Pluto
Pluto is a dwarf planet located in the Kuiper Belt, a doughnut-shaped region of icy bodies beyond the orbit of Neptune in the outer Solar System.[1] It is the largest known object in the Kuiper Belt and the ninth-largest body directly orbiting the Sun, with an average distance of about 3.7 billion miles (5.9 billion kilometers or 39 astronomical units) from the Sun.[2] Discovered on February 18, 1930, by American astronomer Clyde Tombaugh at Lowell Observatory in Arizona using photographic plates and a blink comparator, Pluto was initially considered the ninth planet in the Solar System.[2] The name "Pluto" was suggested by 11-year-old Venetia Burney from Oxford, England, after the Roman god of the underworld, and was selected from several proposals forwarded to the observatory.[2] In 2006, the International Astronomical Union (IAU) reclassified Pluto as a dwarf planet, defining it as a celestial body that orbits the Sun, has sufficient mass to assume a nearly round shape due to hydrostatic equilibrium, but has not cleared its orbital neighborhood of other debris.[3] This decision, adopted via Resolution B5 at the IAU's General Assembly in Prague, resulted in the Solar System having eight planets (Mercury through Neptune) and distinguished dwarf planets as a separate category, with Pluto serving as the prototype for trans-Neptunian objects.[4] Pluto's highly elliptical orbit, which takes 248 Earth years to complete and carries it between 30 and 49 AU from the Sun, intersects the paths of other Kuiper Belt objects, preventing it from meeting the full planet criteria.[2] Its rotation is retrograde, with a day lasting approximately 153 hours (6.4 Earth days) and a significant axial tilt of approximately 120 degrees.[5] Physically, Pluto has an equatorial diameter of about 1,477 miles (2,377 kilometers), roughly one-fifth the width of Earth and two-thirds that of the Moon, with a mass about one-sixth of the Moon's.[2] Its surface, revealed in detail by NASA's New Horizons spacecraft flyby in July 2015, features rugged mountains up to 6,500–9,800 feet (2–3 kilometers) high, vast plains of frozen nitrogen, valleys, craters, and possible cryovolcanoes, all covered in water ice, nitrogen ice, and frozen methane.[2] Pluto maintains a thin atmosphere composed primarily of nitrogen, with traces of methane and carbon monoxide, which seasonally expands as it approaches the Sun and freezes onto the surface when farther away; its average surface temperature is around -387°F (-232°C).[2] The dwarf planet has five confirmed moons: Charon, the largest and nearly half Pluto's size, forming a binary system where the two bodies tidally lock and orbit a common center of gravity; and the smaller moons Styx, Nix, Kerberos, and Hydra.[2] The New Horizons mission, launched by NASA in 2006, provided the first close-up images and data of Pluto, confirming its geological complexity and hinting at active processes beneath its icy exterior, thus reshaping understanding of dwarf planets and the Kuiper Belt's role in Solar System formation.[2] Pluto's reclassification sparked ongoing debate among astronomers about planetary definitions, but it remains a key object for studying the early Solar System's icy remnants and potential building blocks of larger worlds.[6]History
Discovery
In the early [20th century](/page/20th century), Percival Lowell proposed the existence of a hypothetical ninth planet, dubbed Planet X, to explain perceived irregularities in the orbit of Uranus, publishing his predictions in a 1915 Lowell Observatory memoir that estimated its mass to be several times that of Earth and its location in the constellation Gemini.[7] William H. Pickering contributed early orbital predictions for this body, suggesting in 1910 a highly elliptical orbit with a perihelion near 35 astronomical units and specific right ascension and declination coordinates that guided subsequent searches.[8] To pursue Lowell's hypothesis, Lowell Observatory hired 23-year-old Clyde Tombaugh in 1929, tasking him with a systematic sky survey using a 13-inch astrograph telescope to capture photographic plates of regions beyond Neptune, which he then compared pairwise with a blink comparator to detect any moving objects against the fixed star field.[7] Tombaugh's methodical examination of these plates, taken nights or weeks apart, spanned from April 1929 to early 1930, covering millions of stars in targeted areas.[9] On February 18, 1930, Tombaugh identified a faint moving object on plates exposed on January 23 and 29 in the constellation Gemini, approximately 6 degrees from Lowell's predicted position, confirming its planetary nature through subsequent observations that revealed its slow eastward motion.[7] Initial photometric and positional analyses suggested the object was unexpectedly large and massive, with early estimates placing its diameter up to 7,000 miles—comparable to or exceeding Earth's—and a mass potentially sufficient to perturb outer planets, aligning loosely with Planet X expectations despite discrepancies.[10]| Discovery details |
