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

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Abstract

HD 219134 is a K-dwarf star at a distance of 6.5 parsecs around which several low-mass planets were recently discovered1,2. The Spitzer Space Telescope detected a transit of the innermost of these planets, HD 219134 b, whose mass and radius (4.5 M and 1.6 R respectively) are consistent with a rocky composition1. Here, we report new high-precision time-series photometry of the star acquired with Spitzer revealing that the second innermost planet of the system, HD 219134c, 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 M and 1.602 ± 0.055 R for HD 219134 b, and of 4.36 ± 0.22 M and 1.511 ± 0.047 R 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, these two transiting exoplanets — the nearest to the Earth yet found — are well suited for a detailed characterization (for example, precision of a few per cent on mass and radius, and 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.

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Figure 1: Spitzer transit photometry of the planets HD 219134 b and c.
Figure 2: Mass–radius relationship for small planets with precisions on the masses better than 20%.

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Acknowledgements

This work is based in part on observations made with the Spitzer Space Telescope, which is operated by the Jet Propulsion Laboratory, California Institute of Technology, under a contract with NASA. Support for this work was provided by NASA. M.G. is grateful to NASA and SSC Director for having supported his searches for RV planets with Spitzer. M.G. and V.V.G. are Research Associates at the Belgian Scientific Research Fund (F.R.S.-FNRS). The research leading to these results has received funding from the ARC grant for Concerted Research Actions, financed by the Wallonia–Brussels Federation. The authors thank N. Lewis for information on the potential for atmospheric characterization of HD 219134 b and c with JWST.

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M.G. led the HD 219134 b+c transit search with Spitzer, planned and analysed the Spitzer observations, performed the global analysis of the Spitzer and HARPS-N data, and wrote most of the manuscript. B.-O.D. performed an independent analysis of the Spitzer data to verify M.G.’s results. V.V.G. performed the stellar evolutionary modelling of the host star. A.C.C., D.C., D.L., C.L., E.M., F.M., G.M., M.M., F.A.P., G.P., D.Sa., D.Sé., A.S. and S.U. form the HARPS-N science team which managed the RV monitoring of the system. All authors contributed to the writing of the manuscript.

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Correspondence to Michaël Gillon.

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The authors declare no competing financial interests.

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Supplementary Figures 1–2 and Supplementary Tables 1–3. (PDF 707 kb)

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Gillon, M., Demory, B., Van Grootel, V. et al. Two massive rocky planets transiting a K-dwarf 6.5 parsecs away. Nat Astron 1, 0056 (2017). https://doi.org/10.1038/s41550-017-0056

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