1.
Geography of Pluto
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The geography of Pluto entails the delineation and characterization of regions on Pluto. Plutonian geography is focused on what is called physical geography on Earth, that is. On 14 July 2015, the New Horizons spacecraft became the first spacecraft to fly by Pluto, during its brief flyby, New Horizons made detailed geographical measurements and observations of Pluto and its moons. Pluto may be defined as having retrograde rotation and a tilt of 60 degrees, or prograde rotation. Following the latter convention, the currently in daylight is the northern one. This is the used by the International Astronomical Union and the New Horizons team. However, older sources may define Plutos rotation as retrograde and therefore the sunlit side as the southern hemisphere, east and west are also swapped between the two conventions. The following names are names and nicknames, proposed by the New Horizons discovery team. They were taken from various deities or demons, some from ancient mythology and others from modern popular culture. Cthulhu Regio, nicknamed The Whale after its shape, is an elongated and it is 2,990 km long and is the largest dark feature on Pluto. Tombaugh Regio, nicknamed The Heart, is a large, light-colored region in the shape of a named after the discoverer of Pluto. Two lobes of the heart are distinct geological features sharing a bright appearance, the heart is about 1,590 km across. The region contains 3,400 m mountains made of ice, Hillary Montes and Norgay Montes. The lack of craters suggests that its there is less than 100 million years old. Subsequent data indicated that features near the edges of the region show evidence of ice flow such as glaciers, a series of semi-regularly spaced dark spots with irregular boundaries are nicknamed the Brass Knuckles. They average about 480 km in diameter and are located along the equator between the Heart and the tail of the Whale, the brass knuckles are separated from one another by tall uplands. There are also many canyons running through them and through the mountains that are hundreds of miles long. It does not appear as a ring in higher-resolution images, the prime meridian of Pluto is defined as the longitude facing Charon
2.
Pluto
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Pluto is a dwarf planet in the Kuiper belt, a ring of bodies beyond Neptune. It was the first Kuiper belt object to be discovered, Pluto was discovered by Clyde Tombaugh in 1930 and was originally considered to be the ninth planet from the Sun. After 1992, its planethood was questioned following the discovery of objects of similar size in the Kuiper belt. In 2005, Eris, which is 27% more massive than Pluto, was discovered and this led the International Astronomical Union to define the term planet formally in 2006, during their 26th General Assembly. That definition excluded Pluto and reclassified it as a dwarf planet, Pluto is the largest and second-most-massive known dwarf planet in the Solar System and the ninth-largest and tenth-most-massive known object directly orbiting the Sun. It is the largest known trans-Neptunian object by volume but is less massive than Eris, like other Kuiper belt objects, Pluto is primarily made of ice and rock and is relatively small—about one-sixth the mass of the Moon and one-third its volume. It has an eccentric and inclined orbit during which it ranges from 30 to 49 astronomical units or AU from the Sun. This means that Pluto periodically comes closer to the Sun than Neptune, light from the Sun takes about 5.5 hours to reach Pluto at its average distance. Pluto has five moons, Charon, Styx, Nix, Kerberos. Pluto and Charon are sometimes considered a system because the barycenter of their orbits does not lie within either body. The IAU has not formalized a definition for binary dwarf planets, on July 14,2015, the New Horizons spacecraft became the first spacecraft to fly by Pluto. During its brief flyby, New Horizons made detailed measurements and observations of Pluto, on October 25,2016, at 05,48 pm ET, the last bit of data was received from New Horizons from its close encounter with Pluto on July 14,2015. In the 1840s, Urbain Le Verrier used Newtonian mechanics to predict the position of the then-undiscovered planet Neptune after analysing perturbations in the orbit of Uranus. Subsequent observations of Neptune in the late 19th century led astronomers to speculate that Uranuss orbit was being disturbed by another planet besides Neptune, by 1909, Lowell and William H. Pickering had suggested several possible celestial coordinates for such a planet. Lowell and his observatory conducted his search until his death in 1916, unknown to Lowell, his surveys had captured two faint images of Pluto on March 19 and April 7,1915, but they were not recognized for what they were. There are fourteen other known prediscovery observations, with the oldest made by the Yerkes Observatory on August 20,1909. Percivals widow, Constance Lowell, entered into a legal battle with the Lowell Observatory over her late husbands legacy. Tombaughs task was to image the night sky in pairs of photographs, then examine each pair
3.