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Naming and symbol
Following the discovery of the new celestial body on February 18, 1930, at Lowell Observatory, the naming process began promptly, with the observatory receiving hundreds of suggestions from the public and astronomers alike. Among these, the name "Pluto" was proposed by 11-year-old Venetia Burney of Oxford, England, on March 14, 1930, during breakfast with her mother and grandfather, Falconer Madan, a former librarian at the Bodleian Library. Inspired by the Roman god of the underworld—equivalent to the Greek Hades—Burney chose the name to evoke the object's distant, cold, and shadowy nature in the outer solar system; additionally, the initial "P" honored Percival Lowell, whose search for "Planet X" had motivated the discovery. Madan forwarded the suggestion to Herbert Hall Turner, the Astronomer Royal of the University of Oxford, who relayed it to the Lowell Observatory staff, where it quickly gained favor as one of over 150 similar proposals.[11][12] Other prominent suggestions included "Minerva," the Roman goddess of wisdom, which topped a public contest with about 200,000 entries but was rejected because it had already been assigned to an asteroid discovered in 1852; "Cronus," the Greek Titan and father of Zeus, was one of the final three names considered by the observatory team alongside Pluto and Minerva. On May 1, 1930, V.M. Slipher, director of Lowell Observatory, officially announced "Pluto" as the name in the Lowell Observatory Circular, crediting Burney as the originator; the choice aligned with astronomical tradition of mythological nomenclature while distinguishing the body from inner planets. Burney received a £5 reward (equivalent to about £420 in 2025) for her contribution, though she later expressed that the recognition came mostly in her later years.[13][12][6][14]| Designations |
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Classification
Upon its discovery on February 18, 1930, by Clyde Tombaugh at Lowell Observatory, Pluto was immediately classified as the ninth planet in the solar system, fulfilling the long-sought Planet X hypothesized by Percival Lowell to explain perturbations in the orbits of Uranus and Neptune.[7][21] The announcement was made on March 13, 1930, and Pluto was accepted as a major planet due to its position beyond Neptune and the era's limited understanding of the outer solar system.[2] By the mid-20th century, observations revealed Pluto to be significantly smaller than anticipated for Planet X, with a diameter of about 2,377 kilometers—about two-thirds that of the Moon's 3,475 kilometers—and insufficient mass to cause the predicted orbital perturbations, leading to doubts about its planetary status as early as the 1940s.[22][10][23] These findings, combined with improved measurements showing Pluto's density and composition more akin to icy bodies than the terrestrial or gas giant planets, prompted astronomers to question its uniqueness, though it retained planetary designation for decades.[24] The 1992 discovery of the first Kuiper Belt object, 1992 QB1, by David Jewitt and Jane Luu marked a turning point, revealing a vast population of trans-Neptunian objects (TNOs) similar to Pluto in composition and orbit, challenging its outlier status among planets.[25] Subsequent finds, including larger bodies like Eris in 2005, highlighted Pluto as merely the brightest and largest known member of this scattered disk within the Kuiper Belt, eroding the notion of it as a solitary ninth planet.[26][27] On August 24, 2006, the International Astronomical Union (IAU) formalized a new definition of a planet as a celestial body that orbits the Sun, is nearly spherical due to its own gravity, and has cleared its orbital neighborhood of other debris; Pluto met the first two criteria but failed the third, as it shares its orbit with numerous TNOs, resulting in its reclassification as the prototype dwarf planet.[28][29] This decision, passed by a vote of 237 in favor, 157 against, and 17 abstentions among attending members in Prague, reduced the solar system's planets to eight and introduced the dwarf planet category for objects like Pluto, Ceres, and Eris.[30][31] Debates persist, with planetary scientists advocating alternative definitions, such as geophysical criteria emphasizing roundness and hydrostatic equilibrium over orbital clearance, which would reinstate Pluto as a planet alongside Earth's Moon and other rounded bodies.[32][33] These arguments gained traction post-New Horizons flyby in 2015, revealing Pluto's complex geology, while cultural backlash includes public campaigns like petitions and the "Pluto planet" advocacy by groups such as the New Mexico House of Representatives, which passed a 2007 resolution declaring Pluto "an official planet" of their state.[34][35] In July 2024, a group of astronomers proposed a new planet definition based on geophysical properties like hydrostatic equilibrium, which would classify thousands more bodies as planets, including some dwarf planets, but this has not been endorsed by the IAU, and Pluto remains a dwarf planet as of November 2025.[33]Orbit and rotation
