Centauri Dreams
Imagining and Planning Interstellar Exploration
Pandora: Exoplanet Atmospheres via Smallsat
I’ve been digging into NASA’s Small Spacecraft Strategic Plan out of continuing interest in missions that take advantage of miniaturization to do things once consigned to large-scale craft. And I was intrigued to learn about the small spacecraft deployed on Apollo 15 and 16, two units developed by TRW in a series called Particles and Fields Subsatellites. Each weighed 35 kilograms and was powered by six solar panels and rechargeable batteries. The midget satellites were deployed from the Apollo Command and Service Module via a spring-loaded container giving the units a four foot-per-second velocity. Apollo 15’s operated for six months before an electronics failure ended the venture. The Apollo 16 subsatellite crashed on the lunar surface 34 days into its mission after completing 424 orbits.
Here I thought I knew Apollo history backwards and forwards and I had never run into anything about these craft. It turns out that smallsats – usually cited as spacecraft with weight up to 180 kilograms – have an evocative history in support of larger missions, and current planning includes support for missions with deep space applications. Consider Pandora, which is designed to complement operations of the James Webb Space Telescope, extending our knowledge of exoplanet atmospheres with a different observational strategy.
JWST puts transmission spectroscopy to work, analyzing light from the host star as a transiting planet moves across the disk. A planet’s spectral signature can thus be derived and compared to the spectrum taken when the planet is out of transit and only the star is visible. This is helpful indeed, but despite JWST’s obvious successes, detecting the atmosphere of planets as small as Earth is a challenge. The chief culprit is magnetic activity on the star itself, contaminating the spectral data. The Pandora mission, a partnership between NASA and Lawrence Livermore National Laboratory, mitigates the problem by collecting long-duration observations at simultaneous visible and infrared wavelengths.
Image: A transmission spectrum made from a single observation using Webb’s Near-Infrared Imager and Slitless Spectrograph (NIRISS) reveals atmospheric characteristics of the hot gas giant exoplanet WASP-96 b. A transmission spectrum is made by comparing starlight filtered through a planet’s atmosphere as it moves across the star, to the unfiltered starlight detected when the planet is beside the star. Each of the 141 data points (white circles) on this graph represents the amount of a specific wavelength of light that is blocked by the planet and absorbed by its atmosphere. In this observation, the wavelengths detected by NIRISS range from 0.6 microns (red) to 2.8 microns (in the near-infrared). The amount of starlight blocked ranges from about 13,600 parts per million (1.36 percent) to 14,700 parts per million (1.47 percent). Credit: European Space Agency.
Stellar contamination produces spectral noise that mimics features in a planetary atmosphere, or else obscures them, a problem that has long frustrated scientists. Collecting data at shorter wavelengths than JWST’s shortest wavelengths (0.6 microns) helps get around this problem. Pandora’s visible light channel will track the spot-covering fractions of surface stellar activity while its Near-Infrared channel will simultaneously measure the variation in spectral features as the star rotates. A more fine-grained correction for stellar contamination thus becomes possible, and as the new paper on this work explains, the ultimate objective then becomes “…to robustly identify exoplanets with hydrogen- or water-dominated atmospheres, and determine which planets are likely covered by clouds and hazes.”
Pandora will operate concurrently with JWST, complementing JWST’s deep-dive, high-precision spectroscopy measurements with broad wavelength, long-baseline observations. Pandora’s science objectives are well-suited for a SmallSat platform and illustrate how small missions can be used to truly maximize the science from larger flagship missions.
The plan is for the mission to select 20 primary exoplanet host stars and collect data from a minimum of 10 transits per host star, with each observation lasting about 24 hours, producing 200 days of science observations. The lengthy data acquisition time for each star means an abundance of out-of-transit data can be collected to address the problem of stellar contamination. The primary mission has a lifetime of one year, which allows for a significant range of science operations in addition to the above.
