# Pluto <!-- SOLSIM:BEGIN g31 — Solar System explorer state (hand-built on wt-core, specs/solar/); do not hand-edit inside --> **Microsim — three.js (Wikitube framework):** *Pluto in the Solar System explorer* <div class="wt-sim" data-src="https://wikitube-3d-microsims.netlify.app/solar/Solar_System.html?obj=Pluto&embed=1" data-title="Pluto in the Solar System explorer"></div> *The Solar System explorer locked on this article's state (`?obj=Pluto`); every object and population of the [[PORTAL_Solar_System|Solar System portal]] has its own state in the same scene.* <!-- SOLSIM:END --> *Try: drag the year slider to 1989 and find Pluto at the near end of its orbit, just inside Neptune's path, then set the speed to 100 years/s and count how Neptune circles the Sun three times while Pluto goes round twice; switch the scale to true to open Pluto's own frame and see Charon circling it at its true distance.* **Pluto** (minor-planet designation 134340 Pluto) is a [[Dwarf_planet|dwarf planet]] of the [[Kuiper_belt|Kuiper belt]], an icy, rock-bearing body counted as the ninth planet from its discovery in 1930 until the [[IAU_definition_of_planet|International Astronomical Union's 2006 definition of a planet]] placed it among the dwarf planets.[^fs][^iau2006] Its radius is about 1,188 km and its mass 1.30 × 10²² kg, less than a fifth of the [[Moon]]'s (derived).[^fs][^nasa-fs] Among [[Trans-Neptunian_object|trans-Neptunian objects]] it is the largest known by volume, although [[Eris_(dwarf_planet)|Eris]] is more massive.[^sicardy2011][^holler2021] Its orbit, tilted about 17° to the [[Ecliptic|ecliptic]], runs from 29.6 to 49.6 [[Astronomical_unit|AU]] from the [[Sun]], so that for about twenty years in every 248 Pluto is closer to the Sun than [[Neptune]]; a 2:3 resonance keeps the two far apart.[^sbdb][^jpl1999][^malhotra1997] Of its five moons, [[Charon_(moon)|Charon]] is massive enough that the pair circles a point outside Pluto.[^fs] The 2015 New Horizons flyby found nitrogen-ice plains, water-ice mountains, glaciers and a thin, layered haze.[^stern2015] The explorer at the top of this page is locked on Pluto: it draws the orbit from JPL elements, with perihelion inside Neptune's orbit, and its true-scale mode opens Pluto's own frame with Charon.[^sbdb] ## History ### Discovery After Neptune was found in 1846 from its pull on [[Uranus]], small unexplained residuals in Uranus's motion suggested a further planet.[^croswell1997] Percival Lowell searched for this "Planet X" from 1906 until his death in 1916 without success.[^tombaugh1946] His plates had in fact caught Pluto on 19 March and 7 April 1915; the earliest known image is a Yerkes Observatory plate of 20 August 1909.[^hoyt1976][^buchwald2000] A dispute over Lowell's estate delayed a new search until 1929, when Vesto Slipher gave the task to Clyde Tombaugh, a 23-year-old newcomer to the observatory.[^croswell1997] Tombaugh photographed each field twice, days apart, and flipped between the plates in a blink comparator so that anything that had moved appeared to jump. On 18 February 1930 he found such a jump on plates from 23 and 29 January; the observatory announced the discovery on 13 March 1930.[^croswell1997][^hoyt1976] ### Name More than a thousand names were suggested.[^rao2005] The winner, after the Roman god of the underworld, came first from Venetia Burney, an eleven-year-old in Oxford, whose suggestion reached Lowell Observatory by cable on 16 March; the staff adopted it unanimously and published it on 1 May 1930.[^tombaugh1980][^rincon2006][^schindler2018] Its first two letters are Percival Lowell's initials.[^schindler2018] ### Symbol Pluto's planetary symbol is a monogram of P and L, now rarely used in astronomy; astrologers mostly use an orb above a bident, a sign dating from the early 1930s.[^rudhyar1936] In 1941 Glenn Seaborg named the element [[Plutonium|plutonium]] after Pluto, continuing the sequence of uranium and neptunium.[^clark2000] ### Planet X disproved Pluto was far too faint to be Lowell's planet, and estimates of its mass fell for half a century: about one Earth mass in 1931 and still 0.11 Earth masses in 1971, both from assumed perturbations of the giant planets.