Editing Planets beyond Neptune (section)

== Planet X ==
{{Redirect|Planet X|the conspiracy theory|Nibiru cataclysm|the hypothetical planet first proposed in 2014|Planet Nine||Planet X (disambiguation)}}
In 1894, with the help of William Pickering, [[Percival Lowell]] (a wealthy Bostonian) 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,<ref name=tombaugh/> which he named ''Planet&nbsp;X'', a name previously used by Gabriel Dallet.<ref name=grosser/> The ''X'' in the name represents an unknown and is pronounced as the letter, as opposed to the [[Roman numeral]] for 10 (at the time, Planet&nbsp;X would have been the ninth planet). Lowell's hope in tracking down Planet&nbsp;X was to establish his scientific credibility, which had eluded him due to his widely derided belief that channel-like features visible on the surface of [[Mars]] were [[Martian canals|canals constructed by an intelligent civilization]].<ref>Croswell (1997), p. 43.</ref>

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, and found no planets. At that time Pluto was too far above the ecliptic to be imaged by the survey.<ref name=tombaugh>Tombaugh (1946).</ref>  After revising his predicted possible locations, Lowell conducted a second search from 1914 to 1916.<ref name=tombaugh/> In 1915, he published his ''Memoir of a Trans-Neptunian Planet'', in which he concluded that Planet&nbsp;X had a mass roughly seven times that of Earth—about half that of Neptune<ref name="ley195608">{{Cite magazine  |last=Ley  |first=Willy  |date=August 1956  |title=The Demotion of Pluto  |department=For Your Information  |url=https://archive.org/stream/galaxymagazine-1956-08#page/n79/mode/2up  |magazine=Galaxy Science Fiction  |pages=79–91  }}</ref>—and a mean distance from the Sun of 43&nbsp;AU. He assumed Planet&nbsp;X would be a large, low-density object with a high [[albedo]], like the giant planets. As a result, it would show a disc with diameter of about one arcsecond and an [[apparent magnitude]] between 12 and 13—bright enough to be spotted.<ref name=tombaugh/><ref>Littman (1990), p. 70.</ref>

{{anchor|Planet O}}Separately, in 1908, Pickering announced that, by analysing irregularities in Uranus's orbit, he had found evidence for a ninth planet. His hypothetical planet, which he termed "Planet&nbsp;O" (because it came after "N", i.e. Neptune),<ref>{{cite book|title=The Hunt For Planet X|author=Govert Schilling |date=2009 |publisher=Springer |page=34 | isbn= 978-0387778044 }}</ref> possessed a mean orbital radius of 51.9&nbsp;AU and an orbital period of 373.5 years.<ref name=grosser/> Plates taken at his observatory in [[Arequipa]], Peru, showed no evidence for the predicted planet, and British astronomer [[Philip Herbert Cowell|P. H. Cowell]] showed that the irregularities observed in Uranus's orbit virtually disappeared once the planet's displacement of longitude was taken into account.<ref name=grosser/> Lowell himself, despite his close association with Pickering, dismissed Planet&nbsp;O out of hand, saying, "This planet is very properly designated "O", [for it] is nothing at all."<ref name=cros50>Croswell p. 50</ref> Unbeknownst to Pickering, four of the photographic plates taken in the search for "Planet O" by astronomers at the Mount Wilson Observatory in 1919 captured images of [[Pluto]], though this was only recognised years later.<ref name="Hoyt" /> Pickering went on to suggest many other possible trans-Neptunian planets up to the year 1932, which he named ''P'', ''Q'', ''R'', ''S'', ''T'', and ''U''; none were ever detected.<ref name=ketakar/>

=== Discovery of Pluto ===
{{main|Pluto}}
[[File:Clyde W. Tombaugh.jpeg|thumb|right|upright|Clyde William Tombaugh]]
Lowell's sudden death in 1916 temporarily halted the search for Planet&nbsp;X. Failing to find the planet, according to one friend, "virtually killed him".<ref>Croswell (1997), p. 49.</ref> Lowell's widow, Constance, engaged in a legal battle with the observatory over Lowell's legacy which halted the search for Planet&nbsp;X for several years.<ref name=cros2>Croswell (1997), pp. 32–55.</ref> In 1925, the observatory obtained glass discs for a new {{convert|13|in|cm|abbr=on}} wide-field telescope to continue the search, constructed with funds from [[Abbott Lawrence Lowell]],<ref>{{Cite web |last=Schindler |first=Kevin |date=2015-05-14 |title=Percival Lowell's three early searches for Planet X |url=https://www.astronomy.com/science/percival-lowells-three-early-searches-for-planet-x/ |access-date=2024-09-11 |website=Astronomy Magazine |language=en-US}}</ref> Percival's brother.<ref name=tombaugh/> 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.<ref name=cros2/>

