In recent years, computers have become powerful enough to simulate the formation and evolution of planetary systems over many billions of years.
One of the surprises to come out this work is that planets are regularly kicked out of these systems by slingshot effects. By some calculations, this fate may still await planets in our own Solar System.
One interesting question is whether these so-called “rogue planets” could ever support life in the cold dark reaches of interstellar space.
Today, Dorian Abbot and Eric Switzer at the University of Chicago give us an answer. The generally accepted criteria for life is the presence of liquid water. They calculate that an Earth-like rogue planet could support liquid oceans if the water were heated from below by the planet’s core and insulated from above by a thick layer of ice.
Their reasoning is straightforward. They define an Earth-like planet to have dimensions within an order of magnitude of Earth’s and having a similar composition. They then calculate the heat flux from the core and suggest that the thickness of the ice above would reach a steady state in about a million years. That’s much shorter than the lifetime of a hot core.
Note that this is somewhat different from the mechanism that keeps the subglacial ocean on Europa liquid. Here tidal forces play an important role and this generates heat within the ocean itself. By contrast, all the heat must come from the core of a rogue planet and travel through the ocean,
One important unknown is the role that convection and conduction play in the less viscous regions of ice. Since convection carries heat much more quickly than conduction, this is an important factor and could potentially make the difference between the existence of liquid oceans or solid ice.
But with reasonable assumptions Abbot and Switzer say that a planet just 3.5 times the mass of Earth could maintain a liquid ocean. Even more surprising is their conclusion that a planet with a higher fraction of water need only be 0.3 times the size of Earth and still have a liquid ocean. That’s smaller than Venus but bigger than Mars.
They call such a body a Steppenwolf planet “since any life in this strange habitat would exist like a lone wolf wandering the galactic steppe.” It’s not hard to imagine the possibility of life evolving around hydrothermal vents before the planet’s ejection or even afterwards.
These are exciting calculations. Steppenwolf planets would provide one way for life to spread through the galaxy. And if any come within a 1000 AU of our Sun, the reflected sunlight from them ought to be visible in the far infrared to the next generation of telescopes.
That raises an interesting idea: the possibility of visiting such a place. Any passers by would certainly be easier to get to than planets orbiting other stars.
Time to get out the binoculars and lens cloths and start looking.
Ref: arxiv.org/abs/1102.1108: The Steppenwolf: A Proposal For A Habitable Planet in Interstellar Space