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Free-Floating Planets as Interstellar Arks

We haven’t found any technosignatures among the stars, but the field is young and our observational tools are improving steadily. It’s worth asking how likely an advanced civilization will be to produce the kind of technosignature we usually discuss. A Dyson swarm should produce evidence for its existence in the infrared, but not all advanced technologies involve megastructures. Even today we can see the movement of human attention into cyberspace. Would a civilization living primarily within virtual worlds produce a detectable signature, or would it more or less wink out of observability?

In 2020, Valentin Ivanov (ESO Paranal) and colleagues proposed a modification to the Kardashev scale based on how a civilization integrates with its environment (citation below). The authors offered a set of classes. Class 0 is a civilization that uses the environment without substantially changing it. Class 1 modifies its environment to fit its needs, while Class 2 modifies itself to fit its environment. A Class 3 civilization under this scheme would be maddeningly difficult to find because it is indistinguishable from its environment.

This gets speculative indeed, as the Ivanov paper illustrates:

The new classification scheme allows for the existence of quiet advanced civilizations that may co-exist with us, yet remain invisible to our radio, thermal or transit searches. The implicit underlying assumption of Hart (1975) is that the hypothetical ETC [Extraterrestrial Civilization] is interacting with matter on a similar level as us. We cannot even speculate if it is possible to detect a heat leak or a transiting structure build by an ETC capable of interacting with matter at sub-quark level, but the answer is more likely negative and not because that ETC would function according to some speculative physics laws, but because such an ETC would probably be vastly more efficient than us controlling its energy wastes and minimizing its construction projects. Would such an advanced ETC even need megastructures and vast astroengineering projects?

‘Rogue’ Planets and Their Uses

Apart from reconsideration of Kardashev assumptions about available energy as a metric of civilizational progress, it’s always useful to be reminded that we need to question our anthropocentric leanings. We need to consider the range of possibilities advanced civilizations may have before them, which is why a new paper from Irina Romanovskaya catches my eye. The author, a professor of physics and astronomy in the Houston Community College System, argues for planetary and interstellar migration as drivers for the kind of signature we might be able to spot. A star undergoing the transition to a red giant is a case in point: Here we would find a habitable zone being pushed out further from the star, and conceivably evidence of the migration of a culture to the more distant planets and moons of its home system.

Evidence for a civilization expanding to occupy the outer reaches of its system could come in the form of atmospheric technosignatures or infrared-excess, among other possibilities. But it’s in moving to other stars that Romanovskaya sees the likeliest possibility of a detectable signature, noting that stellar close passes could be times to expect movement on a large scale between stars. Other mechanisms also come to mind. We’ve discussed stellar engines in these pages before (Shkadov thrusters, for example), which can move entire stars. Romanovskaya introduces the idea that free-floating planets could be an easier and more efficient way to migrate.

Consider the advantages, as the author does in this passage:

Free-floating planets can provide constant surface gravity, large amounts of space and resources. Free-floating planets with surface and subsurface oceans can provide water as a consumable resource and for protection from space radiation. Technologies can be used to modify the motion of free-floating planets. If controlled nuclear fusion has the potential to become an important source of energy for humankind (Ongena and Ogawa, 2016; Prager, 2019), then it may also become a source of energy for interstellar travelers riding free-floating planets.

What a free-floating, or ‘rogue’ planet offers is plenty of real estate, meaning that a culture dealing with an existential threat may find it useful to send large numbers of biological or post-biological populations to nearby planetary systems. The number of free-floating planets is unknown, but recent studies have suggested there may be billions of these worlds, flung into the interstellar deep by gravitational interactions in their parent systems. We would expect some to move through the cometary clouds of planetary systems, just as stars like Scholz’s Star (W0720) did in our system 70,000 years ago, remaining within 100,000 AU of the Sun for a period of roughly 10,000 years.

A sufficiently advanced culture could also take advantage of events within its own system to ride an object likely to be ejected by a dying star. Here’s one science fictional scenario among many in this paper:

Extraterrestrial civilizations may ride Oort-cloud objects of their planetary systems, which become free-floating planets after being ejected by their host stars during the red giant branch (RGB) evolution and the asymptotic giant branch (AGB) evolution. For example, if a host star is a sun-like star and the critical semimajor axis acr ≈ 1000 AU, then extraterrestrials may use spacecraft to travel from their home planet to an object similar to 2015 TG387, when it is close to its periastron ~60–80 AU. They would ride that object, and they would leave the object when it would reach its apastron ~2100 AU. Then, they would use their spacecraft to transfer to another object of the Oort cloud that would be later ejected by its post-main-sequence star.

