An Ancient Extrasolar System with Five Sub-Earth-size Planets

Campante, T. L.; Barclay, T.; Swift, J. J.; Huber, D.; Adibekyan, V. Zh.; Cochran, W.; Burke, C. J.; Isaacson, H.; Quintana, E. V.; Davies, G. R.; Silva Aguirre, V.; Ragozzine, D.; Riddle, R.; Baranec, C.; Basu, S.; Chaplin, W. J.; Christensen-Dalsgaard, J.; Metcalfe, T. S.; Bedding, T. R.; Handberg, R.; Stello, D.; Brewer, J. M.; Hekker, S.; Karoff, C.; Kolbl, R.; Law, N. M.; Lundkvist, M.; Miglio, A.;  Rowe, J. F.; Santos, N. C.; Van Laerhoven, C.; Arentoft, T.; Elsworth, Y. P.; Fischer, D. A.; Kawaler, S. D.; Kjeldsen, H.; Lund, M. N.; Marcy, G. W.; Sousa, S. G.; Sozzetti, A.; White, T. R.

An Ancient Extrasolar System with Five Sub-Earth-size Planets,
The Astrophysical Journal, Vol. 799, Issue 2

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.