On the correlation between metallicity and the presence of giant planets
  The correlation between stellar metallicity and the presence of giant planets
is well established. It has been tentatively explained by the possible increase
of planet formation probability in stellar disks with enhanced amount of
metals. However, there are two caveats to this explanation. First, giant stars
with planets do not show a metallicity distribution skewed towards metal-rich
objects, as found for dwarfs. Second, the correlation with metallicity is not
valid at intermediate metallicities, for which it can be shown that giant
planets are preferentially found orbiting thick disk stars.
  None of these two peculiarities is explained by the proposed scenarios of
giant planet formation. We contend that they are galactic in nature, and
probably not linked to the formation process of giant planets. It is suggested
that the same dynamical effect, namely the migration of stars in the galactic
disk, is at the origin of both features, with the important consequence that
most metal-rich stars hosting giant planets originate from the inner disk, a
property that has been largely neglected until now. We illustrate that a
planet-metallicity correlation similar to the observed one is easily obtained
if stars from the inner disk have a higher percentage of giant planets than
stars born at the solar radius, with no specific dependence on metallicity. We
propose that the density of molecular hydrogen in the inner galactic disk (the
molecular ring) could play a role in setting the high percentage of giant
planets that originate from this region.
