The Quiet Galaxy

The equation · piece 5 of 7 · fi

The share of planets with life on which it becomes intelligent

Given a planet with life, how likely it is that some of that life eventually becomes intelligent in the sense that matters here: able to build technology. On Earth it took about four billion years, most of it spent as single cells, and happened in one lineage out of billions of species, which the biologist Ernst Mayr took as proof that it is a fluke. Others read the steady rise in the complexity of life as a sign that intelligence is close to inevitable given time. There is no second example, and the only new evidence is statistical: reasoning from how late in Earth's habitable life we arrived.

The range this site uses 0.0001 to 1 reviewed
set on 16 October 2026

When the number on the home page is computed, a value for this piece is picked at random from this range each time, and every tenfold step between the two ends is equally likely.

Why that range

Starts from the 2018 paper's span of 0.001 to 1, which they took from the published estimates, and lowers the floor one power of ten to 0.0001 so that Pascal Lee's 0.0002 falls inside it. The top is 1, Drake's and Westby and Conselice's assumption and Conway Morris's direction of travel. Mayr's one in a billion is recorded but not used for the floor: it counts species, not planets, and no one has turned it into a chance per living world. Kipping's weak three-to-two for 'intelligence is rare' argues against bunching the weight at the top but does not say how far down to go. Evenly spread across its powers of ten, as in the 2018 paper.

Rests on: Sandberg and others, 2018.

The published estimates

6 estimates on the record, oldest first. Each says who made it, when, how, and where to find it. A range is set from these by hand, with the reason above, never computed from them: the reason is the arguable part.

WhenWhoEstimateHow it was reached
1961 Frank Drake and the Green Bank meetingGreen Bank meeting of November 1961, as recorded in Drake and Sobel, Is Anyone Out There? (Delacorte, 1992), pages 55 to 62 (drake-1961) 1 Assumed: the meeting took it that intelligence follows from life given enough time.
1995 Ernst MayrMayr, 'Can SETI Succeed? Not Likely', Bioastronomy News 7(3) (1995), and his reply to Carl Sagan, reprinted by The Planetary Society as 'Ernst Mayr on SETI' (text read in a blog reproduction on 2026-10-16; the Planetary Society page was not reached). Mayr gives a count, not a chance: the span here is our reading of it 1 in 50 billion to 1 in 1000 million Argued from the fossil record: of the billions of species that have lived on Earth, perhaps as many as 50 billion, exactly one developed the kind of intelligence that builds radio telescopes, so the chance per planet is read as between one in 50 billion and one in a billion. A count of species, not of planets, which is why it is recorded but not used for the range's floor.
2005 Simon Conway Morris, as reviewed by CampbellConway Morris, Life's Solution: Inevitable Humans in a Lonely Universe (Cambridge University Press, 2003), as reviewed by Campbell, 'Human-like intelligence is inevitable' (2005); and Bonner, The Evolution of Complexity by Means of Natural Selection (Princeton, 1988) 1 Argued that evolution keeps finding the same solutions (eyes, flight, social behaviour) so that something like human intelligence is close to inevitable wherever complex life has time; a direction rather than a value, recorded as 'approaching 1'.
June 2018 Anders Sandberg, Eric Drexler and Toby OrdSandberg, Drexler and Ord, 'Dissolving the Fermi Paradox', arXiv:1806.02404 (2018), Table 1 (sandberg-2018) 0.001 to 1 A span evenly spread across its powers of ten, taken from the published estimates of 0.001 to 1, with the paper's note that this treats the previous factor as the chance of 'evolutionarily competent' life rather than life of any kind.
June 2020 Tom Westby and Christopher ConseliceWestby and Conselice, 'The Astrobiological Copernican Weak and Strong Limits for Intelligent Life', Astrophysical Journal 896, 58 (2020) (westby-2020) 1 Assumed: in their approach intelligence follows on any planet with life after four and a half to five and a half billion years, as on Earth.
October 2020 Pascal LeeLee, 'N ~ 1: Alone in the Milky Way', talk to the Mount Tamalpais astronomy programme, 2020-10-24, and to the Kalamazoo Astronomical Society, 2021-03-06 0.0002 Took the one million years since the first humans against the four and a half billion years since Earth formed as the share of a planet's life during which intelligence is present, about 0.0002, and read it as the chance.

What would move this

The discovery that would narrow the range

A second example of complex or intelligent life anywhere, a change in the fossil record's timeline for the major steps (complex cells, animals, brains), or a published analysis that puts a new number on how often intelligence follows from life.

When a paper or a survey of that kind is published, it is logged in the story so far with a note saying whether it moved this range. A move is written as a dated revision below, with the old number, the new one and the reason.

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 second example of intelligent life and no new number for this piece. Three results bear on its test and were not in the log: Mills and others (Science Advances, 2025-02-14) argue the long wait for intelligence on Earth needs no improbable steps, only windows of habitability opening in turn; Bernabeu and others (Nature, 2026-06-10) find the complex cell arose through a long succession of microbial partnerships rather than one freak capture; and Rueda-Alaña and others (Science, 2025-02-13) find birds and mammals built similar sensory brain circuits by separate routes, read as complex cognition evolving at least twice. All three lean toward the optimistic end of a range that already reaches 1; none gives a number, so the range stays.

