I’m catching up with a lot of papers in my backlog, prompted by a rereading yesterday of David Kipping’s 2022 paper on the Wow! Signal, the intriguing, one-off reception at the Big Ear radio telescope in Ohio back in 1977 (Kipping citation below). I had just finished checking Abel Mendez’ work at Arecibo, where the Arecibo Wow! project has announced a new analysis based on study of previously unpublished observations using updated signal analysis techniques. No huge surprises here, but both Kipping’s work and Arecibo Wow! are evidence of our continuing fascination with what Kipping calls “the most compelling candidate for an alien radio transmission we have ever received.”
They also remind us that no matter how many times this intriguing event has been looked at, there are still new ways to approach it. I give the citation for the Mendez paper, written with a team of collaborators (one of whom is Kipping) below. Let me just pull this from Mendez’ statement on the Arecibo Wow! site, showing how the new work has refined the original Wow! Signal’s properties:
Location: Two adjacent sky fields, centered at right ascensions 19h 25m 02s ± 3s or 19h 27m 55s ± 3s, and declination –26° 57′ ± 20′ (J2000). This is both more precise and slightly shifted from earlier estimates.
Intensity: A peak flux density exceeding 250 Janskys, more than four times higher than the commonly cited value.
Frequency: 1420.726 MHz, placing it firmly in the hydrogen line but with a greater radial velocity than previously assumed.
Leading Mendez (University of Puerto Rico at Arecibo) to comment:
“Our results don’t solve the mystery of the Wow! Signal. “But they give us the clearest picture yet of what it was and where it came from. This new precision allows us to target future observations more effectively than ever before…. This study doesn’t close the case,” Méndez said. “It reopens it, but now with a much sharper map in hand.”
Image: Comparison of the previously estimated locations of the Wow! Signal (gray boxes) with the refined positions from the Arecibo Wow! Project (yellow boxes). The signal’s source is presumed to lie within one of these boxes and beyond the foreground Galactic hydrogen clouds shown in bright red. Credit: PHL @ UPR Arecibo.
Homing in on interesting anomalies is of course one way for SETI to proceed, although the host of later one-off detections from other locations (none evidently as powerful a signal as Wow!) doesn’t yield optimism that one of these will eventually repeat. A sweeping beam that by sheer chance swept across Earth from some kind of ETI installation? Works for me, if only we had repeating evidence. In the absence of it, we continue to dig into existing data using new techniques.
We can also proceed with targeted searches of nearby stars of interest both because of their proximity as well as the presence of unusual planetary configurations. The TRAPPIST-1 system isn’t remotely like ours, with its seven Earth-sized planets crammed into tight orbit around an M8V red dwarf star, but the fact that each of these transits makes the system of huge value. Now a team led by Guang-Yuan Song (Dezhou University, China) has used the FAST instrument (Five hundred meter Aperture Spherical Telescope) to search for SETI signals, delving into the frequency range 1.05–1.45GHz with a spectral resolution of ~7.5Hz. No signals detected, but the scientists plan to continue to search other nearby systems and do not rule out a return to this one. Citation below.
The Factors Leading to Technology
The frustration over lack of success at finding an extraterrestrial civilization is understandable, so it’s no surprise that theoretical work explaining it from an entirely opposite direction continues to appear. As witness a new study just presented at the EPSC-DPS Joint Meeting 2025 in Helsinki. Here we’re asking what factors go into making a technological society possible, assuming an evolutionary history something like our own, and probing whether changes to any of the parameters that helped us emerge would have made us impossible.
All this goes back to the 1960s and the original Drake Equation, which makes a loose attempt at sizing up the possibilities. Manuel Scherf and Helmut Lammer of the Space Research Institute at the Austrian Academy of Sciences in Graz paint a distressing picture for those intent on plucking a signal from ETI out of the ether. Their work has focused on plate tectonics and its relationship with the critical gas carbon dioxide, which governs the carbon-silicate cycle.
CO2 is a huge factor in sustaining photosynthesis, but too much of it creates greenhouse effects that can likewise spell the end of life on a planet like ours. The fine-tuning that goes on through the carbon-silicate cycle ensures that CO2 gets released back into the atmosphere through plate tectonics and volcanic emissions, recycling it back out of the rock in which it had been previously locked. Scherf pointed out to the EPSC-DPS gathering that if we wait somewhere between 200 million and one billion years, loss of atmospheric CO2 will bring an end to photosynthesis. The Sun may have another five billion years of life ahead, but the environment that sustains us won’t last nearly as long.
