by Paul Gilster | Jun 28, 2024 | Astrobiology and SETI |
Searching for biosignatures in the atmospheres of nearby exoplanets invariably opens up the prospect of folding in a search for technosignatures. Biosignatures seem much more likely given the prospect of detecting even the simplest forms of life elsewhere – no technological civilization needed – but ‘piggybacking’ a technosignature search makes sense. We already use this commensal method to do radio astronomy, where a primary task such as observation of a natural radio source produces a range of data that can be investigated for secondary purposes not related to the original search.
So technosignature investigations can be inexpensive, which also means we can stretch our imaginations in figuring out what kind of signatures a prospective civilization might produce. The odds may be long but we do have one thing going for us. Whereas a potential biosignature will have to be screened against all the abiotic ways it could be produced (and this is going to be a long process), I suspect a technosignature is going to offer fewer options for false positives. I’m thinking of the uproar over Boyajian’s Star (KIC 8462852), where the false positive angles took a limited number of forms.
If we’re doing technosignature screening on the cheap, we can also worry less about what seems at first glance to be the elephant in the room, which is the fact that we have no idea how long a technological society might live. The things that mark us as tool-using technology creators to distant observers have not been apparent for long when weighed against the duration of life itself on our planet. Or maybe I’m being pessimistic. Technosignature hunter Jason Wright at Penn State makes the case that we simply don’t know enough to make statements about technology lifespans.
On this point I want to quote Edward Schwieterman (UC-Riverside) and colleagues from a new paper, acknowledging Wright’s view that this argument fails because the premise is untested. We don’t actually know whether non-technological biosignatures are the predominant way life presents itself. Consider:
In contrast to the constraints of simple life, technological life is not necessarily limited to one planetary or stellar system, and moreover, certain technologies could persist over astronomically significant periods of time. We know neither the upper limit nor the average timescale for the longevity of technological societies (not to mention abandoned or automated technology), given our limited perspective of human history. An observational test is therefore necessary before we outright dismiss the possibility that technospheres are sufficiently common to be detectable in the nearby Universe.
So let’s keep looking, which is what Schwieterman and team are advocating in a paper focusing on terraforming. In previous articles on this site we’ve looked at the prospect of detecting pollutants like chlorofluorocarbons (CFCs), which emerge as byproducts of industrial activity, but like nitrogen dioxide (NO₂) these industrial products seem a transitory target, given that even in our time the processes that produce them are under scrutiny for their harmful effect on the environment. What the new paper proposes is that gases that might be produced in efforts to terraform a planet would be longer lived as an expanding civilization produced new homes for its culture.
Enter the LIFE mission concept (Large Interferometer for Exoplanets), a proposed European Space Agency observatory designed to study the composition of nearby terrestrial exoplanet atmospheres. LIFE is a nulling interferometer working at mid-infrared wavelengths, one that complements NASA’s Habitable Worlds Observatory, according to its creators, by following “a complementary and more versatile approach that probes the intrinsic thermal emission of exoplanets.”
Image: The Large Interferometer for Exoplanets (LIFE), funded by the Swiss National Centre of Competence in Research, is a mission concept that relies on a formation of flying “collector telescopes” with a “combiner spacecraft” at their center to realize a mid-infrared interferometric nulling procedure. This means that the light signal originating from the host star of an observed terrestrial exoplanet is canceled by destructive interference. Credit: ETH Zurich.
In search of biosignatures, LIFE will collect data that can be screened for artificial greenhouse gases, offering high resolutions for studies in the habitable zones of K- and M-class stars in the mid-infrared. The Schwieterman paper analyzes scenarios in which this instrument could detect fluorinated versions of methane, ethane, and propane, in which one or more hydrogen atoms have been replaced by fluorine atoms, along with other gases. The list includes Tetrafluoromethane (CF₄), Hexafluoroethane (C₂F₆), Octafluoropropane (C₃F₈), Sulfur hexafluoride (SF₆) and Nitrogen trifluoride (NF₃). These gases would not be the incidental byproducts of other industrial activity but would represent an intentional terraforming effort, a thought that has consequences.
After all, any attempt to transform a planet the way some people talk about terraforming Mars would of necessity be dealing with long-lasting effects, and terraforming gases like these and others would be likely to persist not just for centuries but for the duration of the creator civilization’s lifespan. Adjusting a planetary atmosphere should present a large and discernable spectral signature precisely in the infrared wavelengths LIFE will specialize in, and it’s noteworthy that gases like those studied here have long lifetimes in an atmosphere and could be replenished.
LIFE will work via direct imaging, but the study also takes in detection through transits by calculating the observing time needed with the James Webb Space Telescope’s instruments as applied to TRAPPIST-1 f. The results make the detection of such gases with our current technologies a clear possibility. As Schwieterman notes, “With an atmosphere like Earth’s, only one out of every million molecules could be one of these gases, and it would be potentially detectable. That gas concentration would also be sufficient to modify the climate.”
Indeed, working with transit detections for TRAPPIST-1 f produces positive results with JWST’s MIRI Low Resolution Spectrometer (LRS) and NIRSpec instrumentation (with “surprisingly few transits”). But while transits are feasible, they’re also more scarce, whereas LIFE’s direct imaging in the infrared takes in numerous nearby stars.
From the paper:
We also calculated the MIR [mid infrared] emitted light spectra for an Earth-twin planet with 1, 10, and 100 ppm of CF₄, C₂F₆, C₃F₈, SF₆, and NF₃… and the corresponding detectability of C₂F₆, C₃F₈, and SF₆ with the LIFE concept mission… We find that in every case, the band-integrated S/Ns were >5σ for outer habitable zone Earths orbiting G2V, K6V, or TRAPPIST-1-like (M8V) stars at 5 and 10 pc and with integration times of 10 and 50 days. Importantly, the threshold for detecting these technosignature molecules with LIFE is more favorable than standard biosignatures such as O₃ and CH₄ at modern Earth concentrations, which can be accurately retrieved… indicating meaningfully terraformed atmospheres could be identified through standard biosignatures searches with no additional overhead.
Image: Qualitative mid-infrared transmission and emission spectra of a hypothetical Earth-like planet whose climate has been modified with artificial greenhouse gases. Credit: Sohail Wasif/UCR.
The choice of TRAPPIST-1 is sensible, given that the system offers seven rocky planet targets aligned in such a way that transit studies are possible. Indeed, this is one of the most highly studied exoplanetary systems available. But the addition of the LIFE mission’s instrumentation shows that direct imaging in the infrared expands the realm of study well beyond transiting worlds. So whereas CFCs are short lived and might flag transient industrial activity, the fluorinated gases discussed in this paper are chemically inert and represent potentially long-lived signatures for a terraforming civilization.
