The Quiet Galaxy

The equation · piece 2 of 7 · fp

The share of stars that have planets

Of all the stars in the galaxy, what fraction have at least one planet. In 1961 this was a guess; since the first exoplanet discoveries in the 1990s and the Kepler mission of 2009 to 2018 it has become one of the two best-measured factors in the equation. The current reading is that planets are the rule, not the exception: every method that counts them, whether by the dip in a star's light as a planet crosses it, by the wobble of the star, or by the bending of light from a background star, finds planets around most or all stars.

The range this site uses 0.5 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

Started on 2026-10-16 from the range Sandberg, Drexler and Ord used in 2018 (0.1 to 1) and raised the same evening once the full record was read: no survey since 2011 finds fewer than half of stars with a planet. The microlensing count (Cassan 2012) puts the floor at 0.62 for the stars it covers; the wobble survey (Mayor 2011) finds more than half of Sun-like stars with a close planet before counting the distant ones; the Kepler red-dwarf count (Dressing and Charbonneau 2015) finds two and a half planets per star for the three quarters of stars that are red dwarfs. The floor is rounded down to 0.5 to leave room for the kinds of star no survey covers well. Evenly spread across its powers of ten for consistency with the other factors; the span is so narrow that the shape barely matters.

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, and in Drake, Vakoch and Dowd, The Drake Equation: Estimating the Prevalence of Extraterrestrial Life through the Ages (Cambridge University Press, 2015); the meeting's record is the founding entry of the publications log (drake-1961) 0.2 to 0.5 Educated guess before any planet outside the solar system had been found.
12 January 2012 Arnaud Cassan and colleaguesCassan et al., 'One or more bound planets per Milky Way star from microlensing observations', Nature 481, 167–169 (2012) 0.62 to 1 Gravitational microlensing over six years of survey data; 62% of stars found to host a planet of a few Earth masses at 0.5 to 10 times the Earth-Sun distance, and the planets per star add to more than one, so the share of stars with any planet lies between the two.
1980 Carl SaganSagan, Cosmos (Random House, 1980), chapter 12, 'Encyclopaedia Galactica' 0.33 A round guess, one third, in a worked example of the equation written for a general audience before any exoplanet was known.
September 2011 Michel Mayor and colleaguesMayor et al., 'The HARPS search for southern extra-solar planets XXXIV. Occurrence, mass distribution and orbital properties of super-Earths and Neptune-mass planets', arXiv:1109.2497 (2011) 0.5 to 1 Eight years of measuring the wobble of 822 Sun-like stars (376 with the HARPS instrument, the rest from an earlier survey with the CORALIE instrument); 51 to 57 percent were found to host at least one planet with an orbit shorter than 100 days (50.6 plus or minus 7.4 percent counting published planets, 57.1 plus or minus 8.0 percent with candidates), after correcting for the planets missed. Planets on longer orbits were not counted, so the share with any planet is higher.
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) 0.9 to 1 Kepler's four-year count of planets crossing red dwarf stars, corrected for the ones it would have missed: 2.5 plus or minus 0.2 planets of one to four Earth radii per red dwarf with orbits under 200 days. Red dwarfs are three quarters of all stars, and with that many planets per star almost every one has at least one; the lower figure here allows for planets bunching around some stars.
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 A span evenly spread across its powers of ten, chosen to cover the published argument; the paper notes that 'different methods are converging on fp ≈ 1'.

What would move this

The discovery that would narrow the range

A survey that counts planets around a large, well-understood sample of stars, including the dim red dwarfs that make up most of the galaxy, and reports the share of stars with none.

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 Nothing new By hand

    No survey in the window reports the share of stars with no planet. The red dwarf occurrence work found is from 2013 to 2022 and already consistent with the range; Gaia's fourth data release, expected in December 2026, is forecast to add about 7,500 planets, mostly giants, and is the next thing to read for this piece.

How the range has moved

DateWhatFromToWhy
16 October 2026 range.low 0.1 0.5 The published record read in full the same day the worked example was written: no survey since 2011 finds fewer than half of stars with a planet (Cassan 2012, Mayor 2011, Dressing and Charbonneau 2015). The 2018 paper's floor of 0.1 covered an argument that the surveys have since closed.
16 October 2026 estimate (Michel Mayor and colleagues, 2011).method Eight years of measuring the wobble of 376 Sun-like stars with the HARPS instrument; more than half were found to host at least one planet with an orbit shorter than 100 days, a lower limit since longer orbits and smaller planets were missed. 822 Sun-like stars (376 with HARPS, the rest with CORALIE); 51 to 57 percent host at least one planet with an orbit shorter than 100 days. The estimate's low (0.5) and high (1.0) are unchanged and the range is untouched. Checked against Mayor et al., arXiv:1109.2497, on 2026-10-16: the statistical sample is 822 stars, and the corrected share is 50.6 plus or minus 7.4 percent (published planets) to 57.1 plus or minus 8.0 percent (with candidates).
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

No explanation on the record bears on this piece yet.

The papers that bear on it

Notes

Checked 2026-10-16 against the primary sources: Sagan's one third and the Mayor period limit of 100 days are borne out; Mayor's statistics use 822 stars, not 376 (corrected in the estimate and in a revision below). Drake's 1961 value of 0.2 to 0.5 could not be checked against Drake and Sobel or the 2015 volume tonight: other secondary accounts give a single 0.5 (Glade and colleagues 2012, arXiv:1112.1506, Table 1; the Astronomical Society of the Pacific article by Scoles and Heatherly, 2011), so the 1961 range is still unconfirmed; it has no weight in the range, which rests on the surveys. Saturday session C writes the publication records for the 2012 microlensing paper and the others cited here; when they exist, each estimate gets a 'publication' link to its record. Written 2026-10-16. Outside review, 2026-10-17: the proposal to change the shape to uniform (item 5) was declined for consistency with the other log-uniform ranges, the headline being the same within noise either way (58.7% against 58.8% alone in the galaxy on a scratch copy); the range is unchanged. 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.

  • Zhu and Dong (2021, zhu-2021), the Annual Review of Astronomy and Astrophysics survey of planet statistics, is logged for this factor, but the reviewer's statement that it concludes the share of stars with planets is close to 100 percent was not found in its abstract or in coverage, and the full text has not been read for it; until it is, there is no estimate to add. Raised 2026-10-17 by the outside review (item 6); open.
  • Drake's 1961 range of 0.2 to 0.5 for fp is quoted from the Wikipedia article on the Drake equation, which cites Drake and Sobel's 1992 book, pages 55 to 62; check it against the book or the 2015 Drake, Vakoch and Dowd volume. Tried 2026-10-16 and still open: neither book could be opened (restricted lending copy, no snippets); other secondary accounts give a single 0.5 for 1961 (Glade and colleagues 2012, Table 1; Scoles and Heatherly 2011 in the Astronomical Society of the Pacific's magazine), and the SETI Institute's page says attendees thought the share was close to 100 percent.

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/planets.toml in the public repository.