Liquid water may survive on free-floating planets that have no star to warm them. If they also support life, they could act as stepping stones to spread life around the galaxy.
Gravitational tussles with other planets or passing stars can eject planets from their solar systems. But even in the cold of space, these wayward worlds could stay warm, thanks to the decay of radioactive elements in their rocky cores.
Dorian Abbot and Eric Switzer of the University of Chicago calculate that rocky planets with a similar mass to Earth could remain warm enough to keep water liquid under thick, insulating ice sheets for over a billion years. A planet with the same fraction of water as Earth could keep a subsurface ocean liquid if it was 3.5 times Earth's mass. But a planet with 10 times Earth's water concentration could do this if it weighed just one-third as much as Earth, they say (arxiv.org/abs/1102.1108).
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