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American public's reaction

Reception to the IAU decision was mixed. While some accepted the reclassification, others seek 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.Acer Travelmate 2200 Adapter

The U.S. state of New Mexico's House of Representatives passed a resolution in honor of Tombaugh, a longtime resident of that state, which declared that Pluto will always be considered a planet while in New Mexican skies and that March 13, 2007 was Pluto Planet Day.HP Compaq 6735b Adapter

The Illinois State Senate passed a similar resolution in 2009, on the basis that Clyde Tombaugh, the discoverer of Pluto, was born in Illinois. The resolution asserted that Pluto was "unfairly downgraded to a 'dwarf' planet" by the IAU. Dell Vostro 1000 Battery

Some members of the public have also rejected the change, citing the disagreement within the scientific community on the issue, or for sentimental reasons, maintaining that they have always known Pluto as a planet and will continue to do so regardless of the IAU decision.HP Compaq nx7300 Adapter

Plutoed

The verb to pluto (preterite and past participle: plutoed) was coined in the aftermath of the 2006 IAU decision. In January 2007, the American Dialect Societychose plutoed as its 2006 Word of the Year, defining to pluto as "to demote or devalue someone or something,Asus f82 Battery

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."

Society president Cleveland Evans stated the reason for the organization's selection of plutoed: Dell XPS M1330 Battery

"Our members believe the great emotional reaction of the public to the demotion of Pluto shows the importance of Pluto as a name. We may no longer believe in the Roman god Pluto, but we still have a sense of connection with the former planet."HP Compaq Business 6910p Battery

Following the discovery of the planet Neptune in 1846, there was considerable speculation that another planet might exist beyond its orbit. The search began in the mid-19th century but culminated at the start of the 20th with Percival Lowell's quest for Planet X.Compaq PRESARIO R3000 Battery

Lowell proposed the Planet X hypothesis to explain apparent discrepancies in the orbits of the gas giants, particularly Uranus and Neptune,[1] speculating that the gravity of a large unseen ninth planet could have perturbed Uranus enough to account for the irregularities.HP Compaq Business NX6110 Battery

Clyde Tombaugh's discovery of Pluto in 1930 appeared to validate Lowell's hypothesis, and Pluto was officially considered the ninth planet until 2006. In 1978, however, Pluto was found to be too small for its gravity to affect the gas giants, resulting in a brief search for a tenth planet. Dell Latitude D610 Adapter

The search was largely abandoned in the early 1990s, when a study of measurements made by the Voyager 2 spacecraft found that the irregularities observed in Uranus's orbit were due to a slight overestimation of Neptune's mass.[3] After 1992, the discovery of numerous small icy objects with similar or even wider orbits than Pluto led to a debate over whether Pluto should remain a planet,Asus Eee PC 701 Adapter

or whether it and its neighbours should, like the asteroids, be given their own separate classification. Although a number of the larger members of this group were initially described as planets, in 2006 the International Astronomical Unionreclassified Pluto and its largest neighbours as dwarf planets, leaving only eight planets in the Solar System.HP Compaq Business NX6325 Battery

Today, the astronomical community widely agrees that Planet X, as originally envisioned, does not exist. However, the concept of Planet X has been revived by a number of astronomers to explain other anomalies observed in the outer Solar System. Dell Studio XPS 16 Battery

In popular culture, and even among some astronomers,[5] Planet X has become a stand-in term for any undiscovered planet in the outer Solar System, regardless of its relationship to Lowell's hypothesis. Other trans-Neptunian planets have also been suggested, based on different evidence.Acer Aspire 5520 Battery

Early speculation

In the 1840s, the French mathematician Urbain Le Verrier used Newtonian mechanics to analyse perturbations in the orbit of Uranus, and hypothesised that they were caused by the gravitational pull of a yet-undiscovered planet. Le Verrier predicted the position of this new planet and sent his calculations to German astronomer Johann Gottfried Galle. Acer TravelMate 5720 Adapter

On 23 September 1846, the night following his receipt of the letter, Galle and his student Heinrich d'Arrest discoveredNeptune, exactly where Le Verrier had predicted.[6] There remained, however, some slight discrepancies in the gas giants' orbits. These were taken to indicate the existence of yet another planet orbiting beyond Neptune.HP Compaq 6735s Adapter

Even before Neptune's discovery, some speculated that one planet alone would not be enough to explain the discrepancy. On 17 November 1834, the British amateur astronomer the Reverend Thomas John Hussey reported a conversation he had had with French astronomer Alexis Bouvard to George Biddell Airy, the British Astronomer Royal.Dell XPS M1330 Battery

Hussey reported that when he suggested to Bouvard that the unusual motion of Uranus might be due to the gravitational influence of an undiscovered planet, Bouvard replied that the idea had occurred to him, and that he had corresponded with Peter Andreas Hansen, director of the Seeberg Observatory in Gotha, about the subject. HP Compaq nc6230 Adapter

Hansen's opinion was that a single body could not adequately explain the motion of Uranus, and theorised that two planets lay beyond Uranus.[7]

In 1848, Jacques Babinet raised an objection to Le Verrier's calculations, claiming that Neptune's observed mass was smaller and its orbit larger than Le Verrier had initially predicted.Dell T117C Battery

He postulated, based largely on simple subtraction from Le Verrier's calculations, that another planet of roughly 12 Earth masses, which he named "Hyperion", must exist beyond Neptune.[7] Le Verrier denounced Babinet's hypothesis, saying, " absolutely nothing by which one could determine the position of another planet, barring hypotheses in which imagination played too large a part."Asus x5dij-sx039c Battery