Long-duration measurements like those planned for Pandora contrast with data collection on large missions like JWST, which often focus on one or a small number of transits per target. Such complementarity is a worthy goal, and a reminder of the lower cost and high adaptability of using the smallsat platform in conjunction with a primary mission. In addition, smallsats rely on standardized and commercial parts to reduce risk and avoid solutions specific to any single mission. Cost savings can be substantial.
Image: The Pandora observatory shown with the solar array deployed. Pandora is designed to be launched as a ride-share attached to an ESPA Grande ring [(Evolved Expendable Launch Vehicle) Secondary Payload Adapter ring]. Very little customization was carried out on the major hardware components of the mission such as the telescope and spacecraft bus. This enabled the mission to minimize non-recurring engineering costs. Credit: Barclay et al.
Operating at these scales has clear deep space applications. This is a fast growing, innovative part of spacecraft design that has implications for all kinds of missions, and I’m reminded of the interesting work ongoing at the Jet Propulsion Laboratory in terms of designing a mission to the Sun’s gravity lens. Smallsats and self-assembly enroute may prove to be a game-changer there.
For the technical details on Pandora, see the just released paper. The project completed its Critical Design Review in October of 2023 and is slated for launch into a Sun-synchronous orbit in the Fall of this year. Launch is another smallsat benefit, for many smallsats are being designed to fit into a secondary payload adapter ring on the launch vehicle, allowing them to be ‘rideshare’ missions that launch with other satellites.
The paper is Barclay et al., “The Pandora SmallSat: A Low-Cost, High Impact
Mission to Study Exoplanets and Their Host Stars,” accepted for the IEEE Aerospace Conference 2025. The preprint is here.
A Three-Dimensional Look at an Exoplanet Atmosphere
Some 900 light years away in the constellation Puppis, the planet WASP-121b is proving an interesting test case as we probe ever deeper into exoplanetary atmospheres. As has been the case with so many early atmosphere studies, WASP-121b, also known as Tylos, is a hot-Jupiter, with a year lasting about thirty Earth hours, in a vise-like tidal lock that leaves one side always facing the star, the other away. What we gain in two new studies of this world is an unprecedented map of the atmosphere’s structure.
At stake here is a 3D look into what goes on as differing air flows move from one side of the planet to the other. A jet stream moves material around its equator, but there is a separate flow at lower altitudes that pumps gas from the hottest regions to the dark side. “This kind of climate has never been seen before on any planet,” says Julia Victoria Seidel (European Southern Observatory), lead author of a paper that appears today in Nature. Seidel points out that we have nothing in the Solar System to rival the speed and violence of the jet stream as it crosses the hot side of Tylos.
The astronomers used the European Southern Observatory’s Very Large Telescope, combining all four units to parse out the movement of chemical elements like iron and titanium in the weather patterns produced by these layered winds. What’s particularly significant here is the fact that we are now able to delve into an exoplanet atmosphere at three levels, analyzing variations in altitude as well as across varying regions on the world, and finally the interactions that produce weather patterns, form clouds and induce precipitation. Such 3D models take us to the greatest level of complexity yet.
“The VLT enabled us to probe three different layers of the exoplanet’s atmosphere in one fell swoop,” says study co-author Leonardo A. dos Santos, an assistant astronomer at the Space Telescope Science Institute in Baltimore. Tracking the movements of iron, sodium and hydrogen, the researchers could follow the course of winds at different layers in the planet’s atmosphere. A second paper, published in Astronomy & Astrophysics, announced the discovery of titanium in the atmosphere.
Image: Structure and motion of the atmosphere of the exoplanet Tylos. Astronomers have peered through the atmosphere of a planet beyond the Solar System, mapping its 3D structure for the first time. By combining all four telescope units of the European Southern Observatory’s Very Large Telescope (ESO’s VLT), they found powerful winds carrying chemical elements like iron and titanium, creating intricate weather patterns across the planet’s atmosphere. The discovery opens the door for detailed studies of the chemical makeup and weather of other alien worlds. Credit: ESO.
