The Quiet Galaxy

The equation · piece 3 of 7 · ne

Planets per star that could support life

For a star with planets, how many of them could support life: in practice, how many rocky planets of roughly Earth's size orbit where water could be liquid on the surface. The Kepler mission turned this from a guess into a measurement for Sun-like stars and for red dwarfs, with the answer coming out between a tenth and one such planet per star. What the surveys cannot count is whether such a planet also has the other things Earth has, such as a large moon, moving continents and a quiet neighbourhood, which is where the argument over this factor now lives.

The range this site uses 0.01 to 1 planets per star 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

The Kepler counts (Petigura 2013, Dressing and Charbonneau 2015, Hsu and others 2019, Bryson 2020) put rocky planets in the habitable zone at between a tenth and one per star for both Sun-like stars and red dwarfs, with the most careful study's error bars reaching one and a half. The top of the range is 1, as in the 2018 paper. The floor is a tenth of theirs, 0.01, to leave room for the requirements the surveys cannot count: a planet in the right orbit is not yet a planet that keeps liquid water for billions of years, and the TRAPPIST-1 atmosphere results of 2023 to 2025 show rocky planets around red dwarfs can lose their air. It stops at 0.01 and does not follow Ward and Brownlee down to one in a hundred thousand, and the reason is not that their figure rests on the extras: it does not. Their one percent to one thousandth of a percent (Rare Earth, 2000, pages 269 to 270) is the share of all stars with an Earth-like habitat, their own reading of the ordinary equation, written before any count of habitable-zone planets existed. The Kepler counts since have overtaken the part of it that counts orbits, and what survives of Rare Earth is the argument about extras (a large moon, plate tectonics, a quiet neighbourhood), which is an argument and not a count, and which the explanation page carries. Evenly spread across powers of ten, as in the 2018 paper. The reason was rewritten on 2026-10-17 (ruled by John 2026-10-17 on the outside review, item 7); the number was not changed.

Rests on: Sandberg and others, 2018; Ward and others, 2000.

The published estimates

8 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 to 5 planets per star Educated guess from the solar system, where Venus, Earth and Mars all orbit near the zone where water can be liquid.
1980 Carl SaganSagan, Cosmos (Random House, 1980), chapter 12, 'Encyclopaedia Galactica' 2 planets per star A round guess in a worked example of the equation for a general audience, before any exoplanet was known.
2000 Peter Ward and Donald BrownleeWard and Brownlee, Rare Earth: Why Complex Life Is Uncommon in the Universe (Copernicus, 2000), pages 269 to 270, chapter 'Assessing the Odds' (ward-2000) 1 in 100 thousand to 0.01 planets per star Said that, after allowing for how common planets are and for the location and lifetime of the habitable zone, the ordinary Drake equation suggests that only between 1% and 0.001% of all stars might have planets with habitats similar to Earth's, and that even these numbers are probably too high. They also ruled out red dwarfs as hosts. Their further extras (a large moon, plate tectonics, few mass extinctions) come after this passage and are not counted in the figure. Taken here as planets per star, since nearly every star has planets.
November 2013 Erik Petigura, Andrew Howard and Geoffrey MarcyPetigura, Howard and Marcy, 'Prevalence of Earth-size planets orbiting Sun-like stars', Proceedings of the National Academy of Sciences 110, 19273–19278 (2013) 0.14 to 0.3 planets per star Kepler's count of planets crossing 42,000 Sun-like stars, corrected for the planets it would have missed by injecting fake ones into the data: 22 plus or minus 8 percent of Sun-like stars have a planet of one to two Earth radii receiving between a quarter and four times the light Earth gets (the paper's abstract gives the directly measured 11 plus or minus 4 percent for one to four times the light Earth gets; the 22 percent extends that down to a quarter, assuming equal numbers of planets in each step of the logarithm of the light received).
July 2015 Courtney Dressing and David CharbonneauDressing and Charbonneau, 'The occurrence of potentially habitable planets orbiting M dwarfs estimated from the full Kepler dataset and an empirical measurement of the detection sensitivity', Astrophysical Journal 807, 45 (2015) 0.09 to 0.33 planets per star Kepler's count of planets crossing red dwarf stars, the most common kind, corrected for the ones it would have missed: 0.16 (between 0.09 and 0.33) Earth-size planets per red dwarf in the strictly defined habitable zone, plus another 0.12 planets somewhat larger than Earth; the wider habitable zone that allows for early Venus and early Mars gives 0.24 and 0.21.
August 2019 Danley Hsu, Eric Ford, Darin Ragozzine and Keir AshbyHsu, Ford, Ragozzine and Ashby, 'Occurrence Rates of Planets Orbiting FGK Stars: Combining Kepler DR25, Gaia DR2, and Bayesian Inference', Astronomical Journal 158, 109 (2019) (hsu-2019) · in the log 0.03 to 0.4 planets per star Kepler's final planet catalogue combined with Gaia's measurements of the stars: for planets of 0.75 to 1.5 Earth radii on orbits of 237 to 500 days around Sun-like stars, a rate of less than 0.27 per star, which could be lower by a factor of about two because some candidates are false alarms; the authors recommend planning for a true rate anywhere from 0.03 to 0.40, which is the span used here. Added on 2026-10-17 (outside review, item 8).
November 2020 Steve Bryson and 81 colleagues (the Kepler team)Bryson et al., 'The occurrence of rocky habitable zone planets around solar-like stars from Kepler data', Astronomical Journal 161, 36 (2021); posted 2020-10-28 (bryson-2020) 0.16 to 1.5 planets per star The Kepler mission's final planet catalogue combined with Gaia's measurements of the stars, corrected for what the catalogue missed and for false detections: between 0.37 (0.16 to 0.85) and 0.60 (0.24 to 1.50) planets of half to one and a half Earth radii per Sun-like star in the strictly defined habitable zone, the two figures reflecting two ways of extrapolating to orbits Kepler could not see; the generous habitable zone gives 0.58 to 0.88. The nearest such planet is estimated to be about 20 light-years away.
June 2018 Anders Sandberg, Eric Drexler and Toby OrdSandberg, Drexler and Ord, 'Dissolving the Fermi Paradox', arXiv:1806.02404 (2018), Table 1 (sandberg-2018) 0.1 to 1 planets per star A span evenly spread across its powers of ten, taking an Earth-like planet to mean a rocky planet in the habitable zone; the paper notes that the published argument runs from below one in a trillion (the Rare Earth case) to more than one (counting icy moons).

