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Pluto designated (134340) Pluto in the Minor Planet Centre catalogue, is the second-largest identified dwarf planet in the solar system and the tenth biggest physical structure orbiting the Sun. It orbits between 29 and 49 AU from the Sun, and was the first Kuiper Belt object to be observed.


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About one-fifth the mass of the Earth's Moon, Pluto is principally made of rock and ice. It has an eccentric celestial orbit that is highly inclined with respect to the planets and takes it nearer to the Sun than Neptune during a part of its orbit. Pluto and its biggest satellite, Charon, could be thought of as a binary system because they are nearer in size than any of the other noted celestial pair collections in the solar system, and because the barycentre of their paths does not lie within either body.

Nevertheless, the International Astronomical Union (IAU) has so far to adjudge a definition for binary dwarf planets, so Charon is presently regarded as a moon of Pluto. Two smaller moons, Nix and Hydra, were observed in 2005. Pluto is smaller than many of the natural satellites or moons in our solar system.

From its finding by Clyde Tombaugh in 1930, Pluto was reasoned to be the Solar System's ninth planet. In the late 20th and early 21st century, numerous related objects were revealed in the outer solar system, most noteworthy the trans-Neptunian object Eris which is slightly larger than Pluto.

On August 24, 2006 the IAU defined the word "planet" for the first time. This definition omitted Pluto, which was then reclassified under the new class of dwarf planet along with Eris and Ceres. Pluto is also categorised as an example of a family of trans-Neptunian objects. After the reclassification, Pluto was added to the listing of minor planets and granted the number 134340.

Pluto - Planet or Not?

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1930 saw Clyde Tombaugh employed on a task probing for a ninth planet at Lowell Observatory. Tombaugh's job was to systematically take images of the celestial sky in pairs, one to two weeks apart, then look for objects that had changed between images. On February 18, 1930, he revealed a possible moving target on photographic plates taken on January 23 and January 29. A lesser-quality picture taken on January 20 helped affirm the movement. Later the observatory proceeded to acquire more confirmatory images, word of the find was wired to the Harvard College Observatory on March 13, 1930. Pluto would later be found on photos going back to 1915.

Relations to Neptune and Uranus

The account of how Pluto was revealed is intertwined with the finds of Neptune and Uranus. In the 1840s, using Newtonian mechanics, Urbain Le Verrier, and John Couch Adams had rightly foreseen the point of the then-undiscovered planet Neptune after studying perturbations in the orbit of Uranus. Conjecturing the perturbations were created by the gravitational force of another planet, Johann Gottfried Galle revealed Neptune in September 1846.

Measurements of Neptune in the late 19th century had astronomers beginning to theorise that Neptune's orbit too was also being disturbed by some other planet in a similar way that Neptune was disturbing Uranus. By 1909, William H. Pickering and Percival Lowell had suggested many possible celestial coordinates for such a planet. In May 1911, the Bulletin of the Astronomical Society of France publicised computations by Indian astronomer V.B. Ketakar which foretold a position for an undetected planet.

Discovered by: Clyde W. Tombaugh
Discovery date: February 18, 1930
MPC designation: 134340 Pluto
Minor planet category: dwarf planet
Orbital characteristics
Epoch J2000
  7,375,927,931 km
49.305 032 87 AU
4,436,824,613 km
29.658 340 67 AU
Sidereal period: 90,613.3055 d
(248.09 a)
Synodic period: 366.73 d
Avg. orbital speed: 4.666 km/s
Max. orbital speed: 6.112 km/s
Min. orbital speed: 3.676 km/s
Satellites: 3
Physical characteristics
Mean radius: 1195 km (19% of Earth, or 742.5 mi)
Surface area: 1.795×107 km² (0.033 Earths)
Volume: 7.15×109 km³ (0.0066 Earths)
Mass: (1.305±0.007)×1022 kg (0.0021 Earths)
Equatorial surface gravity: 0.58 m/s² (0.059 gee)
Surface temp.:
min mean max
33 K 44 K 55 K
Adjectives: Plutonian
Surface pressure: 0.30 pascals (summer maximum)
Composition: nitrogen, methane

Percival Lowell's influence

Percival Lowell would have important outcome on Pluto's discovery. In 1905, the Lowell Observatory began an extended program in a search for the possibility of a ninth planet. This activity continued after Lowell's death in 1916. Lowell was probing for a theoretical Planet X to match measurements seen in Uranus and Neptune.

