Source: http://aeon.co/magazine/science/can-life...

o have any chance of life – at least life like our own – a free-floating Earth would need liquid water. And to have liquid water, a planet needs to keep warm. But space is ridiculously cold, just a few degrees above absolute zero. How could a rogue planet stay warm with no Sun? All planets generate heat in their interiors. Most of Earth’s internal heat was delivered by the giant collisions that built the planet, a large portion of which remains locked inside its crust. This heat slowly trickles to the surface, providing a source of internal energy that has endured since the Earth formed. This interior heat will last for billions of years to come, but it’s a puny amount of energy, 3,000 times smaller than the sunlight that blasts the Earth daily. A free-floating planet with no Sun cannot afford to lose any internal heat. Like a person suffering from hypothermia, a rogue planet needs a really warm blanket.

A layer of ice on a planet’s surface can act as a strong insulator, locking in a planet’s heat. If the ice layer is thick enough, then a planet can maintain an ocean of liquid water beneath the ice. But to prevent the ocean from freezing for billions of years, the ice layer needs to be at least 10 km (6 miles) thick. Two of Jupiter’s large moons – Europa and Ganymede – have oceans lurking under miles-deep ice layers and might be analogs for these icy rogue planets. Could Earth, if frozen, transition into this type of planet and still maintain an abode for life in the deep ocean? Unfortunately, no. Earth is too dry; a global ice layer would only be a few kilometres thick, too thin to act as a strong insulator. At best, Earth could possibly maintain local liquid water – for example, near a strong heat source such as a volcano, but not a global ocean.

A thick atmosphere can also act to retain a planet’s internal heat and allow an ice-free rogue planet to maintain liquid water at its surface.