Note what we have in the image above. The paper describes it this way:
…a unilateral flow from the hot starfacing side to the cooler space-facing side of the planet sits below an equatorial super-rotational jet stream. By resolving the vertical structure of atmospheric dynamics, we move beyond integrated global snapshots of the atmosphere, enabling more accurate identification of flow patterns and allowing for a more nuanced comparison to models.
And that’s the key here – refining existing models to pave the way for future work. Digging into the 3D structure of the atmosphere required the VLT’s ESPRESSO spectrograph, collecting four times the light of an individual instrument to reveal the planet as it transited its star, a F-class star with mass and radius close to that of the Sun. Planet Tylos is named after the ancient Greek name for Bahrain, as part of the NameExoWorlds project. The host star bears the name Dilmun after the ancient civilization emergent on a trade route in the region after the 3rd millennium BC.
The Seidel et al. paper notes that existing Global Circulation Models (3D) do not fully capture what is observed at WASP-121b, making scenarios like these valuable testbeds for advancing the state of the art. Extremely Large Telescopes now under development will be able to put these refined models to work as they broaden the study of exoplanet atmospheres in extreme conditions:
The discrepancy between GCMs and the provided observations highlight the impact of high signal-to-noise ratio data of extreme worlds such as ultra-hot Jupiters in benchmarking our current understanding of atmospheric dynamics. This study marks a shift in our observational understanding of planetary atmospheres beyond our solar system. By probing the atmospheric winds in unprecedented precision, we unveil the 3D structure of atmospheric flows, most importantly the vertical transitions between layers from the deep sub-to-anti-stellar-point winds to a surprisingly pronounced equatorial jet stream. These benchmark observations made possible by ESPRESSO’s 4-UT mode serve as a catalyst for the advancement of global circulation models across wider vertical pressure ranges thus significantly advancing our knowledge on atmospheric dynamics.
The papers are Seidel et al., “Vertical structure of an exoplanet’s atmospheric jet stream,” Nature 18 February 2025 (abstract) and Prinoth et al., “Titanium chemistry of WASP-121 b with ESPRESSO in 4-UT mode,” in process at Astronomy & Astrophysics (preprint).
What Would Surprise You?
Someone asked me the other day what it would take to surprise me. In other words, given the deluge of data coming in from all kinds of observatories, what one bit of news would set me back on my heels? That took some reflection. Would it surprise me, my interlocutor persisted, if SETI fails to find another civilization in my lifetime?
The answer to that is no, because I approach SETI without expectations. My guess is that intelligence in the universe is rare, but it’s only a hunch. How could it be anything else? So no, continuing silence via SETI does not surprise me. And while a confirmed signal would be fascinating news, I can’t say it would truly surprise me either. I can work out scenarios where civilizations much older than ours do become known.
Some surprises, of course, are bigger than others. Volcanoes on Io were a surprise back in the Voyager days, and geysers on Enceladus were not exactly expected, but I’m talking here about an all but metaphysical surprise. And I think I found one as I pondered this over the last few days. What would genuinely shock me – absolutely knock the pins out from under me – would be if we learn through future observation and even probes that Proxima Centauri b is devoid of life.
I’m using Proxima b as a proxy for the entire question of life on other worlds. We have no idea how common abiogenesis is. Can life actually emerge out of all the ingredients so liberally provided by the universe? We’re here, so evidently so, but are we rare? I would be stunned if Proxima b and similar planets in the habitable zone around nearby red dwarfs showed no sign of life whatsoever. And of course I don’t limit this to M-class stars.
Forget intelligence – that’s an entirely different question. I realize that my core assumption, without evidence, is that abiogenesis happens just about everywhere. And I think that most of us share this assumption.
