Earth, Venus, Mars and some extrasolar terrestrial planets1 have a mass and radius that is consistent with a mass fraction of about 30% metallic core and 70% silicate mantle2. At the inner frontier of the Solar System, Mercury has a completely different composition, with a mass fraction of about 70% metallic core and 30% silicate mantle3. Several formation or evolution scenarios are proposed to explain this metal-rich composition, such as a giant impact4, mantle evaporation5 or the depletion of silicate at the inner edge of the protoplanetary disk6. These scenarios are still strongly debated. Here, we report the discovery of a multiple transiting planetary system (K2-229) in which the inner planet has a radius of 1.165 ± 0.066 Earth radii and a mass of 2.59 ± 0.43 Earth masses. This Earth-sized planet thus has a core-mass fraction that is compatible with that of Mercury, although it was expected to be similar to that of Earth based on host-star chemistry7. This larger Mercury analogue either formed with a very peculiar composition or has evolved, for example, by losing part of its mantle. Further characterization of Mercury-like exoplanets such as K2-229 b will help to put the detailed in situ observations of Mercury (with MESSENGER and BepiColombo8) into the global context of the formation and evolution of solar and extrasolar terrestrial planets.

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We are grateful to the HARPS observers who conducted part of the visitor-mode observations at La Silla Observatory: R. I. Bustos, N. Astudillo, A. Wyttenbach, E. Linder, X. Bonfils, E. Hébrard and A. Suarez. A.S. thanks E. Hugot for comments on the manuscript. This publication is based on observations collected at the European Organisation for Astronomical Research in the Southern Hemisphere under ESO programme 198.C-0168. This publication makes use of The Data & Analysis Center for Exoplanets (DACE), which is a facility based at the University of Geneva (CH) dedicated to extrasolar planet data visualization, exchange and analysis. DACE is a platform of the Swiss National Centre of Competence in Research (NCCR) PlanetS, federating the Swiss expertise in exoplanet research. The DACE platform is available at https://dace.unige.ch. This research has made use of the NASA (National Aeronautics and Space Administration) Exoplanet Archive, which is operated by the California Institute of Technology, under contract with NASA under the Exoplanet Exploration Program. This research has made use of the VizieR catalogue access tool, CDS (http://vizier.u-strasbg.fr/vizier/surveys.htx). The original description of the VizieR service was published in ref. 76. This publication makes use of data products from the Two-Micron All-Sky Survey, which is a joint project of the University of Massachusetts and the Infrared Processing and Analysis Center/California Institute of Technology, funded by NASA and the National Science Foundation. This publication makes use of data products from the Wide-field Infrared Survey Explorer, which is a joint project of the University of California, Los Angeles, and the Jet Propulsion Laboratory/California Institute of Technology, funded by NASA. The Porto group acknowledges support from Fundação para a Ciência e a Tecnologia (FCT) through national funds and from FEDER through COMPETE2020 by the grants UID/FIS/04434/2013 & POCI–01–0145-FEDER–007672, PTDC/FIS-AST/1526/2014 & POCI–01–0145-FEDER–016886 and PTDC/FIS-AST/7073/2014 & POCI-01-0145-FEDER-016880. FCT is further acknowledged through the Investigador FCT contracts IF/01312/2014/CP1215/CT0004 (S.C.C.B.), IF/00849/2015/CP1273/CT0003 (E.D.M.), IF/00650/2015/CP1273/CT0001 (V.A.), IF/01037/2013/CP1191/CT0001 (P.F.), IF/00169/2012/CP0150/CT0002 (N.C.S.) and IF/00028/2014/CP1215/CT0002 (S.G.S.) and for the fellowships SFRH/BD/93848/2013 (J.P.F.), PD/BD/128119/2016 (S.H.) and PD/BD/52700/2014 (J.J.N.), which are funded by FCT (Portugal) and POPH/FSE (EC). J.L.-B. acknowledges support from the Marie Curie Actions of the European Commission (FP7-COFUND). D.Bar. has been supported by the Spanish grant ESP2015-65712-C5-1-R. D.J.A. is funded under STFC consolidated grant reference ST/P000495/1. D.J.A.B. acknowledges support from the University of Warwick and the UKSA. E.F. is funded by the Qatar National Research Foundation (programme QNRF-NPRP-X-019-1). X.D. is grateful to the Society in Science–The Branco Weiss Fellowship for its financial support. R.L. thanks CNES for financial support through its postdoctoral programme. The project leading to this publication has received funding from Excellence Initiative of Aix-Marseille University–A*MIDEX, a French Investissements d’Avenir programme. The French group acknowledges financial support from the French Programme National de Planétologie (PNP, INSU). This work has been carried out in the frame of the NCCR PlanetS supported by the Swiss National Science Foundation (SNSF).

