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 cuilminated at the start of the 20th with Percival Lowell's quest for a Planet X. Lowell proposed the Planet X hypothesis to explain apparent discrepancies in the orbits of the gas giants, particularly Uranus and Neptune, speculating that the gravity of a large unseen planet could have perturbed Uranus enough to account for the irregularities.
Clyde Tombaugh's discovery of Pluto in 1930 initially appeared to validate Lowell's hypothesis, and Pluto was 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. 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. After 1992, the discovery of numerous small icy objects within or near Pluto's orbit led to a debate over whether Pluto should remain a planet, 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 Union reclassified Pluto and its largest neighbours as dwarf planets, leaving only eight planets in the Solar System.
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. In popular culture, and even among some astronomers, Planet X has become a stand-in term for any undiscovered planet in the outer Solar System, regardless of its gravitational effect. Other trans-Neptunian planets have also been suggested, based on different evidence.
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 September 23, 1846, the night following his receipt of the letter, Galle and his student Heinrich d'Arrest discovered Neptune, exactly where Le Verrier had predicted. 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. Even before Neptune's discovery, in 1834, amateur astronomer Dr. Thomas Hussey speculated that one planet alone might not be enough to resolve the discrepancies in Uranus's orbit, and that a ninth planet would be required to resolve the issue. 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, largely on simple subtraction from Le Verrier's calculations, that another planet of roughly 12 Earth masses, which he named "Hyperion", must exist beyond it. 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."
In 1850, James Furguson of 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. 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.
In 1879, Camille Flammarion noted that the comets 1862iii and 1889iii had aphelia of 47 and 49 AU, respectively, suggesting that they might mark the orbital radius of an unknown planet that had dragged it into an ecliptic orbit. 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. 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. The first was instigaed by Danish astronomer Hans Emil Lau who, after observing the orbit of Uranus from 1690 and 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 for the discprepancy. 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 planet found.
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. After revising his predicted possible locations, Lowell conducted a second search from 1913 to 1915. 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.
Separately, in 1908, Pickering announced that, by analyising "kinks" in Uranus's orbit, he had found evidence for a ninth planet. His hypothetical planet, which he termed "Planet O", possessed a mean orbital radius of 51.9 AU and an orbital period of 373.5 years. 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. 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. 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.
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". 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. In 1925, the observatory obtained glass discs for a new 13-inch wide-field telescope to continue the search. The telescope was 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, 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. 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.
By the beginning of 1930, Tombaugh's search had reached the constellation of Gemini. On February 18, 1930, after searching for nearly a year and examining nearly 2 million stars, Tombaugh discovered a moving object on photographic plates taken on January 23 and January 29 of that year. A lesser-quality photograph taken on January 21 confirmed the movement. Upon confirmation, Tombaugh walked into Slipher's office and declared, "Doctor Slipher, I have found your Planet X." 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. 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 March 19, 1915. 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.
To the observatory's disappointment and surprise, Pluto showed no visible disc; it appeared fuzzy and indistinct, 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. Since 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. This would give Pluto an assumed diameter of about 8,000 km, or about 60% that of Earth.
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 conclusion that it was midway in size between Mercury and Mars and that its mass was most probably about 0.1 Earth mass. Dale Cruikshank, Carl Pilcher and David Morrison of the University of Hawaii analysed spectra from Pluto's surface in 1976 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.
American astronomer James W. Christy discovered Pluto's moon Charon in 1978. 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 roughly 0.002 that of the Earth (1/6 that of the Moon), far too small to account for the observed discrepancies. Lowell's "prediction" had been a coincidence; if there was a Planet X, it was not Pluto.
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 from the sun than Neptune; 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. 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.
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.
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". However, further analysis revealed that of several unidentified objects, nine were distant galaxies and the tenth was "intergalactic 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. 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, however the parameter space is quite large and a large variety objects could have existed beyond Pluto and not disturb 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.
rect 646 1714 2142 1994 The Earth
circle 1786 614 142 Makemake
circle 2438 616 155 (136108) Haumea
circle 342 1305 137 (90377) Sedna
circle 1088 1305 114 (90482) Orcus
circle 1784 1305 97 (50000) Quaoar
circle 2420 1305 58 (20000) Varuna
Then in 2005, astronomer Mike Brown and his team announced the discovery of (later named after the Greek goddess of discord and strife), a trans-Neptunian object just barely larger than Pluto. Soon afterwards, a NASA Jet Propulsion Laboratory press release described the object as the "tenth planet". However, 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. 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 , , and , should be considered planets in their own right. In any case, Eris is not Planet X, as it is far too small to have significant effects on the outer planets' orbits.
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.
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 is 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. However, the possibility of a Mars-like planetoid in a circular orbit at 60 AU can be safely discounted; such an object would be so bright that it would have been discovered by now. In addition, its influence leads to a TNO population incompatible with observations. For instance, it would severely deplete the plutino population. However, astronomers have not excluded the possibility of a more massive Earth-like planetoid located further than 100 AU with an eccentric and inclined orbit. Computer simulations have suggested that a body roughly the size of Earth, ejected outward by Neptune early in the Solar System's formation and currently in an elongated orbit between 80 and 170 AU from the Sun, could explain not only the Kuiper cliff but also the peculiar "detached" TNOs such as Sedna. While some astronomers have cautiously supported these claims, others have dismissed them as "contrived".
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.
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. 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." 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.