The universe is going to seem like a pretty barren place if we discover that it’s wildly unlikely for life to emerge in any form. I’ve mentioned before my hunch that when it comes to intelligent civilizations, the number of these in the galaxy is somewhere between 1 and 10. At any given time, that is. Who knows what the past has held, or what the future will bring? But if we find that life itself doesn’t take hold to run the experiment, it’s going to color this writer’s entire philosophy and darken his mood.
We want life to thrive. Notice, for example, how we keep reading about potentially habitable planets, our fixation with the habitable zone being natural because we live in one and would like to find places like ours. Out of Oxford comes a news release with the headline “Researchers confirm the existence of an exoplanet in the habitable zone.” That’s the tame version of more lively stories that grow out of such research with titles like “Humans could live here” and “A Home for ET.” I’m making those up, but you know the kind of headlines I mean, and they can get more aggressive still. We hunger for life.
Here’s one from The Times: “‘Super-Earth’ discovered — and it’s a prime candidate for alien life.’” But is it?
Image: Artist’s depiction of an exoplanet like HD 20794 d in a conceivably habitable orbit. It may or may not be rocky. It may or may not be barren. How much do our expectations drive our thinking about it? Credit: University of Oxford.
That Oxford result is revealing, so let’s pause on it. HD 20794 d is about 20 light years from us, orbiting a G-class star like the Sun, which gives it that extra cachet of being near a familiar host. Three confirmed planets and a dust disk orbit this star in Eridanus, the most interesting being the super-Earth in question, which appears to be about twice Earth’s radius and 5.8 times its mass. The HARPS (High Accuracy Radial Velocity Planet Searcher) and ESPRESSO spectrographs at La Silla (Chile) have confirmed the planet, quite a catch given that the original signal detected in radial velocity studies was at the limit of the HARPS spectrograph’s capabilities.
Habitable? Maybe, but we can’t push this too far. The paper notes that “HD 20794 d could also be a mini-Neptune with a non-negligible H/He atmosphere.” And keep an eye on that elliptical orbit, which means climate on such a world would be, shall we say, interesting as it moves among the inner and outer edges of the habitable zone during its 647-day year. I think Oxford co-author Michael Cretignier is optimistic when he refers to this planet as an ‘Earth analogue,’ given that orbit as well as the size and mass of the world, but I get his point that its proximity to Sol makes this an interesting place to concentrate future resources. Again, my instincts tell me that some kind of life ought to show up if this is a rocky world, even if it’s nothing more than simple vegetation.
Because it’s so close, HD 20794 d is going to get attention from upcoming Extremely Large Telescopes and missions like the Habitable Worlds Observatory. The level of stellar activity is low, which is what made it possible to tease this extremely challenging planetary signal out of the noise – remember the nature of the orbit, and the interactions with two other planets in this system. Probing its atmosphere for biosignatures will definitely be on the agenda for future missions.
Obviously we don’t know enough about HD 20794 d to talk meaningfully about it in terms of life, but my point is about expectation and hope. I think we’re heavily biased to expect life, to the point where we’re describing habitable zone possibilities in places where they’re still murky and poorly defined. That tells me that the biggest surprises for most of us will be if we find no life of any kind no matter which direction we look. That’s an outcome I definitely do not expect, but we can’t rule it out. At least not yet.
The paper is Nari et al., “Revisiting the multi-planetary system of the nearby star HD 20794 Confirmation of a low-mass planet in the habitable zone of a nearby G-dwarf,” Astronomy & Astrophysics Vol. 693 (28 January 2025), A297 (full text).
Pondering Life in an Alien Ocean
No one ever said Europa Clipper would be able to detect life beneath the ice, but as we look at the first imagery from the spacecraft’s star-tracking cameras, it’s helpful to keep the scope of the mission in mind. We’re after some critical information here, such as the thickness of the ice shell, the interactions between shell and underlying ocean, the composition of that ocean. All of these should give us a better idea of whether this tiny world really can be a home for life.
