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 can not be accounted for by a simple correlation between metallicity and planet probability .