  With the goal of confirming the metallicity ``excess'' observed in stars with
planetary mass companions, we have conducted a high-precision spectroscopic
study of a ``comparison'' sample of dwarfs included in the CORALIE extra-solar
planet survey (Santos, Israelian, & Mayor 2001). The targets were chosen
following two basic criteria: they make part of a limited volume and they do
not present the signature of a planetary host companion. The spectroscopic
analysis, done using the very same technique as previous works on the
metallicity of stars with planets, permitted a direct and non-biased comparison
of the samples. The results have revealed that metallicity plays an impressive
role on the giant planet formation. The chemical composition of the molecular
cloud is probably the key parameter to form giant planets. Some evidences
exist, however, showing the possibility of accretion of matter in the stellar
outer convective zone. These conclusions impose serious constraints on the
planetary systems formation and evolution models.