One recent study finds that simulations of terrestrial planet formation around stars like the Sun produce about 2.5 terrestrial-mass planets per star that are ejected during the planet formation process, many of these most likely near Mars in size. Louis Strigari (Stanford University) calculated in 2012 that for each main sequence star there may be up to 105 unbound objects, an enormous number that would argue for frequent passage of such worlds near other star systems. Let’s be more conservative and just say that free-floating planets likely outnumber stars in the galaxy. Some of these worlds may be ejected by later scattering interactions in multi-planet systems or by stellar evolution.

These planets are tricky observational targets, as the recent discovery of 70 of them in the Upper Scorpius OB association (420 light-years away from Earth) reminds us. They may exist in their countless billions, but we rely on chance and the momentary alignments with a background star to spot their passage via gravitational microlensing.

Image: This image shows the locations of 115 potential rogue planets, highlighted with red circles, recently discovered by a team of astronomers in a region of the sky occupied by Upper Scorpius and Ophiucus. Rogue planets have masses comparable to those of the planets in our Solar System, but do not orbit a star and instead roam freely on their own. The exact number of rogue planets found by the team is between 70 and 170, depending on the age assumed for the study region. This image was created assuming an intermediate age, resulting in a number of planet candidates in between the two extremes of the study. Credit: ESO/N. Risinger (skysurvey.org)

If we do find a free-floating planet in our data, does it become a SETI target? Romanovskaya thinks the idea has merit, suggesting several strategies for examining such worlds for technosignatures. One thing we might do is home in on post-main sequence stars with previously stable habitable zones, looking for signs of technology near them, under the assumption that a local civilization under duress might need a way out, whether via transfer to a passing free-floating planet or by other means.

Thus the stellar neighborhoods of red giants and white dwarfs that formed from G- and K-class stars merit study. A so-called ‘Dyson slingshot’ (a white dwarf binary gravitational assist) could accelerate a free-floating planet, and as David Kipping has shown, binaries with neutron stars and black holes are likewise candidates for such a maneuver. Thus we open up the technosignature space to white dwarf binaries and their neutron star counterparts being used by civilizations as planet accelerators.

To a Passing Star

Close passes by other stars likewise merit study. A smattering of such attempts have already been made. In one recent study, Bradley Hansen (UCLA) looked at close stellar encounters near the Sun, using the Gaia database within 100 parsecs and identifying 132 pairs of stars passing within 10,000 AU of one another. No infrared excess of the sort that could flag migratory efforts appeared in the data around Sun-like stars.

Two years earlier, Hansen worked with UCLA colleague Ben Zuckerman on survival of technological civilizations given problematic stellar evolution, both papers appearing in the Astronomical Journal (I won’t cite all these papers below, as they’re cited in Romanovskaya’s paper, which is available in full-text online). In a system that has experienced interstellar migration, we would expect to see atmospheric technosignatures and possible evidence of terraforming on colonized planets. A clip from their 2020 paper:

…we associate the migration with a particular astrophysical event that is, in principle, observable, namely a close passage of two stars. One could reduce the vast parameter space of a search for evidence of technology with a focus on such a sample of stars in a search for communication signals or signs of activity such as infrared excesses or transient absorptions of stellar photospheres. However, our estimates suggest that the density of such systems is low compared to the confusing foreground of truly bound stars, and a substantial program of vetting false positives would be required.

Indeed, the list of technosignatures mentioned in the Romanovskaya paper, mostly culled from the literature, takes us far from the original SETI paradigm of listening for radio communications. It introduces the SETI potential of free-floating planets but then goes on to include infrared detection of self-reproducing probes, stellar engines (hypervelocity stars become SETI candidates), interstellar spacecraft communications or cyclotron radiation emitted by magnetic sails and other technologies, and the search for potential artifacts of other civilizations here in the Solar System, as examined by Robert Freitas and others and recently re-invigorated by Jim Benford’s work.

The whole sky seems to open up for search if we accept these premises; technosignatures rain down like confetti, especially given the free-floating planet hypothesis. Thus:

Unexplained emissions of electromagnetic radiation observed only once or a few times along the lines of observation of planetary systems, groups of stars, galaxies and seemingly empty regions of space may be technosignatures produced on free-floating planets located along the lines of observation; the search for free-floating planets is recommended in regions where unexplained emissions or astronomical phenomena occur.