The paper is Schwieterman et al., “Artificial Greenhouse Gases as Exoplanet Technosignatures,” Astrophysical Journal Vol. 969, No. 1 (25 June 2024), 20 (full text).
by Paul Gilster | Jun 21, 2024 | Astrobiology and SETI |
The search for life on planets beyond our Solar System is too often depicted as a binary process. One day, so the thinking goes, we’ll be able to directly image an Earth-mass exoplanet whose atmosphere we can then analyze for biosignatures. Then we’ll know if there is life there or not. If only the situation were that simple! As Alex Tolley explains in his latest essay, we’re far more likely to run into results that are so ambiguous that the question of life will take decades to resolve. Read on as Alex delves into the intricacies of life detection in the absence of instruments on a planetary surface.
by Alex Tolley
“People tend to believe that their perceptions are veridical representations of the world, but also commonly report perceiving what they want to see or hear.” [17]
Evolution has likely selected us to see dangerous things whether they are there or not. Survival favors avoiding a rustling bush that may hide a saber-toothed cat. We see what we are told to see, from gods in the sky that may become etched as a group of bright stars in the sky. The post-Enlightenment world has not eradicated those motivated perceptions, as the history of astronomy and astrobiology demonstrates.
There have been some famous misperceptions of life and ETI in the past. Starting with Giovanni Schiaparelli’s perceptions of channels (canali) on Mars, followed by Lowell’s creation of a Martian civilization from whole cloth based on his interpretation of canali as canals. As a consensus seemed to be building that plants existed on Mars, in 1957 and later in 1959 Sinton claimed that he had detected absorption bands from Mars indicating organic matter probably pointing to plant life. These “Sinton bands” later proved to be the detection of deuterium in the Earth’s atmosphere.
I note in passing that Dr. Wernher Von Braun assumed that the Martian atmosphere had a surface pressure 1/12th of Earth’s, based on a few telescopic and spectrographic observations and calculations. This assumption was used in his The Mars Project [23] and Project Mars: A Technical Tale [24], to design the winged landers that were depicted in the movie “The Conquest of Space”. How wrong those assumptions proved!
Briefly, in 1967, regular radio pulses discovered by astrophysicist Jocelyn Bell were thought to be a possible ET beacon, perhaps influenced by the interest in Frank Drake’s initial Project Ozma search for radio signals that was started in 1960. They were quickly identified as emitted by a pulsar, a new degenerate stellar type.
Not to be outdone, nn 1978, Fred Hoyle and Chandra Wickramasinghe published their popular science book – Lifecloud: The Origin of Life in the Universe [5]. In the chapter “Planets of Life”, they made the inference that the spectra they observed around stars most closely matched cellulose (a macromolecule of simple hexose sugars, composed of the common elements carbon, hydrogen, and oxygen, and the major material of plants). This became the basis of their claims of the ubiquity of life, panspermia, and cometary delivery of viruses to Earth. Again, this assertion of the ubiquity of life proved to be incorrect based on faulty logical inference, which was in turn based on the incorrect interpretation of cellulose as the molecule identified from the light. It remains a cautionary tale.
Figure 1. A) Illusory lines between objects are interpreted as canals. – Source [16] B) The assumption that lichen-like plants were abundant and produced teh dark areas on Mars. Source [16] C) The spectral fit that convinced Hoyle and Wickramasinghe that they had detected cellulose around other stars. – Source [5] D) View of a purported “fossil” in the famous Mars meteorite, Allan Hills 84001. Doubters argue that the feature is too small to be a sign of Mars life. (Image credit: NASA).
The latest possible misinterpretation may be the results of the Hephaistos Project that claimed 7 stars had anomalous longer wavelength intensities that might indicate a technosignature of a Dyson sphere or swarm [19]. Almost immediately a natural explanation appeared suggesting a data contamination issue with a distant galaxy in the same line of sight.
These historical observational misinterpretations are worth bearing in mind, as the odds are we’ll find life beyond our Solar System, if only because of the vast number of planets that have conditions that could bear life on their surfaces, perhaps even an “Earth 2.0” We will be restricted to data from the electromagnetic (em) spectrum with no hope of acquiring the ground truth from probes sent to those systems.
Returning to the early search for life, the limitations of 1960s technology optical astronomy were highlighted in the US publication of Intelligent Life in the Universe by Iosif Samuilovich Shklovsky, with additional content by Sagan [4], as well as by earlier papers by Sagan [3]. Carl Sagan noted that hypothetical Martian astronomers would not be able to confirm the detection of life and intelligence on Earth using the many terrestrial techniques available at the time, and highlighted some of the issues, including the resolution needed to detect human artifacts, and the ambiguity of spectral data. He asserted that Martian astronomers required ground truth, i.e. a probe in Earth’s orbit or a lander.
From the paper [3]
Moreover, with Earth at 1 km resolution “no seasonal variations in the contrast of vegetation could be detected…. [I]t is estimated that better than 35 m resolution, with global coverage, would be needed to detect life on a hypothetical Earth with no intelligent life.
The best telescopic resolution based on a hypothetical solar gravitational line telescope (SGL) would be far too low to detect life without ambiguity. A simulated image is shown in Figure 2 below.
Figure 2. A 1024×1024 pixel image simulation of an exoplanet at a distance of up to 30 parsecs, imaged with a possible solar gravitational lens telescope, has a surface resolution of a few 10s of km per pixel, Credit: Turyshev et al., “Direct Multipixel Imaging and Spectroscopy of an Exoplanet with a Solar Gravity Lens Mission,” Final Report NASA Innovative Advanced Concepts Phase II. – Source [26]
Given the limitations outlined by Sagan and Shklovskii, how might we detect that exoplanet life soon? In an influential 1967 paper, “Life Detection by Atmospheric Analysis”, Hitchcock and Lovelock argued that a living planet would create disequilibria in the atmospheric gases of the planet [1].
“Living systems maintain themselves in a state of relatively low entropy at the expense of their nonliving environments. We may assume that this general property is common to all life in the solar system. On this assumption, evidence of a large chemical free energy gradient between surface matter and the atmosphere in contact with it is evidence of life. Furthermore, any planetary biota which interacts with its atmosphere will drive that atmosphere to a state of disequilibrium which, if recognized, would also constitute direct evidence of life, provided the extent of the disequilibrium is significantly greater than abiological processes would permit. It is shown that the existence of life on Earth can be inferred from knowledge of the major and trace components of the atmosphere, even in the absence of any knowledge of the nature or extent of the dominant life forms. Knowledge of the composition of the Martian atmosphere may similarly reveal the presence of life there.”
This proxy has become the core approach for searching for biosignatures of exoplanets, as we are rapidly evolving the technology to detect gas mixtures via transmission spectroscopy of these worlds.
We have no hope of sending probes to those worlds within the foreseeable future, and the speed of light limits when we could even receive information from a probe that landed on such a planet. Therefore, unlike our system, where samples can be both locally analyzed or returned to Earth, only remote observation is currently possible for exoplanets.
