View source for Planet Nine

{{Short description|Hypothetical Solar System planet}}
{{Distinguish||text=the [[Planet X]] proposed in 1906 by [[Percival Lowell]]}}
{{About|the hypothetical planet first suggested in 2014|the dwarf planet previously classified as the Solar System's ninth planet|Pluto||Ninth planet (disambiguation)}}
{{Featured article}}
{{Use dmy dates|date=November 2024|cs1-dates=l}}
{{Infobox planet
| name = Planet Nine
| image = Planet nine artistic plain.png
| image_alt = Planet Nine depicted as a dark sphere distant from the Sun with the Milky Way in the background.
| caption = {{longitem|Artist's impression of Planet Nine eclipsing the central Milky Way, with the Sun in the distance; Neptune's orbit is shown as a small ellipse around the Sun (See [[:File:Planet nine artistic plain labeled.png|labelled version]])}}
| orbit_ref =
| aphelion = {{val|370|30|u=AU}}<ref name=Siraj2025/>
| perihelion = {{val|200|50|u=AU}}<ref name=Siraj2025/>
| semimajor = {{val|290|30|ul=AU}}<ref name=Siraj2025/>
| eccentricity = {{val|0.29|0.13}}<ref name=Siraj2025/>
| period = {{val|4940|790|750|fmt=commas|u=yr}}<ref name=Siraj2025/>{{efn-ua|As of 2024 the semi-major axis of Planet Nine is estimated by Siraj, Chyba and Tremaine to be 290 AU, with a margin or error of {{±|30}} allowing for a range between 260 and 320 AU; this implies that the planet has an orbital period of 4,190 (260<sup>1.5</sup>) to 5,720 (320<sup>1.5</sup>) years.}}
| avg_speed =
| inclination = {{val|6.8|5.0|u=deg}}<ref name=Siraj2025/>
| asc_node =
| arg_peri = {{val|150|u=°}} (est.)<ref name=TAJ201601/>
| mean_anomaly =
| satellites =
| mean_radius =
| mass = {{val|4.4|1.1|ul=Earth mass}}<ref name=Siraj2025>{{Cite journal |last1=Siraj |first1=Amir |last2=Chyba |first2=Christopher F. |last3=Tremaine |first3=Scott |date=January 2025 |title=Orbit of a Possible Planet X |journal=The Astrophysical Journal |language=en |volume=978 |issue=2 |pages=139 |doi=10.3847/1538-4357/ad98f6 |doi-access=free |issn=0004-637X}}</ref>
| density =
| surface_grav =
| escape_velocity =
| sidereal_day =
| axial_tilt =
| magnitude = ≈21<ref name=Batygin2021/>
| angular_size =
| temperatures =
| temp_name1 =
| min_temp_1 =
| mean_temp_1 =
| max_temp_1 =
| temp_name2 =
| min_temp_2 =
| mean_temp_2 =
| max_temp =
| pronounced =
}}

'''Planet Nine''' <!-- Planet X AKA has a hatnote -->is a [[List of hypothetical Solar System objects|hypothetical ninth planet]] in the [[outer region of the Solar System]].<ref name=P9H2019/><ref name=TAJ201601/> Its gravitational effects could explain the peculiar clustering of [[orbit]]s for a group of [[extreme trans-Neptunian object]]s (ETNOs), bodies beyond [[Neptune]] that orbit the Sun at distances averaging more than 250&nbsp;times that of the Earth i.e. over 250&nbsp;[[astronomical unit]]s (AU). These ETNOs tend to make their closest approaches to the Sun in one sector, and their orbits are similarly tilted. These alignments suggest that an undiscovered planet may be shepherding the orbits of the most distant known [[Solar System]] objects.<ref name=TAJ201601/><ref name=Sheppard2014/><ref name=fatty/> Nonetheless, some astronomers question this conclusion and instead assert that the clustering of the ETNOs' orbits is due to observational biases, resulting from the difficulty of discovering and tracking these objects during much of the year.<ref name=TC-20200525/>

Based on earlier considerations, this hypothetical [[super-Earth]]-sized planet would have had a predicted mass of five to ten times that of the Earth, and an [[elongated orbit]] 400–800&nbsp;[[astronomical unit|AU]]. The orbit estimation was refined in 2021, resulting in a somewhat smaller semimajor axis of {{nobr|380{{±|140|80}} AU.<ref name=Batygin2021/>}} This was shortly thereafter updated to {{nobr|460 {{±|160|100}} AU,<ref name=Brown_Batygin_2021z/>}} and to {{val|290|30|ul=AU}} in 2025.<ref name=Siraj2025/> [[Konstantin Batygin|Batygin]] & [[Michael E. Brown|Brown]] suggested that Planet Nine may be the [[Planetary core|core]] of a [[giant planet]] that was ejected from its original orbit by [[Jupiter]] during the [[nebular hypothesis|genesis]] of the Solar System. Others proposed that the planet was [[gravitational capture|captured]] from another star,<ref name=Mustill_etal_2016/> was once a [[rogue planet]], or that it formed on a distant orbit and was pulled into an eccentric orbit by a passing star.<ref name=TAJ201601/>

Although sky surveys such as [[Wide-field Infrared Survey Explorer]] (WISE) and [[Pan-STARRS]] did not detect Planet Nine, they have not ruled out the existence of a Neptune-diameter object in the outer Solar System.<ref name=bnb20160617/><ref name=Luhman2014/> The ability of these past sky surveys to detect Planet Nine was dependent on its location and characteristics. Further surveys of the remaining regions are ongoing using [[NEOWISE]] and the 8&nbsp;meter [[Subaru Telescope]].<ref name=Meisner_etal_2017b/><ref name=sciencemag201601/> Unless Planet Nine is observed, its existence remains purely conjectural. Several alternative hypotheses have been proposed to explain the observed clustering of [[trans-Neptunian object]]s (TNOs).

==History==
{{see also|Planets beyond Neptune#Orbits of distant objects|Detached object#Orbits}}

Following the [[discovery of Neptune]] in 1846, there was considerable speculation that another planet might exist beyond its orbit. The best-known of these theories predicted the existence of a distant planet that was influencing the orbits of [[Uranus]] and [[Neptune]]. After extensive calculations, [[Percival Lowell]] predicted the possible orbit and location of the hypothetical trans-Neptunian planet and began an extensive search for it in 1906. He called the hypothetical object {{nobr|''[[Planet X]]'',}} a name previously used by Gabriel Dallet.<ref name=grosser/><ref name=tombaugh/> [[Clyde Tombaugh]] continued Lowell's search and in 1930 [[discovery of Pluto|discovered Pluto]], but it was soon determined to be too small to qualify as Lowell's Planet&nbsp;X.<ref name=Croswell1997/> After ''[[Voyager 2]]''{{'}}s flyby of Neptune in 1989, the difference between Uranus' predicted and observed orbit was determined to have been due to the use of a previously inaccurate mass of Neptune.<ref name=Browne_1993/>

Attempts to detect [[planets beyond Neptune]] by indirect means such as orbital perturbation date to before the discovery of Pluto. Among the first was [[George Forbes (scientist)|George Forbes]] who postulated the existence of two trans-Neptunian planets in 1880. One would have an average distance from the Sun, or [[semi-major axis]], of 100&nbsp;[[astronomical unit|AU]], 100&nbsp;times that of the Earth. The second would have a semi-major axis of 300&nbsp;AU. His work is considered similar to more recent Planet Nine theories in that the planets would be responsible for a clustering of the orbits of several objects, in this case the clustering of [[Perihelion and aphelion|aphelion]] distances of periodic [[comets]] near about 100–300&nbsp;AU. This is similar to how the aphelion distances of [[Short-period comet|Jupiter-family comets]] cluster near its orbit.<ref name=Millholland_Laughlin_2017/><ref name=Forbes_1880/>

The discovery of [[Sedna (dwarf planet)|Sedna]], a dwarf planet with a highly peculiar orbit in 2004, led to speculation that it had encountered a massive body other than one of the known planets. Sedna's orbit is [[detached orbit|detached]], with a [[perihelion]] distance of 76&nbsp;AU that is too large to be due to gravitational interactions with Neptune. Several authors proposed that Sedna entered this orbit after encountering a massive body such as an unknown planet on a distant orbit, a member of the [[open cluster]] that formed with the Sun, or another star that later passed near the Solar System.<ref name=spaceBrown/><ref name=iopsciencesedna/> The announcement in March&nbsp;2014 of the discovery of a second [[sednoid#Known members|sednoid]] with a perihelion distance of 80&nbsp;AU, {{mpl|2012 VP|113}}, in a similar orbit led to renewed speculation that an unknown super-Earth remained in the distant Solar System.<ref name=guardianSuperEarth/><ref name=searchformoreSedna/>

At a conference in 2012, Rodney Gomes proposed that an undetected planet was responsible for the orbits of some ETNOs with detached orbits and the large semi-major axis [[Centaur (minor planet)|Centaurs]], [[small Solar System bodies]] that cross the orbits of the giant planets.<ref name=llmysteries/><ref name=nosmokingGun/> The proposed Neptune-massed planet would be in a distant {{nobr|([[semimajor axis|{{mvar|a}}]] ≈ {{gaps|1|500}} AU),}} eccentric {{nobr|([[orbital eccentricity|{{mvar|e}}]] ≈ 0.4),}} and steeply inclined {{nobr|([[orbital inclination|{{mvar|i}}]] ≈ 40°)}} orbit. Like Planet Nine it would cause the perihelia of objects with semi-major axes greater than 300&nbsp;AU to oscillate, delivering some into planet-crossing orbits and others into detached orbits like that of Sedna. An article by Gomes, Soares, and Brasser was published in 2015, detailing their arguments.<ref name=Gomes_2015/>

In 2014, astronomers [[Chad Trujillo]] and [[Scott S. Sheppard]] noted the similarities in the orbits of Sedna and {{mp|2012 VP|113}} and several other ETNOs. They proposed that an unknown planet in a circular orbit between 200 and 300&nbsp;AU was perturbing their orbits.<ref name=Sheppard2014/> Later that year, Raúl and Carlos de la Fuente Marcos argued that two massive planets in [[orbital resonance]] were necessary to produce the similarities of so many orbits, 13&nbsp;known at the time.<ref name=Marcos_2014/> Using a larger sample of 39&nbsp;ETNOs, they estimated that the nearer planet had a semi-major axis in the range of 300–400&nbsp;AU, a relatively low eccentricity, and an inclination of nearly 14°.<ref name=randomsearch/>

==Batygin and Brown hypothesis==
[[File:Planet nine path in orion2.png|upright=3.2|thumb|One hypothetical path through the sky of Planet Nine near [[aphelion]] crossing [[Orion (constellation)|Orion]] west to east with about {{nobr|{{gaps|2|000}} years}} of motion. It is derived from that employed in the artistic conception on Brown's blog.<ref name=findplanet9/>|alt=Starfield with hypothetical path of Planet Nine]]

In early 2016, [[California Institute of Technology]]'s Batygin and Brown described how the similar orbits of six ETNOs could be explained by Planet Nine and proposed a possible orbit for the planet.<ref name=TAJ201601/> This hypothesis could also explain ETNOs with orbits [[perpendicular]] to the [[inner planets]]<ref name=TAJ201601/> and others with extreme inclinations,<ref name=Batygin_Brown_high_inc/> and had been offered as an explanation of the [[axial tilt|tilt]] of the Sun's [[rotation around a fixed axis|axis]].<ref name=Gomes_Deienno_Morbidelli_2016/>

===Orbit===

Planet Nine was initially hypothesized to follow an [[elliptical orbit]] around the Sun with an eccentricity of {{val|0.2|–|0.5}}, and its [[semi-major axis]] was estimated to be {{val|400|–|800|u=AU}},{{efn-ua|
A range of semi-major axes extending from 400–1000&nbsp;AU produce the observed clustering in simulations.<ref name=bnb20160617/>
}}
roughly 13–26&nbsp;times the distance from Neptune to the Sun. It would take the planet between {{nobr|{{gaps|10|000}} – {{gaps|20|000}} years}} to make one full orbit around the Sun, and its inclination to the [[ecliptic]], the plane of the Earth's orbit, was projected to be {{val|15|to|25|u=°}}.<ref name=P9H2019/><ref name=NASA_PlanetX/>{{efn-ua|
''[[The New Yorker]]'' put the average orbital distance of Planet Nine into perspective with an apparent allusion to one of the magazine's most famous cartoons, ''[[View of the World from 9th Avenue]]'': "If the Sun were on [[Fifth Avenue]] and Earth were one block west, Jupiter would be on the [[West Side Highway]], Pluto would be in [[Montclair, New Jersey]], and the new planet would be somewhere near [[Cleveland]]."<ref name=fatty/>
}}
The aphelion, or farthest point from the Sun, would be in the general direction of the [[constellation]] of [[Taurus (constellation)|Taurus]],<ref name=Brown_YouTube2017/> whereas the perihelion, the nearest point to the Sun, would be in the general direction of the southerly areas of [[Serpens]] (Caput), [[Ophiuchus]], and [[Libra (constellation)|Libra]].<ref name=B-BCharts/><ref name=Lemonick2016/> Brown thinks that if Planet Nine exists, a [[space probe|probe]] could reach it in as little as 20&nbsp;years by using a [[Oberth effect|powered slingshot]] [[trajectory]] around the Sun.<ref name=newscientist/>

=== Mass and radius===

The planet is estimated to have 5–10&nbsp;times the mass and 2–4&nbsp;times the radius of the Earth.<ref name=P9H2019/> Brown thinks that if Planet Nine exists, its mass is sufficient to [[clearing the neighbourhood|clear its orbit]] of large bodies in 4.5&nbsp;billion years, the age of the Solar System, and that its gravity dominates the outer edge of the Solar System, which is sufficient to make it a [[Definition of planet#IAU definition|planet by current definitions]].<ref name=lurking/> Astronomer [[Jean-Luc Margot]] has also stated that Planet Nine satisfies his criteria and would qualify as a planet if and when it is detected.<ref name=Margot_2016/><ref name=Margot_2015/>

Later simulations by [[Amir Siraj]] and colleagues in 2025 proposed that Planet Nine's mass would instead be 4.4 ± 1.1 times that of Earth.<ref name="Siraj et al 2025"/><ref name="Ferreira 2025"/>

====Internal composition====
Given a hypothesized ~10 Earth masses and using a theory of exoplanet sizes in the Kepler-454 system, Esther Linder and Christoph Mordasini assumed that Planet Nine's radius would be 3.66 times that of Earth's (23,300 km versus 6,378 km), and that its internal composition would be similar to [[Uranus]] and Neptune's: Planet Nine would likely have a hydrogen-helium atmosphere averaging 47&nbsp;kelvins, with a core composed of [[iron]] and a mantle filled with [[magnesium silicate]] and water ice.<ref name="Linder_Mordasini_2016"/><ref name="BBC 2016">{{cite web | last=Rincon | first=Paul | title=Planet Nine's profile fleshed out | website=BBC News | date=April 8, 2016 | url=https://www.bbc.com/news/science-environment-35996813 | access-date=January 12, 2025}}</ref>

However, Siraj et al. (2025) suggest that Planet Nine's mass and orbital characteristics would render its composition closer to that of a rocky planet like Earth.<ref name="Siraj et al 2025"/>

===Origin===
{{See also|Nice model|Five-planet Nice model|Planetary migration}}

Several possible origins for Planet Nine have been examined, including its ejection from the neighborhood of the known giant planets, capture from another star, and ''[[in situ]]'' formation. In their initial article, Batygin and Brown proposed that Planet Nine formed closer to the Sun and was ejected into a distant eccentric orbit following a close encounter with [[Jupiter]] or [[Saturn]] during the nebular epoch.<ref name=TAJ201601/> Then, either the gravity of a nearby star or drag from the gaseous remnants of the [[Solar nebula]]<ref name=Bromley_Kenyon_2016/> reduced the eccentricity of its orbit. This process raised its perihelion, leaving it in a very wide but stable orbit beyond the influence of the other planets.<ref name=nytimes9thplanet/><ref name=caltechFAQ/>

The odds of this occurring has been estimated at a few percent.<ref name=Bailey_Fabrycky/> If it had not been flung into the Solar System's farthest reaches, Planet Nine could have accreted more mass from the [[proto-planetary disk]] and developed into the core of a [[gas giant]] or [[ice giant]].<ref name=lurking/><ref name=dl2018/> Instead, its growth was halted early, leaving it with a lower mass than Uranus or Neptune.<ref name=Izidoro_etal_2015/>

[[Dynamical friction]] from a massive belt of [[planetesimal]]s also could have enabled Planet Nine's capture into a stable orbit. Recent models propose that a {{Earth mass|60–130|link=yes|sym=yes}} disk of planetesimals could have formed as the gas was cleared from the outer parts of the proto-planetary disk.<ref name=Carrera_etal_2017/> As Planet Nine passed through this disk its gravity would alter the paths of the individual objects in a way that reduced Planet Nine's velocity relative to it. This would lower the eccentricity of Planet Nine and stabilize its orbit. If this disk had a distant inner edge, 100–200&nbsp;AU, a planet encountering Neptune would have a 20% chance of being captured in an orbit similar to that proposed for Planet Nine, with the observed clustering more likely if the inner edge is at 200&nbsp;AU. Unlike the gas nebula, the planetesimal disk is likely to have been long lived, potentially allowing a later capture.<ref name=Eriksson_etal_2017/>

An encounter with another star could also alter the orbit of a distant planet, shifting it from a circular to an eccentric orbit. The ''in situ'' formation of a planet at this distance would require a very massive and extensive disk,<ref name=TAJ201601/> or the outward drift of solids in a dissipating disk forming a narrow ring from which the planet accreted over a billion years.<ref name=Kenyon_Bromley_2016/> If a planet formed at such a great distance while the Sun was in its original cluster, the probability of it remaining bound to the Sun in a highly eccentric orbit is roughly 10%.<ref name=Li_Adams_2016/> However, while the Sun remained in the open cluster where it formed, any extended disk would have been subject to gravitational disruption by passing stars and by mass loss due to photoevaporation.<ref name=P9H2019/>

Planet Nine could have been captured from outside the Solar System during a close encounter between the Sun and another star. If a planet was in a distant orbit around this star, [[Euler's three-body problem|three-body]] interactions during the encounter could alter the planet's path, leaving it in a stable orbit around the Sun. A planet originating in a system without Jupiter-massed planets could remain in a distant eccentric orbit for a longer time, increasing its chances of capture.<ref name=Mustill_etal_2016/> The wider range of possible orbits would reduce the odds of its capture in a relatively low inclination orbit to 1–2%.<ref name=Li_Adams_2016/> Amir Siraj and [[Avi Loeb]] found that the odds of the Sun capturing Planet Nine increases by 20× if the Sun once had a distant, equal-mass binary companion.<ref name=Siraj_Loab_2020/><ref name=Passant_2020/> This process could also occur with rogue planets, but the likelihood of their capture is much smaller, with only 0.05–0.10% being captured in orbits similar to that proposed for Planet Nine.<ref name=Parker_etal_2017/>

===Evidence===
{{TNO}}
The gravitational influence of Planet Nine would explain four peculiarities of the Solar System:<ref name=Brennan_2017/>
* the clustering of the orbits of ETNOs;
* the high perihelia of objects like Sedna that are [[detached object|detached]] from Neptune's influence;
* the high inclinations of ETNOs with orbits roughly perpendicular to the orbits of the eight known planets;
* high-inclination [[trans-Neptunian object]]s (TNOs) with semi-major axis less than 100 AU.

