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A compact system of small planets around a former red-giant star


Planets that orbit their parent star at less than about one astronomical unit (1 au is the Earth–Sun distance) are expected to be engulfed when the star becomes a red giant1. Previous observations have revealed the existence of post-red-giant host stars with giant planets2,3,4 orbiting as close as 0.116 au or with brown dwarf companions5,6 in tight orbits, showing that these bodies can survive engulfment. What has remained unclear is whether planets can be dragged deeper into the red-giant envelope without being disrupted and whether the evolution of the parent star itself could be affected7,8,9. Here we report the presence of two nearly Earth-sized bodies orbiting the post-red-giant, hot B subdwarf star KIC 05807616 at distances of 0.0060 and 0.0076 au, with orbital periods of 5.7625 and 8.2293 hours, respectively. These bodies probably survived deep immersion in the former red-giant envelope. They may be the dense cores of evaporated giant planets that were transported closer to the star during the engulfment and triggered the mass loss necessary for the formation of the hot B subdwarf, which might also explain how some stars of this type did not form in binary systems.

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Figure 1: Brightness variations detected in KIC 05807616.
Figure 2: The calculated properties of the first planet candidate (KOI 55.01) depend on the inclination of the system.


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S.C. thanks the Programme National de Physique Stellaire (PNPS, CNRS/INSU, France) for support. G.F. acknowledges the support of the NSERC of Canada and the contribution of the Canada Research Chair Program. MMT spectra for KIC 05807616 were obtained at the MMT Observatory, a joint facility of the University of Arizona and the Smithsonian Institution. A.S.B. acknowledges funding from the Polish Ministry of Science and Higher Education. S.C. thanks R. Gilliland for help in managing Kepler's procedures. We acknowledge the Kepler team and all who have contributed to making this mission possible. Funding for the Kepler mission is provided by NASA’s Science Mission Directorate.

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S.C. wrote the manuscript, and analysed and interpreted the data from which the presence of planetary bodies was inferred. S.C., G.F., P.B. and V.V.G. derived and checked the calculations to estimate the planets’ properties. G.F., P.B. and S.K.R. computed the cut-off frequencies and the theoretical mode visibilities based on a model of the star from V.V.G. E.M.G. obtained and analysed radial velocity measurements. S.D.K., A.S.B., R.H.Ø, R.S. and J.H.T. from the KASC WG11 group independently checked the detection of the orbital signals in the data. All authors discussed the results and contributed to their interpretation.

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Correspondence to S. Charpinet.

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The authors declare no competing financial interests.

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Supplementary Information

This file comprises Supplementary Sections A-E, which contain Supplementary Text and Data, Supplementary Figures with legends Supplementary Tables and additional references – see Contents on page 1 for more details. (PDF 2452 kb)

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Charpinet, S., Fontaine, G., Brassard, P. et al. A compact system of small planets around a former red-giant star. Nature 480, 496–499 (2011).

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