Planet Nine

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Planet Nine is a hypothetical planet in the outer region of the Solar System. Its gravitational influence could explain a statistical anomaly in the distribution of orbits of a group of distant trans-Neptunian objects (TNOs) found mostly beyond the Kuiper belt in the scattered disc region. This undiscovered super-Earth-sized planet would have an estimated mass of ten Earths, a diameter two to four times that of Earth, and an elongated orbit lasting approximately 15,000 years. To date, efforts to detect Planet Nine have failed.

Speculation about the possible existence of a ninth planet began in 2014. Astronomers Chad Trujillo and Scott S. Sheppard wrote in the journal Nature and compared the similar orbits of trans-Neptunian objects Sedna and 2012 VP113. In early 2016, Konstantin Batygin and Michael E. Brown described how the similar orbits of six TNOs could be explained by Planet Nine and proposed a possible orbit for the planet. This hypothesis could also explain TNOs with orbits perpendicular to the inner planets and those with an extreme tilt, as well as the tilt of the Sun's axis.

Planet Nine is presumed to be the core of a primordial giant planet that was ejected from its original orbit, after encountering Jupiter, during the genesis of the Solar System. Others have proposed that the planet was either captured from another star, a captured rogue planet, or its orbit may have been influenced by a distant encounter with a passing star.

Video Planet Nine

Naming

Planet Nine does not have an official name and will not receive one unless its existence is confirmed, typically through optical imaging. Once confirmed, the International Astronomical Union will certify a name, with priority usually given to a name proposed by its discoverers. It is likely to be a name chosen from Roman or Greek mythology.

In their original article, Batygin and Brown simply referred to the object as "perturber", and only in later press releases did they use "Planet Nine". They have also used the names "Jehoshaphat" and "George" for Planet Nine. Brown has stated: "We actually call it Phattie when we're just talking to each other."

Maps Planet Nine

Hypothetical characteristics

Orbit

Planet Nine is hypothesized to follow a highly elliptical orbit around the Sun lasting 10,000-20,000 years. The planet's semi-major axis is estimated to be 700 AU, roughly 20 times the distance from Neptune to the Sun, and its inclination to be about 30°±10°. The high eccentricity of Planet Nine's orbit could bring it as close as 200 AU at its perihelion and take it as far away as 1,200 AU at its aphelion.

The aphelion, or farthest point from the Sun, would be in the general direction of the constellation of Taurus, 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.

Brown thinks that if Planet Nine is confirmed to exist, a probe could fly by it in as little as 20 years, with a powered slingshot around the Sun.

Size and composition

The planet is estimated to have 10 times the mass and two to four times the diameter of Earth. An object with the same diameter as Neptune has not been excluded by previous surveys in visible light. However, the infrared survey by the Wide-field Infrared Survey Explorer (WISE) may have the capabilities to detect such a planet.

Brown thinks that if Planet Nine exists, its mass is large enough to clear its feeding zone in 4.6 billion years (with possible exceptions for some combinations of semi-major axis and mass) and that its gravity dominates the outer edge of the Solar System, which is sufficient to make it a planet by current definitions. Jean-Luc Margot has also stated that Planet Nine satisfies his criteria and would qualify as a planet--if and when it is detected.

Brown speculates that the predicted planet is most probably an ejected ice giant, similar in composition to Uranus and Neptune: a mixture of rock and ice with a small envelope of gas. In fact, if it once orbited the region of the gas/ice giants, the planet probably acquired an atmosphere of hydrogen and helium. Based on its size, the considerable gravitational pull of Planet Nine could theoretically promote life in subsurface oceans of its moons, were it to have any. Subsurface oceans have been discovered on Europa of Jupiter, Enceladus of Saturn, and subsurface water is postulated for Triton of Neptune.

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Hypothesis and the calculated effects

The gravitational influence of Planet Nine would explain five peculiarities of the Solar System:

• the clustering of the orbits of extreme trans-Neptunian objects (eTNOs);
• the high perihelia of objects like Sedna that are 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 objects with semi-major axis less than 100 AU,
• and the obliquity, or tilt, of the Sun's axis relative to the orbits of the planets.