Planet Nine was initially proposed to explain the clustering of orbits, via a mechanism that would also explain the high perihelia of objects like Sedna. The evolution of some of these objects into perpendicular orbits was unexpected, but found to match objects previously observed. The orbits of some objects with perpendicular orbits were later found to evolve toward smaller semi-major axes when the other planets were included in simulations. Although other mechanisms have been offered for many of these peculiarities, the gravitational influence of Planet Nine is the only one that explains all four. The gravity of Planet Nine would also increase the inclinations of other objects that cross its orbit, however, which could leave the [[scattered disc object|scattered disk objects]],<ref name=Kaib_etal_2019/> bodies orbiting beyond Neptune with semi-major axes greater than 50&nbsp;AU, and [[short-period comet]]s with a broader inclination distribution than is observed.<ref name=Nesvorny_etal_2017/> Previously Planet Nine was hypothesized to be responsible for the 6° tilt of the Sun's axis relative to the orbits of the planets,<ref name="Stirone_sun tilt"/> but recent updates to its predicted orbit and mass limit this shift to ≈1°.<ref name=P9H2019/>

====Observations: Orbital clustering of high perihelion objects====

[[File:Orbit1.svg|thumb|upright=1.2|Diagram illustrating the true anomaly, argument of periapsis, longitude of ascending node, and inclination of a celestial body.|alt=Orbit of a celestial body is shown as a tilted ellipse intersecting the ecliptic.]]
The clustering of the orbits of TNOs with large semi-major axes was first described by Trujillo and Sheppard, who noted similarities between the orbits of Sedna and {{mp|2012 VP|113}}. Without the presence of Planet Nine, these orbits should be distributed randomly, without preference for any direction. Upon further analysis, Trujillo and Sheppard observed that the [[argument of perihelion|arguments of perihelion]] of 12 TNOs with perihelia greater than {{val|30|u=AU}} and semi-major axes greater than {{val|150|u=AU}} were clustered near 0°, meaning that they rise through the ecliptic when they are closest to the Sun. Trujillo and Sheppard proposed that this alignment was caused by a massive unknown planet beyond Neptune via the [[Kozai mechanism]].<ref name=Sheppard2014/> For objects with similar semi-major axes the Kozai mechanism would confine their arguments of perihelion near to either 0° or 180°. This confinement allows objects with eccentric and inclined orbits to avoid close approaches to the planet because they would cross the plane of the planet's orbit at their closest and farthest points from the Sun, and cross the planet's orbit when they are well above or below its orbit.<ref name=Marcos_2014/><ref name=KopanyasKozai/> Trujillo and Sheppard's hypothesis about how the objects would be aligned by the Kozai mechanism has been supplanted by further analysis and evidence.<ref name=TAJ201601/>

Batygin and Brown, looking to refute the mechanism proposed by Trujillo and Sheppard, also examined the orbits of the TNOs with large semi-major axes.<ref name=TAJ201601/> After eliminating the objects in Trujillo and Sheppard's original analysis that were unstable due to close approaches to Neptune or were affected by Neptune's [[mean-motion resonance]]s, Batygin and Brown determined that the arguments of perihelion for the remaining six objects (Sedna, {{mp|2012 VP|113}}, [[474640 Alicanto]], {{mpl|2010 GB|174}}, {{mpl-|148209|2000 CR|105}}, and {{mpl-|445473|2010 VZ|98}}) were clustered around {{val|318|8|u=°}}. This finding did not agree with how the Kozai mechanism would tend to align orbits with arguments of perihelion at 0° or 180°.<ref name=TAJ201601 />{{efn-ua|
Two types of protection mechanisms are possible:<ref name=KopanyasKozai/>

# For bodies whose values of {{mvar|a}} and {{mvar|e}} are such that they could encounter the planets only near perihelion (or aphelion), such encounters may be prevented by the high inclination and the libration of {{mvar|ω}} about 90° or 270° (even when the encounters occur, they do not affect much the minor planet's orbit due to comparatively high relative velocities).
# Another mechanism is viable when at low inclinations when {{mvar|ω}} oscillates around 0° or 180° and the minor planet's semi-major axis is close to that of the perturbing planet: In this case the °node crossing occurs always near perihelion and aphelion, far from the planet itself, provided the eccentricity is high enough and the orbit of the planet is almost circular.
}}

[[File:Planet Nine animation.gif|upright=2|thumb|Orbital correlations among six distant trans-Neptunian objects led to the hypothesis. (See: [[:File:Planet Nine - black background.png|Final frame orbits]].)|alt=animated diagram zooms out from the orbits of the inner and outer planets to the greatly extended orbits of the outermost objects, which point towards the left of the screen. Planet Nine's hypothetical orbit appears as a broken line ]]

Batygin and Brown also found that the orbits of the six ETNOs with semi-major axis greater than 250&nbsp;AU and perihelia beyond 30&nbsp;AU (Sedna, {{mp|2012 VP|113}}, Alicanto, {{mp|2010 GB|174}}, {{mpl|2007 TG|422}}, and {{mpl|2013 RF|98}}) were aligned in space with their perihelia in roughly the same direction, resulting in a clustering of their [[longitude of perihelion|longitudes of perihelion]], the location where they make their closest approaches to the Sun. The orbits of the six objects were also tilted with respect to that of the [[Ecliptic plane|ecliptic]] and approximately [[coplanarity|coplanar]], producing a clustering of their [[longitude of ascending node|longitudes of ascending nodes]], the directions where they each rise through the ecliptic. They determined that there was only a 0.007% likelihood that this combination of alignments was due to chance.<ref name=TAJ201601 /><ref name=cbcRadio/><ref name="planetaryblog201601"/> These six objects had been discovered by six different surveys on six telescopes. That made it less likely that the clumping might be due to an observation bias such as pointing a telescope at a particular part of the sky. The observed clustering should be smeared out in a few hundred million years due to the locations of the perihelia and the ascending nodes changing, or [[Stellar precession|precessing]], at differing rates due to their varied semi-major axes and eccentricities.{{efn-ua|The precession rate is slower for objects with larger semi-major axes and inclinations and with smaller eccentricities: <math>\dot{\varpi}=\frac{3}{4} \sqrt{\frac{GM}{a^3}} \frac{1}{(1-e^2)^2}\sum_{k=5}^8 \frac{m_k a_k^2}{M a^2}cos^2(i)</math>
where <math> m_k a_k</math> are the masses and semi-major axes of the planets Jupiter through Neptune.}} This indicates that the clustering could not be due to an event in the distant past,<ref name=TAJ201601 /> for example a passing star,<ref name="Hands_etal_2019"/> and is most likely maintained by the gravitational field of an object orbiting the Sun.<ref name=TAJ201601 />

Two of the six objects ({{mp|2013 RF|98}} and Alicanto) also have very similar orbits and spectra.<ref name="gtcres"/><ref name="deLeon"/> This has led to the suggestion that they were a [[binary asteroid|binary object]] disrupted near aphelion during an encounter with a distant object. The disruption of a binary would require a relatively close encounter, which becomes less likely at large distances from the Sun.<ref name="Marcos_binary_disruption"/>

In a later article Trujillo and Sheppard noted a correlation between the longitude of perihelion and the argument of perihelion of the TNOs with semi-major axes greater than 150 AU. Those with a longitude of perihelion of 0–120° have arguments of perihelion between 280 and 360°, and those with longitude of perihelion between {{formatnum:180}}° and {{formatnum:340}}° have arguments of perihelion between {{formatnum:0}}° and {{formatnum:40}}°. The statistical significance of this correlation was 99.99%. They suggested that the correlation is due to the orbits of these objects avoiding close approaches to a massive planet by passing above or below its orbit.<ref name="trujillo20160912"/>

A 2017 article by Carlos and Raúl de la Fuente Marcos noted that distribution of the distances to the ascending nodes of the ETNOs, and those of centaurs and comets with large semi-major axes, may be [[bimodal]]. They suggest it is due to the ETNOs avoiding close approaches to a planet with a semi-major axis of 300–400&nbsp;AU.<ref name=nodes/><ref name="nodes_phys.org"/> With more data (40 objects), the distribution of mutual nodal distances of the ETNOs shows a statistically significant asymmetry between the shortest mutual ascending and descending nodal distances that may not be due to observational bias but likely the result of external perturbations.<ref name=nodes2/><ref name=nodes3/>
{{multiple image
 |direction = horizontal
 |align = left
 |total_width = 1000
 |header = The extreme trans-Neptunian object orbits
 |image1 = Planet nine-etnos now-new3.png
 |image2 = Planet_nine-etnos_now-close-new.png
 |alt1 = Orbits of extreme trans-Neptunian objects and Planet Nine
 |alt2 = Close up of extreme trans-Neptunian objects' and planets' orbits
 |caption1 = Six original and [[extreme trans-Neptunian objects|eight additional ETNO objects]] orbits with current positions near their perihelion in purple, with hypothetical Planet Nine orbit in green
 |caption2 = Close up view of 13 ETNO current positions
 |alt = The orbits of the extreme trans-Neptunian objects are shown in various colors, with Planet Nine's orbit shown in bright green. Most of the orbits are aligned to the right of Planet Nine.
}}
{{Clear}}

====Simulations: Observed clustering reproduced====

The clustering of the orbits of ETNOs and raising of their perihelia is reproduced in simulations that include Planet Nine. In simulations conducted by Batygin and Brown, swarms of scattered disk objects with semi-major axes up to 550 AU that began with random orientations were sculpted into roughly [[collinear]] and coplanar groups of spatially confined orbits by a massive distant planet in a highly eccentric orbit. This left most of the objects' perihelia pointed in similar directions and the objects' orbits with similar tilts. Many of these objects entered high-perihelion orbits like Sedna and, unexpectedly, some entered perpendicular orbits that Batygin and Brown later noticed had been previously observed.<ref name=TAJ201601 />

In their original analysis Batygin and Brown found that the distribution of the orbits of the first six ETNOs was best reproduced in simulations using a {{Earth mass|10|link=yes}}{{efn-ua|
Batygin and Brown provide an order of magnitude estimate for the mass.
* If {{mvar|M}}{{sub|9}} were equal to {{Earth mass|0.1|link=yes}}, then the dynamical evolution would proceed at an exceptionally slow rate, and the lifetime of the Solar System would likely be insufficient for the required orbital sculpting to transpire.
* If {{mvar|M}}{{sub|9}} were equal to {{Earth mass|1}}, then long-lived apsidally anti-aligned orbits would indeed occur, but removal of unstable orbits would happen on a much longer timescale than the current evolution of the Solar System. Hence, even though they would show preference for a particular apsidal direction, they would not exhibit true confinement like the data.
* They also note that {{mvar|M}}{{sub|9}} greater than {{Earth mass|10}} would imply a longer semi-major axis.
Hence they estimate that the mass of the object is likely in the range of 5–{{Earth mass|15}}.
}}
planet in the following orbit:{{efn-ua|calculated values in parentheses.}}
* [[semi-major axis]] {{mvar|a}} ≈ {{val|700|u=AU}} ([[orbital period]] 700<sup>1.5</sup> {{=}} {{nobr|{{gaps|18|520}} years}})
* [[orbital eccentricity|eccentricity]] {{mvar|e}} ≈ 0.6, ([[perihelion]] ≈ {{val|280|u=AU}}, [[aphelion]] ≈ {{val|1120|u=AU|fmt=gaps}})
* [[inclination]] {{mvar|i}} ≈ 30° to the [[ecliptic]]
* [[longitude of the ascending node]] {{mvar|Ω}} ≈ {{val|100||u=°}}.{{efn-ua|
The average of longitude of the ascending node for the six objects is about 102°. In a blog published later, Batygin and Brown constrained their estimate of the longitude of the ascending node to {{val|94||u=°}}.
}}
* [[argument of perihelion]] {{mvar|ω}} ≈ 140° and [[longitude of perihelion]] {{mvar|ϖ}} ≡ {{mvar|ω}} + {{math|Ω}} ≈ {{val|240|u=°}}<ref name="P9 where are you P1"/>

These parameters for Planet Nine produce different simulated effects on TNOs. Objects with semi-major axis greater than 250&nbsp;AU are strongly anti-aligned with Planet Nine, with perihelia opposite Planet Nine's perihelion. Objects with semi-major axes between 150 and 250&nbsp;AU are weakly aligned with Planet Nine, with perihelia in the same direction as Planet Nine's perihelion. Little effect is found on objects with semi-major axes less than 150&nbsp;AU.<ref name=bnb20160617/> The simulations also revealed that objects with semi-major axes greater than {{val|250|u=AU}} could have stable, aligned orbits if they had lower eccentricities. These objects have yet to be observed.<ref name=TAJ201601/>

Other possible orbits for Planet Nine were also examined, with semi-major axes between {{val|400|u=AU}} and {{val|1500|u=AU|}}, eccentricities up to 0.8, and a wide range of inclinations. These orbits yield varied results. Batygin and Brown found that orbits of the ETNOs were more likely to have similar tilts if Planet Nine had a higher inclination, but anti-alignment also decreased.<ref name=bnb20160617/> Simulations by Becker et al. showed that their orbits were more stable if Planet Nine had a smaller eccentricity, but that anti-alignment was more likely at higher eccentricities.<ref name=Becker_etal_2017/> Lawler ''et al''. found that the population captured in orbital resonances with Planet Nine was smaller if it had a circular orbit, and that fewer objects reached high inclination orbits.<ref name=Lawler_etal_2016/> Investigations by Cáceres et al. showed that the orbits of the ETNOs were better aligned if Planet Nine had a lower perihelion orbit, but its perihelion would need to be higher than 90&nbsp;AU.<ref name=Caceres_etal_2018/> Later investigations by Batygin ''et al''. found that higher eccentricity orbits reduced the average tilts of the ETNOs' orbits.<ref name=P9H2019/> While there are many possible combinations of orbital parameters and masses for Planet Nine, none of the alternative simulations were better at predicting the observed alignment of the original ETNOs. The discovery of additional distant Solar System objects would allow astronomers to make more accurate predictions about the orbit of the hypothesized planet. These may also provide further support for, or refutation of, the Planet Nine hypothesis.<ref name=discoveryDeBrief/><ref name=thestarKateAllen/>

Simulations that included the migration of giant planets resulted in a weaker alignment of the ETNOs' orbits.<ref name=Nesvorny_etal_2017/> The direction of alignment also switched, from more aligned to anti-aligned with increasing semi-major axis, and from anti-aligned to aligned with increasing perihelion distance. The latter would result in the sednoids' orbits being oriented opposite most of the other ETNOs.<ref name=Kaib_etal_2019/>

====Dynamics: How Planet Nine modifies the orbits of ETNOs====
[[File:Secular evolution of eTNOs induced by Planet Nine.png|thumb|upright=1.6|Long term evolution of ETNOs induced by Planet Nine for objects with semi-major axis of 250 AU.<ref name="Beust_2016" /><ref name="Batygin_Morbidelli_2017" /> Blue: anti-aligned, Red: aligned, Green: metastable, Orange: circulating. Crossing orbits above black line.{{efn-ua|Similar figures in articles by Beust<ref name="Beust_2016" /> and Batygin and Morbidelli<ref name="Batygin_Morbidelli_2017" /> are plots of the Hamiltonian, showing combinations of orbital eccentricities and orientations that have equal energy. If there are no close encounters with Planet Nine, which would change the energy of the orbit, the object's [[orbital elements]] remain on one of these curves as the orbits evolve.}}|alt=the aligned orbits appear as red contour lines on either side of a parabolic black line, while the anti-aligned orbits appear as blue contour lines within the parabola.]]

{{Main|Effects of Planet Nine on trans-Neptunian objects}}

Planet Nine modifies the orbits of ETNOs via a combination of effects. On very long timescales Planet Nine exerts a [[torque]] on the orbits of the ETNOs that varies with the alignment of their orbits with Planet Nine's. The resulting exchanges of [[angular momentum]] cause the perihelia to rise, placing them in Sedna-like orbits, and later fall, returning them to their original orbits after several hundred million years. The motion of their directions of perihelion also reverses when their eccentricities are small, keeping the objects anti-aligned, see blue curves on diagram, or aligned, red curves. On shorter timescales mean-motion resonances with Planet Nine provides phase protection, which stabilizes their orbits by slightly altering the objects' semi-major axes, keeping their orbits synchronized with Planet Nine's and preventing close approaches. The gravity of Neptune and the other giant planets, and the inclination of Planet Nine's orbit, weaken this protection. This results in a [[Chaos theory|chaotic]] variation of semi-major axes as objects hop between resonances, including high-order resonances such as 27:17, on million-year timescales.<ref name="Batygin_Morbidelli_2017" /> The mean-motion resonances may not be necessary for the survival of ETNOs if they and Planet Nine are both on inclined orbits.<ref name="Li_etal_2018"/> The orbital poles of the objects precess around, or circle, the pole of the Solar System's [[Laplace plane]]. At large semi-major axes the Laplace plane is warped toward the plane of Planet Nine's orbit. This causes orbital poles of the ETNOs on average to be tilted toward one side and their longitudes of ascending nodes to be clustered.<ref name="Batygin_Morbidelli_2017"/>

In 2024, Brown and Batygin completed a simulation which showed that the presence of Planet Nine, over time, would increase the eccentricities of a significant subset of objects with semi-major axes above 100 AU until their perihelion reduced under 30 AU, which would mean that their orbits cross that of Neptune. They also conducted a survey of Neptune-crossing objects with inclinations below 40 degrees and semi-major axes between 100 and 1000 AU and argued that the results aligned with the presence of Planet Nine, which would produce a ratio of Neptune-crossers to objects with a perihelion beyond Neptune's orbit of 3%, compared to 0.5% in the absence of Planet Nine.<ref>{{cite journal |author=Batygin |first1=Konstantin |last2=Morbidelli |first2=Alessandro |last3=Brown |first3=Michael E. |last4=Nesvorny |first4=David |name-list-style=and |date=24 April 2024 |title=Generation of Low-inclination, Neptune-crossing Trans-Neptunian Objects by Planet Nine |journal=The Astrophysical Journal Letters |volume=966 |pages=L8 |arxiv=2404.11594 |bibcode=2024ApJ...966L...8B |doi=10.3847/2041-8213/ad3cd2 |doi-access=free |number=1}}</ref>

====Objects in perpendicular orbits with large semi-major axis====

[[File:Planet nine TNO orbits.svg|upright=1.5|thumb|The orbits of five of the objects with high-inclination orbits (nearly perpendicular to the ecliptic) are shown here as cyan ellipses with the hypothetical Planet Nine in orange.|alt=Planet Nine's orbit is seen pointing towards the top, while the clustered comets are seen towards the bottom.]]

Planet Nine can deliver ETNOs into orbits roughly perpendicular to the ecliptic.<ref name=extremeTech/><ref name=Siegel /> Several objects with high inclinations, greater than 50°, and large semi-major axes, above 250 AU, have been observed.<ref name=mpc-6/> These orbits are produced when some low inclination ETNOs enter a [[secular resonance]] with Planet Nine upon reaching low eccentricity orbits. The resonance causes their eccentricities and inclinations to increase, delivering the ETNOs into perpendicular orbits with low perihelia where they are more readily observed. The ETNOs then evolve into [[Retrograde and prograde motion|retrograde]] orbits with lower eccentricities, after which they pass through a second phase of high eccentricity perpendicular orbits, before returning to low eccentricity and inclination orbits. The secular resonance with Planet Nine involves a [[linear combination]] of the orbit's arguments and longitudes of perihelion: {{nobr| {{math| Δ''ϖ'' – 2''ω''}} .}} Unlike the Kozai mechanism this resonance causes objects to reach their maximum eccentricities when in nearly perpendicular orbits. In simulations conducted by Batygin and Morbidelli this evolution was relatively common, with 38% of stable objects undergoing it at least once.<ref name="Batygin_Morbidelli_2017" /> The arguments of perihelion of these objects are clustered near or opposite Planet Nine's and their longitudes of ascending node are clustered around 90° in either direction from Planet Nine's when they reach low perihelia.<ref name=TAJ201601 /><ref name="Li_etal_2018" /> This is in rough agreement with observations with the differences attributed to distant encounters with the known giant planets.<ref name=TAJ201601 />

====Orbits of high-inclination objects====

A population of high-inclination TNOs with semi-major axes less than 100 AU may be generated by the combined effects of Planet Nine and the other giant planets. The ETNOs that enter perpendicular orbits have perihelia low enough for their orbits to intersect those of Neptune or the other giant planets. An encounter with one of these planets can lower an ETNO's semi-major axis to below 100 AU, where the object's orbits is no longer controlled by Planet Nine, leaving it in an orbit like {{mpl|2008 KV|42}}. The predicted orbital distribution of the longest lived of these objects is nonuniform. Most would have orbits with perihelia ranging from 5 AU to 35 AU and inclinations below 110°; beyond a gap with few objects are would be others with inclinations near 150° and perihelia near 10 AU.<ref name="Batygin_Brown_high_inc"/> Previously it was proposed that these objects originated in the [[Oort cloud]],<ref name="Brasser_etal_2012"/> a theoretical cloud of icy planetesimals surrounding the Sun at distances of 2,000 to 200,000 AU.<ref name="Williams_OortCloud"/> In simulations without Planet Nine an insufficient number are produced from the Oort cloud relative to observations, however.<ref name="Kaib_etal_2019" /> A few of the high-inclination TNOs may become [[514107 Kaʻepaokaʻawela|retrograde Jupiter Trojans]].<ref name="Kohne_Batygin_2020"/>

====Oort cloud and comets====

Planet Nine would alter the source regions and the inclination distribution of comets. In simulations of the migration of the giant planets described by the [[Nice model]] fewer objects are captured in the [[Oort cloud]] when Planet Nine is included. Other objects would be captured in a cloud of objects dynamically controlled by Planet Nine. This Planet Nine cloud, made up of the ETNOs and the perpendicular objects, would extend from semi-major axes of {{nobr|200–{{gaps|3|000}} AU}} and contain roughly {{Earth mass|0.3–0.4|link=yes}}.<ref name=Nesvorny_etal_2017/><ref name=Lawler_etal_2016/> When the perihelia of objects in the Planet Nine cloud drop low enough for them to encounter the other planets some would be scattered into orbits that enter the inner Solar System where they could be observed as comets. If Planet Nine exists these would make up roughly one third of the [[Halley-type comet]]s. Interactions with Planet Nine would also increase the inclinations of the scattered disk objects that cross its orbit. This could result in more with moderate inclinations of 15–30° than are observed.<ref name=Kaib_etal_2019/> The inclinations of the [[Short-period comet|Jupiter-family comets]] derived from that population would also have a broader inclination distribution than is observed.<ref name=Nesvorny_etal_2017/><ref name=Gibbs_IEEE/> Recent estimates of a smaller mass and eccentricity for Planet Nine would reduce its effect on these inclinations.<ref name=P9H2019/>

====2019 estimate====
In February&nbsp;2019, the total of ETNOs that fit the original hypothesis of having semi-major axis of over 250&nbsp;AU had increased to fourteen objects. The orbit parameters for Planet Nine favored by Batygin and Brown after an analysis using these objects were:<ref name=P9H2019/>

* semi-major axis of 400–500&nbsp;AU;
* orbital eccentricity of 0.15–0.3;
* orbital inclination around 20°;
* mass of about {{Earth mass|5|link=yes}}.

