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Dwarf planet
(134340) Pluto  Astronomical symbol of Pluto
Global LORRI mosaic of Pluto in true color.jpg
View of the northern polar region and the far side of Pluto in natural colors, taken by the New Horizons spacecraft on July 14, 2015 from a distance of approximately 450,000 km
Properties of the orbit
( animation )
Major semi-axis 39.482  AU
(5,906.4 million km)
Perihelion - aphelion 29.658-49.304 AU
eccentricity 0.2488
Inclination of the orbit plane 17.1418 °
Sidereal period 247  a  343  d
Synodic period 366.73 days
Mean orbital velocity 4.67 km / s
Smallest - largest distance to earth 28.641-50.321 AU
Physical Properties
Equatorial diameter * 2374 km
Pole diameter * 2376 km
Dimensions 1.303 · 10 22  kg
Medium density 1.854 g / cm 3
Gravitational acceleration * 0.62 m / s 2
Escape speed 1.21 km / s
Rotation period 6 d 9 h 43 min 12 s
Inclination of the axis of rotation 122.53 °
Geometric albedo 0.52
Max. Apparent brightness +13.65 m
Properties of the atmosphere
Pressure * ≈ 13 · 10 −6  bar
Temperature *
min. - average - max.
24  K  (−249  ° C )
31 K (−242 ° C)
38 K (−235 ° C)
Main ingredients
* based on the zero level of the dwarf planet
Moons 5
Explorer C. Tombaugh
Date of discovery February 18, 1930
Size comparison
Size comparison between the earth - moon and Pluto - Charon pairs (bottom right) on the same scale (photo montage)

Pluto is the largest and second most massive known dwarf planet in our solar system and the longest known object of the Kuiper Belt . It moves in an even more eccentric orbit around the sun than the planet Mercury . Its volume corresponds to about a third of the earth's moon .

The astronomical symbol of Pluto is ; is also used in astrologyAstrological symbol of Pluto . It is named after the Roman god of the underworld . The new classes of Plutoids and Plutinos were named after the dwarf planet .

From its discovery on February 18, 1930 to the redefinition of the term " planet " on August 24, 2006 by the International Astronomical Union (IAU), Pluto was considered the ninth and outermost planet in the solar system . After more and more plutoids - bodies of similar size in the Kuiper belt - had been found, a more precise definition of the term was developed. Since then it has been assigned to the dwarf planet category and received the minor planet number (134340) Pluto .

In January 2006, a space probe was sent to Pluto for the first time with New Horizons ; it passed him on July 14, 2015, 12,500 km away.

Orbit and rotation


It takes Pluto 247.94 years to orbit the sun  . Compared to the planets, Pluto's orbit is significantly more eccentric, with an eccentricity of 0.2488. This means that the distance to the sun is up to 24.88% smaller or larger than the major semi-axis .

The orbit of Pluto (red) compared to that of Neptune (blue); Object sizes not to scale. The light orbit areas are north of the ecliptic , the dark areas south. The yellow route connects the sun with the equinox .

The point of Pluto's orbit furthest from the sun, the aphelion , is 49.305  AU , while the point closest to the sun, the perihelion , is closer to the sun at 29.658 AU than the very slightly eccentric orbit of Neptune . Pluto passed through this area for the last time, in which it is closer to the sun than the orbit of Neptune, from February 7, 1979 to February 11, 1999. Perihelion happened to Pluto in 1989. He will reach aphelion in the year 2113. There the solar radiation is only about 0.563 W / m². It is 2,430 times as high on earth . For an observer on Pluto, the apparent diameter of the sun is only about 50 and looks like a blindingly bright star about −19. Size class. It fluctuates around 1.1 magnitudes during a Pluto year.

Due to Pluto's eccentric orbit and its fluctuating albedo , its brightness, seen from the earth, changes between 13.8  mag (near the earth) and 16.5 mag (distant from the earth).

It is noticeable that Pluto moves exactly twice around the sun during the time in which Neptune moves three times around the sun. One speaks therefore of a 3: 2 orbit resonance . Many of the Kuiper Belt objects, like Pluto, are in a 3: 2 orbital resonance with Neptune and are referred to as Plutinos. With methods of celestial mechanics it can be shown that their typically very eccentric orbits are stable over millions of years.

