  Recent studies showed that at low metallicities Doppler-detected
planet-hosting stars have preferably high alpha-content and 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 phenomena.
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 %. The binomial statistics gives a probability of 0.008 that this
could have occurred by chance. Combining the two samples (HARPS + Kepler)
reinforces the significance of this result (P ~ 99.97 %). Since most of these
stars are harboring small-mass/size planets we can assume that, although
terrestrial planets can be found at low-iron regime, they are mostly enhanced
by alpha-elements. This implies that early formation of rocky planets could get
started in the Galactic thick disk, where the chemical conditions for their
formation were more favorable.