====2021 estimate====
In August&nbsp;2021, Batygin and Brown reanalyzed the data related to ETNO observations while accounting for observational biases, they found that observations were more likely in some directions than others. They stated that the orbital clustering observed "remains significant at a 99.6% confidence level".<ref name=Batygin2021/> Combining observational biases with numerical simulations, they predicted the characteristics of Planet Nine:<ref name=Batygin2021/>

* semi-major axis of {{nobr|380{{±|140|80}} AU}} (300–520&nbsp;AU);
* perihelion of {{nobr|300{{±|85|60}} AU}} (240–385&nbsp;AU);
* orbital inclination of {{nobr|16{{±|5}}°}} (11°–21°);
* mass of 6.2{{±|2.2|1.3}} Earth masses {{nobr|({{mvar|M}}{{sub|9}} ≈ 4.9–8.4 {{mvar|M}}{{sub|⊕}}).}}

===Reception===

Batygin was cautious in interpreting the results of the simulation developed for his and Brown's research article, saying, "Until Planet Nine is caught on camera it does not count as being real. All we have now is an echo."<ref name=TomLevensonAtlantic/> In 2016, Brown put the odds for the existence of Planet Nine at about 90%.<ref name=lurking /> [[Gregory P. Laughlin|Greg Laughlin]], one of the few researchers who knew in advance about this article, gives an estimate of 68.3%.<ref name=fatty /> Other skeptical scientists demand more data in terms of additional KBOs to be analyzed or final evidence through photographic confirmation.<ref name=vergeSlamDunkandWaterTightEvidence/><ref name=thestarKateAllen/><ref name=scantEvidence/> Brown, though conceding the skeptics' point, still thinks that there is enough data to mount a search for a new planet.<ref name=pbsBrown/>

The Planet Nine hypothesis is supported by several astronomers and academics. In January&nbsp;2016 [[James L. Green|Jim Green]], director of NASA's [[Science Mission Directorate]], said, "the evidence is stronger now than it's been before".<ref name=popsci/> But Green also cautioned about the possibility of other explanations for the observed motion of distant ETNOs and, quoting [[Carl Sagan]], he said, "extraordinary claims require extraordinary evidence."<ref name=lurking /> [[Massachusetts Institute of Technology]] Professor [[Thomas Levenson|Tom Levenson]] concluded that, for now, Planet Nine seems the only satisfactory explanation for everything now known about the outer regions of the Solar System.<ref name=TomLevensonAtlantic/> Astronomer [[Alessandro Morbidelli (astronomer)|Alessandro Morbidelli]], who reviewed the research article for ''[[The Astronomical Journal]]'', concurred, saying, "I don't see any alternative explanation to that offered by Batygin and Brown."<ref name=fatty/><ref name=lurking/>

Astronomer [[Renu Malhotra]] remains agnostic about Planet Nine, but noted that she and her colleagues have found that the orbits of ETNOs seem tilted in a way that is difficult to otherwise explain. "The amount of warp we see is just crazy," she said. "To me, it's the most intriguing evidence for Planet Nine I've run across so far."<ref name=sciamGiantGhost/>

Other experts have varying degrees of skepticism. American astrophysicist [[Ethan Siegel]], who previously speculated that planets may have been ejected from the Solar System during an early dynamical instability, is skeptical of the existence of an undiscovered planet in the Solar System.<ref name=Siegel/><ref name=Siegel_2015/> In a 2018 article discussing a survey that did not find evidence of clustering of the ETNOs' orbits, he suggests the previously observed clustering could have been the result of observational bias and claims most scientists think Planet Nine does not exist.<ref name=Siegel_2018a/> Planetary scientist [[Harold F. Levison|Hal Levison]] thinks that the chance of an ejected object ending up in the inner Oort cloud is about 2%, and speculates that many objects must have been thrown past the Oort cloud if one has entered a stable orbit.<ref name=skyntelescopePlanetX/>

Further skepticism about the Planet Nine hypothesis arose in 2020, based on results from the [[Outer Solar System Origins Survey]] and the [[Dark Energy Survey]], with the OSSOS documenting over 800&nbsp;trans-Neptunian objects and the DES discovering 316&nbsp;new ones.<ref name=Bernardinelli_2020a/> Both surveys adjusted for observational bias and concluded that of the objects observed there was no evidence for clustering.<ref name=Lawler2020/> The authors go further to explain that practically all objects' orbits can be explained by physical phenomena rather than a ninth planet as proposed by Brown and Batygin.<ref name=Lawler2021/> An author of one of the studies, Samantha Lawler, said the hypothesis of Planet Nine proposed by Brown and Batygin "does not hold up to detailed observations" pointing out the much larger sample size of 800&nbsp;objects compared to the much smaller 14 and that conclusive studies based on said objects were "premature". She went further to explain the phenomenon of these extreme orbits could be due to gravitational occultation from Neptune when it migrated outwards earlier in the Solar System's history.<ref name=Universetoday_2020/>

==Alternative hypotheses==

=== Nice Planet #5 ===
Planet Nine has been proposed as a potential remnant of the [[Formation and evolution of the Solar System|early Solar System's evolution]].<ref>{{Cite web |last=Strickland |first=Ashley |date=2020-12-11 |title=Hubble spots distant exoplanet similar to potential 'Planet Nine' |url=https://www.cnn.com/2020/12/11/world/hubble-exoplanet-planet-nine-scn-trnd/index.html |access-date=2025-01-01 |website=CNN |language=en}}</ref> According to the [[Five-planet Nice model]], the early Solar System contained five [[giant planet]]s: Jupiter, Saturn, Uranus, Neptune, and a [[Fifth Giant|fifth]], now-missing [[ice giant]].<ref>{{Cite web |last=#author.fullName} |title=Missing planet explains solar system's structure |url=https://www.newscientist.com/article/dn20952-missing-planet-explains-solar-systems-structure/ |access-date=2025-01-01 |website=New Scientist |language=en-US}}</ref><ref>{{Cite web |date=2022-06-30 |title=The giant planet instability (the "Nice model") – planetplanet |url=https://planetplanet.net/2022/06/30/the-giant-planet-instability-the-nice-model/ |access-date=2025-01-01 |language=en-US}}</ref> Simulations of the [[Nice model]] suggest that gravitational interactions among these planets, coupled with interactions with a disk of [[planetesimal]]s, led to the ejection of the fifth giant from the Solar System approximately 4 billion years ago.<ref>{{Cite web |date=2015-08-21 |title=The Early Solar System Could Have Hosted a Fifth Giant Planet, According to New Study - AmericaSpace |url=https://www.americaspace.com/2015/08/20/the-early-solar-system-could-have-hosted-a-fifth-giant-planet-according-to-new-study/ |access-date=2025-01-01 |website=www.americaspace.com |language=en-US}}</ref><ref>{{Citation |last=Raymond |first=Sean N. |title=Nice Model |date=2011 |encyclopedia=Encyclopedia of Astrobiology |pages=1116–1117 |editor-last=Gargaud |editor-first=Muriel |url=https://link.springer.com/referenceworkentry/10.1007/978-3-642-11274-4_1058 |access-date=2025-01-01 |place=Berlin, Heidelberg |publisher=Springer |language=en |doi=10.1007/978-3-642-11274-4_1058 |isbn=978-3-642-11274-4 |editor2-last=Amils |editor2-first=Ricardo |editor3-last=Quintanilla |editor3-first=José Cernicharo |editor4-last=Cleaves |editor4-first=Henderson James (Jim)}}</ref><ref>{{Cite web |last=Kruesi |first=Liz |date=2012-09-24 |title=Understanding the Nice model {{!}} Astronomy.com |url=https://www.astronomy.com/science/understanding-the-nice-model/ |access-date=2025-01-01 |website=Astronomy Magazine |language=en-US}}</ref> Some researchers propose that Planet Nine could be this fifth giant, lingering in a distant, eccentric orbit far [[Planets beyond Neptune|beyond Neptune]] instead of being entirely ejected from the Solar System.<ref>{{Cite web |author1=Nola Taylor Tillman |date=2017-11-29 |title=Planet Nine: Theories About the Hypothetical Planet |url=https://www.space.com/33480-planet-nine.html |access-date=2025-01-01 |website=Space.com |language=en}}</ref> This hypothesis aligns with observations suggesting Planet Nine's orbit would be stable over the Solar System's lifetime, supporting its survival as an [[Solar System#Outer Solar System|outer-system object]].<ref>{{Cite web |last=Byrd |first=Deborah |date=2018-06-10 |title=No Planet Nine? Collective gravity might explain weird orbits at solar system's edge {{!}} Space {{!}} EarthSky |url=https://earthsky.org/space/no-planet-9-collective-gravity-orbits-outer-solar-system/ |access-date=2025-01-01 |website=earthsky.org |language=en-US}}</ref>

The hypothesis that Planet Nine may be the fifth giant is bolstered by its proposed mass and orbital characteristics, which are consistent with those of an ice giant.<ref>{{Cite web |date=2016-01-20 |title=Caltech Researchers Find Evidence of a Real Ninth Planet |url=https://www.caltech.edu/about/news/caltech-researchers-find-evidence-real-ninth-planet-49523 |access-date=2025-01-01 |website=California Institute of Technology |language=en}}</ref> Numerical simulations of the Nice model show that the ejection of the fifth giant often leaves a gravitational signature in the form of altered orbits for the remaining planets and [[Small Solar System body|small bodies]].<ref>{{Cite web |last=Sanders |first=Ray |date=2011-11-10 |title=Was a Fifth Giant Planet Expelled from Our Solar System? |url=https://www.universetoday.com/90857/was-a-fifth-giant-planet-expelled-from-our-solar-system/ |access-date=2025-01-01 |website=Universe Today |language=en-US}}</ref> The observed clustering of certain [[trans-Neptunian object]]s (TNOs) has been cited as indirect evidence of Planet Nine's gravitational influence, possibly originating from its early interactions with the outer Solar System.<ref>{{Cite web |title=Research - Planet Nine |url=http://www.astro.yale.edu/malenarice/research_planetnine.html |access-date=2025-01-01 |website=www.astro.yale.edu}}</ref>

===Temporary or coincidental clustering===
The results of the Outer Solar System Survey (OSSOS) suggest that the observed clustering is the result of a combination of observational bias and small number statistics. OSSOS, a well-characterized survey of the outer Solar System with known biases, observed eight objects with semi-major axis {{nobr|{{mvar|a}} > 150 AU}} with orbits oriented in a wide range of directions. After accounting for the observational biases of the survey, no evidence for the arguments of perihelion ({{mvar|ω}}) clustering identified by Trujillo and Sheppard was seen,{{efn-ua|
Of the eight objects with a semi-major axis {{nobr|{{mvar|a}} > 150 AU,}} OSSOS found three with arguments of perihelion ({{mvar|ω}}) outside the cluster previously identified by Trujillo and Sheppard (2014):<ref name=Sheppard2014/> {{mpl|2015 GT|50}}, {{mpl|2015 KH|163}}, and {{mpl|2013 UT|15}}.<ref name=Shankman_etal_2017/>
}}
and the orientation of the orbits of the objects with the largest semi-major axis was statistically consistent with being random.<ref name=Shankman_etal_2017/><ref name=Siegel_2018b/> Pedro Bernardinelli and his colleagues also found that the orbital elements of the ETNOs found by the [[Dark Energy Survey]] showed no evidence of clustering. However, they also noted that the sky coverage and number of objects found were insufficient to show that there was no Planet Nine.<ref name=Ratner_2020/><ref name=Bernardelli_2020b/> A similar result was found when these two surveys were combined with a survey by Trujillo and Sheppard.<ref name=Napier_etal_2021/> These results differed from an analysis of discovery biases in the previously observed ETNOs by Mike Brown. He found that after observation biases were accounted for, the clustering of longitudes of perihelion of 10 known ETNOs would be observed only 1.2% of the time if their actual distribution was uniform. When combined with the odds of the observed clustering of the arguments of perihelion, the probability was 0.025%.<ref name="Brown_discovery biases"/> A later analysis of the discovery biases of fourteen ETNOs used by Brown and Batygin determined the probability of the observed clustering of the longitudes of perihelion and the orbital pole locations to be 0.2%&nbsp;.<ref name=Brown_Batygin_2019/>

Simulations of 15&nbsp;known objects evolving under the influence of Planet Nine also revealed differences from observations. Cory Shankman and his colleagues included Planet Nine in a simulation of many clones (objects with similar orbits) of 15&nbsp;objects with semi-major axis {{nobr|{{mvar|a}} > 150 AU}} and perihelion {{nobr|{{mvar|ω}} > 30 AU.}}{{efn-ua|
A link to the plots of the orbital evolution of all 15 is included in the arxiv version of the article.
}}
While they observed alignment of the orbits opposite that of Planet Nine's for the objects with semi-major axis greater than 250&nbsp;AU, clustering of the arguments of perihelion was not seen. Their simulations also showed that the perihelia of the ETNOs rose and fell smoothly, leaving many with perihelion distances between 50 and 70&nbsp;AU where none had been observed, and predicted that there would be many other unobserved objects.<ref name=Shankman_etal_2016_10/> These included a large reservoir of high-inclination objects that would have been missed due to most observations being at small inclinations,<ref name=Lawler_etal_2016/> and a large population of objects with perihelia so distant that they would be too faint to observe. Many of the objects were also ejected from the Solar System after encountering the other giant planets. The large unobserved populations and the loss of many objects led Shankman ''et al''. to estimate that the mass of the original population was tens of Earth masses, requiring that a much larger mass had been ejected during the early Solar System.{{efn-ua|
Shankman ''et al''. estimated the mass of this population at tens of Earth masses, and that hundreds to thousands of Earth masses would need to be ejected from the vicinity of the giant planets for this mass to have remained. In the Nice model 20–50&nbsp;Earth masses is estimated to have been ejected, a significant mass is also ejected from the neighborhoods of the giant planets during their formation.
}}
Shankman ''et al''. concluded that the existence of Planet Nine is unlikely and that the currently observed alignment of the existing ETNOs is a temporary phenomenon that will disappear as more objects are detected.<ref name=sciamGiantGhost/><ref name=Shankman_etal_2016_10/>

===Inclination instability in a massive disk===

Ann-Marie Madigan and Michael McCourt postulate that an [[inclination instability]] in a distant massive belt hypothetically termed a Zderic-Madigan, or ZM&nbsp;belt is responsible for the alignment of the arguments of perihelion of the ETNOs.<ref name=Madigan_McCourt_2016/> An inclination instability could occur in such a disk of particles with high eccentricity orbits {{nobr|([[orbital eccentricity|{{mvar|e}}]] > 0.6)}} around a central body, such as the Sun. The self-gravity of this disk would cause its spontaneous organization, increasing the inclinations of the objects and aligning the arguments of perihelion, forming it into a cone above or below the original plane.<ref name=Madigan_etal_2018/> This process would require an extended time and significant mass of the disk, on the order of a billion years for a 1–10&nbsp;Earth-mass disk.<ref name=Madigan_McCourt_2016/> Ann-Marie Madigan argues that some already discovered trans-neptunian objects like Sedna and [[2012 VP113]] may be members of this disk.<ref name=inverse_com_P9>{{cite web |title=Planet Nine: The answer to this cosmic mystery may come sooner than you think |url=https://www.inverse.com/science/planet-nine-vera-rubin-telescope |access-date=25 March 2023 |website=Inverse.com |date=16 June 2022 |language=en}}</ref> If this is the case there would likely be thousands of similar objects in the region.<ref name=inverse_com_P9/> Mike Brown considers Planet Nine a more probable explanation, noting that current surveys have not revealed a large enough scattered-disk to produce an "inclination instability".<ref name=spaceplanet9/><ref name=planet9ornot/> In Nice model simulations of the Solar System that included the self-gravity of the planetesimal disk an inclination instability did not occur. Instead, the simulation produced a rapid precession of the objects' orbits and most of the objects were ejected on too short of a timescale for an inclination instability to occur.<ref name=Fan_Batygin_2017/>

Madigan and colleagues have shown that the inclination instability would require 20&nbsp;Earth masses in a disk of objects with semi-major axes of a few hundred AU.<ref name=Skibba2020/> An inclination instability in this disk could also reproduce the observed gap in the perihelion distances of the extreme TNOs,<ref name=Zderic_etal_2020/> and the observed apsidal alignment following the inclination instability given sufficient time.<ref name=Zderic_Madigan_2020/><ref name=Zedric_etal_2021/> Simulations show that the [[Vera C. Rubin Observatory]] Legacy Survey of Space and Time (LSST) project should be able to supply strong evidence for or against the ZM&nbsp;belt.<ref name="Siraj et al 2025">{{cite journal | last1=Siraj | first1=Amir | last2=Chyba | first2=Christopher F. | last3=Tremaine | first3=Scott | title=Orbit of a Possible Planet X | journal=The Astrophysical Journal | volume=978 | issue=2 | date=January 10, 2025 | issn=0004-637X | doi=10.3847/1538-4357/ad98f6 | doi-access=free | page=139}}</ref>

===Shepherding by a massive disk===

Antranik Sefilian and Jihad Touma propose that a massive disk of moderately eccentric TNOs is responsible for the clustering of the longitudes of perihelion of the ETNOs. This disk would contain 10&nbsp;Earth-mass of TNOs with aligned orbits and eccentricities that increased with their semi-major axes ranging from zero to 0.165. The gravitational effects of the disk would offset the forward precession driven by the giant planets so that the orbital orientations of its individual objects are maintained. The orbits of objects with high eccentricities, such as the observed ETNOs, would be stable and have roughly fixed orientations, or longitudes of perihelion, if their orbits were anti-aligned with this disk.<ref name=Seilian_Touma_2019/> Although Brown thinks the proposed disk could explain the observed clustering of the ETNOs, he finds it implausible that the disk could survive over the age of the Solar System.<ref name=Patel_2019/> Batygin thinks that there is insufficient mass in the Kuiper belt to explain the formation of the disk, and asks "why would the protoplanetary disk end near 30&nbsp;AU and restart beyond 100&nbsp;AU?"<ref name=Dvorsky_2019/>

===Planet in lower eccentricity orbit===
{| class="wikitable sortable" style="text-align:center; float:right;"
|+ Proposed resonant objects for<br />[[Semimajor axis|{{mvar|a}}]]&nbsp;>&nbsp;150&nbsp;AU, [[perihelion|{{mvar|q}}]]&nbsp;>&nbsp;40&nbsp;AU<ref name="Malhotra_Volk_Wang"/>
!Body || [[Orbital period|Barycentric period]]<br />(years) ||Ratio
|-
|{{mpl|2013 GP|136}} || 1,830 || 9:1
|-
|{{mpl|2000 CR|105}} || 3,304 || 5:1
|-
|{{mpl|2012 VP|113}}|| 4,300 || 4:1
|-
|[[474640 Alicanto]]|| 5,900 || 3:1
|-
|{{mpl|2010 GB|174}}|| 6,500 || 5:2
|-
|[[90377 Sedna]] || ≈ 11,400 || 3:2
|-
! Hypothetical planet || ≈ 17,000 || 1:1 (by definition)
|}

The Planet Nine hypothesis includes a set of predictions about the mass and orbit of the planet. An alternative hypothesis predicts a planet with different orbital parameters. Renu Malhotra, Kathryn Volk, and Xianyu Wang have proposed that the four detached objects with the longest orbital periods, those with perihelia beyond {{val|40|u=AU}} and semi-major axes greater than {{val|250|u=AU}}, are in ''n'':1 or ''n'':2 mean-motion resonances with a hypothetical planet. Two other objects with semi-major axes greater than {{val|150|u=AU}} are also potentially in resonance with this planet. Their proposed planet could be on a lower eccentricity, low inclination orbit, with [[Orbital eccentricity|eccentricity ''e'']]&nbsp;<&nbsp;0.18 and [[Orbital inclination|inclination ''i'']]&nbsp;≈&nbsp;11°. The eccentricity is limited in this case by the requirement that close approaches of {{mp|2010 GB|174}} to the planet be avoided. If the ETNOs are in periodic orbits of the third kind,{{efn-ua|This is often referred (perhaps erroneously) to as Kozai within mean-motion resonance.<ref name="Malhotra_2017"/>}} with their stability enhanced by the libration of their arguments of perihelion, the planet could be in a higher inclination orbit, with [[Orbital inclination|''i'']]&nbsp;≈&nbsp;48°. Unlike Batygin and Brown, Malhotra, Volk and Wang do not specify that most of the distant detached objects would have orbits anti-aligned with the massive planet.<ref name="Malhotra_Volk_Wang"/><ref name="Malhotra_TED_2017"/>

===Alignment due to the Kozai mechanism===

Trujillo and Sheppard argued in 2014 that a massive planet in a circular orbit with an average distance between {{val|200|u=AU}} and {{val|300|u=AU}} was responsible for the clustering of the arguments of perihelion of twelve TNOs with large semi-major axes. Trujillo and Sheppard identified a clustering near zero degrees of the arguments of perihelion of the orbits of twelve TNOs with perihelia greater than {{val|30|u=AU}} and semi-major axes greater than {{val|150|u=AU}}.<ref name=TAJ201601 /><ref name=Sheppard2014/> After numerical simulations showed that the arguments of perihelion should circulate at varying rates, leaving them randomized after billions of years, they suggested that a massive planet in a circular orbit at a few hundred astronomical units was responsible for this clustering.<ref name=Sheppard2014/><ref name=sciencenews/> This massive planet would cause the arguments of perihelion of the TNOs to librate about 0° or 180° via the [[Kozai mechanism]] so that their orbits crossed the plane of the planet's orbit near perihelion and aphelion, the closest and farthest points from the planet.<ref name="Sheppard2014"/><ref name="Marcos_2014" /> In numerical simulations including a 2–15 Earth mass body in a circular low-inclination orbit between {{val|200|u=AU}} and {{val|300|u=AU}} the arguments of perihelia of Sedna and {{mp|2012 VP|113}} librated around 0° for billions of years (although the lower perihelion objects did not) and underwent periods of libration with a Neptune mass object in a high inclination orbit at 1,500 AU.<ref name="Sheppard2014" /> Another process such as a passing star would be required to account for the absence of objects with arguments of perihelion near 180°.<ref name=TAJ201601 />{{efn-ua|Assuming that the orbital elements of these objects have not changed, Jílková et al. proposed an encounter with a passing star might have helped acquire these objects – dubbed sednitos ([[Extreme trans-Neptunian object|ETNO]]s with {{nowrap|''q'' > 30}} and {{nowrap|''a'' > 150}}) by them. They also predicted that the ''sednitos'' region is populated by 930 planetesimals and the inner Oort cloud acquired ≈440 planetesimals through the same encounter.<ref name=JilkovaSednito/><ref name=stealingsedna/>}}

These simulations showed the basic idea of how a single large planet can shepherd the smaller TNOs into similar types of orbits. They were basic proof of concept simulations that did not obtain a unique orbit for the planet as they state there are many possible orbital configurations the planet could have.<ref name=sciencenews/> Thus they did not fully formulate a model that successfully incorporated all the clustering of the ETNOs with an orbit for the planet.<ref name=TAJ201601 /> But they were the first to notice there was a clustering in the orbits of TNOs and that the most likely reason was from an unknown massive distant planet. Their work is very similar to how [[Alexis Bouvard]] noticed Uranus' motion was peculiar and suggested that it was likely gravitational forces from an unknown 8th planet, which led to the discovery of Neptune.<ref name="Bouvard"/>

Raúl and Carlos de la Fuente Marcos proposed a similar model but with two distant planets in resonance.<ref name="Marcos_2014"/><ref name=time2planets/> An analysis by Carlos and Raúl de la Fuente Marcos with [[Sverre Aarseth|Sverre J. Aarseth]] confirmed that the observed alignment of the arguments of perihelion could not be due to observational bias. They speculated that instead it was caused by an object with a mass between that of Mars and Saturn that orbited at some {{val|200|u=AU}} from the Sun. Like Trujillo and Sheppard they theorized that the TNOs are kept bunched together by a Kozai mechanism and compared their behavior to that of [[96P/Machholz|Comet 96P/Machholz]] under the influence of [[Jupiter]].<ref name=mnrasflippingcomets/> They also struggled to explain the orbital alignment using a model with only one unknown planet, and therefore suggested that this planet is itself in resonance with a more-massive world about {{val|250|u=AU}} from the Sun.<ref name=sciencenews/><ref name=universetoday2planets/> In their article, Brown and Batygin noted that alignment of arguments of perihelion near 0° or 180° via the Kozai mechanism requires a ratio of the semi-major axes nearly equal to one, indicating that multiple planets with orbits tuned to the data set would be required, making this explanation too unwieldy.<ref name=TAJ201601 />