Until the discovery of many other, similar objects, Pluto was considered an escaped moon of Neptune. Its pronounced eccentric and, at 17 °, strongly inclined orbit towards the ecliptic and its small size suggested this. The great Neptune moon Triton is said to have been captured by Neptune and in the process disrupted the original lunar system considerably: Pluto was catapulted out of the Neptune system and the considerable eccentricity of the Neptune moon Nereid was created. The reverse sense of rotation speaks in favor of trapping the Triton, which is why this is still the current theory for Triton. However, the hypothesis that Pluto escaped has since been dropped. The discovery of numerous other trans-Neptunian objects on the edge of the planetary system has shown that Pluto is the largest and in any case the brightest representative of the Kuiper belt, an accumulation of thousands of asteroids and cometary nuclei in a disc-shaped region behind Neptune's orbit. The genesis of Pluto is therefore closely linked to that of the Kuiper Belt, which consists of remnants of the formation of the outer planetary system. Triton is also said to have been a member of this belt before his suspected capture.

Size comparison of the 10 largest TNOs


Pluto in a rotation animation

Pluto rotates once on its axis in 6,387 days. The equatorial plane is inclined by 122.53 ° to the plane of the orbit, so Pluto rotates in reverse . Its axis of rotation is therefore even more inclined than that of Uranus . In contrast to Uranus and Venus , the reason for this is generally evident, as is the cause of Pluto's large period of rotation compared to other celestial bodies , because the rotation of the dwarf planet is tied to the orbital motion of its very large moon Charon due to the tidal forces . Pluto and Charon were the first and the only known body in the solar system with a double over time synchronous rotation until in the Kuiper Belt and in the asteroid belt similar types of systems have been found, such as (90) Antiope with their companions Antiope B .

The determination of the poles for the dwarf planet was carried out in such a way that its north pole is the point of rotation at which the rotation of the surface runs counterclockwise. Due to Pluto's retrograde sense of rotation, the axis direction of its north pole points south of the ecliptic, in contrast to the planets.


Shell structure of Pluto

With a diameter of only 2,374 km, Pluto is significantly smaller than the seven largest moons in the solar system. Its structure is probably similar to that of the larger and even colder Triton. It is of similar density, has a very thin atmosphere of nitrogen , is also of a rather reddish color, has polar ice caps , and darker areas predominate towards the equator.

internal structure

Pluto's mean density is 1.860 g / cm³; at a density of around 2 g / cm³, a composition of around 70% rock and 30% water ice is likely.

According to the current model of Pluto's structure, its interior has by heat from radioactive shell structure core decay processes in a differentiated . The core consists mostly of rock and measures 70% of Pluto's diameter. Beneath the surface of predominantly nitrogen ice, the core is encased in a coat of water ice. In the transition zone between core and mantle, the internal melting processes could have formed a global extraterrestrial ocean that may still exist today .


With a size of 17.6 million square kilometers, Pluto's surface is just about the size of South America . After that of Saturn's moon Iapetus , it shows the greatest brightness contrasts among all other bodies in the solar system. This explains the pronounced fluctuations in brightness that were measured between 1985 and 1990 during eclipses caused by its large moon Charon.

From 2004 Pluto and Charon were observed with the Spitzer Space Telescope in the thermal infrared . The light curves showed that Pluto with around 40 Kelvin is around 10 Kelvin colder than Charon. The causes are higher albedo , which means that less sunlight is absorbed, and greater thermal inertia, which means that the rotation transports more heat to the back.

The New Horizons flyby enabled the northern hemisphere and the southern equatorial zone to be photographed from Pluto's surface; the rest of the time ruled the seasonal polar night. The most detailed images were obtained from the areas in the middle of the side constantly facing away from the moon Charon around the 180th degree of longitude. There is a light, approximately heart-shaped, homogeneous appearing region. The larger part of it is located north of the equator and was named Tombaugh Regio after the discoverer of Pluto, Clyde Tombaugh . Within the Tombaugh region is an area that has been named Sputnik Planitia . The Sputnik Plain - named after the first artificial satellite Sputnik 1  - is a very large ice surface that covers the western half of the Tombaugh region. Because it is free of impact craters , some researchers believe that it is less than 100 million years old and may still be in a state of active geological formation. At first sight, they are reminiscent of frozen mud. Visible streaks in this area could be caused by winds. Other researchers assume that there is a geologically inactive, old surface on which only atmospheric processes affect and precipitate. In glaciers, nitrogen ice flows from a light area from the east into the Sputnik Planum. It is believed that it previously evaporated in the center of the Sputnik plain and precipitated east of the plain from where it flows back.

