Pluto
Discovery | |||||||||
---|---|---|---|---|---|---|---|---|---|
Discovered by | Clyde W. Tombaugh | ||||||||
Discovery site | Lowell Observatory | ||||||||
Discovery date | February 18, 1930 | ||||||||
Designations | |||||||||
Designation | (134340) Pluto | ||||||||
Pronunciation | /ˈpluːtoʊ/ ⓘ | ||||||||
Named after | Pluto | ||||||||
Aphelion |
| ||||||||
Perihelion | |||||||||
| |||||||||
Eccentricity | 0.2488 | ||||||||
366.73 days[3] | |||||||||
Average orbital speed | 4.743 km/s[3] | ||||||||
14.53 deg | |||||||||
Inclination |
| ||||||||
110.299° | |||||||||
113.834° | |||||||||
Known satellites | 5 | ||||||||
Physical characteristics | |||||||||
Dimensions | 2,376.6±1.6 km (observations consistent with a sphere, predicted deviations too small to be observed)[5] | ||||||||
Mean radius | |||||||||
Flattening | <1%[7] | ||||||||
| |||||||||
Volume |
| ||||||||
Mass | |||||||||
Mean density | 1.854±0.006 g/cm3[6][7] | ||||||||
Synodic rotation period |
Sidereal rotation period
| ||||||||
Equatorial rotation velocity | 47.18 km/h | ||||||||
122.53° (to orbit)[3] | |||||||||
North pole right ascension | 132.993°[9] | ||||||||
North pole declination | −6.163°[9] | ||||||||
Albedo | 0.52 geometric[3] 0.72 Bond[3] | ||||||||
| |||||||||
13.65[3] to 16.3[10] (mean is 15.1)[3] | |||||||||
−0.44[11] | |||||||||
0.06″ to 0.11″[3][g] | |||||||||
Atmosphere | |||||||||
Surface pressure | 1.0 Pa (2015)[7][12] | ||||||||
Composition by volume | Nitrogen, methane, carbon monoxide[13] |
Pluto (
Pluto has a moderately
Pluto has
Pluto was discovered in 1930 by
History
Discovery
In the 1840s, Urbain Le Verrier used Newtonian mechanics to predict the position of the then-undiscovered planet Neptune after analyzing perturbations in the orbit of Uranus. Subsequent observations of Neptune in the late 19th century led astronomers to speculate that Uranus's orbit was being disturbed by another planet besides Neptune.[15]
In 1906,
Percival's widow, Constance Lowell, entered into a ten-year legal battle with the Lowell Observatory over her husband's legacy, and the search for Planet X did not resume until 1929.
Tombaugh's task was to systematically image the night sky in pairs of photographs, then examine each pair and determine whether any objects had shifted position. Using a blink comparator, he rapidly shifted back and forth between views of each of the plates to create the illusion of movement of any objects that had changed position or appearance between photographs. On February 18, 1930, after nearly a year of searching, Tombaugh discovered a possible moving object on photographic plates taken on January 23 and 29. A lesser-quality photograph taken on January 21 helped confirm the movement.[21] After the observatory obtained further confirmatory photographs, news of the discovery was telegraphed to the Harvard College Observatory on March 13, 1930.[17]
One Plutonian year corresponds to 247.94 Earth years;[3] thus, in 2178, Pluto will complete its first orbit since its discovery.
Name and symbol
The name Pluto came from the Roman god of the underworld; and it is also an epithet for Hades (the Greek equivalent of Pluto).