Pluto is too small to have an outcome on Neptune's orbit that started the investigation. Later after a flyby of Neptune by Voyager 2 in 1989, it was once and for all shown that the differences in Neptune's orbit discovered by 19th century astronomers were due rather to imprecise calculations of Neptune's mass.

Once found, Pluto's faintness and want of a seeable disk cast uncertainty on the idea that it could be Percival Lowell's Planet X. Lowell had made an observation of Pluto's place in 1915 which was reasonably next to its real place at that time. Ernest W. Brown thought almost straight-away that this was happen-stance, and this perspective is still held today.


The privilege to name the new object belonged to the Lowell Observatory and its director, Vesto Melvin Slipher. Tombaugh urged Slipher to propose a name promptly before somebody else did. Ideas flowed in from all over the world. Constance Lowell, Percival Lowell's widow, suggested Zeus, then Lowell, and lastly her own first name, none of which met with any exuberance. Mythical names, such as Cronus and Minerva, were top on a list of names.

The name Pluto was first recommended by Venetia Burney (later Venetia Phair), an eleven-year-old girl from Oxford, England. Venetia, who was fascinated by Classical mythology as well as astronomy, suggested the name, the Roman equivalent of Hades, in a conversation to her grandfather Falconer Madan, a onetime librarian of the Bodleian Library. Madan passed the idea to Professor Herbert Hall Turner, Turner then wired the suggestion to colleagues in America. After favourable consideration which was almost unanimous, the name Pluto was formally adopted in May, 1930. Madan gave Venetia five pounds as a reward.

The name is that of the Roman god Pluto, and it is also intentional to elicit the initials of the astronomer Percival Lowell. In the Chinese, Japanese, and Korean languages, the name was interpreted as dark king star (冥王星), suggested by Houei Nojiri in 1930. In Vietnamese it is named after Yama (Sao Diêm Vương), the Guardian of Hell in Buddhist mythology. Yama (Devanāgarī यम) is also used in India, as it is the deity of Hell in Hindu mythologies.

Physical characteristics

Much information about Pluto is sparse, chiefly owing to the reality that it has not yet been seen up close by spacecraft.

Pluto's distance from Earth makes in-depth research very hard.


Pluto's apparent magnitude is fainter than 14 m and hence a scope is needed for observation. To find it, a telescope of about 30 cm aperture is desired. It appears star-like even in very large telescopes because of its angular diameter being only 0.15". The colour of Pluto is light brown with a very slight shade of yellow.

Charon's find resulted in the figuring of Pluto's albedo's being amended up; since Pluto was now seen as being far smaller than earlier guessed, its capacity to reflect light must be greater than at one time thought. Present-day approximations place Pluto's albedo as marginally less than that of Venus, which is moderately high.

Distance and limits on telescope engineering make it presently impracticable to directly photograph surface information on Pluto. Pictures from the Hubble Space Telescope scarcely show any distinct surface definitions or markings. The foremost pictures of Pluto come from brightness maps made from close observations of eclipses by its biggest moon, Charon. Using computer processing, observances are made in brightness factors as Pluto is eclipsed by Charon.

For example, eclipsing a bright place on Pluto makes a larger total brightness change than eclipsing a grey spot. Using this method, one can quantify the whole average brightness of the Pluto-Charon system and trail occurrences in brightness over a period of time.

Mass and size

Pluto's diameter and mass were wrongly overestimated for many years after its discovery. It was believed to be quite big similar to the Earth's mass, but as observations got better the calculations of its size were revised sharply downward.

A satellite of Pluto, named Charon was found in 1978 and by the use of Newton' formulation of Keplar's third law the mass of the Pluto Charon system could be worked out. After it was found that Pluto was not on it's own its size was downgraded because the two objects were originally thought to be just one.

Further measurements were able to work out Pluto's diameter when at occultation with Charon. Pluto's diameter is 2,390 km, making it the biggest known object in the Kuiper Belt.

Pluto is small compared to other planets and moons in our solar system, less than 0.2 solar masses.