Charon (moon)
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Charon, also known as Pluto I, is the largest of the five known natural satellites of the dwarf planet Pluto. It was discovered in 1978 at the United States Naval Observatory in Washington, with half the diameter and one eighth the mass of Pluto, it is a very large moon in comparison to its parent body. Its gravitational influence is such that the barycenter of the Pluto–Charon system lies outside Pluto, the New Horizons spacecraft is the only probe that has visited the Pluto system. It approached Charon to within 27,000 km, Charon was discovered by United States Naval Observatory astronomer James Christy, using the 1. 55-meter Flagstaff telescope two months prior. Christy noticed that a slight elongation appeared periodically, later, the bulge was confirmed on plates dating back to April 29,1965. Subsequent observations of Pluto determined that the bulge was due to an accompanying body. The periodicity of the bulge corresponded to Plutos rotation period, which was known from Plutos light curve. This indicated a synchronous orbit, which suggested that the bulge effect was real. Doubts about Charons existence were erased when it and Pluto entered a period of mutual eclipses. This occurs when the Pluto–Charon orbital plane is edge-on as seen from Earth and it was fortuitous that one of these intervals happened to occur soon after Charons discovery. Charon was originally known by the temporary designation S/1978 P1, on June 24,1978, Christy first suggested the name Charon as a scientific-sounding version of his wife Charlenes nickname, Char. The IAU officially adopted the name in late 1985 and it was announced on January 3,1986, there is minor debate over the preferred pronunciation of the name. These indicate only one pronunciation of Charon when referring specifically to Plutos moon, speakers of many languages other than English, and many English-speaking astronomers as well, follow this pronunciation. However, Christy himself pronounced the ch as sh, after his wife Charlene, simulation work published in 2005 by Robin Canup suggested that Charon could have been formed by a collision around 4.5 billion years ago, much like Earth and the Moon. In this model, a large Kuiper belt object struck Pluto at high velocity, destroying itself and blasting off much of Plutos outer mantle, however, such an impact should result in an icier Charon and rockier Pluto than scientists have found. It is now thought that Pluto and Charon might have been two bodies that collided before going into orbit about each other, the collision would have been violent enough to boil off volatile ices like methane but not violent enough to have destroyed either body. The very similar density of Pluto and Charon implies that the parent bodies were not fully differentiated when the impact occurred, Charon and Pluto orbit each other every 6.387 days. The two objects are locked to one another, so each keeps the same face towards the other
4.
Cthulhu Regio
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Cthulhu Regio /kəˈθuːluː, ˈkθuː- ˈrɛdʒi. oʊ/ is a prominent surface feature of the dwarf planet Pluto, that is reminiscent of a whale in shape. It is a dark region along Plutos equator,2,990 km long. It is west of the Sputnik Planitia region of Tombaugh Regio, also known as Plutos heart, and to the east of Meng-Po, as of 30 July 2015, the eastern head region had been imaged in much higher resolution than the western tail region. NASA initially referred to it as the Whale in reference to its overall shape, by 14 July 2015, the provisional name Cthulhu was being used by the New Horizons team. It was named after the deity from the works of H. P. Lovecraft. The character Cthulhu initially appeared in Lovecrafts 1928 short story The Call of Cthulhu and it is the subject of worship by a number of human cults asserting that while it is currently trapped, Cthulhu will eventually return. Cthulhu was the most popular name in this category of the poll, the name received positive reaction from the press and social media, with a Chicago Tribune editorial supporting the name and its democratic origin. The name Cthulhu may be submitted to the IAU as an official name, Cthulhu Regio contains many craters and linear features that have also been given informal names. Geography of Pluto Media related to Cthulhu Regio at Wikimedia Commons
5.