Image: This mosaic of a star field was made from three images captured Dec. 4, 2024, by star tracker cameras aboard NASA’s Europa Clipper spacecraft. The pair of star trackers (formally known as the stellar reference units) captured and transmitted Europa Clipper’s first imagery of space. The picture, composed of three shots, shows tiny pinpricks of light from stars 150 to 300 light-years away. The starfield represents only about 0.1% of the full sky around the spacecraft, but by mapping the stars in just that small slice of sky, the orbiter is able to determine where it is pointed and orient itself correctly. The starfield includes the four brightest stars – Gienah, Algorab, Kraz, and Alchiba – of the constellation Corvus, which is Latin for “crow,” a bird in Greek mythology that was associated with Apollo. Besides being interesting to stargazers, the photos signal the successful checkout of the star trackers. The spacecraft checkout phase has been going on since Europa Clipper launched on a SpaceX Falcon Heavy rocket on Oct. 14, 2024. Credit: NASA/JPL-Caltech.
Seen in one light, this field of stars is utterly unexceptional. Fold in the understanding that the data are being sent from a spacecraft enroute to Jupiter, and it takes on its own aura. Naturally the images that we’ll be getting at the turn of the decade will far outdo these, but as with New Horizons, early glimpses along the route are a way of taking the mission’s pulse. It’s a long hike out to our biggest gas giant.
I bring this up, though, in relation to new work on Enceladus, that other extremely interesting ice world. You would think Enceladus would pose a much easier problem when it comes to examining an internal ocean. After all, the tiny moon regularly spews material from its ocean out through those helpful cracks around its south pole, the kind of activity that an orbiter or a flyby spacecraft can readily sample, as did Cassini.
Contrast that with Europa, which appears to throw the occasional plume as well, though to my knowledge, these plumes are rare, with evidence for them emerging in Hubble data no later than 2016. It’s possible that Europa Clipper will find more, or that reanalysis of Galileo data may point to older activity. But there’s no question that in terms of easy access to ocean material, Enceladus offers the fastest track.
Enceladus flybys by the Cassini orbiter revealed ice particles, salts, molecular hydrogen and organic compounds. But according to a new paper from Flynn Ames (University of Reading) and colleagues, such snared material isn’t likely to reveal life no matter how many times we sample it. Writing in Communications Earth and Environment, the authors make the case that the ocean inside Enceladus is layered in such a way that microbes or other organic materials would likely break down as they rose to the surface.
In other words, Enceladus might have a robust ecosystem on the seafloor and yet produce jets of material which cannot possibly yield an answer. Says Ames:
“Imagine trying to detect life at the depths of Earth’s oceans by only sampling water from the surface. That’s the challenge we face with Enceladus, except we’re also dealing with an ocean whose physics we do not fully understand. We’ve found that Enceladus’ ocean should behave like oil and water in a jar, with layers that resist vertical mixing. These natural barriers could trap particles and chemical traces of life in the depths below for hundreds to hundreds of thousands of years.”
The study relies on theoretical models that are run through global ocean numerical simulations, plugging in a timescale for transporting material to the surface across a range of salinity and mixing (mostly by tidal effects). Remarkably, there is no choice of variables that offers an ocean that is not stratified from top to bottom. In this environment, given the transport mechanisms at work, hydrothermal materials would take centuries to reach the plumes, with obvious consequences for their survival.
From the paper:
Stable stratification inhibits convection—an efficient mechanism for vertical transport of particulates and dissolved substances. In Earth’s predominantly stably stratified ocean this permits the marine snow phenomena, where organic matter, unable to maintain neutral buoyancy, undergoes ’detrainment’, settling down to the ocean bottom. Meanwhile, the slow ascent of hydrothermally derived, dissolved substances provides time for scavenging processes and usage by life, resulting in surface concentrations far lower than those present nearer source regions at depth.