There is not much of an atmosphere on Pluto. It does have a thin of surrounding gas which is thought to consist of nitrogen, methane and carbon monoxide that changes into solid nitrogen and carbon monoxide ice on the surface as it moves away from the sun.

Pluto was found to have an atmosphere from an occultation observation in 1985 (IAU Circ. 4097; MNRAS 276, 571); the finding was confirmed and significantly strengthened by extensive observations of another occultation in 1988. When an object with no atmosphere occults a star, the star abruptly disappears; in the case of Pluto, the star dimmed out gradually. From the rate of dimming, the atmosphere was determined to have a pressure of 0.15 Pa, roughly 1/700,000 that of Earth.

In 2002, another occultation of a star by Pluto was observed and analyzed by teams led by Bruno Sicardy of the Paris Observatory and by James Elliot of MIT and Jay Pasachoff of Williams College. Surprisingly, the atmosphere was estimated to have a pressure of 0.3 Pa, even though Pluto was further from the Sun than in 1988, and hence should be colder and have a less dense atmosphere. The current best hypothesis is that the south pole of Pluto came out of shadow for the first time in 120 years in 1987, and extra nitrogen sublimated from a polar cap. It will take decades for the excess nitrogen to condense out of the atmosphere.

In October, 2006, the spectroscopic discovery of ethane (C2H6) on Pluto's surface, presented by Dale Cruikshank of NASA/Ames Research Center (a New Horizons co-investigator) and colleagues was announced. This ethane is produced from the photolysis or radiolysis (i.e., the chemical conversion driven by sunlight and charged particles) of frozen methane (CH4) on Pluto's surface and suspended in its atmosphere.

The MIT-Williams College team of James Elliot and Jay Pasachoff and a Southwest Research Institute team led by Leslie Young observed a further occultation of a star by Pluto on 12 June 2006 from sites in Australia. (Elliot, J. L., Person, M. J., Gulbis, A. A. S., Adams, E. R., Kramer, E. A., Zuluaga, C. A., Pike, R. E., Pasachoff, J. M., Souza, S. P., Babcock, B. A., Gangestad, J. W., Jaskot, A. E., Francis, P. J., Lucas, R., Bosh, A. S. 2006, "The Size of Pluto's Atmosphere As Revealed by the 2006 June 12 Occultation," Pasadena Division of Planetary Sciences meeting, October 2006.)


The surface of Pluto is remarkably heterogeneous, as evidenced by its lightcurve, maps of its surface constructed from Hubble Space Telescope observations, and by periodic variations in its infrared spectra. The face of Pluto oriented toward Charon has more methane ice, while the opposite face has more ices of nitrogen and carbon monoxide. This makes Pluto the second most contrasted body in the Solar System after Iapetus.


Pluto's orbit is very unusual in comparison to the planets of the solar system. The planets orbit the Sun close to an imaginary flat plane called the plane of the ecliptic, and have nearly circular orbits. In contrast, Pluto's orbit is highly inclined above the ecliptic (up to 17° above it) and very eccentric (non-circular). Owing to the orbit’s inclination, Pluto's perihelion is well above (~8.0 AU) the ecliptic. The high eccentricity means that part of Pluto's orbit is closer to the Sun than Neptune's.

Heliocentric distance

Near perihelion, Pluto gets closer to the Sun than Neptune; the most recent occurrence of this phenomenon lasted from February 7, 1979 through February 11, 1999. Mathematical calculations indicate that the previous occurrence lasted only fourteen years from July 11, 1735 to September 15, 1749. However, the same calculations indicate that Pluto was closer to the Sun than Neptune between April 30, 1483 and July 23, 1503, which is almost exactly the same length as the 1979 to 1999 period. Recent studies suggest each crossing of Pluto to inside Neptune's orbit lasts alternately for approximately thirteen and twenty years with minor variations.

Pluto orbits in a 3:2 orbital resonance with Neptune. When Neptune approaches Pluto from behind their gravity starts to pull on each other slightly, resulting in an interaction between their positions in orbit of the same sort that produces Trojan points. Since the orbits are eccentric, the 3:2 periodic ratio is favoured because this means Neptune always passes Pluto when they are almost farthest apart. Half a Pluto orbit later, when Pluto is nearing its closest approach, it initially seems as if Neptune is about to catch up with Pluto. But Pluto speeds up due to the gravitational acceleration from the Sun, stays ahead of Neptune, and pulls ahead until they meet again on the other side of Pluto's orbit.