Upon the announcement of the discovery, Lowell Observatory received over a thousand suggestions for names.[22] Three names topped the list: Minerva, Pluto and Cronus. 'Minerva' was the Lowell staff's first choice[23] but was rejected because it had already been used for an asteroid; Cronus was disfavored because it was promoted by an unpopular and egocentric astronomer, Thomas Jefferson Jackson See. A vote was then taken and 'Pluto' was the unanimous choice. To make sure the name stuck, and that the planet would not suffer changes in its name as Uranus had, Lowell Observatory proposed the name to the American Astronomical Society and the Royal Astronomical Society; both approved it unanimously.[14]: 136 [24] The name was published on May 1, 1930.[25][26]
The name Pluto had received some 150 nominations among the letters and telegrams sent to Lowell. The first[h] had been from Venetia Burney (1918–2009), an eleven-year-old schoolgirl in Oxford, England, who was interested in classical mythology.[14][25] She had suggested it to her grandfather Falconer Madan when he read the news of Pluto's discovery to his family over breakfast; Madan passed the suggestion to astronomy professor Herbert Hall Turner, who cabled it to colleagues at Lowell on March 16, three days after the announcement.[23][25]
The name 'Pluto' was mythologically appropriate: the god Pluto was one of six surviving children of
The name 'Pluto' was soon embraced by wider culture. In 1930, Walt Disney was apparently inspired by it when he introduced for Mickey Mouse a canine companion named Pluto, although Disney animator Ben Sharpsteen could not confirm why the name was given.[38] In 1941, Glenn T. Seaborg named the newly created element plutonium after Pluto, in keeping with the tradition of naming elements after newly discovered planets, following uranium, which was named after Uranus, and neptunium, which was named after Neptune.[39]
Most languages use the name "Pluto" in various transliterations.
Planet X disproved
Once Pluto was found, its faintness and lack of a viewable disc cast doubt on the idea that it was Lowell's Planet X.[16] Estimates of Pluto's mass were revised downward throughout the 20th century.[43]
Year | Mass | Estimate by |
---|---|---|
1915 | 7 Earths
|
Planet X)[16]
|
1931 | 1 Earth
|
Nicholson & Mayall[44][45][46] |
1948 | 0.1 (1/10) Earth
|
Kuiper[47] |
1976 | 0.01 (1/100) Earth
|
Cruikshank, Pilcher, & Morrison[48] |
1978 | 0.0015 (1/650) Earth
|
Christy & Harrington[49] |
2006 | 0.00218 (1/459) Earth
|
Buie et al.[50]
|
Astronomers initially calculated its mass based on its presumed effect on Neptune and Uranus. In 1931, Pluto was calculated to be roughly the mass of Earth, with further calculations in 1948 bringing the mass down to roughly that of Mars.[45][47] In 1976, Dale Cruikshank, Carl Pilcher and David Morrison of the University of Hawaiʻi calculated Pluto's albedo for the first time, finding that it matched that for methane ice; this meant Pluto had to be exceptionally luminous for its size and therefore could not be more than 1 percent the mass of Earth.[48] (Pluto's albedo is 1.4–1.9 times that of Earth.[3])
In 1978, the discovery of Pluto's moon
Classification
From 1992 onward, many bodies were discovered orbiting in the same volume as Pluto, showing that Pluto is part of a population of objects called the
IAU classification
The debate came to a head in August 2006, with an IAU resolution that created an official definition for the term "planet". According to this resolution, there are three conditions for an object in the Solar System to be considered a planet:
- The object must be in orbit around the Sun.
- The object must be massive enough to be rounded by its own gravity. More specifically, its own gravity should pull it into a shape defined by hydrostatic equilibrium.
- It must have cleared the neighborhood around its orbit.[58][59]
Pluto fails to meet the third condition.
There has been some resistance within the astronomical community toward the reclassification.
