How metallicity affects volatile abundances: implications for planetary system formation

Abstract

ABSTRACT Astronomers have confirmed the existence of several thousand extrasolar planetary systems having a wide range of orbital and compositional characteristics. A host star’s metallicity, defined as the abundance of all elements heavier than helium (metals), appears to play a role in determining whether an exoplanetary system is more likely to include Jupiter-sized gas and ice giants. Here, we show how molecular cloud (MC) metallicity is likely to significantly affect the initial conditions of planetary formation by affecting the abundances of volatile ices (H2O, CO, etc.) in parent MCs. Through analytic and numerical treatments of molecular chemical lifetimes, we show that volatile elements are more likely to be found as ices in metal-rich clouds compared to metal-poor ones. These correlations, in turn, may impact the characteristics of planetary systems as a function of their metallicity as suggested by the systematic shifts in snowline distances as a function of metallicity. We evaluate the ‘wet Earth’ hypothesis for the origins of Earth’s water and find that elevated protoplanetary disc pressures are required to retain the required partial (∼2 per cent) monolayer of water on interstellar dust grain surfaces with MRN distribution.