  HD 219134 is a K-dwarf star at a distance of 6.5 parsecs around which several
low-mass planets were recently discovered. The Spitzer space telescope detected
a 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 rocky
composition. Here, we report new highprecision time-series photometry of the
star acquired with Spitzer revealing that the second innermost planet of the
system, HD 219134 c, is also transiting. 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 4.74+-0.19 MEarth and 1.602+-0.055 REarth for HD
219134 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 the
proximity and the small size of their host star (0.778+-0.005 Rsun), these two
transiting exoplanets - the nearest to the Earth to date - are well-suited for
a detailed characterization (precision of a few percent on mass and radius,
constraints on the atmospheric properties...) that could give important
constraints on the nature and formation mechanism of the ubiquitous
short-period planets of a few Earth masses.
