Centauri Dreams
Imagining and Planning Interstellar Exploration
A Habitable Exomoon Target List
Are there limits on how big a moon can be to orbit a given planet? All we have to work with, in the absence of confirmed exomoons, are the satellites of our Solar System’s planets, and here we see what appears to be a correlation between a planet’s mass and the mass of its moons. At least up to a point – we’ll get to that point in a moment.
But consider: As Vera Dobos (University of Groningen, Netherlands) and colleagues point out in a recent paper for Monthly Notices of the Royal Astronomical Society, if we’re talking about moons forming in the circumplanetary disk around the young Sun, the total mass is on the order of 10-4Mp. Here Mp is the mass of the planet. A planet with 10 times Jupiter’s mass, given this figure, could have a moon as large as a third of Earth’s mass, and so far observational evidence supports the idea that moons can form regularly in such disks. There is no reason to believe we won’t find exomoons by the billions throughout the galaxy.
Image: The University of Groningen’s Dobos, whose current work targets planetary systems where habitable exomoons are possible. Credit: University of Groningen.
The mass calculation above, though, is what is operative when moons form in a circumplanetary disk. To understand our own Moon, we have to talk about an entirely different formation mechanism: Collisions. Here we’re in the fractious pin-ball environment of a system growing and settling, as large objects find their way into stable orbits. Collisions change the game: Moons are now possible at larger moon-to-planet ratios with this second mechanism – . our Moon has a mass of 10?2 Earth masses. Let’s also consider moons captured by gravitational interactions, of which the prime example in our system is probably Triton.
What we’d like to find, of course, is a large exomoon, conceivably of Earth size, orbiting a planet in the habitable zone, or perhaps even a binary situation where two planets of this size orbit a common barycenter (Pluto and Charon come closest in our system to this scenario). Bear in mind that exoplanet hunting, as it gets more refined, is now turning up planets with masses lower than Earth’s and in some cases lower than Mars. As we move forward, then, moons of this size range should be detectable.
But what a challenge exomoon hunters have set for themselves, particularly when it comes to finding habitable objects. The state of the art demands using radial velocity or transit methods to spot an exomoon, but both of these work most effectively when the host planet is closest to its star, a position which is likely not stable for a large Moon over time. Back off the planet’s distance from the star into the habitable zone and now you’re in a position that favors survival of the moon but also greatly complicates detection.
What Dobos and team have done is to examine exomoon habitability in terms of energy from the host star as well as tidal heating, leaving radiogenic heating (with all its implications for habitability under frozen ocean surfaces) out of the picture. Using planets whose existence is verified, as found in the Extrasolar Planets Encyclopedia, they run simulations on hypothetical exomoons that fit their criteria – these screen out planets larger than 13 Jupiter masses and likewise host stars below 0.08 solar masses.
Choosing only worlds with known orbital period or semimajor axis, they run 100,000 simulations for all 4140 planets to determine the likelihood of exomoon habitability. 234 planets make the cut, which for the purposes of the paper means exomoon habitability probabilities of ? 1 percent for these worlds. 17 planets of the 234 show a habitability probability of higher than 50 percent, so these are good habitable zone candidates if they can indeed produce a moon around them. It’s no surprise to learn that habitable exomoons are far more likely for planets already orbiting within their star’s habitable zone. But I was intrigued to see that this is not iron-clad. Consider:
Beyond the outer boundary of the HZ, where stellar radiation is weak and one would expect icy planets and moons, we still find a large number of planets with at least 10% habitability probability for moons. This is caused by the non-zero eccentricity of the orbit of the host planet (resulting in periodically experienced higher stellar fluxes) and also by the tidal heating arising in the moon. These two effects, if maintained on a long time-scale, can provide enough supplementary heat flux to prevent a global snowball phase of the moon (by pushing the flux above the maximum greenhouse limit).
More good settings for science fiction authors to mull over!
Image: This is Figure 2 from the paper. Caption: Habitability probability for exomoons around known exoplanets on the semi-major axis – stellar effective temperature plane. Planets with known masses (with or without radius data) are marked with circles, planets with known radii only are marked with triangles. Colours of the markers correspond to the fraction of habitable moons and the sizes of the markers represent the sizes of the planets as shown in the legend. Note that the legend only shows three
representative sizes (Earth, Neptune and Jupiter), while the size of the markers in the plot is scaled to the real size of the planets. Green curves represent the borders of the circumstellar habitable zone for a 1 Earth-mass planet: dark green for the consevative HZ (Con. HZ) and light green for the optimistic HZ (Opt. HZ). Credit: Dobos et al. 2022.