The conspicuous Tombaugh region is probably not by chance at the point of the equator that faces exactly away from Charon. According to the preferred explanation, they must have placed the centrifugal force of Pluto's rotation and the tidal force of Charon in this particular position. The Sputnik Planitia, however, as a lowland plain has a negative topology and, due to a lack of mass, would therefore not be considered at first glance for a positive gravity anomaly on which these forces could attack. The lowlands are believed to be less than 100 million years old impact basin, through whose perforated bottom water of a possible hidden ocean could penetrate. The ice layer formed by the ascending water is thinner than the surrounding ice crust, but it has a higher mass density, so that a positive gravity anomaly developed. Subsequent deposits of nitrogen ice intensified this effect.

On the southern edge of the Sputnik plain, the Norgay Montes, named after Tenzing Norgay , rise up to a height of 3500 meters , alongside Edmund Hillary, one of the two first climbers of Mount Everest . A little further north, on the western edge of the Sputnik plain, the Hillary Montes rise up to 1,500 meters above their surroundings. The high mountains are most likely made of water ice, as this is hard as rock at the low temperatures. Methane and nitrogen ice, which cover most of Pluto's surface, are not stable enough for such structures - although their weight on the dwarf planet is only a fifteenth of what they would have on Earth. The cause of their formation is still completely in the dark, because the dwarf planet is not under the gravitational influence of an even more massive celestial body that could deform its crust in such a way. In the vicinity of the Norgay Mountains there are two 3 and 5 kilometer high elevations, Wright Mons and Piccard Mons, with central depressions, probably ice volcanoes .

In the eastern neighborhood of the Tombaugh region lies the Tartarus Dorsa region  - named after Tartarus in Greek mythology, the deepest part of the underworld . The terrain of these mountain ridges was nicknamed "snakeskin" by the interpreters because of its strange relief. The rugged Tartarus ridges stretch for hundreds of kilometers and are traversed by almost parallel grooves that could have been created by tectonic movements. The backs are covered with blade-like ridges that may have formed over a long period of time from volatile and repeatedly frozen material.

1070 impact craters were counted on the hemisphere recorded with the highest resolution. They show very different states of conservation. The areas with the highest crater density are estimated to be 4 billion years old.

For the naming of Pluto's formations, the IAU has restricted the possibilities within the scope of its nomenclature to mythological names for the underworld and associated gods, dwarfs, heroes and explorers, of space vehicles, as well as of writers, scientists and engineers who were familiar with Pluto and the Kuiper Belt. The first names were officially confirmed on September 21, 2017.

the atmosphere

An almost true color picture of Pluto out of its shadow. Numerous layers of blue haze float in Pluto's atmosphere. Mountains and their shadows are visible along and near its edge. (New Horizons, April 14, 2015)

Pluto's very thin atmosphere consists mostly of nitrogen, also some carbon monoxide and about 0.5% methane . According to measurements at the James Clerk Maxwell Telescope , the atmosphere was 3000 km high in 2011 and the carbon monoxide it contained had a temperature of −220 ° C. The atmospheric pressure on Pluto's surface is around 0.3  Pa according to the US space agency NASA and around 1.5 Pa according to the European Southern Observatory (ESO). Assumptions about the freezing out of Pluto's atmosphere after passing the orbit area closer to the sun could not be confirmed so far. A comparison of spectroscopic measurements from 1988 and 2002 even revealed a slight expansion of the gas envelope. It is also assumed that the mass is twice as large.

According to the absorption measurements of the New Horizons mission, the atmosphere reaches an altitude of 1,600 kilometers.