In 1850 James Ferguson, Assistant Astronomer at the US Naval Observatory, noted that he had "lost" a star he had observed, GR1719k, which Lt. Matthew Maury, the superintendent of the Observatory, claimed was evidence that it must be a new planet.Dell Inspiron 1464 Battery

However, subsequent searches failed to recover the "planet" in a different position, and in 1878, CHF Peters, director of the Hamilton College Observatory in New York, showed that the star had not in fact vanished, and that the previous results had been due to human error.HP Compaq 6720s Adapter

In 1879, Camille Flammarion noted that the comets 1862 III and 1889 III had aphelia of 47 and 49 AU, respectively, suggesting that they might mark the orbital radius of an unknown planet that had dragged them into an elliptical orbit. Astronomer Georges Forbes concluded on the basis of this evidence that two planets must exist beyond Neptune. Dell Latitude D630 Adapter

He calculated, based on the fact that four comets possessed aphelia at around 100 AU and a further six with aphelia clustered at around 300 AU, the orbital elements of a pair of hypothetical trans–Neptunian planets. These elements concorded suggestively with those made independently by another astronomer named David Peck Todd, suggesting to many that they might be valid.HP Pavilion dm4 Battery

However, skeptics argued that the orbits of the comets involved were still too uncertain to produce meaningful results.

In 1900 and 1901, Harvard Observatory director William Henry Pickering led two searches for trans-Neptunian planets. Sony NP-FS11 Battery

The first was begun by Danish astronomer Hans Emil Lau who, after studying the data on the orbit of Uranus from 1690 to 1895, concluded that one trans-Neptunian planet alone could not account for the discrepancies in its orbit, and postulated the position of two planets he believed were responsible. Dell Inspiron 1520 Adapter

The second was launched when Gabriel Dallet suggested that a single trans-Neptunian planet lying at 47 AU could account for the motion of Uranus. Pickering agreed to examine plates for any suspected planets. In neither case was any found.Toshiba PA3399U-1BRS Battery

In 1909, Thomas Jefferson Jackson See, an astronomer with a reputation as an egocentric contrarian, opined "that there is certainly one, most likely two and possibly three planets beyond Neptune". Tentatively naming the first planet "Oceanus", he placed their respective distances at 42, 56 and 72 AU from the Sun. Acer Aspire 5920G Adapter

However, he gave no indication as to how he determined their existence, and no known searches were mounted to locate them.

In 1911, Indian astronomer Venkatesh P. Ketakar suggested the existence of two trans-Neptunian planets, which he named Brahma and Vishnu, Acer Aspire 5920 Battery

by reworking the patterns observed by Pierre-Simon Laplace in the planetary satellites of Jupiter and applying them to the outer planets.[9] The three inner Galilean moons of Jupiter, Io, Europa and Ganymede, are locked in a complicated 1:2:4 resonance called a Laplace resonance.IBM Thinkpad T40 Adapter

Ketakar suggested that Uranus, Neptune and his hypothetical trans-Neptunian planets were locked in Laplace-like resonances. His calculations predicted a mean distance for Brahma of 38.95 AU and an orbital period of 242.28 Earth years (3:4 resonance with Neptune). Nikon COOLPIX 4300 Battery

When Pluto was discovered 19 years later, its mean distance of 39.48 AU and orbital period of 248 Earth years would be quite close to Ketakar's prediction (Pluto in fact has a 2:3 resonance with Neptune). However Ketakar made no predictions for the orbital elements other than mean distance and period.Apple MB771 Battery

It is not clear how Ketakar arrived at these figures, and his second planet, Vishnu, was never located.[9]

Planet X

In 1894, with the help of William Pickering, Percival Lowell, a wealthy Bostonian aristocrat, founded the Lowell Observatory in Flagstaff, Arizona.Sony DSC-P5 Battery

In 1906, convinced he could resolve the conundrum of Uranus's orbit, he began an extensive project to search for a trans-Neptunian planet,[11] which he named Planet X. The X in the name represents an unknown and is pronounced as the letter, as opposed to the Roman numeral for 10 (Planet X would, at the time, have been the ninth planet). Dell Inspiron 6400 Adapter

Lowell's hope in tracking down Planet X was to establish his scientific credibility, which had eluded him thanks to his widely derided belief that channel-like features visible on the surface of Mars were canals constructed by an intelligent civilisation.Toshiba PA3534U-1BRS Battery

Lowell's first search focused on the ecliptic, the plane encompassed by the zodiac where the other planets in the Solar System lie. Using a 5-inch photographic camera, he manually examined over 200 three-hour exposures with a magnifying glass, but found no planets. At that time Pluto was too far above the ecliptic to be imaged by the survey.Sony NP-FP50 Battery

After revising his predicted possible locations, Lowell conducted a second search from 1914 to 1916. In 1915, he published his Memoir of a Trans-Neptunian Planet, in which he concluded that Planet X had a mass roughly seven times that of the Earth—about half that of Neptune—and a mean distance from the Sun of 43 AU. HP Compaq nx7300 Adapter

He assumed Planet X would be a large, low-density object with a high albedo, like the gas giants. As the result it would show a disc with diameter of about one arcsecond and an apparent magnitude of between 12 and 13—bright enough to be spotted. Dell Latitude D820 Adapter

Separately, in 1908, Pickering announced that, by analysing "kinks" in Uranus's orbit, he had found evidence for a ninth planet. His hypothetical planet, which he termed "Planet O" (because it came after "N", i.e. Neptune),[14] possessed a mean orbital radius of 51.9 AU and an orbital period of 373.5 years.Toshiba PA3399U-1BRS Battery