Public reception to the IAU decision was mixed. A resolution introduced in the
Researchers on both sides of the debate gathered in August 2008, at the Johns Hopkins University
In April 2024, Arizona (where Pluto was first discovered in 1930) passed a law naming Pluto as the official state planet.[83]
Orbit
Pluto's orbital period is about 248 years. Its orbital characteristics are substantially different from those of the planets, which follow nearly circular orbits around the Sun close to a flat reference plane called the ecliptic. In contrast, Pluto's orbit is moderately inclined relative to the ecliptic (over 17°) and moderately eccentric (elliptical). This eccentricity means a small region of Pluto's orbit lies closer to the Sun than Neptune's. The Pluto–Charon barycenter came to perihelion on September 5, 1989,[4][l] and was last closer to the Sun than Neptune between February 7, 1979, and February 11, 1999.[84]
Although the 3:2 resonance with Neptune (see below) is maintained, Pluto's inclination and eccentricity behave in a chaotic manner. Computer simulations can be used to predict its position for several million years (both forward and backward in time), but after intervals much longer than the Lyapunov time of 10–20 million years, calculations become unreliable: Pluto is sensitive to immeasurably small details of the Solar System, hard-to-predict factors that will gradually change Pluto's position in its orbit.[85][86]
The
Relationship with Neptune
Despite Pluto's orbit appearing to cross that of Neptune when viewed from north or south of the Solar System, the two objects' orbits do not intersect. When Pluto is closest to the Sun, and close to Neptune's orbit as viewed from such a position, it is also the farthest north of Neptune's path. Pluto's orbit passes about 8 AU north of that of Neptune, preventing a collision.[88][89][90][m]
This alone is not enough to protect Pluto;
The 2:3 resonance between the two bodies is highly stable and has been preserved over millions of years.
Other factors
Numerical studies have shown that over millions of years, the general nature of the alignment between the orbits of Pluto and Neptune does not change.[88][87] There are several other resonances and interactions that enhance Pluto's stability. These arise principally from two additional mechanisms (besides the 2:3 mean-motion resonance).
First, Pluto's
Second, the longitudes of ascending nodes of the two bodies—the points where they cross the
Orcus
The 2nd-largest known plutino, 90482 Orcus, has a diameter around 900 km and is in a very similar orbit to that of Pluto. However, the orbits of Pluto and Orcus are out of phase, so that the two never approach each other. It has been termed the "anti-Pluto", and is named for the Etruscan counterpart to the god Pluto.
Rotation
Pluto's
Geology
Surface
The plains on Pluto's surface are composed of more than 98 percent
Sputnik Planitia, the western lobe of the "Heart", is a 1,000 km-wide basin of frozen nitrogen and carbon monoxide ices, divided into polygonal cells, which are interpreted as convection cells that carry floating blocks of water ice crust and sublimation pits towards their margins;[106][107][108] there are obvious signs of glacial flows both into and out of the basin.[109][110] It has no craters that were visible to New Horizons, indicating that its surface is less than 10 million years old.[111] Latest studies have shown that the surface has an age of 180000+90000
−40000 years.[112]
The New Horizons science team summarized initial findings as "Pluto displays a surprisingly wide variety of geological landforms, including those resulting from glaciological and surface–atmosphere interactions as well as impact, tectonic, possible cryovolcanic, and mass-wasting processes."[7]
In Western parts of Sputnik Planitia there are fields of
-
Multispectral Visual Imaging Camera image of Pluto in enhanced color to bring out differences in surface composition.
-
Distribution of numerous impact craters and basins on both Pluto and Charon. The variation in density (with none found in Sputnik Planitia) indicates a long history of varying geological activity. Precisely for this reason, the confidence of numerous craters on Pluto remain uncertain.[114] The lack of craters on the left and right of each map is due to low-resolution coverage of those anti-encounter regions.
-
Geologic map of Sputnik Planitia and surroundings (context), with convection cell margins outlined in black
-
Regions where water ice has been detected (blue regions)
Internal structure
Pluto's density is 1.860±0.013 g/cm3.