Indeed, the researchers argue, surface partial pressures and mixing ratios of CO2, O2, and N2 likewise affect such things as combustion, needed for the smelting of metals that underpins the growth of a technological civilization. Imagine a planet with 10 percent of its atmosphere taken up by CO2 (as opposed to the 0.042 percent now found on Earth). This world produces a biosphere that can sustain itself against a runaway greenhouse if further away from its star than we are from the Sun, but it would also require no less than 18 percent oxygen (Earth now has 21 percent) to ensure that combustion can occur.
If we do away with plate tectonics, so critical to the carbon-silicate cycle, we likewise limit habitable conditions at the surface. So we need this as well as enough oxygen to provide combustion to make technology possible. In other words, we have astrophysical, geophysical, and biochemical criteria that have to be met even when a planet is in the habitable zone if we are hoping to find lifeforms that have survived long enough to create technology. Rare Earth?
Scherf and Lammer weigh these factors against the amount of time it takes technology to emerge, assuming that the longer an ETI civilization exists, the more likely we are to observe it. Here I don’t have a paper to work with, so I can only report the conclusions presented at the EPSC-DPS conference, which are stated bluntly by Scherf:
“For 10 civilizations to exist at the same time as ours, the average lifetime must be above 10 million years. The numbers of ETIs are pretty low and depend strongly upon the lifetime of a civilization.”
I can also fall back on a 2024 paper from the same team discussing these matters, which delves not only into the question of the perhaps rare combination of circumstances which allows for technological civilizations to emerge but also our use of the Copernican Principle in framing the issues:
…our study is agnostic about life originating on hypothetical habitats other than EHs. Any more exotic habitats (e.g., subsurface ocean worlds) could significantly outnumber planets with Earthlike atmospheres, at least in principle. Finally, we argue that the Copernican Principle of Mediocrity cannot be valid in the sense of the Earth and consequently complex life being common in the Galaxy. Certain requirements must be met to allow for the existence of EHs and only a small fraction of planets indeed meet such criteria. It is therefore unscientific to deduce complex aerobic life to be common in the Universe, at least based on the Copernican Principle. Instead, we argue, at maximum, for a combined Anthropic-Copernican Principle stating that life as we know it may be common, as long as certain criteria are met to allow for its existence. Extremophiles, anaerobic and simple aerobic lifeforms, however, could be more common.
Image: An artist’s impression of our Milky Way Galaxy, showing the location of the Sun. Our Solar System is about 27,000 light years from the centre of the galaxy. The nearest technological species could be 33,000 light years away. Credit: NASA/JPL–Caltech/R. Hurt (SSC–Caltech).
All of which illuminates the paucity of data. We could say that it took four and a half billion years for technology to emerge on Earth, but that is our only reference point. We also have no data on how long technological societies exist. It is clear, though, that the longer the survival period, the more likely we are to be present in the cosmos at the same time they are. For there to be even one technological civilization in the galaxy coinciding with our existence, ETI would have to have survived in a technological phase for at least 280,000 years. I think that matches up with the case Brian Lacki has been making for some time now, which emphasizes the ‘windows’ of time within which we view the cosmos.
But Scherf adds this:
“Although ETIs might be rare there is only one way to really find out and that is by searching for it. If these searches find nothing, it makes our theory more likely, and if SETI does find something, then it will be one of the biggest scientific breakthroughs ever achieved as we would know that we are not alone in the Universe.”
Image: This graph shows the maximum number of ETIs presently existing in the Milky Way. The solid orange line describes the scenario of planets with nitrogen–oxygen atmospheres with 10 per cent carbon dioxide. In this case the average lifetime of a civilization must be at least 280,000 years for a second civilization to exist in the Milky Way. Changing the amount of atmospheric carbon dioxide produces different results. Credit: Manuel Scherf and Helmut Lammer.