Orbit
Pluto's orbit is highly elliptical, with a semi-major axis of 39.482 AU, an eccentricity of 0.2488, and an orbital inclination of 17.16° relative to the ecliptic plane. This eccentricity results in a perihelion distance of 29.658 AU, which brings Pluto temporarily inside the orbit of Neptune, and an aphelion of 49.305 AU.[5][2] The orbital period is 247.94 Earth years, during which Pluto travels at an average speed of 4.743 km/s.[2]| Orbital Parameter | Value |
|---|---|
| Semi-major axis | 39.482 AU |
| Eccentricity | 0.2488 |
| Orbital period (sidereal) | 247.94 Earth years |
| Aphelion | 49.305 AU |
| Perihelion | 29.658 AU |
| Inclination | 17.16° |
| Average orbital speed | 4.743 km/s |
Rotation
Pluto exhibits retrograde rotation, spinning from east to west opposite to its orbital direction around the Sun. Its sidereal rotation period is 6.387 Earth days, equivalent to approximately 153.29 hours.[41] This rotation period is exactly synchronized with the orbital period of its largest moon, Charon, due to mutual tidal locking in the Pluto-Charon binary system. In this configuration, Pluto and Charon always present the same face to each other, a result of their close proximity and significant mass ratio, where Charon comprises about 12% of Pluto's mass.[5] Pluto's axial tilt, or obliquity, measures 122.5° relative to the normal of its orbital plane, confirming its retrograde sense of rotation and classifying it among the most extremely tilted bodies in the Solar System, similar to Uranus. This pronounced tilt causes dramatic seasonal variations in solar insolation across Pluto's surface, with polar regions enduring extended periods of either continuous sunlight or perpetual darkness lasting up to a century during its 248-year orbit. Such dynamics influence the distribution of volatiles like nitrogen and methane, driving atmospheric and surface changes over long timescales.[43][44][5] The history of Pluto's rotational dynamics is shaped by intense tidal interactions with Charon, which have evolved the system from an initially more eccentric orbit to its current synchronous state. These tides dissipated angular momentum, slowing Pluto's spin and expanding Charon's orbit until achieving 1:1 spin-orbit resonance, a process modeled to have occurred over billions of years following the system's formation from a giant impact.[45][46] Ground-based photometric observations, spanning decades, have utilized rotational light curves to precisely determine Pluto's period and detect subtle variations attributable to its irregular shape and surface albedo contrasts. These light curves, obtained from telescopes monitoring brightness changes over multiple rotations, reveal a double-peaked profile with an amplitude of about 0.05 magnitudes, confirming the period and indicating deviations from a perfect sphere due to tidal and rotational forces.Physical characteristics
| Parameter | Value | Unit | Notes/Source |
|---|---|---|---|
| Equatorial diameter | 2,376.6 ± 1.6 | km | New Horizons data |
| Mean radius | 1,188.3 ± 0.8 | km | New Horizons data |
| Mass | (1.303 ± 0.003) × 10²² | kg | Equivalent to 0.00218 Earth masses |
| Surface area | 16.7 | million km² | Comparable to Russia's land area |
| Density | 1.854 ± 0.004 | g/cm³ | Consistent with icy composition |
| Primary composition | Water ice, nitrogen ice, methane, carbon monoxide | - | Surface volatiles and rocky core |
Size and mass
Pluto has a mean radius of 1188.3 ± 1.6 km, resulting in an equatorial diameter of 2376.6 km, or about one-fifth the diameter of Earth.[41] This makes Pluto the largest known trans-Neptunian object, with a surface area of approximately 16.7 million square kilometers, comparable to the land area of Russia. The mass of Pluto is (1.303 ± 0.003) × 10^{22} kg, equivalent to 0.00218 Earth masses or about 17.7% of the Moon's mass. This value is primarily derived from the gravitational perturbations observed in the orbit of its largest satellite, Charon, which orbits a common barycenter with Pluto at a distance of about 19,591 km with a period of 6.387 days, allowing Kepler's third law to yield the system's total mass; the individual masses are then apportioned based on their mass ratio of approximately 8:1. Measurements from the New Horizons spacecraft's radio science experiment in 2015 further refined this estimate by analyzing Doppler shifts in radio signals during the flyby, achieving precision on the order of 0.1%.