[^nicholson1930][^seidelmann1971] In 1976 Cruikshank, Pilcher and Morrison detected methane frost, which implied a bright surface and therefore a small body of at most about 1 percent of Earth's mass.[^cruikshank1976] Charon's discovery in 1978 allowed a direct measurement, about 0.2 percent of Earth's mass, too little to disturb Uranus.[^christy1978] In 1993 Myles Standish showed that the residuals vanish once Neptune's mass, found by Voyager 2 in 1989 to be about 0.5 percent lower than assumed, is corrected; no Planet X is needed.[^standish1993] ### Classification From 1992, beginning with 1992 QB1, surveys found many bodies orbiting in Pluto's region, and Pluto came to be seen as the largest member of a population.[^jewitt1993] In 2005 Michael Brown's team announced Eris, a body now known to be more massive than Pluto.[^nasa2005] In August 2006 the IAU defined a planet as a body that orbits the Sun, is massive enough for gravity to make it nearly round, and has [[Clearing_the_neighbourhood|cleared the neighbourhood]] of its orbit; bodies meeting only the first two conditions became dwarf planets.[^iau2006] Pluto fails the third test by far: it has 0.07 times the mass of the other bodies in its orbital zone, against 1.7 million for Earth.[^soter2006] It received the minor-planet number 134340 in September 2006.[^green2006] The decision remains contested. Alan Stern, principal investigator of New Horizons, criticised it and the small number of astronomers who voted, and some planetary scientists argue that geology, not orbital dominance, should define a planet.[^shiga2006][^metzger2022] In 2024 Arizona, where Pluto was found, named it the official state planet.[^npr2024] ## Orbit Pluto takes 247.94 years to circle the Sun. With an eccentricity near 0.25 and an inclination of 17°, it ranges between about 29.6 AU at perihelion and 49.6 AU at aphelion.[^fs][^sbdb] It was inside Neptune's orbit from 7 February 1979 to 11 February 1999, and the Pluto–Charon barycentre passed perihelion on 5 September 1989.[^jpl1999][^horizons] At the mean distance of 39.5 AU, sunlight takes about 5.5 hours to arrive (derived). Over long times the motion is [[Chaos_theory|chaotic]]: Sussman and Wisdom's 1988 integration showed that small errors in Pluto's position grow by a factor of e every 10–20 million years, so its place along the orbit cannot be forecast beyond that horizon, although the orbit's shape and resonance persist.[^sussman1988] The semi-major axis oscillates between about 39.3 and 39.6 AU over roughly 20,000 years.[^williams1971] The explorer draws Pluto's present osculating orbit from JPL's Small-Body Database, so the crossing inside Neptune's path and the 17° tilt are real; the dot is enlarged for visibility.[^sbdb] ### Relationship with Neptune From above, the orbits appear to cross, but the bodies never meet. The main protection is a mean-motion [[Resonance|resonance]]: Pluto completes two orbits for every three of Neptune's, a pattern that repeats about every 495 years and keeps Neptune far away whenever Pluto is near perihelion. At the 1989 perihelion Neptune was about 57° of longitude ahead, and the two never come closer than about 17 AU, farther than Pluto's closest approach to Uranus, about 11 AU.[^horizons][^malhotra1997] Pluto is the prototype of the [[Plutino|plutinos]], the [[Resonant_trans-Neptunian_object|resonant trans-Neptunian objects]] sharing this ratio. The resonance corrects itself. If Pluto drifts ahead of the exact ratio, it approaches Neptune from behind near perihelion, gains [[Angular_momentum|angular momentum]], moves to a slightly larger orbit and, by [[Kepler's_laws_of_planetary_motion|Kepler's third law]], slows until it falls back; one such libration takes about 20,000 years.[^malhotra1997][^cohen1965] In addition, Pluto's argument of perihelion librates about 90°, so perihelion always falls about 8 AU above Neptune's orbital plane, and the nodes of the two orbits move in step with that libration, a "1:1 superresonance" to which all four giant planets contribute.[^williams1971][^wan2001] ## Rotation Pluto turns once every 6.387 days, the period of Charon's orbit, and spins retrograde with its axis tilted about 120° to its orbit.