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 many -3formed 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 13.8billion-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.

From stellar to planetary composition: Galactic chemical evolution of Mg/Si mineralogical ratio

“…Taking into account the fact that planet-hosting stars at low metallicities mostly belong to the Galactic thick disk (Haywood 2008;Adibekyan et al 2012b) one can speculate that the oldest terrestrial planetary systems (e.g. Kepler-444;Campante et al 2015) may have different compositions to the terrestrial solar system planets; however, for the exact derivation of the chemical properties of these planets precise abundances of C and O are also needed 4 . While we already derived oxygen abundance for most of the stars in this sample (Bertran de Lis et al 2015) the derivation of carbon abundances is still in progress (Suárez-Andrés et al, in prep.…”

Synergies Between Asteroseismology and Exoplanetary Science

“…One of the most remarkable discoveries so far is Kepler-444, which consists of a K dwarf of age 11.2 ± 1.0 Gyr hosting five sub-Earth-size planets with orbital periods of less than 10 days (Campante et al, 2015). Kepler-444 demonstrated that sub-Earth-size planets have existed for most of the history of our Universe, and the discovery of a pair of low-mass companions in a highly eccentric orbit furthermore showed that the formation of small planets appears to be robust against early truncation of the protoplanetary disk (Dupuy et al, 2016).…”

The unstable fate of the planet orbiting the A star in the HD 131399 triple stellar system

“…Old main-sequence systems (e.g. Campante et al 2015), dust and planets around giant branch stars (Bonsor et al 2014;Trifonov et al 2015;Lillo-Box, Barrado & Correia 2016;Wittenmyer et al 2016), and polluted white dwarfs at a variety of cooling ages (Koester, Gänsicke & Farihi 2014;Hollands et al, in preparation) all provide necessary constraints. Main-sequence planetary studies have now begun to utilize white dwarf atmosphere chemical signatures in their planetary formation models (Carter-Bond et al 2012;Morlok et al 2014;Bergin et al 2015;Ramírez et al 2015;Mordasini et al 2016;Spina, Meléndez & Ramírez 2016), and future missions such as PLATO will further enable comparisons with well-constrained stellar ages (Veras et al 2015b).…”