As the ESO announced on March 2, 2009, there is mostly an inversion weather situation on Pluto caused by atmospheric methane , which increases the temperature by 3 to 15 K per altitude kilometer. In the lower atmosphere the temperature is −180 ° C and in the upper atmosphere −170 ° C, while on the ground it is only about −220 ° C. It is believed that this low value is caused, among other things, by the evaporation of methane, which changes from the solid to the gaseous state.

The presence of an atmosphere was again proven on June 29, 2015 with the help of the Stratospheric Observatory for Infrared Astronomy , when the star UCAC 4347-1165728 , as seen from Earth, was covered by Pluto for 90 seconds. However, the first data from New Horizons show that there is a discrepancy between the atmospheric pressure measured by the probe and the atmospheric pressure observed and calculated by the earth. The previous atmospheric pressure is measured during earth observation at an altitude of around 50 to 75 km and calculated down to the Pluto surface using assumptions. This gives a pressure of 2.2 Pa, while New Horizons was able to measure directly on the surface and thus got a value of 0.5 Pa.

After the first pictures of the flyby, New Horizons discovered aerosols up to 130 km in the Pluto's atmosphere . These are mainly concentrated on two layers of fog, the first around 50 km above ground and the second at around 80 km above sea level. In the meantime, over 12 layers of fog are known, the first of which is close to the ground. In addition, Pluto's atmosphere is continually losing nitrogen, which is ionized and blown away by the solar wind .


Five natural satellites are known from Pluto . Their orbits are approximately circular and coplanar to one another . They lie in Pluto's strongly inclined equatorial plane, not in its plane. At the orbital period of the dominant Charon, the orbital periods of the other, outer moons are approximately resonant; the ratios are around 1: 3: 4: 5: 6.

Pluto and Charon circle each other (graphic). The barycenter (white point in the middle) is outside of Pluto.

With Charon, Pluto has a relatively large moon, which is why it is sometimes referred to as the " double system Pluto-Charon". The size ratio is even smaller than that of the Earth-Moon system and is less than 2: 1 in terms of diameter. Due to the mass ratio of a good 8: 1 and a distance of almost 15 Pluto radii, the common center of gravity , the barycentre of the system, is outside of Pluto. Charon and Pluto are thus orbiting each other.

The four smaller satellites move approximately around the common center of gravity of Pluto and Charon, so in the long term the orbit of all of them is probably unpredictable ( three-body problem ). Because of the non-uniformly cooperating gravitational fields of Pluto and Charon, they also have no bound rotation like Charon; In addition, their axes of rotation are very strongly inclined and their rotational behavior is also not constant over a long period of time. The figure of Kerberos and Hydra speaks for a fusion of two smaller bodies.

The origin of the Pluto moons is explained according to the model of the collision theory of the origin of the Earth's moon through the grazing collision of Pluto with another large body of the Kuiper belt, through which debris got into orbits around Pluto, from which the moons formed there. This popular theory was used for Charon before the discovery of the four little ones. The coplanar orbits with the approximately resonant orbital times and the uniformly colored surfaces speak for a common formation of all Pluto moons. If caught, a different color would have been expected.

Pluto and its moons in the Kuiper Belt are exposed to constant bombardment by mini meteorites that knock dust and ice particles out of the surfaces. While the gravity of Pluto and Charon ensures that all the debris falls back onto the celestial bodies, the attraction of the smaller moons is not enough. The scientists therefore suspect that, in astronomical periods, they lose so much material through further impacts that a ring of dust will gradually form around Pluto.

The discovery of further Pluto moons came unexpectedly, as no celestial body with more than one satellite had been observed beyond Neptune; however, a month later a second moon was found at (136108) Haumea . Since Pluto and Charon can with some justification be viewed as double (dwarf) planets, Nix and Hydra can also be seen as the first evidence of circumbinary satellites with reasonably stable orbits in a double system.

Before the flyby, New Horizons was intensively searched for satellites and dust rings for safety reasons; however, no further Pluto moons could be discovered. Should they still exist, they could have at most about a quarter of the brightness of the small, dark Kerberos .