I’ll note in passing that Adam Frank (Rochester Institute of Technology) and Amedeo Balbi (University of Rome Tor Vergata) have analyzed the question of an ‘oxygen bottleneck’ for the emergence of technology in a recent paper in Nature Astronomy. The memorable thought that if there are no other civilizations in the galaxy, it’s a tremendous waste of space sounds reasonable only if we have fully worked out how likely any planet is to be habitable. This new direction of astrobiological research tells me we have a long way to go.
The Mendez paper is Mendez et al., “Arecibo Wow! II: Revised Properties of the Wow! Signal from Archival Ohio SETI Data,” currently available as a preprint but submitted to The Astrophysical Journal. The Kipping paper from 2022 is Kipping & Gray, “Could the ‘Wow’ signal have originated from a stochastic repeating beacon?” Monthly Notices of the Royal Astronomical Society, Volume 515, Issue 1 (September 2022), pp.1122-1129 (full text). Thanks to my friend Antonio Tavani for the heads-up on the Mendez paper. The paper on the FAST search of TRAPPIST-1 is Guang-Yuan Song et al., “A Deep SETI Search for Technosignatures in the TRAPPIST-1 System with FAST,” submitted to The Astrophysical Journal and available as a preprint.
The Scherf and Lammer presentation is titled “How common are biological ETIs in the Galaxy?” EPSC-DPS Joint Meeting 2025, Helsinki, Finland, 7–12 Sep 2025, EPSC-DPS2025-1512. The abstract is available here. The same team’s 2024 paper on these matters is “Scherf et al. “Eta-Earth Revisited II: Deriving a Maximum Number of Earth-Like Habitats in the Galactic Disk,” Astrobiology 24 (2023), e916 (full text). The paper from Adam Frank is Frank & Balbi, “The oxygen bottleneck for technospheres,” Nature Astronomy 8 (2024), pp. 39–43 (abstract / preprint).
And I want to be sure to mention Robert Gray, Kipping’s co-author on his 2022 paper, who devoted years to the study of the Wow! Signal and was kind enough to write about his quest in these pages. If you’re not familiar with Gray’s work, I hope you’ll read my An Appreciation of SETI’s Robert Gray (1948-2021). I only wish I had gotten to know him better. His death was a loss to the entire community. David Kipping’s fine video covering his work with Gray is available at Cool Worlds. Keith Cooper also explicates this paper in One Man’s Quest to Investigate the Mysterious “Wow!” Signal.
(Hi, Paul. Hope you are well!)
The conclusion of Lammer, Scherf and Sproß seems reasonable. The required extreme longevity of ETI is often taken by optimists as a given. Sometimes under the assumption that advanced civilizations will know how to prevent their own demise. Sometimes under the naive(and oft repeated) thesis that a migration to the stars will somehow prevent or massively delay the demise of the ETI.
It’s easy to plug in generous values for each parameter and imagine a galaxy buzzing with ETIs. But as soon as you apply realistic constraints like the rarity of Earth like atmospheres in the sense the authors define, or the fragility of technological civilizations, the product of probabilities collapses fast.
The single biggest determinant of how many coexisting ETIs exist isn’t how many habitable planets there are, but how long civilizations last. If the average longevity is short (thousands of years), the galaxy could have hosted miriad civilizations that never overlap with us. If longevity is long (hundreds of thousands to millions of years), then overlap becomes plausible, but the extreme longevity requires a number of dubious explanations to achieve.
The SETI community often leans toward optimism ala “absence of evidence isn’t evidence of absence”. But that only works up to a point. Papers like Lammer’s are important because they remind us that the numbers are not actually our friends when the calculations are made with less wishful values.
Common counterarguments attempting to extend the ETI longevity window often invoke interstellar migration and or a transition to a post-biological existence.
But Fermi’s paradox cuts deeper especially if longevity and migration are common.
If advanced civilizations can last for hundreds of thousands to millions of years, or migrate, or build machine colonies, then we should expect a galaxy full of detectable signatures. Transmissions, engineering projects, artifacts. Thus far(grasp the straw!), nothing. Detection methods will improve, then you’ll see, we are told. Yet, the fact remains, so far, we do not. The fact that we see nothing makes one of two possibilities more likely. Those trajectories (post-biological, star-faring) are either much less likely than we imagine or civilizations that achieve them all deliberately avoid detectable activity.