What would move this

The discovery that would narrow the range

A new count of rocky planets in the habitable zone per star from Kepler, TESS, Gaia or PLATO data, or a measurement of how often such planets keep an atmosphere and liquid water (the TRAPPIST-1 and K2-18b results bear on this).

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 new count of rocky planets in the habitable zone. On whether such planets keep an atmosphere: the JWST DREAMS team's four transits of TRAPPIST-1 e (Glidden and others, 2025-09-05) give no strong evidence for or against an atmosphere, with a bare rock and a nitrogen-rich atmosphere both fitting; the fifteen-transit follow-up has no published result as of 2026-10-16. A press claim (2026-07-09) that the JWST Rocky Worlds programme found GJ 3929 b airless is not on the programme's own site, which on 2026-07-10 still said the analysis was ongoing, so it is not counted. Logged: Glidden 2025.

    Glidden, 2025 source 1source 2

How the range has moved

DateWhatFromToWhy
16 October 2026 estimate (Peter Ward and Donald Brownlee, 2000).method Argued that Earth-like habitats need a long list of extras beyond the right orbit (a large moon, plate tectonics, a giant planet in the right place, a quiet galactic neighbourhood) and that only between 1% and 0.001% of all stars might have planets with habitats similar to Earth's Said that, after allowing for how common planets are and for the location and lifetime of the habitable zone, the ordinary Drake equation suggests only 1% to 0.001% of stars might have such planets; their further extras come later and are not in the figure. Source page corrected from 270 to 269-270. The estimate's numbers and the range are unchanged. Checked against Rare Earth (Copernicus, 2000), pages 269 to 271, on 2026-10-16: the figure is their reading of the ordinary equation, not of the extras.Because of Rare Earth: Why Complex Life Is Uncommon in the Universe.
17 October 2026 range.why described the one-in-a-hundred-thousand end as resting on Ward and Brownlee's extras describes Ward and Brownlee's figure as their reading of the ordinary equation (the share of all stars with Earth-like habitats, written before any count of habitable-zone planets), overtaken in its orbit-counting part by the Kepler counts, and keeps the argument about extras as the reason the floor sits at 0.01 Ruled by John 2026-10-17 on the outside review (item 7: decline the lower floor, accept the corrected reason); the 2026-10-16 check of pages 269 to 271 of Rare Earth had found the old wording wrong. The range itself is unchanged.Because of Rare Earth: Why Complex Life Is Uncommon in the Universe.
17 October 2026 estimate seven published estimates eight: Hsu, Ford, Ragozzine and Ashby (2019), 0.03 to 0.40 per Sun-like star, added Ruled by John 2026-10-17 on the outside review (item 8): the paper's own planning range, not the reviewer's 'about 0.2'. The range is unchanged.Because of Occurrence Rates of Planets Orbiting FGK Stars: Combining Kepler DR25, Gaia DR2, and Bayesian Inference.
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. 2 explanations say this piece is smaller than it looks; 1 says it is larger.

The papers that bear on it

Books and films that take it up

Notes

Lineweaver, Fenner and Gibson (Science, 2004) argued that only about a tenth of the galaxy's stars sit in a 'galactic habitable zone' with enough heavy elements and few enough nearby supernovae for complex life; that is a multiplier on this factor rather than an estimate of it, and the surveys above count planets without it. The Rare Earth argument (Ward and Brownlee 2000, ward-2000) is written up as an explanation in its own right; its number is recorded here because it is the published pessimistic extreme. Checked 2026-10-16: the 1% to 0.001% is Ward and Brownlee's reading of the ordinary equation, before their further extras (pages 269 to 270; the extras come on page 271), so the range's reason below, which describes the one-in-a-hundred-thousand end as resting on the extras, was written before this was checked and should be revisited at the outside review; the range itself is unchanged. Drake's 1961 range of 1 to 5 could not be checked against the book tonight: other secondary accounts give a single 2 (Glade and colleagues 2012, Table 1; Scoles and Heatherly 2011) or 'approximately one' (SETI Institute). Written 2026-10-16. Outside review, 2026-10-17: the proposal to lower the floor from 0.01 to 0.00001 to match Ward and Brownlee (item 7) was declined and the reason in the range rewritten, as this note had asked; the headline at 0.00001 on a scratch copy was 69% alone in the galaxy against 59%. Ruled by John 2026-10-17.

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.

  • Drake's 1961 range of 1 to 5 planets per star is quoted from the Wikipedia article on the Drake equation, which cites Drake and Sobel 1992; the same article notes that Glade and colleagues' 2012 table of the 1961 values differs from the book. Tried 2026-10-16 and still open: neither book could be opened; other secondary accounts give a single 2 (Glade and colleagues 2012, Table 1; Scoles and Heatherly 2011) or 'approximately one' (SETI Institute).

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/habitable-planets.toml in the public repository.