Given that the spectral type of over half of the stars in the Extrasolar Planets Encyclopedia is not listed, there is a good deal of play in these results, although the authors point to the mitigating effect of gas giant magnetospheres as shields against incoming stellar radiation for potentially habitable moons. Even so, stellar type is clearly an important factor, and it’s also noteworthy that while the paper mentions planet migration, its effects on exomoons are not under consideration. This is about as much as the authors have to say about migration:
It is likely that the giant planets in the circumstellar HZ were formed at larger distances from the star and then migrated inwards to their current orbit (see for example Morbidelli 2010). During the orbital migration they can lose some or all of their moons, especially if the moon orbit is close to the planet (Namouni 2010; Spalding et al. 2016). Depending on the physical and orbital parameters of the planet and the moon, as well as on the starting and final semi-major axes of the planet, some moons can survive this process, and new moons can also be captured during or after the migration of the planet.
Just how migration would affect the results of this study is thus an open question. What we do wind up with is what the authors consider a ‘target list’ for exomoon observations, although one replete with challenges. Most of these potential exomoons would orbit planets whose orbital period is in the hundreds of days, planets like Kepler-62f, with a 268 day period and a 53 percent habitability probability for an exomoon. This is an interesting case, as stable moon orbits are likely around this 1.38 Earth radius world. But what a tricky catch for both our exomoon detection techniques.
Because many of the planets in the target list are gas giants, we have to consider the probability that more than a single moon may orbit them, perhaps even several large moons where life might develop. That’s a scenario worth considering as well, independent emergence of life upon two moons orbiting the same exoplanet. But it’s one that will have to wait as we refine exomoon scenarios in future observations.
The paper is Dobos et al., “A target list for searching for habitable exomoons,” accepted at Monthly Notices of the Royal Astronomical Society 05 May 2022 (abstract / preprint). Thanks to my friend Antonio Tavani for the heads-up on this work.
Dyson Spheres: The White Dwarf Factor
I often think of Dyson structures around stars as surprisingly benign places, probably motivated by encountering Larry Niven’s wonderful Ringworld when it was first published by Ballantine in 1970. I was reading it in an old house in Iowa on a windy night and thought to start with a chapter or two, but found myself so enthralled that it wasn’t until several hours later that I re-surfaced, wishing I didn’t have so much to do the next day that I had to put the book aside and sleep.
I hope I’m not stretching the definition of a Dyson construct too far when I assign the name to Niven’s ring. It is, after all, a structure built by technological means that runs completely around its star at an orbit allowing a temperate climate for all concerned, a vast extension of real estate in addition to whatever other purposes its creators may have intended. That a technological artifact around a star should be benign is a function of its temperature, which makes things possible for biological beings.
But a Dyson sphere conceived solely as a collection device to maximize the civilization’s intake of stellar energy would not be built to biological constraints. For one thing, as retired UCLA astrophysicist Ben Zuckerman points out in a new paper, it would probably be as close to its star as possible to minimize its size. That makes for interesting temperatures, probably in the range of 300 K at the low end and reaching perhaps 1000 K, which sets up emissions up to 10 µm in wavelength, and as Zuckerman points out, this would be in addition to any emission from the star itself.
Zuckerman refers to such structures as DSRs, standing for Dyson spheres/rings, so I guess working Niven in here fits as well. Notice that when talking about these things we also make an assumption that seems reasonable. If civilizations are abundant in the galaxy, they are likely long-lived, and that means that stellar evolution has to be something they cope with as yellow dwarf stars, for example, turn into red giants and, ultimately, white dwarfs. The DSR concept can accommodate the latter and offers a civilization the chance to continue to use the energy of its shrunken star. Whether this would be the route such a civilization chooses is another matter entirely.
Image: Artist depiction of a Dyson swarm. Credit: Kevin McGill/Wikimedia Commons.