How do we construct a coherent observational program from the enormous list of possibilities? The author makes no attempt to produce such, but brainstorming the possibilities has its own virtues that may prove useful as we try to make sense of future enigmatic data to ask whether what we see is of natural or technological origin.

The paper is Romanovskaya, “Migrating extraterrestrial civilizations and interstellar colonization: implications for SETI and SETA,” published online by Cambridge University Press (28 April 2022). Full text. The Ivanov et al. paper cited at the beginning is “A qualitative classification of extraterrestrial civilizations,” Astronomy & Astrophysics Vol. 639, A94 (14 July 2020). Abstract.

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{ 48 comments… add one }
  • Michael Fidler May 6, 2022, 9:18

    Interesting idea, I would think the best planets would be free floating planets that have a large magnetosphere. This would keep cosmic rays and any other hard radiation at bay. A deep atmosphere to keep x-rays and gamma rays down would be good also.
    That would seem to point to objects with possibly a large salty oceans at the bottom end and up to large super earths, that can develop large magnetospheres. Geologically active would also help create desired atmospheres and geothermal energy. So a little smaller then earth too a large super earth that could sustain a warm earth like environment from internal heat either directly or by power plants.

    This points to higher temperatures that may be taken as a cool brown dwarf but is actually a ET enhanced rocky planet. The ice that originally covered the object would be melted and a normal rotation rate of 10 to 20 hours since no tidal locking.

    By logic we should be able to develop a model for how such worlds should look and behave, the problem with small magnetospheres detection is no solar wind or flares to cause it to emit electromagnetic energy.

    The other possibility is large free floating planets Neptune to brown dwarf size that has a large moon. The large gas giants would have the large magnetosphere that may protect the the planet from radiation.

    • Michael Fidler May 6, 2022, 21:06

      I see that the consensus is still looking inward not out.

      What will be the most likely place we will first reach outside the known solar system? Planet 9, nearby cool to cold brown dwarfs or even earth to Mars size free floating planets…

      These planets would be ideal to develop and use for further exploration since not in the deep gravitational well of our Sun. Like the Forts that were used in Canada and the USA to explore and settle North America. These may be the most common type of alien outpost for trade since no deep gravity well and no developed mega weapons or Navy that would be protecting the original home world. Large super earths could be developed into exotic earth like environments with strange and unusual alien life forms that may be the most unusual vacation hotspot. Trade, vacations and alien intelligence in a duty free zone that does not bring diseases or unfriendly species to the protected home worlds.

      Maybe the perfect place to put an interstellar wormhole! ;-}

      • Jeff Wright May 8, 2022, 5:30

        Gliese 710 and any associated worlds would be a better focus. It is heading inbound.

        • Michael Fidler May 8, 2022, 10:18

          Yes, very interesting, 0.1663 light years in about 1.3 million years and reach the outskirts of the inner Oort cloud. Hopefully we can make the best of it and maybe use the large comets it throws at us for colonies.

  • Alex Tolley May 6, 2022, 9:47

    While there may be many, many free-floating worlds, there may be few, or none, ETCs using them. Of those worlds, how many are of the warm variety versus small worlds that have frozen solid (although Pluto is not entirely solid as once thought).

    As for migration rates, ETCs have a long time to migrate a global population to these wandering worlds. 10 millennia is enough to migrate just one million per year of a 10 bn population, far less than our terrestrial aircraft flights. The transports needn’t be rockets, but simple containers with life support that are flung out to the world in one shot, or in stages. Could we detect such passive carriers?

    More plausible to me are civilizations that have maintained technology but at a much more sophisticated level than we have, voluntarily or by circumstance ending their use of the gross, energy-consuming, environment-destroying technologies we use. The population may be much smaller too. This seems far more sustainable over the long term than our energy and resource-hogging setup. But how would we detect such a civilization?

    We also need to rid ourselves of the Star Trek universe of many, broadly technology-comparable civilizations. Intelligent species are either technologically far behind us, like Moonwatcher, or vastly older than us with technologies indistinguishable from magic. We have no idea what physical form they may have – whether biologically evolved, biological or machine designed. For star travel, a journey as an uploaded mind might well be the best state to be in, with reincorporation at the destination if desired.