For terrestrial worlds like our contemporary Earth, the signature would be the presence of both oxygen (O2) from oxygenic photosynthesis and methane (CH4) from methanogenic bacteria or archaea. So strong is this idea, that my first post on Centauri Dreams was to review a paper that discussed what the atmospheric biosignature would be for an early Earth before photosynthesis had made O2 the 2nd most common gas in the atmosphere, a period that encompassed most of Earth’s history [2].
However, there are increasing concerns from the astrobiology community about this approach because of possible false positives. For example, O2 could be entirely generated by photolysis of water, and even CH4 might be sufficiently produced by geologic means to maintain that disequilibrium, creating false positives or at best ambiguity of the spectral analysis as a biosignature.
Astrobiologists have continued to explore other possible biosignatures, usually with terrestrial life as the template. For example, Sara Seager published a catalog of small, detectable molecules that included some that are only made by life forms. This included phosphine (AKA phosphane, PH3). In 2021, Greaves reported that PH3 had been discovered in the spectra of Venus’s atmosphere [27]. As PH3 is principally produced by life on Earth, this set off a flurry of observations, experiments, and even a soon-to-be-launched probe to the temperate zone of the Venusian atmosphere. Unfortunately, in this case, confirmation of the signal was not made. It was also suggested that the very similar spectral signature of sulfur dioxide was the culprit. The original observation remains controversial, but at least we will eventually get the ground truth we need. But as we will see later, there is a theoretical abiotic route to PH3 production via Venusian volcanic emissions which adds ambiguity to the finding as a biosignature.
In 2018, Sara Walker published a long paper on the issue of dealing with false positives for any biosignature [7]. Much of the paper relied on the use of Bayesian statistical methods, although a potential flaw was the issue of assigning the prior probabilities. Much of the paper dealt with data that would need some sort of sample, even ground truth, such as a sample taken by an in situ probe, as originally suggested by Sagan and Shklovskii. Purely electromagnetic spectrum (em) data would exclude these sample analyses.
A 2021 paper by Green et al [8] suggested that the detection of life should be viewed on a scale of increasing certainty, rather than a binary true or false determination. In other words, ambiguity was to be encompassed:
“The Community Workshop Report argues (with reasonable grounds) that the first detection of an extraterrestrial biosignature will likely be ambiguous and require significant follow-on work.”
They suggested a new Confidence of Life Detection scale (CoLD), to indicate reliability based on NASA’s Technology Readiness Level (TRL) approach: Confidence is increased with confirmation and as abiotic causes are ruled out. A modified chart is shown in Figure 3.
Figure 3. CoLD scale. Printed with permission in the Vickers et al paper.
At about the same time, NASA sponsored a biosignature workshop [9] to assess and report on life detection to address the recognized problems of reliability of life detection as an immature science. Their effort was especially targeted at communicating a possible life detection. It should be noted that NASA did a very poor job of this in the past, notably the press conferences on the “Martian microbes” from the ALH84001 meteorite in 2006, and the arsenic-based bacteria in 2010, billed as a possible different alien biology that could exist on another world [28]. Nasa clearly wanted to avoid such premature announcements and tread a more cautious approach. [In an ironic twist, researchers have discovered arsenic metabolism in some deep sea marine microbes [29].
This has gained importance because of the increasingly attention-grabbing approach of articles on the discovery of exoplanets that resemble Earth, often using the “Earth 2.0” label. It is highly unlikely any exoplanet with similar dimensions and orbits in the habitable zone (HZ) is a terrestrial-type verdant world suitable for eventual colonization if or when our starships can reach them.
The workshop produced this Standards of Evidence scale published in 2022:
Table 1. Standards of Evidence Life Detection scale, produced by a community wide Effort [9] Credit: Walker et al [14]
In 2023, Smith, Harrison, and Mathis published an extensive critique of the reliability of biosignatures for life detection. In their essay [10] they state in the abstract that:
“Our limited access to otherworlds suggests this observation is more likely to reflect out-of-equilibrium gases than a writhing octopus. Yet, anything short of a writhing octopus will raise skepticism about what has been detected.”
In the introduction, they state that atmospheric gas disequilibria are byproducts of life on Earth, and not unique, for example, abiotic production of O2 in the atmosphere. The following includes a critique of the Krissansen-Totton et al paper that I sourced in my first Centauri Dreams post:
“Often these models don’t rely on any underlying theory of life, and instead consider specific sources and sinks of chemical species, and rules of their interactions. Astrobiologists label these sources, sinks, or transformations as being due to life or nonlife, tautologically defined by the fluxes they influence. For example, defining life via biotic fluxes of methane from methanogenesis [6,7] and defining abiotic fluxes via rates of serpentinization and impacts.”
And:
“This leads to the conclusion that most exoplanet biosignatures are futile if our goal is to detect life outside the solar system with confidence.”
Figure 4 below shows the 4 approaches they suggest to firm up the strength of biosignatures.
1. Biological research on terrestrial life,
2. Looking for life with probes sent to planets in our system
3. Experimental work on abiogenesis and molecular outcomes
4. SETI via technosignatures
Figure 4. The 4 approaches to explore biosignatures. Credit: Smith, Harrison & Mathis.
They conclude:
“As astrobiologists we believe the search for life beyond Earth is one of the most pressing scientific questions of our time. But if we as a community can’t decide how to formalize our ideas into testable hypotheses to motivate specific measurements or observational goals, we are taking valuable observational time and resources away from other disciplines and communities that have clearly articulated goals and theories. It’s one thing to grope around in the dark, or explore uncharted territory, but do so at the cost of other scientific endeavors become increasingly difficult to justify. One of the most significant unification of biological phenomena–Darwin’s theory of natural selection–emerged only after Darwin went on exploratory missions around the world and documented observations. It’s possible the data required to develop a theory of life that can make predictions about living worlds simply has not been documented sufficiently. But if that’s the case we should stop aiming to detect something we cannot understand, and instead ask what kinds of exploration are needed to help us formalize such a theory.”
One month later, Vickers, Peter, et al. published “Confidence of Life Detection: The Problem of Unconceived Alternatives.” [11]. This paper aimed to demolish the idea of using Bayesian probabilities as there was little hope of even conceiving of novel ways life may arise, nor the abiotic mimics of possible signatures.
From the abstract:
“It is argued that, for most conceivable potential biosignatures, we currently have not explored the relevant possibility space very thoroughly at all. Not only does this severely limit the circumstances in which we could reasonably be confident in our detection of extraterrestrial life, it also poses a significant challenge to any attempt to quantify our degree of (un)certainty.”
From the introduction:
“(…) the problem of unconceived abiotic explanations for phenomena of interest……., we stress that articulating our uncertainty requires an assessment of the extent to which we have explored the relevant possibility space. It is argued that, for most conceivable potential biosignatures, we currently have not explored the relevant possibility space very thoroughly at all.”