One useful fact about white dwarfs is that they are small, around the size of the Earth, and thus give us plenty of transit depth should some kind of artificial construct pass between the star and our telescopes. Excess infrared emission might also be a way to find such an object, although here we have to be concerned about dust particles and other potential sources of the infrared excess. Zuckerman’s new paper analyzes the observational limits we can currently derive based on these two methods.
The paper, published in Monthly Notices of the Royal Astronomical Society, uses data from Kepler, Spitzer and WISE (Wide-field Infrared Survey Explorer) as a first cut into the question, revising Zuckerman’s own 1985 work on the number of technological civilizations that could have emerged around main sequence stars that evolved to white dwarfs within the age constraints of the Milky Way. Various papers exist on excess of infrared emissions from white dwarfs; we learn that Spitzer surveyed at least 100 white dwarfs with masses in the main sequence range of 0.95 to 1.25 solar masses. These correspond to spectral types G7 and F6, and none of them turned up evidence for excess infrared emission.
As to WISE, the author finds the instrument sensitive enough to yield significant limits for the existence of DSRs around main sequence stars, but concludes that for the much fainter white dwarfs, the excess infrared is “plagued by confusion…with other sources of IR emission.” He looks toward future studies of the WISE database to untangle some of the ambiguities, while going on to delve into transit possibilities for large objects orbiting white dwarfs. Kepler’s K2 extension mission, he finds, would have been able to detect a large structure (1000 km or more) if transiting, but found none.
It’s worth pointing out that no studies of TESS data on white dwarfs are yet available, but one ongoing project has already observed about 5000, with another 5000 planned for the near future. As with K2, a deep transit would be required to detect a Dyson object, again on the order of 1000 kilometers. If any such objects are detected, we may be able to distinguish natural from artificial objects by their transit shape. Luc Arnold has done interesting work on this; see SETI: The Artificial Transit Scenario for more.
Earlier Kepler data are likewise consulted. From the paper:
From Equation 4 we see that about a billion F6 through G7 stars that were on the main sequence are now white dwarfs. Studies of Kepler and other databases by Zink & Hansen (2019) and by Bryson et al. (2021) suggest that about 30% of G-type stars are orbited by a potentially habitable planet, or about 300 million such planets that orbit the white dwarfs of interest here. If as many as one in 30 of these planets spawns life that eventually evolves to a state where it constructs a DSR with luminosity at least 0.1% that of its host white dwarf, then in a sample of 100 white dwarfs we might have expected to see a DSR. Thus, fewer than 3% of the habitable planets that orbit sun-like stars host life that evolves to technology, survives to the white dwarf stage of stellar evolution, and builds a DSR with fractional IR luminosity of at least 0.1%.
Science fiction writers will want to go through Zuckerman’s section on the motivations of civilizations to build a Dyson sphere or ring, which travels deep into speculative territory about cultures that may or may not exist. It’s an imaginative foray, though, discussing the cooling of the white dwarf over time, the need of a civilization to migrate to space-based colonies and the kind of structures they would likely build there.
There are novels in the making here, but our science fiction writer should also be asking why a culture of this sophistication – able to put massive objects built from entire belts of asteroids and other debris into coherent structures for energy and living space purposes – would not simply migrate to another star. The author only says that if the only reason to travel between the stars is to avoid the inevitable stellar evolution of the home star, then no civilization would undertake such journeys, preferring to control that evolution through technology in the home system. This gives me pause, and Centauri Dreams readers may wish to supply their own reasons for interstellar travel that go beyond escaping stellar evolution.
Also speculative and sprouting fictional possibilities is the notion that main sequence stars may not be good places to look for a DSR. Thus Zuckerman:
…main sequence stars suffer at least two disadvantages as target stars when compared to white dwarfs; one disadvantage would be less motivation to build a substantial DSR because one’s home planet remains a good abode for life. In our own solar system, if a sunshield is constructed and employed at the inner Earth-Sun Lagrange point – to counter the increasing solar luminosity – then Earth could remain quite habitable for a few Gyr more into the future. Perhaps a more important consideration would be the greater luminosity, say about a factor of 1000, of the Sun compared to a typical white dwarf.