The New York Times
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The New York Times is an American daily newspaper, founded and continuously published in New York City since September 18,1851, by The New York Times Company. The New York Times has won 119 Pulitzer Prizes, more than any other newspaper, the papers print version in 2013 had the second-largest circulation, behind The Wall Street Journal, and the largest circulation among the metropolitan newspapers in the US. The New York Times is ranked 18th in the world by circulation, following industry trends, its weekday circulation had fallen in 2009 to fewer than one million. Nicknamed The Gray Lady, The New York Times has long been regarded within the industry as a newspaper of record. The New York Times international version, formerly the International Herald Tribune, is now called the New York Times International Edition, the papers motto, All the News Thats Fit to Print, appears in the upper left-hand corner of the front page. On Sunday, The New York Times is supplemented by the Sunday Review, The New York Times Book Review, The New York Times Magazine and T, some other early investors of the company were Edwin B. Morgan and Edward B. We do not believe that everything in Society is either right or exactly wrong, —what is good we desire to preserve and improve, —what is evil, to exterminate. In 1852, the started a western division, The Times of California that arrived whenever a mail boat got to California. However, when local California newspapers came into prominence, the effort failed, the newspaper shortened its name to The New-York Times in 1857. It dropped the hyphen in the city name in the 1890s, One of the earliest public controversies it was involved with was the Mortara Affair, the subject of twenty editorials it published alone. At Newspaper Row, across from City Hall, Henry Raymond, owner and editor of The New York Times, averted the rioters with Gatling guns, in 1869, Raymond died, and George Jones took over as publisher. Tweed offered The New York Times five million dollars to not publish the story, in the 1880s, The New York Times transitioned gradually from editorially supporting Republican Party candidates to becoming more politically independent and analytical. In 1884, the paper supported Democrat Grover Cleveland in his first presidential campaign, while this move cost The New York Times readership among its more progressive and Republican readers, the paper eventually regained most of its lost ground within a few years. However, the newspaper was financially crippled by the Panic of 1893, the paper slowly acquired a reputation for even-handedness and accurate modern reporting, especially by the 1890s under the guidance of Ochs. Under Ochs guidance, continuing and expanding upon the Henry Raymond tradition, The New York Times achieved international scope, circulation, in 1910, the first air delivery of The New York Times to Philadelphia began. The New York Times first trans-Atlantic delivery by air to London occurred in 1919 by dirigible, airplane Edition was sent by plane to Chicago so it could be in the hands of Republican convention delegates by evening. In the 1940s, the extended its breadth and reach. The crossword began appearing regularly in 1942, and the section in 1946
6.
