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. 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.[2]
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, Pluto was found to be too small for its gravity to affect the gas giants, resulting in a brief search for a tenth planet. 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, or whether it and its neighbors 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 Union reclassified Pluto and its largest neighbours as dwarf planets, leaving only eight planets in the Solar System.[4]
Today, the astronomical community widely agrees that Planet X, as originally envisioned, does not exist, but the concept of Planet X has been revived by a number of astronomers to explain other anomalies observed in the outer Solar System. 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.
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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. On 23 September 1846, the night following his receipt of the letter, Galle and his student Heinrich d'Arrest discovered Neptune, exactly where Le Verrier had predicted.[6] There remained some slight discrepancies in the gas giants' orbits. These were taken to indicate the existence of yet another planet orbiting beyond Neptune.
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. 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. 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. 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, "[There is] absolutely nothing by which one could determine the position of another planet, barring hypotheses in which imagination played too large a part."[7]
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. 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.[7]
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.[7] Astronomer Georges Forbes concluded on the basis of this evidence that two planets must exist beyond Neptune. 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.[7] However, skeptics argued that the orbits of the comets involved were still too uncertain to produce meaningful results.[7]
In 1900 and 1901, Harvard Observatory director William Henry Pickering led two searches for trans-Neptunian planets. 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. 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.[7]
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".[8] Tentatively naming the first planet "Oceanus", he placed their respective distances at 42, 56 and 72 AU from the Sun. He gave no indication as to how he determined their existence, and no known searches were mounted to locate them.[8]
In 1911, Indian astronomer Venkatesh P. Ketakar suggested the existence of two trans-Neptunian planets, which he named Brahma and Vishnu, 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.[10] 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). 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). Ketakar made no predictions for the orbital elements other than mean distance and period. It is not clear how Ketakar arrived at these figures, and his second planet, Vishnu, was never located.[9]
In 1894, with the help of William Pickering, Percival Lowell, a wealthy Bostonian aristocrat, founded the Lowell Observatory in Flagstaff, Arizona. 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). 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.[12]
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.[11] After revising his predicted possible locations, Lowell conducted a second search from 1914 to 1916.[11] 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. 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.[11][13]
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.[7] 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.[7] 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."[15] Unbeknown to Pickering, four of the photographic plates taken in the search for "Planet O" by astronomers at the Mount Wilson Observatory in 1919 would capture Pluto, 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]
Lowell's sudden death in 1916 temporarily halted the search for Planet X. 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.[18] 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.[11] In 1929 the observatory's director, Vesto Melvin Slipher, summarily handed the job of locating the planet to Clyde Tombaugh, 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.[18]
Tombaugh's task was to systematically capture sections of the night sky in pairs of images. 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, 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. Tombaugh decided to image the entire zodiac, rather than focus on those regions suggested by Lowell.[11]
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.[19] 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.[18] 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 was later precovered on photographs dating back to 19 March 1915.[16] 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 of variable stars and asteroids, as well as two comets, but no further planets.[21]
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.[11] 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.[1] 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. 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, or it may be one of many long-period planetary objects yet to be discovered, or a bright cometary object."[23] 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."[24]
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,[25] while in 1949, measurements of Pluto's diameter led to the conclusion that 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. This meant that Pluto, far from being dark, was in fact exceptionally bright, and thus was probably no more than 0.01 Earth mass.[27]
| Year | Mass | Notes |
|---|---|---|
| 1931 | 1 Earth | Nicholson & Mayall[25] |
| 1948 | .1 (1/10 Earth) | Kuiper [26] |
| 1976 | .01 (1/100 Earth) | Cruikshank, Pilcher, & Morrison [27] |
| 1978 | .002 (2/1000 Earth) | Christy & Harrington [28] |
Pluto's size was finally determined conclusively in 1978, when American astronomer James W. Christy discovered its moon Charon. 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.[28] 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, 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.[29]
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.[30]
In the 1980s and 1990s, Robert Harrington led a search to determine the real cause of the apparent irregularities.[30] 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—the planet's orbit would be at roughly a 32-degree angle from the orbital plane of the other known planets.[31] 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.[32]
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. 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.[33]
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".[34] 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.[35]
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. 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, and 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. 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.[36]
Harrington died in January 1993, without having found Planet X.[37] 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.[38] 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.[3] 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.[40]
After the discovery of Pluto and Charon, no more trans-Neptunian objects (TNOs) were found until (15760) 1992 QB1 in 1992.[41] Since then, 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. 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".[42]
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.[43] Soon afterwards, a NASA Jet Propulsion Laboratory press release described the object as the "tenth planet".[44]
Eris was never officially classified as a planet, and the 2006 definition of planet defined both Eris and Pluto not as planets but as dwarf planets because they have not cleared their neighbourhoods.[4] 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, 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.[45]
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.[46]
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.[47][48]
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),[49] 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.[50] 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.[51] Astronomers have not excluded the possibility of a more massive Earth-like planet located further than 100 AU with an eccentric and inclined orbit. 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, could explain not only the Kuiper cliff but also the peculiar detached objects such as Sedna.[51] While some astronomers have cautiously supported these claims, others have dismissed them as "contrived".[48]
Another hypothesis argues that long-period comets, rather than arriving from random points across the sky as is commonly thought, 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.[52][53] This hypothesis was first proposed by John Matese of the University of Louisiana at Lafayette in 1999.[54] He also suggested that the gravitational pull of such an object might explain Sedna's peculiar orbit.[55] 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.[56] 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, 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[56] (-73°C), due to residual heat from its formation and Kelvin–Helmholtz heating.
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. 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.[57] Mike Brown, the discoverer of Sedna, noted in his 2007 Lowell Lecture that "Sedna is about three-quarters the size of Pluto. If there are sixty objects three-quarters the size of Pluto [out there] 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, 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.[58]
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. 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.[59]
Based on computer simulations, David Nesvorny suggests the current make-up of the outer solar system can be explained by the early presence of a 5th large gaseous body once located between Saturn and Uranus that was ejected into interstellar space after an encounter with Jupiter. Nesvorny bolsters this theory noting recent discoveries of a large number free-floating planets in interstellar space suggesting planet ejection may be a relatively common occurrence.[60][61]
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