THE SURVIVAL OF WATER WITHIN EXTRASOLAR MINOR PLANETS

Abstract

We compute that extrasolar minor planets can retain much of their internal\nH_2O during their host star's red giant evolution. The eventual accretion of a\nwater-rich body or bodies onto a helium white dwarf might supply an observable\namount of atmospheric hydrogen, as seems likely for GD 362. More generally, if\nhydrogen pollution in helium white dwarfs typically results from accretion of\nlarge parent bodies rather than interstellar gas as previously supposed, then\nH_2O probably constitutes at least 10% of the aggregate mass of extrasolar\nminor planets. One observational test of this possibility is to examine the\natmospheres of externally-polluted white dwarfs for oxygen in excess of that\nlikely contributed by oxides such as SiO_2. The relatively high oxygen\nabundance previously reported in GD 378 plausibly but not uniquely can be\nexplained by accretion of an H_2O-rich parent body or bodies. Future\nultraviolet observations of white dwarf pollutions can serve to investigate the\nhypothesis that environments with liquid water that are suitable habitats for\nextremophiles are widespread in the Milky Way.\n