Tombaugh Regio
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Tombaugh Regio /ˈtɒmbaʊ ˈrɛdʒi. oʊ/, nicknamed The Heart after its shape, is the largest bright surface feature of the dwarf planet Pluto. It is just north of the equator, to the northeast of Cthulhu Regio and to the northwest of Krun Macula and its western lobe, a 1000 km-wide plain of nitrogen and other ices, is named Sputnik Planitia. The eastern lobe is thought to consist of high-albedo uplands coated by nitrogen transported through the atmosphere from Sputnik Planitia, some of this nitrogen ice then returns to Sputnik Planitia via glacial flow. It is named after Clyde Tombaugh, the discoverer of Pluto, Tombaugh Regio is a large, light-colored region about 1,590 km across. The two lobes of the feature are geologically distinct, the western lobe, Sputnik Planitia, is smoother than the eastern, and they are of slightly different colors. Early speculation was that the western lobe may be an impact crater filled with nitrogen snow. Bright spots within the region were initially speculated to be mountain peaks, photos, released on 15 July 2015, revealed 3,400 m mountains made of water ice in the feature, they also showed no craters in this same region. The surface of Sputnik Planitia is divided into polygonal convection cells and is less than 10 million years old, indicating that Pluto is geologically active. The feature had been identified as a spot for six decades prior to the New Horizons flyby. Over these six decades the spot had been observed to be dimming, Tombaugh Regio was first identified in the initial image of Pluto returned after the New Horizons probe recovered from an anomaly that temporarily sent it into safe mode. NASA initially referred to it as a heart in reference to its overall shape, on 15 July 2015, the region was named Tombaugh Regio by the New Horizons team in honor of astronomer Clyde Tombaugh, the discoverer of Pluto, regio being Latin for region. Some people find the feature also resembles the Disney character Pluto, the Walt Disney Company acknowledged this perceived likeness in a short animation. Geography of Pluto List of geological features on Pluto Media related to Tombaugh Regio at Wikimedia Commons
7.
Sputnik Planitia
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Sputnik Planitia /ˈspʌtnɪk pləˈnɪʃiə/, originally Sputnik Planum, is a high-albedo ice-covered basin on Pluto, about 1,050 by 800 km in size. It lies mostly in the hemisphere, but extends across the equator. Much of it has a surface of irregular polygons separated by troughs, the polygons are on average about 33 km across. In some cases troughs are populated by blocky mountains or hills, Sputnik Planitia constitutes the western lobe of the heart-shaped Tombaugh Regio. There appear to be wind streaks on the surface evidence of sublimation. The dark streaks are a few kilometers long and all aligned in the same direction, the basin also contains pits that have possibly formed by sublimation. Sputnik Planitia is named after Earths first artificial satellite, the planitia has no craters that were detectable by New Horizons, leading to the conclusion that its surface must be less than 10 million years old. The ice composing the basin is thought to consist primarily of ice, with smaller fractions of carbon monoxide and methane ice. At Plutos ambient temperature of 38 K, nitrogen and carbon monoxide ices are denser and much less rigid water ice, making glacial-like flows possible. Sputnik Planitia may have originated as a basin that subsequently collected volatile ices. Alternatively, it has suggested that the accumulation of ices in this location depressed the surface there. The accumulation of several kilometers of ice in the basin was in part a consequence of its higher surface pressure. The positive temperature gradient of Plutos atmosphere contributes to making a topographic depression a cold trap, a high seasonal thermal inertia of Plutos surface is an important driver of deposition of nitrogen ice at low latitudes. These latitudes receive less insolation than Plutos polar regions due to its high obliquity. However, other effects of an event could have also contributed to such an anomaly. A positive gravity anomaly could have caused polar wander, reorienting the spin axis of Pluto to put the planitia near the Pluto-Charon tidal axis, Sputnik Planitia is presently close to the anti-Charon point on Pluto, a result that has less than a 5% probability of arising by chance. Gradual freezing of such an ocean, in combination with polar wander, convection cells have about 100 m of vertical relief, with the highest points being at their centers. Modeling of nitrogen ice convection cells suggests a depth of one tenth their width, or 3–4 km for most of the planitia
8.