From section 2 – The challenges of known and unknown false positives:
“As Meadows et al. (2022, p. 26) note, “[I]f the scope of possible abiotic explanations is known to be poorly explored, it suggests we cannot adequately reject abiotic mechanisms.” Conversely, if it is known to be thoroughly explored, we probably can reject abiotic mechanisms.”
In effect, they are reiterating the problems of inference. For example, it was thought since Roman times that all swans were white as no examples of differently colored swans had been seen in the European, Asian, and African continents. This remained the case until black swans were discovered in Australia.
Today the more popular phrase that covers the issue of inference is “Absence of Evidence is not Evidence of Absence”.
They critique Green’s CoLD scale and suggest that the Intergovernmental Panel on Climate Change (IPCC) approach using a 2D scale of scientific consensus and strength of evidence may be more suitable.
Figure 5. IPCC framework for climate as a biosignature confidence scale. Credit: Vickers et al.
As Vickers was sowing doubts about biosignature detection reliability and how best to handle the uncertainties, Sara Walker and collaborators published “False Positives and the Challenge of Testing the Alien Hypothesis” [12] repeating their earlier argument for Bayesian methods, and the need for definitive biosignatures to avoid false positives. Those definitive biosignatures may be based on the Assembly Theory of the composition of organic molecules [13]. I would include the complementary approach, even though it does allow for false positives [14]. However, these approaches require samples that will not be available for exoplanets. Walker ends with the suggestion that the approach of levels (or ladders) of certainty such as the Confidence of Life Detection (CoLD) scale, and the community Standards of Evidence Life Detection scale should be used because we need more data to understand planetary types and life, and that data will improve the probabilities of evaluating the specific probability of life on a planet, especially with the rapidly increasing number of exoplanets.
Whatever the pros and cons of different approaches, it appears that continuing research and cataloging of exoplanets will help narrow down the uncertainties of life detection. Ideally, several orthogonal approaches can be used to triangulate the probability that the biosignature is a true positive.
Sagan was right in that we need the ground truth of close observation and samples to validate electromagnetic data. While ground truth can eventually be acquired for planets in our solar system, we don’t have that for exoplanets, nor will we have that for the foreseeable future, unless that low probability SETI radio or optical signal is detected. In time, with a catalog of exoplanet data, it might be possible to collect enough examples to determine if there are abiotic mimics of different gas disequilibria, or other phenomena like the chlorophyll “red edge”. But we cannot know with certainty, and any abiotic mimic reduces the confidence of biotic interpretations.
Therefore, biosignatures from exoplanets will remain uncertain indications of life. We cannot escape from this. It will be up to the community and the responsible media to make this clear. [And good luck with the media].
As if this issue were not relevant, Payne and Kalteneggar just published a paper indicating that the O2 + CH4 signature is stronger in the last 100-300m than today, principally due to the greater partial pressure of O2 in the atmosphere during that period [15]. It was covered by the press as “Earth Was More Attractive to Aliens Back When Dinosaurs Roamed” [21]. C’est la vie.
References
1. Hitchcock, D. R., and J. E. Lovelock. “Life Detection by Atmospheric Analysis.” Icarus, vol. 7, no. 1–3, Jan. 1967, pp. 149–59. https://doi.org/10.1016/0019-1035(67)90059-0.
2. Tolley, Alex Detecting Early Life on Exoplanets. Centauri Dreams February 23, 2018.
https://www.centauri-dreams.org/2018/02/23/detecting-early-life-on-exoplanets/
3. Kilston, S. D., Drummond, R. R., & Sagan, C. (1966). A search for life on Earth at kilometer resolution. Icarus, 5(1–6), 79–98. https://doi.org/10.1016/0019-1035(66)90010-8
4. Shklovskii, I.S., and Carl Sagan. Intelligent Life in the Universe. San Francisco, CA, United States of America, Holden-Day, Inc., 1966.
5. Hoyle, Fred, and N. Chandra Wickramasinghe. Lifecloud: The Origin of Life in the Universe. HarperCollins Publishers, 1978.
6. Stirone, S., Chang, K., & Overbye, D. (2020). Life on Venus? Astronomers see a signal in its clouds. The New York Times. https://www.nytimes.com/2020/09/14/science/venus-life-clouds.html
7. Walker SI, et al Exoplanet Biosignatures: Future Directions. Astrobiology. 2018 Jun;18(6):779-824. doi: 10.1089/ast.2017.1738. PMID: 29938538; PMCID: PMC6016573.
8. J. Green, T. Hoehler, M. Neveu, S. Domagal-Goldman, D. Scalice, and M. Voytek, 2021, “Call for a Framework for Reporting Evidence for Life Beyond Earth,” Nature 598:575-579, https://doi.org/10.1038/s41586-021-03804-9.
9. “Independent Review of the Community Report From the Biosignature Standards of Evidence Workshop.” National Academies Press eBooks, 2022, https://doi.org/10.17226/26621.
10. Smith, Harrison B., and Cole Mathis. “Life Detection in a Universe of False Positives.” BioEssays, vol. 45, no. 12, Oct. 2023, https://doi.org/10.1002/bies.202300050.
11, Vickers, Peter, et al. “Confidence of Life Detection: The Problem of Unconceived Alternatives.” Astrobiology, vol. 23, no. 11, Nov. 2023, pp. 1202–12. https://doi.org/10.1089/ast.2022.0084.
12, Foote, Searra, Walker, Sara, et al. “False Positives and the Challenge of Testing the Alien Hypothesis.” Astrobiology, vol. 23, no. 11, Nov. 2023, pp. 1189–201. https://doi.org/10.1089/ast.2023.0005.
13. Tolley, A (2024) “Alien Life or Chemistry? A New Approach Alien Life or Chemistry? A New Approach, Centauri Dreams https://www.centauri-dreams.org/2024/01/24/alien-life-or-chemistry-a-new-approach/
14, Tolley, A (2018) “Detecting Life On Other Worlds”, Centauri Dreams https://www.centauri-dreams.org/2018/08/10/detecting-life-on-other-worlds/
15. R C Payne, L Kaltenegger, Oxygen bounty for Earth-like exoplanets: spectra of Earth through the Phanerozoic, Monthly Notices of the Royal Astronomical Society: Letters, Volume 527, Issue 1, January 2024, Pages L151–L155, https://doi.org/10.1093/mnrasl/slad147
16. Ley, Willy, and Wernher Von Braun. The Exploration of Mars. 1956
17. Leong, Y.C., Hughes, B.L., Wang, Y. et al. Neurocomputational mechanisms underlying motivated seeing. Nat Hum Behav 3, 962–973 (2019). https://doi.org/10.1038/s41562-019-0637-z
18. BBC News. “Arsenic-loving Bacteria May Help in Hunt for Alien Life.” BBC News, 2 Dec 2010, https://www.bbc.co.uk/news/science-environment-11886943.