Pluto is also affected by the Kozai mechanism, which causes its closest approach to the Sun to occur when it is farthest out of the plane of the Solar System, again keeping Pluto from getting too close to Neptune.

Beginning in the 1990s, other trans-Neptunian objects (TNOs) were discovered, and a certain number of these also have a 3:2 orbital resonance with Neptune. TNOs with this orbital resonance are named "plutinos", after Pluto.

Trans-Neptunian object

Pluto's orbit is often described as 'crossing' that of Neptune. In fact, Pluto's nodes (the points at which the orbit crosses the ecliptic) are both situated outside Neptune’s orbit and are separated by a distance of 6.4 AU (that is, over six times the distance of the Earth from the Sun). Furthermore, due to the orbital resonance between them, Pluto executes 2 full cycles while Neptune makes 3; this means that when Neptune reaches the 'closest' point on the orbit, Pluto remains far behind and when Pluto in turn reaches that point, Neptune is far (over 50°) ahead. During the following orbit of Pluto, Neptune is half an orbit away. Consequently, Pluto never gets closer than 30 AU to Neptune at this point in its orbit.

The actual closest approach between Neptune and Pluto occurs at the opposite part of the orbit, some 30 years after Pluto's aphelion (its last aphelion was in 1866) when Neptune catches up with Pluto (i.e. Neptune and Pluto have similar longitudes). The minimum distance was 18.9 AU in June 1896. In other words, Pluto never approaches Neptune much closer than it approaches Saturn.

In the 1950s it was suggested that Pluto was an escaped moon of Neptune, knocked out of orbit by Triton, Neptune's largest moon. This notion has since been discredited. Triton shares many atmospherical and geological composition similarities with Pluto and is believed to be a captured Kuiper belt object.

Comet comparison

The Kuiper belt is believed to be the source for all short-period comets, and Pluto, like other Kuiper Belt objects, shares features in common with comets. The solar wind is gradually blowing Pluto's surface into space, in the manner of a comet. If Pluto were placed near the Sun, it would develop a tail, like comets do.


Pluto has three known natural satellites: Charon, first identified in 1978 by astronomer James Christy; and two smaller moons, Nix and Hydra, both discovered in 2005.


The Pluto-Charon system is noteworthy for being the largest of the solar system's few binary systems, i.e. whose barycenter lies above the primary's surface (617 Patroclus is a smaller example). This and the large size of Charon relative to Pluto lead some astronomers to call it a dwarf double planet. The system is also unusual among planetary systems in that they are both tidally locked to each other: Charon always presents the same face to Pluto, and Pluto also always presents the same face to Charon.

Pluto and Charon, compared to Earth's Moon

(Pronunciation key)

Diameter (km) Mass (kg) Orbital radius (km)
Orbital period (d)
Pluto ploo'-toe
(65% Moon)
(18% Moon)
(0.6% Moon)
(25% Moon)
Charon shair'-ən
(35% Moon)
(2% Moon)
(5% Moon)

Nix and Hydra

Two additional moons of Pluto were imaged by astronomers working with the Hubble Space Telescope on May 15, 2005, and received provisional designations of S/2005 P 1 and S/2005 P 2. The International Astronomical Union officially christened Pluto's newest moons Nix (or Pluto II, the inner of the two moons, formerly P 2) and Hydra (Pluto III, the outer moon, formerly P 1), on June 21, 2006.

These small moons orbit Pluto at approximately two and three times the distance of Charon: Nix at 48,700 kilometres and Hydra at 64,800 kilometers from the barycenter of the system. They have nearly circular prograde orbits in the same orbital plane as Charon, and are very close to (but not in) 4:1 and 6:1 mean motion orbital resonances with Charon.

Observations of Nix and Hydra are ongoing to determine individual characteristics. Hydra is sometimes brighter than Nix, speculating that it either is larger in dimension or different parts of its surface may vary in brightness. Sizes are estimated from albedos. The moons' spectral similarity with Charon suggests a 35% albedo similar to Charon's; this results in diameter estimates of 46 kilometers for Nix and 61 kilometers for brighter Hydra. Upper limits on their diameters can be estimated by assuming the 4% albedo of the darkest Kuiper Belt objects; these bounds are 137 ± 11 km and 167 ± 10 km respectively. At the larger end of this range, the inferred masses are less than 0.3% of Charon's mass, or 0.03% of Pluto's.

