Abstract
The first discoveries of exoplanets around Sun-like stars have fueled efforts to find ever smaller worlds evocative of Earth and other terrestrial planets in the Solar System. While gas-giant planets appear to form preferentially around metal-rich stars, small planets (with radii less than four Earth radii) can form under a wide range of metallicities. This implies that small, including Earth-size, planets may have readily formed at earlier epochs in the Universe's history when metals were far less abundant. We report Kepler spacecraft observations of KOI-3158, a metal-poor Sun-like star from the old population of the Galactic thick disk, which hosts five planets with sizes between Mercury and Venus. We used asteroseismology to directly measure a precise age of 11.2+/-1.0 Gyr for the host star, indicating that KOI-3158 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, providing scope for the existence of ancient life in the Galaxy.
Highlights
The NASA Kepler spacecraft [1] uses precise photometry to measure the periodic dips in starlight due to transits of planets across the face of their stars
During spectroscopic observations with the Keck telescope, a fainter companion was visually detected on the HIRES guide camera at an angular distance of 1.8arcsec, being unresolved in Kepler observations
Follow-up observations using the Robo-AO system [4] on the Palomar 60-inch telescope were used to determine the level of resulting contamination in the Kepler bandpass
Summary
The NASA Kepler spacecraft [1] uses precise photometry to measure the periodic dips in starlight due to transits of planets across the face of their stars. KOI-3158 is a cool main-sequence star (spectral type K0V) with apparent magnitude V = 8.86 and lying at a distance of 36 pc [3] in the constellation Lyra, making it the brightest and closest multipleplanet host detected by Kepler. It is a high-proper-motion star [3], with an annual motion across the celestial sphere in excess of 0.5 arcsec. A high-resolution spectrum obtained with the Keck/HIRES spectrograph showed that KOI-3158 has a paucity of heavy elements with only about 30 % of the Sun’s iron abundance It is overabundant in α-process elements (such as silicon and titanium) for a star of its metallicity
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