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Between the Silent Universe and the Fragile Earth Exo-Earths, Cosmic Silence, and Civilisational Responsibility

Submitted:

05 October 2026

Posted:

08 October 2026

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Abstract
Several million narrowly defined exo-Earth candidates may exist in the Milky Way’s 7–9 kpc annulus from the Galactic Centre, according to the conditional model estimate underlying this essay. The number is large, but its interpretation must remain narrow. Similarity in radius, stellar flux, and po-sition within a conservative habitable zone does not show whether such planets have suitable atmos-pheres, stable climates, life, or technological civilisations. This raises a problem. If potentially suitable worlds are numerous, why does the universe still appear silent? Distance alone may not explain this. Civilisations could emerge at very different times, remain detectable only briefly, disappear before another civilisation arises, or never develop the capacity for sustained interstellar expansion. The absence of detected traces therefore leaves substantial uncertainty. We currently know of only one active biosphere and one technological civilisation, whose future is not guaranteed. The ability to control violence, technological risks, ecological degradation, and destruction of the biological systems on which civilisation depends may be as important to long-term survival as technological progress itself. Regardless of how many living worlds may exist elsewhere, preserving Earth is not only a matter of environmental policy, but a question of the long-term continuity of life and civilisation.
Keywords: 
exo-Earths
;  
astrobiology
;  
Fermi paradox
;  
extraterrestrial intelligence
;  
technosignatures
;  
interstellar civilisations
;  
biosphere
;  
civilisational responsibility
Subject: 
Physical Sciences  -   Astronomy and Astrophysics
Based on the modelling framework used (DOI: 10.5281/zenodo.22938807), the available astronomical data, and the selected statistical and Galactic models, the result can be interpreted as a conditional model estimate of the expected number of hypothetical exo-Earth candidates around G dwarfs (5300–6000 K) at least 4.57 Gyr old in the 7–9 kpc Galactocentric annulus of the Milky Way [1], with some basic astronomical properties that broadly resemble Earth’s. Across the two completeness scenarios of the reference Model 1, median projections range from approximately 3.23 to 4.58 million such candidates [2]. These central values inherit the uncertainty of the underlying occurrence rates of [3], whose 68% credible intervals already span a factor of about five to six.
Their radii and the received stellar flux are within 10% of Earth’s values, and they lie within the selected conservative habitable zones of their stars. This means only that their basic radiative configurations are not clearly incompatible with conditions that might permit liquid water and perhaps some forms of life, provided the planets have suitable compositions, atmospheres, and other physical characteristics [4]. This definition does not, by itself, in any way imply that these planets are actually habitable, still less that they are inhabited. The model does not determine their atmospheres, masses, geology, surface conditions, magnetic fields, chemical composition, or biological state. The validity and interpretation of this estimate depend on the quality of the input data, the assumptions built into the analysis, the chosen statistical functional form, and the way results from the observed Kepler sample are extrapolated to the wider Galactic population.
The difference between an estimate of this kind and an actual census of worlds will remain important. However extensive and accurate the data gathered by our successors may become, equations like these will probably continue to serve mainly as research tools for estimating the expected number of worlds. They will allow us to connect different datasets, test assumptions, and indicate the range within which the actual number most likely lies. How close our successors might come to a complete census remains an uncertain and open question. Even the distinction between detecting a planet and understanding its conditions in detail shows how different the levels of knowledge implied by such a census can be. With technology far beyond our own, it would still be extraordinarily difficult to survey every star in the Galaxy and obtain complete information about its planets, their atmospheres, geological histories, and possible biospheres. Doing the same for stars in other galaxies would be far more difficult.
A simple thought experiment illustrates the scale of the problem. If the Milky Way contains at least 100 billion stars, our successors would have to examine, on average, roughly 2.74 million stars per day to inspect each one physically within a century. Even with an unlimited number of probes travelling simultaneously in every direction at speeds arbitrarily close to the speed of light, the most idealised upper limit of their reach during that century would approach 100 light-years from the point of departure. Given the local stellar density (0.0799 stars per cubic parsec), this would amount to roughly 9,600 reachable stars. If the survey results also had to return to the point of origin within the same century, the maximum theoretical radius would be about 50 light-years, corresponding to around 1,200 stars at approximately the same local density [5]. If we allowed the purely hypothetical possibility of faster-than-light transport, for which there is currently neither experimental confirmation nor any known physically feasible mechanism, the travel-time constraint would change, but much of the problem would remain. An enormous number of probes would still be needed, their routes would have to be coordinated, energy supplied, and enough observation time devoted to every planetary system. A complete empirical survey of the Galaxy would therefore be exceptionally difficult; extending it to the entire universe is even harder to imagine.
Such a thought experiment assumes, however, that one civilisation must complete the entire research task within a limited time span, whereas a more speculative possibility is that no civilisation would need to survey the Galaxy alone. If long-lived interstellar civilisations existed, catalogues or maps of worlds where life had been detected might be assembled and expanded over many generations. Even more speculatively, different technological civilisations might exchange these records. A map of a living Galaxy would then not necessarily be the product of one civilisation or the achievement of a single era. It could instead grow through observations accumulated over very long periods by terrestrial and numerous extraterrestrial interstellar intelligent species. A civilisation might consequently know about worlds that it could not observe or explore directly.