With the discovery of the two small moons, Pluto may possess a variable ring system. Small body impacts can create debris that can form into a ring system. Data from a deep optical survey by the Advanced Camera for Surveys on the Hubble Space Telescope suggests that no ring system is present. If such a system exists, it is either tenuous like the Rings of Jupiter, or it is tightly confined to less than 1000km in width.


The distribution of Plutonian moons is highly unusual compared to other observed systems. Moons could potentially orbit Pluto up to 53% (or 69%, if retrograde) of the Hill sphere radius (stable gravitational zone of influence) of 6.0 million kilometers. In simple terms, an imaginary sphere is drawn around an object to represent the potential of an object to have other objects orbit it stably. For example, Psamathe orbits Neptune at 40% of the Hill radius. In the case of Pluto, only the inner 3% of the zone is known to be occupied by satellites. In the discoverers’ terms, the Plutonian system appears to be "highly compact and largely empty."

Additional moons?

In imaging the Plutonian system, observations from Hubble placed limits on any additional moons. With 90% confidence, no additional moons larger than 12 km (or a maximum of 37 km with an albedo of 0.041) exist beyond the glare of Pluto 5 arcseconds from the dwarf planet. This assumes a Charon-like albedo of 0.38; at a 50% confidence level the limit is 8 kilometers.

Exploration of Pluto

Pluto presents significant challenges for space craft because of its small mass and great distance from Earth. Voyager 1 could have visited Pluto, but controllers opted instead for a close flyby of Saturn's moon Titan, which resulted in a trajectory incompatible with a Pluto flyby. Voyager 2 never had a plausible trajectory for reaching Pluto. In 2000, NASA cancelled the Pluto Kuiper Express mission, citing increasing costs and launch vehicle delays.

The first spacecraft to visit Pluto will be NASA's New Horizons, launched on January 19, 2006. The craft will benefit from a gravity assist from Jupiter, and the closest approach to Pluto will be on July 14, 2015. Scientific observations of Pluto will begin 5 months prior to closest approach and will continue for at least a month after the encounter. New Horizons captured its first images of Pluto in late September 2006, during a test of the Long Range Reconnaissance Imager (LORRI). The images, taken from a distance of approximately 4.2 billion kilometres (2.6 billion miles), confirm the spacecraft's ability to track distant targets, critical for manoeuvring toward Pluto and other Kuiper Belt objects.

New Horizons will use a remote sensing package that includes imaging instruments and a radio science investigation tool, as well as spectroscopic and other experiments, to characterize the global geology and morphology of Pluto and its moon Charon, map their surface composition and characterize Pluto's neutral atmosphere and its escape rate. New Horizons will also photograph the surfaces of Pluto and Charon. Some of the ashes of Pluto's discoverer, Clyde W. Tombaugh, are aboard the spacecraft.

Discovery of moons Nix and Hydra may present unforeseen challenges for the probe. With the relatively low escape velocity of Nix and Hydra, collisions with Kuiper belt debris may produce a tenuous dusty ring. Were New Horizons to fly through such a ring system, there would be an increased potential for micrometeorite damage that could damage or disable the probe.

Planetary status controversy

Pluto's official status as a planet has been a constant subject of controversy, fueled by the past lack of a clear definition of planet, since at least as early as 1992, when the first Kuiper Belt Object, (15760) 1992 QB1, was discovered. Since then, further discoveries intensified the debate in the 21st century.

Omission from museum models

Museum and planetarium directors occasionally created controversy by omitting Pluto from planetary models of the solar system. Some omissions were intentional; the Hayden Planetarium reopened after renovation in 2000 with a model of 8 planets without Pluto. The controversy made headlines in the media at the time.

Commemoration as a planet

Pluto is shown as a planet on the Pioneer plaque, an inscription on the space probes Pioneer 10 and Pioneer 11, launched in the early 1970s. The plaque, intended to give information about the origin of the probes to any alien civilization that might in the future encounter the vehicles, includes a diagram of our solar system, showing nine planets. Similarly, an analog image contained within the Voyager Golden Record included on the probes Voyager 1 and Voyager 2 (also launched in the 1970s) includes data regarding Pluto and again shows it as the ninth planet.