Surname discovery object Path parameters brightness
picture By-mes-ser
(10 18  kg)
Large half-
axis (km)
Rotation time
Max. O Min.
Charon 1978
Charon by New Horizons on 13 July 2015.png
1212 1587 17 536 ± 3 * 06,387 230 0.002 2 0.001 ° 15.8 m 16.8 m 18.3 m
Nothing 2005
Nix best view.jpg
≈ 49.8 × 33.2 × 31.1 ≤ 0.09 48 690 ± 3 * 24,854 800 0.000 00 0.0 ° 23.3 m 24.4 m 24.9 m
Hydra 2005
Hydra imaged by LORRI from 231 000 kilometers.jpg
≈ 50.9 × 36.1 × 30.9 ≤ 0.09 64 721 ± 3 * 38.202 100 0.005 54 0.3 ° 22.9 m 24.0 m 25.2 m
Kerberos 2011
Kerberos (moon) .jpg
≈ 12 × 4.5 ≤ 0.03 57 750 ± 3 * 32.167 900 0.000 00 0.4 ° 25.7 m 26.8 m 28.1 m
Styx 2012
Styx (moon) .jpg
≈ 16 × 9 × 8 0.007 5 42 656 ± 78 20.161 550 0.000 01 0.0 ° 26.6 m 27.7 m 29.0 m
  • Max .: Pluto-perihelion, conjunction, brightest side facing the earth
    Min .: Pluto-aphelion, opposition, darkest side facing the earth
Pluto and its moons to scale with barycenter



The history of the discovery of Pluto is somewhat similar to that of Neptune, which was found 83 years earlier. For both celestial bodies an attempt was made to predict their discovery based on orbital disturbances of the neighboring planet. In the case of Pluto, however, the actual finding of an object in the corresponding search area is ultimately a pure coincidence, since Pluto's mass is not sufficient for the disturbances on which the calculation is based.

Pluto was discovered on February 18, 1930 at the Lowell Observatory by comparing some sky images on the blink comparator after around 25 years of searching, but not at the exact position predicted. The young explorer Clyde Tombaugh had only recently been hired for the photographic search for the legendary Trans-Neptune . The Mars explorer Percival Lowell had been looking for such a celestial body himself since 1905 and financed the Lowell Observatory. As it turned out later, Pluto was already recognizable on two of the photographic plates that Lowell had made in 1915. Since Lowell was looking for a much brighter object, he had missed this discovery.

The discovery was announced to the very interested public on March 13, 1930, the 149th anniversary of the discovery of Uranus by William Herschel in 1781 and the 75th birthday of Percival Lowell, who had died in 1916.

Now they looked for a suitable name. The privilege of naming lay with the Lowell Observatory. A large number of suggestions soon arrived there. The name of the ruler of the underworld for this heavenly body so far from the sun was suggested by Venetia Burney , an eleven-year-old Oxford girl who was very interested in classical mythology. She found out about the news of the discovery and the search for a name in the Times from her grandfather, Falconer Madan, the morning after the discovery was made. He was a retired librarian from the Bodleian Library and thought her suggestion was so good that he told Herbert Hall Turner , an astronomer friend and professor of astronomy at Oxford University , about it. Via this he reached the Lowell Observatory by telegram on March 15, where he was accepted in May of the same year. According to the IAU regulations, the naming had to be based on mythological criteria. Venetia's great-uncle Henry Madan, a Science Master at Eton College , had already suggested the names Phobos and Deimos for the moons of Mars . The name Pluto for the sought-after ninth planet was first suggested in 1919 by the French doctor and amateur astronomer P. Reynaud, but apparently hardly anyone outside of France could remember it in 1930. For the author Richard Buschick, however, only three years before Pluto's discovery, Pluto's future and present name was already taken for granted. The fact that the astronomical symbol was composed of Lowell's initials may also have played a role in the final, official choice of name. Percival, Lowell, and even her own name Constance had previously been suggested by his widow .

Because of its relative proximity and size, Pluto was discovered more than 60 years earlier than the next distinct Trans-Neptunian object: (15760) Albion . It is now assumed that the orbital deviations of Neptune and Uranus determined at the time were only simulated by a small, inevitable measurement deviation . In addition, the mass of Neptune was misjudged prior to Voyager 2's flyby. Since the exact mass of Neptune is known, the orbits of the outer planets can be well explained, that is, another planet would have to be much further away.