[^fs] Its poles therefore lie close to the orbital plane, as Uranus's do, and around the solstices much of each hemisphere spends decades in continuous daylight or darkness. The tilt may not be primordial. A spinning body settles with excess mass near its equator. Keane and colleagues argued in 2016 that nitrogen ice loaded into the Sputnik Planitia basin created such an excess and rolled Pluto over until the basin lay near the Pluto–Charon axis, a process called true polar wander, and that the stresses explain faults across the surface.[^keane2016] Sputnik Planitia now sits near 180° longitude, on the hemisphere facing directly away from Charon, where such a reorientation would place it.[^keane2016][^grundy2013] Because Pluto and Charon are tidally locked to each other, Charon hangs motionless in Pluto's sky over one hemisphere and never rises over the other.[^fs] ## Geology ### Surface Pluto's surface is dominated by volatile ices. Infrared [[Spectroscopy|spectroscopy]] in 1993 showed that the bright plains are more than 98 percent [[Nitrogen|nitrogen]] ice, with some methane and carbon monoxide; later monitoring found nitrogen and carbon monoxide concentrated on the hemisphere facing away from Charon and methane strongest near 300° east.[^owen1993][^grundy2013] These ices flow and sublimate readily, whereas the mountains are [[Water|water]] ice, rigid enough at surface temperatures below 40 K to act as bedrock.[^stern2015][^fs] Sputnik Planitia, the western lobe of the bright, heart-shaped Tombaugh Regio, is a basin about 1,000 km across and 3 km deep, generally read as an ancient impact site filled with nitrogen, carbon monoxide and methane ice.[^schenk2018] Its ice is divided into polygonal cells tens of kilometres across, which two independent studies explained as the tops of slow [[Convection|convection]] cells in a nitrogen-ice layer a few kilometres thick.[^mckinnon2016][^trowbridge2016] New Horizons saw no craters on the plain, capping its age at 10 million years; the pattern of sublimation pits implies ice creeping at about 10 cm per year and a surface about 180,000 years old.[^marchis2016][^buhler2017] Nitrogen glaciers flow into the basin from the uplands,[^umurhan2016] and on its western side transverse dunes with crests 0.4–1 km apart are probably built of methane-ice grains 200–300 μm across.[^telfer2018] Dark, reddish equatorial terrain such as Belton Regio contrasts with bright frosted areas, and southwest of Sputnik Planitia the broad mountains Wright Mons and Piccard Mons may be clusters of cryovolcanic domes.[^stern2018][^singer2022] ### Internal structure A density of 1,854 kg/m³ implies a mixture of rock and ice.[^fs] Heat from [[Radioactive_decay|radioactive decay]] should have separated the two early on, leaving a rocky core under a water-ice mantle; pre-flyby models put the core at about 1,700 km across, 70 percent of Pluto's diameter, and allowed an ocean 100–180 km thick above it.[^hussmann2006] Evidence since favours an ocean: Sputnik Planitia's position near the tidal axis suggests a mass excess, most easily explained if dense liquid water rose beneath the basin after the impact.[^keane2016] Bierson, Nimmo and Stern argued in 2020 that extensional faults record an ocean present since Pluto formed, implying a "hot start".[^bierson2020] ## Mass and size Pluto's mass, 1.30 × 10²² kg, is about 0.2 percent of Earth's and 18 percent of the Moon's (derived); seven moons of other planets, among them [[Triton_(moon)|Triton]], [[Titan_(moon)|Titan]], [[Europa_(moon)|Europa]] and [[Io_(moon)|Io]], are more massive.[^fs][^jplsats] It became measurable only after Charon was found, because the moon's distance and period give the pair's combined mass through [[Newton's_law_of_universal_gravitation|Newton's form of Kepler's third law]]: with Charon at 19,596 km and a period of 6.387 days, the system comes to about 1.46 × 10²² kg, 89 percent of it in Pluto (derived).[^fs][^christy1978] The size was harder, because the atmosphere and haze blur the limb in stellar occultations. Before the flyby, Lellouch and colleagues used atmospheric methane to argue for a diameter above 2,360 km, most likely 2,368 km.