  Stars with planets at intermediate metallicities ([-0.7,-0.2] dex) exhibit
properties that differ from the general field stars. Thirteen stars with
planets reported in this metallicity range belong to the thick disc, while only
one planet have been detected among stars of the thin disc. Although this
statistics is weak, it contradicts the known correlation between the presence
of planet and metallicity. We relate this finding to the specific property of
the thin disc in this metallicity range, where stars are shown to rotate around
the Galaxy faster than the Sun. Their orbital parameters are conveniently
explained if they are contaminants coming from the outer Galactic disc, as a
result of radial mixing. This must be considered together with the fact that
metal-rich stars ([Fe/H]>+0.1 dex) found in the solar neighbourhood, which are
the hosts of most of the detected planets, are suspected of being wanderers
from the inner Galactic disc. It is then questionned why stars that originate
in the inner and outer thin disc show respectively the highest and lowest rate
of detected planets. It is suggested that the presence of giant planets might
be primarily a function of a parameter linked to galactocentric radius, but not
metallicity. Combined with the existing radial metallicity gradient, then
radial mixing explains the correlation at high metallicity observed locally,
but also the peculiarity found at low metallicity, which cannot be accounted
for by a simple correlation between metallicity and planet probability.