Earthbound exploration

With its angular diameter of less than 0.1 ", Pluto avoided a direct determination of its diameter for a long time, as it could only be seen as a point in the telescope. Until the 1960s, its diameter was postulated at 14,000 km, so that the orbit disturbances were postulated of Neptune, which required a mass of 2 to 11 earth masses (however, based on calculation errors and wrong assumptions) (Pickering and Lowell). So that the whole thing was compatible with its apparent brightness of 15 m , Pluto was measured with an albedo of 2 % classified as extremely dark object. Kuiper observed Pluto in 1950 and determined the disk diameter to be 0.23 ± 0.01 "and thus the diameter to be almost 6000 km. The source of the accuracy and why the 0.58 "distant Charon was not recognized is unclear. A star occultation in 1966 (Halliday et all.) Could only be agreed with a maximum diameter of 6800 km. Further doubts came from spectroscopic investigations in 1976 Dale P. Cruikshank , David Morrison and Carl B. Pilcher , who used characteristic absorption lines to indicate methane ice and a rather very bright object with an albedo of more than 50 to 80% and thus only allowed diameters of less than 3000 km. With the discovery by Charon in 1978 and the coverages of Pluto by Charon from 1985 to 1990, the diameter was further corrected to 2306 km, values ​​that were essentially confirmed by the HST in 1994 with 2390 km. Since New Horizon the diameter has been 2370 km pretty well known.

The two hemispheres of Pluto in the best possible resolution of the HST from 1994 and the surface contrasts calculated from them
Pluto in rotation. Computer calculated single images from 2010.

With the development of powerful telescopes , the diameter and mass of Pluto had to be continuously revised downwards, initially around 1950 after measurements by the Mount Palomar observatory to half the size of the earth. It was soon joked that if the measured values ​​were extrapolated , Pluto would soon disappear completely. Unconventional theories have been postulated: Pluto is really big, but you only see a small, bright spot on the surface. The astronomer Fred Whipple calculated an exact orbit for the first time. For this purpose, photo plates could be used on which Pluto could be traced back to 1908. The discovery of the moon Charon in 1978 then made it possible to determine the exact mass using the gravitational dynamics of the system. From 1985 to 1990 there was mutual occlusion between the two, with which the diameter of Pluto was finally determined to be 2390 km. Subsequent measurements with adaptive optics, with the Hubble Space Telescope (HST) and with star obscurations have shown values ​​of around 2280 to 2320 km. Images from the New Horizons space probe in July 2015 showed a diameter of 2370 km.

Combinations of images taken with the Hubble Space Telescope have shown that Pluto's northern hemisphere became brighter in 2002 and 2003 and that the dwarf planet appears reddish overall.

In 1994 NASA published the first global images of Pluto, in which images from the Hubble space telescope were converted into a surface map. With great effort, scientists working with Marc W. Buie generated a surface map of Pluto in 2010, which was the most accurate map of the dwarf planet for a good five years. To do this, they used 384 images of only a few pixels in size from the Hubble Space Telescope, which had been created between 2002 and 2003. Using deconvolution and other sophisticated algorithms, a surface map of Pluto was calculated on 20 computers within 4 years.

Exploration with space probes

New Horizons preparing for launch

NASA planned a rapid since the early 1990s under the name "Pluto Fast Flyby" flyby mission to Pluto before its thin atmosphere freezes out - nearest the Sun path point was passed in 1989 Pluto and it is only in 2247 reached again. After the first concepts failed due to technical difficulties and insufficient funding, the implementation of the mission, now called “New Horizons”, was approved in 2001 as part of the New Frontiers program . The spacecraft launched on January 19, 2006 and passed Pluto and Charon on July 14, 2015. Images from the probe in April 2015 already exceeded those from the Hubble telescope. There were

  • global maps of the dwarf planet and its moon created, with spectral resolution in the visible and IR,
  • studied the atmosphere of Pluto in transmission, with spectral resolution in the UV range,
  • High resolution photos with up to 25 m per pixel resolution obtained,
  • the electric and magnetic fields as well as ions, neutral particles and dust are measured.

The complete transfer of all data took more than 15 months and ended on October 25, 2016.