It is possible that such heating continues, creating a
Mass and size
Pluto's diameter is 2376.6±3.2 km[5] and its mass is (1.303±0.003)×1022 kg, 17.7% that of the Moon (0.22% that of Earth).[123] Its surface area is 1.774443×107 km2, or just slightly bigger than Russia or Antarctica. Its surface gravity is 0.063 g (compared to 1 g for Earth and 0.17 g for the Moon).[3] This gives Pluto an escape velocity of 4,363.2 km per hour / 2,711.167 miles per hour (as compared to Earth's 40,270 km per hour / 25,020 miles per hour). Pluto is more than twice the diameter and a dozen times the mass of Ceres, the largest object in the asteroid belt. It is less massive than the dwarf planet Eris, a trans-Neptunian object discovered in 2005, though Pluto has a larger diameter of 2,376.6 km[5] compared to Eris's approximate diameter of 2,326 km.[124]
With less than 0.2 lunar masses, Pluto is much less massive than the terrestrial planets, and also less massive than seven moons: Ganymede, Titan, Callisto, Io, the Moon, Europa, and Triton. The mass is much less than thought before Charon was discovered.[125]
The discovery of Pluto's satellite Charon in 1978 enabled a determination of the mass of the Pluto–Charon system by application of Newton's formulation of Kepler's third law. Observations of Pluto in occultation with Charon allowed scientists to establish Pluto's diameter more accurately, whereas the invention of adaptive optics allowed them to determine its shape more accurately.[126]
Determinations of Pluto's size have been complicated by its atmosphere[127] and hydrocarbon haze.[128] In March 2014, Lellouch, de Bergh et al. published findings regarding methane mixing ratios in Pluto's atmosphere consistent with a Plutonian diameter greater than 2,360 km, with a "best guess" of 2,368 km.[129] On July 13, 2015, images from NASA's New Horizons mission Long Range Reconnaissance Imager (LORRI), along with data from the other instruments, determined Pluto's diameter to be 2,370 km (1,473 mi),[124][130] which was later revised to be 2,372 km (1,474 mi) on July 24,[131] and later to 2374±8 km.[7] Using radio occultation data from the New Horizons Radio Science Experiment (REX), the diameter was found to be 2376.6±3.2 km.[5]
Graphs are unavailable due to technical issues. There is more info on Phabricator and on MediaWiki.org. |
Atmosphere
In July 2019, an occultation by Pluto showed that its atmospheric pressure, against expectations, had fallen by 20% since 2016.[138] In 2021, astronomers at the Southwest Research Institute confirmed the result using data from an occultation in 2018, which showed that light was appearing less gradually from behind Pluto's disc, indicating a thinning atmosphere.[139]
The presence of methane, a powerful greenhouse gas, in Pluto's atmosphere creates a temperature inversion, with the average temperature of its atmosphere tens of degrees warmer than its surface,[140] though observations by New Horizons have revealed Pluto's upper atmosphere to be far colder than expected (70 K, as opposed to about 100 K).[135] Pluto's atmosphere is divided into roughly 20 regularly spaced haze layers up to 150 km high,[7] thought to be the result of pressure waves created by airflow across Pluto's mountains.[135]
Natural satellites
Pluto has five known
The orbital periods of all Pluto's moons are linked in a system of orbital resonances and near-resonances.[145][147] When precession is accounted for, the orbital periods of Styx, Nix, and Hydra are in an exact 18:22:33 ratio.[145] There is a sequence of approximate ratios, 3:4:5:6, between the periods of Styx, Nix, Kerberos, and Hydra with that of Charon; the ratios become closer to being exact the further out the moons are.[145][148]
The Pluto–Charon system is one of the few in the Solar System whose barycenter lies outside the primary body; the
Pluto's moons are hypothesized to have been formed by a collision between Pluto and a similar-sized body, early in the history of the Solar System. The collision released material that consolidated into the moons around Pluto.[153]
Quasi-satellite