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 photography|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.<ref name=tombaugh/>

[[File:Pluto discovery plates.png|thumb|left|Discovery photographs of Pluto]]
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&nbsp;million stars, Tombaugh discovered a moving object on photographic plates taken on 23 January and 29 January of that year.<ref>Tombaugh (1946), p. 79</ref> A lesser-quality photograph taken on January 21 confirmed the movement.<ref name=cros2/> Upon confirmation, Tombaugh walked into Slipher's office and declared, "Doctor Slipher, I have found your Planet&nbsp;X."<ref name=cros2/> The object was just six degrees from one of two locations for Planet&nbsp;X  Lowell had suggested; thus it seemed he had at last been vindicated.<ref name=cros2/> 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 [[Precovery|precovered]] on photographs dating back to 19 March 1915.<ref name="Hoyt">{{Cite journal |last=Hoyt |first=William Graves |date=December 1976 |title=W. H. Pickering's Planetary Predictions and the Discovery of Pluto |journal=Isis |language=en |volume=67 |issue=4 |pages=551–564 |doi=10.1086/351668 |issn=0021-1753 |jstor=230561 |pmid=794024 |s2cid=26512655}} p. 563.</ref> 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.<ref>{{cite web|title= NASA's Solar System Exploration: Multimedia: Gallery: Pluto's Symbol |url= http://sse.jpl.nasa.gov/multimedia/display.cfm?IM_ID=263 |publisher=NASA |access-date=2007-03-25 |url-status=dead |archive-url= https://web.archive.org/web/20061001015053/http://sse.jpl.nasa.gov/multimedia/display.cfm?IM_ID=263 |archive-date=2006-10-01 }}</ref> After discovering Pluto, Tombaugh continued to search the ecliptic for other distant objects. He found hundreds of [[variable star]]s and [[Clyde Tombaugh#Asteroids discovered|asteroids]], as well as two [[comet]]s, but no further planets.<ref>{{cite web|title=Clyde W. Tombaugh|publisher=New Mexico Museum of Space History |url=http://www.nmspacemuseum.org/halloffame/detail.php?id=51|access-date=2008-06-29}}</ref>

=== Pluto loses Planet X title ===
[[File:Charon Discovery.jpg|thumb|Discovery image of [[Charon (moon)|Charon]]]]
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.<ref name=tombaugh/> Because of Lowell's predictions, astronomers thought that Pluto would be massive enough to [[Perturbation (astronomy)|perturb]] planets. This led them to assume that its [[Albedo#Astronomical albedo|albedo]] could be no less than 0.07 (meaning that, at minimum, it would reflect 7% of the light that hit it), which would have made Pluto about as dark as asphalt, and similar in reflectivity to the least reflective planet, which is [[Mercury (planet)|Mercury]].<ref name="bower">{{Cite journal |last=Bower |first=Ernest Clare |date=1931 |title=On the orbit and mass of Pluto with an ephemeris for 1931-1932 |journal=Lick Observatory Bulletins |language=en |volume=15 |issue=437 |pages=171–178 |bibcode=1931LicOB..15..171B |doi=10.5479/ADS/bib/1931LicOB.15.171B |issn=0075-9317}}</ref> This would have given Pluto an estimated mass of no more than 70% that of Earth.<ref name=bower/> Observations also revealed that Pluto's orbit was very elliptical, far more than that of any other planet.<ref name="lauch">{{Cite book |last1=Davies |first1=John K. |title=The Solar System Beyond Neptune |last2=McFarland |first2=John |last3=Bailey |first3=Mark E. |last4=Marsden |first4=Brian G. |last5=Ip |first5=Wing-Huen |date=2008 |publisher=University of Arizona Press |editor-last=Baracci |editor-first=M. Antonietta |pages=11–23 |chapter=The Early Development of Ideas Concerning the Transneptunian Region |display-authors=4 |access-date=2014-11-05 |editor-last2=Boenhardt |editor-first2=Hermann |editor-last3=Cruikchank |editor-first3=Dale |editor-last4=Morbidelli |editor-first4=Alissandro |chapter-url=http://www.arm.ac.uk/preprints/2008/522.pdf |archive-url=https://web.archive.org/web/20150220182134/http://www.arm.ac.uk/preprints/2008/522.pdf |archive-date=2015-02-20 |url-status=dead}}</ref>