The possibility of these long-lived interstellar civilisations makes their apparent absence more puzzling. If even one technological civilisation could sustain interstellar expansion for a very long time, or if it used autonomous or self-replicating probes, its presence, artefacts, or accumulated knowledge might spread through much of the Galaxy. Models of self-replicating interstellar probes suggest that a large part of the Galaxy, and in some scenarios all of it, could be explored on timescales far shorter than the Galaxy’s age [6]. Interstellar distances alone are not a sufficient explanation for cosmic silence. Still, from the absence of detected traces we cannot directly infer that such civilisations do not exist; any such conclusion would rest on further assumptions about their longevity, willingness to expand, use of self-replicating probes, the survival of their artefacts, and our own ability to recognise the evidence. The same hypothetical possibility that could produce a cumulative map of a living Galaxy also sharpens Fermi’s question: if long-lived and expansionist interstellar civilisations were common, why have we not found their traces?
This brings us back to the question of how common and long-lived the civilisations capable of creating such knowledge might be. If planets with conditions actually suitable for sustaining life over long periods are rare in the Galaxy, then there are also fewer environments in which life could arise and later develop into a technological civilisation. Astronomical habitability is only the beginning of a long, poorly understood chain of evolutionary events and historical contingencies. We do not know how likely abiogenesis is, how often complex organisms develop, how frequently intelligence appears, or how often a technological civilisation develops [7,8]. Nor do we know what proportion of such civilisations survives long enough to reach an interstellar stage. A further civilisational selection filter may also operate, linked to whether a civilisation can manage the risks created by its own technological power [9,10]. Limiting violence and internal conflict may be one factor in such a filter.
A civilisation’s longevity also does not mean that it remains detectable from afar at all times. If another technological civilisation were detected in the future through its unintentional radio emissions, the signal might come mainly from an early phase of technological development, when it uses transmitters powerful enough for radiation to leak into interstellar space; this is not, however, an absolute requirement. Earth itself shows that such an electromagnetic signature is not fixed. In roughly a century of technological development, we have moved from a relatively small number of powerful analogue radio and television transmitters to large numbers of weaker, broadband, and often more directional digital communication systems [11]. This does not mean that an older or more advanced civilisation would necessarily become radio-silent. Powerful radars, interplanetary communications, or purpose-built interstellar transmitters could be much easier to detect. It does mean, however, that a civilisation’s unintended radio detectability need not be permanent and may change greatly as its technology develops [12]. The question of detectability thus leads us to temporal as well as spatial distances. Two civilisations may arise in the same Galaxy yet be separated by a million years. One may disappear before the other even emerges. They may never meet, and they may never detect one another, especially if they leave no long-lived detectable traces, probes, colonies, or other artefacts. The apparent silence of the universe therefore does not by itself show that the universe is empty [13].
Amid this uncertainty, I personally lean towards the view that we are probably not alone in the universe. My intuition is that abiogenesis may be a common natural process, but only within a relatively narrow range of physical and chemical conditions that make it possible. Intelligence recognisably similar to ours, particularly in its capacity to understand the world and build a technological civilisation, may be much rarer. The sheer number of potentially suitable planets does not by itself tell us how often life emerges on them, or how often intelligence comparable to ours, or even far beyond it, develops. This expectation therefore remains a personal intuition.
Although we still have no answer to the question of whether other civilisations exist, we can already consider what their discovery might mean. Conclusive evidence of even one second biosphere that arose independently of Earth would show that life is not unique to our planet. The discovery of a single extraterrestrial technological civilisation would mark a far more profound turning point, demonstrating for the first time that intelligence capable of developing technology and exploring the universe has arisen independently on more than one occasion. To take the thought further: what would it actually mean for humanity to learn that the universe contains not just one other technological civilisation, but perhaps hundreds, thousands, or even more, including some that might be thousands or millions of years older and technologically far more advanced than ours? Such knowledge would represent one of the greatest epistemological, anthropological, and existential turning points in human history and would transform our understanding of our place in the universe at a far deeper level.
It is difficult to know how people would respond to such a realisation. We would have answered one of the oldest questions about our existence and satisfied part of our desire for knowledge. At the same time, we would have to rethink our sense of human exceptionalism and our place both in the universe and here on Earth. If technological intelligence had arisen independently on many worlds, humanity would for the first time unambiguously recognise itself as one of potentially many independent manifestations of consciousness and intelligence across immense space and time. The idea of humanity as a central or especially privileged phenomenon in the cosmos would become difficult to sustain. This would not, in itself, diminish the value of human life. After all, the value of the biosphere and of humanity is not determined solely by how common or rare they are in the universe. Our understanding of our own role also depends on our capacity to understand, create, preserve, and care for others, as well as on the way we connect our knowledge to moral and ethical responsibility, not only towards other people but also towards animals and the biosphere.
