@article{a4137fa3798347c8be2d33057a9738bf,
title = "An ancient extrasolar system with five sub-Earth-size planets",
abstract = "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.",
keywords = "Galaxy: disk, planetary systems, stars: individual (HIP 94931), stars: late-type, stars: oscillations, techniques: photometric",
author = "Campante, {T. L.} and T. Barclay and Swift, {J. J.} and D. Huber and Adibekyan, {V. Zh.} and W. Cochran and Burke, {C. J.} and H. Isaacson and Quintana, {E. V.} and Guy Davies and {Silva Aguirre}, V. and D. Ragozzine and R. Riddle and C. Baranec and S. Basu and Chaplin, {W. J.} and J. Christensen-dalsgaard and Metcalfe, {T. S.} and Bedding, {T. R.} and R. Handberg and D. Stello and Brewer, {J. M.} and S. Hekker and C. Karoff and R. Kolbl and Law, {N. M.} and M. Lundkvist and A. Miglio and Rowe, {J. F.} and Santos, {N. C.} and {Van Laerhoven}, C. and T. Arentoft and Elsworth, {Y. P.} and Fischer, {D. A.} and Kawaler, {S. D.} and H. Kjeldsen and Lund, {M. N.} and Marcy, {G. W.} and Sousa, {S. G.} and A. Sozzetti and White, {T. R.}",
year = "2015",
month = jan,
day = "27",
doi = "10.1088/0004-637X/799/2/170",
language = "English",
volume = "799",
journal = "The Astrophysical Journal",
issn = "0004-637X",
publisher = "Institute of Physics Publishing Ltd",
number = "2",
}