  We present a uniform study of the chemical abundances of 12 elements (Si, Ca,
Sc, Ti, V, Cr, Mn, Co, Ni, Na, Mg, and Al) derived from the spectra of 451
stars observed as part of one of the HARPS GTO planet search programs. Sixty
eight of these are planet-bearing stars. The main goals of our work are: i) the
investigation of possible differences between the abundances of stars with and
without planets; ii) the study of the possible differences in the abundances of
stars in the thin and the thick disc. We confirm that there is a systematically
higher metallicity in planet host stars, when compared to non planet-hosts,
common to all studied species. We also found that there is no difference in the
galactic chemical evolution trends of the stars with and without planets. Stars
that harbour planetary companions simply appear to be in the high metallicity
tail of the distribution. We also confirm that Neptunian and super-Earth class
planets may be easier to find at lower metallicities. A statistically
significative abundance difference between stars of the thin and the thick disc
was found for [Fe/H] $<$ 0. However, the populations from the thick and the
thin disc cannot be clearly separated.