Atmosphere of Pluto
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The atmosphere of Pluto is the tenuous layer of gases surrounding Pluto. It consists mainly of nitrogen, with amounts of methane and carbon monoxide. It contains layered haze, probably consisting of compounds which form from these gases due to high-energy radiation. The atmosphere of Pluto is notable for its strong and not completely clear seasonal changes caused by peculiarities of the orbital and axial rotation of Pluto. Surface pressure of the atmosphere of Pluto, measured by New Horizons in 2015, is about 1 Pa, temperature on the surface is 40 to 60 K, but it quickly grows with altitude due to methane-generated greenhouse effect. Near the altitude 30 km it reaches 110 K, and then slowly decreases, Pluto is the only trans-Neptunian object with a known atmosphere. Its closest analog is the atmosphere of Triton, although in some aspects it resembles even the atmosphere of Mars, the atmosphere of Pluto has been studied since the 1980s by way of earth-based observation of occultations of stars by Pluto and spectroscopy. In 2015, it was studied from a distance by the spacecraft New Horizons. The main component of the atmosphere of Pluto is nitrogen, content of methane, according to measurements by New Horizons, is 0. 25%. For carbon monoxide, the Earth-based estimates are 0. 025–0. 15% and 0. 05–0. 075%, under influence of high-energy cosmic radiation, these gases react to form more complex compounds, including ethane, ethylene, acetylene, heavier hydrocarbons and nitriles and hydrogen cyanide. These compounds slowly precipitate on the surface, probably, they also include tholins, which are responsible for the brown color of Pluto. The most volatile compound of the atmosphere of Pluto is nitrogen, the second is carbon monoxide, the indicator of volatility is saturated vapor pressure. At temperature 40 K it is about 10 Pa for nitrogen,1 Pa for carbon monoxide and 0.001 Pa for methane and it quickly increases with temperature, and at 60 K approaches to 10000 Pa,3000 Pa and 10 Pa respectively. For heavier-than-methane hydrocarbons, water, ammonia, carbon dioxide and hydrogen cyanide, this pressure remains negligibly low, but actually concentration of, at least, methane, does not depends noticeably of height, longitude and time. But temperature dependence of volatility of methane and nitrogen suggest that concentration of methane will decrease during moving of Pluto further from the Sun, reasons of this discrepancy are unknown. It can be due to existence of separate patches of relatively clean methane ice, seasonal and orbital changes of insolation results in migration of surface ices, they sublimate in some places and condensate in another. According to some estimates, it gives meter-sized changes of their thickness and this results in appreciable changes of brightness and color of Pluto. Methane and carbon monoxide, despite their low abundance, are significant for thermal structure of the atmosphere, methane is a strong heating agent and carbon monoxide is a cooling one
9.
Moons of Pluto
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The dwarf planet Pluto has five moons down to a detection limit of about 1 km in diameter. In order of distance from Pluto, they are Charon, Styx, Nix, Kerberos, Charon, the largest of the five moons, is mutually tidally locked with Pluto, and is massive enough that Pluto–Charon is sometimes considered a double dwarf planet. The innermost and largest moon, Charon, was discovered by James Christy on 22 June 1978 and this led to a substantial revision in estimates of Plutos size, which had previously assumed that the observed mass and reflected light of the system were all attributable to Pluto alone. The International Astronomical Union officially named these moons Nix and Hydra, Styx, announced on 7 July 2012, was discovered while looking for potential hazards for New Horizons. Charon is about half the diameter of Pluto and is so massive that the barycenter lies between them, approximately 960 km above Plutos surface. Charon and Pluto are also locked, so that they always present the same face toward each other. The IAU General Assembly in August 2006 considered a proposal that Pluto and Charon be reclassified as a double planet, but the proposal was abandoned. Plutos four small moons orbit Pluto at two to four times the distance of Charon, ranging from Styx at 42,700 kilometres to Hydra at 64,800 kilometres from the barycenter of the system and they have nearly circular prograde orbits in the same orbital plane as Charon. All are much smaller than Charon, Nix and Hydra, the two larger, are roughly 42 and 55 kilometers on their longest axis respectively, and Styx and Kerberos are 7 and 12 kilometers respectively. The Pluto system is compact and largely empty. Moons could potentially orbit Pluto at up to 53% of the Hill radius, for example, Psamathe orbits Neptune at 40% of the Hill radius. An intense search conducted by New Horizons confirmed that no larger than 4.5 km in diameter exist at the distances up to 180,000 km from Pluto. Further information is expected as more data from the New Horizons flyby is sent back to Earth. The orbits of the moons are confirmed to be circular and coplanar, with inclinations differing less than 0. 4°, as seen from Earth, these circular orbits appear foreshortened into ellipses depending on Plutos position. The discovery of Nix and Hydra suggested that Pluto could have a ring system, small-body impacts can create debris that can form into a ring system. However, data from a survey by the Advanced Camera for Surveys on the Hubble Space Telescope, by occultation studies. Styx, Nix, and Hydra are thought to be in a 3-body orbital resonance with orbital periods in a ratio of 18,22,33, the ratios should be exact when orbital precession is taken into account. This means that in a recurring cycle there are 11 orbits of Styx for every 9 of Nix and 6 of Hydra, putting Nix and Hydra into a simple 2,3 resonance
10.
