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A Living Planet Between the Stars?

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).


Comments on this entry are closed.

  • Alex Tolley February 10, 2011, 11:22

    Why does this planet need an ocean? We know that bacterial life exists in the hot rocks deep in the crust. All that is needed is just enough liquid water percolating between the grains. This would avoid the need for a liquid ocean unless you demand multi-cellular organisms as a life form.

    As for the “The Man from Planet X” scenario, we never saw the details of their planet, but since the alien had a space ship and some advanced technology, the planet could have been quite artificial.

  • Marc G Millis February 10, 2011, 11:32

    And dare I mention “Space 1999” about our wandering Moon? Them Eagles blowwed up real good!

  • Mike February 10, 2011, 11:32

    If there were any rogue planets wandering nearby I wonder if WISE could spot them. There could be numerous rogue planets if certain models of stellar system formation are correct. Those models that predict the ejection of planets, particulary the lower mass planets.
    If that is the case then billions of rogue planets wander the Milky Way. Are they a hazard to navigation?
    I wonder at what distance WISEs’ mid-infrared detector could spot one. Or will that have to be a job for lensing observation projects.

  • Istvan February 10, 2011, 12:54

    I find the natural occurrence of a life-bearing rogue planet fairly unlikely, but I certainly remember being impressed by Larry Niven’s vision, in Ringworld and related works, of inhabited planets being moved artificially.

    The situation was expressed as a solution for dealing with the civilization’s excess heat generation. They moved their planet outward from their primary, apparently a number of times, to keep it in thermal balance, until they recognized they really didn’t need the star anymore for much beyond an anchor. When they had reason to migrate elsewhere, they already had experience moving their world. A fun thought-experiment in ultratech.

  • kzb February 10, 2011, 14:29

    Here is a link to the arxiv version of the Debes and Sigurosson paper


    The topic of free-floating planets is a fascinating one in itself, when you consider that the current models predict more planets being ejected than retained. You also can consider the planetary-mass objects (PMOs) formed directly like stars. These are likely massive Jovian-plus objects and could have satellites with significant tidal heating.

    One question though. Most water in the inner solar system planets is said to have come from comet and asteriod impacts much later than all the planet ejection was going on?

  • John Gallagher February 10, 2011, 19:47

    What a stable platform for a long term stable civilization! Free from the variations and orbital impacters of its system of birth. Perhaps not enough excitement to drive the evolution of complex forms of life but still prime reale estate for technical exploitation. Could be Seti needs to think about the between places and move its gaze from the crucibles.

  • TomB February 10, 2011, 20:26

    The concept of rogue planets is hardly new. A science fiction story from the 1930’s predates “The man from planet X”…that was “When Worlds Collide” and a movie based on the book was done in the 1950’s. Rumor has it that a remake will be out in the next year ot so. The TV series,, “Star Trek Enterprise” likewise had a story called “Rogue Planet” which was based on this idea. How true is this..who knows..will probably be along time before we find the answer. Very interesting though.

  • spaceman February 10, 2011, 21:58

    The existence of free-floating planets may no longer be a matter of speculation. In this 2010 presentation by OSU astronomer Scott Gaudi, have a look at page 45 in which the paper (in prep) says that there may be wide or free-floating planets in high numbers.

  • Michael Spencer February 11, 2011, 7:30

    Are we living in interesting times, or what?

    Here we have top-flight scientists speculating on seemingly outlandish notions. But are they? Heady days indeed, in advance of what is sure to come: time will slowly wrap her arms around the the facts, slowly limiting the upper and lower bounds of possibilities.

    Until then, the speculation is invigorating as hell.

  • Alex Tolley February 11, 2011, 10:28

    @ Tom B. In “When Worlds Collide” (the movie version), the planet was not rogue, it was in orbit about the approaching star, Bellatrix.

  • kzb February 11, 2011, 12:17

    Mike: I don’t think WISE could detect a planet as in this paper. It is only sensitive to objects warmer than 70K. If you look at the plot in the paper, the ice surface temperature is less than 50K even for a planet ten times as massive as Earth.

    Having said that, WISE is said to be able to detect a Neptune at 1000AU or a Jupiter at 1 light-year. That is based on calculated cooling curve from formation of those kinds of planets.

  • TomB February 11, 2011, 14:09

    You are correct for the movie version, it was Bullus (star) and Zyra planet. I thank you for the clarification. In the book it was Bronson Alpha and Bronson Beta both planets. IMHO the book When Worlds Collide and its sequal After Worlds Collide were much better then the movie….Tom

  • Kenneth Harmon February 11, 2011, 14:59

    Question for All,

    Now that it seems possible to have “Steppenwolf Planets” as well ejected single Stars roaming between Galaxies is there any reason why there could not be entire ejected Trinary Stars Systems such as an Alpha Centauri like Star System but with multiple habitable planets roaming between Galaxies? For some reason I have always been fascinated by the idea of a single 3-5 Star System that has just the right separations and complexion to support a significant number of habitable planets and moons, roaming between Galaxies. Do you believe that it is possible for such a Star System to be ejected in its entirety from a Galaxy and maintain its overall integrity?

  • Tarmen February 12, 2011, 14:58

    Nature often demonstrates a survival and reproduction strategy of casting thousands and millions of seeds or larvae into the wind or the ocean, knowing full well that only a very few will survive and find a warm wet patch to grow.
    So , I imagine that many frozen seeds/pod/eggs are to be found aboard interstellar planets, comets , or purpose-built craft. Most never to awake.
    It would be natural.

  • Adam February 13, 2011, 6:10

    Then there’s the theory of Carl Gibson that stars form from “primordial planets” – originally small over-densities from the Big Bang, which collapsed quickly and brewed up “The Stuff of Life”. The Journal of Cosmology covers his ideas – he’s one of the founders, so that’s inevitable.

  • kzb February 14, 2011, 14:22

    Adam, I’ve been reading the articles of Carl Gibson & Rudy Schild with some fascination for a while now.

    Certainly it gives one an alternative point of view of the universe, and it apparently explains a great deal. As far as I can tell however, they seem to be ignored by the professional astronomical community. What is missing is some professional critique of their theories.

    The biggest problems I have with their ideas are
    (1) if there are 30,000 planetary-sized frozen planets per star, how come we don’t detect them, even by accident? (Schild & Gibson maintain that we DO detect them -in the Helix Nebula and in quasar microlensing -however I’d have thought they’d be more obvious than this)

    (2) The idea of star formation by merging of these planetary bodies does not see to be in accord with observations.

    If anyone is interested in these ideas, these two articles are a good start:

    Gravitational hydrodynamics vs observations of voids, Jeans clusters and MACHO dark matter

    Do micro brown dwarf detections explain the galactic dark matter?

  • Adam February 18, 2011, 22:21

    Hi kzb
    I know what you mean. Does seem odd if they’re so prevalent that they’re not more observable. Thus I read their papers with a measure of scepticism.

  • Dex July 20, 2012, 13:49

    I wonder with all the ejected mass in the interstellar medium, that mankind would make the journey to the stars using slower than light transport. Our travelers would be like arabs on slow camels going from one oasis to another. They would be settleing on each worldlett and then some would continue on twords a star. This would take a few generations. It would be like a route 66 in space.