An Ancient Extrasolar System with Five Sub-Earth-size Planets
The chemical composition of stars hosting small exoplanets (with radii less than four Earth radii) appears to be more diverse than that of gas-giant hosts, which tend to be metal-rich. This implies that small, including Earth-size, planets may have readily formed at earlier epochs in the universe's history when metals were more scarce. We report Kepler spacecraft observations of Kepler-444, a metal-poor Sun-like star from the old population of the Galactic thick disk and the host to a compact system of five transiting planets with sizes between those of Mercury and Venus. We validate this system as a true five-planet system orbiting the target star and provide a detailed characterization of its planetary and orbital parameters based on an analysis of the transit photometry. Kepler-444 is the densest star with detected solar-like oscillations. We use asteroseismology to directly measure a precise age of 11.2 ± 1.0 Gyr for the host star, indicating that Kepler-444 formed when the universe was less than 20% of its current age and making it the oldest known system of terrestrial-size planets. We thus show that Earth-size planets have formed throughout most of the universe's 13.8 billion year history, leaving open the possibility for the existence of ancient life in the Galaxy. The age of Kepler-444 not only suggests that thick-disk stars were among the hosts to the first Galactic planets, but may also help to pinpoint the beginning of the era of planet formation.
© 2015 American Astronomical Society. Received 2014 October 22; accepted 2014 December 24; published 2015 January 27. Funding for the Kepler mission is provided by NASA's Science Mission Directorate. The authors thank the entire Kepler team, without whom these results would not be possible. We thank David W. Latham for helpful comments on the manuscript. Funding for the Stellar Astrophysics Centre is provided by The Danish National Research Foundation (grant DNRF106). The research is supported by the ASTERISK project (ASTERoseismic Investigations with SONG and Kepler) funded by the European Research Council (grant agreement No. 267864). The research leading to the presented results has received funding from the European Research Council under the European Community's Seventh Framework Programme (FP7/2007-2013)/ERC grant agreement No. 338251 (StellarAges). This research made use of APLpy, an open-source plotting package for Python hosted at http://aplpy.github.com. T.L.C., G.R.D., W.J.C., R.H., A.M., and Y.P.E. acknowledge the support of the UK Science and Technology Facilities Council (STFC). T.B. was supported by a NASA Keck PI Data Award, administered by the NASA Exoplanet Science Institute. Some of the data presented herein were obtained at the W. M. Keck Observatory from telescope time allocated to the National Aeronautics and Space Administration through the agency's scientific partnership with the California Institute of Technology and the University of California. The Observatory was made possible by the generous financial support of the W. M. Keck Foundation. The authors wish to recognize and acknowledge the very significant cultural role and reverence that the summit of Mauna Kea has always had within the indigenous Hawaiian community. We are most fortunate to have the opportunity to conduct observations from this mountain. D.H. and E.V.Q. acknowledge support by an appointment to the NASA Postdoctoral Program at Ames Research Center administered by Oak Ridge Associated Universities. D.H. also acknowledges NASA grant NNX14AB92G issued through the Kepler Participating Scientist Program and support by the Australian Research Council's Discovery Projects funding scheme (project No. DE40101364). V. Zh. A., N.C.S., and S.G.S. acknowledge support from the European Research Council/European Community under FP7 through Starting grant agreement No. 239953, and from FCT (Portugal) through FEDER funds in program COMPETE, as well as through national funds in the form of grants RECI/FIS-AST/0176/2012 (FCOMP-01-0124- FEDER-027493) and RECI/FIS-AST/0163/2012 (FCOMP- 01-0124-FEDER-027492). V.Zh.A. and S.G.S. also acknowledge grants SFRH/BPD/70574/2010 and SFRH/BPD/47611/ 2008 from FCT (Portugal), respectively. The Robo-AO system is supported by collaborating partner institutions, the California Institute of Technology and the Inter-University Centre for Astronomy and Astrophysics, and by the National Science Foundation under grant Nos. AST-0906060 and AST-0960343, by the Mount Cuba Astronomical Foundation, and by a gift from Samuel Oschin. C.B. acknowledges support from the Alfred P. Sloan Foundation. S.B. acknowledges NSF grant AST-1105930 and NASA grant NNX13AE70G. T.S.M. acknowledges NASA grant NNX13AE91G. C.K. acknowledges support from the Villum Foundation. N.C.S. also acknowledges support in the form of contract reference IF/00169/2012 funded by FCT/MCTES (Portugal) and POPH/FSE (EC). Facilities: Kepler, Keck:I (HIRES), Keck:II (NIRC2), PO:1.5m (Robo-AO)
Published - 0004-637X_799_2_170.pdf
Submitted - 1501.06227v1.pdf