19. Sankaran, Vishwam. “Dyson Spheres: Alien Power Plants May Be Drawing Energy From 7 Stars in the Milky Way.” The Independent, 17 May 2024, https://www.independent.co.uk/space/aliens-dyson-spheres-milky-way-power-plants-b2546601.html.
20. “Astrobiology at Ten.” Nature, vol. 440, no. 7084, Mar. 2006, p. 582. https://doi.org/10.1038/440582a.
21. Nield, David. “Earth Was More Attractive to Aliens Back When Dinosaurs Roamed.” ScienceAlert, 10 Nov. 2023 https://www.sciencealert.com/earth-was-more-attractive-to-aliens-back-when-dinosaurs-roamed.
22. Choi, Charles Q. “Mars Life? 20 Years Later, Debate Over Meteorite Continues.” Space.com, 10 Aug. 2016, https://www.space.com/33690-allen-hills-mars-meteorite-alien-life-20-years.html.
23, Von Braun, Wernher. The Mars Project. University of Illinois Press, 1953.
24, Von Braun, Wernher. Project Mars: A Technical Tale. Apogee Books, 2006.
25. Sinton, W M, “Radiometric Observations of Mars,” Astrophysical Journal, vol. 131, p. 459-469 (1960).
26. Turyshev et al., “Direct Multipixel Imaging and Spectroscopy of an Exoplanet with a Solar Gravity Lens Mission,” Final Report NASA Innovative Advanced Concepts Phase II. https://arxiv.org/abs/2002.11871
27. Greaves, J.S., Richards, A.M.S., Bains, W. et al. “Phosphine gas in the cloud decks of Venus.” Nat Astron 5, 655–664 (2021). https://doi.org/10.1038/s41550-020-1174-4
28. NASA (2010) NASA-Funded Research Discovers Life Built With Toxic Chemical URL: https://www.prnewswire.com/news-releases/nasa-funded-research-discovers-life-built-with-toxic-chemical-111207604.html
29. Saunders, J. K., Fuchsman, C. A., McKay, C., & Rocap, G. (2019). “Complete arsenic-based respiratory cycle in the marine microbial communities of pelagic oxygen-deficient zones.” Proceedings of the National Academy of Sciences, 116(20), 9925-9930. https://doi.org/10.1073/pnas.1818349116
by Paul Gilster | May 18, 2024 | Astrobiology and SETI |
I’m enjoying the conversation about Project Hephaistos engendered by the article on Dyson spheres. In particular, Al Jackson and Alex Tolley have been kicking around the notion of Dyson sphere alternatives, ways of preserving a civilization that are, in Alex’s words, less ‘grabby’ and more accepting of their resource limitations. Or as Al puts it:
One would think that a civilization that can build a ‘Dyson Swarm’ for energy and natural resources would have a very advanced technology. Why then does that civilization not deploy an instrumentality more sly? Solving its energy needs in very subtle ways…
As pointed out in the article, a number of Dyson sphere searches have been mounted, but we are only now coming around to serious candidates, and at that only seven out of a vast search field. Two of these are shown in the figure below. We’re a long way from knowing what these infrared signatures actually represent, but let’s dig into the Project Hephaistos work from its latest paper in 2024 and also ponder what astronomers can do as they try to learn more.
Image: This is Figure 7 from the paper. Caption: SEDs [spectral energy distributions] of two Dyson spheres candidates and their photometric images. The SED panels include the model and data, with the dashed blue lines indicating the model without considering the emission in the infrared from the Dyson sphere and the solid black line indicating the model that includes the infrared flux from the Dyson sphere. Photometric images encompass one arcmin. All images are centered in the position of the candidates, according to Gaia DR3. All sources are clear mid-infrared emitters with no clear contaminators or signatures that indicate an obvious mid-infrared origin. The red circle marks the location of the star according to Gaia DR3. Credit: Suazo et al.
We need to consider just how much we can deduce from photometry. Measuring light from astronomical sources across different wavelengths is what photometry is about, allowing us to derive values of distance, temperature and composition. We’re also measuring the object’s luminosity, and this gets complicated in Dyson sphere terms. Just how does the photometry of a particular star change when a Dyson sphere either partially or completely encloses it? We saw previously that the latest paper from this ongoing search for evidence of astroengineering has developed its own models for this.
The model draws on earlier work from some of the co-authors of the paper we’re studying now. It relies on two approaches to the effect of a Dyson sphere on a star’s photometry. First, we need to model the obscuration of the star by the sphere itself. Beyond this, it’s essential to account for the re-emission of absorbed radiation at much longer wavelengths, as the megastructure – if we can call it that – gives off heat.
“[W]e model the stellar component as an obscured version of its original spectrum and the DS component as a blackbody whose brightness depends on the amount of radiation it collects,” write the authors of the 2022 paper I discussed in the last post. The modeling process is worth a post of its own, but instead I’ll send those interested to an even earlier work, a key 2014 paper from Jason Wright and colleagues, “The Ĝ Infrared Search for Extraterrestrial Civilizations with Large Energy Supplies. II. Framework, Strategy, and First Result.” The citation is at the end of the text.
The recently released 2024 paper from Hephaistos examined later data from Gaia (Data Release 3) while also incorporating the 2MASS and WISE photometry of some 5 million sources to create a list of stars that could potentially host a Dyson sphere. In the new paper, the authors home in on partial Dyson spheres, which will partially obscure the star’s light and would show varying effects depending on the level of completion. The waste heat generated in the mid-infrared would depend upon the degree to which the structure (or more likely, ‘swarm’) was completed as well as its effective temperature.
So we have a primary Dyson sphere signature in the form of excess heat, thermal emission that shows up at mid-infrared wavelengths, and that means we’re in an area of research that also involves other sources of such radiation. The dust in a circumstellar disk is one, heated by the light of the star and re-emitted at longer wavelengths. As we saw yesterday, all kinds of contamination are thus possible, but the data pipeline used by Project Hephaistos aims at screening out the great bulk of these.
Seven candidates for Dyson spheres survive the filter. All seven appear to be actual infrared sources that are free of contamination from dust or other sources. The researchers subjected the data to over 6 million models that took in 391 Dyson sphere effective temperatures. They modeled Dyson spheres in temperature ranges from 100 to 700 K, with covering factors (i.e., the extent of completion of the sphere) from 0.1 to 0.9. Among many factors considered here, they’re also wary of Hα (hydrogen alpha) emissions, which could flag the early stage of star growth and might be implicated in observations of infrared radiation.