===Primordial black hole===
In 2019, Jakub Scholtz and James Unwin proposed that a [[primordial black hole]] was responsible for the clustering of the orbits of the ETNOs. Their analysis of [[Optical Gravitational Lensing Experiment|OGLE]] gravitational lensing data revealed a population of planetary mass objects in the direction of the galactic bulge more numerous than the local population of stars. They propose that instead of being free floating planets, these objects are primordial black holes. Since their estimate of the size of this population is greater than the estimated population of free floating planets from planetary formation models they argue that the capture of a hypothetical primordial black hole would be more probable than the capture of a free floating planet. This could also explain why an object responsible for perturbing the orbits of the ETNOs, if it exists, has yet to be seen.<ref name="Unwin2020"/><ref name="NYT-20200911"/> A detection method was proposed in the paper, stating that the black hole is too cold to be detected over the [[Cosmic microwave background|CMB]], but interaction with surrounding [[dark matter]] would produce [[gamma ray]]s detectable by the [[Fermi Gamma-ray Space Telescope|FERMILAT]]. Konstantin Batygin commented on this, saying while it is possible for Planet Nine to be a primordial black hole, there is currently not enough evidence to make this idea more plausible than any other alternative.<ref name="AM-20191001"/> [[Edward Witten]] proposed a fleet of probes accelerated by radiation pressure that could discover a Planet Nine primordial black hole's location, however Thiem Hoang and [[Avi Loeb]] showed that any signal would be dominated by noise from the [[interstellar medium]].<ref name="Letzer2020"/><ref name="Hoang2020"/> Amir Siraj and Avi Loeb proposed a method for the [[Vera C. Rubin Observatory]] to detect flares from any low-mass black hole in the outer Solar System, including a possible Planet Nine primordial black hole.<ref name="Overbye2020"/><ref name="Siraj_2020"/>

===Modified Newtonian dynamics===
In 2023, it was shown that a gravity theory known as [[modified Newtonian dynamics]] (MOND), which can explain galactic rotation without invoking [[dark matter]], can provide an alternative explanation using secular approximations. It predicts that the major axes of the KBO orbits will be aligned with the direction toward the [[Galactic Center]] and that the orbits cluster in phase space, in agreement with observations.<ref>{{cite journal |last1=Brown |first1=Katherine |last2=Mathur |first2=Harsh |date=23 September 2023 |title=Modified Newtonian Dynamics as an Alternative to the Planet Nine Hypothesis – Iopscience |journal=The Astrophysical Journal |publisher=Iopscience.iop.org |volume=166 |issue=4 |page=168 |arxiv=2304.00576 |doi=10.3847/1538-3881/acef1e |s2cid= |doi-access=free}}</ref>

==Detection attempts==

===Visibility and location===
Due to its extreme distance from the Sun, Planet Nine would reflect little sunlight, potentially evading telescope sightings.<ref name=lurking /> It is expected to have an [[apparent magnitude]] fainter than 22, making it at least 600&nbsp;times fainter than [[Pluto]].<ref name="bnb20160617" />{{efn-ua|The 8&nbsp;meter [[Subaru Telescope]] has achieved a 27.7&nbsp;magnitude photographic limit with a ten-hour exposure,<ref name="FaintestObject"/> which is about 100&nbsp;times dimmer than Planet Nine is expected to be. For comparison, the [[Hubble Space Telescope]] has detected objects as faint as 31st&nbsp;magnitude with an exposure of about 2&nbsp;million seconds (555&nbsp;hours) during [[Hubble Ultra-Deep Field]] photography.<ref name="Illingworth2012"/> Hubble’s field of view is very narrow, as is the [[Keck Observatory]]'s [[Large Binocular Telescope]].<ref name=sciencemag201601/> Brown hopes to make a request for use of the ''[[Hubble Space Telescope]]'' the day the planet is spotted.<ref name="DeepAstronomy2016"/>}} If Planet Nine exists and is close to perihelion, astronomers could identify it based on existing images. At aphelion, the largest telescopes would be required, but if the planet is currently located in between, many [[observatory|observatories]] could spot Planet Nine.<ref name=CaltechJan2016 /> Statistically, the planet is more likely to be close to its aphelion at a distance greater than 600&nbsp;AU.<ref name="NatGeo201601" /> This is because objects move more slowly when near their aphelion, in accordance with [[Kepler's second law]]. A 2019 study estimated that Planet Nine, if it exists, may be smaller and closer than originally thought. This would make the hypothetical planet brighter and easier to spot, with an apparent magnitude of 21–22.<ref name="P9H2019" /><ref name="more support"/> Observation and analysis of the orbital dynamics of Kuiper Belt objects constrain the possible orbital parameters of a Planet Nine,<ref name=Batygin2021/> and at the current rate of new observations, [[University of Michigan]] professor Fred Adams believes enough data will have been gathered to pinpoint Planet Nine or rule out its existence by 2035.<ref name="Carter2019"/><ref name="boost"/>

===Searches of existing data===

The search of [[Star catalogue|databases of stellar objects]] by Batygin and Brown has already excluded much of the sky along Planet Nine's predicted orbit. The remaining regions include the direction of its aphelion, where it would be too faint to be spotted by these surveys, and near the plane of the [[Milky Way]], where it would be difficult to distinguish from the numerous stars.<ref name=B-BCharts/> This search included the archival data from the [[Catalina Sky Survey]] to magnitude 21–22, [[Pan-STARRS]] to magnitude 21.5, and infrared data from the [[Wide-field Infrared Survey Explorer]] (WISE) satellite.<ref name=findplanet9/><ref name="bnb20160617" /><ref name=B-BCharts/> In 2021, they also searched the first three years of data from the [[Zwicky Transient Facility]] (ZTF) without identifying Planet Nine.<ref name="Brown_Batygin_2021z"/> The search of the ZTF data alone has ruled out 56% of the [[parameter space]] for possible Planet Nine positions. As a result of ruling out mostly objects with small semi-major axes, the expected orbit of Planet Nine was pushed slightly further away.<ref name="Brown_Batygin_2021z"/>

Other researchers have been conducting searches of existing data. David Gerdes, who helped develop the camera used in the [[Dark Energy Survey]], claims that software designed to identify distant Solar System objects such as {{mpl|2014 UZ|224}} could find Planet Nine if it was imaged as part of that survey, which covered a quarter of the southern sky.<ref name=npr2014UZ224/><ref name=Shannon_NewSci/> Michael Medford and Danny Goldstein, graduate students at the [[University of California, Berkeley]], are also examining archived data using a technique that combines images taken at different times. Using a [[supercomputer]] they will offset the images to account for the calculated motion of Planet Nine, allowing many faint images of a faint moving object to be combined to produce a brighter image.<ref name="Gibbs_IEEE"/> A search combining multiple images collected by WISE and NEOWISE data has also been conducted without detecting Planet Nine. This search covered regions of the sky away from the [[galactic plane]] at the "W1" wavelength (the 3.4&nbsp;μm wavelength used by WISE) and is estimated to be able to detect a 10-Earth mass object out to 800–900 AU.<ref name="Meisner_etal_2017b" /><ref name="Meisner_etal_2017a"/>

Malena Rice and Gregory Laughlin applied a targeted shift-stacking search algorithm to analyze data from [[TESS]] sectors 18 and 19 looking for Planet Nine and candidate outer Solar System objects.<ref name="Rice_Laughlin_TESS_2020"/> Their search generated no serious evidence for the presence of a distant planet, but it produced 17 new outer Solar System body candidates located at geocentric distances in the range 80–200 AU, that need follow-up observations with ground-based telescope resources for confirmation. Early results from a survey with [[William Herschel Telescope|WHT]] aimed at recovering these distant TNO candidates have failed to confirm two of them.<ref name="WHT_2022" /><ref name="ING_PR" />

By 2022, a comparison between [[IRAS]] and [[AKARI]] data yielded no Planet Nine detection. It was noted that far-infrared data in the major portion of the sky are heavily contaminated by the emission from the galactic nebulae, making detection of Planet Nine thermal emission problematic close to the galactic plane or bulge.<ref>{{Cite web |url=http://oro.open.ac.uk/84507/1/stac2044.pdf |title=Searching for giant planets in the outer Solar System with far-infrared all-sky surveys}}</ref>

===Ongoing searches===

Because the planet is predicted to be visible in the [[Northern Hemisphere]], the primary search is expected to be carried out using the Subaru Telescope, which has both an [[Objective (optics)|aperture]] large enough to see faint objects and a wide field of view to shorten the search.<ref name=searchformoreSedna/> Two teams of astronomers—Batygin and Brown, as well as Trujillo and Sheppard—are undertaking this search together, and both teams expect the search to take up to five years.<ref name=sciencemag201601/><ref name=SpaceCom/> Brown and Batygin initially narrowed the search for Planet Nine to roughly 2,000 [[square degree]]s of sky near [[Orion (constellation)|Orion]], a swath of space that Batygin thinks could be covered in about 20 nights by the Subaru Telescope.<ref name=crockettnewclues/> Subsequent refinements by Batygin and Brown have reduced the search space to 600–800 square degrees of sky.<ref name="Choi2016"/> In December 2018, they spent four half–nights and three full nights observing with the Subaru Telescope.<ref name="Stirone_2019"/> Due to the elusiveness of the hypothetical planet, it has been proposed that different detection methods be used when looking for a [[super-Earth]] mass planet ranging from using differing telescopes to using multiple spacecraft. In late April and early May 2020, Scott Lawrence and Zeeve Rogoszinski proposed the latter method for finding it as multiple spacecraft would have advantages that land-based telescopes do not have.<ref name="Lawrence_2020"/>

===Radiation===
Although a distant planet such as Planet Nine would reflect little light, due to its large mass it would still be radiating the heat from its formation as it cools. At its estimated temperature of {{convert|47|K|C F}}, the peak of its emissions would be at [[infrared]] wavelengths; its apparent magnitude in the [[UBV photometric system|V filter]] (540 nm wavelength) would be 21.7.<ref name="Linder_Mordasini_2016"/> This radiation signature could be detected by Earth-based [[submillimeter telescope]]s, such as [[Atacama Large Millimeter Array|ALMA]],<ref name=discoverPerspective/> and a search could be conducted by [[cosmic microwave background]] experiments operating at [[Microwave#Radio astronomy|mm wavelengths]].<ref name="Cowan2016"/><ref name="Aron_2016"/><ref name="Wood2018"/>{{efn-ua|It is estimated that to find Planet Nine, telescopes that can resolve a 30&nbsp;[[Jansky|mJy]] point source are needed, and that can also resolve an annual [[stellar parallax|parallax motion]] of ≈5&nbsp;[[arcminute]]s.<ref name=aasnovacmb/>}} A search of part of the sky using archived data of the [[Atacama Cosmology Telescope]] has not detected Planet Nine.<ref name="Naess_etal_2021"/> Jim Green of NASA's Science Mission Directorate is optimistic that it could be observed by the [[James Webb Space Telescope#Infrared astronomy|James Webb Space Telescope]], the successor to the [[Hubble Space Telescope]].<ref name="popsci"/>

===Citizen science===
{{See also|Citizen science}}
{{Excerpt|Backyard Worlds: Planet 9|hat=no}}

The [[Zooniverse]] "Catalina Outer Solar System Survey" project, operating from August 2020 to April 2023, was using archived data from the [[Catalina Sky Survey]] to search for TNOs.<ref>{{Cite web |title=Zooniverse |url=https://www.zooniverse.org/projects/fulsdavid/catalina-outer-solar-system-survey/talk/3716/2886710 |access-date=31 October 2024 |website=zooniverse.org}}</ref><ref name=findp9/><ref name="Zooniverse"/><ref name="Comb"/>

==Attempts to predict location==
===Measurements of Saturn's orbit by the ''Cassini'' probe===
Precise observations of Saturn's orbit using data from [[Cassini–Huygens|''Cassini'']] suggest that Planet Nine could not be in certain sections of its proposed orbit because its gravity would cause a noticeable effect on Saturn's position. This data neither proves nor disproves that Planet Nine exists.<ref name="Fienga_2016" />

An initial analysis by Fienga, Laskar, Manche, and Gastineau using Cassini data to search for Saturn's orbital residuals, small differences with its predicted orbit due to the Sun and the known planets, was inconsistent with Planet Nine being located with a [[true anomaly]], the location along its orbit relative to perihelion, of −130° to −110° or −65° to 85°. The analysis, using Batygin and Brown's orbital parameters for Planet Nine, suggests that the lack of perturbations to Saturn's orbit is best explained if Planet Nine is located at a true anomaly of {{val|117.8|+11|-10|u=deg}}. At this location, Planet Nine would be approximately {{val|630|u=AU}} from the Sun,<ref name=Fienga_2016/> with [[right ascension]] close to 2<sup>h</sup> and [[declination]] close to −20°, in [[Cetus]].<ref name=findingNine/> In contrast, if the putative planet is near aphelion it would be located near right ascension 3.0<sup>h</sup> to 5.5<sup>h</sup> and declination −1° to 6°.<ref name=peripole/>

A later analysis of ''Cassini'' data by astrophysicists Matthew Holman and Matthew Payne tightened the constraints on possible locations of Planet Nine. Holman and Payne developed a more efficient model that allowed them to explore a broader range of parameters than the previous analysis. The parameters identified using this technique to analyze the Cassini data was then intersected with Batygin and Brown's dynamical constraints on Planet Nine's orbit. Holman and Payne concluded that Planet Nine is most likely to be located within 20° of RA = 40°, Dec = −15°, in an area of the sky near the constellation Cetus.<ref name="Shannon_NewSci" /><ref name="Holman_Payne_Cassini"/>

William Folkner, a planetary scientist at the [[Jet Propulsion Laboratory]] (JPL), has stated that the ''Cassini'' spacecraft was not experiencing unexplained deviations in its orbit around Saturn. An undiscovered planet would affect the orbit of Saturn, not ''Cassini''. This could produce a signature in the measurements of ''Cassini'', but JPL has seen no unexplained signatures in ''Cassini'' data.<ref name="JPL"/>

===Analysis of Pluto's orbit===
An analysis in 2016 of Pluto's orbit by Holman and Payne found perturbations much larger than predicted by Batygin and Brown's proposed orbit for Planet Nine. Holman and Payne suggested three possible explanations: systematic errors in the measurements of Pluto's orbit; an unmodeled mass in the Solar System, such as a small planet in the range of 60–{{val|100|u=AU}} (potentially explaining the [[Kuiper cliff]]); or a planet more massive or closer to the Sun instead of the planet predicted by Batygin and Brown.<ref name="sciamGiantGhost" /><ref name=Holman-Payne/>

===Orbits of nearly parabolic comets===
An analysis of the orbits of comets with nearly [[parabolic orbit]]s identifies five new comets with [[hyperbolic orbit]]s that approach the nominal orbit of Planet Nine described in Batygin and Brown's initial article. If these orbits are hyperbolic due to close encounters with Planet Nine the analysis estimates that Planet Nine is currently near aphelion with a right ascension of 83–90° and a declination of 8–10°.<ref name="Medvedev_etal_2017"/> Scott Sheppard, who is skeptical of this analysis, notes that many different forces influence the orbits of comets.<ref name=sciamGiantGhost/>

===Occultations by Jupiter trojans===
Malena Rice and Gregory Laughlin have proposed that a network of telescopes be built to detect [[Minor-planet occultation|occultation]]s by [[Jupiter trojan]]s. The timing of these occultations would provide precise astrometry of these objects enabling their orbits to be monitored for variations due to the tide from Planet Nine.<ref name="Rice_Laughlin_2019"/>

===Possible encounter with interstellar meteor===
In May 2022, it was suggested that the peculiar meteor [[CNEOS 2014-01-08]] may have entered Earth-crossing orbit after a swing-by of Planet Nine. If that hypothesis is true, the trajectory back-tracing of CNEOS 2014-01-08 means Planet Nine may be currently located in the constellation of [[Aries (constellation)|Aries]], at right ascension 53{{±|4.3}}°, and declination 9.2{{±|1.3}}°.<ref>{{cite journal |arxiv=2205.07675 |year=2023 |title=A Candidate Location for Planet Nine from an Interstellar Meteoroid: The Messenger Hypothesis |last1=Socas-Navarro |first1=Hector |journal=The Astrophysical Journal |volume=945 |issue=1 |page=22 |doi=10.3847/1538-4357/acb817 |bibcode=2023ApJ...945...22S |s2cid=256416556 |doi-access=free}}</ref>

==Attempts to predict the semi-major axis==

An analysis by Sarah Millholland and Gregory Laughlin identified a pattern of [[Commensurability (astronomy)|commensurabilities]] (ratios between orbital periods of pairs of objects consistent with both being in resonance with another object) of the ETNOs. They identify five objects that would be near resonances with Planet Nine if it had a semi-major axis of 654 AU: Sedna (3:2), [[474640 Alicanto]] (3:1), {{mp|2012 VP|113}} (4:1), {{mpl|2000 CR|105}} (5:1), and {{mpl|2001 FP|185}} (5:1). They identify this planet as Planet Nine but propose a different orbit with an eccentricity ''e'' ≈ 0.5, inclination ''i'' ≈ 30°, argument of perihelion ω ≈ 150°, and longitude of ascending node Ω ≈ 50° (the last differs from Brown and Batygin's value of 90°).<ref name="Millholland_Laughlin_2017"/>{{efn-ua|A 3-D version of the image of the orbit and those of several ETNOs shown in figure 14 of "Constraints on Planet Nine's Orbit and Sky Position within a Framework of Mean-motion Resonances" is available.<ref name="Millholland_3D_orbit"/>}}

Carlos and Raúl de la Fuente Marcos also note commensurabilities among the known ETNOs similar to that of the Kuiper belt, where accidental commensurabilities occur due to objects in resonances with Neptune. They find that some of these objects would be in 5:3 and 3:1 resonances with a planet that had a semi-major axis of ≈700 AU.<ref name=moreplanets/>

[[File:Hubble Space Telescope image showing one possible orbit (dashed ellipse) of the 11-Jupiter-mass exoplanet HD 106906 b.png|thumb|upright=1.8|A possible orbit of the 11-Jupiter-mass exoplanet [[HD 106906 b]]<ref name=HD106906b/>]]
Three objects with smaller semi-major axes near 172 AU ({{mpl|2013 UH|15}}, {{mpl|2016 QV|89}} and [[List of minor planets: 594001–595000#594301–594400|(594337) 2016 QU<sub>89</sub>]]) have also been proposed to be in resonance with Planet Nine. These objects would be in resonance and anti-aligned with Planet Nine if it had a semi-major axis of 315 AU, below the range proposed by Batygin and Brown. Alternatively, they could be in resonance with Planet Nine, but have orbital orientations that circulate instead of being confined by Planet Nine if it had a semi-major axis of 505 AU.<ref name="Kaine_etal_2018"/>

A later analysis by Elizabeth Bailey, Michael Brown, and Konstantin Batygin found that if Planet Nine is in an eccentric and inclined orbit the capture of many of the ETNOs in higher-order resonances and their chaotic transfer between resonances prevent the identification of Planet Nine's semi-major axis using current observations. They also determined that the odds of the first six objects observed being in N/1 or N/2 period ratios with Planet Nine are less than 5% if it has an eccentric orbit.<ref name="Bailey_etal_2018"/>

A 2025 study by Amir Siraj, Christopher F. Chyba, and Scott Tremaine using an expanded sample of 51 ETNOs to inform 300 simulations in the Rebound program, proposed new orbital characteristics for Planet Nine: that its semi-major axis is 290 ± 30 AU, its eccentricity is 0.29 ± 0.13, and its inclination is roughly 6°. The authors noted that it would put Planet Nine in the field of view of the Rubin Observatory's early observations.<ref name="Siraj et al 2025"/><ref name="Ferreira 2025">{{cite web | last=Ferreira | first=Becky | title=If Planet X Exists, It's Running Out of Places to Hide | website=404 Media | date=January 11, 2025 | url=https://www.404media.co/if-planet-x-exists-its-running-out-of-places-to-hide-3/ | access-date=January 12, 2025}}</ref>

In late 2020 it was determined [[HD 106906 b]], a candidate exoplanet, had an eccentric orbit that took it outside the debris disk of its binary host stars. Its orbit appears to be similar to the predictions made for Planet Nine's semi-major axis and it may serve as a proxy for Planet Nine that helps explain how such planetary orbits evolve,<ref name=HD106906b/> although this exoplanet is well over ten times as massive as Jupiter.

