  Observational studies show that the probability of finding gas giant planets
around a star increases with the star's metallicity. Our latest simulations of
disks undergoing gravitational instabilities (GIs) with realistic radiative
cooling indicate that protoplanetary disks with lower metallicity generally
cool faster and thus show stronger overall GI-activity. More importantly, the
global cooling times in our simulations are too long for disk fragmentation to
occur, and the disks do not fragment into dense protoplanetary clumps. Our
results suggest that direct gas giant planet formation via disk instabilities
is unlikely to be the mechanism that produced most observed planets.
Nevertheless, GIs may still play an important role in a hybrid scenario,
compatible with the observed metallicity trend, where structure created by GIs
accelerates planet formation by core accretion.
