Planets are sometimes ejected from their solar systems as other planets or passing stars whip up extreme gravitational forces. These starless worlds become cosmic wanderers, freezing slowly to death in the vast emptiness of space. But even that doesn't have to be the end - under the right conditions, these wandering planets could sustain life for billions and billions of years.
University of Chicago astrophysicists Dorian Abbot and Eric Switzer have calculated that an Earth-sized planet could remain habitable for at least a billion years. The heat would come from the decay of radioactive elements inside the planet's core, which would be enough heat to keep the oceans liquid beneath a thick layer of ice. The surface would likely be uninhabitable, but life could still thrive beneath the ice for practically indefinite amounts of time.
The more water the planet has, the warmer it can be. A planet with the same relative amount of water as Earth would need to be about 3.5 times Earth's mass to keep the water liquid, whereas a planet with ten times our planet's water concentration would only need to weigh about a third as much as our planet.
It's a fascinating thought, although we're unlikely to spot one of these planets without a star to point us in the right direction. Actually, it might be for the best if they remain undetected - they're all probably just full of Cybermen anyway.