    If nothing else, it is a huge make-work program to extend SETI to SMETI.

  • Tiens van Greuning May 6, 2022, 14:20

    While it is anybody’s guess how a technological civilization will progress and what great feats they will attempt as they grow which we cpuld hope to detect evidence of, of course, if a civilization like ours find themselves in the fortunate position that another star is passing 10000 AU from them, they will almost certainly make the effort to send some hardware and some carbon units over.

    However, if they find themselves in our position, i. e. the next close stellar encounter is in 40000 years, and even then it is a red dwarf passing around 3 light years away, then I can not fathom them waiting countless millenia for the next close stellar pass to occur.

    I’d put my money on them having a sizable portion of their population living off planet in a few cemturies, probavly in O’Neill type habitats constructed from mined out ateroids and comets. I’d also venture that they will have mastered fusion at some stage, and that they would have to develop the capability to move small bodies like asteroids and comets around.

    They could use fusion and other technologies to send probes and possibly robots to star systems several light years away.

    They could use fusion to put the brakes on comets passing through their inner Solar system and mine them for building materials.

    They could conceivably build huge craft filled with hydrogen fuel in liquid form and slow boat them to other stars, building bridges of such vessels using fusion to power laser arrays with which to propel and brake beamed sailcraft between the stars.

    So what I’d look for is stars with swarms of small bodies (the O’Neill type habitats), stars with gas giant-mass planets but stripped of the gas, stars stripped of their outer hydrogen layers but sans the presence of a white dwarf or other such companion which could have done the stripping.

    No civilization will wait 40k years for another star to pass close by, or 5 billion years for their star to become a red giant, before they start migrating to other stars.

    If they decide that migrate and / or colonize is what they want to do, they will develop the tech and the infrastructure to start doing that ASAP.

    • Antonio May 6, 2022, 16:00

      Yep, the study is awkwardly based on the unwarranted assumption that almost all extraterrestrial civilizations are static and don’t travel outside their home system until they are forced to or have a very easy opportunity.

  • Robin Datta May 6, 2022, 15:47

    An effective propulsion system for such rogue planets would likely be a high priority: few would care to be helplessly adrift. The technologies needed are mostly speculative from our current perspective. And sources of energy is yet another issue.

    Such a colony/civilization would be well ahead of where we are now. Yes, that could make them hard to detet with our present technology.

    • Ron S. May 6, 2022, 20:04

      “few would care to be helplessly adrift”

      Earth (along with the rest of the solar system) is helplessly adrift. Should we be concerned about it? Does it affect our quality of life? If we could so something about it, what trajectory should we choose?

      • Robin Datta May 7, 2022, 14:26

        Ancient cosmologies see an order to the universe that does not carry into today’s astronomy.

        Surya is the sun and the solar deity in Hinduism

        The Sun and the Earth
        The Sun causes day and night on the earth,
        because of revolution,
        when there is night here, it is day on the other side,
        the sun does not really rise or sink.
        —Aitareya Brahmana III.44

        “Should we be concerned about it?”
        Only if one is not satisfied with a. perception of order.

        • Mike Serfas May 8, 2022, 10:10

          A perception of order needs to match the complexity of nature. Often we destroy true order (a prairie) to create the most superficial appearance of one (a lawn). Disorder sprays out as phosphates, herbicides, and invasive species; stability gives way to decay.

          The superficial comparison of atoms to planets seems to have found some reality recently – at least an approximate notion of solar systems following Schroedinger’s equation. ( https://academic.oup.com/mnras/article/475/4/5070/4817553 ). I haven’t made a millimeter of headway with that one I’m afraid; I still didn’t figure out even if their x-axis curves around the star. But I wonder what hidden patterns might have arisen spontaneously among rogue planets in our galaxy that could make them easier to understand.

  • Eric Maddox May 6, 2022, 21:13

    I’m feeling a little lonely here.

    Looks like we’re finding that nearly all red dwarfs have planets, and that there are LOTS of rogue planets. Since the birth of the solar system, these planets have been sweeping by. For potential visitors, also add in any galaxy wide and more localized societies.

    What I’m driving at, is that over the last 3-4 billion years, visitors should have been able to calculate that our system has a good chance to be stable, and may eventually have technological life. On the far (less active) side of the moon, and/or on the surface of Callisto, they could have placed a crater or magnetic anomaly based record of sufficient size to last for billions of years (designed to be extremely unlikely to be natural). For example, define a unit, then make a square or rectangular pattern showing segments the exact measurements of pi and/or e.

