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Do you remember the TV series "Space: 1999"? An enormous explosion of nuclear waste on the Moon pushes our satellite out of its orbit, marking the start of a long cosmic odyssey for the inhabitants of Moonbase Alpha, in search of habitable worlds and civilizations. Oddly, the base continues to be illuminated by some unknown light source, in stark contrast to the freezing darkness that would actually have afflicted the travelers.
The same thing would happen, if, for some reason, the Earth found itself far from the Sun: it would become an ice planet like that illustrated above.
But what seems like science fiction could be reality for a large number of planets. Various models of the evolution of planetary systems predict, that during the phase of planetary orbital migration, some planets can actually be ejected from the system, escaping the gravitational pull of the central star, to drift through interstellar space.
Taking this as a starting point, Dorian Abbot and Eric Switzer, of the University of Chicago, propose that these "rogue" planets can nonetheless retain a heat source sufficient to maintain liquid water, beneath a frozen surface, and may even provide a habitat suitable for the development of life.
The calculations by the two researchers investigated under what circumstances such a planet might be able to support an ecosystem. They found, that for a planet with a quantity of water similar to that of Earth, it would require a total mass of 3.5 Earth masses to maintain a liquid ocean, under a thick ice layer. If the quantity of water were 10 times that of Earth, then 0.3 Earth masses would be sufficient.
For these limiting cases, and those in between, a thick frozen atmosphere helps to form an insulating layer. Underneath the thick ice layer, within the rocks and mantle, geothermal heat both left over from its formation and produced by the decay of radioactive elements (K-40, U-238, Th-232) can keep water in the liquid state for between 1 and 5 billion years (depending on the mass of the planet).
This is more than enough for the onset of primitive life, perhaps near geothermal vents, just as may have happened on Earth.
All this means that the diffusion of life in the Cosmos may need neither the light of a nearby star nor an orbit in the so-called "habitable zone". A real conceptual revolution!
Although it would be possible to detect a rogue planet, using existing infrared instruments, if it passed within 1000 astronomical units of the Earth, such an event must be exceptionally rare given the mean separation between stars. Nonetheless, the possibility that a planet harbouring life could pass by our solar system is certainly intriguing.
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