The Quiet Galaxy

Explanations · They are gone

Natural catastrophes: asteroid strikes, giant eruptions and nearby exploding stars end civilisations before they can last long enough to be noticed

Proposed by No single author; the modern treatment is due to Milan Ćirković and colleagues, 2004 L smaller

Earth's history is punctuated by disasters that killed most of the species alive at the time: the rock that ended the dinosaurs, the eruptions that caused the worst extinction of all, and perhaps the radiation from nearby dying stars. A civilisation is a far more fragile thing than a biosphere. If disasters of this kind strike every few tens of millions of years, and smaller ones far more often, then a civilisation needs not just to arise but to survive long enough between blows to be heard, and most may not. On this view the galaxy is quiet because the universe keeps knocking its inhabitants down.

The idea that the natural world could end a civilisation is as old as the stories of floods and fires, but it became a scientific position in 1980, when Luis and Walter Alvarez showed that the extinction which ended the age of dinosaurs coincided with the impact of a large asteroid or comet. The fossil record has since been read as a series of such shocks: five great mass extinctions and many smaller ones, caused variously by impacts, by vast volcanic eruptions that poisoned the air and sea for thousands of years, by sudden changes of climate, and perhaps by bursts of radiation from exploding stars nearby. Each cleared the field and let something new grow in the space, which is why mammals, and eventually humans, exist at all. The same process, applied to a species that has only just learned to build telescopes, is less kind.

The Serbian astronomer Milan Ćirković and his colleagues brought this to bear on the Fermi question in the 2000s. Their point was that habitability is not a fixed property of a planet but something that comes and goes, so that the right question is not whether Earth-like planets are rare in space but whether long, uninterrupted stretches of habitability are rare in time. A planet may be the right size, in the right orbit, with water and air, and still be hit hard enough every so often that nothing complicated lasts. The frequency of the blows sets a ceiling on how long a civilisation can expect to have. Ćirković argued that this ceiling, together with the galaxy's violent youth, explains why civilisations might all be young and few.

The bearing on the Drake Equation is on the last factor, the detectable lifetime, which natural disasters would cut short. The sister explanation on this site, the hostile galaxy, applies the same disasters to the earlier steps, where they prevent intelligence from appearing at all; this page is about what they do to a civilisation that has already appeared.

The main objection is that a civilisation at our level or beyond is less fragile than a species. Humans already track asteroids and could, with effort, deflect one. A civilisation spread across even a few worlds would survive any single disaster. And impacts large enough to end a civilisation are rare, roughly one every hundred million to a few hundred million years for the dinosaur-killer class, which is long compared with the time it takes to get off a planet. The reply is that smaller events, a large volcanic eruption or a modest impact, could collapse a civilisation without ending its species, that recovery is not guaranteed, and that most of the galaxy's history was more violent than the recent past. Few researchers think disasters alone explain the silence, but most grant that they shorten the expected lifetime in the equation.

What it says about the equation

Every explanation is a claim about one or more of the seven pieces that are multiplied to give the number of detectable civilisations. This one bears on 1 piece:

The test: what would strengthen or weaken it

Would strengthen it: A better record of how often civilisation-ending events strike a typical planet, showing them to be more frequent than Earth's recent history suggests; geological evidence linking more of Earth's extinctions to astronomical causes; signs of a civilisation around another star that ended suddenly.

Would weaken it: Evidence that most planetary systems are calmer than ours; a civilisation detected that has survived for millions of years; a demonstration that a civilisation at roughly our level can reliably protect itself against impacts and eruptions.

These two lines are what the record is read against when a new paper or discovery is logged. A result of the kind named here, published with its evidence, is what would settle it; a press claim on its own settles nothing.

The last reading

Once a week the records are read against the news, each against the test above. Open a line to see what that week held. Every reading ever made of this record is on the reading page.

  • 2026-10-16 Logged, no change By hand

    No new estimate of how often civilisation-ending events strike a typical planet. Quintana, Wright and Martínez García (2025-03-11) find the nearby supernova rate matches the rate of Earth's unexplained extinctions, and Finke and Razzaque (2024-07-22) raise the possible rate of extinction-level gamma-ray bursts; both logged. On the weakening side, the 2022 DART impact remains the one demonstration of deflection, with China's first test planned before 2030.

    Quintana, 2025Finke, 2024 source 1source 2

How seriously it is taken

Accepted as a real contributor to a shorter average lifetime, and part of most serious discussions of the question; rarely offered as a full explanation on its own, because disasters large enough to end a spacefaring civilisation are rare.

The papers that bear on it

Books, films and other work that take it up

Read next

Notes

Corrected 2026-10-16. (1) The title of Ćirković's paper was given as 'Earths: rare in time, not space?'; the British Interplanetary Society's catalogue lists it as 'Earths – Rare in Time, Not in Space', Journal of the British Interplanetary Society 57, 53-59 (2004), reference 2004.57.53 (https://bis-space.com/shop/?p=7666). (2) The text put the dinosaur-killer impact rate at about one per hundred million years. A NASA Jet Propulsion Laboratory briefing gives 'every 100 million years (or so)' (Chodas, https://kiss.caltech.edu/workshops/neo/presentations/ShortCourse_Chodas-Protecting_Earth.pdf) and a 2022 National Academies workshop gives 'every few hundred million years' (https://www.nationalacademies.org/read/26698/chapter/9), so the text now gives that range. Confirmed unchanged: Alvarez, Alvarez, Asaro and Michel, Science 208, 1095-1108 (1980-06-06). Not changed but flagged in verify: the paper's abstract describes a comparison of the rare-Earth idea with Annis's phase-transition idea, which is about timing across the whole galaxy, not about disasters ending civilisations.

Claims still to be checked

These were written from memory and have not yet been checked against a source. A later pass clears each one.

  • That Ćirković's 2004 paper argues what the body says it does: that natural disasters set a ceiling on how long a civilisation lasts, and that this explains why civilisations may be young and few. Checked 2026-10-16: the paper's abstract (British Interplanetary Society catalogue, https://bis-space.com/shop/?p=7666) says only that it compares Ward and Brownlee's rare-Earth idea with Annis's phase-transition scenario, examines how long the galaxy stays habitable and a four-dimensional galactic habitable zone, and slightly favours the phase-transition picture. The full text was not reached, so whether it makes the civilisation-lifetime argument is unconfirmed, and the attribution to Ćirković in proposed_by and the body may overstate.

Think this page gets the explanation wrong, or misses an objection? Say so, or read how disagreement gets folded in. The record itself is data/explanations/natural-catastrophes.toml in the public repository. This page's address will never change; if the explanation is ever retired, the address will say what replaced it.