Author information


  1. Aix Marseille Univ., CNRS, LAM, Laboratoire d’Astrophysique de Marseille, Marseille, France

    • A. Santerne
    • , B. Brugger
    • , H. Gosselin
    • , A. Aguichine
    • , I. Boisse
    • , M. Deleuil
    • , M. Hobson
    • , R. Ligi
    • , O. Mousis
    • , H. P. Osborn
    •  & A. Vigan
  2. Department of Physics, University of Warwick, Coventry, UK

    • D. J. Armstrong
    • , D. J. A. Brown
    • , A. Doyle
    • , F. Faedi
    • , E. Foxell
    • , J. Jackman
    • , G. King
    • , J. Kirk
    • , K. W. F. Lam
    • , T. Louden
    • , J. McCormac
    • , H. P. Osborn
    •  & D. Pollacco
  3. Instituto de Astrofísica e Ciências do Espaço, Universidade do Porto, CAUP, Porto, Portugal

    • V. Adibekyan
    • , S. C. C. Barros
    • , E. Delgado Mena
    • , O. Demangeon
    • , J. P. Faria
    • , P. Figueira
    • , S. Hojjatpanah
    • , J. J. Neal
    • , N. C. Santos
    •  & S. G. Sousa
  4. European Southern Observatory (ESO), Santiago de Chile, Chile

    • J. Lillo-Box
    •  & P. Figueira
  5. Université de Toulouse, UPS-OMP, IRAP, Toulouse, France

    • H. Gosselin
  6. Paris-Saclay Université, ENS Cachan, Cachan, France

    • A. Aguichine
  7. Observatoire Astronomique de l’Université de Genève, Versoix, Switzerland

    • J.-M. Almenara
    • , D. Bayliss
    • , F. Bouchy
    • , R. F. Díaz
    • , X. Dumusque
    • , H. Giles
    • , C. Lovis
    • , F. Pepe
    •  & S. Udry
  8. Depto. de Astrofísica, Centro de Astrobiología (CSIC-INTA), Madrid, Spain

    • D. Barrado
  9. INAF—Osservatorio Astrofisico di Torino, Pino Torinese, Italy

    • A. S. Bonomo
  10. Universidad de Buenos Aires, Facultad de Ciencias Exactas y Naturales, Buenos Aires, Argentina

    • R. F. Díaz
  11. CONICET—Universidad de Buenos Aires, Instituto de Astronomía y Física del Espacio (IAFE), Buenos Aires, Argentina

    • R. F. Díaz
  12. INAF—Osservatorio Atrofisico di Catania, Catania, Italy

    • F. Faedi
  13. Departamento de Física e Astronomia, Faculdade de Ciencias, Universidade do Porto, Porto, Portugal

    • J. P. Faria
    • , S. Hojjatpanah
    • , J. J. Neal
    •  & N. C. Santos
  14. Institut d’Astrophysique de Paris, UMR7095 CNRS, Universite Pierre & Marie Curie, Paris, France

    • G. Hébrard
  15. Aix Marseille Univ., CNRS, OHP, Observatoire de Haute Provence, Saint Michel l’Observatoire, France

    • G. Hébrard


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The Warwick group (D.J.A., D.J.A.B., A.D., F.F., E.F., J.J., G.K., J.K., K.W.F.L., T.L., J.McC., H.P.O., D.P.) detected the candidates. S.C.C.B., O.D. and M.D. reduced the POLAR–K2 light curves used in the candidate search. The Geneva group (D.Bay., F.B., R.F.D., X.D., H.Gi., C.L., F.P., S.U.) organized the HARPS runs, reduced the HARPS data and developed the DACE tool. A.S.B., S.G.S., D.J.A. and J.P.F. performed part of the visitor-mode runs on HARPS. M.H. derived the spectroscopic indices. V.A., E.D.M., N.C.S. and S.G.S. derived the stellar parameters and chemical composition of the star. J.L.-B. and D.Bar. performed the AstraLux observations and derived the background source confidence. H.Go. and A.S. performed the Bayesian analysis of the data with the PASTIS code that was initially developed by R.F.D., J.-M.A. and A.S. B.B., M.D., O.M. and A.A. developed the planet composition model and discussed the mantle evaporation. V.A., J.-M.A., D.Bar., S.C.C.B., D.Bay., I.B., A.S.B., F.B., D.J.A.B., M.D., E.D.M., O.D., J.P.F., P.F., H.Gi., G.H., S.H., R.L., C.L., J.J.N., H.P.O., F.P., D.P., N.C.S., S.G.S., S.U. and A.V. are co-investigators of the ESO–K2 large programme that is coordinated by A.S., who led the ESO proposal. A.S. led this study and wrote most of the manuscript. All authors contributed to the discussion of the paper.

Competing interests

The authors declare no competing interests.

Corresponding author

Correspondence to A. Santerne.

Supplementary information

  1. Supplementary Information

    Supplementary Figures 1–7, Supplementary Tables 1–7