[47] Pluto's mean density is 1.854 ± 0.004 g/cm³, a value consistent with an icy composition comprising water ice, frozen volatiles, and a substantial rocky core, as inferred from the balance of its mass and volume.[48] This density is similar to that of other large trans-Neptunian objects, such as Eris at 1.98 g/cm³, reflecting a common formation history in the outer solar system where ice dominates but rock contributes significantly to the interior.[49] Early estimates of Pluto's mass in the 1930s, based on assumed perturbations in the orbits of Uranus and Neptune, were significantly overestimated, ranging from 0.5 to 1.0 Earth masses, as astronomers sought to explain discrepancies later attributed to observational errors rather than a massive perturber.[50] These figures were progressively refined in the mid-20th century through improved planetary ephemerides, and dramatically reduced after the 1978 discovery of Charon, which enabled direct mass calculation from their binary orbit; stellar occultations in the 1980s and 1990s provided better size constraints, while the New Horizons flyby in 2015 confirmed the current parameters with high accuracy.[50]Internal structure
Pluto's internal structure is modeled as a differentiated body consisting of a rocky core enveloped by a water-ice mantle and an outer crust primarily composed of nitrogen and carbon dioxide ices. The rocky core is estimated to have a radius of approximately 850 km, comprising silicates and possibly iron, which accounts for a significant portion of Pluto's mass given its overall density of around 1.86 g/cm³.[48][2] The overlying mantle, roughly 300-400 km thick, is dominated by water ice in various phases, potentially including high-pressure forms at depth.[51] The thin crust, estimated at 10-50 km thick, features volatile ices that can sublimate and redistribute seasonally.[52] A key aspect of these models is the potential presence of a subsurface liquid water ocean, estimated at 40-80 km thick in recent models, maintained beneath the icy mantle.[53] This ocean is inferred from evidence of cryovolcanic activity and the need for internal heat to sustain geological processes, with the ocean layer potentially thinner or absent in some regions due to localized freezing. Recent models suggest this ocean is highly saline, with a density approximately 8% greater than Earth's seawater.[53] Tidal heating from Pluto's large moon Charon, combined with radiogenic decay in the rocky core, provides the primary heat sources to prevent complete freezing and drive past differentiation into layers.[54] These mechanisms likely facilitated early separation of rock from ice, with tidal interactions in the binary system enhancing heat flux compared to isolated bodies.[46] Data from the New Horizons spacecraft reveal gravity field anomalies, particularly a positive anomaly over the Sputnik Planitia basin, indicating an uplifted, denser subsurface layer consistent with a rocky core or concentrated silicates beneath the ices.[52] Additionally, interpretations of impact crater morphologies and antipodal terrains suggest seismic wave propagation from ancient impacts, implying a thick subsurface ocean that transmitted energy to deform the opposite hemisphere and a hydrated core with altered minerals.[55] Pluto's internal evolution, driven by its binary orbit with Charon, contrasts with other icy bodies like Europa, where tidal heating from Jupiter's satellites sustains a global ocean but without the mutual tidal locking and spin-orbit resonance that uniquely shaped Pluto's differentiation and potential ocean persistence.[54][46]Surface
Pluto's surface is a complex mosaic of icy terrains shaped by geological processes, dominated by volatile ices and featuring a variety of landforms revealed in detail by the New Horizons spacecraft flyby in 2015. The most prominent feature is Sputnik Planitia, a vast, glacier-like plain of nitrogen ice approximately 1,000 km across, forming the western lobe of the heart-shaped Tombaugh Regio near Pluto's equator. This basin, likely an ancient impact crater, exhibits convective overturn of its icy surface, with blocks of water ice embedded in the nitrogen layer, contributing to its smooth, reddish appearance due to tholins—organic compounds formed by irradiation of methane. Surrounding Sputnik Planitia are rugged mountainous regions, such as Tenzing Montes and Hillary Montes, rising up to 3.5 km above the surrounding plains and composed primarily of water ice, which is more rigid than the volatile ices at lower elevations. These mountains lack significant impact cratering, suggesting relatively recent tectonic uplift or exhumation from the subsurface. Cryovolcanic features, including the large mound Wright Mons with a central depression resembling a caldera, indicate past eruptions of icy slurries, possibly driven by subsurface heat sources, adding to the diverse topography. The surface