  The positive correlation between planet detection rate and host star iron
abundance lends strong support to the core accretion theory of planet
formation. However, iron is not the most significant mass contributor to the
cores of giant planets. Since giant planet cores are thought to grow from
silicate grains with icy mantles, the likelihood of gas giant formation should
depend heavily on the oxygen and silicon abundance of the planet formation
environment. Here we compare the silicon and oxygen abundances of a set of 76
planet hosts and a control sample of 80 metal-rich stars without any known
giant planets. Our new, independent analysis was conducted using high
resolution, high signal-to-noise data obtained at McDonald Observatory. Because
we do not wish to simply reproduce the known planet-metallicity correlation, we
have devised a statistical method for matching the underlying [Fe/H]
distributions of our two sets of stars. We find a 99% probability that planet
detection rate depends on the silicon abundance of the host star, over and
above the observed planet-metallicity correlation. We do not detect any such
correlation for oxygen. Our results would thus seem to suggest that grain
nucleation, rather than subsequent icy mantle growth, is the important limiting
factor in forming giant planets via core accretion. Based on our results and
interpretation, we predict that planet detection should correlate with host
star abundance for refractory elements responsible for grain nucleation and
that no such trends should exist for the most abundant volatile elements
responsible for icy mantle growth.