    Mills, 2025Bernabeu, 2026Rueda-Alaña, 2025 source 1source 2source 3

How the range has moved

DateWhatFromToWhy
16 October 2026 estimate (Carl Sagan, 1980) value 0.1, 'a round guess, one tenth' removed Checked against Cosmos (1980), chapter 12, on 2026-10-16: Sagan writes 'let us choose fi x fc = 1/100' and gives no value for the intelligence factor alone. The range is untouched.
16 October 2026 estimate (Ernst Mayr, 1995) value 1e-9 (the Wikipedia article's rendering) low 2e-11, high 1e-9 Checked on 2026-10-16 against a reproduction of Mayr's essay and reply: he counts billions of species, perhaps as many as 50 billion, one of them intelligent, and gives no chance of his own. The range is untouched (its floor is 0.0001); the published pessimistic extreme in the headline's estimate table moves with this.
17 October 2026 range.status first-pass reviewed The outside review of the seven ranges ran on 2026-10-17 and John ruled on every item the same day (docs/reviews/2026-10-17-rulings.md).

The explanations that lean on this piece

An explanation for the silence is, in the end, a claim about one or more of the seven pieces. 7 explanations say this piece is smaller than it looks.

The papers that bear on it

Books and films that take it up

Notes

Kipping's 2020 Bayesian analysis (kipping-2020) found a weak preference, three-to-two, for intelligence rarely emerging, from the fact that it took almost all of Earth's habitable lifetime to appear; the paper strongly excludes intelligence arising quickly (in much less than a billion years) on an Earth-like world, and gives no single value for this factor. Frank and Sullivan (2016, Astrobiology 16, 359) turned the question round: for Earth to be the only technological species ever in the observable universe, the combined chance of life, intelligence and technology per habitable planet would have to be below 2.5 in ten to the 24, and below 1.7 in ten to the 11 for the galaxy. Hanson and colleagues' 2021 'grabby aliens' model (hanson-2021-grabby) constrains the product of this factor and the next from the other end: if expanding civilisations exist and we are early among them, quiet civilisations that never expand can be at most a few thousand times as common as loud ones, else we would have seen a loud one by now. Checked 2026-10-16 against the sources: Carl Sagan's one tenth was removed (Cosmos, chapter 12, gives only the product of this factor and the next, one in a hundred); Mayr's point value of one in a billion was replaced by the span his own counts give, 2 in 100 billion to 1 in a billion (the range is unchanged, and its floor of 0.0001 stays well above it); Westby and Conselice's assumption of 1 is borne out (their section 2.2; the strong version only requires 4.5 to 5.5 billion years). Lineweaver (2009, 'Paleontological tests: human-like intelligence is not a convergent feature of evolution', in From Fossils to Astrobiology, arXiv:0711.1751) argues from the fossil record that human-like intelligence is not convergent, and gives no number, so it is not an estimate; the 'Planet of the Apes' label belongs to the opposite view, which he argues against. Written 2026-10-16. Outside review, 2026-10-17, ruled by John the same day: lowering the floor from 0.0001 to one in 10 billion or below, to include the hard-steps conclusions (item 12), was declined. The hard-steps papers (Carter 1983; Snyder-Beattie and colleagues 2021) conclude that intelligence is rare but give no chance per planet, so there is no published number to put the floor at; the only published figure below 0.0001 is Mayr's count of species, which is kept above and not used. Worth keeping visible because the floor is the one place the site's judgment, not a measurement, decides the headline: at one in 10 billion the chance we are alone in the galaxy would be 77% (scratch copy) against 59%, and if the published pessimist is to be covered the principled step is Mayr's top, one in a billion (75%), not one in 10 billion. The reviewer's suggestion to add Barrow and Tipler (1986) as an estimate (item 13) was declined: their figure counts one genome rather than giving a chance of intelligence per living planet, and is named here among the arguments without a usable number. A survey of living planets against a silent sky (item 14) bounds this factor only together with its neighbours and is not added to the test line.

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.

  • Barrow and Tipler (The Anthropic Cosmological Principle, Oxford University Press, 1986) are said, second-hand only, to put the chance of the human genome evolving at about one in ten to the 24 million; the book has not been read, and the figure, if it is theirs, counts one genome and is not a chance of intelligence per living planet. Raised 2026-10-17 by the outside review (item 13); open.
  • Pascal Lee's figure of 0.0002, from a 2020 talk, is quoted from the Wikipedia article on the Drake equation, which cites recordings of the talks; check it. Tried 2026-10-16 and still open: the recordings could not be reached. The only source found is a Centauri Dreams post (2023-10-18) reporting a talk at the Interstellar Research Group symposium in Montreal, which gives 0.0002 and derives it from the median duration of Homo erectus (about a million years) over the age of Earth, not from 'the first humans', and also gives his detectable factor as 0.1 and his lifetime as 10,000 years; so the number is supported second-hand but the talk, date and derivation in the record are not confirmed.

Think the range should move? Argue it, with a source. Spotted an error on this page? Say so. Or read how disagreement gets folded in. The record itself is data/factors/intelligence.toml in the public repository.