    If WE develop the ability to do this (totally automated?), why wouldn’t we? Finally, this sort of record would seem to give a push for any society that finds it, to search for the source.

  • James Franklin May 7, 2022, 4:11

    It is good to see the scientific community taking a far more holistic view to the speculation regarding the detection of technosignatures.

    We must be careful not to get carried away though, logic must be applied to each and every potential search criteria.

    Free floating planets may be plentiful, and free floating dwarf ones even more so, but without some mechanism to keep subsurface oceans liquid, the bulk are likely frozen as hard as granite.

    We know, or at least have good evidence for the subsurface ocean models for Ceres, Europa, Ganymede, Callisto, Pluto and several Saturnian moons, but we can speculate that these have hot cores driven by gravitational tidal interactions causing volcanism or a similar mechanism, no free floating planet will have this mechanism, unless it has a substantial satellite orbiting it which is not tidally locked, and this will be very uncommon.

    I’m all for any look at any potential evidence we are not alone, but at the same time we need to ensure resources are directly expeditiously and we don’t get lost in the weeds of ‘what if, oh, what if’ speculations.

    Each potential technosignature source needs to be examined fully, detail the levels of technology required to make it real, look at the obstacles needed to overcome to realise the dream, then grade then on a scale from most probable to least likely, that way we can direct the necessary efforts in the most efficient direction, after all, taking advantage of free floating planets is one thing, but having the technology to redirect their course, or take advantage of hypervelocity stars is one a whole other level.

    • Antonio May 7, 2022, 9:52

      “We know, or at least have good evidence for the subsurface ocean models for Ceres, Europa, Ganymede, Callisto, Pluto and several Saturnian moons, but we can speculate that these have hot cores driven by gravitational tidal interactions causing volcanism or a similar mechanism”

      No tidal heating for Pluto.

      • Michael May 8, 2022, 3:59

        It does have a highly elliptical orbit though, maybe enough to enduced tidal heating but it would be tiny I would think.

        • Antonio May 8, 2022, 16:10

          Totally negligible.

      • James Franklin May 8, 2022, 18:03

        We are assuming it has no tidal heating, but clearly something is causing it to have a, at least, partially melted core core or some similar heat source. The most likely candidate is Charon, but that would also mean rethinking how the two interact with each other as Charon and Pluto are believed to be tidally locked, the remaining Plutonian system bodies lack sufficient mass to gravitationally heat Pluto’s core.

        • Antonio May 9, 2022, 14:47

          “as Charon and Pluto are believed to be tidally locked”

          Not only believed but measured.

          “We are assuming it has no tidal heating”

          No, we are deducing it has no tidal heating.

  • Michael May 8, 2022, 2:43

    Estimates for rogue planets is up to 100 000 per star in our gallaxy, that brings the average distance taking our nearest stars distance into account of around 20 to 30 thousand AU. Spotting them would be incredibly difficult but if we had a SFL craft spotting them would be much easier though.

    • Michael Fidler May 8, 2022, 10:28

      What is a SFL craft abd why would it make it easier?

    • Alex Tolley May 8, 2022, 11:23

      Wouldn’t WISE have detected some if there were so many?

      If there were 10^5 planets per star, wouldn’t that imply that such planets would be making frequent encounters inside the solar system purely by chance? Could we detect evidence of this? If we looked at other systems, wouldn’t we see a few examples of such planets having some effect on the stability of the orbits of their planets?

      • Juraj May 8, 2022, 13:50

        If they are in thermal equilibrium with space, which small ones do, Wise would not see them. We could see only occultations. No idea if any actual observations support this.

        Also 10^5 isn’t really such a big number in this context. I think the planet would have to move throught cisneptunian space to have any lasting effects detectable to us. That volume is really small compared to volume of interstellar space. Perhaps Uranus had such collision and that’s all.

        • Alex Tolley May 8, 2022, 17:02

          If they are in thermal equilibrium with space, which small ones do, Wise would not see them.

          Earth0-sized and larger planets wouldn’t be in thermal equilibrium with space. They would be emitting in the IR.

          Pluto emits in IR as shown by this false-color IR image.
          If Pluto can be seen in IR, so should other planets. Webb should be able to see these wandering planets.

