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.
@article{arxiv.1703.01430,
title = {Two massive rocky planets transiting a K-dwarf 6.5 parsecs away},
author = {Michael Gillon and Brice-Olivier Demory and Valerie Van Grootel and Fatemeh Motalebi and Christophe Lovis and Andrew Collier Cameron and David Charbonneau and David Latham and Emilio Molinari and Francesco A. Pepe and Damien Segransan and Dimitar Sasselov and Stephane Udry and Michel Mayor and Giuseppina Micela and Giampaolo Piotto and Alessandro Sozzetti},
journal= {arXiv preprint arXiv:1703.01430},
year = {2017}
}