While other mechanisms have been offered for many of these peculiarities, the gravitational influence of Planet Nine is the only one that explains all five. The gravity of Planet Nine also excites the inclinations of scattering objects, which in numerical simulations leaves the short-period comets with a broader inclination distribution than is observed.

Observations: Orbital clustering amongst high perihelion objects

The clustering of the orbits of extreme trans-Neptunian objects was first described by Chad Trujillo and Scott S. Sheppard, who noted similarities between the orbits of Sedna and 2012 VP₁₁₃. Upon further analysis they observed that the arguments of perihelion (which indicate the orientation of elliptical orbits within their orbital planes) of 12 eTNOs with perihelia greater than 30 AU and semi-major axes greater than 150 AU were clustered near zero degrees. Trujillo and Sheppard proposed that this alignment was caused by a massive unknown planet beyond Neptune via the Kozai mechanism. (see Trujillo and Sheppard (2014) for more details.)

Caltech's Konstantin Batygin and Michael E. Brown, looking to refute the mechanism proposed by Trujillo and Sheppard, also examined the orbits of the extreme trans-Neptunian objects. 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 mean-motion resonances, they determined that the arguments of perihelion for the remaining six objects (namely Sedna, 2012 VP113, 2004 VN₁₁₂, 2010 GB174, 2000 CR₁₀₅, and 2010 VZ₉₈) were clustered around 318°±8°. This was out of alignment with how the Kozai mechanism would align these orbits, at c. 0° or 180°.

Batygin and Brown also found that the orbits of the six objects with semi-major axes greater than 250 AU and perihelia beyond 30 AU (namely Sedna, 2012 VP113, 2004 VN₁₁₂, 2010 GB174, 2007 TG422, and 2013 RF98) were aligned in space with their perihelia in roughly the same direction, resulting in a clustering of their longitudes of perihelion. The orbits of the six objects were also tilted with respect to that of the ecliptic and approximately co-planar, producing a clustering of their longitudes of ascending nodes. They determined that there was only a 0.007% likelihood that this combination of alignments was due to chance. These six objects had been discovered by six different surveys on six different 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 by the object's varied precession rates in a few hundred million years. This indicates that it couldn't be due to an event in the distant past, like a passing star, and is most likely being maintained by an object orbiting the Sun.

In a later article Trujillo and Sheppard noted a correlation between the longitude of perihelion and the argument of perihelion of the eTNOs with semi-major axes greater than 150 AU. Those with a longitude of perihelion of 0-120° have arguments of perihelion between 280-360°, and those with longitude of perihelion of 180-340° have argument of perihelion 0-40°. 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.

Extreme trans-Neptunian objects

Among the extreme trans-Neptunian objects are two high-perihelion objects: Sedna and 2012 VP113. Sedna and 2012 VP113 are distant detached objects with perihelia greater than 70 AU. Their high perihelia keep them at a sufficient distance to avoid significant gravitational perturbations from Neptune. Previous explanations for the high perihelion of Sedna include a close encounter with an unknown planet on a distant orbit and a distant encounter with a random star or a member of the Sun's birth cluster that passed near the Solar System.

Since early 2016, seven more extreme trans-Neptunian objects have been discovered with orbits that have a perihelion greater than 30 AU and a semi-major axis greater than 250 AU bringing the total to thirteen. Most eTNos have perihelia significantly beyond Neptune, which orbits 30 AU from the Sun. Generally, TNOs with perihelia smaller than 36 AU experience strong encounters with Neptune. Most of the eTNOs are relatively small, but currently relatively bright because they are near their closest distance to the Sun in their elliptical orbits. These are also included in the orbital diagrams and tables below.

• (*) longitude of perihelion, ?, outside expected range;
•    are the 7 TNO objects discovered by Trujillo and Sheppard
•    are the 2 objects discovered by DES.