composition varies regionally, with volatile ices like nitrogen (N₂), methane (CH₄), and carbon monoxide (CO) dominating the equatorial lowlands, where they form bright, reflective plains and seasonal dunes up to 300 meters high, as imaged by New Horizons. In contrast, the higher latitudes and rugged highlands are rich in water ice (H₂O), which forms the backbone of Pluto's crust and appears in bladed terrains—sharp, knife-edge ridges up to 500 meters tall, possibly resulting from sublimation or freeze-thaw cycles. Glacial flows, such as those flowing from the highlands into Sputnik Planitia, demonstrate active mass wasting, with nitrogen ice behaving like a fluid over geological timescales. Impact craters on Pluto are sparse and predominantly young, with fewer than expected for its age, implying widespread resurfacing by cryovolcanism, glaciation, or volatile transport; notable examples include the 270-km-wide Burney Basin, filled with dark, tholin-rich material, and smaller craters like those on Viking Terræ, which show ejecta blankets modified by ice flows. Polar regions feature darker, volatile-depleted caps, while the equatorial plains remain brighter due to nitrogen frost accumulation, influencing Pluto's overall albedo and undergoing seasonal variations as volatiles migrate with its 248-year orbit. These patterns highlight the dynamic interplay of sublimation, deposition, and geological activity in maintaining Pluto's varied surface.Atmosphere
Pluto's atmosphere is a tenuous envelope, primarily composed of molecular nitrogen (N₂), which constitutes over 99% of its volume near the surface, with trace amounts of methane (CH₄) at approximately 0.5% and carbon monoxide (CO) at less than 0.1%.[56] The surface pressure is roughly 10 μbar, equivalent to about one ten-thousandth of Earth's sea-level pressure, as measured during the New Horizons flyby in 2015.[57] This low pressure results from the sublimation of surface nitrogen ices, which directly feeds the gaseous layer.[58] Photochemical processes in the upper atmosphere, driven by ultraviolet solar radiation, produce complex organic compounds known as tholins from the interaction of nitrogen and methane molecules.[59] These reactions form aerosol particles that create multiple haze layers extending up to 200 km above the surface, scattering blue light and giving Pluto's sky its observed bluish hue.[60] The hazes are dynamic, with particles undergoing continuous formation and sedimentation, and atmospheric escape occurs mainly through thermal Jeans escape, where high-velocity molecules in the cold upper atmosphere (around 70 K) overcome Pluto's weak gravity.[61] Due to Pluto's highly eccentric orbit, the atmosphere exhibits pronounced seasonal variability: it expands significantly near perihelion as increased solar insolation sublimates volatile ices, increasing pressure and extent, while at aphelion it contracts and largely collapses onto the surface as temperatures drop.[62] These changes have been tracked over decades using ground-based stellar occultations, which reveal fluctuations in atmospheric density and radius, with the expansion phase observed from the 1980s through the 2015 New Horizons encounter.[63] Data from New Horizons' instruments, including the Alice ultraviolet spectrograph and REX radio science experiment, provided the first direct measurements of the atmospheric structure, revealing a temperature profile that decreases from about -180°C in the lower layers to -220°C near the surface, with a stratopause at around 110 K.[64] The mission also detected zonal wind patterns driven by sublimation from surface volatiles, circulating up to altitudes of 100 km and influencing global transport of haze and gases.[65]Satellites
Main satellites
Pluto's primary satellite system consists of five known moons, with Charon being the largest and most significant, orbiting the common barycenter of the Pluto-Charon binary. The smaller moons—Styx, Nix, Kerberos, and Hydra—also orbit this barycenter at greater distances, forming a compact and dynamically complex arrangement. This configuration likely originated from a giant impact between Pluto and another Kuiper Belt object approximately 4.5 billion years ago, which ejected debris that coalesced into the moons.[2] The following table summarizes key parameters of Pluto's five main satellites, based on data from NASA's New Horizons mission and ground-based observations:| Moon | Discovery Date | Discoverer/Method | Mean Diameter (km) | Orbital Period (days) | Semi-major Axis (km) |
|---|---|---|---|---|---|
| Charon | June 22, 1978 | James W. Christy (ground) | 1,213 | 6.4 | 19,596 |
| Styx | 2012 | Hubble Space Telescope | 10–13 | 20.5 | 42,456 |
| Nix | 2005 | Hubble Space Telescope | ~40 | 24.9 | 48,687 |
| Kerberos | 2011 | Hubble Space Telescope | 10–13 | 32.2 | 57,783 |
| Hydra | 2005 | Hubble Space Telescope | ~40 | 38.2 | 64,738 |