          As for interactions within a system, I would imagine that there would be orbital perturbations. There may even be more drastic effects, after all, we saw the impact of the Shoemaker-Levy comet stream on Jupiter. When viewing many other systems, one might expect statistically rare interactions to show up in some way.

          WISE didn’t detect anything, and if Webb also does not see any rogue planets, then I would infer that they are nowhere near as common as suggested.

          • Adam Crowl May 10, 2022, 4:37

            Hi Alex
            Earth’s thermal equilibrium temperature for its current energy output of 0.08 W per square metre is 34 K. Even the Early Earth, with a much higher load of U, Th & K was ~64 K.
            To quote Wikipedia…

            WISE was not able to detect Kuiper belt objects, because their temperatures are too low.[21] Pluto is the only Kuiper belt object that was detected.[22] It was able to detect any objects warmer than 70–100 K. A Neptune-sized object would be detectable out to 700 Astronomical unit (AU), a Jupiter mass object out to 1 light year (63,000 AU), where it would still be within the Sun’s zone of gravitational control. A larger object of 2–3 Jupiter masses would be visible at a distance of up to 7–10 light years.[21]
            So rogue terrestrials would be nigh invisible, unless warmed well above equilibrium.

            • Alex Tolley May 10, 2022, 9:48

              Do you have comparable figures for the Webb telescope? Would it have a greater capability of detecting rogue planets, and if so, by how much?

              With the instruments we have, this implies that our best hope is to either find a rogue wandering into our solar system with sufficient illumination/heating to be detectable or to discover one of the solar system bodies is actually an intruder from elsewhere.

      • Michael May 8, 2022, 13:51

        It’s highly dependant on the mass limits of what is perceived as a ‘planet’

        https://www.google.com/url?sa=t&rct=j&q=&esrc=s&source=web&cd=&ved=2ahUKEwiBw5LtuND3AhV2QkEAHTddDEAQFnoECAoQAQ&url=https%3A%2F%2Farxiv.org%2Fpdf%2F1201.2687&usg=AOvVaw1QchtJf0hJ-cqySF75uyzm

        SFL, Solar Focal Line, opps I should have said gravity focal line.

        Everything is magnified in the field of view.

      • Martin May 9, 2022, 19:02

        At 10^5 objects/star, ignoring gravitational focusing (because I don’t know how to do it!), using 280 cubic light years per star, average relative motion of 20 km/sec and a radius of the solar system of 30 AU I get one encounter every 59 million years, or 77 over the lifetime of the solar system. Oh, and an average separation of the objects of 5000 AU.
        These would be very dark, cold objects and very hard to detect. As for the effect on the solar system, there are lots of odd things in the solar system, perhaps we haven’t just interpreted them correctly. As an (unlikely) example: maybe Triton is a formerly extra solar object!

    • Michael Fidler May 8, 2022, 21:31

      I would love to see a 100,000 rogue planets per star, but there is a BIG problem with that! These objects are not stationary but moving like the stars and that means we would be having large objects entering the solar system on a regular basis. We have a K7 star at 62 light years coming as close as 20,000 AU in 1.3 million years. How many rogue planets would be coming into 30 AU??? The other problem is just how many objects smaller then Mars are flying around out there. We should be seeing 50 percent of the micrometeorites off your roofs rain gutter being older then 4.5 billion years! So what happened to all this material? We should be seeing huge meteor showers every night and bolides every day. Something is sweeping up the small debris, but at what level? These would all be from interstellar space but we may see something like this happening when passing through the milky way’s arms and it’s dense gas/dust clouds. But what of all the large interstellar comets and moon size objects that should be out there if they increase in numbers as they shrink in size? Either the aliens have colonized and cloaked every one of them (which isn’t a bad idea) and (maybe that’s why there are so many UAPs) or the vacuum sucked them up! ;-}

      • kzb May 9, 2022, 13:21

        I believe the 100,000 free floating planets per star comes from quasar microlensing studies. Viewing a quasar through an intervening galaxy detects this number of planetary mass lenses.
        However, lensing studies in our own galaxy do not detect this high number.
        There is a discrepancy between the two methods which has never been explained to my knowledge.

  • Deanna Shaw May 8, 2022, 5:16

    JF wrote “…but without some mechanism to keep subsurface oceans liquid, the bulk are likely frozen as hard as granite.”

