Exploring the<i>α</i>-enhancement of metal-poor planet-hosting stars. The<i>Kepler</i>and HARPS samples

Recent studies have shown that at low metallicities Doppler-detected planet-hosting stars tend to have high α-content and to belong to the thick disk. We used the reconnaissance spectra of 87 Kepler planet candidates and data available from the HARPS planet search survey to explore this phenomenon. Using the traditional spectroscopic abundance analysis methods, we derived Ti, Ca, and Cr abundances for the Kepler stars. In the metallicity region –0.65 < [Fe/H] < –0.3 dex, the fraction of Ti-enhanced thick-disk HARPS planet harboring stars is 12.3 ± 4.1%, and for their thin-disk counterparts this fraction is 2.2 ± 1.3%. Binomial statistics give a probability of 0.008 that this could have occurred by chance. Combining the two samples (HARPS and Kepler) reinforces the significance of this result (P ~ 99.97%). Since most of these stars harbor small sized or low-mass planets we can assume that, although terrestrial planets can be found in a low-iron regime, they are mostly enhanced by α-elements. This implies that early formation of rocky planets could start in the Galactic thick disk, where the chemical conditions for their formation are more favorable.