  We study the prospects for life on planets with subsurface oceans, and find
that a wide range of planets can exist in diverse habitats with ice envelopes
of moderate thickness. We quantify the energy sources available to these
worlds, the rate of production of prebiotic compounds, and assess their
potential for hosting biospheres. Life on these planets is likely to face
challenges, which could be overcome through a combination of different
mechanisms. We estimate the number of such worlds, and find that they may
outnumber rocky planets in the habitable zone of stars by a few orders of
magnitude.
