Dwarf_planet - Wikiparency

# Dwarf 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 pick out the few bodies coloured as dwarf planets among the planets; press l to name them, then scroll out and drag the view to follow Pluto, Haumea and Makemake just beyond Neptune, Eris and Gonggong farther out on steep orbits, and Ceres alone among the asteroids; switch show to small bodies to see how many smaller neighbours share their zones.* A **dwarf planet** is a body that orbits the [[Sun]] directly and is massive enough for its own [[Gravity|gravity]] to pull it into a nearly round shape, but not massive enough to dominate the region around its orbit as the eight planets do.[^iau-res] The class was created by the International Astronomical Union in 2006, in the same vote that set out the [[IAU_definition_of_planet|IAU definition of planet]], and its prototype is [[Pluto]], counted as a planet from 1930 until then.[^iau-res] The IAU treats dwarf planets as a separate class, not a kind of planet; many planetary geologists disagree and count dwarf planets and large moons as planets.[^metzger2022] Five bodies are usually listed as the dwarf planets: [[Ceres_(dwarf_planet)|Ceres]] in the [[Asteroid_belt|asteroid belt]], and Pluto, [[Eris_(dwarf_planet)|Eris]], [[Haumea]] and [[Makemake]] beyond [[Neptune]].[^wgpsn] Most astronomers add at least [[Gonggong_(dwarf_planet)|Gonggong]], [[Quaoar]], [[Sedna_(dwarf_planet)|Sedna]] and [[Orcus_(dwarf_planet)|Orcus]], making nine, and some add many more.[^tancredi2008][^emery2024] Two have been visited by spacecraft, Ceres by Dawn and Pluto by New Horizons, both in 2015, and both proved geologically active.[^landau2015][^nimmo2017] The explorer at the top of this page opens on its census view, which colours every body by class; the dwarf planets form a small group, set apart from the eight planets and from the far larger population of minor bodies. ## History of the concept The problem the category solves first arose in the nineteenth century. Ceres, found in 1801, was counted as a planet until so many similar bodies had been found between [[Mars]] and [[Jupiter]] that, by about 1851, astronomers moved them into a separate class of asteroids or minor planets.[^murzi2007] Pluto went through the same process over a longer span. After its discovery in 1930 it was thought for decades to be larger than [[Mercury_(planet)|Mercury]], until the orbit of its moon [[Charon_(moon)|Charon]], found in 1978, gave a mass far below expectations.[^buie2006][^cuk2007] Pluto's eccentric, inclined orbit also set it apart from the other planets.[^weintraub2006] In the 1990s surveys found more bodies in Pluto's region, the [[Kuiper_belt|Kuiper belt]], many sharing its orbit type, and Pluto came to look like the largest of a population.[^brown2004] Terms such as "planetoid" and "subplanet" came into use for the larger ones.[^eicher2007][^hubble2004] The discovery of Eris in January 2005, at first thought slightly larger than Pluto, forced the issue at the IAU General Assembly of August 2006.[^brown-lila][^brown-what] The first draft there would have made Ceres, Charon and Eris planets; the rival proposal by Julio Ángel Fernández and Gonzalo Tancredi introduced an intermediate class for bodies round enough but not dominant in their orbits, and that approach prevailed.[^britt2006] Reaction was divided. Mike Brown, Eris's discoverer, supported the reduction to eight planets.[^brown-eight] NASA said it would follow the IAU's usage.[^nasa2006] Alan Stern, principal investigator of New Horizons, rejected both the separation of dwarf planets from planets and the use of an orbital test; years before the vote he had proposed "überplanets" and "unterplanets", distinguished by their dynamical dominance but all counted as planets.[^stern2006][^stern2000] ## Name "Dwarf planet" was not a new phrase. It had long been used for the smallest planets, including the terrestrial ones, and in 2006 the IAU gave it the opposite sense of a body that is not a planet at all. Brown called the choice illogical, and has written that "planetoid" would have served, but the plenary session changed the draft's wording to "dwarf planet" without dissent.