However, plates taken at his observatory in Arequipa, Peru, showed no evidence for the predicted planet, and British astronomer PH Cowell showed that the "kinks" observed in Uranus's orbit virtually disappeared once the planet's displacement of longitude was taken into account.Dell Studio XPS 1640 Battery

Lowell himself, despite his close association with Pickering, dismissed Planet O out of hand, saying, "This planet is very properly designated "O", [for it] is nothing at all." Unbeknownst to Pickering, four of the photographic plates taken in the search for "Planet O" by astronomers at the Mount Wilson Observatory in 1919 would in fact capture Pluto,Dell Latitude D830 Battery

though this was only recognised years later.[16]Pickering would go on to suggest many other possible trans-Neptunian planets up to the year 1932, which he named P, Q, R, S, T and U, but none were ever detected.[9]

Discovery of Pluto

Lowell's sudden death in 1916 temporarily halted the search for Planet X. Dell Inspiron E1505 Adapter

Failing to find the planet, according to one friend, "virtually killed him".[17] Constance Lowell, Percival Lowell's widow, subsequently embroiled the observatory in a long legal battle to secure its million-dollar portion of Lowell's legacy for herself, which meant that the search for Planet X could not resume for several years.Asus Eee PC 1201PN Battery

In 1925, the observatory obtained glass discs for a new 13-inch wide-field telescope to continue the search, constructed with funds from George Lowell, Percival's brother. In 1929 the observatory's director, Vesto Melvin Slipher, summarily handed the job of locating the planet to Clyde Tombaugh, Asus Eee PC 701 Adapter

a 22-year-old Kansas farm boy who had only just arrived at the Lowell Observatory after Slipher had been impressed by a sample of his astronomical drawings.

Tombaugh's task was to systematically capture sections of the night sky in pairs of images. Dell XPS M1330 Battery

Each image in a pair was taken two weeks apart. He then placed both images of each section in a machine called a blink comparator, which by exchanging images quickly created a time lapse illusion of the movement of any planetary body. To reduce the chances that a faster-moving (and thus closer) object be mistaken for the new planet, fujifilm np95 Battery

Tombaugh imaged each region near its opposition point, 180 degrees from the Sun, where the apparent retrograde motion for objects beyond Earth's orbit is at its strongest. He also took a third image as a control to eliminate any false results caused by defects in an individual plate.Asus Eee PC 900 Adapter

Tombaugh decided to image the entire zodiac, rather than focus on those regions suggested by Lowell.

By the beginning of 1930, Tombaugh's search had reached the constellation of Gemini. On 18 February 1930, after searching for nearly a year and examining nearly 2 million stars, Tombaugh discovered a moving object on photographic plates taken on 23 January and 29 January of that year.Sony VAIO PCG-713 Battery

A lesser-quality photograph taken on January 21 confirmed the movement.[18] Upon confirmation, Tombaugh walked into Slipher's office and declared, "Doctor Slipher, I have found your Planet X."[18] The object lay just six degrees from one of two locations for Planet X Lowell had suggested; thus it seemed he had at last been vindicated.Dell Latitude D830 Adapter

After the observatory obtained further confirmatory photographs, news of the discovery was telegraphed to the Harvard College Observatory on March 13, 1930. The new object would later be found on photographs dating back to 19 March 1915.Dell Studio XPS 1640 Battery

The decision to name the object Pluto was intended in part to honour Percival Lowell, as his initials made up the word's first two letters.[20] After discovering Pluto, Tombaugh continued to search the ecliptic for other distant objects. He found hundreds ofvariable stars and asteroids, as well as two comets, but no further planets.Panasonic LUMIX DMC-TZ4S  Battery

Pluto loses Planet X title

To the observatory's disappointment and surprise, Pluto showed no visible disc; it appeared as a point, no different from a star, and, at only 15th magnitude, was six times dimmer than Lowell had predicted, which meant it was either very small, or very dark.HP Compaq Business NX6320 Battery

Since Lowell astronomers thought Pluto was massive enough to perturb planets, they assumed that it should have an albedo of 0.07 (meaning that it reflected only 7% of the light that hit it); about as dark as asphalt and similar to that of Mercury, the least reflective planet known.Canon POWERSHOT G2 Battery

This would give Pluto an assumed diameter of about 8,000 km, or about 60% that of Earth.[22] Observations also revealed that Pluto's orbit was very elliptical, far more than for any planet.[23]

Some astronomers expressed skepticism at the result. Shortly after its discovery in 1930, Armin O. Dell Latitude E6500 Battery

Leuschner disputed Pluto's status, suggesting that its dimness and high orbital eccentricity made it more similar to an asteroid or comet; "The Lowell result confirms the possible high eccentricity announced by us on April 5. Among the possibilities are a large asteroid greatly disturbed in its orbit by close approach to a major planet such as Jupiter, Dell LATITUDE D630 Battery

or it may be one of many long-period planetary objects yet to be discovered, or a bright cometary object." In 1931, Ernest W. Brown asserted, using a mathematical formula, that the observed irregularities in the orbit of Uranus could not be due to the gravitational effect of a more distant planet, and thus that Lowell's supposed prediction was "purely accidental."Asus R33030 Adapter

Throughout the mid-20th century, estimates of Pluto's mass were revised downward. In 1931, Nicholson and Mayall calculated its mass, based on its supposed effect on the gas giants, as roughly that of the Earth, while in 1949, measurements of Pluto's diameter led to the conclusion that Dell Mini 1011 Battery

it was midway in size between Mercury and Mars and that its mass was most probably about 0.1 Earth mass.[26] In 1976, Dale Cruikshank, Carl Pilcher and David Morrison of the University of Hawaii analysed spectra from Pluto's surface and determined that it must contain methane ice, which is highly reflective.toshiba pa3332u-1brs Battery

This meant that Pluto, far from being dark, was in fact exceptionally bright, and thus was probably no more than 0.01 Earth mass.

