  Rogue planets have been ejected from their planetary system. We investigate
the possibility that a rogue planet could maintain a liquid ocean under layers
of thermally-insulating water ice and frozen atmosphere as a result of
geothermal heat flux. We find that a rogue planet of Earth-like composition and
age could maintain a subglacial liquid ocean if it were ~3.5 times more massive
than Earth. If a rogue planet had about ten times higher water mass fraction or
a thick cryo-atmospheric layer, it would need to be only ~0.3 times the mass of
Earth to maintain a liquid ocean. Such a planet could be detected from
reflected solar radiation and its thermal emission could be characterized in
the far-IR if it passed within O(1000) AU of Earth.
