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HD 219134 is a K-dwarf star at a distance of 6.5 parsecs around which severallow-mass planets were recently discovered. The Spitzer space telescope detecteda transit of the innermost of these planets, HD 219134 b, whose mass and radius(4.5 MEarth and 1.6 REarth respectively) are consistent with a rockycomposition. Here, we report new highprecision time-series photometry of thestar acquired with Spitzer revealing that the second innermost planet of thesystem, HD 219134 c, is also transiting. A global analysis of the Spitzertransit light curves and the most up-to-date HARPS-N velocity data set yieldsmass and radius estimations of 4.74+-0.19 MEarth and 1.602+-0.055 REarth for HD219134 b, and of 4.36+-0.22 MEarth and 1.511+-0.047 REarth for HD 219134 c.These values suggest rocky compositions for both planets. Thanks to theproximity and the small size of their host star (0.778+-0.005 Rsun), these twotransiting exoplanets - the nearest to the Earth to date - are well-suited fora detailed characterization (precision of a few percent on mass and radius,constraints on the atmospheric properties...) that could give importantconstraints on the nature and formation mechanism of the ubiquitousshort-period planets of a few Earth masses.

Two massive rocky planets transiting a K-dwarf 6.5 parsecs away