    It’s likely that an Earth sized (or larger) rogue terrestrial planet can generate enough internal heat though radioactive decay of Uranium, Thorium and etc to liquefy any water ocean’s bottom. Add anti freeze….. ammonia, salts….. to depress the solidus state further. A thick blanket of primordial Hydrogen in the atmosphere would make for a terrific insulator, maybe even enough for liquid water on it’s surface.

    The problem is how to locate and identify such planets besides rare occultations.

    So around our Sol…. Some kind of very VERY!!!! bright flash of photons could illuminate these bodies so that we at least know where to aim our telescopes (radar, IR) to better characterize them. And as Freeman Dyson suggested….. we JUST might see eyes staring back at us too.

    • Michael Fidler May 8, 2022, 10:34

      Maybe one of the all sky deep imaging projects could pick up a reflection from a large nearby rouge earth size planet if frozen. What may work better is solar flaring, UV to X-ray that would light up any nearby object weeks to months later.

    • Alex Tolley May 8, 2022, 11:18

      Without an atmosphere, Earth’s temperature would be 120K at 1 AU. If a thick hydrogen atmosphere would raise the surface temperature of the Earth in deep space, the same should be obvious on planets like Saturn, but the atmospheric temperature profile at 1 bar is still less than 160K, and even with extrapolation looks to be well under 273K at 10 bar.

      From this, I would suggest that a rogue/wandering Earth-sized rocky world with even a dense H2 atmosphere as well as other GHGs would still have a surface temperature well below freezing. The habitable volume would have to be in the lithosphere, at a depth where heat from the core maintains a temperate zone.

      • Mike Serfas May 9, 2022, 3:09

        At least going by the Wikipedia article on equilibrium temperature, we have 255 K for Earth, versus actual average temperature of 288 K. An image of the Moon ( https://sos.noaa.gov/catalog/datasets/moon-surface-temperature/ ) seems roughly in line with this equilibrium temperature at the terminator.

        Dense atmospheres can hold in the heat, but the heat flux seems like the larger issue. If life forms have access to only a tiny amount of geothermal free energy to build new chemicals, the excess thermal energy at liquid water temperatures seems like it would only break them down faster. I suspect a sphere like Titan might actually have a better chance of evolving life on its surface that can grow and move very slowly, but stably. But if it also has cryovolcanoes, then within that limited territory it might have access to Earthlike levels of free energy in the presence of liquid water.

        • Alex Tolley May 9, 2022, 11:01

          I don’t know what happened there with the Earth’s temperature w/o an atmosphere. I do recall doing a calculation to convert the degrees C to degrees K to match the Earth’s and the Saturn’s temperature. Sloppy keying or something.

  • henry cordova May 8, 2022, 10:20

    If these dead, solitary worlds are as common as they appear to be, then it would seem they would be ideal places to hide for civilizations which seek to avoid contact with anyone else. They are the perfect hideout, in plain sight in a vast cloud of inert objects.

    If that is the case, then it can be expected that these hermits would use every tool in their technology kit to disguise their ark, to make it look like just another floating lump of ice, slag and ash. There will be no technosignatures.

    On the other hand, if these cultures have no objection to meeting others, then it will be likely that they will advertise their presence by making themselves look as ‘unnatural’ as possible, by broadcasting or signaling, or maneuvering in an erratic or artificial fashion.

    To put it another way, if these solitary, floating worlds are indeed suitable SETI targets, then they will stick out like a sore thumb.

    • tesh May 9, 2022, 11:43

      This, but coupled to the following:
      1. Once a sufficient understanding of the universe is achieved, there is no reason to expand/advertise
      2. The escape is into a more interesting universe – constructed or imagined, i.e. can be actual or virtual
      3. Are these free floaters then ideal to keep expanding and avoid unwanted distractions…

  • andy May 8, 2022, 10:33

    An extraterrestrial intelligence at a rogue planet does feature in my favourite first contact story, Blindsight by Peter Watts. I would definitely recommend giving it a read, one of the more thought-provoking takes on the subject.

    • Paul Gilster May 8, 2022, 12:31

      Was not aware of this one. Thanks!