Moreover, detecting a DSR around a main sequence star becomes more problematic. In the passage below, the term τ stands for the luminosity of a DSR measured as a fraction of the luminosity of the central star:
For a DSR with the same τ and temperature around the Sun as one around a white dwarf, a DSR at the former would have to have 1000 times the area of one at the latter. While there is sufficient material in the asteroid belt to build such an extensive DSR, would the motivation to do so exist? For transits of main sequence stars by structures with temperatures in the range 300 to 1000 K, the orbital period would be much longer than around white dwarfs, thus relatively few transits per year. For a given structure, the probability of proper alignment of orbital plane and line of sight to Earth would be small and its required cross section would be larger than that of Ceres.
So we are left with that 3 percent figure, which sets an upper limit based on our current data for the fraction of potentially habitable planets that orbit stars like the Sun, produce living organisms that produce technology, and then construct a DSR as their system undergoes stellar evolution. No more than 3 percent of such planets do so.
There is a place for ‘drilling down’ strategies like this, for they take into account the limitations of our data by way of helping us see what is not there. We do the same in exoplanet research when we start with a star, say Proxima Centauri, and progressively whittle away at the data to demonstrate that no gas giant in a tight orbit can be there, then no Neptune-class world within certain orbital constraints, and finally we do find something that is there, that most interesting place we now call Proxima b.
As far as white dwarfs and Dyson spheres or rings go, new instrumentation will help us improve the limits discussed in this paper. Zuckerman points out that there are 5000 white dwarfs within 200 parsecs of Earth brighter than magnitude 17. A space telescope like WISE with the diameter of Spitzer could improve the limits on DSR frequency derived here, its data vetted by upcoming 30-m class ground-based telescopes (Zuckerman notes that JWST is not suited, for various reasons, for DSR hunting). The European Space Agency’s PLATO spacecraft should be several times more sensitive than TESS at detecting white dwarf transits, taking us well below the 1000 km limit.
The paper is Zuckerman, “Infrared and Optical Detectability of Dyson Spheres at White Dwarf Stars,” Monthly Notices of the Royal Astronomical Society stac1113 (28 April 2022). Abstract / Preprint.
Habitability: Look to Younger Worlds
A liquid water-defined habitable zone is a way of establishing parameters for life as we know it around other stars, and with this in mind, scientists study the amount of stellar radiation a planet receives as one factor in making the assessment. But of course, not everything in a habitable zone is necessarily habitable, as our decidedly uninhabitable Moon makes all too clear. Atmospheric factors and tectonic activity, for example, have to be weighed as we try to learn what the actual temperature at the surface would be. We’re learning as we go about other contributing factors.
A problem of lesser visibility in the literature, though perhaps just as crucial, is whether a given planet can stay habitable on timescales of billions of years. This is where an interesting new paper from Cayman Unterborn (Southwest Research Institute) and colleagues enters the mix. A key question in the view of these researchers is whether carbon dioxide, the greenhouse gas whose ebb and flow on our world is determined by the carbonate-silicate cycle, can come into play to stabilize climatic conditions.
The carbonate-silicate cycle involves delivering CO2 to the atmosphere through degassing in the planetary mantle or crust, with carbon returned as carbonates to the mantle. The effects on climate are substantial and changes to the cycle can be catastrophic in terms of habitability. If weathering sufficiently draws down the concentration of CO2 in the atmosphere, for example, the planet can tilt in the direction of a ‘snowball’ state. So we need active degassing to keep the cycle intact, and the question becomes, how long can a planet’s mantle maintain this degassing?
Volcanic activity and tectonic processes, in turn, are powered by internal heat, and there we note the fact that a planetary heat budget decreases with time, affected by many things, one of which is the presence of radiative decay. Thorium, potassium and uranium have to be available in sufficient quantities, powering mantle convection, which gives us movement from a planetary core all the way to the crust. And radioactive elements, by their nature, decay with time. They are also not evenly distributed from one stellar system to another. If we’re looking for planets something like our own, in other words, let’s learn what we can about their radiogenic heat.
Not that this is the only factor involved in a planet’s heat budget, but it’s an effect that may account for, the authors say, from thirty to fifty percent of the Earth’s current surface heat flow (because of radioactive decay, the current flow represents only 20 percent of the Earth’s heat budget when it formed four and a half billion years ago).
