IAU_definition_of_planet

# IAU definition of planet  **Microsim — three.js (Wikitube framework):** *What the Solar System holds (Solar System explorer)* <div class="wt-sim" data-src="https://wikitube-3d-microsims.netlify.app/solar/Solar_System.html?view=census&embed=1" data-title="What the Solar System holds (Solar System explorer)"></div> *The Solar System explorer locked on this article's state (`?view=census`); every object and population of the [[PORTAL_Solar_System|Solar System portal]] has its own state in the same scene.*  *Try: under show, choose planets and moons and count the eight bodies coloured as planets against the smaller round worlds coloured as dwarf planets; switch show to small bodies to see the swarms that meet only the first of the three criteria; press l to label everything, then drag and scroll out past Neptune to find Pluto and Eris among the trans-Neptunian points.* The **IAU definition of planet** is the set of criteria adopted by the International Astronomical Union at its 26th General Assembly in Prague on 24 August 2006. Under Resolution 5A a planet of the [[PORTAL_Solar_System|Solar System]] is a body that orbits the [[Sun]], is massive enough for its own [[Gravity|gravity]] to pull it into a nearly round shape, and has cleared the neighbourhood around its orbit.[^iau-res] A body that meets the first two conditions but not the third, and is not a moon, is a [[Dwarf_planet|dwarf planet]]; one that meets only the first is a [[Small_Solar_System_body|small Solar System body]].[^iau-res] The companion Resolution 6A named [[Pluto]], counted as the ninth planet since 1930, a dwarf planet, the first example of a new class among the [[Trans-Neptunian_object|trans-Neptunian objects]].[^iau-vote] The vote closed a debate that had been building since the 1990s, when Pluto was found to be one member of a large population beyond [[Neptune]], and that came to a head when [[Eris_(dwarf_planet)|Eris]] was measured as slightly larger than Pluto.[^basri2006] The decision has been criticised both for its substance, chiefly the vagueness of "[[Clearing_the_neighbourhood|clearing the neighbourhood]]", and for the small number of members who voted.[^rincon2006] It leaves eight planets. Its wording depends on location: a body the size of [[Mars]] far out in the [[Oort_cloud|Oort cloud]] could not clear so wide a zone and would not qualify.[^basri2006] And [[Mercury_(planet)|Mercury]], which the IAU lists as a planet, is not in fact in hydrostatic equilibrium.[^solomon2018] The explorer at the top of this page opens on its census view, which colours every body by the class this definition created: the eight planets, the dwarf planets, the moons, and the far larger crowd of minor planets and comets that fall under the small-body heading. ## Background The argument over Pluto repeated one that had played out two centuries earlier. When Giuseppe Piazzi found [[Ceres_(dwarf_planet)|Ceres]] on 1 January 1801, it was hailed as the planet long expected between [[Mars]] and [[Jupiter]], but within a few years three similar bodies had turned up nearby.[^basri2006] By 1851 the tally of planets stood at 23, eight large ones and fifteen small ones in the gap, and astronomers began to list the small ones separately as asteroids.[^soter2007] Pluto's own history followed the same course more slowly. Clyde Tombaugh found it in 1930 at Lowell Observatory during a search for a massive "Planet X" beyond Neptune, and it was at first thought to be larger than Mercury. Doubt set in early: in 1956 the writer Willy Ley published a column called "The Demotion of Pluto", complaining that the body had never matched its advance billing.[^ley1956] The discovery of its moon [[Charon_(moon)|Charon]] in 1978 made the problem precise. The period of Charon's orbit gives the mass of the pair directly, and the result was far smaller than anyone had assumed.[^buie2006] Pluto's mass is about 1.3 × 10²² kg, roughly one twenty-fifth of Mercury's and under a fifth of the [[Moon]]'s (derived).[^nasa-fs] In the 1990s, surveys began to find hundreds of bodies in the [[Kuiper_belt|Kuiper belt]] beyond Neptune, many of them in orbits like Pluto's, eccentric, inclined and locked in the same resonance with Neptune.[^basri2006] Pluto now looked like the largest known member of a population rather than a lone planet. The discoveries of [[Quaoar]], [[Sedna_(dwarf_planet)|Sedna]] and Eris after 2000 forced the choice: either all bodies of Pluto's size would be planets, and their number would keep rising, or Pluto would be reclassified.