Almost immediately, some astronomers questioned Pluto's status as a planet. Barely a month after its discovery was announced, on April 14, 1930, in an article in ''[[The New York Times]]'', [[Armin Otto Leuschner|Armin O. Leuschner]] suggested that Pluto's 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."<ref name=lauch/><ref name=nyt/> In that same article, [[Harvard Observatory]] director [[Harlow Shapley]] wrote that Pluto was a "member of the Solar System not comparable with known asteroids and comets, and perhaps of greater importance to cosmogony than would be another major planet beyond Neptune."<ref name=nyt>{{cite news|journal=The New York Times|date=April 14, 1930|url= https://timesmachine.nytimes.com/timesmachine/1930/04/14/96096499.pdf|title="Planet X" Orbit Raises More Doubt}}</ref> In 1931, after examining the structure of the [[Errors and residuals|residuals]] of Uranus' longitude using a trigonometric formula, [[Ernest W. Brown]] asserted (in agreement with [[E. C. Bower]]) that the presumed 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".<ref>{{Cite journal |last=Brown |first=Ernest W. |date=November 1931 |title=On a Criterion for the Prediction of an Unknown Planet |journal=Monthly Notices of the Royal Astronomical Society |language=en |volume=92 |issue=1 |pages=80–100 |bibcode=1931MNRAS..92...80B |doi=10.1093/mnras/92.1.80 |issn=0035-8711 |doi-access=free}}</ref>

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 giant planets, as roughly that of Earth;<ref name="The Discovery of Pluto">{{cite journal|title= The Discovery of Pluto |journal= Monthly Notices of the Royal Astronomical Society |volume=91 |issue= 4 |date=February 1931 |pages= 380–385 |bibcode= 1931MNRAS..91..380. |doi=10.1093/mnras/91.4.380|doi-access= free }}</ref> a value somewhat in accord with the 0.91 Earth mass calculated in 1942 by [[Lloyd R. Wylie]] at the [[US Naval Observatory]], using the same assumptions.<ref name="wylie">{{Cite book |last=Weintraub |first=David A. |url=https://books.google.com/books?id=dW1_AwAAQBAJ |title=Is Pluto a Planet?: A Historical Journey through the Solar System |date=2014 |publisher=Princeton University Press |isbn=978-1-4008-5297-0 |page=141 |language=en}}</ref> In 1949, [[Gerard Kuiper]]'s measurements of Pluto's diameter with the 200-inch telescope at [[Mount Palomar Observatory]] led him 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.<ref name="Gerard P. Kuiper 1950 133–137">{{cite journal|title=The Diameter of Pluto |first=Gerard P. |last=Kuiper |journal=Publications of the Astronomical Society of the Pacific |volume=62 |pages= 133–137 |date=August 1950 |bibcode= 1950PASP...62..133K|doi= 10.1086/126255 |issue=366|doi-access=free }}</ref>

In 1973, based on the similarities in the periodicity and amplitude of brightness variation with [[Triton (moon)|Triton]], Dennis Rawlins conjectured Pluto's mass must be similar to Triton's. In retrospect, the conjecture turns out to have been correct; it had been argued by astronomers [[Walter Baade]] and E.C. Bower as early as 1934.<ref>{{Cite journal |last=Baade |first=W. |author-link=Walter Baade |year=1934 |title=The Photographic Magnitude and Color Index of Pluto |journal=Publications of the Astronomical Society of the Pacific |language=en |volume=46 |page=218 |bibcode=1934PASP...46..218B |doi=10.1086/124467 |issn=0004-6280 |doi-access=free |number=272}}</ref> However, because Triton's mass was then believed to be roughly 2.5% of the Earth–Moon system (more than ten times its actual value), Rawlins's determination for Pluto's mass was similarly incorrect. It was nonetheless a meagre enough value for him to conclude Pluto was not Planet X.<ref name="Mass and Position Limits for an Hypothetical Tenth Planet of the Solar System">{{Cite journal |last1=Rawlins |first1=D. |last2=Hammerton |first2=M. |date=June 1973 |title=Mass and Position Limits for an Hypothetical Tenth Planet of the Solar System |journal=Monthly Notices of the Royal Astronomical Society |language=en |volume=162 |issue=3 |pages=261–270 |bibcode=1973MNRAS.162..261R |doi=10.1093/mnras/162.3.261 |issn=0035-8711 |doi-access=free}} Rawlins also took into account Pluto's stellar occultation failure as reported by {{Cite journal |last1=Halliday |first1=Ian |last2=Hardie |first2=R. H. |last3=Franz |first3=Otto G. |last4=Priser |first4=John B. |year=1966 |title=An Upper Limit for the Diameter of Pluto |journal=Publications of the Astronomical Society of the Pacific |language=en |volume=78 |issue=461 |pages=113 |bibcode=1966PASP...78..113H |doi=10.1086/128307 |issn=0004-6280 |s2cid=121483531}}</ref> 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]] [[Volatile (astrogeology)|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 {{frac|1|100}}&nbsp;Earth mass.<ref>{{cite journal |doi=10.1126/science.194.4267.835-a |pmid=17744185 |journal=Science |volume=194 |issue=4267 |pages=835–837 |year=1976 |title=Pluto: Evidence for methane frost }}</ref><ref name="Croswell 1997, p. 57">Croswell (1997), p. 57.</ref>

