A planet that wanders through the night far from any star is a fascinating notion, one that resonates on some primal level with me because of my childhood viewing of the 1951 film The Man from Planet X. In the movie, a scientist on a remote Scottish moor observes a rogue planet as it approaches the Earth, and deals with a visitor from that world whose apparent good intentions are brought to ruin by a self-serving character intent on exploiting the situation. I doubt similar viewing of this old classic motivated many of my readers, but evidently the idea of a rogue planet does inspire thought, given how many people wrote me about new work on the idea of wandering planets.
The paper is by Dorian Schuyler Abbot and Eric Switzer (University of Chicago) and follows up studies of similar ‘dark’ planets by John Debes (Carnegie Institution) and Steinn Sigurðsson (Penn State) — more about the latter duo in a moment. For now, focus on the process. We know that planets can be thrown out of their solar systems because of their gravitational interactions with gas giants. Indeed, the idea of planetary migration, which implies accompanying ejection events at least some of the time, has become a standard in explaining system formation.
Habitability Between the Stars
The question that interests the researchers above is whether or not such a planet might become a home for life. If so, it could be a potential carrier of life everywhere it went, an example of interstellar panspermia. One possibility for habitability is a planet with an extremely high pressure hydrogen atmosphere, which could result in a greenhouse effect strong enough to maintain liquid water on the planetary surface thanks to geothermal heat. But Abbot and Switzer focus on sub-glacial liquid water on a planetary body roughly like the Earth. And let’s qualify that further: “By Earth-like, we mean specifically within an order of magnitude in mass and water complement, similar in composition of radionuclides in the mantle, and of similar age.”
Energy from an active mantle could create a habitable environment deep below the ice of this ‘lone wolf’ planet. The authors go on to comment:
We can imagine that the ice layer on top of an ocean on a Steppenwolf planet will grow until either it reaches steady-state or all available water freezes. Geothermal heat from the interior of the Steppenwolf planet will be carried through the ice layer by conduction, and potentially by convection in the lower, warmer, and less viscous portion of the ice layer. Since convection transports heat much more efficiently than conduction, the steady-state ice thickness will be much larger if convection occurs, making it harder to maintain a subglacial ocean.
To arrive at a world with an internal ocean, the scientists first calculate the thickness of the ice layer under the assumption that heat is lost solely through conduction, then go on to show that heat loss dominated by conduction would be a reasonable assumption. The micro-scale composition of the ice is the wild card here, and the authors acknowledge that the convection issue is problematic. They call the resulting world a ‘Steppenwolf’ planet because ‘any life in this strange habitat would exist like a lone wolf wandering over the galactic steppe.’ It’s an enchanting thought, this dark world moving through the deep carrying the spark of life, yet by calculating the heat flux from the core, the authors show that it is not beyond possibility.
Characterizing the Rogue Planet
So what would it take to produce the Steppenwolf planet? Assuming a world similar to Earth in water mass fraction, radionuclide composition and age, and assuming it has no frozen CO2 layer, the world would have to be roughly 3.5 times as massive as the Earth. Supply it with ten times the amount of water or a thick, frozen CO2 layer, and a mass only slightly larger than Earth’s is required for the liquid ocean to exist. But what really surprises me are the authors’ calculations on the potential lifetime of such a planet. Take a look:
A Steppenwolf planet’s lifetime will be limited by the decay of the geothermal heat flux, which is determined by the half-life of its stock of radioisotopes (40 K, 238U, 232Th) and by the decay of its heat of formation. These decay times are ∼1−5 Gyr, so its lifetime is thus comparable to planets in the traditional habitable zone of main-sequence stars.
We can imagine various ways for life to have found a home here, the most obvious being that it could originate when the planet was still within the solar system that gave it birth. But we also know that life can form around hydrothermal vents, another possibility the authors suggest. In either case, a rogue planet of this kind would be a spectacular mission destination, assuming we could find one passing close enough to our system. Abbot and Switzer calculate that a detection would be possible within roughly 1000 AU of the Sun, where ‘detection of reflected sunlight in the optical wavebands and IR follow-up present the only viable observational choices in the near term.’ What a detection it would be, and what a strange laboratory for interstellar life.
I mentioned the work of John Debes and Steinn Sigurðsson earlier because their own work suggests that internal heat could maintain an atmosphere and sustain a liquid ocean under the ice of a wandering planet. Debes went on to show through simulations that a planet with a large moon could actually survive the ejection process with the moon still in orbit, an additional factor re life because it would supply tidal energies that could cause the interior of the planet to warm. The case for life wandering the interstellar dark may not be so far fetched after all.
For more, see Abbot and Switzer, “The Steppenwolf: A proposal for a habitable planet in interstellar space” (preprint). The Debes and Sigurðsson work is “The Survival Rate of Ejected Terrestrial Planets with Moons,” Astrophysical Journal 668 (October 20, 2007), L167-L170 (abstract).