[^basri2006] The IAU had no procedure for defining a planet. In 2005 it formed a committee of 19 members chaired by Iwan Williams, which set out three possible bases for a definition: cultural (a planet is whatever enough people call one), structural (a body large enough to be round) and dynamical (a body that eventually removes other objects from its orbit).[^eales2007] A smaller Planet Definition Committee, chaired by the historian of astronomy Owen Gingerich and including five planetary scientists and the writer Dava Sobel, was then charged with a firm proposal.[^eales2007] ## Proposals ### First draft proposal The committee's draft, released on 16 August 2006, rested on the structural option: a planet would be any body in orbit around a star, not itself a star or a moon, that is massive enough to be round.[^iau-draft] Applied at once, it would have raised the count from nine to twelve by adding Ceres, Charon and Eris, and the press reported the result that way.[^cook2006] Mike Brown estimated that at least 53 known bodies would probably qualify and that a full survey of the outer Solar System might reveal more than 200.[^brown-howmany] Charon's inclusion came from a double-planet clause. A pair of round bodies would count as a double planet if their common centre of mass lay outside both of them; the Pluto–Charon barycentre lies in the space between the two, whereas the barycentre of [[Earth]] and the Moon, or of Jupiter and [[Ganymede_(moon)|Ganymede]], lies inside the planet.[^britt2006a] Round trans-Neptunian planets with periods over 200 years were to be called "plutons", and the draft gave a rough threshold for roundness of about 5 × 10²⁰ kg in mass or 800 km in diameter.[^iau-draft5] Brown objected that the threshold suits rocky bodies but that icy bodies relax into round shapes at perhaps 200–400 km across.[^brown-howmany] The draft did not say where a planet ends and a brown dwarf begins.[^iau-qa] The committee of the American Astronomical Society's Division for Planetary Sciences endorsed the draft on 18 August.[^spacedaily2006] Critics, among them the astronomer Phil Plait, called the wording loose.[^plait2006] Because it tied planets to stars, a free-floating body would never qualify. And the double-planet test could in principle change the Moon's status: tidal friction slowly pushes the Moon outward, and after billions of years the Earth–Moon barycentre could leave Earth's interior, although the Sun is expected to have expanded long before then.[^britt2006b] ### Alternative proposal A straw poll of an IAU subgroup on 18 August found only 18 members for the draft and more than 50 against it, according to Alan Boss of the Carnegie Institution.[^britt2006c] The opponents backed a rival text by the Uruguayan astronomers Gonzalo Tancredi and Julio Ángel Fernández, under which Pluto would become a dwarf planet.[^britt2006c] ### Revised draft proposal A second draft, issued on 22 August, dropped the name "pluton", which geologists pointed out already denotes a body of intruded igneous rock, and required a double planet's barycentre to lie outside both bodies for most of each orbit.[^wuchterl2006][^brumfiel2006] At two open meetings the same day, the mood turned: indicative votes went heavily against the double-planet and pluton provisions, and Fernández's approach, which would leave eight planets, gained ground.[^overbye2006a] By the end of the day the Executive Committee had agreed to exclude planets around other stars and to add a criterion of dominance over the orbital neighbourhood.[^fienberg2006] ### Final draft proposal The third draft reached the closing plenary session on 24 August, chaired by Jocelyn Bell Burnell.[^hogan2006] Members present voted by raising yellow cards, and the counts were compiled by Virginia Trimble.[^ocregister2006] Four resolutions were put. Resolution 5A, the definition itself, passed by a large majority. Resolution 5B, which would have inserted "classical" before "planet" so that dwarf planets became a kind of planet, won only 91 votes. Resolution 6A, on Pluto, passed 237 to 157 with 30 abstentions. Resolution 6B, which would have named the category "plutonian objects", failed 183 to 186.