<div style="float:right; margin:2px;">
{| class=wikitable style="text-align:center; font-size:11px"
|+ Mass estimates for Pluto
!Year!! Mass!!Notes
|-
|1931 || 1 Earth ||Nicholson & Mayall<ref name="The Discovery of Pluto"/>
|-
|1942 || 0.91 Earth || Wylie<ref name=wylie/>
|-
|1948||  0.1 (1/10 Earth) || Kuiper<ref name="Gerard P. Kuiper 1950 133–137"/>
|-
|1973|| 0.025 (1/40 Earth) || Rawlins<ref name="Mass and Position Limits for an Hypothetical Tenth Planet of the Solar System" />
|-
|1976||0.01 (1/100 Earth)||Cruikshank, Pilcher, & Morrison<ref name="Croswell 1997, p. 57"/>
|-
|1978||0.002 (1/500 Earth) || Christy & Harrington<ref name="Christy">{{Cite journal |last1=Christy |first1=J. W. |last2=Harrington |first2=R. S. |name-list-style=amp |date=August 1978 |title=The satellite of Pluto |journal=The Astronomical Journal |volume=83 |issue=8 |pages=1005 |bibcode=1978AJ.....83.1005C |doi=10.1086/112284}}</ref>
|-
|2006||0.00218 (1/459 Earth) || Buie et al.<ref name="Buie">{{Cite journal |last1=Buie |first1=Marc W. |last2=Grundy |first2=William M. |last3=Young |first3=Eliot F. |last4=Young |first4=Leslie A. |last5=Stern |first5=S. Alan |name-list-style=amp |date=July 2006 |title=Orbits and Photometry of Pluto's Satellites: Charon, S/2005 P1, and S/2005 P2 |journal=The Astronomical Journal |language=en |volume=132 |issue=1 |pages=290–298 |arxiv=astro-ph/0512491 |bibcode=2006AJ....132..290B |doi=10.1086/504422 |issn=0004-6256 |s2cid=119386667}}</ref>
|}
</div>

Pluto's size was finally determined conclusively in 1978, when American astronomer [[James W. Christy]] discovered its moon [[Charon (moon)|Charon]]. This enabled him, together with [[Robert Sutton Harrington]] of the U.S. Naval Observatory, to measure the mass of the Pluto–Charon system directly by observing the moon's orbital motion around Pluto.<ref name="Christy" /> They determined Pluto's mass to be 1.31×10<sup>22</sup>&nbsp;kg; roughly one five-hundredth that of 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&nbsp;X, it was not Pluto.<ref>Croswell (1997), pp. 57–58.</ref>

=== Further searches for Planet X ===
After 1978, a number of astronomers kept up the search for Lowell's Planet&nbsp;X, convinced that, because Pluto was no longer a viable candidate, an unseen tenth planet must have been perturbing the outer planets.<ref name=cros />

In the 1980s and 1990s, Robert Harrington led a search to determine the real cause of the apparent irregularities.<ref name=cros>Croswell, pp. 56–71</ref> He calculated that any Planet&nbsp;X would be at roughly three times the distance of Neptune from the Sun; its orbit would be highly [[orbital eccentricity|eccentric]], and strongly [[orbital inclination|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.<ref>{{Cite journal |last=Harrington |first=R. S. |date=October 1988 |title=The location of Planet X |journal=The Astronomical Journal |volume=96 |pages=1476 |bibcode=1988AJ.....96.1476H |doi=10.1086/114898}}</ref> This hypothesis was met with a mixed reception. Noted Planet&nbsp;X skeptic [[Brian G. Marsden]] of the [[Minor Planet Center]] pointed out that these discrepancies were a hundredth the size of those noticed by Le Verrier, and could easily be due to observational error.<ref>Croswell (1997), pp. 62–63.</ref>