Simulations: Reproduce observed clustering and Inference

The clustering of the orbits of extreme trans-Neptunian objects and raising of their perihelia is reproduced in simulations that include Planet Nine. In simulations conducted by Batygin and Brown swarms of large semi-major axis scattered disk objects that began with random orientations were sculpted into roughly collinear groups of spatially confined orbits by a massive distant planet in a highly eccentric orbit. These surviving objects, about 10% of the population with initial semi-major axes of 250-550 AU, were in orbits that were oriented with their long axes anti-aligned with respect to the massive planet and were roughly co-planar with it. The objects were also found to be in resonance with the massive planet. The resonances included high-order resonances, for example 27:17, and were interconnected, yielding an orbital evolution that was fundamentally chaotic, causing their semi-major axes to vary unpredictably on million-year timescales. The perihelia of these objects were also raised temporarily, producing Sedna-like orbits, before being returned to orbits more typical of typical trans-Neptunian objects after several hundred million years. Some of the eTNOs also evolved into orbits perpendicular to the plane of the Solar System, which Batygin and Brown later discovered had also been observed. Similar results also occur if simulations began with the objects initially interacting with multiple planets, like in various versions of the Nice model. Lawler et al. and Nesvorny et al. both found that objects scattered outward by the giant planets would be captured in a cloud centered around Planet Nine's semi-major axis with most objects at greater semi-major axis. A significant fraction of these objects had inclinations of greater than 60°, and for an initial 20 Earth mass planetesimal disk roughly 0.3-0.4 Earth masses remained in the Planet Nine cloud at the end of a 4 billion year simulation.

Batygin and Brown found that the distribution of the orbits of known extreme trans-Neptunian objects is best reproduced in simulations using a 10 M_? planet in the following orbit:

• semi-major axis a ? 700 AU, orbital period 18,000 years
• eccentricity e ? 0.6, perihelion ? 280 AU, aphelion ? 1,120 AU
• inclination i ? 30° to the ecliptic
• longitude of the ascending node ? ? 94°.
• argument of perihelion ? ? 139° and longitude of perihelion ? = 235°±12°

This orbit results in strong anti-alignment beyond 250 AU, weak alignment between 150 AU and 250 AU, and little effect inside 150 AU. Anti-alignment occurs with variable success using a semi-major axis between 400 AU and 1500 AU and an eccentricity between 0.5 and 0.8. Anti-alignment weakens as Planet Nine's inclination is increased. Simulations conducted by Becker et al. found a similar range for the stability of eTNOs, semi-major axes ranging from 500 to 1200 AU and eccentricities ranging from 0.3 to 0.6 with lower eccentricities being favored at smaller semi-major axes. They noted that while stability was favored with smaller eccentricities, anti-alignment was more likely at higher eccentricities near the borders of stability. Lawler et al. found that the population captured was smaller in simulations with a planet in a circular orbit which also produced few high inclination objects.

Dynamics: Effect on other objects in the Solar System

Planet Nine modifies the orbits of extreme trans-Neptunian objects via a combination of effects. On very long timescales exchanges of angular momentum with Planet Nine causes the perihelia of anti-aligned objects to rise until their precession reverses direction, maintaining their anti-alignment, and later fall, returning them to their original orbits. 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 inclination of Planet Nine's orbit weakens this protection, resulting in a chaotic variation of semi-major axes as objects hop between resonances. The orbital poles of the objects circle that of the Solar System's Laplace plane, which at large semi-major axes is warped toward the plane of Planet Nine's orbit, causing their poles to be clustered toward one side.

Apsidal anti-alignment

The anti-alignment and the raising of the perihelia of extreme trans-Neptunian objects with semi-major axes greater than 250 AU is produced by the secular effects of Planet Nine. Secular effects act on timescales much longer than orbital periods so the perturbations two objects exert on each other are the average between all possible configurations. Effectively the interactions become like those between two wires of varying thickness, thicker where the objects spend more time, that are exerting torques on each other, causing exchanges of angular momentum but not energy. Thus secular effects can alter the eccentricities, inclinations and orientations of orbits but not the semi-major axes.

Exchanges of angular momentum with Planet Nine cause the perihelia of the anti-aligned objects to rise and fall while their longitudes of perihelion librate, or oscillate within a limited range of values. When the angle between an anti-aligned object's perihelion and Planet Nine's (delta longitude of perihelion on diagram) climbs beyond 180° Planet Nine exerts a positive average torque on the object's orbit. This torque increases the object's angular momentum, causing the eccentricity of its orbit to decline (see blue curves on diagram) and its perihelion to rise away from Neptune's orbit. The object's precession then slows and eventually reverses as its eccentricity declines. After delta longitude of perihelion drops below 180° the object begins to feel a negative average torque and its eccentricity grows and perihelion falls. When the object's eccentricity is once again large it precesses forward, returning the object to its original orbit after several hundred million years.