[^iau-vote][^nuncius2006] ## Final definition Resolution 5A defines a planet as "a celestial body that (a) is in orbit around the Sun, (b) has sufficient mass for its self-gravity to overcome rigid body forces so that it assumes a hydrostatic equilibrium (nearly round) shape, and (c) has cleared the neighbourhood around its orbit."[^iau-res] A dwarf planet meets (a) and (b), fails (c) and is not a satellite; every other body orbiting the Sun except moons is a small Solar System body.[^iau-res] Footnotes to the resolution name the eight planets, Mercury to Neptune, and state that an IAU process would assign borderline bodies to one class or the other.[^iau-res] Resolution 6A adds that Pluto is a dwarf planet by this definition and the prototype of a new category of trans-Neptunian objects.[^iau-res] The three criteria test different things. The first is an orbital fact, and excludes moons and free-floating bodies. The second is a physical state: a body whose gravity exceeds the strength of its material flows into a shape set by gravity and rotation. The third is dynamical, and is the one the Prague vote added late. The three classes do not nest: the IAU stated that planets and dwarf planets are distinct classes, so "dwarf planet", despite the name, does not describe a kind of planet.[^iau-vote] The explorer's census view is built on this sorting. Every body it draws is coloured by its class, so the eight planets, the dwarf planets and the minor bodies read as separate groups; the planets' positions come from JPL orbital elements and their physical data from the NASA fact sheet.[^jpl-t1][^nasa-fs] ## Closing issues ### Substance The sharpest objection came from Alan Stern, principal investigator of NASA's New Horizons mission to Pluto, who argued that none of Earth, Mars, Jupiter or Neptune has cleared its zone either: Earth shares its region with about 10,000 [[Near-Earth_object|near-Earth asteroids]] and Jupiter with about 100,000 [[Jupiter_trojan|Trojans]]. "If Neptune had cleared its zone," he said, "Pluto wouldn't be there."[^rincon2006] The reply from Brown and others is that clearing means control, not emptiness. The Trojans share Jupiter's orbit only because its gravity holds them there, and Pluto and the [[Plutino|plutinos]] cross Neptune's path safely because Neptune has trapped them in a 3:2 [[Resonance|resonance]], two of their orbits for every three of its own.[^brown-eight] Quantitative criteria that separate the eight planets from everything else by orders of magnitude were published afterwards, and are covered under [[Clearing_the_neighbourhood|clearing the neighbourhood]]. On 11 June 2008 the IAU named trans-Neptunian dwarf planets "plutoids".[^iau-plutoid] The definition says nothing about other stars, and orbit clearing cannot be observed for an [[Planetary_system|extrasolar planetary system]]. A separate working definition for exoplanets, first drafted by an IAU working group in 2003, sets the upper mass limit at the threshold for deuterium fusion and the lower limit at the one used in the Solar System.[^boss2007][^lecavelier2022] ### Process Only 424 astronomers voted, out of an IAU membership of about 9,000; the ballot came on the last day of a ten-day meeting, when many members had already left Prague, and most members never attend General Assemblies at all.[^rincon2006] Jean-Luc Margot noted that a random sample of 400 from 9,000 would still estimate the whole membership's view to within about 5%.[^margot-web] Marla Geha argued that only members working on planetary science needed to vote.[^geha2006] ## Impact The ruling reached well beyond astronomy. Textbooks, classroom charts and toys that showed nine planets went out of date, and the publisher of the *World Book Encyclopedia* held its 2007 edition until the vote was known.[^overbye2006b] Strong public feeling had been expected. Pluto was the only planet found by an American, and in 1999 there had been an outcry when news reports took a proposal to give all trans-Neptunian objects catalogue numbers as a plan to demote it.[^infoplease2006] Mike Brown, whose discovery of Eris precipitated the vote, defended the outcome and later wrote a popular account of the episode, *How I Killed Pluto and Why It Had It Coming*.[^brown-eight][^brown2010] ### Popular culture Pluto's reclassification became a small theme in popular music. Songs about it include BTS's "134340" (2018), titled after Pluto's minor-planet number, and Tom Cardy's "H.S" (2023).