A multiple system of transiting super-Earths at 6.5pc

Abstract

HD 219134 is a K-dwarf at a distance of 6.5 pc around which several low-mass planets were recently discovered by the radial velocity method1,2 18 . With the Spitzer space telescope we detected a transit of the innermost of these planets, the 3.1d-period super- Earth HD 219134 b. Its absolute mass and radius were measured to be 4.5 MEarth and 1.6 REarth, respectively, consistent with a rocky composition1 21 . Here, we report new high- precision time-series photometry of the star acquired with Spitzer that not only confirms the transiting nature of HD 219134 b, but also reveals the transiting nature of the second innermost planet of the system, the 6.8d-period super-Earth HD 219134 c. A global analysis of the Spitzer transit light curves and the most up-to-date HARPS-N velocity data set yields mass and radius estimations of 4.62 ± 0.16 MEarth and 1.602 ± 0.053 REarth for 219134 b, and of 4.24 ± 0.20 MEarth and 1.515 ± 0.047 REarth for HD 219134 c. These values suggest rocky compositions for both planets. Thanks to the proximity and the small size of their host star (0.778 ± 0.005 R¤3 29 ), these two transiting exoplanets - the nearest to the Earth - are well-suited for a detailed characterization (precision of a few percent on mass and radius, constraints on the atmospheric properties) that could shed interesting light on the nature and formation mechanism of the ubiquitous short-period super-Earths.