The behavior of the orbits of other objects varies with their initial orbits. Stable orbits exist for aligned objects with small eccentricities. Although objects in these orbits have high perihelia and have yet to be observed, they may have been captured at the same time as Planet Nine due to perturbations from a passing star. Aligned objects with lower perihelia are only temporarily stable, their orbits precess until parts of the orbits are tangent to that of Planet Nine, leading to frequent close encounters. After crossing this region the perihelia of their orbits decline causing them to encounter the other planets leading to their ejection. The curves the orbits follow vary with semi-major axis of the object and if the object is in resonance. At smaller semi-major axes the aligned and anti-aligned regions shrink and eventually disappear below 150 AU, leaving typical Kuiper belt objects unaffected by Planet Nine. The anti-alignment of resonant objects, for example if Sedna is in a 3:2 resonance with Planet Nine as proposed by Malhotra, Volk and Wang, is maintained by similar secular effects inside the mean-motion resonances. The secular dynamics is more complex if Planet Nine and the eTNOs are in inclined orbits, alignment and anti-alignment in this case is more the result of sticky chaos rather than confinement, with orbits evolving widely but spending more time in regions of relative stability associated with secular resonances.

Evolution and long-term stability of anti-aligned orbits

The long term stability of anti-aligned extreme trans-Neptunian objects with orbits that intersect that of Planet Nine is due to their being captured in mean-motion resonances. Objects in mean-motion resonances with a massive planet are phase protected, preventing them from making close approaches to the planet. When the orbit of a resonant object drifts out of phase, causing it to make closer approaches to a massive planet, the gravity of the planet modifies its orbit, altering its semi-major axis in the direction that reverses the drift. This process repeats as the drift continues in the other direction causing the orbit to appear to rock back and forth, or librate, about a stable center when viewed in a rotating frame of reference. In the example at right, when the orbit of a plutino drifts backward it loses angular momentum when it makes closer approaches ahead of Neptune, causing its semi-major axis and period to shrink, reversing the drift.

In a simplified model where all objects orbit in the same plane and the giant planets are represented by rings, objects captured in strong resonances with Planet Nine could remain in them for the lifetime of the Solar System. At large semi-major axes, beyond a 3:1 resonance with Planet Nine, most of these objects would be in anti-aligned orbits. At smaller semi-major axes the longitudes of perihelia of an increasing number of objects could circulate, passing through all values ranging from 0° to 360°, without being ejected, reducing the fraction of objects that are anti-aligned. 2015 GT50 may be in one of these circulating orbits.

If this model is modified with Planet Nine and the eTNOs in inclined orbits the objects alternate between extended periods in stable resonances and periods of chaotic diffusion of their semi-major axes. The distance of the closest approaches varies with the inclinations and orientations of the orbits, in some cases weakening the phase protection and allowing close encounters. The close encounters can then alter the eTNO's orbit, producing stochastic jumps in its semi-major axis as it hops between resonances, including higher order resonances. This results in a chaotic diffusion of an object's semi-major axis until it is captured in a new stable resonance and the secular effects of Planet Nine shift its orbit to a more stable region. The chaotic diffusion reduces the range of longitudes of perihelion that anti-aligned objects can reach while remaining in stable orbits.

Neptune's gravity can also drive a chaotic diffusion of semi-major axes when all objects are in the same plane. Distant encounters with Neptune can alter the orbits of the eTNOs, causing their semi-major axes to vary significantly on million year timescales. These perturbations can cause the semi-major axes of the anti-aligned objects to diffuse chaotically while occasionally sticking in resonances with Planet Nine. At semi-major axes larger than Planet Nine's, where the objects spend more time, anti-alignment may be due to the secular effects outside mean-motion resonances, and for objects with a > 1000 AU perihelia near 40 AU are stable.

The phase protection of Planet Nine's resonances stabilizes the orbits of objects that interact with Neptune, via its resonances, for example 2013 FT28, or by close encounters for objects with low perihelia like 2007 TG422 and 2013 RF98. Instead of being ejected following a series of encounters these objects can hop between resonances with Planet Nine and evolve into orbits no longer interacting with Neptune. A shift in the position of Planet Nine in simulations from the location favored by an analysis of Cassini data to a position near aphelion has been shown to increase the stability of some of the observed objects, possibly due to this shifting the phases of their orbits to a stable range.