[^brown2010][^iau-res] A companion resolution, 5B, would have made dwarf planets a subtype of planet beside eight "classical planets"; it was defeated.[^iau-vote] Alternatives such as "subplanet" and "nanoplanet" were later discussed by the IAU's small-bodies nomenclature committee without agreement.[^bowell2008] Resolution 6A made Pluto the prototype of a new category of [[Trans-Neptunian_object|trans-Neptunian objects]] without naming it. "Plutons" and "plutonian objects" had both been floated in the debate and dropped, partly because geologists already use "pluton" for a body of intruded igneous rock.[^iau-res] On 11 June 2008 the IAU Executive Committee announced the term "plutoid" for all trans-Neptunian dwarf planets.[^iau-plutoid] Ceres, the one dwarf planet inside Neptune's orbit, is therefore not a plutoid. Earlier writers had drawn the same line between the rocky "terrestrial dwarf" Ceres and the "ice dwarfs" of the outer Solar System.[^carson2011] Since Dawn found that Ceres itself is rich in ice and may have formed farther out, it too has been called an ice dwarf.[^castillo2017][^carroll2019] ## Criteria The class sits between two ideas of what makes a planet. A dynamicist sorts bodies by their effect on their surroundings: Mercury through Neptune control their orbital zones, and everything else is part of a swarm. A geologist sorts them by what they are like inside: once a body is massive enough to be round, it can have mantles, cryovolcanoes and an internally driven geology like a planet's. Because roundness sets in at a much lower mass than dominance, a set of bodies is round but not dominant, and the dwarf planet class holds them.[^lakdawalla2020] ### Orbital dominance Several measures of orbital dominance have been proposed. Alan Stern and Harold Levison defined a parameter Λ that grows with the square of a body's mass and falls with its orbital period; a body with Λ above 1 will in time scatter the small bodies near its orbit.[^stern2000] Steven Soter proposed a planetary discriminant µ, the ratio of a body's mass to the total mass of everything else in its orbital zone.[^soter2006] Jean-Luc Margot derived a third, Π, from theory alone.[^margot2015] All three separate the two groups by orders of magnitude: Soter's µ is about 1.7 × 10⁶ for [[Earth]] but about 0.33 for Ceres and 0.08 for Pluto, and Margot's Π is about 810 for Earth against 0.04 for Ceres and 0.03 for Pluto.[^soter2006][^margot2015] These criteria are discussed in detail under [[Clearing_the_neighbourhood|clearing the neighbourhood]]. ### Hydrostatic equilibrium A small body keeps whatever irregular shape its formation and collisions gave it, because the strength of its material exceeds its self-gravity. As mass grows, the pressure inside rises until the material yields and flows, high ground sinks and hollows fill, and the body settles into the shape a fluid would take, a sphere if it did not rotate and an ellipsoid if it does; this is [[Hydrostatics|hydrostatic equilibrium]].[^lineweaver2010] Rapid rotation stretches the shape: [[Haumea]], which spins in about four hours, is far longer along one axis than through its poles.[^ortiz2017] No size or mass is written into the definition. The IAU's 2006 question-and-answer sheet suggested that bodies above about 5 × 10²⁰ kg and 400 km in radius would normally be round, while leaving borderline cases to observation.[^iau-qa] The real threshold depends on composition and thermal history. Icy bodies relax at smaller sizes than rocky ones, but Grundy and colleagues argued in 2019 that dark, low-density trans-Neptunian objects smaller than about 900–1,000 km across never collapsed into solid bodies and keep internal pores, which would disqualify them.