Elements 92, 93, and 94 are named uranium, neptunium, and plutonium respectively after Uranus, Neptune, and Pluto. The Disney character Pluto, introduced in 1930, was also named in honour of the planet.

New discoveries ignite debate

Continuing advances in telescope technology allowed for further discoveries of Trans-Neptunian objects in the 21st century, some of comparable size to that of Pluto. In 2002, 50000 Quaoar was discovered, with a diameter of 1,280 kilometers, about half that of Pluto. In 2004, the discoverers of 90377 Sedna placed an upper limit of 1,800 kilometers on its diameter, near Pluto's diameter of 2,320 kilometers.

On July 29, 2005, a Trans-Neptunian object later named Eris was announced, which on the basis of its magnitude and simple albedo considerations is assumed to be slightly larger than Pluto. This was the largest object discovered in the solar system since Neptune in 1846. Discoverers and media initially called it the "tenth planet", although there was no official consensus at the time on whether to call it a planet. Others in the astronomical community considered the discovery to be the strongest argument for reclassifying Pluto as a minor planet.

The last remaining distinguishing feature of Pluto was now its large moon, Charon, and its atmosphere; these characteristics are probably not unique to Pluto: several other Trans-Neptunian objects have satellites; and Eris' spectrum suggests that it has a similar surface composition to Pluto, as well as a moon, Dysnomia, discovered in September 2005. Trans-Neptunian object 2003 EL61 (nicknamed "Santa") has two moons (one of which is nicknamed "Rudolph") and is the fourth largest TNO behind Eris, Pluto, and 2005 FY9 (nicknamed "Easterbunny").

IAU Decision

The debate came to a head in 2006 with an IAU resolution that created an official definition for the term "planet". According to this resolution, there are three main conditions for an object to be considered a 'planet':

  1. The object must be in orbit around the Sun.
  2. The object must be massive enough to be a sphere by its own gravitational force. More specifically, its own gravity should pull it into a shape of hydrostatic equilibrium.
  3. It must have cleared the neighborhood around its orbit.

Pluto fails to meet the third condition. The IAU further resolved that Pluto be classified in the simultaneously created dwarf planet category, and that it act as prototype for a yet-to-be-named category of trans-Neptunian objects, in which it would be separately, but concurrently, classified.

Impact of the IAU decision

There has been resistance amongst the astronomical community towards the reclassification, dubbed the "Great Pluto War" by some astronomers. Alan Stern, principal investigator with NASA's "New Horizons" mission to Pluto, has publicly derided the IAU resolution, stating that "the definition stinks" albeit "for technical reasons." Stern's current contention is that by the terms of the new definition Earth, Mars, Jupiter and Neptune, all of which share their orbits with asteroids would be excluded. However, his own published writing has supported the new list of planets, as "our solar system clearly contains" eight planets that have cleared their neighborhoods, however, he does not deny planetary classification to objects like Pluto & Ceres that have not "cleared their neighborhood". Marc W. Buie of the Lowell observatory has voiced his opinion on the new definition on his website and is one of the petitoners against the definition. Others have supported the IAU. Mike Brown, the astronomer who discovered Eris, said "through this whole crazy circus-like procedure, somehow the right answer was stumbled on. It’s been a long time coming. Science is self-correcting eventually, even when strong emotions are involved."

Among the general public, reception is mixed amidst widespread media coverage. Some have accepted the reclassification, while some are seeking to overturn the decision, with online petitions urging the IAU to consider reinstatement. A resolution introduced by some members of the California state assembly light-heartedly denounces the IAU for "scientific heresy," among other crimes. Others reject the change for sentimental reasons, citing that they have always known Pluto as a planet and will continue to do so regardless of the IAU decision.

The words "plutoed" and its variant "to pluto" were coined in the aftermath of the decision. In January 2007, the American Dialect Society chose "plutoed" as its 2006 Word of the Year, defining "to pluto" as "to demote or devalue someone or something", an example being "as happened to the former planet Pluto when the General Assembly of the International Astronomical Union decided Pluto no longer met its definition of a planet."

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