Clustering of orbital poles (nodal alignment)

The clustering of the orbital poles, which produces an apparent clustering of the longitude of the ascending nodes and arguments of perihelion of the extreme TNOs, is the result of a warping of the Laplace plane of the Solar System toward that of Planet Nine's orbit. The Laplace plane defines the center around which the pole of an object's orbit precesses with time. At larger semi-major axes the angular momentum of Planet Nine causes the Laplace plane to be warped toward that of its orbit. As a result, when the poles of the eTNO orbit precess around the Laplace plane's pole they tend to remain on one side of the ecliptic pole. For objects with small inclination relative to Planet Nine, which were found to be more stable in simulations, this off-center precession produces a libration of the longitudes of ascending nodes with respect to the ecliptic making them appear clustered. In simulations the precession is broken into short arcs by encounters with Planet Nine and the positions of the poles are clustered in an off-center elliptical region. In combination with the anti-alignment of the longitudes of perihelion this can also produce clustering of the arguments of perihelion.

Objects in perpendicular orbits with large semi-major axis

Planet Nine can deliver extreme trans-Neptunian objects into orbits roughly perpendicular to the plane of the Solar System. Several objects with high inclinations, greater than 60°, and large semi-major axes, above 250 AU, have been observed. Their high inclination orbits can be generated by a high order secular resonance with Planet Nine involving a linear combination of the orbit's arguments and longitudes of perihelion: ?? - 2?. Low inclination eTNOs can enter this resonance after first reaching low eccentricity orbits. The resonance causes their eccentricities and inclinations to increase, delivering them into perpendicular orbits with low perihelia where they are more readily observed. The orbits then evolve into retrograde orbits with lower eccentricities after which they pass through a second phase of high eccentricity perpendicular orbits before returning to low eccentricity, low inclination orbits. Unlike the Kozai mechanism this resonance causes objects to reach their maximum eccentricities when in nearly perpendicular orbits. In simulations conducted by Batygin and Brown this evolution was relatively common, with 38% of stable objects undergoing it at least once. Saillenfest et al. also observed this behavior in their study of the secular dynamics of eTNOs and noted that it caused the perihelia to fall below 30 AU for objects with semi-major axes greater than 300 AU, and with Planet Nine in an inclined orbit it could occur for objects with semi-major axes as small as 150 AU. The arguments of perihelion and longitudes of ascending nodes of the objects that reach low perihelia in simulations are in rough agreement with observations with the differences attributed to distant encounters with the known giant planets. Six objects with semi-major axes greater than 250 AU and perihelia beyond Jupiter's orbit are currently known:

High inclination TNOs with smaller semi-major axis orbits

A population of high inclination trans-Neptunian objects with semi-major axes less than 100 AU may be generated by the combined effects of Planet Nine and the other giant planets. The extreme trans-Neptunian objects that enter perpendicular orbits have perihelia low enough for their orbits to intersect those of Neptune or the other giant planets. Encounters with one of these planets can lower their semi-major axes to below 100 AU where their evolution would no longer be controlled by Planet Nine, leaving them on orbits like 2008 KV42. The orbital distribution of the longest lived of these objects is nonuniform. Most objects have orbits with perihelia ranging from 5 AU to 35 AU and inclinations below 110 degree, beyond a gap with few objects are others with inclinations near 150 degrees and perihelia near 10 AU. Previously it was proposed that these objects originated in the Oort cloud.

Solar obliquity

Analyses conducted contemporarily and independently by Bailey, Batygin and Brown; by Gomes, Deienno and Morbidelli; and later by Lai suggest that Planet Nine could be responsible for inducing the spin-orbit misalignment of the Solar System. The Sun's axis of rotation is tilted approximately six degrees from the orbital plane of the giant planets. The exact reason for this discrepancy remains an open question in astronomy. The analyses used analytical models and computer simulations to show that both the magnitude and direction of tilt can be explained by the gravitational torques exerted by Planet Nine. The torques would cause the orbits of the other planets to precess, similar to but slower than the eTNOs, covering short arcs over the lifetime of the Solar System. These observations are consistent with the Planet Nine hypothesis, but do not prove that Planet Nine exists, as there are other potential explanations, for the spin-orbit misalignment of the Solar System.