==Naming==
Planet Nine does not have an official name and will not receive one unless its existence is confirmed via imaging. Only two planets, Uranus and Neptune, have been discovered in the Solar System during recorded history.<ref name="MIT2019"/> However, many [[minor planet]]s, including [[dwarf planet]]s such as Pluto, [[asteroid]]s, and comets have been discovered and named. Consequently, there is [[Astronomical naming conventions|a well-established process]] for naming newly discovered Solar System objects. If Planet Nine is observed, the [[International Astronomical Union]] will certify a name, with priority usually given to a name proposed by its discoverers.<ref name="IAU"/> It is likely to be a name chosen from [[Roman mythology|Roman]] or [[Greek mythology]].<ref name=planetnames/>

In their original article, Batygin and Brown simply referred to the object as "perturber",<ref name=TAJ201601 /> and only in later press releases did they use "Planet Nine".<ref name="Fesemaier_2016"/> They have also used the names "[[Jehoshaphat]]" and "George" (a reference to [[William Herschel]]'s proposed name for [[Uranus]]) for Planet Nine. Brown has stated: "We actually call it [[wikt:phattie|Phattie]]{{efn-ua|Most news outlets reported the name as ''[[wikt:phattie|Phattie]]'' (a slang term for "cool" or "awesome"; also, a marijuana cigarette)<ref name=sciencemag201601/> but ''The New Yorker'' quote cited above uses "fatty" in what appears to be a nearly unique variation. The apparently correct spelling has been substituted.}} when we're just talking to each other."<ref name=fatty /> In 2018, Batygin has also informally suggested, based on a petition on [[Change.org]], to name the planet after singer [[David Bowie]], and to name any potential moons of the planet after characters from Bowie's song catalogue, such as [[Ziggy Stardust (character)|Ziggy Stardust]] and [[Major Tom]].<ref name="Katwala"/>

Jokes have been made connecting "Planet Nine" to [[Ed Wood]]'s 1959 science-fiction horror film ''[[Plan 9 from Outer Space]]''.<ref name=findp9/> In connection with the Planet Nine hypothesis, the film title recently found its way into academic discourse. In 2016, an article titled ''Planet Nine from Outer Space'' about the hypothesized planet in the [[Solar System#Trans-Neptunian region|outer region of the Solar System]] was published in ''[[Scientific American]]''.<ref name="Lemonick"/> Several [[Academic conference|conference talks]] since then have used the same [[word play]],<ref name="Batygin1"/><ref name="Batygin2"/> as did a lecture by [[Michael Brown (astronomer)|Mike Brown]] given in 2019.<ref name="Brown"/>

[[Persephone]], the wife of the deity Pluto, had been a popular name commonly used in [[science fiction]] for [[Fictional planets of the Solar System#Trans-Neptunian planets|a planet beyond Neptune]], most notably in the works of [[Arthur C. Clarke]] and [[Larry Niven]].<ref>{{Cite book |last=Westfahl |first=Gary |title=Science fiction literature through history: an encyclopedia |date=2021 |publisher=ABC-CLIO |isbn=978-1-4408-6617-3 |location=Santa Barbara, California}}</ref> However, it is unlikely that Planet Nine or any other conjectured planet beyond Neptune will be given the name Persephone once its existence is confirmed, as it is already the name for asteroid [[399 Persephone]].<ref name="TechRepublic2006"/> In 2017, physicist [[Lorenzo Iorio]] informally suggested to name the hypothetical planet as ″Telisto″, from the [[ancient Greek]] word "τήλιστος" for "farthest" or "most remote".<ref>
{{cite journal |last1=Iorio |first1=L. |year=2017 |title=Preliminary constraints on the location of the recently hypothesized new planet of the Solar System from planetary orbital dynamics |journal=[[Astrophysics and Space Science]] |volume=362 |issue=1 |page=11 |arxiv=1512.05288 |bibcode=2017Ap&SS.362...11I |doi=10.1007/s10509-016-2993-8 |s2cid=254264344}}
</ref> Another [[Classical mythology|classical mythological]] name, suggested by [[Jet Propulsion Laboratory]] physicist Makan Mohageg, is [[Chronos]], after the Greek personification of time; Mohageg's method of finding Planet Nine would revolve around precision timing.<ref name="Katwala" />

In 2018, planetary scientist [[Alan Stern]] objected to the name ''Planet Nine'', saying, "It is an effort to erase [[Clyde Tombaugh]]'s legacy and it's frankly insulting", suggesting the name Planet X until its discovery.<ref name="Xv9"/> He signed a statement with 34 other scientists saying, "We further believe the use of this term [Planet Nine] should be discontinued in favor of culturally and [[Taxonomy (general)|taxonomically]] neutral terms for such planets, such as Planet X, Planet Next, or Giant Planet Five."<ref name="Abell2018"/> According to Brown, {{"'}}[[Planet X]]' is not a generic reference to some unknown planet, but a specific prediction of Lowell's which led to the (accidental) discovery of Pluto. Our prediction is not related to this prediction."<ref name="Xv9"/>

==See also==
* [[List of hypothetical Solar System objects#Planets|Hypothetical planets of the Solar System]]
* [[Nemesis (hypothetical star)]]
* [[Planets beyond Neptune]]
* [[Tyche (hypothetical planet)]]
* [[Five-planet Nice model]]

==Notes==
{{Reflist|group=upper-alpha}}

==References==
{{Reflist|refs=
<ref name="bnb20160617">{{cite journal |last1=Brown |first1=Michael E. |last2=Batygin |first2=Konstantin |title=Observational Constraints on the Orbit and Location of Planet Nine in the Outer Solar System |journal=[[The Astrophysical Journal Letters]] |year=2016 |volume=824 |issue=2 |page=L23 |doi=10.3847/2041-8205/824/2/L23 |arxiv=1603.05712 |bibcode=2016ApJ...824L..23B |s2cid=10904017 |doi-access=free}}</ref>

<ref name="lurking">{{cite news |last1=Achenbach |first1=Joel |author-link=Joel Achenbach |last2=Feltman |first2=Rachel |date=20 January 2016 |title=New Evidence Suggests a Ninth Planet Lurking at the Edge of the Solar System |url=https://www.washingtonpost.com/news/speaking-of-science/wp/2016/01/20/new-evidence-suggests-a-ninth-planet-lurking-at-the-edge-of-the-solar-system/ |newspaper=The Washington Post|access-date=20 January 2016 |archive-url=https://web.archive.org/web/20160120172051/https://www.washingtonpost.com/news/speaking-of-science/wp/2016/01/20/new-evidence-suggests-a-ninth-planet-lurking-at-the-edge-of-the-solar-system/ |archive-date=20 January 2016 |url-status=live}}</ref>

<ref name="Gomes_Deienno_Morbidelli_2016">{{cite journal |last1=Gomes |first1=Rodney |last2=Deienno |first2=Rogerio |last3=Morbidelli |first3=Alessandro |title=The Inclination of the Planetary System Relative to the Solar Equator May Be Explained by the Presence of Planet 9 |year=2016 |journal=[[The Astronomical Journal]] |volume=153 |issue=1 |page=27 |arxiv=1607.05111 |doi=10.3847/1538-3881/153/1/27 |bibcode=2017AJ....153...27G |s2cid=118330945 |doi-access=free}}</ref>

<ref name=CaltechJan2016>{{cite web |last=Fesenmaier |first=Kimm |date=20 January 2016 |title=Caltech Researchers Find Evidence of a Real Ninth Planet |url=https://www.caltech.edu/news/caltech-researchers-find-evidence-real-ninth-planet-49523 |website=[[California Institute of Technology|Caltech]] |access-date=20 January 2016 |archive-url=https://web.archive.org/web/20160120165807/https://www.caltech.edu/news/caltech-researchers-find-evidence-real-ninth-planet-49523 |archive-date=20 January 2016 |url-status=live}}</ref>

<ref name=TAJ201601>{{cite journal |last1=Batygin |first1=Konstantin |author-link1=Konstantin Batygin |last2=Brown |first2=Michael E. |author-link2=Michael E. Brown |year=2016 |title=Evidence for a Distant Giant Planet in the Solar System |journal=[[The Astronomical Journal]] |volume=151 |issue=2 |page=22 |arxiv=1601.05438 |bibcode=2016AJ....151...22B |doi=10.3847/0004-6256/151/2/22 |s2cid=2701020 |doi-access=free}}</ref>

<ref name="Beust_2016">{{cite journal |last1=Beust |first1=H. |title=Orbital Clustering of Distant Kuiper Belt Objects by Hypothetical Planet 9. Secular or Resonant? |journal=[[Astronomy & Astrophysics]] |year=2016 |volume=590 |page=L2 |doi=10.1051/0004-6361/201628638 |arxiv=1605.02473 |bibcode=2016A&A...590L...2B |s2cid=53994177}}</ref>

<ref name=P9H2019>{{cite journal |last1=Batygin |first1=Konstantin |last2=Adams |first2=Fred C. |last3=Brown |first3=Michael E. |last4=Becker |first4=Juliette C. |title=The Planet Nine Hypothesis |year=2019 |journal=[[Physics Reports]] |volume=805 |pages=1–53 |doi=10.1016/j.physrep.2019.01.009 |arxiv=1902.10103 |bibcode=2019PhR...805....1B |s2cid=119248548}}</ref>

<ref name=NatGeo201601>{{cite magazine |first=Nadia |last=Drake |author-link=Nadia Drake |date=20 January 2016 |title=Scientists Find Evidence for Ninth Planet in Solar System |url=http://news.nationalgeographic.com/2016/01/150119-new-ninth-planet-solar-system-space/ |magazine=[[National Geographic]] |access-date=15 July 2016 |archive-url=https://web.archive.org/web/20160629022424/http://news.nationalgeographic.com/2016/01/150119-new-ninth-planet-solar-system-space/ |archive-date=29 June 2016 |url-status=dead}}</ref>

<ref name="Meisner_etal_2017b">{{cite journal |last1=Meisner |first1=A.M. |last2=Bromley |first2=B.C. |last3=Kenyon |first3=S.J. |last4=Anderson |first4=T.E. |year=2017 |title=A 3π Search for Planet Nine at 3.4μm with WISE and NEOWISE |journal=[[The Astronomical Journal]] |volume=155 |issue=4 |page=166 |arxiv=1712.04950 |bibcode=2018AJ....155..166M |doi=10.3847/1538-3881/aaae70 |s2cid=119504867 |doi-access=free}}</ref>

<ref name=Batygin2021>{{cite journal |last1=Brown |first1=Michael E. |author-link1=Michael E. Brown |last2=Batygin |first2=Konstantin |author-link2=Konstantin Batygin |arxiv=2108.09868 |title=The Orbit of Planet Nine |journal=The Astronomical Journal |date=26 August 2021 |volume=162 |issue=5 |page=219 |doi=10.3847/1538-3881/ac2056 |bibcode=2021AJ....162..219B |s2cid=230196275 |doi-access=free}}</ref>

<ref name=fatty>{{cite magazine |last=Burdick |first=Alan |date=20 January 2016 |title=Discovering Planet Nine |url=http://www.newyorker.com/tech/elements/discovering-planet-nine |magazine=[[The New Yorker]] |access-date=20 January 2016 |archive-url=https://web.archive.org/web/20160121034701/http://www.newyorker.com/tech/elements/discovering-planet-nine |archive-date=21 January 2016 |url-status=live}}</ref>

<ref name="TC-20200525">{{cite news |last=Lawler |first=Samantha |title=Why astronomers now doubt there is an undiscovered 9th planet in our solar system |url=https://theconversation.com/why-astronomers-now-doubt-there-is-an-undiscovered-9th-planet-in-our-solar-system-127598 |date=25 May 2020 |work=[[The Conversation (website)|The Conversation]] |access-date=26 May 2020 |archive-date=29 May 2020 |archive-url=https://web.archive.org/web/20200529051746/https://theconversation.com/why-astronomers-now-doubt-there-is-an-undiscovered-9th-planet-in-our-solar-system-127598 |url-status=live}}</ref>

<ref name="Luhman2014">{{cite journal |last=Luhman |first=Kevin L. |year=2014 |title=A Search for a Distant Companion to the Sun with the Wide-Field Infrared Survey Explorer |journal=[[The Astrophysical Journal]] |volume=781 |issue=4 |page=4 |bibcode=2014ApJ...781....4L |doi=10.1088/0004-637X/781/1/4 |s2cid=122930471}}</ref>

<ref name=grosser>{{cite journal |title=The Search for a Planet Beyond Neptune |author=Morton Grosser |journal=[[Isis (journal)|Isis]] |volume=55 |issue=2 |year=1964 |pages=163–183 |doi=10.1086/349825 |jstor=228182 |s2cid=144255699}}</ref>

<ref name=tombaugh>{{cite journal |title=The Search for the Ninth Planet, Pluto |last=Tombaugh |first=Clyde W. |year=1946 |journal=[[Astronomical Society of the Pacific|Astronomical Society of the Pacific Leaflets]] |volume=5 |issue=209 |pages=73–80 |bibcode=1946ASPL....5...73T}}</ref>

<ref name="Croswell1997">{{anchor|CITEREFCroswell1997}}{{cite book |title=Planet Quest: The Epic Discovery of Alien Solar Systems |author=Ken Croswell |location=New York |publisher=The Free Press |date=1997 |isbn=978-0-684-83252-4 |pages=57–58}}</ref>

<ref name="Browne_1993">{{cite news |last1=Browne |first1=Malcolm W. |title=Evidence for Planet X Evaporates in Spotlight of New Research |url=https://www.nytimes.com/1993/06/01/science/evidence-for-planet-x-evaporates-in-spotlight-of-new-research.html |website=The New York Times |date=1 June 1993 |access-date=9 February 2019 |archive-date=1 April 2019 |archive-url=https://web.archive.org/web/20190401192455/https://www.nytimes.com/1993/06/01/science/evidence-for-planet-x-evaporates-in-spotlight-of-new-research.html |url-status=live}}</ref>

<ref name="Forbes_1880">{{cite journal |last1=Kirkwood |first1=D. |title=On Comets and Ultra-Neptunian Planets |journal=[[The Observatory (journal)|The Observatory]] |year=1880 |volume=3 |pages=439–447 |bibcode=1880Obs.....3..439K |url=http://adsbit.harvard.edu/full/seri/Obs../0003//0000439.000.html |access-date=17 January 2019 |archive-date=3 November 2019 |archive-url=https://web.archive.org/web/20191103181725/http://adsbit.harvard.edu/full/seri/Obs../0003//0000439.000.html |url-status=live}}</ref>

<ref name=spaceBrown>{{cite web |last=Wall |first=Mike |date=24 August 2011 |title=A Conversation With Pluto's Killer: Q & A With Astronomer Mike Brown |url=http://www.space.com/12711-pluto-killer-mike-brown-dwarf-planets-interview.html |website=[[Space.com]] |access-date=7 February 2016 |archive-url=https://web.archive.org/web/20160202232821/http://www.space.com/12711-pluto-killer-mike-brown-dwarf-planets-interview.html |archive-date=2 February 2016 |url-status=live}}</ref>

<ref name=iopsciencesedna>{{cite journal |last1=Brown |first1=Michael E. |last2=Trujillo |first2=Chadwick |last3=Rabinowitz |first3=David |year=2004 |title=Discovery of a Candidate Inner Oort Cloud Planetoid |journal=[[The Astrophysical Journal]] |volume=617 |issue=1 |pages=645–649 |arxiv=astro-ph/0404456 |bibcode=2004ApJ...617..645B |doi=10.1086/422095 |s2cid=7738201}}</ref>

<ref name=guardianSuperEarth>{{cite news |last=Sample |first=Ian |date=26 March 2014 |title=Dwarf Planet Discovery Hints at a Hidden Super Earth in Solar System |url=https://www.theguardian.com/science/2014/mar/26/dwarf-planet-super-earth-solar-system-2012-vp113 |newspaper=The Guardian |access-date=18 July 2016 |archive-url=https://web.archive.org/web/20160429200749/https://www.theguardian.com/science/2014/mar/26/dwarf-planet-super-earth-solar-system-2012-vp113 |archive-date=29 April 2016 |url-status=live}}</ref>

<ref name=llmysteries>{{cite web |last=Wolchover |first=Natalie |date=25 May 2012 |title=Planet X? New Evidence of an Unseen Planet at Solar System's Edge |url=http://www.livescience.com/20583-planet-edge-solar-system.html |website=[[LiveScience]] |access-date=7 February 2016 |quote=More work is needed to determine whether Sedna and the other scattered disc objects were sent on their circuitous trips round the Sun by a star that passed by long ago, or by an unseen planet that exists in the solar system right now. Finding and observing the orbits of other distant objects similar to Sedna will add more data points to astronomers' computer models. |archive-url=https://web.archive.org/web/20160130192931/http://www.livescience.com/20583-planet-edge-solar-system.html |archive-date=30 January 2016 |url-status=live}}</ref>

<ref name=nosmokingGun>{{cite web |last=Lovett |first=Richard A. |date=12 May 2012 |title=New Planet Found in Our Solar System? |url=http://news.nationalgeographic.com/news/2012/05/120511-new-planet-solar-system-kuiper-belt-space-science/ |website=[[National Geographic News]] |access-date=18 July 2016 |archive-url=https://web.archive.org/web/20160710060039/http://news.nationalgeographic.com/news/2012/05/120511-new-planet-solar-system-kuiper-belt-space-science/ |archive-date=10 July 2016 |url-status=dead}}</ref>

<ref name="Gomes_2015">{{cite journal |last1=Gomes |first1=Rodney |year=2015 |title=The Observation of Large Semi-Major Axis Centaurs: Testing for the Signature of a Planetary-Mass Solar Companion |journal=[[Icarus (journal)|Icarus]] |volume=258 |pages=37–49 |bibcode=2015Icar..258...37G |doi=10.1016/j.icarus.2015.06.020}}</ref>

<ref name="randomsearch">{{cite journal |last1=de la Fuente Marcos |first1=C. |last2=de la Fuente Marcos |first2=R. |title=Memories of past close encounters in extreme trans-Neptunian space: Finding unseen planets using pure random searches |journal=Astronomy and Astrophysics Letters |volume=646 |page=L14 (9 pp) |date=18 February 2021 |doi=10.1051/0004-6361/202140311 |arxiv=2102.02220 |bibcode=2021A&A...646L..14D |s2cid=231802033 |url=https://www.aanda.org/articles/aa/full_html/2021/02/aa40311-21/aa40311-21.html |access-date=18 February 2021 |archive-date=18 February 2021 |archive-url=https://web.archive.org/web/20210218162631/https://www.aanda.org/articles/aa/full_html/2021/02/aa40311-21/aa40311-21.html |url-status=live}}</ref>

<ref name="findplanet9">{{cite web |url=http://www.findplanetnine.com/p/blog-page.html |title=Where is Planet Nine? |date=20 January 2016 |website=The Search for Planet Nine |type=Blog |archive-url=https://web.archive.org/web/20160130091255/http://www.findplanetnine.com/p/blog-page.html |archive-date=30 January 2016 |url-status=live}}</ref>

<ref name="NASA_PlanetX">{{Cite web |url=https://solarsystem.nasa.gov/planets/hypothetical-planet-x/in-depth |title=Planet X |website=NASA Solar System Exploration |access-date=14 May 2019 |archive-url=https://web.archive.org/web/20191019012033/https://solarsystem.nasa.gov/planets/hypothetical-planet-x/in-depth/ |archive-date=19 October 2019 |url-status=live}}</ref>

<ref name="Brown_YouTube2017">{{cite web |author=Michael E. Brown |date=3 March 2017 |title=Planet Nine |url=https://www.youtube.com/watch?v=v-ktWBtt7sc&t=1147 |at=19:06 |via=YouTube |access-date=15 March 2017 |archive-url=https://web.archive.org/web/20170406153739/https://www.youtube.com/watch?v=v-ktWBtt7sc&t=1147 |archive-date=6 April 2017 |url-status=live |author-link=Michael E. Brown}}</ref>

<ref name=B-BCharts>{{cite web |last1=Batygin |first1=Konstantin |last2=Brown |first2=Mike |date=20 January 2016 |title=Where is Planet Nine? |url=http://www.findplanetnine.com/p/blog-page.html |at=RA/Dec chart |website=The Search for Planet Nine |publisher=[[Michael E. Brown]] and [[Konstantin Batygin]] |access-date=24 January 2016 |archive-url=https://web.archive.org/web/20160130091255/http://www.findplanetnine.com/p/blog-page.html |archive-date=30 January 2016 |url-status=live}}</ref>

<ref name="Lemonick2016">{{cite web |last=Lemonick |first=Michael D. |date=20 January 2016 |title=Strong Evidence Suggests a Super Earth Lies beyond Pluto |url=http://www.scientificamerican.com/article/strong-evidence-suggests-a-super-earth-lies-beyond-pluto1/ |at=video |website=[[Scientific American]] |access-date=22 January 2015 |archive-url=https://web.archive.org/web/20160122201344/http://www.scientificamerican.com/article/strong-evidence-suggests-a-super-earth-lies-beyond-pluto1/ |archive-date=22 January 2016 |url-status=live}}</ref>

<ref name=newscientist>{{cite web |last1=Becker |first1=Adam |last2=Grossman |first2=Lisa |last3=Aron |first3=Jacob |date=22 January 2016 |title=How Planet Nine May Have Been Exiled to Solar System's Edge |url=https://www.newscientist.com/article/2074961-how-planet-nine-may-have-been-exiled-to-solar-systems-edge/ |website=[[New Scientist]] |access-date=25 January 2016 |archive-url=https://web.archive.org/web/20160124033216/https://www.newscientist.com/article/2074961-how-planet-nine-may-have-been-exiled-to-solar-systems-edge/ |archive-date=24 January 2016 |url-status=live}}</ref>

<ref name="Margot_2016">{{cite web |last=Margot |first=Jean-Luc |date=22 January 2016 |title=Would Planet Nine Pass the Planet Test? |url=http://mel.epss.ucla.edu/jlm/epo/planet/nine.html |publisher=[[University of California at Los Angeles]] |access-date=18 July 2016 |archive-url=https://web.archive.org/web/20160401040546/http://mel.epss.ucla.edu/jlm/epo/planet/nine.html |archive-date=1 April 2016 |url-status=live}}</ref>

<ref name="Margot_2015">{{cite journal |last1=Margot |first1=Jean-Luc |title=A Quantitative Criterion for Defining Planets |journal=[[The Astronomical Journal]] |year=2015 |volume=150 |issue=6 |page=185 |doi=10.1088/0004-6256/150/6/185 |arxiv=1507.06300 |bibcode=2015AJ....150..185M |s2cid=51684830}}</ref>

<ref name="Bromley_Kenyon_2016">{{cite journal |last1=Bromley |first1=Benjamin C. |last2=Kenyon |first2=Scott J. |date=22 July 2016 |title=Making Planet Nine: A Scattered Giant in the Outer Solar System |journal=[[The Astrophysical Journal]] |volume=826 |issue=1 |page=64 |arxiv=1603.08010 |bibcode=2016ApJ...826...64B |doi=10.3847/0004-637X/826/1/64 |s2cid=118448057 |doi-access=free}}</ref>

<ref name=nytimes9thplanet>{{cite news |first=Kenneth |last=Chang |date=20 January 2016 |title=Ninth Planet May Exist Beyond Pluto, Scientists Report |url=https://www.nytimes.com/2016/01/21/science/space/ninth-planet-solar-system-beyond-pluto.html |newspaper=The New York Times |access-date=18 July 2016 |archive-url=https://web.archive.org/web/20160124105438/http://www.nytimes.com/2016/01/21/science/space/ninth-planet-solar-system-beyond-pluto.html |archive-date=24 January 2016 |url-status=live}}</ref>

<ref name=caltechFAQ>{{cite news |first=Sanden |last=Totten |date=20 January 2016 |title=Caltech Researchers Answer Skeptics' Questions about Planet 9 |url=http://www.scpr.org/news/2016/01/20/56964/caltech-researchers-answer-skeptics-questions-abou/ |publisher=[[KPCC (radio station)|89.3 KPCC]] |access-date=18 July 2016 |archive-url=https://web.archive.org/web/20160706171901/http://www.scpr.org/news/2016/01/20/56964/caltech-researchers-answer-skeptics-questions-abou/ |archive-date=6 July 2016 |url-status=live}}</ref>

<ref name="Bailey_Fabrycky">{{cite journal |last1=Bailey |first1=Nora |last2=Fabrycky |first2=Daniel |title=Stellar Flybys Interrupting Planet-Planet Scattering Generates Oort Planets |journal=The Astronomical Journal |year=2019 |volume=158 |issue=2 |pages=94 |doi=10.3847/1538-3881/ab2d2a |arxiv=1905.07044 |bibcode=2019AJ....158...94B |s2cid=158047152 |doi-access=free}}</ref>

<ref name=dl2018>{{cite book |last=D'Angelo |first=G. |author2=Lissauer, J.J. |chapter=Formation of Giant Planets |bibcode=2018haex.bookE.140D |title=Handbook of Exoplanets |publisher=[[Springer International Publishing AG]] |editor=Deeg H., Belmonte J. |pages=2319–2343 |date=2018 |doi=10.1007/978-3-319-55333-7_140 |arxiv=1806.05649 |isbn=978-3-319-55332-0 |s2cid=116913980}}</ref>

