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Life on Earth as we know it requires water and reliable access to phosphorus to drive basic biological processes. Studies of polluted white dwarf stars are able to detect both water and phosphorus in the accreted material from their extant planetary systems. In this proposal we seek to carefully examine the phosphorus content of a set of polluted white dwarf stars that are accreting the crust and mantle regions (the surface) of massive comma differentiated rocky bodies. COS FUV spectroscopic observations of three white dwarf stars -- one water-rich and two relatively water-poor -- along with available data for another water-rich white dwarf will allow us to robustly assess their phosphorus content and compare the biological potential of very wet and more Earth-like worlds.