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 at 59 AU that is in a [[retrograde orbit]] around the Sun.<ref name=Brady1972>{{cite journal |last=Brady |first=Joseph L. |title=The Effect of a Trans-Plutonian Planet on Halley's Comet |journal=Publications of the Astronomical Society of the Pacific |volume=84 |issue=498 |pages=314–322 |date=1972 |bibcode=1972PASP...84..314B |doi=10.1086/129290|s2cid=122053270 |doi-access=free }}</ref> However, both Marsden and Planet&nbsp;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&nbsp;X would have severely affected the orbits of known outer planets.<ref>Croswell (1997), p. 63.</ref>

Although its mission did not involve a search for Planet&nbsp;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".<ref>{{Cite news |last=O'Toole |first=Thomas |date=December 29, 1983 |title=Possibly as Large as Jupiter |url=https://www.washingtonpost.com/archive/politics/1983/12/30/possibly-as-large-as-jupiter/1075b265-120a-4d40-9493-a8c523b76927/ |access-date=2024-09-11 |newspaper=[[Washington Post]] |language=en-US |issn=0190-8286 |archive-date=2023-04-01 |archive-url=https://web.archive.org/web/20230401061239/https://www.washingtonpost.com/archive/politics/1983/12/30/possibly-as-large-as-jupiter/1075b265-120a-4d40-9493-a8c523b76927/ |url-status=dead }}</ref> Further analysis revealed that of several unidentified objects, nine were distant galaxies and the tenth was "[[infrared cirrus|interstellar cirrus]]"; none were found to be Solar System bodies.<ref>{{Cite journal |last1=Houck |first1=J. R. |last2=Schneider |first2=D. P. |last3=Danielson |first3=G. E. |last4=Neugebauer |first4=G. |last5=Soifer |first5=B. T. |last6=Beichman |first6=C. A. |last7=Lonsdale |first7=C. J. |display-authors=3 |name-list-style=vanc |date=March 1985 |title=Unidentified IRAS sources - Ultrahigh-luminosity galaxies |url=https://authors.library.caltech.edu/74762/1/1985ApJ___290L___5H.pdf |journal=The Astrophysical Journal |language=en |volume=290 |pages=L5 |bibcode=1985ApJ...290L...5H |doi=10.1086/184431 |issn=0004-637X}}</ref>

In 1988, A. A. Jackson and R. M. 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 axis]]. 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 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 semi-major axes of 52.5 and 62.5 AU disrupt the four-million-year libration of Pluto's argument of perihelion.<ref>{{Cite journal |last1=Jackson |first1=A. A. |last2=Killen |first2=R. M. |name-list-style=amp |date=October 1988 |title=Planet X and the stability of resonances in the Neptune-Pluto system |journal=Monthly Notices of the Royal Astronomical Society |language=en |volume=235 |issue=2 |pages=593–601 |bibcode=1988MNRAS.235..593J |doi=10.1093/mnras/235.2.593 |issn=0035-8711 |doi-access=free}}</ref>

=== Planet X disproved ===
Harrington died in January 1993, without having found Planet X.<ref name="croswell66">Croswell (1997), p. 66.</ref> Six months before, [[E. 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<ref name="croswell66" />—to recalculate its gravitational effect on Uranus.<ref>{{Cite journal |last=Standish |first=E. M. |date=May 1993 |title=Planet X - No dynamical evidence in the optical observations |journal=The Astronomical Journal |volume=105 |issue=5 |pages=2000 |bibcode=1993AJ....105.2000S |doi=10.1086/116575}}</ref> 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&nbsp;X, vanished.<ref name="standage" /> 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.<ref>Littman (1990), p. 204.</ref> Today, most astronomers agree that Planet&nbsp;X, as Lowell defined it, does not exist.<ref>{{Cite book |last=Standage |first=Tom |url=https://archive.org/details/neptunefilestory00stan/ |title=The Neptune file: a story of astronomical rivalry and the pioneers of planet hunting |date=2000 |publisher=Walker |isbn=978-0-8027-1363-6 |location=New York |page=[https://archive.org/details/neptunefilestory00stan/page/168 168] |url-access=registration}}</ref>