And this loops back to the point of wishful thinking. While the public and parts of the research community often lean toward hopeful extrapolation, the empirical silence plus the hard numbers in this paper suggest less fanciful grounds for that silence. As sobering and unpalatable as it may seem, the Galaxy is probably* empty of co-existing technological civilizations right now.
The irony is sharp. If civilizations are fragile and short-lived, the numbers explain the silence. There is no “overlap”. If civilizations are long-lived and expansionist, the silence is inexplicable(or only explicable with very presumptuous assumptions about ETI mentalities).
The conclusion is both depressing and liberating. Depressing because the hope of “neighbors” fades. Liberating because it divests the mind of what amounts to a collection of wishful assumptions – ones that seem more aligned to science fiction tropes ala Star Trek. I would love to be proven wrong. But in the absence of evidence, I won’t invoke any pseudo-scientific analogs to a magic wand.
If the Scherf et al paper is correct, then habitable, Earth-like worlds are very rare. This suggests that searching for biosignatures will result in almost entirely null detection cases.
Even worse, SETI may never receive a radio/optical message. The last image in teh post suggests that for just 2 civilizations in the galaxy, us and one ETI, technological, communicating civilizations have to be extant for at least 10,000 years, almost as long as there have been civilizations on Earth, and that is the optimistic case.
If technological artifacts last considerably longer, much like the Egyptian pyramids lasted much longer than the dominant Egyptian civilization, then perhaps we should look for technosignatures rather than hoping for a message or transient energy beam.
We know that early civilizations (i.e., humans living in cities and no longer nomadic) existed about 10 millennia ago. Human cultures are perhaps 10x longer, based on rock carvings and paintings. This makes our Industrial Revolution technological civilization almost an eyeblink in comparison. Clearly, if we could detect intelligent species or civilizations that were pre-modern technological, then the window opens up from centuries to millennia. Our civilization, even at our current level of technology and population, will exhaust fossil carbon fuels fairly quickly, no more than a few centuries at best. Had we only maintained small, localized civilizations, we might even have only needed renewable forests as fuels, allowing these civilizations far longer to exist. How much development these small civilizations can manage is debatable, but clearly, progress was very slow and incremental until our industrial age.
If our civilization is going to have a long future, then we will have to successfully transition to space-based solar energy as our primary, long-term energy source if we continue to expand our economy and spatial distribution. This would seem to be the case for other similar civilizations, whether they send messages or not. Can we detect them? But note that the Scherf analysis suggests that our galaxy might only have one other extant civilization if such civilizations survive for 100,000,000 years! (When dinosaurs ruled the Earth.) For biosignatures, these may only exist for 1 in 10,000 or 100,000 worlds. If so, we will really need to be able to narrow down our search. Would any of the current techniques be able to manage this on a reasonably economic basis, or will we need to be able to do this by “casting a net” on patches of the sky to survey thousands of stars simultaneously, much like Kepler did for transiting planets, and Gaia for astrometry?
If communicating civilizations are extremely rare, it does suggest that the Wow! signal and other transients were probably not signals or power beams, and that either RFI or natural phenomena were the cause.
In my more cynical moments, I think of these transients as being interpreted at ETI as similar to seeing rare phenomena as signs of the supernatural, something that seems almost baked into humanity’s cognition. (And Avi Loeb is “Asking questions” about whether 3I/ATLAS also could be an alien spaceship. Is this really science?)
A counterargument to my suggestion that transients are unlikely to be signs of ETI based on the Scherf analysis of life and civilizations is to turn the Fermi Question on its head. If the transients are signs of ETI, then this implies that ETI is far more common than we suppose. Maybe there were very few civilizations, but at least one spread through the galaxy, populating the stars, extending the extant number of civilizations by uncoupling the L term from the Drake equation that assumes civilizations are bound to their home world or system. This assumes that interstellar travel by any means to seed star systems with “new life and new civilizations” is both possible and has been done.
This seems analogous to sci-fi stories where an STL ship arrives at its destination star, only to find an Earth civilization that arrived on a faster ship welcoming the travelers. We may be like those STL crews, assuming the galaxy is almost empty of ETI, only to find that ETI[s] have already spread throughout the galaxy before us. Hopefully, they are friendly, and we have passed the test to join their Federation.