[^lellouch2015] New Horizons images settled it: Nimmo and colleagues derived a mean radius of 1,188.3 km, a diameter of 2,376.6 km, with no measurable flattening.[^nimmo2017] Pluto is thus slightly larger than Eris, about 2,326 km across from a 2010 occultation, although Eris has about a quarter more mass (derived).[^sicardy2011][^holler2021] ## Atmosphere Pluto's thin atmosphere is nitrogen with about half a percent methane and a trace of carbon monoxide, in [[Vapor_pressure|vapour-pressure]] balance with the surface ices; the surface pressure is about 13 microbar, roughly 1/78,000 of Earth's (derived).[^fs] Gas sublimates where sunlight falls and condenses where it is colder, so the atmosphere moves heat and frost around the globe and its pressure follows the seasons. Methane absorbs sunlight and warms the gas above the surface, producing a temperature inversion tens of kelvin warmer than the ice below.[^lellouch2009] New Horizons found the upper atmosphere colder than predicted, about 70 K rather than 100 K, and nitrogen escaping about 10,000 times more slowly than expected.[^beatty2016] Radio occultation showed at most a thin boundary layer, about 4 km thick at the site measured, at 37 ± 3 K.[^gladstone2016] Backlit haze formed about 20 regularly spaced layers up to about 150 km high, probably shaped by gravity waves raised by wind over the mountains.[^stern2015][^beatty2016] Stellar occultations monitor the atmosphere from Earth. Those up to 2013 showed no sign of the collapse some models predicted as Pluto receded from perihelion.[^olkin2015] Ten occultations from 2017 to 2023 showed the pressure steady from 2015 to 2021 and lower in 2022, by about 7 percent at a reference radius of 1,275 km.[^sickafoose2026] ## Natural satellites Pluto has five known moons. Charon, discovered by James Christy in 1978, has a radius of 606 km and about an eighth of Pluto's mass; Nix and Hydra were found in Hubble Space Telescope images in 2005, Kerberos in 2011 and Styx in 2012.[^christy1978][^fs][^weaver2006][^nasa2011][^wall2012] The four small moons are irregular, from about 10 km (Styx) to about 62 km (Hydra) across.[^fs] All the orbits are nearly circular, with eccentricities below 0.006, and lie within 1° of Pluto's equator, sharing its 120° tilt.[^buie2012][^showalter2015] The family is compact, orbiting within the innermost few percent of the region where Pluto could hold a prograde moon.[^stern2005] With Charon at 19,596 km and a mass ratio of 0.122, the barycentre lies about 2,100 km from Pluto's centre, some 900 km above its surface (derived), so both bodies circle a point in empty space and the small moons are circumbinary.[^fs] Each of the pair is tidally locked to the other, a state confirmed elsewhere only for Eris and its moon Dysnomia.[^szakats2023] The small moons tumble chaotically under the shifting pull of the binary.[^showalter2015] The favoured origin is a giant collision early in Solar System history whose debris formed the moons.[^nasa2015] ### Quasi-satellite The plutino 15810 Arawn shares Pluto's period and for part of the time appears, in Pluto's frame, to loop around it without being bound. De la Fuente Marcos and de la Fuente Marcos calculated in 2012 that this lasts about 350,000 years in every 2 million, and New Horizons observations refined the orbit.[^delafuente2012][^porter2016] Because Neptune controls Arawn's path far more than Pluto does, some authors doubt that the label applies.[^porter2016] ## Origin An early hypothesis, discussed by Gerard Kuiper, made Pluto an escaped moon of Neptune; it was dropped when dynamical studies showed that Pluto never comes near Neptune.[^kuiper1961][^stern1997] The discoveries from 1992 onward instead placed Pluto as the largest known body of the Kuiper belt, a leftover planetesimal of the [[Protoplanetary_disk|protoplanetary disc]].[^jewitt1993] Its resonant orbit is best explained by Neptune's outward migration. Malhotra showed in 1995 that a migrating Neptune would sweep its 2:3 resonance through the planetesimals, capture bodies on near-circular orbits and raise their eccentricities; Pluto could have started near 33 AU.