Silicon and oxygen abundances in planet-host stars

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 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 60 planet hosts and a control sample of 60 metal-rich stars without giant planets. 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. Due to our large error bars on oxygen abundances, we do not yet observe any correlation between oxygen abundance and planet detection rate. We predict that a correlation between planet occurrence and oxygen abundance should emerge when we can measure [O/Fe] at 0.05 dex precision. Since up to 20% of the carbon in the universe may be in refractory grains, we also predict that planet detection rate should correlate positively with host star carbon abundance for any population of planets formed by core accretion.