[^grundy2019] Nor is there an upper bound: a body larger than Mercury far out in the Solar System might not have had time to clear its zone, and would count as a dwarf planet. ## Population of dwarf planets Applying the definition is harder than stating it, because for most candidates neither shape nor interior can be observed directly. Of the three bodies argued over in 2006, only Pluto has been measured closely enough to confirm that its shape matches equilibrium; its mean diameter is 2,376.6 km.[^nimmo2017] Ceres is close to equilibrium, with some gravity anomalies still unexplained.[^raymond2018] Eris, 2,326 km across, is accepted because it is about 27% more massive than Pluto (derived).[^sicardy2011][^brown2007][^brozovic2024] The IAU assigned naming to a joint committee for any unnamed trans-Neptunian object brighter than absolute magnitude +1, which corresponds to a diameter of at least 838 km even at the highest possible albedo.[^iau-plutoid][^bruton] Haumea and Makemake passed that test and were named as dwarf planets in 2008, making the commonly listed five, although neither has been shown directly to be in equilibrium.[^wgpsn] ### Most likely dwarf planets Beyond the five, JPL called Gonggong a dwarf planet in 2016, and the IAU's own Division F has referred to Quaoar as one.[^dyches2016][^iau-divf] Tancredi and Favre recommended in 2008 that the IAU accept Orcus, Sedna and Quaoar.[^tancredi2008] Brown keeps a longer list ranked by estimated size, from "nearly certain" to "possible".[^brown-dps] Spectra taken by the James Webb Space Telescope in 2022 suggest that Sedna, Gonggong and Quaoar melted and differentiated inside, as the larger dwarf planets did.[^emery2024] Salacia, about 838 km across, is the usual borderline case.[^kiss2025] | Body | Region | Semi-major axis (AU) | Eccentricity | Inclination | Diameter (km) | |---|---|---|---|---|---| | Ceres | Asteroid belt | 2.77 | 0.080 | 10.6° | 939.4[^park2019] | | Pluto | Kuiper belt, 2:3 resonance | 39.6 | 0.252 | 17.1° | 2,376.6[^nimmo2017] | | Orcus | Kuiper belt, 2:3 resonance | 39.4 | 0.221 | 20.6° | 910[^brown2018] | | Haumea | Kuiper belt | 43.1 | 0.194 | 28.2° | — | | Quaoar | Kuiper belt | 43.2 | 0.035 | 8.0° | — | | Makemake | Kuiper belt | 45.6 | 0.159 | 29.0° | — | | Gonggong | Scattered disc | 66.9 | 0.504 | 30.9° | — | | Eris | Scattered disc | 67.9 | 0.438 | 43.9° | 2,326[^sicardy2011] | | Sedna | Detached | 544 | 0.86 | 11.9° | — | Orbital elements are osculating values from the JPL Small-Body Database, rounded; Sedna's semi-major axis in particular varies with the epoch.[^jpl-sbdb] ### Symbols Ceres and Pluto received planetary symbols (⚳ and ♇) because they were counted as planets when found. The later dwarf planets were discovered after such symbols had fallen out of astronomical use; symbols for them devised by the software engineer Denis Moskowitz were added to Unicode in 2022, and NASA has used several of them.[^unicode2022][^miller2021] ## Exploration Two missions have studied dwarf planets at close range. Dawn entered orbit around Ceres on 6 March 2015, the first spacecraft to orbit a dwarf planet, after first orbiting the large asteroid [[4_Vesta|Vesta]].[^landau2015] New Horizons flew through the Pluto system in July 2015 and returned images from which Pluto's and Charon's radii and shapes were measured.[^nimmo2017] Both worlds turned out to be active. Ceres shows bright salt deposits and signs of cryovolcanism, with brine below its surface.[^castillo2017] Pluto has mountains of water ice, plains and glaciers of nitrogen ice and an extended atmosphere.[^stern2015] From the Kuiper belt, New Horizons has also imaged Haumea, Eris, Makemake and Quaoar at phase angles impossible from Earth, which constrains how their surfaces scatter light.[^verbiscer2022] China's proposed pair of Shensuo probes to the outer Solar System has considered Quaoar as a flyby target.[^jones2021] The explorer draws the dwarf planets as individual bodies on their catalogued orbits, taken from the JPL Small-Body Database; their sizes on screen follow the explorer's display size law rather than true scale.