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.