  We compute that extrasolar minor planets can retain much of their internal
H_2O during their host star's red giant evolution. The eventual accretion of a
water-rich body or bodies onto a helium white dwarf might supply an observable
amount of atmospheric hydrogen, as seems likely for GD 362. More generally, if
hydrogen pollution in helium white dwarfs typically results from accretion of
large parent bodies rather than interstellar gas as previously supposed, then
H_2O probably constitutes at least 10% of the aggregate mass of extrasolar
minor planets. One observational test of this possibility is to examine the
atmospheres of externally-polluted white dwarfs for oxygen in excess of that
likely contributed by oxides such as SiO_2. The relatively high oxygen
abundance previously reported in GD 378 plausibly but not uniquely can be
explained by accretion of an H_2O-rich parent body or bodies. Future
ultraviolet observations of white dwarf pollutions can serve to investigate the
hypothesis that environments with liquid water that are suitable habitats for
extremophiles are widespread in the Milky Way.