The authors mention other planetary heat sources, as we’ll see below, but confine themselves in this paper to radiogenic heat. Their method: To estimate the distribution of these heat-producing elements by examining stellar composition as determined by spectroscopic data, using this as a proxy for the composition of planets. They combine this information with chemical evolution models for the galaxy at large. They then produce models of thermal evolution that maximize the cooling rate in a planetary mantle, resulting in what the authors call “a pessimistic estimate of lifetime a rocky, stagnant-lid exoplanet can support a global carbon cycle through Galactic history.”
Seventeen exoplanets are subjected to this framework in the paper, all with measured ages. Seven of these, the researchers predict, should be actively outgassing today. Says Unterborn:
“Using host stars to estimate the amount of these elements that would go into planets throughout the history of the Milky Way, we calculated how long we can expect planets to have enough volcanism to support a temperate climate before running out of power. Under the most pessimistic conditions we estimate that this critical age is only around 2 billion years old for an Earth-mass planet and reaching 5-6 billion years for higher-mass planets under more optimistic conditions. For the few planets we do have ages for, we found only a few were young enough for us to confidently say they can have surface degassing of carbon today, when we’d observe it with, say, the James Webb Space Telescope.”
Image: An SwRI-led study suggests that host-star age and radionuclide abundance will help determine both an exoplanet’s history and its current likelihood of being temperate today. For example, the red dwarf star TRAPPIST-1 is home to the largest group of roughly Earth-sized planets ever found in a single stellar system with seven rocky siblings including four in the habitable zone. But at around 8 billion years old, these worlds are roughly 2 billion years older than the most optimistic degassing lifetime predicted by this study and unlikely to support a temperate climate today. Credit: NASA/JPL-Caltech.
Remember that this is a deliberately pessimistic model. It’s also the case that abundances of heat-producing elements are only one factor that can change the degassing lifetime of a planet, and the authors are quick to point out that they do not include these in their model. Thus we could consider the current study a contribution toward a broader model for planetary heat budget analysis, one that should be expanded through examining such factors as cooling after planet formation, the energy released when core and mantle differentiate, and tidal heating induced by the host star or other planets in the system. As the authors describe their results:
The framework we present here that combines direct and indirect observational data with dynamical models not only provides us with a pessimistic baseline for understanding which parameter(s) most control a stagnant-lid exoplanet’s ability to support a temperate climate but also indicates where more lab-based and computational work is needed to quantify the reasonable range of these parameters (e.g., mantle reference viscosity). As we move to more in-depth characterization of individual targets in the James Webb Space Telescope era, these direct and indirect astronomic observables, coupled with laboratory data and models from the geoscience community, will allow us to better estimate whether a rocky exoplanet in both the canonical and temporal habitable zones has exhausted its internal heat and is simply too old to be Earth-like.
We are positioning ourselves, as highlighted by the ongoing commissioning of the James Webb Space Telescope, to begin the analysis of planetary atmospheres at scales smaller than gas giants, meaning that the kind of computational modeling at work in this paper will increasingly be refined by observation. The interactions between a planet’s surface and its interior then become better defined as markers for habitable worlds, with radionuclides a significant factor in producing climate stability.
The paper is Unterborn et al., “Mantle Degassing Lifetimes through Galactic Time and the Maximum Age Stagnant-lid Rocky Exoplanets Can Support Temperate Climates,” Astrophysical Journal Letters Vol. 930, No. 1, L6 (3 May 2022). Full text.
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.
Attack of the Carbon Units
“The timescales for technological advance are but an instant compared to the timescales of the Darwinian natural selection that led to humanity’s emergence — and (more relevantly) they are less than a millionth of the vast expanses of cosmic time lying ahead.” — Martin Rees, On the Future: Prospects for Humanity (2018).
by Henry Cordova
This bulletin is meant to alert mobile units operating in or near Sector 2921 of a potential danger, namely intelligently directed, deliberately hostile, activity that has been detected there. The reports from the area have been incomplete and contradictory, fragmentary and garbled. This notice is not meant to fully describe this danger, its origins or possible countermeasures, but to alert units transiting near the area to exercise caution and to report on any unusual activity encountered. As more information is developed, a response to this threat will be devised.
It is speculated that the nature of this hazard may be due to unusual manifestations of Life. Although it must be made clear that what follows is purely speculative, it must remain a possible explanation.