<ref name="Izidoro_etal_2015">{{cite journal |last1=Izidoro |first1=André |last2=Morbidelli |first2=Alessandro |last3=Raymond |first3=Sean N. |last4=Hersant |first4=Franck |last5=Pierens |first5=Arnaud |title=Accretion of Uranus and Neptune from Inward-Migrating Planetary Embryos Blocked by Jupiter and Saturn |journal=[[Astronomy & Astrophysics]] |year=2015 |volume=582 |page=A99 |doi=10.1051/0004-6361/201425525 |arxiv=1506.03029 |bibcode=2015A&A...582A..99I |s2cid=118356267}}</ref>

<ref name="Carrera_etal_2017">{{cite journal |last1=Carrera |first1=Daniel |last2=Gorti |first2=Uma |last3=Johansen |first3=Anders |last4=Davies |first4=Melvyn B. |title=Planetesimal Formation by the Streaming Instability in a Photoevaporating Disk |journal=[[The Astrophysical Journal]] |year=2017 |volume=839 |issue=1 |page=16 |doi=10.3847/1538-4357/aa6932 |arxiv=1703.07895 |bibcode=2017ApJ...839...16C |s2cid=119472343 |doi-access=free}}</ref>

<ref name="Eriksson_etal_2017">{{Cite journal |last1=Eriksson |first1=Linn E.J. |last2=Mustill |first2=Alexander J. |last3=Johansen |first3=Anders |title=Circularizing Planet Nine through Dynamical Friction with an Extended, Cold Planetesimal Belt |journal=[[Monthly Notices of the Royal Astronomical Society]] |volume=475 |issue=4 |page=4609 |year=2017 |arxiv=1710.08295 |doi=10.1093/mnras/sty111 |doi-access=free |bibcode=2018MNRAS.475.4609E |s2cid=119230823}}</ref>

<ref name="Kenyon_Bromley_2016">{{cite journal |last1=Kenyon |first1=Scott J. |last2=Bromley |first2=Benjamin C. |year=2016 |title=Making Planet Nine: Pebble Accretion at 250–750 AU in a Gravitationally Unstable Ring |journal=[[The Astrophysical Journal]] |volume=825 |issue=1 |page=33 |arxiv=1603.08008 |bibcode=2016ApJ...825...33K |doi=10.3847/0004-637X/825/1/33 |s2cid=119212968 |doi-access=free}}</ref>

<ref name="Mustill_etal_2016">{{cite journal |last1=Mustill |first1=Alexander J. |last2=Raymond |first2=Sean N. |last3=Davies |first3=Melvyn B. |date=21 July 2016 |title=Is there an exoplanet in the Solar System? |journal=[[Monthly Notices of the Royal Astronomical Society: Letters]] |volume=460 |issue=1 |pages=L109–L113 |arxiv=1603.07247 |bibcode=2016MNRAS.460L.109M |doi=10.1093/mnrasl/slw075 |doi-access=free |s2cid=119229382}}</ref>

<ref name="Li_Adams_2016">{{cite journal |last1=Li |first1=Gongjie |last2=Adams |first2=Fred C. |year=2016 |title=Interaction Cross Sections and Survival Rates for Proposed Solar System Member Planet Nine |journal=[[The Astrophysical Journal Letters]] |volume=823 |issue=1 |page=L3 |arxiv=1602.08496 |bibcode=2016ApJ...823L...3L |doi=10.3847/2041-8205/823/1/L3 |s2cid=15890864 |doi-access=free}}</ref>

<ref name="Siraj_Loab_2020">{{Cite journal |last1=Siraj |first1=Amir |last2=Loeb |first2=Abraham |date=18 August 2020 |title=The Case for an Early Solar Binary Companion |journal=The Astrophysical Journal|volume=899 |issue=2 |pages=L24 |doi=10.3847/2041-8213/abac66 |issn=2041-8213 |arxiv=2007.10339 |bibcode=2020ApJ...899L..24S |s2cid=220665422 |doi-access=free}}</ref>

<ref name="Passant_2020">{{cite web |last1=Rabie |first1=Passant |title=Did the Sun Have a Twin? New Study Rewrites the Star's Early History |url=https://www.inverse.com/science/long-lost-twin-sun-study |website=Inverse |date=20 August 2020 |access-date=28 August 2020 |archive-date=22 August 2020 |archive-url=https://web.archive.org/web/20200822033224/https://www.inverse.com/science/long-lost-twin-sun-study |url-status=live}}</ref>

<ref name="Parker_etal_2017">{{cite journal |last1=Parker |first1=Richard J. |last2=Lichtenberg |first2=Tim |last3=Quanz |first3=Sascha P. |year=2017 |title=Was Planet 9 Captured in the Sun's Natal Star-Forming Region? |journal=[[Monthly Notices of the Royal Astronomical Society: Letters]] |volume=472 |issue=1 |pages=L75–L79 |arxiv=1709.00418 |doi=10.1093/mnrasl/slx141 |doi-access=free |bibcode=2017MNRAS.472L..75P |s2cid=10792152}}</ref>

<ref name="Brennan_2017">{{cite web |last1=Brennan |first1=Pat |title=The Super-Earth that Came Home for Dinner |url=https://www.jpl.nasa.gov/news/news.php?release=2017-259 |website=[[Jet Propulsion Laboratory]] |access-date=13 October 2017 |archive-url=https://web.archive.org/web/20171016045431/https://www.jpl.nasa.gov/news/news.php?release=2017-259 |archive-date=16 October 2017 |url-status=live}}</ref>

<ref name="Nesvorny_etal_2017">{{cite journal |last1=Nesvorny |first1=D. |last2=Vokrouhlicky |first2=D. |last3=Dones |first3=L. |last4=Levison |first4=H.F. |last5=Kaib |first5=N. |last6=Morbidelli |first6=A. |journal=[[The Astrophysical Journal]] |volume=845 |issue=1 |page=27 |title=Origin and Evolution of Short-Period Comets |arxiv=1706.07447 |year=2017 |doi=10.3847/1538-4357/aa7cf6 |bibcode=2017ApJ...845...27N |s2cid=119399322 |doi-access=free}}</ref>

<ref name="Stirone_sun tilt">{{cite web |last1=Stirone |first1=Shannon |title=Planet Nine May Be Responsible for Tilting the Sun |url=http://www.astronomy.com/news/2016/10/planet-nine-tilting-the-sun |website=[[Astronomy (magazine)|Astronomy]] |date=19 October 2016 |access-date=29 July 2017 |archive-url=https://web.archive.org/web/20170810194731/http://www.astronomy.com/news/2016/10/planet-nine-tilting-the-sun |archive-date=10 August 2017 |url-status=live}}</ref>

<ref name=KopanyasKozai>{{cite web |first=Barbara |last=Koponyás |date=10 April 2010 |title=Near-Earth Asteroids and the Kozai-Mechanism |url=http://www.univie.ac.at/adg/Conferences/ahw5/talks/KoponysBarbara.pdf.pdf |website=5th Austrian Hungarian Workshop in Vienna |access-date=18 July 2016 |archive-url=https://web.archive.org/web/20160314110041/http://www.univie.ac.at/adg/Conferences/ahw5/talks/KoponysBarbara.pdf.pdf |archive-date=14 March 2016 |url-status=live}}</ref>

<ref name=cbcRadio>{{cite news |last=McDonald |first=Bob |date=24 January 2016 |title=How Did We Miss Planet 9? |url=https://www.cbc.ca/news/technology/planet-9-bob-macdonald-1.3414268 |publisher=[[CBC News]] |access-date=18 July 2016 |quote="It's like seeing a disturbance on the surface of water but not knowing what caused it. Perhaps it was a jumping fish, a whale or a seal. Even though you didn't actually see it, you could make an informed guess about the size of the object and its location by the nature of the ripples in the water." |archive-url=https://web.archive.org/web/20160205174831/https://www.cbc.ca/news/technology/planet-9-bob-macdonald-1.3414268 |archive-date=5 February 2016 |url-status=live}}</ref>

<ref name="planetaryblog201601">{{cite web |first=Emily |last=Lakdawalla |date=20 January 2016 |title=Theoretical Evidence for an Undiscovered Super-Earth at the Edge of Our Solar System |url=http://www.planetary.org/blogs/emily-lakdawalla/2016/01200955-theoretical-evidence-for-planet-9.html |website=[[The Planetary Society]] |access-date=18 July 2016 |archive-url=https://web.archive.org/web/20160423222919/http://www.planetary.org/blogs/emily-lakdawalla/2016/01200955-theoretical-evidence-for-planet-9.html |archive-date=23 April 2016 |url-status=live}}</ref>

<ref name="Hands_etal_2019">{{cite journal |last1=Hands |first1=T. O. |last2=Dehnen |first2=W. |last3=Gration |first3=A. |last4=Stadel |first4=J. |last5=Moore |first5=B. |title=The fate of planetesimal discs in young open clusters: implications for 1I/'Oumuamua, the Kuiper belt, the Oort cloud and more |journal=Monthly Notices of the Royal Astronomical Society |pages=21–36 |year=2019 |volume=490 |issue=1 |doi=10.1093/mnras/stz1069 |doi-access=free |arxiv=1901.02465 |bibcode=2019MNRAS.490...21H |s2cid=118597453}}</ref>

<ref name="gtcres">{{Cite journal |first1=Julia |last1=de León |first2=Carlos |last2=de la Fuente Marcos |last3=de la Fuente Marcos |first3=Raúl |year=2017 |title=Visible Spectra of (474640) 2004 VN112-2013 RF98 with OSIRIS at the 10.4 M GTC: Evidence for Binary Dissociation near Aphelion Among the Extreme Trans-Neptunian Objects |journal=[[Monthly Notices of the Royal Astronomical Society: Letters]] |volume=467 |issue=1 |pages=L66–L70 |arxiv=1701.02534 |bibcode=2017MNRAS.467L..66D |doi=10.1093/mnrasl/slx003 |doi-access=free |s2cid=119419889}}</ref>

<ref name="deLeon">{{Cite journal |last1=de León |first1=J. |last2=de la Fuente Marcos |first2=C. |last3=de la Fuente Marcos |first3=R. |date=1 May 2017 |title=Visible spectra of (474640) 2004 VN 112 –2013 RF 98 with OSIRIS at the 10.4 m GTC: evidence for binary dissociation near aphelion among the extreme trans-Neptunian objects |url=https://academic.oup.com/mnrasl/article/467/1/L66/2870017 |journal=Monthly Notices of the Royal Astronomical Society: Letters|volume=467 |issue=1 |pages=L66–L70 |doi=10.1093/mnrasl/slx003 |doi-access=free |issn=1745-3925 |arxiv=1701.02534 |bibcode=2017MNRAS.467L..66D}}</ref>

<ref name="Marcos_binary_disruption">{{cite journal |title=Binary Stripping as a Plausible Origin of Correlated Pairs of Extreme Trans-Neptunian Objects |first1=C. |last1=de la Fuente Marcos |first2=R. |last2=de la Fuente Marcos |first3=S.J. |last3=Aarseth |date=1 November 2017 |journal=[[Astrophysics and Space Science]] |volume=362 |issue=11 |page=198 |arxiv=1709.06813 |bibcode=2017Ap&SS.362..198D |doi=10.1007/s10509-017-3181-1 |s2cid=118890903}}</ref>

<ref name="trujillo20160912">{{cite journal |last1=Sheppard, Scott S. |first1=Scott S. |last2=Trujillo |first2=Chadwick |title=New Extreme Trans-Neptunian Objects: Toward a Super-Earth in the Outer Solar System |journal=[[The Astronomical Journal]] |year=2016 |volume=152 |issue=6 |page=221 |doi=10.3847/1538-3881/152/6/221 |arxiv=1608.08772 |bibcode=2016AJ....152..221S |s2cid=119187392 |doi-access=free}}</ref>

<ref name=nodes>{{cite journal |last1=de la Fuente Marcos |first1=Carlos |last2=de la Fuente Marcos |first2=Raúl |year=2017 |title=Evidence for a Possible Bimodal Distribution of the Nodal Distances of the Extreme Trans-Neptunian Objects: Avoiding a Trans-Plutonian Planet or Just Plain Bias? |journal=[[Monthly Notices of the Royal Astronomical Society Letters]] |volume=471 |issue=1 |pages=L61–L65 |arxiv=1706.06981 |bibcode=2017MNRAS.471L..61D |doi=10.1093/mnrasl/slx106 |doi-access=free |s2cid=55469849}}</ref>

<ref name="nodes_phys.org">{{cite web |author=Spanish Foundation for Science and Technology (FECYT) |title=New Evidence in Support of the Planet Nine Hypothesis |url=https://phys.org/news/2017-07-evidence-planet-hypothesis.html |website=phys.org |access-date=29 July 2017 |archive-url=https://web.archive.org/web/20170730014118/https://phys.org/news/2017-07-evidence-planet-hypothesis.html |archive-date=30 July 2017 |url-status=live}}</ref>

<ref name=nodes2>{{cite journal |last1=de la Fuente Marcos |first1=Carlos |last2=de la Fuente Marcos |first2=Raúl |title=Peculiar orbits and asymmetries in extreme trans-Neptunian space |journal=[[Monthly Notices of the Royal Astronomical Society]] |url=https://academic.oup.com/mnras/article-abstract/506/1/633/6307523 |volume=506 |issue=1 |pages=633–649 |arxiv=2106.08369 |bibcode=2021MNRAS.506..633D |doi=10.1093/mnras/stab1756 |date=1 September 2021 |doi-access=free |access-date=8 July 2021 |archive-date=19 October 2021 |archive-url=https://web.archive.org/web/20211019195919/https://academic.oup.com/mnras/article-abstract/506/1/633/6307523 |url-status=live}}</ref>

<ref name=nodes3>{{cite journal |last1=de la Fuente Marcos |first1=Carlos |last2=de la Fuente Marcos |first2=Raúl |title=Twisted extreme trans-Neptunian orbital parameter space: statistically significant asymmetries confirmed |journal=Monthly Notices of the Royal Astronomical Society Letters |url=https://academic.oup.com/mnrasl/article-abstract/512/1/L6/6524836 |volume=512 |issue=1 |pages=L6–L10 |arxiv=2202.01693 |bibcode=2022MNRAS.512L...6D |doi=10.1093/mnrasl/slac012 |date=1 May 2022 |doi-access=free}}</ref>

<ref name="P9 where are you P1">{{cite web |last1=Brown |first1=Michael E. |title=Planet Nine: Where Are You? (Part 1) |url=http://www.findplanetnine.com/2017/09/planet-nine-where-are-you-part-1.html |website=The Search for Planet Nine |publisher=[[Michael E. Brown]] and [[Konstantin Batygin]] |access-date=19 October 2017 |archive-url=https://web.archive.org/web/20171020083937/http://www.findplanetnine.com/2017/09/planet-nine-where-are-you-part-1.html |archive-date=20 October 2017 |url-status=live}}</ref>

<ref name="Becker_etal_2017">{{cite journal |last1=Becker |first1=Juliette C. |last2=Adams |first2=Fred C. |last3=Khain |first3=Tali |last4=Hamilton |first4=Stephanie J. |last5=Gerdes |first5=David |title=Evaluating the Dynamical Stability of Outer Solar System Objects in the Presence of Planet Nine |journal=[[The Astronomical Journal]] |year=2017 |volume=154 |issue=2 |page=61 |doi=10.3847/1538-3881/aa7aa2 |arxiv=1706.06609 |bibcode=2017AJ....154...61B |s2cid=111384673 |doi-access=free}}</ref>

<ref name="Lawler_etal_2016">{{cite journal |last1=Lawler |first1=S.M. |last2=Shankman |first2=C. |last3=Kaib |first3=N. |last4=Bannister |first4=M.T. |last5=Gladman |first5=B. |last6=Kavelaars |first6=J.J. |orig-year=21 May 2016 |title=Observational Signatures of a Massive Distant Planet on the Scattering Disk |journal=[[The Astronomical Journal]] |date=29 December 2016 |volume=153 |issue=1 |page=33 |bibcode=2017AJ....153...33L |doi=10.3847/1538-3881/153/1/33 |arxiv=1605.06575 |s2cid=54854087 |doi-access=free}}</ref>

<ref name="Caceres_etal_2018">{{cite journal |last1=Cáceres |first1=Jessica |last2=Gomes |first2=Rodney |title=The Influence of Planet 9 on the Orbits of Distant TNOs: The Case for a Low Perihelion Planet |journal=[[The Astronomical Journal]] |volume=156 |issue=4 |page=157 |year=2018 |arxiv=1808.01248 |doi=10.3847/1538-3881/aad77a |bibcode=2018AJ....156..157C |s2cid=119064276 |doi-access=free}}</ref>

<ref name=discoveryDeBrief>{{cite magazine |last=Scharping |first=Nathaniel |date=20 January 2016 |title=Planet Nine: A New Addition to the Solar System? |url=http://blogs.discovermagazine.com/d-brief/2016/01/20/planet-9-discovery-shakes-our-understanding-of-the-solar-system/ |magazine=[[Discover (magazine)|Discover]] |access-date=18 July 2016 |archive-url=https://web.archive.org/web/20160716105822/http://blogs.discovermagazine.com/d-brief/2016/01/20/planet-9-discovery-shakes-our-understanding-of-the-solar-system/ |archive-date=16 July 2016 |url-status=live}}</ref>

<ref name=thestarKateAllen>{{cite news |last=Allen |first=Kate |date=20 January 2016 |title=Is a Real Ninth Planet out There Beyond Pluto? |url=https://www.thestar.com/news/world/2016/01/20/is-a-real-ninth-planet-out-there-beyond-pluto.html |newspaper=[[Toronto Star]] |access-date=18 July 2016 |archive-url=https://web.archive.org/web/20160417232029/http://www.thestar.com/news/world/2016/01/20/is-a-real-ninth-planet-out-there-beyond-pluto.html |archive-date=17 April 2016 |url-status=live}}</ref>

<ref name="Li_etal_2018">{{cite journal |last1=Li |first1=Gongjie |last2=Hadden |first2=Samuel |last3=Payne |first3=Matthew |last4=Holman |first4=Matthew J. |title=The Secular Dynamics of TNOs and Planet Nine Interactions |journal=[[The Astronomical Journal]] |year=2018 |volume=156 |issue=6 |page=263 |doi=10.3847/1538-3881/aae83b |arxiv=1806.06867 |bibcode=2018AJ....156..263L |s2cid=118898729 |doi-access=free}}</ref>

<ref name="Batygin_Morbidelli_2017">{{cite journal |last1=Batygin |first1=Konstantin |last2=Morbidelli |first2=Alessandro |title=Dynamical Evolution Induced by Planet Nine |journal=[[The Astronomical Journal]] |year=2017 |volume=154 |issue=6 |page=229 |doi=10.3847/1538-3881/aa937c |arxiv=1710.01804 |bibcode=2017AJ....154..229B |s2cid=119704953 |doi-access=free}}</ref>

<ref name=extremeTech>{{cite web |last=Hruska |first=Joel |date=20 January 2016 |title=Our Solar System May Contain a Ninth Planet, Far beyond Pluto |url=http://www.extremetech.com/extreme/221595-new-research-suggests-our-solar-system-may-contain-a-ninth-planet-far-beyond-pluto |website=[[ExtremeTech]] |access-date=18 July 2016 |archive-url=https://web.archive.org/web/20160728074926/http://www.extremetech.com/extreme/221595-new-research-suggests-our-solar-system-may-contain-a-ninth-planet-far-beyond-pluto |archive-date=28 July 2016 |url-status=live}}</ref>

<ref name=mpc-6>{{cite web |title=MPC list of ''a'' > 250, ''i'' > 40, and ''q'' > 6 |url=http://minorplanetcenter.net/db_search/show_by_properties?semimajor_axis_min=250&inclination_min=40&perihelion_distance_min=6 |website=[[Minor Planet Center]] |access-date=4 February 2016 |archive-url=https://web.archive.org/web/20170802041218/http://minorplanetcenter.net/db_search/show_by_properties?semimajor_axis_min=250&inclination_min=40&perihelion_distance_min=6 |archive-date=2 August 2017 |url-status=live}}</ref>

<ref name="Batygin_Brown_high_inc">{{cite journal |last1=Batygin |first1=Konstantin |last2=Brown |first2=Michael E. |title=Generation of Highly Inclined Trans-Neptunian Objects by Planet Nine |year=2016 |journal=[[The Astrophysical Journal Letters]] |volume=833 |issue=1 |page=L3 |arxiv=1610.04992 |bibcode=2016ApJ...833L...3B |doi=10.3847/2041-8205/833/1/L3 |s2cid=6751947 |doi-access=free}}</ref>

<ref name="Brasser_etal_2012">{{cite journal |last1=Brasser |first1=R. |last2=Schwamb |first2=M.E. |last3=Lykawka |first3=P.S. |last4=Gomes |first4=R.S. |title=An Oort Cloud Origin for the High-Inclination, High-Perihelion Centaurs |journal=[[Monthly Notices of the Royal Astronomical Society]] |year=2012 |volume=420 |issue=4 |pages=3396–3402 |doi=10.1111/j.1365-2966.2011.20264.x |doi-access=free |arxiv=1111.7037 |bibcode=2012MNRAS.420.3396B |s2cid=56403467}}</ref>

<ref name="Williams_OortCloud">{{cite web |last1=Williams |first1=Matt |title=What is the Oort Cloud? |url=https://www.universetoday.com/32522/oort-cloud/ |website=Universe Today |date=10 August 2015 |access-date=25 February 2019 |archive-date=23 January 2018 |archive-url=https://web.archive.org/web/20180123211102/https://www.universetoday.com/32522/oort-cloud/ |url-status=live}}</ref>

<ref name="Kohne_Batygin_2020">{{cite journal |last1=Köhne |first1=Tobias |last2=Batygin |first2=Konstantin |title=On the Dynamical Origins of Retrograde Jupiter Trojans and their Connection to High-Inclination TNOs |journal=Celestial Mechanics and Dynamical Astronomy |year=2020 |volume=132 |issue=9 |page=44 |doi=10.1007/s10569-020-09985-1 |arxiv=2008.11242 |bibcode=2020CeMDA.132...44K |s2cid=221319701}}</ref>

<ref name="Kaib_etal_2019">{{cite journal |last1=Kaib |first1=Nathan A. |last2=Pike |first2=Rosemary |last3=Lawler |first3=Samantha |last4=Kovalik |first4=Maya |last5=Brown |first5=Christopher |last6=Alexandersen |first6=Mike |last7=Bannister |first7=Michele T. |last8=Gladman |first8=Brett J. |last9=Petit |first9=Jean-Marc |title=OSSOS XV: Probing the Distant Solar System with Observed Scattering TNOs |year=2019 |arxiv=1905.09286 |doi=10.3847/1538-3881/ab2383 |journal=The Astronomical Journal |volume=158 |issue=1 |page=43 |pmid=31379385 |pmc=6677154 |bibcode=2019AJ....158...43K |doi-access=free}}</ref>