In 2012, it was calculated that 15810 Arawn could be a quasi-satellite of Pluto, a specific type of co-orbital configuration.[154] According to the calculations, the object would be a quasi-satellite of Pluto for about 350,000 years out of every two-million-year period.[154][155] Measurements made by the New Horizons spacecraft in 2015 made it possible to calculate the orbit of Arawn more accurately,[156] and confirmed the earlier ones.[157] However, it is not agreed upon among astronomers whether Arawn should be classified as a quasi-satellite of Pluto based on its orbital dynamics, since its orbit is primarily controlled by Neptune with only occasional perturbations by Pluto.[158][156][157]
Origin
Pluto's origin and identity had long puzzled astronomers. One early hypothesis was that Pluto was an escaped moon of Neptune[159] knocked out of orbit by Neptune's largest moon, Triton. This idea was eventually rejected after dynamical studies showed it to be impossible because Pluto never approaches Neptune in its orbit.[160]
Pluto's true place in the
Though Pluto is the largest Kuiper belt object discovered,[128] Neptune's moon Triton, which is larger than Pluto, is similar to it both geologically and atmospherically, and is thought to be a captured Kuiper belt object.[167] Eris (see above) is about the same size as Pluto (though more massive) but is not strictly considered a member of the Kuiper belt population. Rather, it is considered a member of a linked population called the scattered disc.[168]
Like other members of the Kuiper belt, Pluto is thought to be a residual
Observation and exploration
Observation
Pluto's distance from Earth makes its in-depth study and exploration difficult. Pluto's visual apparent magnitude averages 15.1, brightening to 13.65 at perihelion.[3] To see it, a telescope is required; around 30 cm (12 in) aperture being desirable.[172] It looks star-like and without a visible disk even in large telescopes,[173] because its angular diameter is maximum 0.11".[3]
The earliest maps of Pluto, made in the late 1980s, were brightness maps created from close observations of eclipses by its largest moon, Charon. Observations were made of the change in the total average brightness of the Pluto–Charon system during the eclipses. For example, eclipsing a bright spot on Pluto makes a bigger total brightness change than eclipsing a dark spot. Computer processing of many such observations can be used to create a brightness map. This method can also track changes in brightness over time.[174][175]
Better maps were produced from images taken by the Hubble Space Telescope (HST), which offered higher resolution, and showed considerably more detail,[103] resolving variations several hundred kilometers across, including polar regions and large bright spots.[105] These maps were produced by complex computer processing, which finds the best-fit projected maps for the few pixels of the Hubble images.[176] These remained the most detailed maps of Pluto until the flyby of New Horizons in July 2015, because the two cameras on the HST used for these maps were no longer in service.[176]
Exploration
The New Horizons spacecraft, which flew by Pluto in July 2015, is the first and so far only attempt to explore Pluto directly. Launched in 2006, it captured its first (distant) images of Pluto in late September 2006 during a test of the Long Range Reconnaissance Imager.[177] The images, taken from a distance of approximately 4.2 billion kilometers, confirmed the spacecraft's ability to track distant targets, critical for maneuvering toward Pluto and other Kuiper belt objects. In early 2007 the craft made use of a gravity assist from Jupiter.
New Horizons made its closest approach to Pluto on July 14, 2015, after a 3,462-day journey across the Solar System. Scientific observations of Pluto began five months before the closest approach and continued for at least a month after the encounter. Observations were conducted using a remote sensing package that included imaging instruments and a radio science investigation tool, as well as spectroscopic and other experiments. The scientific goals of New Horizons were to characterize the global geology and morphology of Pluto and its moon Charon, map their surface composition, and analyze Pluto's neutral atmosphere and its escape rate. On October 25, 2016, at 05:48 pm ET, the last bit of data (of a total of 50 billion bits of data; or 6.25 gigabytes) was received from New Horizons from its close encounter with Pluto.[178][179][180][181]
Since the New Horizons flyby, scientists have advocated for an orbiter mission that would return to Pluto to fulfill new science objectives.