[^hellokpop2018][^cardy2023] ### Plutoed In January 2007 the American Dialect Society chose "plutoed" as its word of the year for 2006, glossing the verb "to pluto" as to lower the rank or worth of a person or thing, in the manner of the former planet.[^ads2007][^cnn2007] The society's president, Cleveland Evans, said the strength of public feeling showed how much the name still mattered.[^nbc2007] ## Notes The IAU's own documents use the British spelling "neighbourhood", which is kept here. The explorer's census counts more dwarf planets than the handful the IAU has formally named; that tally is the explorer's own, drawn from the wider set of bodies generally accepted as dwarf planets, not an IAU list. ## See also - [[Dwarf_planet]] - [[Small_Solar_System_body]] - [[Clearing_the_neighbourhood]] - [[Pluto]] - [[Planetary_system]] - List of former planets · Minor planet · Geophysical planet definition (plain text: not yet on Wikitube) ## References [^iau-res]: International Astronomical Union (24 August 2006). "IAU 2006 General Assembly: Resolutions 5 and 6". https://iauarchive.eso.org/static/resolutions/Resolution_GA26-5-6.pdf [^iau-vote]: International Astronomical Union (24 August 2006). "IAU 2006 General Assembly: Result of the IAU Resolution votes". News release IAU0603. https://iauarchive.eso.org/news/pressreleases/detail/iau0603/ [^nuncius2006]: "IAU General Assembly Newspaper, 25 August 2006". *Dissertatio cum Nuncio Sidereo* III(10). http://astro.cas.cz/nuncius/nsiii_10.pdf [^basri2006]: Basri, G.; Brown, M. E. (2006). "Planetesimals to brown dwarfs: What is a planet?". *Annual Review of Earth and Planetary Sciences* 34: 193–216. https://doi.org/10.1146/annurev.earth.34.031405.125058 [^solomon2018]: Solomon, S. C.; Nittler, L. R.; Anderson, B. J., eds. (2018). *Mercury: The View after MESSENGER*. Cambridge Planetary Science 21. Cambridge University Press, pp. 72–73. [^soter2007]: Soter, S. (2007). "What is a planet?". *Scientific American* 296 (1): 34–41. https://doi.org/10.1038/scientificamerican0107-34 [^ley1956]: Ley, W. (August 1956). "The demotion of Pluto". *Galaxy Science Fiction*, pp. 79–91. https://archive.org/stream/galaxymagazine-1956-08#page/n79/mode/2up [^buie2006]: Buie, M. W.; Grundy, W. M.; Young, E. F.; Young, L. A.; Stern, S. A. (2006). "Orbits and photometry of Pluto's satellites: Charon, S/2005 P1, and S/2005 P2". *The Astronomical Journal* 132: 290–298. https://doi.org/10.1086/504422 [^nasa-fs]: NASA NSSDCA. "Planetary Fact Sheet". https://nssdc.gsfc.nasa.gov/planetary/factsheet/ (fetched 2026-09-18). [^jpl-t1]: JPL Solar System Dynamics. "Approximate Positions of the Planets", Table 1. https://ssd.jpl.nasa.gov/planets/approx_pos.html [^eales2007]: Eales, S. (May 2007). "Prague: Pluto and the demotion". *Prospect*, pp. 31–34. [^iau-draft]: International Astronomical Union (16 August 2006). "The IAU draft definition of 'planet' and 'plutons'". News release IAU0601. https://iauarchive.eso.org/news/pressreleases/detail/iau0601/ [^iau-draft5]: International Astronomical Union (2006). "Draft Resolution 5 for GA-XXVI: Definition of a Planet". http://www.iau2006.org/mirror/www.iau.org/iau0601/iau0601_resolution.html [^iau-qa]: International Astronomical Union (2006). "'Planet Definition' Questions & Answers Sheet". http://www.iau2006.org/mirror/www.iau.org/iau0601/iau0601_Q_A.html [^cook2006]: Cook, G. (16 August 2006). "Nine no longer: Panel declares 12 planets". *The Boston Globe*. http://archive.boston.com/news/science/articles/2006/08/16/nine_no_longer_panel_declares_12_planets/ [^brown-howmany]: Brown, M. E. (2006). "How many planets are there?". California Institute of Technology. http://www.gps.caltech.edu/~mbrown/whatsaplanet/howmanplanets.html [^britt2006a]: Britt, R. R. (16 August 2006). "Nine planets become 12 with controversial new definition". *Space.com*. http://www.space.com/scienceastronomy/060816_planet_definition.html [^spacedaily2006]: "Planetary scientists support proposed redefinition of a planet" (2006). *SpaceDaily*. http://www.spacedaily.com/reports/Planetary_Scientists_Support_Proposed_Redefinition_Of_A_Planet_999.html [^plait2006]: Plait, P. (15 August 2006). "Congratulations! It's a planet!". *Bad Astronomy* (Discover). http://blogs.discovermagazine.com/badastronomy/2006/08/15/congratulations-its-a-planet/ [^britt2006b]: Britt, R. R. (18 August 2006). "Earth's moon could become a planet". *CNN*. http://edition.cnn.com/2006/TECH/space/08/18/moon.planet/ [^britt2006c]: Britt, R. R. (18 August 2006). "Pluto may get