Context : Recent studies have shown that at low metallicities Doppler-detected planet-hosting stars tend to have high \alpha -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 \pm 4.1 % , and for their thin-disk counterparts this fraction is 2.2 \pm 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 \sim 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 \alpha -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 .

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