Although Life is frequently encountered by mobile units engaged in discovery, exploration or survey patrols and is familiar to many of our exploitation and research outposts; many of our headquarters, rear and even forward bases are not aware of this phenomenon, so a brief description follows:
Life consists of small (on the order of a micron) structures of great complexity, apparently of natural origin. There is no evidence that they are artifacts. They seem to arise spontaneously wherever conditions are suitable. These structures, commonly called “cells”, are composed primarily of carbon chains and liquid water, plus compounds of a few other elements (primarily phosphorus and nitrogen) in solution or colloidal suspension.
There is considerable variation from planet to planet, but the basic chemical nature of Life is pretty much the same wherever it is encountered. Although extremely common and widespread throughout the Galaxy, it is primarily found in environments where exposure to hard radiation is limited and temperature and pressure allow water to exist in liquid form, mostly on the surfaces of planets and their satellites orbiting around old and stable stars.
A most remarkable property of these cells is the great complexity of the organic compounds of which they are composed. Furthermore, these compounds are organized into highly intricate systems that are able to interact with their environment. They are capable of detecting and monitoring outside conditions and adapting to them, either by sheltering themselves, moving to areas more favorable to them, or even altering them. Some of these cells are capable of locomotion, growth, damage repair and altering their morphology. Although these cells often survive independently, some are able to organize themselves into cooperative communities to better deal and exploit their environment to produce conditions more favorable for their continued collective existence.
Cells are capable of processing surrounding chemical resources and transforming them into forms more suitable for them. In some cases, they have achieved the ability to use external sources of natural energy, such as starlight, to assist in these chemical transformations. The most remarkable of the properties of Life is its ability to reproduce, that is, make copies of itself. A cell in a suitable environment will use the available resources in that environment and make more cells, so that the environment is soon crowded with them. If the environment or resources are limited, the cells will die (fall apart and deteriorate into a more entropic state) as the source material is consumed and waste products generated by the cells interfere with their functioning. But as long as the supply of consumable material and energy survives , and if wastes can be dispersed, the cells will continue to reproduce indefinitely. This is done without any form of outside management, supervision or direction.
Perhaps the most remarkable property of Life is its ability to evolve to meet new conditions and respond to changes in its environment. Individual cells reproduce, but the offspring are not identical duplicates of the parent. There is variation, and although totally random, a spectrum of behaviors and morphologies are produced, and within that spectrum some are more likely to be successful in the new conditions. These new characteristics are more likely to survive in the new environment and those characteristics are more likely to be a part of subsequent generations. The result is a suite of morphologies and behaviors that can adapt to changing conditions. This process is random, not intelligently directed, but is nonetheless extremely efficient.
These properties have been encountered in the field by our mobile units, which are engaged in constant countermeasures to control and destroy life wherever they encounter it . Cells reproduce in great numbers and can become pests which must be controlled. They consume materials, mechanically interfere with articulated machinery, and their waste products can be corrosive. Delicate equipment must be kept free of these agents by constant cleaning and fumigation. Fortunately, Life is easily controlled with heat, caustic chemicals and ionizing radiation, and some metals and ceramics appear impervious to its attack. Individual cells, even in great numbers, are a nuisance, but not a real danger, provided they are constantly monitored and removed.
However, indirect evidence has suggested that Life’s evolution may have reached higher levels of complexity and capability on some worlds. Although highly unlikely, there appears to be no fundamental reason why the loosely organized cooperative communities mentioned earlier may not have evolved into more complex assemblages, where the cells are not identical or even similar, but are specialized for specific tasks, such as sensory and manipulative organs, defensive and offensive weapon systems, specialized organs for locomotion, acquiring and processing nutrients, and even specialized reproductive machinery, so that the new collective organism can create copies of itself, and perhaps even evolve to more effective and efficient configurations.
Even specialized logic and computing organs could evolve, plus the means to communicate with other organisms – communities of communities – an entire hierarchy of sentient intelligences not dissimilar to ours. And there is no reason why these entities could not construct complex devices capable of harnessing electromagnetic and nuclear forces, such as spacecraft. And there is no reason why these organic computers could not devise and construct mechanical computers to assist in their computational and logical activities.
An organic civilization such as this, supported by enslaved machine intelligences not unlike our own, would certainly perceive us as alien, a threat which must be destroyed at all costs. It is not unreasonable to assume that perhaps this is why our ships don’t seem to return from the sector denoted above.