Pluto's size was finally determined conclusively in 1978, when American astronomer James W. Christy discovered its moon Charon.HP Compaq 6710b Adapter

This enabled him, together with Robert Sutton Harrington of the US Naval Observatory, to measure the mass of the Pluto-Charon system directly by observing the moon's orbital motion around Pluto. They determined Pluto's mass to be 1.31×1022 kg; roughly one five–hundredth that of the Earth or one sixth that of the Moon,HP Compaq nx6325 Adapter

and far too small to account for the observed discrepancies in the orbits of the outer planets. Lowell's "prediction" had been a coincidence: if there was a Planet X, it was not Pluto.

Further searches for Planet(s) X

After 1978, a number of astronomers kept up the search for Lowell's Planet X, convinced that, since Pluto was no longer a viable candidate, an unseen tenth planet must have been perturbing the outer planets.Acer Aspire 5720 Adapter

In the 1980s and 1990s, Robert Harrington led a search to determine the real cause of the apparent irregularities. He calculated that any Planet X would be at roughly three times the distance of Neptune from the Sun; its orbit would be highly eccentric, and strongly inclined to the ecliptic—Toshiba Equium A110-252 Battery

the planet's orbit would be at roughly a 32-degree angle from the orbital plane of the other known planets. This hypothesis was met with a mixed reception. Noted Planet X sceptic Brian Marsden of Harvard University's Minor Planet Center pointed out that these discrepancies were a hundred times smaller than those noticed by Le Verrier, and could easily be due to observational error. Acer Aspire 5052ANWXMi Battery

In 1972, Joseph Brady of the Lawrence Livermore National Laboratory studied irregularities in the motion of Halley's Comet. Brady claimed that they could have been caused by a Jupiter-sized planet beyond Neptune that is in a retrograde orbit around the Sun.Dell Mini 1011v Battery

However, both Marsden and Planet X proponent P. Kenneth Seidelmann attacked the hypothesis, showing that Halley's Comet randomly and irregularly ejects jets of material, causing changes to its own orbital trajectory, and that such a massive object as Brady's Planet X would have severely affected the orbits of known outer planets.HP Compaq nx7300 Adapter

While its mission did not involve a search for Planet X, the IRAS space observatory made headlines briefly in 1983 due to an "unknown object" that was at first described as "possibly as large as the giant planet Jupiter and possibly so close to Earth that it would be part of this Solar System".Dell Vostro 1510 Battery

However, further analysis revealed that of several unidentified objects, nine were distant galaxies and the tenth was "interstellar cirrus"; none were found to be Solar System bodies.

In 1988, Jackson and Killen studied the stability of Pluto's resonance with Neptune by placing test "Planet X-es" with various masses and at various distances from Pluto. Toshiba PA3534U-1BRS Battery

Pluto and Neptune's orbits are in a 3:2 resonance, which prevents their collision or even any close approaches, regardless of their separation in the z-dimension. It was found that the hypothetical object's mass had to exceed 5 Earth masses to break the resonance;Asus M50 Battery

however, the parameter space is quite large and a large variety of objects could have existed beyond Pluto without disturbing the resonance. Four test orbits of a trans-Plutonian planet have been integrated forward for four million years in order to determine the effects of such a body on the stability of the Neptune-Pluto 3:2 resonance. Asus Eee PC 901 Battery

Planets beyond Pluto with masses of 0.1 M and 1.0 Earth masses in orbits at 48.3 and 75.5 AU, respectively, do not disturb the 3:2 resonance. Test planets of 5 Earth masses with semimajor axes of 52.5 and 62.5 AU disrupt the four-million-year libration of Pluto's argument of perihelion.Dell Inspiron 6400 Battery

Planet X disproved

Harrington died in January 1993, without having found Planet X.Six months before, Myles Standish had used data from Voyager 2's 1989 flyby of Neptune, which had revised the planet's total mass downward by 0.5%—an amount comparable to the mass of Mars[37]—to recalculate its gravitational effect on Uranus.HP Compaq Business NC6220 Battery

When Neptune's newly determined mass was used in the Jet Propulsion Laboratory Developmental Ephemeris (JPL DE), the supposed discrepancies in the Uranian orbit, and with them the need for a Planet X, vanished.Sony VGP-BPS9 Battery

Moreover, there are no discrepancies in the trajectories of any space probes such as Pioneer 10, Pioneer 11, Voyager 1 and Voyager 2 that can be attributed to the gravitational pull of a large undiscovered object in the outer Solar System.[39]Today, most astronomers agree that Planet X, as Lowell defined it, does not exist.Sony VGP-BPS13 Battery

Discovery of further trans-Neptunian objects

After the discovery of Pluto and Charon, no more trans-Neptunian objects (TNOs) were found until (15760) 1992 QB1 in 1992. Since then, however, hundreds of such objects have been observed. Most are now recognised as part of the Kuiper belt, a swarm of icy bodies left over from the Solar System's formation that orbit near the ecliptic plane beyond Neptune.HP Compaq nx7300 Adapter

Though none were as large as Pluto, some of these distant trans-Neptunian objects, such as Sedna, were initially described in the media as "new planets".

In 2005, astronomer Mike Brown and his team announced the discovery of 2003 UB313 (later named Eris after the Greek goddess of discord and strife), a trans-Neptunian object just barely larger than Pluto.HP Compaq 2510p Adapter

Soon afterwards, a NASA Jet Propulsion Laboratory press release described the object as the "tenth planet".