Although its focus is on Enceladus, the paper offers clear implications for what may be going on at Europa. Have a look at the image below (drawn not from the body of the paper but from the supplementary materials linked after the footnotes) and you’ll see the problem. We’re looking at these findings as applied to what we know of Europa.
Image: From part of Figure S7 in the supplementary materials. Caption: “Tracer age (years) at Europa’s ocean-ice interface, computed using the theoretical model outlined in the main text. Note that age contours are logarithmic.” Credit: Ames et al.
The figure shows the depth of the inversion and age of the ice shell for the same ranges in ocean salinity as inserted for Enceladus. Here we have to be careful about how much we don’t know. The ice thickness, for instance, is assumed as 10 kilometers in these calculations. Given all the factors involved, the transport timescale through the stratified layers of the modeled Europa is, as the figure shows, over 10,000 years. The same stratification layers impede delivery of oxidants from the surface to the ocean.
So there we are. The Ames paper stands as a challenge to the idea that we will be able to find evidence of life in the waters just below the ice, and likewise indicates that even if we do begin to trace more plumes from Europa’s ocean, these would be unlikely to contain any conclusive evidence about biology. Just what we needed – the erasure of evidence due to the length of the journey from the ocean depths to the ice sheet. Icy moons, it seems, are going to remain mysterious even longer than we thought.
The paper is Ames et al., “Ocean stratification impedes particulate transport to the plumes of Enceladus,” Communications Earth & Environment 6 (6 February 2025), 63 (full text).
Putting AI to Work on Technosignatures
As a quick follow-up to yesterday’s article on quantifying technosignature data, I want to mention the SETI Institute’s invitation for applicants to the Davie Postdoctoral Fellowship in Artificial Intelligence for Astronomy. The Institute’s Vishal Gajjar and his collaborators both in the US and at IIT Tirupati in India will be working with the chosen candidate to focus on neural networks optimized for processing image data, so-called ‘CNN architectures’ that can uncover unusual signals in massive datasets.
“Machine learning is transforming the way we search for exoplanets, allowing us to uncover hidden patterns in vast datasets,” says Gajjar. “This fellowship will accelerate the development of advanced AI tools to detect not just conventional planets, but also exotic and unconventional transit signatures including potential technosignatures.”
As AI matures, the exploration of datasets is a critical matter as these results from missions like TESS and Kepler are packed with both exoplanet data as well as stellar activity and systematics that can mislead investigators. Frameworks for sifting out anomalies should help us distinguish unusual candidates including disintegrating objects, planets with rings, exocomets and perhaps even megastructures and other technosignatures, all flagged by their deviation from our widely used transit models.
The data continue to accumulate even as our AI tools sharpen to look for anomalies. I can think of several Centauri Dreams readers who should find this work right up their alley. If you’re interested, you can find everything you need to apply for the fellowship here. The deadline for applications is March 15, 2025.
Quantifying the Chances of a Technosignature
It’s one thing to talk about technology as we humans know it, but applying it to hypothetical extraterrestrials is another matter. We have to paint with a broad brush here. Thus Jason Wright’s explanation of technosignatures as conceived by SETI scientists. The Penn State astronomer and astrophysicist defines technology in that context as “the physical manifestations of deliberate engineering.” That’s saying that a technology produces something that is in principle observable. Whether or not our current detection methods are adequate to the task is another matter.
Image: Artist’s concept of an interesting radio signal from galactic center. But the spectrum of possible technosignature detections is broad indeed, extending far beyond radio. Credit: UCLA SETI Group/Yuri Beletsky, Carnegie Las Campanas Observatory.
A technosignature need not be the sign of industrial or scientific activity. Consider: In a new paper in The Astronomical Journal, Sofia Sheikh (SETI Institute) and colleagues including such SETI notables as Wright himself, Ravi Kopparapu and Adam Frank point out that the extinction of ancient megafauna some 12,800 years ago may have contributed to changes in atmospheric methane that fed into a period of cooling known as the Younger Dryas, to be followed by growing human agricultural activity whose effects on carbon dioxide and methane in the atmosphere would be detectable.