Image: IC 2118, a giant cloud of gas and dust also known as the Witch Head Nebula. H-alpha emissions, which are observed over most of the Orion constellation, are shown in red. This H-alpha image was taken by the MDW Survey, a high-resolution astronomical survey of the entire night sky not affiliated with Project Hephaistos. I’m showing it to illustrate how pervasive and misleading Hα can be in a Dyson sphere search. Credit: Columbia University.
I want to be precise about what the authors are saying in this paper: “…we identified seven sources displaying mid-infrared flux excess of uncertain origin.” They are not, contra some sensational reports, saying they found Dyson spheres. These are candidates. But let’s dig in a bit, because the case is intriguing. From the paper:
Various processes involving circumstellar material surrounding a star, such as binary interactions, pre-main sequence stars, and warm debris disks, can contribute to the observed mid-infrared excess (e.g. Cotten & Song 2016). Kennedy & Wyatt (2013) estimates the occurrence rate of warm, bright dust. The occurrence rate is 1 over 100 for very young sources, whereas it becomes 1 over 10,000 for old systems (> 1 Gyr). However, the results of our variability check suggest that our sources are not young stars.
Are the candidate objects surrounded by warm debris disks? What’s interesting here is that all seven of these are M-class stars, and as the authors note, M-dwarf debris disks are quite rare, with only a few confirmed. Why this should be so is the object of continuing study, but both the temperature and luminosity of the candidate objects differs from typical debris disks. The questions deepen and multiply:
Extreme Debris Disks (EDD) (Balog et al. 2009), are examples of mid-infrared sources with high fractional luminosities (f > 0.01) that have higher temperatures compared to that of standard debris disks (Moór et al. 2021). Nevertheless, these sources have never been observed in connection with M dwarfs. Are our candidates’ strange young stars whose flux does not vary with time? Are these stars M-dwarf debris disks with an extreme fractional luminosity? Or something completely different?
The authors probe the possibilities. They consider chance alignments with distant infrared sources, and offsets in the astrometry when incorporating the WISE data. There is plenty to investigate here, and the paper suggests optical spectroscopy as a way of refuting false debris disks around M-dwarfs, which could help sort between the seven objects here identified. Stellar rotation, age and magnetic activity may also be factors that will need to be probed. But when all is said and done, we wind up with this:
…analyzing the spectral region around Hα can help us ultimately discard or verify the presence of young disks by analyzing the potential Hα emission. Spectroscopy in the MIR [mid-infrared] region would be very valuable when determining whether the emission corresponds to a single blackbody, as we assumed in our models. Additionally, spectroscopy can help us determine the real spectral type of our candidates and ultimately reject the presence of confounders.
So the hunt for Dyson spheres proceeds. Various pieces need to fall into place to make the case still more compelling, and we should remember that “The MIR data quality for these objects is typically quite low, and additional data is required to determine their nature.” This layman’s guess – and I am not qualified to do anything more than guess – is that rather than Dyson spheres we are glimpsing interesting astrophysics regarding M-dwarfs that this investigation will advance. In any case, do keep in mind that among some five million sources, only seven show compatibility with the Dyson sphere model.
If Dyson spheres are out there, they’re vanishingly rare. But finding just one would change everything.
The paper on Dyson sphere modeling is Wright et al., “The Ĝ Infrared Search for Extraterrestrial Civilizations with Large Energy Supplies. II. Framework, Strategy, and First Result,” The Astrophysical Journal Vol. 792, Issue 1 (September, 2014), id 27 (abstract). The 2022 paper from Project Hephaistos is Suazo et al., “Project Hephaistos – I. Upper limits on partial Dyson spheres in the Milky Way,” Vol. 512, Issue 2 (May 2022), 2988-3000 (abstract / preprint). The 2024 paper is Suazo et al., “Project Hephaistos – II. Dyson sphere candidates from Gaia DR3, 2MASS, and WISE,” MNRAS (6 May 2024), stae1186 (abstract / preprint).
by Paul Gilster | May 15, 2024 | Astrobiology and SETI |
For a project looking for the signature of an advanced extraterrestrial civilization, the name Hephaistos is an unusually apt choice. And indeed the leaders of Project Hephaistos, based at Uppsala University in Sweden, are quick to point out that the Greek god (known as Vulcan in Roman times) was a sort of preternatural blacksmith, thrown off Mt. Olympus for variously recounted transgressions and lame from the fall, a weapons maker and craftsman known for his artifice. Consider him the gods’ technologist.
Who better to choose for a project that pushes SETI not just throughout the Milky Way but to myriads of galaxies beyond? Going deep and far is a sensible move considering that we have absolutely no information about how common life is beyond our own Earth, if it exists at all. If the number of extraterrestrial civilizations in any given galaxy is scant, then a survey looking for evidence of Hephaistos-style engineering writ large will comb through existing observational data from our own galaxy but also consider what lies beyond. Which is why Project Hephaistos’s first paper (2015) searched for what the authors called ‘Dysonian astroengineering’ in over 1000 spiral galaxies.
More recent papers have stayed within the Milky Way to incorporate data from Gaia, the 2 Micron All Sky Survey (2MASS) and the accumulated offerings of the Wide-field Infrared Survey Explorer, which now operates as NEOWISE, analyzing the observational signatures of Dyson spheres in the process of construction and calling out upper limits on such spheres-in-the-making in the Milky Way. Such objects could present anomalously low optical brightness levels yet high mid-infrared flux. This is the basic method for searching for Dyson spheres, identifying the signature of waste heat while screening out young stellar objects and other factors that can mimic such parameters.
This article is occasioned by the release of a new paper, one that homes in on Dyson sphere candidates now identified. And it prompts reflection on the nature of the enterprise. Key to the concept is the idea that any flourishing (and highly advanced) extraterrestrial civilization will need to find sources of energy to meet its growing needs. An obvious source is a star, which can be harvested by a sphere of power-harvesting satellites. The notion, which Dyson presented in a paper in Science in 1960, explains how a search could be conducted in its title: “Search for Artificial Stellar Sources of Infrared Radiation.” In other words, comb the skies for infrared anomalies.
I strongly favor this ‘Dysonian’ approach to SETI, which makes no assumptions at all about any decision to communicate. As we have no possible idea of the values that would drive an alien culture to attempt to talk to us – or for that matter to any other civilizations – why not add to the search space the things that we can detect in other ways. However it is constructed, a Dyson sphere should produce waste heat as it obscures the light from the central star. Infrared searches could detect a star that is strangely dim but radiant at infrared wavelengths, and we might also find changes in brightness as such a ‘megastructure’ evolves that vary on relatively short timeframes.
Funding plays into our science in inescapable ways, so the fact that Dysonian SETI can be conducted using existing data is welcome. It’s also helpful that in-depth studies of particular Dyson sphere candidates may prove useful for nailing down astrophysical properties that interest the entire community, especially since there is the possibility of ‘feedback’ mechanisms on the star from any surrounding sphere of technology. We go looking for extraterrestrial megastructures but even if we don’t find them, we produce good science on unusual stellar properties and refine our observational technique. Not a bad way forward even as the traditional SETI effort in radio and optics continues.