Eris, however, was never officially classified as a planet, and the 2006 definition of planet defined both Eris and Pluto not as planets but as dwarf planetsbecause they have not cleared their neighbourhoods.Dell Studio PP39L Battery

In other words, they do not orbit the Sun alone, but as part of a population of similarly sized objects. Pluto itself is now recognized as being a member of the Kuiper belt and the second largest dwarf planet after Eris. A number of astronomers, most notably Alan Stern, the head of NASA's New Horizons mission to Pluto, Nikon COOLPIX 5700  Battery

contend that the IAU's definition is flawed, and that not only Pluto and Eris, but all large trans-Neptunian objects, such as Makemake, Sedna, Quaoar and Varuna, should be considered planets in their own right.Eris is far too small to have significant effects on the outer planets' orbits. It is not Planet X, nor does its discovery rehabilitate the theory.Dell Studio XPS 16 Battery

Subsequent proposed trans-Neptunian planets

Although most astronomers accept that Lowell's Planet X does not exist, a number have revived the idea that a large unseen planet could create observable gravitational effects in the outer Solar System. These hypothetical objects are often referred to as "Planet X", although their link to that world is purely conceptual.HP Compaq nx7400 Adapter

Kuiper cliff

The Kuiper belt terminates suddenly at a distance of 48 astronomical units (AU) from the Sun (by comparison, Neptune lies 30 AU from the Sun), and there has been some speculation that this sudden drop-off, known as the "Kuiper cliff", may be attributed to the presence of an object with a mass between that of Mars and Earth located beyond 48 AU.HP Pavilion DV6500 Adapter

However, the presence of a Mars-like planet in a circular orbit at 60 AU leads to a TNO population incompatible with observations. For instance, it would severely deplete the plutino population. Astronomers have not excluded the possibility of a more massive Earth-like planet located further than 100 AU with an eccentric and inclined orbit. Dell T117C Battery

Computer simulations by Patryk Lykawka of Kobe University have suggested that a body with a mass between 0.3 and 0.7 that of the Earth, ejected outward by Neptune early in the Solar System's formation and currently in an elongated orbit between 101 and 200 AU from the Sun, Acer PA-1650-02 Adapter

could explain not only the Kuiper cliff but also the peculiar detached objects such as Sedna. While some astronomers have cautiously supported these claims, others have dismissed them as "contrived".

Tyche

Another hypothesis argues that long-period comets, rather than arriving from random points across the sky as is commonly thought,Acer Aspire 5920G Adapter

are in fact clustered in a band inclined to the ecliptic. Such clustering could be explained if they were disturbed by an unseen object at least as large as Jupiter, possibly a brown dwarf. The hypothetical planet—or companion of the Sun—would be located in the outer part of the Oort cloud.Asus S1 Adapter

This hypothesis was first proposed by John Matese of the University of Louisiana at Lafayette in 1999. He also suggested that the gravitational pull of such an object might explain Sedna's peculiar orbit. In 2011, Matese and Daniel Whitmire claimed that evidence of this object, which they named Tyche, would be detectable in the archive of data that was collected by NASA's WISE telescope. HP Pavilion dv6000 Battery

The name is derived from the sister of Nemesis in Greek mythology, a reference which they selected to avoid confusion with Nemesis, a similar hypothetical object first proposed by Richard A. Muller in 1984. They believe Tyche's orbit to lie at approximately 500 times the distance of Neptune, Ibm Thinkpad T30 Battery

equivalent to 15,000 AU or roughly a fourth of a light-year, and speculate that Tyche could be from one to four times the mass of Jupiter, and have a relatively high surface temperature of approximately 200 Kelvin (-73°C), due to residual heat from its formation and Kelvin–Helmholtz heating.Dell Studio PP39L Battery

Probability

In addition, probability arguments have also been used to suggest the existence of planet-sized objects in the outer Solar System. Sedna's 12,000-year orbit is so eccentric that it spends only a small fraction of its orbital period near the Sun, where it can be easily observed. HP Pavilion dv5 Adapter

This means that unless its discovery was a freak accident, there are probably a substantial population of objects roughly Sedna's diameter yet to be observed in its orbital region. Mike Brown, the discoverer of Sedna, noted in his 2007 Lowell Lecture that, "Sedna is about three-quarters the size of Pluto. HP Compaq Business NC6400 Battery

If there are sixty objects three-quarters the size of Pluto  then there are probably forty objects the size of Pluto ... If there are forty objects the size of Pluto, then there are probably ten that are twice the size of Pluto. There are probably three or four that are three times the size of Pluto, HP Compaq 490306-001 Battery

and the biggest of these objects ... is probably the size of Mars or the size of the Earth."[58] However, he notes that, should such an object be found, even though it might approach the Earth in size, it would still be a dwarf planet by the current definition, since it will not have cleared its neighborhood sufficiently.Compaq Presario CQ35-100 Battery

Halo of oligarch planets

The oligarch theory of planet formation suggests that there were hundreds of planet-sized objects, known as oligarchs, in the early stages of the Solar System's evolution. In 2005, astronomer Eugene Chiang speculated that while some of these oligarchs became the planets we know today, most would have been flung outward by gravitational interactions. HP Compaq HSTNN-LB51 Battery

Some may have escaped the Solar System altogether to become free-floating planets, while others would be orbiting in a halo around the Solar System, with orbital periods of millions of years. This halo would lie at between 1,000 and 10,000 AU from the Sun, or between a third and a thirtieth the distance to the Oort cloud.HP Pavilion ZE4500 Battery