That responsibility does not begin only when life is discovered elsewhere. Until we have reliable evidence of another biosphere or extraterrestrial intelligence, we must distinguish between what seems probable and what we actually know. This distinction matters not only for our understanding of the universe but also for our relationship with our own planet. The expectation that life exists elsewhere cannot justify indifference to life here. Even if biospheres one day prove common across the universe, their existence would not make up for irreversible losses of life and biodiversity on Earth. Our prospects for the future, and perhaps also our prospects for cooperation with interstellar civilisations elsewhere, are shaped by how we treat our planet, one another, and animals. To some extent, people today are architects of future generations. The decisions we make now will partly determine whether we leave our successors a society that continues to follow an atavistic logic of power, domination, and violence, or one capable of moving beyond its conflicts and perhaps one day becoming an interstellar civilisation. The question of a civilisational selection filter is not only about hypothetical other civilisations; it is also a question we can ask about ourselves.
Apart from our ability to manage relations among people, our capacity for long-term survival also depends on preserving the biosphere that supports us. Life on Earth is the product of billions of years of successful evolutionary history. Technological civilisation is one of its most recent products, and its future remains deeply uncertain. Because its activities are closely bound up with the rest of life on the planet, the consequences of that uncertainty are not limited to humanity [14]. This interdependence can be illustrated by one of many scenarios in which a civilisation collapses abruptly after a prolonged period of gradual disintegration. With supply systems breaking down and food becoming scarce, a large proportion of the population might resort to intensive hunting of wild animals simply to survive. Such widespread and uncontrolled pressure on animal populations could, within a very short time, severely deplete populations of many large mammals, birds, and reptiles and drive some to local or even global extinction, while causing extensive disruption to food webs and ecosystem functioning [15,16]. If the losses were extensive enough, their ecological and evolutionary consequences could persist for a very long time, potentially even millions of years [17]. Combined with the other consequences of civilisational collapse, this could further accelerate the ongoing anthropogenic loss of animal and plant species [18].
The risk to the biosphere is not confined to civilisational collapse. Avoiding collapse is therefore not enough if the means used to preserve stability simultaneously destroy the conditions needed to sustain it in the long term. Humanity thus finds itself within an exceptionally narrow and precarious corridor of civilisational safety: it must remain stable enough to prevent the chaotic breakdown of its systems, yet transform and adapt thoroughly and rapidly enough to avoid sustaining that stability through the continued depletion of the biosphere [19,20]. At the same time, it must ensure that rapid changes intended to preserve long-term stability do not themselves become a source of renewed instability or even collapse. Put plainly, humanity has no guaranteed safe path. Transformation is difficult and uncertain, yet continuing to erode the natural foundations of civilisation is no safe alternative, as it heightens the risk of severe crises that, under extreme conditions, could lead to widespread and long-lasting degradation of the biosphere and profound civilisational collapse. In such an extreme scenario, breakdown or loss of functionality could affect even the largest state and political structures, including those that currently appear the most stable, not only smaller or already vulnerable states. Navigating this corridor also requires a degree of international cooperation, made more difficult by the differing interests, values, political systems, cultural assumptions, and short-term aims of individual societies and elites.
All this also shows that humanity as a whole has no clearly defined common direction or final goal. Its future emerges from a multitude of interconnected and often conflicting decisions, interests, and circumstances. Rather than following a predetermined civilisational path, we largely leave our future to an evolutionary dynamic in which ideas, technologies, social arrangements, and ways of life are continually tested, altered, retained, or lost according to how successfully they persist and spread. As our technological power grows, an important question arises: can this open process continue without a shared goal if some of its failures could endanger the conditions in which further evolutionary development remains possible?
The discussion of ourselves and other worlds thus returns to the world from which we began it. Perhaps, therefore, the most important result of equations like these will not be a definitive answer to the question of how many hypothetical exo-Earth candidates exist. It may instead be a clearer recognition of the limits of our knowledge and of the responsibility we bear as inhabitants of the only planet currently known with confidence to host an active biosphere and the products of its evolution.

Funding

This research received no external funding.

Data Availability Statement

The model, its inputs, and the calculations underlying the numerical estimate discussed in this essay are openly available on Zenodo at https://doi.org/10.5281/zenodo.22938807 [2].

Declaration of Generative AI and AI-Assisted Technologies

I used GPT and Claude to assist with English translation and proofreading. The arguments, interpretations, scientific claims, and conclusions are my own. I reviewed the final manuscript and take full responsibility for its content.

Conflicts of Interest

The author declares no conflicts of interest.

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