Nix (moon)
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Nix is a natural satellite of Pluto. It was discovered along with Hydra in June 2005 and it was imaged along with Pluto and its other moons by the New Horizons spacecraft as it flew by the Pluto system in July 2015. Of the four small Plutonian moons the best pictures are of Nix, the discovery images were taken on 15 May 2005 and 18 May 2005. Nix and Hydra were independently discovered by Max J. Mutchler on 15 June 2005, the discoveries were announced on 31 October 2005, after confirmation by precoveries from 2002. They were provisionally designated S/2005 P1 and S/2005 P2 and casually referred to as P1 and P2, respectively. The formal name Nix, from the Greek goddess of darkness and night and mother of Charon, was announced on 21 June 2006 on IAU Circular 8723, the initials N and H, for Nix and Hydra, come from New Horizons. The original proposal was to use the classical spelling Nyx, but to avoid confusion with the asteroid 3908 Nyx, jürgen Blunck explains it as the Spanish translation of the Greek name. Nix follows an orbit in the same plane as Charon. It is in a 3,2 orbital resonance with Hydra, as a result of this Laplace-like 3-body resonance, it has conjunctions with Styx and Hydra in a 2,3 ratio. Its orbital period of 24.9 days is also close to a 1,4 orbital resonance with Charon, as with Hydra and perhaps the other small Plutonian moons, Nix tumbles chaotically, the moons axial tilt and day length vary greatly over short timescales. The rotation period has increased by 10% since Nix was discovered. Nix has been measured to be 54 by 41 by 36 kilometers in diameter, indicating an elongated shape. In the discovery image, Nix is 6,300 times fainter than Pluto, New Horizons determined that Nix is approximately 32 kilometers across. Early research appeared to show that Nix was reddish like Pluto and unlike the other moons, New Horizons spotted a large region with a distinctive red tint surrounding a large crater, which may explain these conflicting results. Detailed images of the moon have revealed at least 6 impact craters on it, New Horizons is the only spacecraft that has flown through the Pluto system and hence the only one that has explored Nix. Until 13 July 2015, when NASAs Long Range Reconnaissance Imager on board New Horizons determined Nixs size, all, of Plutos smaller satellites, it was the only one clearly imaged. Steffl, A. J. Mutchler, M. J. Weaver, H. A. Stern, New Constraints on Additional Satellites of the Pluto System
11.