New Horizons imaged all of Pluto's northern hemisphere, and the equatorial regions down to about 30° South. Higher southern latitudes have only been observed, at very low resolution, from Earth.[188] Images from the Hubble Space Telescope in 1996 cover 85% of Pluto and show large albedo features down to about 75° South.[189][190] This is enough to show the extent of the temperate-zone maculae. Later images had slightly better resolution, due to minor improvements in Hubble instrumentation.[191] The equatorial region of the sub-Charon hemisphere of Pluto has only been imaged at low resolution, as New Horizons made its closest approach to the anti-Charon hemisphere.[192]
Some albedo variations in the higher southern latitudes could be detected by New Horizons using Charon-shine (light reflected off Charon). The south polar region seems to be darker than the north polar region, but there is a high-albedo region in the southern hemisphere that may be a regional nitrogen or methane ice deposit.[193]
See also
Notes
- ^ The mean elements here are from the Theory of the Outer Planets (TOP2013) solution by the Institut de mécanique céleste et de calcul des éphémérides (IMCCE). They refer to the standard equinox J2000, the barycenter of the Solar System, and the epoch J2000.
- ^ Surface area derived from the radius r: .
- ^ Volume v derived from the radius r: .
- ^ Surface gravity derived from the mass M, the gravitational constant G and the radius r: .
- ^ Escape velocity derived from the mass M, the gravitational constant G and the radius r: .
- ^ Based on geometry of minimum and maximum distance from Earth and Pluto radius in the factsheet
- ^ A French astronomer had suggested the name Pluto for Planet X in 1919, but there is no indication that the Lowell staff knew of this.[27]
- ^ For example, ⟨♇⟩ (in Unicode: U+2647 ♇ PLUTO) occurs in a table of the planets identified by their symbols in a 2004 article written before the 2006 IAU definition,[31] but not in a graph of planets, dwarf planets and moons from 2016, where only the eight IAU planets are identified by their symbols.[32] (Planetary symbols in general are uncommon in astronomy, and are discouraged by the IAU.)[33]
- Uranian astrology.[37]
- ^ The equivalence is less close in languages whose phonology differs widely from Greek's, such as Somali Buluuto and Navajo Tłóotoo.
- barycenter. Charon came to perihelion 4 September 1989. The Pluto–Charon barycenter came to perihelion 5 September 1989. Pluto came to perihelion 8 September 1989.
- satellite of Neptune.[91]
- scattered-disc object, often considered a distinct population from Kuiper-belt objects like Pluto; Pluto is the largest body in the Kuiper belt proper, which excludes the scattered-disc objects.
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Further reading
- Codex Regius (2016), Pluto & Charon, CreateSpace Independent Publishing Platform ISBN 978-1534960749
- Stern, S A and Tholen, D J (1997), Pluto and Charon, University of Arizona Press ISBN 978-0816518401
- Stern, Alan; Grinspoon, David (2018). ISBN 978-125009896-2.
- Stern, Alan (August 10, 2021). The Pluto System After New Horizons. University of Arizona Press. p. 688. ISBN 978-0816540945.
External links
- New Horizons homepage Archived July 26, 2015, at the Wayback Machine
- Pluto Profile at NASA's Solar System Exploration site
- NASA Pluto factsheet Archived November 19, 2015, at archive.today
- Website of the observatory that discovered Pluto Archived March 2, 2011, at the Wayback Machine
- Earth telescope image of Pluto system
- Keck infrared with AO of Pluto system Archived November 9, 2020, at the Wayback Machine
- Video – Pluto – viewed through the years (GIF) Archived July 26, 2015, at the Wayback Machine (NASA; animation; July 15, 2015).
- Video – Pluto – "FlyThrough" (00:22; MP4) Archived September 29, 2021, at the Wayback Machine (YouTube) Archived December 2, 2020, at the Wayback Machine (NASA; animation; August 31, 2015).
- "A Day on Pluto Video made from July 2015 New Horizon Images" Archived February 23, 2016, at the Wayback Machine Scientific American
- NASA CGI video Archived August 1, 2017, at the Wayback Machine of Pluto flyover (July 14, 2017)
- CGI video Archived October 3, 2020, at the Wayback Machine simulation of rotating Pluto by Seán Doran (see album Archived July 27, 2020, at the Wayback Machine for more)
- Google Pluto 3D Archived August 6, 2020, at the Wayback Machine, interactive map of the dwarf planet
- "Interactive 3D gravity simulation of the Plutonian system". Archived from the original on June 11, 2020.