AN ANCIENT EXTRASOLAR SYSTEM WITH FIVE SUB-EARTH-SIZE PLANETS

作者：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; Aguirre V Silva; 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

来源：Astrophysical Journal, 2015, 799(2): 170.

DOI：10.1088/0004-637X/799/2/170

• Galaxy: disk
• planetary systems
• stars: individual (HIP 94931)
• stars: late-type
• stars: oscillations
• techniques: photometric

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 +/- 1.0Gyr 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.

• 出版日期2015-2-1