[^malhotra1995] Later models such as the [[Nice_model|Nice model]] set the migration within an instability of the giant planets that also filled the [[Scattered_disc|scattered disc]] and supplied [[Jupiter_trojan|Jupiter's trojans]]; they need a primordial disc holding about a thousand Pluto-sized bodies.[^levison2008][^hahn2005] Neptune's moon Triton, larger than Pluto and with a similar nitrogen-rich surface, is thought to be one such body captured from the belt.[^planetary-triton] Glein and Waite argued that the amount of nitrogen at Sputnik Planitia fits a Pluto assembled largely from cometary material.[^glein2018] ## Observation and exploration ### Observation Pluto's mean opposition magnitude is 15.1, reaching 13.65 near perihelion; its disc never exceeds 0.11 arcseconds, so even large telescopes show a star-like point.[^fs] The first maps came from the mutual events of 1985–1990, when Pluto and Charon eclipsed each other as seen from Earth and the dips in their combined light revealed bright and dark regions.[^buie1992] Hubble Space Telescope images later resolved features several hundred kilometres across and showed the surface changing between 1994 and 2002–2003.[^buie2010] ### Exploration NASA's New Horizons, launched in 2006, took its first distant images of Pluto that September, swung past [[Jupiter]] in early 2007 and made its closest approach on 14 July 2015, 3,462 days after launch; it remains the only spacecraft to visit the system.[^jhuapl2006][^stern2015][^jhuapl2016] It mapped the geology and composition of Pluto and Charon and measured the atmosphere; the last flyby data, 6.25 gigabytes in all, reached Earth on 25 October 2016.[^jhuapl2016] Its closest pass was over the hemisphere facing away from Charon, so the Charon-facing side was imaged at lower resolution,[^rothery2015] and the dark southern regions only in light reflected from Charon.[^lauer2021] Mission scientists have proposed an orbiter that would map the whole surface and use Charon's gravity to adjust its orbit.[^wall2017] ## See also - [[Charon_(moon)]] - [[Plutino]] - [[Dwarf_planet]] · [[IAU_definition_of_planet]] - [[Kuiper_belt]] · [[Trans-Neptunian_object]] - [[Eris_(dwarf_planet)]] · [[Haumea]] · [[Makemake]] - [[Timeline_of_Solar_System_exploration]] - [[PORTAL_Solar_System|Solar System portal]] ## Notes Derived numbers are computed from the cited values: Pluto's mass relative to the Moon, 1.303 / 7.346 ≈ 0.18, and to Earth, 0.01303 / 5.972 ≈ 0.0022; the light time at 39.5 AU, 39.5 × 499 s ≈ 19,700 s ≈ 5.5 hours; the system mass from Kepler's third law, M = 4π²a³ / (GT²) with a = 1.9596 × 10⁷ m and T = 5.518 × 10⁵ s, ≈ 1.46 × 10²² kg; the Charon–Pluto mass ratio 1.586 / 13.03 ≈ 0.122; the barycentre's distance from Pluto's centre, 19,596 × 0.122 / 1.122 ≈ 2,130 km, which is about 940 km above a surface at 1,188 km; and the surface pressure ratio 13 μbar / 1,013 mbar ≈ 1 / 78,000; the Eris–Pluto mass ratio 1.6466 / 1.303 ≈ 1.26. ## References [^nasa-fs]: NASA NSSDCA. 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(1997). *Pluto and Charon*. University of Arizona Press. ISBN 978-0-8165-1840-1. - Stern, S. A.; Moore, J. M.; Grundy, W. M.; Young, L. A.; Binzel, R. P., eds. (2021). *The Pluto System After New Horizons*. University of Arizona Press. - Brown, M. E. (2010). *How I Killed Pluto and Why It Had It Coming*. Spiegel & Grau. ISBN 978-0-385-53108-5. ## External links - NASA Science: Pluto. https://science.nasa.gov/dwarf-planets/pluto/ - NASA NSSDCA Pluto Fact Sheet. https://nssdc.gsfc.nasa.gov/planetary/factsheet/plutofact.html - New Horizons mission (JHUAPL). https://pluto.jhuapl.edu/ - Lowell Observatory: Pluto research. https://lowell.edu/in-depth/pluto/pluto-research-at-lowell/ - USGS/IAU Gazetteer of Planetary Nomenclature: Pluto. https://planetarynames.wr.usgs.gov/ ## Wikipedia : Wikitube **Strict pair:** [Wikipedia](https://en.wikipedia.org/wiki/Pluto) : [Wikitube](https://en.wikitube.io/wiki/Pluto) · pinned revision [1373664343](https://en.wikipedia.org/w/index.php?oldid=1373664343) · 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-021 · explorer state `?obj=Pluto`.* <!-- hub_tags: Life_Physics · PORTAL_Solar_System -->