a quasi-direct way and with exquisite precision the stellar radii. Moreover, the transit light curve can be used to directly obtain the stellar density, and thus the stellar mass. We apply this technique to the system of HD 219134, which hosts two transiting super-Earths. Using these observational techniques and the correlations between the measured parameters, we directly derive new stellar radius, density and mass: \RStar = 0.726 ± 0.014 \RSun, ρStar = 1.82±0.19 ρSun, \MStar = 0.696±0.078 \MSun. This yields new planetary parameters, and in particular, we find that the two transiting exoplanets show different densities despite similar masses. This can be explained by three hypothesis, among which one suggests that tides heat the internal part of the innermost planet, leading to a molten mantle with lower density.