The Oort cloud (play /??rt/; alternatively the Öpik–Oort cloud IPA: [?øpik]) is a hypothesized spherical cloud of comets which may lie roughly 50,000 AU, or nearly a light-year, from the Sun. This places the cloud at nearly a quarter of the distance to Proxima Centauri, the neareststar to the Sun. HP G62-100EB Battery

The Kuiper belt and scattered disc, the other two reservoirs of trans-Neptunian objects, are less than one thousandth the Oort cloud's distance. The outer extent of the Oort cloud defines the gravitational boundary of our Solar System.HP Pavilion DV6-1211AX Battery

The Oort cloud is thought to comprise two separate regions: a spherical outer Oort cloud and a disc-shaped inner Oort cloud, or Hills cloud. Objects in the Oort cloud are largely composed of ices, such as water, ammonia, and methane. HP Pavilion DV6-1220SO Battery

Astronomers believe that the matter composing the Oort cloud formed closer to the Sun and was scattered far out into space by the gravitational effects of the giant planets early in the Solar System's evolution.

Although no confirmed direct observations of the Oort cloud have been made,HP EliteBook 6930P Battery

astronomers believe that it is the source of all long-period andHalley-type comets entering the inner Solar System and many of the Centaurs and Jupiter-family comets as well.  The outer Oort cloud is only loosely bound to the Solar System, and thus is easily affected by the gravitational pull both of passing stars and of the Milky Way Galaxy itself. HP Pavilion DV6-1210SA Battery

These forces occasionally dislodge comets from their orbits within the cloud and send them towards the inner Solar System.  Based on their orbits, most of the short-period comets may come from the scattered disc, but some may still have originated from the Oort cloud.Compaq Presario CQ71 Battery

Although the Kuiper belt and the farther scattered disc have been observed and mapped, only four currently known trans-Neptunian objects—90377 Sedna, 2000 CR105, 2006 SQ372, and 2008 KV42—are considered possible members of the inner Oort cloud.HP Pavilion DV7-1290EO Battery

Hypothesis

In 1932, Estonian astronomer Ernst Öpik postulated that long-period comets originated in an orbiting cloud at the outermost edge of the Solar System. In 1950, the idea was independently revived by Dutch astronomer Jan Hendrik Oort as a means to resolve a paradox:HP G6000 Battery

over the course of the Solar System's existence, the orbits of comets are unstable; eventually, dynamics dictate that a comet must either collide with the Sun or a planet, or else be ejected from the Solar System by planetary perturbations. Moreover, their volatile composition means that as they repeatedly approach the Sun,HP HDX X18-1005EA Battery

radiation gradually boils the volatiles off until the comet splits or develops an insulating crust that prevents further outgassing. Thus, reasoned Oort, a comet could not have formed on its current orbit, and must have been held in an outer reservoir for almost all of its existence.HP Pavilion DV6-3013SL Battery

There are two main classes of comet: short-period comets (also called ecliptic comets) and long-period comets (also called nearly isotropic comets). Ecliptic comets have relatively small orbits, below 10 AU, and follow the ecliptic plane, the same plane in which the planets lie.HP Compaq HSTNN-IB62 Battery

Nearly all isotropic comets have very large orbits, on the order of thousands of AU, and appear from every corner of the sky. Oort noted that there was a peak in numbers of nearly isotropic comets with aphelia—their farthest distance from the Sun—of roughly 20,000 AU, which suggested a reservoir at that distance with a spherical, isotropic distribution.HP Pavilion DV6100 Battery

Those relatively rare comets with orbits of about 10,000 AU have probably gone through one or more orbits through the Solar System and have had their orbits drawn inward by the gravity of the planets.[9]

Structure and composition

The Oort cloud is thought to occupy a vast space from somewhere between 2,000 and 5,000 AU (0.03 and 0.08 ly)  to as far as 50,000 AU (0.79 ly)[1] from the Sun. HP EliteBook 2730P Battery

Some estimates place the outer edge at between 100,000 and 200,000 AU (1.58 and 3.16 ly).[9] The region can be subdivided into a spherical outer Oort cloud of 20,000–50,000 AU (0.32–0.79 ly), and a doughnut-shaped inner Oort cloud of 2,000–20,000 AU (0.03–0.32 ly). HP Pavilion DV9000 Battery

The outer cloud is only weakly bound to the Sun and supplies the long-period (and possibly Halley-type) comets to inside the orbit of Neptune.[1] The inner Oort cloud is also known as the Hills cloud, named after J. G. Hills, who proposed its existence in 1981.HP Pavilion DV7 Battery

Models predict that the inner cloud should have tens or hundreds of times as many cometary nuclei as the outer halo; it is seen as a possible source of new comets to resupply the relatively tenuous outer cloud as the latter's numbers are gradually depleted. The Hills cloud explains the continued existence of the Oort cloud after billions of years. HP Pavilion DV7-1290EO Battery

The outer Oort cloud is believed to contain several trillion individual objects larger than approximately 1 km (0.62 mi)[1] (with many billions withabsolute magnitudes[14] brighter than 11—corresponding to approximately 20 km (12 mi) diameter), with neighboring objects typically tens of millions of kilometres apart.HP Compaq Business 6735S Battery

Its total mass is not known with certainty, but, assuming that Halley's comet is a suitable prototype for all comets within the outer Oort cloud, the estimated combined mass is 3×1025 kg (7×1025 lb or roughly five times the mass of the Earth) Earlier it was thought to be more massive (up to 380 Earth masses), HP 540 Battery

but improved knowledge of the size distribution of long-period comets has led to much lower estimates. The mass of the inner Oort Cloud is not currently known.