[^jpl-sbdb] ## Similar objects Several bodies resemble dwarf planets physically but fail the orbital part of the definition, or once met the physical part and no longer do. The groups overlap: [[Triton_(moon)|Triton]] belongs to both of the classes below. ### Former dwarf planets [[4_Vesta|Vesta]], the second most massive body in the asteroid belt, was round once but was deformed after it solidified by giant impacts, and its present shape is not in equilibrium.[^thomas1997][^russell2012] Triton, more massive than Pluto or Eris, is thought to be a dwarf planet captured by [[Neptune]], probably from a binary pair, which would explain its backward orbit.[^agnor2006] Saturn's moon Phoebe, a captured body, was probably round early on, when radioactive heating kept it soft.[^cook2012] ### Planetary-mass moons Many moons are massive enough to be round, and the largest of them, among them the [[Moon]], [[Titan_(moon)|Titan]] and [[Ganymede_(moon)|Ganymede]], outweigh Pluto and Eris; the Moon alone has more than five times Pluto's mass (derived).[^nasa-fs] Such moons are physically like dwarf planets but orbit planets rather than the Sun, and some planetary scientists argue that they should be classed as planets for that reason.[^metzger2022] Stern calls them "satellite planets", and the broader term "planemo" covers all planetary-mass objects.[^discovery2010][^basri2006] Charon is a special case: the IAU treats it as Pluto's satellite, although the centre of mass of the pair lies outside Pluto,[^buie2006] and has left open whether it might later qualify as a dwarf planet.[^iau-pluto] ## See also - [[IAU_definition_of_planet]] - [[Clearing_the_neighbourhood]] - [[Small_Solar_System_body]] - [[Trans-Neptunian_object]] - [[Kuiper_belt]] · [[Scattered_disc]] · [[Detached_object]] - List of possible dwarf planets (plain text: not yet on Wikitube) ## Notes Where a figure is marked "(derived)" it is computed here from the cited values: Eris's mass of 1.66 × 10²² kg divided by Pluto's 1.303 × 10²² kg gives 1.27. The census count of dwarf planets shown in the explorer is the explorer's own tally, not an IAU list. ## 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/ [^iau-qa]: International Astronomical Union (2006). "'Planet definition' questions & answers sheet". https://iauarchive.eso.org/static/archives/releases/doc/iau0601_q_answers.doc [^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/ [^iau-pluto]: International Astronomical Union. 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"Dawn at Vesta: Testing the protoplanetary paradigm". *Science* 336: 684–686. https://doi.org/10.1126/science.1219381 [^agnor2006]: Agnor, C. B.; Hamilton, D. P. (2006). "Neptune's capture of its moon Triton in a binary–planet gravitational encounter". *Nature* 441: 192–194. https://doi.org/10.1038/nature04792 [^cook2012]: Cook, J.-R. C.; Brown, D. (26 April 2012). "Cassini finds Saturn moon has planet-like qualities". NASA/JPL. http://saturn.jpl.nasa.gov/news/newsreleases/newsrelease20120426/ [^discovery2010]: "Should large moons be called 'satellite planets'?" (14 May 2010). *Discovery News*. http://news.discovery.com/space/should-large-moons-be-called-satellite-planets.html [^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 ## External links - NASA Science: Dwarf planets. https://science.nasa.gov/dwarf-planets/ - IAU Working Group for Planetary System Nomenclature: Planets and dwarf planets. https://planetarynames.wr.usgs.gov/Page/Planets - JPL Small-Body Database Lookup. https://ssd.jpl.nasa.gov/tools/sbdb_lookup.html - Brown, M. E. "How many dwarf planets are there in the outer solar system?". http://www.gps.caltech.edu/~mbrown/dps.html ## Wikipedia : Wikitube **Strict pair:** [Wikipedia](https://en.wikipedia.org/wiki/Dwarf_planet) : [Wikitube](https://en.wikitube.io/wiki/Dwarf_planet) · pinned revision [1373894932](https://en.wikipedia.org/w/index.php?oldid=1373894932) · 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-031 · explorer state `?view=census`.*