Planets that lose their parent stars can stay warm enough to have water, raising the prospect that they also have life.
A new study shows that heat from the natural decay of radioactive elements could keep a world like Earth relatively toasty - provided it has a layer of insulating ice.
"You just need a thick enough blanket on top," says University of Chicago cosmologist Dr Eric Switzer.
The study began after a chance remark by a colleague, an environmental scientist, who wondered what Earth would be like if the Sun went out. That piqued Switzer's curiosity and he started to do some math.
It turns out that without the Sun's heat, a planet like Earth would need a sheet of ice 15 kilometres thick to hold heat generated by the decay of naturally occurring radioactive elements, such as potassium-40, uranium-238 and thorium-232, as well as the primordial energy left over from the planet's formation.
The ice cap could be thinner and still protect water if it were topped with a layer of frozen atmospheric gas, such as dry ice, which is solid carbon dioxide.
Even without a sun, a planet could stay warm for what Switzer calls "a biologically significant" amount of time - enough time for life to evolve.
"You could get something like this set up and it could take one to 10 billion years until it freezes all the oceans out," says study co-author Dr Dorian Abbot.
Life on Jovian, Saturnian moons?
Switzer doesn't speculate on what type of life could exist on a Sun-less planet, but ice-shrouded moons of Jupiter and Saturn might hold some clues. Scientists believe liquid oceans exist beneath the frozen surfaces of Jupiter's Europa and Callisto, as well as Saturn's Enceladus.
"It's a possible habitat," says Abbot. "There's energy sources and there's water and there are chemicals that life might need."
The idea of rogue planets wandering the galaxy is not as far-fetched as it may sound. Gravitational tugs-of-war with other planets or passing stars can boot a planet out of its solar system.
"The calculations about the formation of planetary systems suggest that a lot of planets are ejected like this - about one or two per system," says Abbot.
Even Earth may have some sister planets that left the solar system. "It's likely that that is the case, but we don't have any evidence for that, of course," he says.
The study has been submitted to The Astrophysical Journal Letters and appears on the pre-press website arxiv.org.