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

TL;DR

This study reports the discovery and characterization of two rocky, transiting exoplanets orbiting the nearby K-dwarf star HD 219134, providing valuable data for understanding small planet compositions and formation.

Contribution

The paper presents the first detection of transits for the second planet in the system and provides precise mass and radius measurements for both planets, enhancing knowledge of rocky exoplanets around nearby stars.

Findings

Both planets are likely rocky based on mass and radius.

The planets are the closest transiting exoplanets to Earth.

High-precision measurements enable detailed characterization.

Abstract

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.