The number of searches for individual Dyson spheres is surprisingly large, and to my knowledge extends back at least as far as 1985, when Russian radio astronomer Vyacheslav Ivanovich Slysh searched using data from the Infrared Astronomical Satellite (IRAS) mission, as did (at a later date) M. Y. Timofeev, collaborating with Nikolai Kardashev. Richard Carrigan, a scientist emeritus at the Fermi National Accelerator Laboratory, looked for Dyson signatures out to 300 parsecs.
But we can go earlier still. Carl Sagan was pondering “The Infrared Detectability of Dyson Civilizations” (a paper in The Astrophysical Journal) back in the 1960s. In more recent times, the Glimpsing Heat from Alien Technologies effort at Pennsylvania State University (G-HAT) has been particularly prominent. What becomes staggering is the realization that the target list has grown so vast as our technologies have improved. Note this, from a Project Hephaistos paper in 2022 (citation below):
Most search efforts have aimed for individual complete Dyson spheres, employing far-infrared photometry (e.g., Slysh 1985; Jugaku & Nishimura 1991; Timofeev et al. 2000; Carrigan 2009) from the Infrared Astronomical Satellite (IRAS: Neugebauer et al. 1984), while a few considered partial Dyson spheres (e.g., Jugaku & Nishimura 2004). IRAS scanned the sky in the far infrared, providing data of ≈ 2.5 × 105 point sources. However, nowadays, we rely on photometric surveys covering optical, near-infrared, and mid-infrared wavelengths that reach object counts of up to ∼109 targets and allow for larger search programs.
The Project Hepaistos work in the 2022 paper homed in on producing upper limits for partial Dyson spheres in the Milky Way by searching Gaia DR2 data and WISE results that showed infrared excess, looking at more than 108 stars. We still have no Dyson sphere confirmations, but the new Hephaistos paper adds 2MASS data and moves to Gaia data release 3, which aids in the rejection of false positives. Gaia also adds to the mix its unique capabilities at parallax, which the authors describe thus:
…Gaia also provides parallax-based distances, which allow the spectral energy distributions of the targets to be converted to an absolute luminosity scale. The parallax data also make it possible to reject other pointlike sources of strong mid-infrared radiation such as quasars, but do not rule out stars with a quasar in the background.
Notable in the new 2024 paper is its description of the data pipeline focusing on separating Dyson sphere candidates from natural sources including circumstellar dust. The authors make the case that it is all but impossible to prove the existence of a Dyson sphere based solely on photometric data, so what is essentially happening is a search for sources showing excess infrared that are consistent with the Dyson sphere hypothesis. The data pipeline runs from data collection through a grid search methodology, image classification for filtering out young stars obscured by dust or associated with dusty nebulae, inspection of the signal to noise ratio, further analysis of the infrared excess and visual inspection from all the sources to reject possible contamination.
This gets tricky indeed. Have a look at some of the ‘confounders,’ as the authors call them. The figure shows three categories of confounders: blends, irregular structures and nebular features. In blends, the target is contaminated by external sources within the WISE coverage. The nebular category is a hazy and disordered false positive without a discernible source of infrared at the target’s location. Irregulars are sources without indication of nebulosity whose exact nature cannot be determined. All of these sources would be considered unreliable at the conclusion of the pipeline:
Image: This is Figure 5 from the paper. Caption: Examples of typical confounders in our search. The top row features a source from the blends category, the middle row a source embedded in a nebular region, and the bottom row a case from the irregular category. On these scales, the irregular and nebular cases cannot be distinguished, but the nebular nature can be established by inspecting the images at larger scales. Credit: Suazo et al.
In the next post, I want to take a look at the results, which involve seven interesting candidates, all of them around a type of star I wouldn’t normally think of in Dyson sphere terms. The papers are Suazo et al., “Project Hephaistos – I. Upper limits on partial Dyson spheres in the Milky Way,” Vol. 512, Issue 2 (May 2022), 2988-3000 (abstract / preprint) and Suazo et al., “Project Hephaistos – II. Dyson sphere candidates from Gaia DR3, 2MASS, and WISE,” MNRAS (6 May 2024), stae1186 (abstract / preprint).
by Paul Gilster | Apr 20, 2024 | Astrobiology and SETI |
Radio and optical SETI look for evidence of extraterrestrial civilizations even though we have no evidence that such exist. The search is eminently worthwhile and opens up the ancillary question: How would a transmitting civilization produce a signal strong enough for us to detect it at interstellar distances? Beacons of various kinds have been considered and search strategies honed to find them. But we’ve also begun to consider new approaches to SETI, such as detecting technosignatures in our astronomical data (Dyson spheres, etc.). To this mix we can now add a consideration of gravitational lensing, and the magnifications possible when electromagnetic radiation is focused by a star’s mass. For a star like our Sun, this focal effect becomes useful at distances beginning around 550 AU.
Theoretical work and actual mission design for using this phenomenon began in the 1990s and continues, although most work has centered on observing exoplanets. Here the possibilities are remarkable, including seeing oceans, continents, weather patterns, even surface vegetation on a world circling another star. But it’s interesting to consider how another civilization might see gravitational lensing as a way of signaling to us. Indeed, doing so could conceivably open up a communications channel if the alien civilization is close enough, for if we detect lensing being used in this way, we would be wise to consider using our own lens to reply.
Or maybe not, considering what happens in The Three Body Problem. But let’s leave METI for another day. A new paper from Slava Turyshev (Jet Propulsion Laboratory) makes the case that we should be considering not just optical SETI, but a gravitationally lensed SETI signal. The chances of finding one might seem remote, but then, we don’t know what the chances of any SETI detection are, and we proceed in hopes of learning more. Turyshev argues that with the level of technology available to us today, a lensed signal could be detected with the right strategy.
Image: Slava Turyshev (Jet Propulsion Laboratory). Credit: Asteroid Foundation.
“Search for Gravitationally Lensed Interstellar Transmissions,” now available on the arXiv site, posits a configuration involving a transmitter, receiver and gravitational lens in alignment, something we cannot currently manage. But recall that the effort to design a solar gravity lens (SGL) mission has been in progress for some years now at JPL. As we push into the physics involved, we learn not only about possible future space missions but also better strategies for using gravitational lensing in SETI itself. We are now in the realm of advanced photonics and optical engineering, where we define and put to work the theoretical tools to describe how light propagates in a gravity field.