As a technosignature, that one has a certain fascination, but it’s not likely to be definitive in ferreting out extraterrestrials, at least not at our stage of detection technology. But Sheikh and team are really after a much less ambiguous question. We know what our own transmission and detection methods are. How far away can our own technosignature be detected? By studying the range of technosignatures we are producing on Earth, the authors produce a scale covering thirteen orders of magnitude in detectability, with radio still at the top of the heap. The work establishes quantitative standards for detectability based on Earth’s current capabilities.
We might, for example, use the James Webb Space Telescope and the upcoming Habitable Worlds Observatory to provide data on atmospheric technosignatures as far out as 5.7 light years away. That takes us interstellar, with that interesting system at Proxima Centauri in range. Let’s tarry a bit longer on this one. While carbon dioxide is implicated in manmade changes to Earth’s atmosphere, the paper points to other sources, zeroing in on one in particular:
…there are other atmospheric technosignatures in Earth’s atmosphere that have very few or even no known nontechnological sources. For example, chlorofluorocarbons (CFCs), a subcategory of halocarbons, are directly produced by human technology (with only very small natural sources), e.g., refrigerants and cleaning agents, and their presence in Earth’s atmosphere constitutes a nearly unambiguous technosignature (J. Haqq-Misra et al. 2022). Nitrogen dioxide (NO2), like CO2, has abiotic, biogenic, and technological sources, but combustion in vehicles and fossil-fueled power plants is a significant contributor to the NO2 in Earth’s atmosphere (R. Kopparapu et al. 2021).
And indeed nitrogen dioxide (NO2) is what the authors plug into this study, drawing on earlier work by some of the paper’s authors. Note the fact that biosignatures and technosignatures overlap here given how much work has proceeded on characterizing exoplanet atmospheres. It turns out that the wavelength bands that the Habitable Worlds Observatory will see best in its search for biosignatures are also those that include the NO2 technosignature, a useful example of piggybacking our searches.
But of course the realm of technosignatures is wide, including everything from the lights of cities to ‘heat islands’ (inferring cities), orbiting satellites, radio transmissions and lasers. I’m aware of no other study that combines the various forms of technosignature in a single analysis. If you start looking at the full range of technosignatures according to distance, you find objects on a planetary surface to be the toughest catch, with heat islands swimming into focus only from a distance as far as outer planetary orbits in the Solar System. The current technology with the most punch is planetary radar, whose pulses should be detectable as much as 12,000 light years away, although such a signal would be a fleeting and non-repeating curiosity.
SETI does find signals like that now and then. But precisely because they are non-repeating, we simply don’t know what to make of them.
Image: The maximum distances that each of Earth’s modern-day technosignatures could be detected at using modern-day receiving technology, in visual form. Also marked are various astronomical objects of interest. Credit: Sheikh et al.
Think back to the early days of SETI and ponder how far we’ve come in trying to understand what other civilizations might do that could get us to notice them. SETI grew directly out of the famous work by Giuseppe Cocconi and Philip Morrison that laid out the case for artificial radio signals in 1959, followed shortly thereafter by Frank Drake’s pioneering work at Green Bank with Project Ozma. Less known is the 1961 paper by Charles Townes and R. N. Schwartz that got us into optical wavelengths.
And while ‘Dysonian SETI’, which explicitly searches for technosignatures, is usually associated with vast engineering projects like Dyson spheres, the point here is that a civilization will produce evidence for an outside observer that will continue to deepen as that observer’s tools increase in sophistication. The search for technosignatures, then, actually grows into a multi-wavelength effort, but one that spans a vast range. Making all this quantitative involves a ‘detectability distance scale.’ The authors choose one known as an ichnoscale. Here’s how the paper describes it:
Using Earth as a mirror in this way, we can employ the concept of the ichnoscale (ι) from H. Socas-Navarro et al. (2021): “the relative size scale of a given technosignature in units of the same technosignature produced by current Earth technology.” An ι value of 1 is defined by Earth’s current technology. This necessarily evolves over time—for this work, we set the ichnoscale to Earth-2024-level technology, including near-future technologies that are already in development.
