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A giant impact as the likely origin of different twins in the Kepler-107 exoplanet system

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Abstract

Measures of exoplanet bulk densities indicate that small exoplanets with radius less than 3 Earth radii (R) range from low-density sub-Neptunes containing volatile elements1 to higher-density rocky planets with Earth-like2 or iron-rich3 (Mercury-like) compositions. Such astonishing diversity in observed small exoplanet compositions may be the product of different initial conditions of the planet-formation process or different evolutionary paths that altered the planetary properties after formation4. Planet evolution may be especially affected by either photoevaporative mass loss induced by high stellar X-ray and extreme ultraviolet (XUV) flux5 or giant impacts6. Although there is some evidence for the former7,8, there are no unambiguous findings so far about the occurrence of giant impacts in an exoplanet system. Here, we characterize the two innermost planets of the compact and near-resonant system Kepler-107 (ref. 9). We show that they have nearly identical radii (about 1.5–1.6R), but the outer planet Kepler-107 c is more than twice as dense (about 12.6 g cm–3) as the innermost Kepler-107 b (about 5.3 g cm−3). In consequence, Kepler-107 c must have a larger iron core fraction than Kepler-107 b. This imbalance cannot be explained by the stellar XUV irradiation, which would conversely make the more-irradiated and less-massive planet Kepler-107 b denser than Kepler-107 c. Instead, the dissimilar densities are consistent with a giant impact event on Kepler-107 c that would have stripped off part of its silicate mantle. This hypothesis is supported by theoretical predictions from collisional mantle stripping10, which match the mass and radius of Kepler-107 c.

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Data availability

The RV data that support the findings of this study and have been used to produce some of the plots are available in the Supplementary Information. Kepler data are available at the Mikulski Archive for Space Telescopes (https://archive.stsci.edu/kepler/).

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Acknowledgements

The authors wish to thank R. D. Haywood, R. Silvotti and D. Charbonneau for useful discussions. The HARPS-N project was funded by the Prodex Program of the Swiss Space Office (SSO), the Harvard-University Origin of Life Initiative (HUOLI), the Scottish Universities Physics Alliance (SUPA), the University of Geneva, the Smithsonian Astrophysical Observatory (SAO), the Italian National Astrophysical Institute (INAF), University of St. Andrews, Queen’s University Belfast and University of Edinburgh. The present work is based on observations made with the Italian Telescopio Nazionale Galileo (TNG) operated on the island of La Palma by the Fundación Galileo Galilei of the INAF (Istituto Nazionale di AstroFisica) at the Spanish Observatorio del Roque de los Muchachos of the Instituto de Astrofísica de Canarias. This paper exploited data collected by the Kepler mission; funding for the Kepler mission is provided by the NASA (National Aeronautics and Space Administration) Science Mission directorate. The research leading to these results received funding from the European Union Seventh Framework Programme (FP7/2007–2013) under grant agreement number 313014 (ETAEARTH). L.Z. acknowledges support from the Simons Foundation (SCOL (award no. 337090)). M.D. acknowledges financial support from Progetto Premiale 2015 FRONTIERA funding scheme of the Italian Ministry of Education, University, and Research. Funding for the Stellar Astrophysics Centre is provided by The Danish National Research Foundation (grant agreement no. DNRF106). V.S.A. acknowledges support from VILLUM FONDEN (research grant 10118). E.C. is funded by the European Union’s Horizon 2020 research and innovation program under the Marie Sklodowska-Curie grant agreement no. 664931. T.D. is supported by a STFC PhD studentship. R.A.G. acknowledges support from CNES. This work has been carried out in the frame of the National Centre for Competence in Research PlanetS supported by the Swiss National Science Foundation (SNSF). C.L., F.B., F.P. and S.U. acknowledge the financial support of the SNSF. M.S.L. is supported by The Independent Research Fund Denmark’s Sapere Aude program (grant agreement no. DFF–5051–00130). S.M. acknowledges support from the Ramon y Cajal fellowship number RYC-2015–17697.

Author information

The underlying radial-velocity observation programme was conceived and organized by F.P., A.C.C., D.W.L., C.L., D.Ségransan, S.U. and E.M. Observations with HARPS-N were carried out by L.A., A.C.C., A.M., C.D.D., M.D., X.D., R.C., A.F.M.F., P.F., A.H., F.M., M.L.-M., L.M., C.N., V.N., K.R. and A.V. C.L. maintained and updated the reduction pipeline, L.M. implemented the correction of radial velocities for moonlight contamination, and X.D. computed the values of the log(R’HK) activity indicator. A.S.B., M.D. and K.R. analysed and modelled the radial velocities. L.M. simulated and compared the radial velocities using both non-interacting and interacting Keplerians. A.S.B. also performed the transit fitting and S.M.M. worked on the analysis of transit timing variations. A.S.B. and A.V. analysed the Kepler light curve in search of the stellar rotational modulation signal. L.B. and A.M. determined the stellar atmospheric parameters from the HARPS-N spectra; A.M. also derived the stellar chemical abundances. T.A., M.B., E.C., J.C.-D., R.A.G., R.H., A.B.J., H.K., M.N.L., M.S.L., S.M. and V.S.A. carried out the asteroseismic analysis of the Kepler light curve and determined the stellar parameters from which A.S.B. and M.D. derived the planetary orbital and physical parameters. L.Z. deduced the planet interior compositions and E.L. estimated the planet atmospheric escapes. K.R. carried out dynamical simulations of the planetary system, and Z.M.L. and T.D. conducted the simulations of giant impact events. A.S.B. was the primary author of the manuscript and received important contributions by L.Z., Z.M.L., M.D., E.L., M.N.L., V.S.A., A.S., A.M. and M.L.-M. All authors have contributed to the interpretation of the data and the results.

Competing interests

The authors declare no competing interests.

Correspondence to Aldo S. Bonomo.

Supplementary information

  1. Supplementary Information

    Supplementary Figures 1–8, Supplementary Tables 1–4.

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Fig. 1: Radial velocity measurements of Kepler-107 with HARPS-N.
Fig. 2: Mass–radius diagram of exoplanets smaller than 3R.
Fig. 3: Smoothed particle hydrodynamical collision simulation.