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Three regimes of extrasolar planet radius inferred from host star metallicities



Approximately half of the extrasolar planets (exoplanets) with radii less than four Earth radii are in orbits with short periods1. Despite their sheer abundance, the compositions of such planets are largely unknown. The available evidence suggests that they range in composition from small, high-density rocky planets to low-density planets consisting of rocky cores surrounded by thick hydrogen and helium gas envelopes. Here we report the metallicities (that is, the abundances of elements heavier than hydrogen and helium) of more than 400 stars hosting 600 exoplanet candidates, and find that the exoplanets can be categorized into three populations defined by statistically distinct (4.5σ) metallicity regions. We interpret these regions as reflecting the formation regimes of terrestrial-like planets (radii less than 1.7 Earth radii), gas dwarf planets with rocky cores and hydrogen–helium envelopes (radii between 1.7 and 3.9 Earth radii) and ice or gas giant planets (radii greater than 3.9 Earth radii). These transitions correspond well with those inferred from dynamical mass estimates2,3, implying that host star metallicity, which is a proxy for the initial solids inventory of the protoplanetary disk, is a key ingredient regulating the structure of planetary systems.

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Figure 1: Host star metallicities and three types of exoplanets with different composition.
Figure 2: The radius of transition from rocky to gaseous exoplanets.


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L.A.B. acknowledges support from the Harvard Origins of Life Initiative. M.B. acknowledges funding from the Danish National Research Foundation (grant number DNRF97) and from the European Research Council under ERC Consolidator grant agreement 616027- STARDUST2ASTEROIDS. D.W.L. acknowledges support from the Kepler Mission under NASA Cooperative Agreements NCC2-1390, NNX11AB99A and NNX13AB58A with the Smithsonian Astrophysical Observatory, and thanks the observers who helped obtain the TRES observations reported here, especially R. Stefanik, G. Esquerdo, P. Berlind and M. Calkins.

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Authors and Affiliations



L.A.B. led the project and developed the classification tools for the metallicity analysis. M.B., D.W.L. and D.S. contributed to the discussion of the theoretical implications of the data. L.A.B., D.W.L., W.D.C., M.E., H.I., D.J. and G.W.M. worked on gathering the spectroscopic observations. All authors discussed the results and commented on the manuscript. L.A.B. and M.B. wrote the paper with input from D.W.L. and D.S.

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Correspondence to Lars A. Buchhave.

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The authors declare no competing financial interests.

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Buchhave, L., Bizzarro, M., Latham, D. et al. Three regimes of extrasolar planet radius inferred from host star metallicities. Nature 509, 593–595 (2014).

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