Debate about planet status and withdrawal

The discussion about whether Pluto even deserved the name “planet” began when, in addition to its highly elliptical and very inclined orbit, its smaller size was recognized. After the third trans-Neptunian object after Pluto and Charon was found in 1992 with QB 1 in 1992 , the astronomers discovered four more Plutinos within four days a year later. This increased the debate about Pluto's status. Brian Marsden's proposal from the MPC in 1998 to give Pluto double status and also to classify it as an asteroid with the outstanding number 10000 in order to prevent a constantly changing number of planets due to new discoveries was not approved.

Hundreds of other objects of the Kuiper Belt have been discovered over time, including some of a size similar to Pluto. Such outstanding discoveries, especially by Eris , have often been referred to by the media as the “ tenth planet ”. With the first scientific definition of a planet along with Pluto, none of these objects were confirmed as such. Instead, the IAU defined the new class of dwarf planets for such bodies in 2006 . Within this new class, Pluto is the second object after Ceres that was first considered a planet. Ceres was downgraded to the newly created class of asteroids along with other objects in the mid-19th century when it became increasingly clear that Mars and Jupiter are members of a belt of very numerous smaller objects. With regard to Pluto as the ninth planet that has been used for over seven decades, however, the controversy among astronomers continues after this decision.

IAU vote on the planet definition on August 23, 2006

The adopted definition with the addition that a body is only a planet if its mass exceeds the total mass of all other bodies in its orbit, takes into account that Pluto has not cleared its orbit to the same extent of other bodies. As the largest plutino, it corresponds more closely to the role of the asteroid (153) Hilda , the largest member of the Hilda group. Hilda and at least 56 other objects move a bit outside the main belt of the asteroids in a 2: 3 ratio to the in this case longer orbital period of the neighboring giant planet.

At the 26th General Assembly of the IAU in August 2006 in Prague, a slightly different definition was proposed without this addition. A planet would therefore be a celestial body, the mass of which is sufficient to assume a hydrostatic equilibrium form ("almost round", that is, approximately spheroidal shape) due to its own gravity, and which is on an orbit around a star, but not itself a star or moon of a planet is. According to this, not only would Pluto be a planet, but also Ceres, Charon and Eris. Charon was added with an addition, according to which it is a double planet if the common center of gravity is outside the main body.

At the same time, the definition of a new class of planets, the so-called “plutons”, was proposed, to which planets should belong that would take longer than 200 years to orbit the star, and which would then also have included Pluto. However, this proposal for the planet definition could not prevail at the general assembly, so that on August 24, 2006, the decision was made through a vote to revoke Pluto’s planet status and classify it in the new class of dwarf planets. The class of plutons was defined (with Pluto as a prototype), but remained unnamed for the time being, as the designation as plutons was discarded like other suggestions. In June 2008 this unnamed subclass of dwarf planets was given the name “ Plutoids ”.

Pluto has had the minor planet number 134340 since September 2006. Such a unique number is usually assigned consecutively as soon as the orbit of an asteroid or dwarf planet is known precisely enough from enough observations. Pluto's orbit had long been sufficiently determined, but due to its previous classification as a planet, no minor planet number was an option for it for around 76 years. The last asteroids discovered before Pluto were given the numbers 1143 and 1144 .

In 2009, the Senate of Illinois , the home state of Pluto discoverer Clyde Tombaugh, decided to keep Pluto a planet. NASA administrator Jim Bridenstine also stated in 2019 that he continues to view Pluto as a planet.


Pluto in October 2009 with an apparent magnitude of 14.1 mag

To see Pluto, a telescope with an opening of at least 200 mm is required. He is currently hiking through the constellation Sagittarius and will switch to Capricorn in 2023/2024 . Since Pluto was in perihelion on September 5, 1989, it has been moving away from the Sun on its elliptical orbit since then; therefore successive oppositions up to the year 2113 take place with ever greater distance, with ever smaller apparent size and with ever decreasing brightness of the dwarf planet.


In the year of discovery 1930, Disney invented the cartoon dog Pluto , which was named after the new celestial body. In 1942, the new chemical element 94 was named plutonium after the astronomical increase that was considered a planet . In 1955, the Pluto Glacier on the Antarctic Alexander I Island was named after the astronomical object. In 2012, a semi-submersible submarine with the name of the dwarf planet was launched in the USA .