If analyses of comets are representative of the whole, the vast majority of Oort-cloud objects consist of various ices such as water, methane,ethane, carbon monoxide and hydrogen cyanide.HP Compaq Business NW9440 Battery

However, the discovery of the object 1996 PW, an asteroid in an orbit more typical of a long-period comet, suggests that the cloud may also be home to rocky objects. Analysis of the carbon and nitrogen isotope ratios in both the Oort cloud and Jupiter-family comets shows little difference between the two, despite their vastly separate regions of origin.HP Compaq Business  TC4400 Battery

This suggests that both originated from the original protosolar cloud, a conclusion also supported by studies of granular size in Oort cloud comets  and by the recent impact study of Jupiter-family comet Tempel 1.

Origin

The Oort cloud is thought to be a remnant of the original protoplanetary disc that formed around the Sun approximately 4.6 billion years ago.HP Compaq 491278-001 Battery

The most widely accepted hypothesis is that the Oort cloud's objects initially coalesced much closer to the Sun as part of the same process that formed the planets and asteroids, but that gravitational interaction with young gas giant planets such as Jupiter ejected the objects into extremely long elliptic or parabolic orbits.HP HSTNN-LB31 Battery

Simulations of the evolution of the Oort cloud from the beginnings of the Solar System to the present suggest that the cloud's mass peaked around 800 million years after formation, as the pace of accretion and collision slowed and depletion began to overtake supply.HP Pavilion DV3-4000  Battery

Models by Julio Ángel Fernández suggest that the scattered disc, which is the main source for periodic comets in the Solar System, might also be the primary source for Oort cloud objects. According to the models, about half of the objects scattered travel outward towards the Oort cloud,HP Pavilion TX1000 Battery

while a quarter are shifted inward to Jupiter's orbit, and a quarter are ejected on hyperbolic orbits. The scattered disc might still be supplying the Oort cloud with material.[24] A third of the scattered disc's population is likely to end up in the Oort cloud after 2.5 billion years.HP Pavilion DV6-1210SA  Battery

Computer models suggest that collisions of cometary debris during the formation period play a far greater role than was previously thought. According to these models, the number of collisions early in the Solar System's history was so great that most comets were destroyed before they reached the Oort cloud. HP 448007-001 Battery

Therefore, the current cumulative mass of the Oort cloud is far less than was once suspected.[26] The estimated mass of the cloud is only a small part of the 50–100 Earth masses of ejected material.[1]

Gravitational interaction with nearby stars and galactic tides modified cometary orbits to make them more circular. attery-7975.htm">HP Compaq Business  6530B  Battery

This explains the nearly spherical shape of the outer Oort cloud.[1] On the other hand, the Hills cloud, which is bound more strongly to the Sun, has yet to acquire a spherical shape. Recent studies have shown that the formation of the Oort cloud is broadly compatible with the hypothesis that the Solar System formed as part of an embedded cluster of 200–400 stars. HP Pavilion DV5-1015EA  Battery

These early stars likely played a role in the cloud's formation, since the number of close stellar passages within the cluster was much higher than today, leading to far more frequent perturbations.

In June 2010 Harold F. Levison and others have suggested on the basis of enhanced computer simulations that the Sun "HP Pavilion DV6-1230SB Battery

captured comets from other stars while it was in its birth cluster." Their results imply that "a substantial fraction of the Oort cloud comets, perhaps exceeding 90%, are from the protoplanetary disks of other stars." [28]

Comets

Comets are believed to have two separate points of origin in the Solar System. HP Pavilion ZD8181EA  Battery

Short-period comets (those with orbits of up to 200 years) are generally accepted to have emerged from the Kuiper belt or scattered disc, two linked flat discs of icy debris beyond Neptune's orbit at 30 AU and jointly extending out beyond 100 AU from the Sun.HP Pavilion DV9500  Battery

Long-period comets, such as comet Hale-Bopp, whose orbits last for thousands of years, are thought to originate in the Oort cloud. The orbits within the Kuiper belt are relatively stable, and so very few comets are believed to originate there. The scattered disc, however, is dynamically active, and is far more likely to be the place of origin for comets.HP Pavilion DV4-1125BR  Battery

Comets pass from the scattered disc into the realm of the outer planets, becoming what are known as centaurs.[29] These centaurs are then sent farther inward to become the short-period comets.

There are two main varieties of short-period comet: Jupiter-family comets (those with semi-major axes of less than 5 AU) and Halley-family comets. HP Pavilion DV5-1111EA Battery

Halley-family comets, named for their prototype, Halley's Comet, are unusual in that while they are short-period comets, it is believed that their ultimate origin lies in the Oort cloud, not in the scattered disc. Based on their orbits, it is believed they were long-period comets that were captured by the gravity of the giant planets and sent into the inner Solar System.HP Pavilion DV6-1234TX Battery

This process may have also created the present orbits of a significant fraction of the Jupiter-family comets, although the majority of such comets are thought to have originated in the scattered disc.