  • Adam Crowl May 10, 2022, 4:15

    Hi Paul
    This topic has stirred up memories of Olaf Stapledon’s “Star Maker” which described interstellar travel thus…

    Actual interstellar voyaging was first effected by detaching a planet from its natural orbit by a series of well-timed and well-placed rocket impulsions, and thus projecting it into outer space at a speed far greater than the normal planetary and stellar speeds. Something more than this was necessary, since life on a sunless planet would have been impossible. For short interstellar voyages the difficulty was sometimes overcome by the generation of sub-atomic energy from the planet’s own substance; but for longer voyages, lasting for many thousands of years, the only method was to form a small artificial sun, and project it into space as a blazing satellite of the living world. For this purpose an uninhabited planet would be brought into proximity with the home planet to form a binary system. A mechanism would then be contrived for the controlled disintegration of the atoms of the lifeless planet, to provide a constant source of light and heat. The two bodies, revolving round one another, would be launched among the stars.

    Which suggests an immediate observable – an artificial sun. Even a planet warmed to habitability would be visible given a big enough NIR telescope to detect it in the chill of interstellar space.

    • Paul Gilster May 10, 2022, 5:25

      You know me, Adam. I always love it when Stapledon is invoked in the context of such mind-bending ideas!

    • Alex Tolley May 10, 2022, 10:05

      This is further evidence of Stapledon’s influence on Clarke. In “The Sands of Mars” (1953), the secret project is to make Phobos into an artificial sun to warm Mars to increase habitability. This was also reflected again in 2010: Odyssey Two with the ignition of Jupiter to thaw Europa.

      As “Starmaker” was published in 1937, Stapledon might have in turn been influenced by Oberth’s 1929 book “Ways to Spaceflight” (1929) which advocated putting large mirrors in space to illuminate parts of the Earth.

      [Starting to feel like James Burke and “Connections”.]

  • Neil Stahl May 11, 2022, 7:56

    If the right planet came by headed in a fortuitous direction it would surely be tempting to attach a settlement to it. The right planet would be an ongoing source of resources and maybe the best place to live (in?). Or maybe much of the population would live in O’Neill cylinders orbiting it if the powerful radiation can be dealt with. And such cylinders properly propelled, would make great people movers at each end of the trip. And in the meantime living in them would keep people attuned to living in space.

    • Michael May 11, 2022, 14:41

      Free floating massive planets should have next to no radiation belts as there is no particles to trap unless they have active moons. However the magnetic field could be tapped for energy by those in orbit.

      • Neil Stahl May 12, 2022, 13:23

        I was referring to the high energy cosmic rays our heliosphere protects us from. The universe gets progressively nastier as we get away from home. :(

  • stephen May 12, 2022, 0:33

    I went to this space travel calculator…
    https://www.omnicalculator.com/physics/space-travel

    I put in some values for propelling a rogue planet.
    Mass of rogue planet 1000 Earth masses
    Distance traveled 1 lightyear
    Duration of trip 1949 years
    Fuel used 1.22e22 metric tons, or about 2 Earth masses, about .002 of the mass of the planet.
    Acceleration .00001 meters/second
    Maximum velocity .001 lightspeed, 308 km/s.

    The motivation could be to take their rogue planet to the nearest blue giant for its solar energy.

    I don’t know if this scenario is to be taken seriously.

    Another issue is, if the inhabitants are hiding, explorers stopping by will feel free to colonize the place, so the inhabitants will want to set up an AI or something to greet/intercept/discourage visitors.

    But a sufficiently advanced AI might get bored, or lonely…

    • Mike Serfas May 12, 2022, 9:08

      If the super-Jupiter gains a motion of 3E5 m/s^2, the exhaust should be moving at half the speed of light, minus a correction for the relativistic mass. At two Earth masses, the exhaust is quite a vessel in its own right, if you can power the engine, but you have to convert roughly a fifth of an Earth mass to energy for that. The Jupiter-like planet would arrive with noticeably more helium, I suspect.

    • Alex Tolley May 12, 2022, 10:14

      The fuel type and exhaust is somewhat hidden, but we can make some approximate estimates using the rocket equation.

      If the trip takes nearly 2000 years, that ,means that the average velocity is 1/2000 the speed of light = 300,000/2000 = 150 km/s.

      The mass ratio, M0/M1 = 1000/998.

      Rearranging, we get Ve = V/(ln(M0/M1) = 150/ln(1000/998)
      ~= 75,000 km/s, 0.25c

      So a pretty high mass conversion ratio to energy.

      How to do this?
      1. If there are 2 Earth masses of fissile material, then perhaps a fission fragment rocket, or perhaps using fusion explosions to create the needed exhaust velocity, rather like an Orion rocket.

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