<ref name=TomLevensonAtlantic>{{cite magazine |last=Levenson |first=Thomas |date=25 January 2016 |title=A New Planet or a Red Herring? |url=https://www.theatlantic.com/science/archive/2016/01/a-new-planet-or-a-red-herring/42681 |magazine=[[The Atlantic]] |access-date=18 July 2016 |quote={{-'}}We plotted the real data on top of the model' Batyagin recalls, and they fell 'exactly where they were supposed to be.' That was, he said, the epiphany. 'It was a dramatic moment. This thing I thought could disprove it turned out to be the strongest evidence for Planet Nine.{{'-}} |archive-date=19 October 2021 |archive-url=https://web.archive.org/web/20211019195840/https://www.theatlantic.com/politics/archive/2007/06/the-capitolist/42681/ |url-status=live}}</ref>

<ref name=vergeSlamDunkandWaterTightEvidence>{{cite news |last=Grush |first=Loren |date=20 January 2016 |title=Our Solar System May Have a Ninth Planet After All – but Not All Evidence Is in (We Still Haven't Seen It Yet) |url=https://www.theverge.com/2016/1/20/10801824/ninth-planet-x-discovered-evidence |newspaper=[[The Verge]] |access-date=18 July 2016 |quote=The statistics do sound promising, at first. The researchers say there's a 1 in 15,000 chance that the movements of these objects are coincidental and don't indicate a planetary presence at all. ... 'When we usually consider something as clinched and air tight, it usually has odds with a much lower probability of failure than what they have,' says Sara Seager, a planetary scientist at MIT. For a study to be a slam dunk, the odds of failure are usually 1 in 1,744,278. ... But researchers often publish before they get the slam-dunk odds, in order to avoid getting scooped by a competing team, Seager says. Most outside experts agree that the researchers' models are strong. And Neptune was originally detected in a similar fashion—by researching observed anomalies in the movement of Uranus. Additionally, the idea of a large planet at such a distance from the Sun isn't actually that unlikely, according to Bruce Macintosh, a planetary scientist at Stanford University. |archive-url=https://web.archive.org/web/20160729141120/http://www.theverge.com/2016/1/20/10801824/ninth-planet-x-discovered-evidence |archive-date=29 July 2016 |url-status=live}}</ref>

<ref name=scantEvidence>{{cite magazine |last=Crocket |first=Christopher |date=31 January 2016 |title=Computer Simulations Heat up Hunt for Planet Nine |url=https://www.sciencenews.org/article/computer-simulations-heat-hunt-planet-nine |magazine=[[Science News]] |access-date=7 February 2016 |quote={{-'}}It's exciting and very compelling work,' says Meg Schwamb, a planetary scientist at Academia Sinica in Taipei, Taiwan. But only six bodies lead the way to the putative planet. 'Whether that's enough is still a question.{{'-}} |archive-url=https://web.archive.org/web/20160206211941/https://www.sciencenews.org/article/computer-simulations-heat-hunt-planet-nine |archive-date=6 February 2016 |url-status=live}}</ref>

<ref name=pbsBrown>{{cite news |date=22 January 2016 |title=We Can't See This Possible 9th Planet, but We Feel Its Presence |url=https://www.pbs.org/newshour/bb/we-cant-see-this-possible-9th-planet-but-we-feel-its-presence/ |newspaper=[[PBS NewsHour]] |access-date=18 July 2016 |quote={{-'}}Right now, any good scientist is going to be skeptical, because it's a pretty big claim. And without the final evidence that it's real, there is always that chance that it's not. So, everybody should be skeptical. But I think it's time to mount this search. I mean, we like to think of it as, we have provided the treasure map of where this ninth planet is, and we have done the starting gun, and now it's a race to actually point your telescope at the right spot in the sky and make that discovery of planet nine.' —Mike Brown |archive-url=https://web.archive.org/web/20160722042941/http://www.pbs.org/newshour/bb/we-cant-see-this-possible-9th-planet-but-we-feel-its-presence/ |archive-date=22 July 2016 |url-status=live}}</ref>

<ref name="popsci">{{cite web |last=Fecht |first=Sarah |date=22 January 2016 |title=Can There Really Be a Planet in Our Solar System That We Don't Know About? |url=http://www.popsci.com/could-there-really-be-9th-planet-that-we-didnt-know-about |website=[[Popular Science]] |access-date=18 July 2016 |archive-date=3 May 2016 |archive-url=https://web.archive.org/web/20160503150851/http://www.popsci.com/could-there-really-be-9th-planet-that-we-didnt-know-about |url-status=live}}</ref>

<ref name=Siegel>{{cite web |author-link=Ethan Siegel |last1=Siegel |first1=Ethan |date=20 January 2016 |title=Not So Fast: Why There Likely Isn't A Large Planet Beyond Pluto |url=https://www.forbes.com/sites/startswithabang/2016/01/20/not-so-fast-why-there-likely-isnt-a-large-planet-beyond-pluto/ |website=[[Forbes]] |access-date=22 January 2016 |archive-url=https://web.archive.org/web/20171014035019/https://www.forbes.com/sites/startswithabang/2016/01/20/not-so-fast-why-there-likely-isnt-a-large-planet-beyond-pluto/#749e30245af4 |archive-date=14 October 2017 |url-status=live}}</ref>

<ref name="Siegel_2015">{{cite news |author-link=Ethan Siegel |last1=Siegel |first1=Ethan |date=3 November 2015 |title=Jupiter May Have Ejected a Planet from Our Solar System |url=https://www.forbes.com/sites/startswithabang/2015/11/03/leading-theory-for-how-the-solar-system-formed-just-disproved/ |work=[[Forbes]] |access-date=22 January 2016 |archive-url=https://web.archive.org/web/20160128152609/http://www.forbes.com/sites/startswithabang/2015/11/03/leading-theory-for-how-the-solar-system-formed-just-disproved/ |archive-date=28 January 2016 |url-status=live}}</ref>

<ref name="Siegel_2018a">{{cite news |author-link=Ethan Siegel |last1=Siegel |first1=Ethan |date=14 September 2018 |title=This Is Why Most Scientists Think Planet Nine Doesn't Exist |url=https://www.forbes.com/sites/startswithabang/2018/09/14/this-is-why-scientists-think-planet-nine-doesnt-exist/ |work=[[Forbes]] |access-date=17 September 2018 |archive-date=18 September 2018 |archive-url=https://web.archive.org/web/20180918012518/https://www.forbes.com/sites/startswithabang/2018/09/14/this-is-why-scientists-think-planet-nine-doesnt-exist/ |url-status=live}}</ref>

<ref name=skyntelescopePlanetX>{{cite magazine |last=Beatty |first=Kelly |date=26 March 2014 |title=New Object Offers Hint of 'Planet X' |url=http://www.skyandtelescope.com/astronomy-news/new-object-offers-hint-ofplanet-x/ |magazine=[[Sky & Telescope]] |access-date=18 July 2016 |archive-date=29 January 2016 |archive-url=https://web.archive.org/web/20160129213741/http://www.skyandtelescope.com/astronomy-news/new-object-offers-hint-ofplanet-x/ |url-status=live}}</ref>

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<ref name="Lawler2020">{{Cite web |url=https://www.sciencealert.com/astronomers-now-doubt-there-is-an-undiscovered-9th-planet-in-our-solar-system |title=Many Astronomers Now Think Planet Nine Might Not Exist After All, Here's Why |first=Samantha |last=Lawler |website=ScienceAlert |date=26 May 2020 |access-date=27 May 2020 |archive-date=30 May 2020 |archive-url=https://web.archive.org/web/20200530185950/https://www.sciencealert.com/astronomers-now-doubt-there-is-an-undiscovered-9th-planet-in-our-solar-system |url-status=live}}</ref>

<ref name="Lawler2021">{{Cite web |url=http://theconversation.com/why-astronomers-now-doubt-there-is-an-undiscovered-9th-planet-in-our-solar-system-127598 |title=Why astronomers now doubt there is an undiscovered 9th planet in our solar system |first=Samantha |last=Lawler |website=The Conversation |date=25 May 2020 |access-date=19 October 2021 |archive-date=7 September 2021 |archive-url=https://web.archive.org/web/20210907121445/https://theconversation.com/why-astronomers-now-doubt-there-is-an-undiscovered-9th-planet-in-our-solar-system-127598 |url-status=live}}</ref>

<ref name=Universetoday_2020>{{Cite web |url=https://www.universetoday.com/146283/maybe-the-elusive-planet-9-doesnt-exist-after-all/ |title=Maybe the Elusive Planet 9 Doesn't Exist After All |date=29 May 2020 |access-date=9 June 2020 |archive-date=9 June 2020 |archive-url=https://web.archive.org/web/20200609023753/https://www.universetoday.com/146283/maybe-the-elusive-planet-9-doesnt-exist-after-all/ |url-status=live}}</ref>

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<ref name="Siegel_2018b">{{cite web |last1=Siegel |first1=Ethan |title=This Is Why Most Scientists Think Planet Nine Doesn't Exist |url=https://www.forbes.com/sites/startswithabang/2018/09/14/this-is-why-scientists-think-planet-nine-doesnt-exist/ |department=Starts with a Bang |website=[[Forbes]] |access-date=17 September 2018 |archive-url=https://web.archive.org/web/20180918012518/https://www.forbes.com/sites/startswithabang/2018/09/14/this-is-why-scientists-think-planet-nine-doesnt-exist/#cd530456e77c |archive-date=18 September 2018 |url-status=live}}</ref>

<ref name="Ratner_2020">{{cite web |last1=Ratner |first1=Paul |title=New study deepens the controversy over Planet Nine's existence |url=https://bigthink.com/surprising-science/planet-nine |website=Big Think |date=23 April 2020 |access-date=25 April 2020 |archive-date=30 April 2020 |archive-url=https://web.archive.org/web/20200430165959/https://bigthink.com/surprising-science/planet-nine |url-status=live}}</ref>

<ref name="Bernardelli_2020b">{{cite journal |last1=Bernardelli |first1=Pedro |display-authors=etal |title=Testing the isotropy of the Dark Energy Survey's extreme trans-Neptunian objects |journal=The Planetary Science Journal |year=2020 |volume=1 |issue=2 |page=28 |doi=10.3847/PSJ/ab9d80 |arxiv=2003.08901 |bibcode=2020PSJ.....1...28B |s2cid=213175490 |doi-access=free}}</ref>

<ref name="Napier_etal_2021">{{cite journal |last1=Napier |first1=J. K. |display-authors=etal |title=No Evidence for Orbital Clustering in the Extreme Trans-Neptunian Objects |journal=The Planetary Science Journal |year=2021 |volume=2 |issue=2 |page=59 |doi=10.3847/PSJ/abe53e |arxiv=2102.05601 |bibcode=2021PSJ.....2...59N |s2cid=231861808 |doi-access=free}}</ref>

<ref name="Brown_discovery biases">{{cite journal |last1=Brown |first1=Michael E. |title=Observational Bias and the Clustering of Distant Eccentric Kuiper Belt Objects |year=2017 |journal=[[The Astronomical Journal]] |volume=154 |issue=2 |page=65 |doi=10.3847/1538-3881/aa79f4 |arxiv=1706.04175 |bibcode=2017AJ....154...65B |s2cid=56043830 |doi-access=free}}</ref>

<ref name="Brown_Batygin_2019">{{cite journal |last1=Brown |first1=Michael E. |last2=Batygin |first2=Konstantin |title=Orbital Clustering in the Distant Solar System |journal=[[The Astronomical Journal]] |year=2019 |volume=157 |issue=2 |page=62 |doi=10.3847/1538-3881/aaf051 |url=http://web.gps.caltech.edu/~mbrown/papers/ps/clustering.pdf |arxiv=1901.07115 |bibcode=2019AJ....157...62B |s2cid=119361145 |access-date=22 January 2019 |archive-date=23 January 2019 |archive-url=https://web.archive.org/web/20190123071330/http://web.gps.caltech.edu/~mbrown/papers/ps/clustering.pdf |url-status=live |doi-access=free}}</ref>

<ref name=sciamGiantGhost>{{cite web |last=Choi |first=Charles Q. |date=25 October 2016 |title=Closing in on a Giant Ghost Planet |url=https://www.scientificamerican.com/article/closing-in-on-a-giant-ghost-planet/ |website=[[Scientific American]] |access-date=21 March 2017 |archive-url=https://web.archive.org/web/20170728115415/https://www.scientificamerican.com/article/closing-in-on-a-giant-ghost-planet/ |archive-date=28 July 2017 |url-status=live}}</ref>

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<ref name=spaceplanet9>{{cite web |last=Wall |first=Mike |date=4 February 2016 |title='Planet Nine'? Cosmic objects' strange orbits may have a different explanation |url=http://www.space.com/31817-planet-nine-existence-question.html |website=[[Space.com]] |quote=We need more mass in the outer solar system," she (Madigan) said. "So it can either come from having more minor planets, and their self-gravity will do this to themselves naturally, or it could be in the form of one single massive planet—a Planet Nine. So it's a really exciting time, and we're going to discover one or the other. |access-date=8 February 2016 |archive-url=https://web.archive.org/web/20160208095024/http://www.space.com/31817-planet-nine-existence-question.html |archive-date=8 February 2016 |url-status=live}}</ref>

<ref name=planet9ornot>{{cite web |last=Snell |first=Jason |date=5 February 2016 |title=This Week in Space: Weird Pluto and No Plan for Mars |url=https://www.yahoo.com/tech/this-week-in-space-weird-pluto-and-no-plan-for-183233179.html |publisher=[[Yahoo Tech]] |access-date=18 July 2016 |archive-url=https://web.archive.org/web/20160818145649/https://www.yahoo.com/tech/this-week-in-space-weird-pluto-and-no-plan-for-183233179.html |archive-date=18 August 2016 |url-status=live}}</ref>

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<ref name=universetoday2planets>{{cite web |last=Atkinson |first=Nancy |date=15 January 2015 |title=Astronomers Are Predicting at Least Two More Large Planets in the Solar System |url=http://www.universetoday.com/118252/astronomers-are-predicting-at-least-two-more-large-planets-in-the-solar-system/ |website=[[Universe Today]] |access-date=7 February 2016 |archive-url=https://web.archive.org/web/20160206145105/http://www.universetoday.com/118252/astronomers-are-predicting-at-least-two-more-large-planets-in-the-solar-system/ |archive-date=6 February 2016 |url-status=live}}</ref>

<ref name="Unwin2020">{{Cite journal |last1=Scholtz |first1=Jakub |last2=Unwin |first2=James |date=29 July 2020 |title=What if Planet 9 is a Primordial Black Hole? |journal=Physical Review Letters |volume=125 |issue=5 |pages=051103 |doi=10.1103/PhysRevLett.125.051103 |pmid=32794880 |arxiv=1909.11090 |bibcode=2020PhRvL.125e1103S |issn=0031-9007 |doi-access=free}}</ref>

<ref name="NYT-20200911">{{cite news |last=Overbye |first=Dennis |author-link=Dennis Overbye |title=Is There a Black Hole in Our Backyard? – Astrophysicists have recently begun hatching plans to find out just how weird Planet Nine might be. |url=https://www.nytimes.com/2020/09/11/science/astronomy-planet-nine-black-hole.html |date=11 September 2020 |work=The New York Times |access-date=11 September 2020 |archive-date=11 September 2020 |archive-url=https://web.archive.org/web/20200911091154/https://www.nytimes.com/2020/09/11/science/astronomy-planet-nine-black-hole.html |url-status=live}}</ref>

<ref name="AM-20191001">{{Cite web |last=Parks |first=Jake |title=Planet Nine may be a black hole the size of a baseball |url=https://astronomy.com/news/2019/10/planet-nine-may-be-a-black-hole-the-size-of-a-baseball |date=1 October 2019 |work=[[Astronomy (magazine)|Astronomy magazine]] |access-date=23 August 2020 |archive-date=13 August 2020 |archive-url=https://web.archive.org/web/20200813151017/https://astronomy.com/news/2019/10/planet-nine-may-be-a-black-hole-the-size-of-a-baseball |url-status=live}}</ref>

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<ref name=sciencemag201601>{{cite journal |first=Eric |last=Hand |date=20 January 2016 |title=Astronomers say a Neptune-sized planet lurks beyond Pluto |url=http://www.sciencemag.org/news/2016/01/feature-astronomers-say-neptune-sized-planet-lurks-unseen-solar-system |journal=[[Science (journal)|Science]] |doi=10.1126/science.aae0237 |access-date=20 January 2016 |archive-url=https://web.archive.org/web/20160120170200/http://www.sciencemag.org/news/2016/01/feature-astronomers-say-neptune-sized-planet-lurks-unseen-solar-system |archive-date=20 January 2016 |url-status=live}}</ref>

<ref name="DeepAstronomy2016">{{cite web |author=Deep Astronomy |date=19 February 2016 |title=Ninth Planet Beyond Neptune? |via=YouTube |publisher=Deep Astronomy |url=https://www.youtube.com/watch?v=1jPxZQoVVZo&t=46m57s |at=46:57 |access-date=18 July 2016 |archive-date=26 June 2020 |archive-url=https://web.archive.org/web/20200626031241/https://www.youtube.com/watch?v=1jPxZQoVVZo&t=46m57s |url-status=live}}</ref>

<ref name="more support">{{cite web |url=https://phys.org/news/2019-02-planet.html |title=More support for Planet Nine |publisher=Phys.org |date=27 February 2019 |access-date=26 June 2019 |archive-date=26 June 2019 |archive-url=https://web.archive.org/web/20190626032137/https://phys.org/news/2019-02-planet.html |url-status=live}}</ref>

<ref name="Carter2019">{{Cite web |url=https://www.techradar.com/news/are-we-getting-closer-to-finding-planet-nine |title=Are we getting closer to finding 'Planet Nine'? |department=Future tech |first=Jamie |last=Carter |date=25 March 2019 |website=TechRadar|access-date=14 May 2019 |archive-date=14 May 2019 |archive-url=https://web.archive.org/web/20190514091808/https://www.techradar.com/news/are-we-getting-closer-to-finding-planet-nine |url-status=live}}</ref>

<ref name="boost">{{cite web |url=https://earthsky.org/space/review-paper-physics-reports-planet-9-feb-2019 |title=Planet 9 hypothesis gets a boost |author=Paul Scott Anderson |publisher=EarthSky |date=3 March 2019 |access-date=26 June 2019 |archive-date=26 June 2019 |archive-url=https://web.archive.org/web/20190626032132/https://earthsky.org/space/review-paper-physics-reports-planet-9-feb-2019 |url-status=live}}</ref>

<ref name="Brown_Batygin_2021z">{{cite journal |last1=Brown |first1=Michael E. |last2=Batygin |first2=Konstantin |title=A search for Planet Nine using the Zwicky Transient Facility public archive |journal=The Astronomical Journal |date=31 January 2022 |volume=163 |issue=2 |page=102 |arxiv=2110.13117 |bibcode=2022AJ....163..102B |doi=10.3847/1538-3881/ac32dd |s2cid=239768690 |doi-access=free}}</ref>

<ref name=npr2014UZ224>{{cite web |title=A Friend For Pluto: Astronomers Find New Dwarf Planet in Our Solar System |publisher=[[NPR]] |first=Joe |last=Palka |url=https://www.npr.org/sections/thetwo-way/2016/10/11/497071139/a-friend-for-pluto-astronomers-find-new-dwarf-planet-in-our-solar-system |access-date=5 April 2018 |archive-url=https://web.archive.org/web/20180405032925/https://www.npr.org/sections/thetwo-way/2016/10/11/497071139/a-friend-for-pluto-astronomers-find-new-dwarf-planet-in-our-solar-system |archive-date=5 April 2018 |url-status=live}}</ref>

<ref name=Shannon_NewSci>{{cite web |last=Hall |first=Shannon |date=20 April 2016 |title=We Are Closing in on Possible Whereabouts of Planet Nine |url=https://www.newscientist.com/article/2084924-we-are-closing-in-on-possible-whereabouts-of-planet-nine/ |website=[[New Scientist]] |access-date=18 July 2016 |archive-url=https://web.archive.org/web/20160617082953/https://www.newscientist.com/article/2084924-we-are-closing-in-on-possible-whereabouts-of-planet-nine/ |archive-date=17 June 2016 |url-status=live}}</ref>

<ref name="Gibbs_IEEE">{{cite web |last1=Gibbs |first1=W. Wayt |title=Is There a Giant Planet Lurking Beyond Pluto? |url=https://spectrum.ieee.org/is-there-a-giant-planet-lurking-beyond-pluto |website=[[IEEE Spectrum]] |date=August 2017 |access-date=1 August 2017 |archive-url=https://web.archive.org/web/20170801005216/https://spectrum.ieee.org/aerospace/satellites/is-there-a-giant-planet-lurking-beyond-pluto |archive-date=1 August 2017 |url-status=live}}</ref>

<ref name="Meisner_etal_2017a">{{cite journal |last1=Meisner |first1=Aaron M. |last2=Bromley |first2=Benjamin B. |last3=Nugent |first3=Peter E. |last4=Schlegel |first4=David J |last5=Kenyon |first5=Scott J. |last6=Schlafly |first6=Edward F. |last7=Dawson |first7=Kyle S. |title=Searching for Planet Nine with Coadded WISE and NEOWISE-Reactivation Images |journal=[[The Astronomical Journal]] |year=2016 |volume=153 |issue=2 |page=65 |doi=10.3847/1538-3881/153/2/65 |arxiv=1611.00015 |bibcode=2017AJ....153...65M |s2cid=118391962 |doi-access=free}}</ref>

<ref name=searchformoreSedna>{{cite web |first=Nicole |last=Mortillaro |date=9 February 2016 |title=Meet Mike Brown: Pluto Killer and the Man Who Brought Us Planet 9 |url=http://globalnews.ca/news/2500010/meet-mike-brown-pluto-killer-and-the-man-who-brought-us-planet-9/ |website=[[Global News]] |quote={{-'}}It was that search for more objects like Sedna ... led to the realization ... that they're all being pulled off in one direction by something. And that's what finally led us down the hole that there must be a big planet out there.' —Mike Brown |access-date=10 February 2016 |archive-url=https://web.archive.org/web/20160210095238/http://globalnews.ca/news/2500010/meet-mike-brown-pluto-killer-and-the-man-who-brought-us-planet-9/ |archive-date=10 February 2016 |url-status=live}}</ref>

<ref name=SpaceCom>{{cite web |last1=Wall |first1=Mike |date=21 January 2016 |title=How Astronomers Could Actually See 'Planet Nine' |url=http://www.space.com/31677-astronomers-could-see-planet-nine.html |website=[[Space.com]] |access-date=24 January 2016 |archive-url=https://web.archive.org/web/20160123220605/http://www.space.com/31677-astronomers-could-see-planet-nine.html |archive-date=23 January 2016 |url-status=live}}</ref>