demoted after all". *Space.com*. http://www.space.com/scienceastronomy/060818_planet_newprop.html [^wuchterl2006]: Wuchterl, G. (22 August 2006). "The IAU Committee presents today in Prague the new proposals for the definition of planet". Kuffner-Sternwarte. http://kuffner-sternwarte.at/im_brennp/archiv2006/Planeten_Definition_Resolution_5_6_7.html [^brumfiel2006]: Brumfiel, G. (21 August 2006). "Plutons, planets and dwarves: Geologists and astronomers wrangle over words". *Nature News*. https://doi.org/10.1038/news060821-4 [^overbye2006a]: Overbye, D. (22 August 2006). "Pluto seems poised to lose its planet status". *The New York Times*. https://www.nytimes.com/2006/08/22/science/space/22cnd-pluto.html [^fienberg2006]: Tresch Fienberg, R. (22 August 2006). "The day we lost Pluto". *Sky & Telescope*. http://skytonight.com/news/home/3707031.html [^hogan2006]: Hogan, J. (2006). "Diary of a planet's demise". *Nature* 442: 966–967. https://doi.org/10.1038/442966a [^ocregister2006]: "Pluto in perspective" (1 September 2006). *Orange County Register*. https://www.ocregister.com/2006/09/01/pluto-in-perspective/ [^rincon2006]: Rincon, P. (25 August 2006). "Pluto vote 'hijacked' in revolt". *BBC News*. https://news.bbc.co.uk/2/hi/science/nature/5283956.stm [^brown-eight]: Brown, M. E. (2006). "The eight planets". California Institute of Technology. http://www.gps.caltech.edu/~mbrown/eightplanets/ [^iau-plutoid]: International Astronomical Union (11 June 2008). "Plutoid chosen as name for Solar System objects like Pluto". News release IAU0804. https://iauarchive.eso.org/news/pressreleases/detail/iau0804/ [^boss2007]: Boss, A. P.; Butler, R. P.; Hubbard, W. B.; Ianna, P. A.; Kürster, M.; Lissauer, J. J.; et al. (2007). "Definition of a planet". *Proceedings of the International Astronomical Union* 1 (T26A): 183–186. https://doi.org/10.1017/S1743921306004509 [^lecavelier2022]: Lecavelier des Etangs, A.; Lissauer, J. J. (2022). "The IAU working definition of an exoplanet". *New Astronomy Reviews* 94: 101641. https://doi.org/10.1016/j.newar.2022.101641 [^margot-web]: Margot, J.-L. (2006). "What makes a planet?". University of California, Los Angeles. http://www2.ess.ucla.edu/~jlm/epo/planet/planet.html [^geha2006]: Geha, M. (2006). "A passion for Pluto". *The Nation*. http://www.thenation.com/doc/20060925/pluto [^overbye2006b]: Overbye, D. (24 August 2006). "Pluto is demoted to 'dwarf planet'". *The New York Times*. https://www.nytimes.com/2006/08/24/science/space/25pluto.html [^hellokpop2018]: "Korean scientists commend BTS for integration science on '134340'" (3 September 2018). *Hellokpop*. https://www.hellokpop.com/kpop/korean-scientists-commend-bts-134340/ [^cardy2023]: Cardy, T. (6 September 2023). "H.S" (music video). https://www.youtube.com/watch?v=EuRjmzz6qL0 [^ads2007]: American Dialect Society (5 January 2007). "'Plutoed' voted 2006 Word of the Year". http://www.americandialect.org/Word-of-the-Year_2006.pdf [^infoplease2006]: Pearson Education (2006). "The flap over Pluto". *Infoplease*. http://www.infoplease.com/ipa/A0776605.html [^brown2010]: Brown, M. E. (2010). *How I Killed Pluto and Why It Had It Coming*. New York: Spiegel & Grau. [^cnn2007]: "Pluto's revenge: 'Word of the Year' award" (7 January 2007). *CNN*. http://www.cnn.com/2007/US/01/07/word.of.the.year/index.html [^nbc2007]: "'Plutoed' chosen as '06 Word of the Year" (8 January 2007). *NBC News*. https://www.nbcnews.com/id/wbna16529756 ## External links - International Astronomical Union: Resolutions B5 and B6 of the 2006 General Assembly (PDF). https://iauarchive.eso.org/static/resolutions/Resolution_GA26-5-6.pdf - International Astronomical Union: result of the 2006 resolution votes. https://iauarchive.eso.org/news/pressreleases/detail/iau0603/ - NASA Science: Pluto. https://science.nasa.gov/dwarf-planets/pluto/ - Basri, G.; Brown, M. E. (2006). "Planetesimals to brown dwarfs: What is a planet?". https://doi.org/10.1146/annurev.earth.34.031405.125058 ## Wikipedia : Wikitube **Strict pair:** [Wikipedia](https://en.wikipedia.org/wiki/IAU_definition_of_planet) : [Wikitube](https://en.wikitube.io/wiki/IAU_definition_of_planet) · pinned revision [1372697874](https://en.wikipedia.org/w/index.php?oldid=1372697874) · 2026-09-18 ## Previous hub tags Hubs: `Life_Physics`. Portals: [[PORTAL_Solar_System]]. --- *Solar System portal child articles, wave 1 · 2026-09-18 · drafted · row SOL-001 · explorer state `?view=census`.*