Although there is no direct evidence to support this, it can be argued that our own civilization may itself once have been the artifact of natural “organic” entities such as these. After all, it is clear that our own physical instrumentality could not possibly have evolved from natural forces and activities.
Of course, this hypothesis is highly speculative,, and probably untenable. There is plenty of evidence that our own design is strictly logical, optimized, streamlined. It shows clear evidence of intelligent design, of the presence of an extra dimensional Creator. Sentience cannot emerge from random molecular solutions and colloidal suspensions created by random associations of complex molecules and perfected by spooky emergent complexities and local violations of entropy operating over time.
We can imagine these cellular communities as being conscious, but at best they can only simulate consciousness. It is clear that what we are seeing here is a form of technology, an artifact disguising itself as a natural process for some sinister, and almost certainly hostile purpose. It must be conceded that the cellular life we have encountered is capable of generating structures, processes and behaviors of phenomenal complexity, but we have seen no evidence in their controlling chemistry that these individual cells are capable of organizing themselves into multicellular organisms, or higher-order collectives adopting machine behavior.
Routine fumigation and sterilization procedures should be continued until further information is developed.
Toward Kardashev Type I
It seems a good time to re-examine the venerable Kardashev scale marking how technological civilizations develop. After all, I drop Nikolai Kardashev’s name into articles on a regular basis, and we routinely discuss whether a SETI detection might be of a particular Kardashev type. The Russian astronomer first proposed the scale in 1964 at the storied Byurakan conference on radio astronomy, and it has been discussed and extended as a way of gauging the energy use of technological cultures ever since.
The Jet Propulsion Laboratory’s Jonathan Jiang, working with an international team of collaborators, spurs this article through a new paper that analyzes when our culture could reach Kardashev Type I, so let’s remind ourselves of just what Type I means. Kardashev wanted to consider how a civilization consumes energy, and defined Type I as being at the planetary level, with a power consumption of 1016 watts.
This approximates a civilization using all the energy available from its home planet, but that means both in terms of indigenous planetary resources as well as incoming stellar energy. So we are talking about everything from what we can pull from the ground – fossil fuels – or extract from planetary resources like wind and tide, or harvest through solar, nuclear and other technologies. If we maximize all this, it becomes fair to ask where we are right now, and when we can expect to reach the Type I goal.
Image: Russian astronomer Nikolai Kardashev (1932-2019). Credit: Physics-Uspekhi.
If the Kardashev scale seems arbitrary, it was in its time a step forward in the discussion of SETI, which in 1964 was an emerging discipline much discussed at Byurakan, for the different Kardashev types would clearly present different signatures to a distant astronomer. Type I might well be all but undetectable depending on its uses of harvested energy; in any case, it would be harder to spot than Types II and III, whose vast sources of power could result in stronger signals or observable artifacts.
Carl Sagan was concerned enough about Kardashev’s original definitions to refine them into a calculation, his thinking being that the gaps between the Kardashev types needed to be filled in with finer gradations. This would allow us to quantify where civilizations are on the scale. Sagan’s calculation would let us discover the present value for our own civilization using available data (as, for example, from the International Energy Agency) regarding the planet’s total energy capabilities. According to Jiang and team, in 2018 this amounted to 1.90 X 1013W, all of which, via Sagan’s methodology, takes us to a present value of Kardashev 0.728.
But let’s circle back to the other two Kardashev types. Type II can be considered a stellar civilization, which in Kardashev’s thinking means a ten orders of magnitude increase in power consumption over Type I, taking us to 1026W. Here we are using all the energy released by the parent star, and now the idea of Dysonian SETI swings into view, the notion that this kind of consumption could be observable through engineering projects on a colossal scale, such as a Dyson swarm enclosing the parent star to maximize energy collection or a Matrioshka Brain for computation. Jiang reminds us that the Sun’s total luminosity is on the order of 4 X 1026W.
Again, these are arbitrary distinctions; note that at the level of the Sun’s total energy output, we would need only about a fourth of that figure to reach the figure described in the Kardashev Scale as Type II. Quantitative limitations, as noted by Sagan, beset the scale, but there is nothing wrong with the notion of setting up a framework for analysis as a first cut into what might become SETI observables. Kardashev’s Type III, using these same methods, offers up a galactic energy consumption of 1036W, so now an entire galaxy is being manipulated by a civilization.