<ref name=crockettnewclues>{{cite magazine |first=Christopher |last=Crockett |date=5 July 2016 |title=New Clues in Search for Planet Nine |url=https://www.sciencenews.org/article/new-clues-search-planet-nine |magazine=[[Science News]] |access-date=6 July 2016 |archive-url=https://web.archive.org/web/20160705130303/https://www.sciencenews.org/article/new-clues-search-planet-nine |archive-date=5 July 2016 |url-status=live}}</ref>

<ref name="Choi2016">{{cite web |last=Choi |first=Charles C. |url=https://www.scientificamerican.com/article/closing-in-on-a-giant-ghost-planet/ |title=Closing in on a Giant Ghost Planet |website=[[Scientific American]] |date=25 October 2016 |access-date=26 October 2016 |archive-url=https://web.archive.org/web/20161026023621/https://www.scientificamerican.com/article/closing-in-on-a-giant-ghost-planet/ |archive-date=26 October 2016 |url-status=live}}</ref>

<ref name="Stirone_2019">{{cite web |last1=Stirone |first1=Shannon |title=The Hunt for Planet Nine |url=https://longreads.com/2019/01/22/the-hunt-for-planet-nine/ |website=Longreads |date=22 January 2019 |access-date=22 January 2019 |archive-date=22 January 2019 |archive-url=https://web.archive.org/web/20190122200521/https://longreads.com/2019/01/22/the-hunt-for-planet-nine/ |url-status=live}}</ref>

<ref name="Lawrence_2020">{{Cite arXiv |eprint=2004.14980 |last1=Lawrence |first1=Scott |last2=Rogoszinski |first2=Zeeve |title=The Brute-Force Search for Planet Nine |year=2020 |class=astro-ph.EP}}</ref>

<ref name="Linder_Mordasini_2016">{{cite journal |last1=Linder |first1=Esther F. |last2=Mordasini |first2=Christoph |year=2016 |title=Evolution and Magnitudes of Candidate Planet Nine |journal=[[Astronomy & Astrophysics]] |volume=589 |issue=134 |pages=A134 |arxiv=1602.07465 |bibcode=2016A&A...589A.134L |doi=10.1051/0004-6361/201628350 |s2cid=53702941}}</ref>

<ref name=discoverPerspective>{{cite magazine |last=Powel |first=Corey S. |date=22 January 2016 |title=A Little Perspective on the New "9th&nbsp;Planet" (and the 10th, and the 11th) |url=http://blogs.discovermagazine.com/outthere/2016/01/22/new-9th-planet/ |magazine=[[Discover (magazine)|Discover]] |access-date=18 July 2016 |archive-url=https://web.archive.org/web/20160714182831/http://blogs.discovermagazine.com/outthere/2016/01/22/new-9th-planet/#.VqYOnZp97iw |archive-date=14 July 2016 |url-status=live}}</ref>

<ref name="Cowan2016">{{cite journal |last1=Cowan |first1=Nicolas B. |last2=Holder |first2=Gil |last3=Kaib |first3=Nathan A. |year=2016 |title=Cosmologists in Search of Planet Nine: the Case for CMB Experiments |journal=[[The Astrophysical Journal Letters]] |volume=822 |issue=1 |pages=L2 |arxiv=1602.05963 |bibcode=2016ApJ...822L...2C |doi=10.3847/2041-8205/822/1/L2 |s2cid=119308822 |doi-access=free}}</ref>

<ref name="Aron_2016">{{cite web |last1=Aron |first1=Jacob |date=24 February 2016 |title=Planet Nine Hunters Enlist Big Bang Telescopes and Saturn Probe |url=https://www.newscientist.com/article/2078757-planet-nine-hunters-enlist-big-bang-telescopes-and-saturn-probe/ |website=[[New Scientist]] |access-date=27 February 2016 |archive-url=https://web.archive.org/web/20160225105639/https://www.newscientist.com/article/2078757-planet-nine-hunters-enlist-big-bang-telescopes-and-saturn-probe/ |archive-date=25 February 2016 |url-status=live}}</ref>

<ref name="Wood2018">{{cite news |title=Is there a mysterious Planet Nine lurking in our Solar system beyond Neptune? |newspaper=The Washington Post|first=Charlie |last=Wood |date=2 September 2018 |url=https://www.washingtonpost.com/national/health-science/is-there-a-mysterious-planet-nine-lurking-in-our-solar-system-beyond-neptune/2018/08/31/1957c8ca-a495-11e8-8fac-12e98c13528d_story.html |access-date=17 January 2019 |archive-url=https://web.archive.org/web/20180902220558/https://www.washingtonpost.com/national/health-science/is-there-a-mysterious-planet-nine-lurking-in-our-solar-system-beyond-neptune/2018/08/31/1957c8ca-a495-11e8-8fac-12e98c13528d_story.html |archive-date=2 September 2018 |url-status=live}}</ref>

<ref name=aasnovacmb>{{cite web |last=Kohler |first=Susanna |date=25 April 2016 |title=Can CMB experiments find Planet Nine? |url=http://aasnova.org/2016/04/25/can-cmb-experiments-find-planet-nine/ |website=AAS Nova |publisher=[[American Astronomical Society]] |access-date=29 April 2016 |archive-url=https://web.archive.org/web/20160531185715/http://aasnova.org/2016/04/25/can-cmb-experiments-find-planet-nine/ |archive-date=31 May 2016 |url-status=live}}</ref>

<ref name="Naess_etal_2021">{{cite journal |last1=Naess |first1=Sigurd |display-authors=et al |title=The Atacama Cosmology Telescope: A Search for Planet 9 |journal=The Astrophysical Journal |year=2021 |volume=923 |issue=2 |page=224 |doi=10.3847/1538-4357/ac2307 |arxiv=2104.10264 |bibcode=2021ApJ...923..224N |s2cid=233324478 |doi-access=free}}</ref>

<ref name=findp9>{{cite web |last=Strom |first=Marcus |url=https://www.smh.com.au/technology/you-can-help-find-planet-nine-from-outer-space-through-citizen-science-20170216-gue44k.html |title=You Can Help Find Planet Nine from Outer Space Through Citizen Science |website=[[The Sydney Morning Herald]] |date=16 February 2017 |access-date=12 November 2018 |archive-url=https://web.archive.org/web/20180618025523/https://www.smh.com.au/technology/you-can-help-find-planet-nine-from-outer-space-through-citizen-science-20170216-gue44k.html |archive-date=18 June 2018 |url-status=live}}</ref>

<ref name="Zooniverse">{{Cite web |title=Catalina Outer Solar System Survey – About |url=https://www.zooniverse.org/projects/fulsdavid/catalina-outer-solar-system-survey/about/faq |access-date=1 September 2020 |website=Catalina Outer Solar System Survey |archive-date=17 September 2021 |archive-url=https://web.archive.org/web/20210917122910/https://www.zooniverse.org/projects/fulsdavid/catalina-outer-solar-system-survey/about/faq |url-status=live}}</ref>

<ref name="Comb">{{Cite web |date=11 August 2020 |title=Comb the Edges of the Solar System with the Catalina Outer Solar System Survey |url=https://science.nasa.gov/citizenscience/news/catalina-outer-solar-system-survey |access-date=1 September 2020 |website=NASA Science |archive-date=29 September 2020 |archive-url=https://web.archive.org/web/20200929165411/https://science.nasa.gov/citizenscience/news/catalina-outer-solar-system-survey |url-status=live}}</ref>

<ref name=Fienga_2016>{{cite journal |last1=Fienga |first1=A. |last2=Laskar |first2=J. |last3=Manche |first3=H. |last4=Gastineau |first4=M. |year=2016 |title=Constraints on the Location of a Possible 9th Planet Derived from the Cassini Data |journal=[[Astronomy & Astrophysics]] |volume=587 |issue=1 |pages=L8 |arxiv=1602.06116 |bibcode=2016A&A...587L...8F |doi=10.1051/0004-6361/201628227 |s2cid=119116589}}</ref>

<ref name=findingNine>{{cite journal |first1=Carlos |last1=de la Fuente Marcos |first2=Raúl |last2=de la Fuente Marcos |year=2016 |journal=[[Monthly Notices of the Royal Astronomical Society Letters]] |volume=459 |issue=1 |pages=L66–L70 |title=Finding Planet Nine: a Monte Carlo Approach |arxiv=1603.06520 |bibcode=2016MNRAS.459L..66D |doi=10.1093/mnrasl/slw049 |doi-access=free |s2cid=118433545}}</ref>

<ref name=peripole>{{cite journal |last1=de la Fuente Marcos |first1=Carlos |last2=de la Fuente Marcos |first2=Raúl |year=2016 |title=Finding Planet Nine: Apsidal Anti-Alignment Monte Carlo Results |journal=[[Monthly Notices of the Royal Astronomical Society]] |volume=462 |issue=2 |pages=1972–1977 |arxiv=1607.05633 |bibcode=2016MNRAS.462.1972D |doi=10.1093/mnras/stw1778 |doi-access=free |s2cid=119212828}}</ref>

<ref name="Holman_Payne_Cassini">{{cite journal |last1=Holman |first1=Matthew J. |last2=Payne |first2=Matthew J. |title=Observational Constraints on Planet Nine: Cassini Range Observations |journal=[[The Astronomical Journal]] |year=2016 |volume=152 |issue=4 |page=94 |doi=10.3847/0004-6256/152/4/94 |arxiv=1604.03180 |bibcode=2016AJ....152...94H |s2cid=118618464 |doi-access=free}}</ref>

<ref name="JPL">{{cite web |date=8 April 2016 |title=Saturn Spacecraft Not Affected by Hypothetical Planet 9 |url=http://www.jpl.nasa.gov/news/news.php?feature=6200 |website=[[NASA]]/[[Jet Propulsion Laboratory]] |access-date=20 April 2016 |archive-url=https://web.archive.org/web/20160416140310/http://www.jpl.nasa.gov/news/news.php?feature=6200 |archive-date=16 April 2016 |url-status=live}}</ref>

<ref name=Holman-Payne>{{cite journal |last1=Holman |first1=Matthew J. |last2=Payne |first2=Matthew J. |date=9 September 2016 |title=Observational Constraints on Planet Nine: Astrometry of Pluto and Other Trans-Neptunian Objects |journal=[[The Astronomical Journal]] |volume=152 |issue=4 |page=80 |arxiv=1603.09008 |bibcode=2016AJ....152...80H |doi=10.3847/0004-6256/152/4/80 |s2cid=119189007 |doi-access=free}}</ref>

<ref name="Medvedev_etal_2017">{{cite journal |last1=Medvedev |first1=Yu D. |last2=Vavilov |first2=D.E. |last3=Bondarenko |first3=Yu S. |last4=Bulekbaev |first4=D.A. |last5=Kunturova |first5=N.B. |title=Improvement of the Position of Planet X Based on the Motion of Nearly Parabolic Comets |journal=[[Astronomy Letters]] |year=2017 |volume=42 |issue=2 |pages=120–125 |doi=10.1134/S1063773717020037 |bibcode=2017AstL...43..120M |s2cid=125957280}}</ref>

<ref name="Rice_Laughlin_2019">{{cite journal |last1=Rice |first1=Malena |last2=Laughlin |first2=Gregory |title=The Case for a Large-Scale Occultation Network |journal=The Astronomical Journal |volume=158 |issue=1 |pages=19 |year=2019 |arxiv=1905.06354 |doi=10.3847/1538-3881/ab21df |bibcode=2019AJ....158...19R |s2cid=155099837 |doi-access=free}}</ref>

<ref name="Millholland_Laughlin_2017">{{cite journal |last1=Millholland |first1=Sarah |last2=Laughlin |first2=Gregory |title=Constraints on Planet Nine's Orbit and Sky Position within a Framework of Mean-Motion Resonances |journal=[[The Astronomical Journal]] |year=2017 |volume=153 |issue=3 |page=91 |doi=10.3847/1538-3881/153/3/91 |arxiv=1612.07774 |bibcode=2017AJ....153...91M |s2cid=119325788 |doi-access=free}}</ref>

<ref name="Millholland_3D_orbit">{{cite journal |last1=Millholland |first1=Sarah |last2=Laughlin |first2=Gregory |title=Constraints on Planet Nine's Orbit and Sky Position within a Framework of Mean-Motion Resonances |journal=[[The Astronomical Journal]] |year=2017 |volume=153 |issue=3 |page=91 |doi=10.3847/1538-3881/153/3/91 |arxiv=1612.07774 |bibcode=2017AJ....153...91M |s2cid=119325788 |doi-access=free}} supplemented by {{cite web |last1=Millholland |first1=Sarah |title=Planet Nine's Orbit in Space |website=[[GitHub]] |url=https://smillholland.github.io/P9_Orbit/ |access-date=8 August 2017 |archive-url=https://web.archive.org/web/20170221005255/https://smillholland.github.io/P9_Orbit/ |archive-date=21 February 2017 |url-status=live}}</ref>

<ref name=moreplanets>{{cite journal |last1=de la Fuente Marcos |first1=Carlos |last2=de la Fuente Marcos |first2=Raúl |year=2016 |title=Commensurabilities between ETNOs: a Monte Carlo survey |journal=[[Monthly Notices of the Royal Astronomical Society Letters]] |volume=460 |issue=1 |pages=L64–L68 |arxiv=1604.05881 |bibcode=2016MNRAS.460L..64D |doi=10.1093/mnrasl/slw077 |doi-access=free |s2cid=119110892}}</ref>

<ref name="Kaine_etal_2018">{{cite journal |last1=Kaine |first1=T. |display-authors=etal |title=Dynamical Analysis of Three Distant Trans-Neptunian Objects with Similar Orbits |journal=The Astronomical Journal |year=2018 |volume=156 |issue=6 |page=273 |doi=10.3847/1538-3881/aaeb2a |arxiv=1810.10084 |bibcode=2018AJ....156..273K |s2cid=85440531 |doi-access=free}}</ref>

<ref name="Bailey_etal_2018">{{cite journal |last1=Bailey |first1=Elizabeth |last2=Brown |first2=Michael E. |last3=Batygin |first3=Konstantin |title=Feasibility of a Resonance-Based Planet Nine Search |journal=[[The Astronomical Journal]] |year=2018 |volume=156 |issue=2 |page=74 |arxiv=1809.02594 |doi=10.3847/1538-3881/aaccf4 |bibcode=2018AJ....156...74B |s2cid=55192116 |doi-access=free}}</ref>

<ref name=HD106906b>{{cite web |publisher=nasa.gov |url=https://www.nasa.gov/feature/goddard/2020/hubble-pins-down-weird-exoplanet-with-far-flung-orbit |title=Hubble Pins Down Weird Exoplanet with Far-Flung Orbit |date=10 December 2020 |access-date=18 December 2020 |archive-date=19 December 2020 |archive-url=https://web.archive.org/web/20201219040919/http://www.nasa.gov/feature/goddard/2020/hubble-pins-down-weird-exoplanet-with-far-flung-orbit/ |url-status=live}}</ref>

<ref name="MIT2019">{{Cite web |url=https://www.technologyreview.com/2019/03/05/136785/theres-probably-another-planet-in-our-solar-system/ |title=There's probably another planet in our solar system |date=5 March 2019 |access-date=8 March 2021 |publisher=MIT Technology Review |archive-date=16 April 2021 |archive-url=https://web.archive.org/web/20210416104929/https://www.technologyreview.com/2019/03/05/136785/theres-probably-another-planet-in-our-solar-system/ |url-status=live}}</ref>

<ref name="IAU">{{cite web |title=Naming of Astronomical Objects |url=https://www.iau.org/public/themes/naming/ |website=[[International Astronomical Union]] |access-date=25 February 2016 |archive-url=https://web.archive.org/web/20160617214812/http://www.iau.org/public/themes/naming/ |archive-date=17 June 2016 |url-status=live}}</ref>

<ref name=planetnames>{{cite web |last=Totten |first=Sanden |date=22 January 2016 |title=Planet 9: What Should Its Name Be If It's Found? |url=http://www.scpr.org/news/2016/01/22/56998/planet-9-what-should-its-name-be-if-it-s-found |website=[[KPCC (radio station)|89.3 KPCC]] |access-date=7 February 2016 |quote={{-'}}We like to be consistent,' said [[Rosaly Lopes]], a senior research scientist at NASA's Jet Propulsion Laboratory and a member of the IAU's Working Group for Planetary System Nomenclature. ... For a planet in our solar system, being consistent means sticking to the theme of giving them names from Greek and Roman mythology. |archive-url=https://web.archive.org/web/20160207085353/http://www.scpr.org/news/2016/01/22/56998/planet-9-what-should-its-name-be-if-it-s-found/ |archive-date=7 February 2016 |url-status=live}}</ref>

<ref name="Fesemaier_2016">{{cite web |last1=Fesenmaier |first1=Kimm |title=Caltech Researchers Find Evidence of a Real Ninth Planet |date=20 January 2016 |url=http://www.caltech.edu/news/caltech-researchers-find-evidence-real-ninth-planet-49523 |website=[[California Institute of Technology|Caltech]] |access-date=15 January 2019 |archive-date=16 January 2019 |archive-url=https://web.archive.org/web/20190116051438/http://www.caltech.edu/news/caltech-researchers-find-evidence-real-ninth-planet-49523 |url-status=live}}</ref>

<ref name="Lemonick">{{Citation |author=Lemonick, M. D. |year=2016 |title=Planet Nine from Outer Space |journal=Scientific American |volume=314 |issue=5 |page=36 |doi=10.1038/scientificamerican0516-36 |pmid=27100252 |bibcode=2016SciAm.314e..36L}}</ref>

<ref name="Batygin1">{{Citation |title=Planet Nine from Outer Space |author=Batygin, Konstantin |journal=American Astronomical Society Meeting Abstracts |year=2017 |volume=230 |pages=211.01 |bibcode=2017AAS...23021101B}}</ref>

<ref name="Batygin2">{{Citation |title=Planet Nine from Outer Space |author1=Batygin, Konstantin |author2=Brown, Michael |journal=42nd COSPAR Scientific Assembly |year=2018 |volume=42 |pages=PIR.1–14–18 |bibcode=2018cosp...42E.229B}}</ref>

<ref name="Brown">{{Citation |title=Planet Nine from Outer Space |author=Brown, Mike |publisher=Caltech Astro |date=15 March 2019 |access-date=8 April 2019 |url=https://www.youtube.com/watch?v=Zw5MDh_wpnI?t=1 |archive-date=28 December 2020 |archive-url=https://web.archive.org/web/20201228100238/https://www.youtube.com/watch?v=Zw5MDh_wpnI |url-status=live}}</ref>

<ref name="TechRepublic2006">{{cite web |title=Planet X Marks the Spot |work=TechRepublic |year=2006 |url=http://www.sunflower-astronomy.com/KCKCC_Docs/NewsArticles/Geek_Trivia_Planet_X_marks_the_spot.pdf |access-date=13 July 2008 |archive-url=https://web.archive.org/web/20080910074246/http://www.sunflower-astronomy.com/KCKCC_Docs/NewsArticles/Geek_Trivia_Planet_X_marks_the_spot.pdf |archive-date=10 September 2008 |url-status=live}}</ref>

<ref name="Xv9">{{cite web |url=http://www.businessinsider.com/planet-nine-x-name-argument-2018-6 |title=Is It Planet 9 or Planet X? Scientists Spar over What to Call the Solar System's Hypothetical Missing World |website=[[Business Insider]] |first=Dave |last=Mosher |date=7 June 2018 |access-date=9 June 2018 |archive-url=https://web.archive.org/web/20180608090727/http://www.businessinsider.com/planet-nine-x-name-argument-2018-6 |archive-date=8 June 2018 |url-status=live}}</ref>

<ref name="Abell2018">{{cite journal |journal=Planetary Exploration Newsletter |url=http://planetarynews.org/archive18/pen_v12_n31_180729.txt |title=On The Insensitive Use of the Term 'Planet 9' for Objects Beyond Pluto |author=Paul Abell |display-authors=etal |volume=12 |number=31 |date=29 July 2018 |access-date=15 January 2019 |archive-date=17 August 2018 |archive-url=https://web.archive.org/web/20180817161646/http://planetarynews.org/archive18/pen_v12_n31_180729.txt |url-status=live}}</ref>

<ref name="Katwala">{{cite magazine |title=The unending hunt for Planet Nine, our solar system's hidden world |url=https://www.wired.co.uk/article/search-for-planet-nine-planet-x-solar-system |author=Amit Katwala |magazine=Wired |date=24 September 2018 |access-date=10 November 2021}}</ref>

<ref name="Rice_Laughlin_TESS_2020">{{cite journal |last1=Rice |first1=Malena |last2=Laughlin |first2=Gregory |title=Exploring Trans-Neptunian Space with TESS: A Targeted Shift-stacking Search for Planet Nine and Distant TNOs in the Galactic Plane |journal=[[The Planetary Science Journal]] |volume=1 |issue=3 |pages=81 (18 pp.) |date=December 2020 |arxiv=2010.13791 |doi=10.3847/PSJ/abc42c |bibcode=2020PSJ.....1...81R |s2cid=225075671 |doi-access=free}}</ref>

<ref name="WHT_2022">{{cite journal |last1=de la Fuente Marcos |first1=Carlos |last2=de la Fuente Marcos |first2=Raúl |last3=Vaduvescu |first3=Ovidiu |last4=Stanescu |first4=Malin |title=Distant trans-Neptunian object candidates from NASA's TESS mission scrutinized: fainter than predicted or false positives? |journal=[[Monthly Notices of the Royal Astronomical Society Letters]] |volume=513 |issue=1 |pages=L78–L82 |arxiv=2204.02230 |bibcode=2022MNRAS.513L..78D |doi=10.1093/mnrasl/slac036 |url=https://academic.oup.com/mnrasl/article-abstract/513/1/L78/6565292 |date=June 2022 |doi-access=free}}</ref>

<ref name="ING_PR">{{cite web |url=https://www.ing.iac.es/PR/press/tess.html |title=Distant Trans-Neptunian Object Candidates: Fainter Than Predicted or False Positives? |date=20 May 2022}}</ref>
 
}}

==External links==
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* {{Commons category-inline}}
* [http://findplanetnine.blogspot.com/ The Search for Planet Nine] – Blog by Brown and Batygin 
* [https://solarsystem.nasa.gov/planets/hypothetical-planet-x/in-depth/ Hypothetical Planet X] – NASA [[Planetary Science Division]]

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[[Category:2016 in outer space]]
[[Category:Astronomical events of the Solar System]]
[[Category:Hypothetical planets]]
[[Category:Hypothetical trans-Neptunian objects]]
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