Definition of planet
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The definition of planet, since the word was coined by the ancient Greeks, has included within its scope a wide range of celestial bodies. Greek astronomers employed the term asteres planetai, wandering stars, for objects which apparently moved over the sky. Over the millennia, the term has included a variety of different objects, from the Sun and the Moon to satellites and asteroids. By the end of the 19th century the word planet, though it had yet to be defined, had become a term applied only to a small set of objects in the Solar System. After 1992, however, astronomers began to discover many additional objects beyond the orbit of Neptune and these discoveries not only increased the number of potential planets, but also expanded their variety and peculiarity. Some were nearly enough to be stars, while others were smaller than Earths moon. These discoveries challenged long-perceived notions of what a planet could be, the issue of a clear definition for planet came to a head in January 2005 with the discovery of the trans-Neptunian object Eris, a body more massive than the smallest then-accepted planet, Pluto. Under this new definition, Pluto and the other trans-Neptunian objects do not qualify as planets, the IAUs decision has not resolved all controversies, and while many scientists have accepted the definition, some in the astronomical community have rejected it outright. While knowledge of the planets history and is common to most civilizations. The five bodies currently called planets that were known to the Greeks were those visible to the eye, Mercury, Venus, Mars, Jupiter. In his Timaeus, written in roughly 360 BC, Plato mentions, the Sun and Moon and five other stars and his student Aristotle makes a similar distinction in his On the Heavens, The movements of the sun and moon are fewer than those of some of the planets. In his Almagest written in the 2nd century, Ptolemy refers to the Sun, Moon, hyginus explicitly mentions the five stars which many have called wandering, and which the Greeks call Planeta. In his Natural History, written in 77 AD, Pliny the Elder refers to the seven stars, nonnus, the 5th century Greek poet, says in his Dionysiaca, I have oracles of history on seven tablets, and the tablets bear the names of the seven planets. Medieval and Renaissance writers generally accepted the idea of seven planets, indicating that the Sun and the Moon were planets. Even Nicolaus Copernicus, who rejected the model, was ambivalent concerning whether the Sun. And of the five other planets, as the historian of science Thomas Kuhn noted in his book, The Structure of Scientific Revolutions, The Copernicans who denied its traditional title planet to the sun. Were changing the meaning of planet so that it would continue to make distinctions in a world where all celestial bodies. Were seen differently from the way they had seen before
12.
Dwarf planet
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A dwarf planet is a planetary-mass object that is neither a planet nor a natural satellite. The International Astronomical Union currently recognizes five dwarf planets, Ceres, Pluto, Haumea, Makemake, another hundred or so known objects in the Solar System are suspected to be dwarf planets. Individual astronomers recognize several of these, and in August 2011 Mike Brown published a list of 390 candidate objects, Stern states that there are more than a dozen known dwarf planets. Only two of these bodies, Ceres and Pluto, have observed in enough detail to demonstrate that they actually fit the IAUs definition. The IAU accepted Eris as a dwarf planet because it is more massive than Pluto and they subsequently decided that unnamed trans-Neptunian objects with an absolute magnitude brighter than +1 are to be named under the assumption that they are dwarf planets. The classification of bodies in other systems with the characteristics of dwarf planets has not been addressed. Starting in 1801, astronomers discovered Ceres and other bodies between Mars and Jupiter which were for some decades considered to be planets. Between then and around 1851, when the number of planets had reached 23, astronomers started using the asteroid for the smaller bodies. With the discovery of Pluto in 1930, most astronomers considered the Solar System to have nine planets and it was roughly one-twentieth the mass of Mercury, which made Pluto by far the smallest planet. Although it was more than ten times as massive as the largest object in the asteroid belt, Ceres. In the 1990s, astronomers began to find objects in the region of space as Pluto. Many of these shared several of Plutos key orbital characteristics, and Pluto started being seen as the largest member of a new class of objects and this led some astronomers to stop referring to Pluto as a planet. Several terms, including subplanet and planetoid, started to be used for the now known as dwarf planets. By 2005, three trans-Neptunian objects comparable in size to Pluto had been reported and it became clear that either they would also have to be classified as planets, or Pluto would have to be reclassified. Astronomers were also confident that more objects as large as Pluto would be discovered, Eris was discovered in January 2005, it was thought to be slightly larger than Pluto, and some reports informally referred to it as the tenth planet. As a consequence, the became a matter of intense debate during the IAU General Assembly in August 2006. The IAUs initial draft proposal included Charon, Eris, and Ceres in the list of planets, dropping Charon from the list, the new proposal also removed Pluto, Ceres, and Eris, because they have not cleared their orbits. The IAUs final Resolution 5A preserved this three-category system for the bodies orbiting the Sun
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