In 1987 Kim Stanley Robinson published the science fiction novel The icy pillars of Pluto, based on the original edition Icehenge from 1987. In it, space travelers discovered an enigmatic, huge monument made of ice in the middle of the third millennium on Pluto.

In 2000, Colin Matthews composed the eighth movement Pluto, the Renewer, to complement the orchestral suite The Planets by Gustav Holst .

Size comparison

Artist's impression of some large Trans-Neptunian objectsTransneptunisches Objekt (136199) Eris (136199) Eris Dysnomia (Mond) Dysnomia (Mond) Pluto Pluto Charon (Mond) Charon (Mond) Styx (Mond) Nix (Mond) Kerberos (Mond) Hydra (Mond) (136472) Makemake Namaka (Mond) Hiʻiaka (Mond) (136108) Haumea (90377) Sedna (225088) Gonggong (50000) Quaoar (50000) Quaoar Weywot (Mond) (90482) Orcus (90482) Orcus Vanth (Mond) Erde
Comparison of some large trans-Neptunian objects with the earth (mostly fantasy drawings. Caption as of June 2015) . To get to the corresponding article, click on the object ( large display ).

See also


  • Alan Stern, Jaqueline Mitton: Pluto and Charon. Ice Worlds on the Ragged Edge of the Solar System. University of Arizona Press, Tucson, AZ 1997, ISBN 0-8165-1840-8 ; 2nd expanded edition, Wiley-VCH, Weinheim 2005, ISBN 3-527-40556-9 (English).
  • David A. Weintraub: Is Pluto a Planet? A Historical Journey through the Solar System. Princeton University Press, Princeton NJ 2007, ISBN 0-691-12348-9 (English).
  • Leif Allendorf: Planet Pluto. The secrets of the outer solar system. Avinus, Berlin 2007, ISBN 978-3-930064-76-2 .
  • Silvia Protopapa: Surface characterization of Pluto, Charon and (47171) 1999 TC36. Copernicus Publishing, Katlenburg-Lindau 2009, ISBN 978-3-936586-96-1 . Dissertation at the Technical University of Braunschweig 2009, 143 pages (English).
  • Sue Ward: The Foundation of Astrology: How the Old Rulers and the New Planets came to their astrological interpretation. Translated by Reinhardt Stiehle, Chiron, Tübingen 2011, ISBN 978-3-89997-195-8 .
  • Tilmann Althaus: First details of Pluto. In: Stars and Space . 2015, 9, pp. 26-37 ( abstract ).

Web links

Commons : Pluto  - collection of pictures, videos and audio files
Wikibooks: Pluto  - learning and teaching materials

Individual evidence

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  8. Emmanuel Lellouch et al .: Thermal properties of Pluto's and Charon's surfaces from observations. Icarus 214, 2011, p. 701, doi: 10.1016 / j.icarus.2011.05.035 ( online ).
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  13. Flight over Pluto. - Astronomy Picture of the Day from July 18, 2015.
  14. NASA's New Horizons Discovers Exotic Ices on Pluto. July 24, 2015.
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  31. NASA / JHUAPL / SWRI: Stunning Nightside Image Reveals Pluto's Hazy Skies. ( Memento of the original from September 20, 2015 in the Internet Archive ) Info: The archive link was inserted automatically and has not yet been checked. Please check the original and archive link according to the instructions and then remove this notice. In: July 23, 2015, accessed September 2, 2015. @1@ 2Template: Webachiv / IABot /
  32. NASA / Johns Hopkins University Applied Physics Laboratory / Southwest Research Institute: PIA19719: Artist's Concept of the Interaction of the Solar Wind. In: July 17, 2015, accessed August 5, 2015.
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  40. ↑ The eccentricity and inclination of Pluto and Charon are the same, since the values ​​refer to the same two-body problem (the gravitational influence of the smaller moons is neglected here).
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  48. 14000 km requires an albedo of almost exactly 2%
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  50. Value is, among other things, to be found in: Universe, Earth, Man; Verlag Neues Leben; 1972 edition
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  54. Kosmos Himmelsjahr 2017: sun, moon and stars in the course of the year; Hans-Ulrich Keller; ISBN 978-3-440-15115-0
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This article was added to the list of excellent articles on April 3, 2006 in this version .