And while we lack the technologies to transmit using these methods ourselves (at least for now), we do have the ability to detect extraterrestrial signals using gravitational lensing. In an email yesterday, Dr. Turyshev offered an overview of what his analysis showed:
Many factors influence the effectiveness of interstellar power transmission. Our analysis, based on realistic assumptions about the transmitter, shows that substantial laser power can be effectively transmitted over vast distances. Gravitational lensing plays a crucial role in this process, amplifying and broadening these signals, thereby increasing their brightness and making them more distinguishable from background noise. We have also demonstrated that modern space- and ground-based telescopes are well-equipped to detect lensed laser signals from nearby stars. Although individual telescopes cannot yet resolve the Einstein rings formed around many of these stars, a coordinated network can effectively monitor the evolving morphology of these rings as it traces the beam’s path through the solar system. This network, equipped with advanced photometric and spectroscopic capabilities, would enable not only the detection but also continuous monitoring and detailed analysis of these signals.
We’re imagining, then, an extraterrestrial civilization placing a transmitter in the region of its own star’s gravitational lens, on the side of its star opposite to the direction of our Solar System. The physics involved – and the mathematics here is quite complex, as you can imagine – determine what happens when light from an optical transmitter is sent to the star so that when it encounters the warped spacetime induced by the star’s mass, the diffracted rays converge and create what scientists call a ‘caustic,’ a pattern created by the bending of the light rays and their resulting focused patterns.
In the case of a targeted signal, the lensing effect emerges in a so-called ‘Einstein ring’ around the distant star as seen from Earth. The signal is brightened by its passage through warped spacetime, and if targeted with exquisite precision, could be detected and untangled by Earth’s technologies. Turyshev asks in this paper how the generated signal appears over interstellar distances.
The answer should help us understand how to search for transmissions that use gravitational lensing, developing the best strategies for detection. We’ve pondered possible interstellar networks of communication in these pages, using the lensing properties of participating stellar systems. Such signals would be far more powerful than the faint and transient signals detectable through conventional optical SETI.
Laser transmissions are inherently directional, unlike radio waves, the beams being narrow and tightly focused. An interstellar laser signal would have to be aimed precisely towards us, an alignment that in and of itself does not resolve all the issues involved. We can take into account the brightness of the transmitting location, working out the parameters for each nearby star and factoring in optical background noise, but we would have no knowledge of the power, aperture and pointing characteristics of a transmitted signal in advance. But if we’re searching for a signal boosted by gravitational lensing, we have a much brighter beam that will have been enhanced for best reception.
Image: Communications across interstellar distances could take advantage of a star’s ability to focus and magnify communication signals through gravitational lensing. A signal from—or passing through—a relay probe would bend due to gravity as it passes by the star. The warped space around the object acts somewhat like a lens of a telescope, focusing and magnifying the light. Pictured here is a message from our Sun to another stellar system. Possible signals from other stars using these methods could become SETI targets. Image credit: Dani Zemba / Penn State. CC BY-NC-ND 4.0 DEED.
Mathematics at this level is something I admire and find beautiful much in the same way I appreciate Bach, or a stunning Charlie Parker solo. I have nowhere near the skill to untangle it, but take it in almost as a form of art. So I send those more mathematically literate than I am to the paper, while relying on Turyshev’s explanation of the import of these calculations, which seek to determine the shape and dimensions of the lensed caustic, using the results to demonstrate the beam propagation affected by the lens geometry, and the changes to the density of the EM field received.
It’s interesting to speculate on the requirements of any effort to reach another star with a lensed signal. Not only does the civilization in question have to be able to operate within the focal region of its stellar lens, but it has to provide propulsion for its transmitter, given the relative motion between the lens and the target star (our own). In other words, it would need advanced propulsion just to point toward a target, and obviously navigational strategies of the highest order within the transmitter itself. As you can imagine, the same issues emerge within the context of exoplanet imaging. From the paper:
…we find that in optical communications utilizing gravitational lenses, precise aiming of the signal transmissions is also crucial. There could be multiple strategies for initiating transmission. For instance, in one scenario, the transmission could be so precisely directed that Earth passes through the targeted spot. Consequently, it’s reasonable to assume that the transmitter would have the capability to track Earth’s movement. Given this precision, one might question whether a deliberately wider beam, capable of encompassing the entire Earth, would be employed instead. This is just [a] few of many scenarios that merit thorough exploration.
Detecting a lensed signal would demand a telescope network optimized to search for transients involving nearby stars. Such a network would be capable of a broad spectrum of measurements which could be analyzed to monitor the event and study its properties as it develops. Current and near-future instruments from the James Webb Space Telescope and Nancy Grace Roman Space Telescope to the Vera C. Rubin Observatory’s LSST, the Thirty Meter Telescope and the Extremely Large Telescope could be complemented by a constellation of small instruments.
Because the lens parameters are known for each target star, a search can be constructed using a combination of possible transmitter parameters. A search space emerges that relies on current technology for each specific laser wavelength. According to Turyshev’s calculations, a signal targeting a specific spot 1 AU from the Sun would be detectable with such a network with the current generation of optical instruments. Again from the paper:
Once the signal is detected, the spatial distribution of receivers is invaluable, as each will capture a distinct dataset by traveling through the signal along a different path… Correlating the photometric and spectral data from each path enables the reconstruction of the beam’s full profile as it [is] projected onto the solar system. Integrating this information with spectral data from multiple channels reveals the transmitter’s specific features encoded in the beam, such as its power, shape, design, and propulsion capabilities. Additionally, if the optical signal contains encoded information, transmitted via a set of particular patterns, this information will become accessible as well.
While microlensing events created by a signal transmitted through another star’s gravitational lens would be inherently transient, they would also be strikingly bright and should, according to these calculations, be detectable with the current generation of instruments making photometric and spectroscopic observations. Using what Turyshev calls “a spatially dispersed network of collaborative astronomical facilities,” it may be possible not only to detect such a signal but to learn if message data are within. The structure of the point spread function (PSF) of the transmitting lens could be determined through coordinated ground- and space-based telescope observations.
We are within decades of being able to travel to the focal region of the Sun’s gravitational lens to conduct high-resolution imaging of exoplanets around nearby stars, assuming we commit the needed resources to the effort. Turyshev advocates a SETI survey along the lines described to find out whether gravitationally lensed signals exist around these stars, pointing out that such a discovery would open up the possibility of studying an exoplanet’s surface as well as initiating a dialogue. “[W]e have demonstrated the feasibility of establishing interstellar power transmission links via gravitational lensing, while also confirming our technological readiness to receive such signals. It’s time to develop and launch a search campaign.“
The paper is Turyshev, “Search for gravitationally lensed interstellar transmissions,” now available as a preprint. You might also be interested in another recent take on detecting technosignatures using gravitational lensing. It’s Tusay et al., “A Search for Radio Technosignatures at the Solar Gravitational Lens Targeting Alpha Centauri,” Astronomical Journal Vol. 164, No. 3 (31 August 2022), 116 (full text), which led to a Penn State press release from which the image I used above was taken.