Considering how fast our detection methods are improving as we build extremely large telescopes (ELTs) and push into ever more sophisticated space observatories, learning the nature of this scale will become increasingly relevant. While we realized in the mid-20th Century that radio was detectable at interstellar distances, we’re now able to detect not just an intentional signal but radio leakage, at least from nearby stellar systems. That’s an extension of the parameter space that involves levels of power we have already demonstrated on Earth. The ichnoscale framework quantifies these signatures that will gradually become possible to detect as our methods evolve.
We see more clearly which methods are most likely to succeed. This is an important scaling because the universe we actually live in may not resemble the one we construct in our imaginations. Let me quote the paper on this important point:
…the focus on planetary-scale technosignatures provides very specific suggestions for which searches to pursue in a Universe where large-scale energy expenditures and/or travel between systems is logistically infeasible. While science fiction is, for example, replete with mechanics for rapid interstellar travel, all current physics implies it would be slow and expensive. We should take that constraint seriously.
And with this in mind we can state key results:
1. Radio remains the way that Earth is most detectable at ι = 1.
2. Investment in atmospheric biosignature searches has opened up the door for atmospheric technosignature searches.
3. Humanity’s remotely detectable impacts on Earth and the solar system span 12 orders of magnitude.
4. Our modern-day planetary-scale impacts are modest compared to what is assumed in many technosignature papers.
5. We have a multiwavelength constellation of technosignatures, with more of the constellation becoming visible the closer the observer becomes.
Let’s pause on item 4. The point here is that most notions of technosignatures assume technologies visible on astronomical scales, and indeed it is usually assumed that our first SETI detections, when and if they come, will involve civilizations vastly older and superior in technology than ourselves. Planets bearing technologies like those we have today are a supremely difficult catch, because the technosignatures we are throwing are tiny and all but trivial compared to the Dyson spheres and starship beaming networks we typically consider. And this point seems overwhelmingly obvious:
We should be careful about extrapolating current technosignatures to scales of ιTS = 10 (or even ιTS = 2) without considering the changing context in which these technologies are being developed, used, and (sometimes) mitigated or phased out (e.g., the recovery of the ozone hole; J. Kuttippurath & P. J. Nair 2017). As another example, we are becoming aware of the negative health effects of the UHI [urban heat index] (as detailed in, e.g., A. Piracha & M. T. Chaudhary 2022); thus, work may be done to mitigate the concentrated regions of high infrared flux discussed in Section 4.3.
Indeed. How many of the technosignatures we are producing are stable? Chlorofluorocarbons in the atmosphere are subject to adjustment on astronomically trivial timeframes. The chances of running into a culture that is about to realize it is polluting itself just before it takes action to mitigate the problem seem remote. So all these factors have to be taken into account as we rank technosignature detection strategies, and it’s clear that in this “multiwavelength constellation of technosignatures” the closer we are, the better we see. All the more reason to continue to pursue not just better telescopes but better ways to get ever improving platforms into the outer Solar System and beyond. Interstellar probes, anyone?
The paper is Sheikh et al., “Earth Detecting Earth: At What Distance Could Earth’s Constellation of Technosignatures be Detected with Present-day Technology?” Astronomical Journal Vol. 169, No. 2 (3 February 2025), 118 (full text). The Cocconi and Morrison paper is “Searching for Interstellar Communications,” Nature 184 (19 September 1959), 844-846 (abstract). The 1961 paper on laser communications is Schwartz and Townes, “Interstellar and Interplanetary Communication by Optical Masers,” Nature 190 (15 April 1961), 205-208 (abstract).
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