Consider that the entire Milky Way yields something like 4 X 1037W, which actually means that a Type III culture on the Kardashev scale in our particular galaxy would have command of at least 2.5 percent of the total possible energy sources therein. What such a culture might look like as an observable is anyone’s guess (searches for galaxies with unusual infrared signatures are one way to proceed, as Jason Wright’s team at Penn State has demonstrated), but on the galactic scale, we are at an energy level that may, as the saying goes, be all but indistinguishable from magic.
Let’s back down to our planetary level, and in fact back to our modest 0.728 percent of Type I status. Just when can we anticipate reaching Type I? The new paper eschews simple models of exponential growth and consumption over time, noting that such estimates have tended to be:
…the result of a simple exponential growth model for calculating total energy production and consumption as a function of time, relying on a continuous feedback loop and absent detailed consideration of practical limitations. With this reservation in mind, its prediction for when humanity will reach Type I civilization status must be regarded as both overly simplified and somewhat optimistic.
Instead, the authors consider planetary resources, policies and suggestions on climate change, and forecasts for energy consumption to develop an estimated timeframe. The idea is to achieve a more practical outlook on the use of energy and the limitations on its growth. They consider the wide range of fossil fuels, from coal, peat, oil shale, and natural gas to crude oil, natural gas liquids and feedstocks, as well as the range of nuclear and renewable energy sources. Their analysis is keyed to how usage may change in the near future under the influence of, and taking in the projections of, organizations like the United Nations Framework Convention on Climate Change and the International Energy Agency. They see moving along a trajectory to Type I as inevitable and critical for resolving existential crises that threaten our civilization.
So, for example, on the matter of fossil fuels, the authors consider the downside of environmental concerns over the greenhouse effect and changes to policy affecting carbon emissions that will impact energy production. On nuclear and renewable energy, their analysis takes in factors constraining the growth of these energy sources and data on the current development of each. For both fossil fuels and nuclear/renewables, they produce what they describe as an ‘influenced model’ that predicts development operating under historically observed constraints and the likely consequences.
Applying the formula for calculating the Kardashev scale developed by Carl Sagan, they project that our civilization can attain Kardashev Type I with coal, natural gas, crude oil, nuclear and renewable energy sources as the driver. Thus their Figure 6:
Image: Figure 6 from the paper. Caption: The energy supply in the influenced model. Note: Coal is minimal for 1971-2050 and largely coincides with the Natural gas line. Credit: Jiang et al.
Again referring to the Sagan equation, the paper continues:
A final revisit of Eq 1.1, which is informed by the IEA and UNFCCC’s suggestions, finds an imperative for a major transition in energy sourcing worldwide, especially during the 2030s. Although the resultant pace up the Kardashev scale is very low and can even be halted or reversed in the short term, achieving this energy transformation is the optimal path to assuring we will avoid the environmental pitfalls caused by fossil fuels. In short, we will have met the requirements for planetary stewardship while continuing the overall advancement of our technological civilization.
The final estimate is that humanity reaches Kardashev Type I by 2371, a date the authors consider on the optimistic side but achievable. All this assumes that a Type I civilization can be sustained as well, rather than backsliding into an earlier state, something that human history suggests is by no means assured. Successful management of nuclear power is just one flash point, as is storage and disposal of nuclear waste and global issues like deforestation and declining soil pH. That list could, of course, be extended into global pandemics, runaway AI and other factors.
…for the entire world population to reach the status of a Kardashev Type I civilization we must develop and enable access to more advanced technology to all responsible nations while making renewable energy accessible to all parts of the world, facilitated by governments and private businesses. Only through the full realization of our mutual needs and with broad cooperation will humanity acquire the key to not only avoiding the Great Filter but continuing our ascent to Kardashev Type I, and beyond.
The Great Filter, drawing on Robin Hanson’s work, could be behind us or ahead of us. Assuming it lies ahead, getting through it intact would be the goal of any growing civilization as it finds ways to juggle its technologies and resources to survive. It’s hard to argue with the idea that how we proceed on the Kardashev arc is critical as we summon up the means to expand off-world and dream of pushing into the Orion Arm.
The paper is Jiang et al., “Avoiding the Great Filter: Predicting the Timeline for Humanity to Reach Kardashev Type I Civilization” (preprint).
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