Oort noted that the number of returning comets was far less than his model predicted, and this issue, known as "cometary fading", has yet to be resolved.HP Pavilion DV3-2008TX  Battery

No known dynamical process can explain this undercount of observed comets. Hypotheses for this discrepancy include the destruction of comets due to tidal stresses, impact or heating; the loss of all volatiles, rendering some comets invisible, or the formation of a non-volatile crust on the surface.HP Pavilion DV8  Battery

Dynamical studies of Oort Cloud comets have shown that their occurrence in the outer planet region is several times higher than in the inner planet region. This discrepancy may be due to the gravitational attraction ofJupiter, which acts as a kind of barrier, trapping incoming comets and causing them to collide with it, just as it did with Comet Shoemaker-Levy 9 in 1994.HP Pavilion DV6-1215SA  Battery

Tidal effects

Most of the comets seen close to the Sun are believed to have reached their current positions through gravitational distortion of the Oort cloud by the tidal force exerted by the Milky Way galaxy. Just as the Moon's tidal force bends and deforms the Earth's oceans,HP Compaq 490306-001  Battery

causing the tides to rise and fall, the galactic tide also bends and distorts the orbits of bodies in the outer Solar System, pulling them towards the galactic centre. In the charted regions of the Solar System, these effects are negligible compared to the gravity of the Sun.Compaq Presario CQ50  Battery

At the outer reaches of the system, however, the Sun's gravity is weaker and the gradient of the Milky Way's gravitational field plays a far more noticeable role. Because of this gradient, galactic tides can deform an otherwise spherical Oort cloud, stretching the cloud in the direction of the galactic centre and compressing it along the other two axes. HP Pavilion DV6-1223EO  Battery

These small galactic perturbations may be enough to dislodge members of the Oort cloud from their orbits, sending them towards the Sun.[33] The point at which the Sun's gravity concedes its influence to the galactic tide is called the tidal truncation radius. It lies at a radius of 100,000 to 200,000 AU, and marks the outer boundary of the Oort cloud.HP Compaq 490306-001  Battery

Some scholars theorise that the galactic tide may have contributed to the formation of the Oort cloud by increasing the perihelia—closest distances to the Sun—of planetesimals with large aphelia.[34]The effects of the galactic tide are quite complex, and depend heavily on the behaviour of individual objects within a planetary system.Compaq Presario CQ35-100 Battery

Cumulatively, however, the effect can be quite significant: up to 90% of all comets originating from the Oort cloud may be the result of the galactic tide.[35] Statistical models of the observed orbits of long-period comets argue that the galactic tide is the principal means by which their orbits are perturbed toward the inner Solar System.HP Compaq HSTNN-LB51  Battery

Star perturbations and stellar companion hypotheses

Besides the galactic tide, the main trigger for sending comets into the inner Solar System is believed to be interaction between the Sun's Oort cloud and the gravitational fields of near-by stars[1] orgiant molecular clouds.[32] The orbit of the Sun through the plane of the Milky Way sometimes brings it in relatively close proximity to other stellar systems. HP Pavilion ZE4500  Battery

For example, during the next 10 million years the known star with the greatest possibility of perturbing the Oort cloud is Gliese 710.[37] This process also serves to scatter the objects out of the ecliptic plane, potentially also explaining the cloud's spherical distribution.HP G62-100EB  Battery

In 1984, Physicist Richard A. Muller postulated that the Sun has a heretofore undetected companion, either a brown dwarf star, a dim red dwarf star or a gaseous giant planet, in an elliptical orbit within the Oort cloud. This object, known as Nemesis, is hypothesized to pass through a portion of the Oort cloud approximately every 26 million years, HP Pavilion DV6-1211AX  Battery

bombarding the inner Solar System with comets. However, to date no direct evidence of Nemesis has been found.[39] One star survey using parallax measurements in order to clarify local star distances, the WISE mission, is currently underway with part of its mission being to help with either proving or disproving the Nemesis hypothesis.HP EliteBook 6930P Battery

The analysis of this survey is expected to be completed by 2013. Two other planned astronomical missions, the Pan-STARRS and the LSST astronomical surveys may also be able to aid in resolving the debate over the existence or non-existence of a Nemesis companion star to our Sun.HP Pavilion DV9500  Battery

A somewhat similar hypothesis was advanced by astronomer John J. Matese of the University of Louisiana at Lafayette in 2002. He contends that more comets are arriving in the inner Solar System from a particular region of the Oort Cloud than can be explained by the galactic tide or stellar perturbations alone, and that the most likely cause is a Jupiter-mass object in a distant orbit.Compaq Evo N610C  Battery

Oort cloud objects (OCOs)

Apart from long-period comets, only four known objects have orbits which suggest that they may belong to the Oort cloud: 90377 Sedna, 2000 CR105, 2006 SQ372 and 2008 KV42. The first two, unlike scattered disc objects, have perihelia outside the gravitational reach of Neptune, and thus their orbits cannot be explained by perturbations from the gas giant planets.HP Pavilion DM3-1050EO Battery

If they formed in their current locations, their orbits must originally have been circular; otherwise accretion (the coalescence of smaller bodies into larger ones) would not have been possible because the large relative velocities between planetesimals would have been too disruptive.Compaq Presario 1245  Battery

Their present-day elliptical orbits can be explained by a number of hypotheses:

  1. These objects could have had their orbits and perihelion distances "lifted" by the passage of a nearby star when the Sun was still embedded in its birth star cluster.HP G6000  Battery
  2. Their orbits could have been disrupted by an as-yet-unknown planet-sized body within the Oort cloud.
  3. They could have been scattered by Neptune during a period of particularly high eccentricity or by the gravity of a far larger primordial trans-Neptunian disc.HP Compaq 451086-661  Battery
  4. They could have been captured from around smaller passing stars.

Of these, the stellar disruption and "lift" hypothesis appears to agree most closely with observations.[4] Some astronomers prefer to refer to Sedna and 2000 CR105 as belonging to the "extended scattered disc" rather than to the inner Oort cloud.HP Pavilion DV2166EA  Battery,HP Pavilion DM4 Battery,Dell Latitude E6400 Battery

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