Oort cloud and comets

Numerical simulations of the migration of the giant planets show that the number of objects captured in the Oort cloud is reduced if Planet Nine was in its predicted orbit at that time. This reduction of objects captured in the Oort cloud also occurred in simulations with the giant planets on their current orbits.

The inclination distribution of Jupiter-family (or ecliptic) comets would become broader under the influence of Planet Nine. Jupiter-family comets originate primarily from the scattering objects, trans-Neptunian objects with semi-major axes that vary over time due to distant encounters with Neptune. In a model including Planet Nine, the scattering objects that reach large semi-major axes dynamically interact with Planet Nine, increasing their inclinations. As a result, the population of the scattering objects, and the population of comets derived from it, is left with a broader inclination distribution. This inclination distribution is broader than is observed, in contrast to a five-planet Nice model without a Planet Nine that can closely match the observed inclination distribution.

In a model including Planet Nine, part of the population of Halley-type comets is derived from the cloud of objects that Planet Nine dynamically controls. This Planet Nine cloud is made up of objects with semi-major axes centered around that of Planet Nine that have had their perihelia raised by the gravitational influence of Planet Nine. The continued dynamical effects of Planet Nine drive oscillations of the perihelia of these objects, delivering some of them into planet-crossing orbits. Encounters with the other planets can then alter their orbits, placing them in low-perihelion orbits where they are observed as comets. The first step of this process is slow, requiring more than 100 million years, compared to comets from the Oort cloud, which can be dropped into low-perihelion orbits in one period. The Planet Nine cloud contributes roughly one-third of the total population of comets, which is similar to that without Planet Nine due to a reduced number of Oort cloud comets.

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Origin

A number of possible origins for Planet Nine have been examined including its ejection from the neighborhood of the current 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. Gravitational interactions with nearby stars in the Sun's birth cluster, or dynamical friction from the gaseous remnants of the Solar nebula, then reduced the eccentricity of its orbit, raising its perihelion, leaving it on a very wide but stable orbit. Had it 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. Instead, its growth was halted early, leaving it with a lower mass of five times Earth's mass, similar to that of Uranus and Neptune. For Planet Nine to have been captured in a distant, stable orbit, its ejection must have occurred early, between three million and ten million years after the formation of the Solar System. This timing suggests that Planet Nine is not the planet ejected in a five-planet version of the Nice model, unless that occurred too early to be the cause of the Late Heavy Bombardment, which would then require another explanation. These ejections, however, are likely to have been two events well separated in time.

Dynamical friction from a massive belt of planetesimals could also enable Planet Nine's capture in a stable orbit. Recent models propose that a 60-130 Earth mass disk of planetesimals could have formed via streaming instabilities following the photoevaporation of the outer parts of the proto-planetary disk. If the disk had a distant inner edge, 100-200 AU, a planet encountering Neptune would have a 20% chance of being captured in an orbit similar to that proposed for Planet Nine. The observed clustering is more likely if the inner edge is at 200 AU. Unlike the gas nebula the planetesimal disk is likely to be long lived, potentially allowing a later capture.

Planet Nine could have been captured from beyond the Solar System during a close encounter between the Sun and another star in its birth cluster. Three-body interactions during these encounters can perturb the path of planets on distant orbits around another star, or free-floating planets, leaving one in a stable orbit around the Sun via a process similar to the capture of irregular satellites around the giant planets. If the planet originated in a system with a number of Neptune-massed planets, and without Jupiter-massed planets, it could be scattered into a more long-lasting distant eccentric orbit, increasing its chances of capture. Although the odds of the Sun capturing another planet from another star can be higher, a wider variety of orbits are possible, reducing the probability of a planet being captured on an orbit like that proposed for Planet Nine to 1-2 percent. In simulations where the planets orbiting the Sun and the other star are in the same plane a large number of other objects are also captured into orbits aligned with the planet, potentially allowing this capture scenario to be distinguished from others. The likelihood of the capture of a free-floating planet is much smaller, with only 5-10 of 10,000 simulated free-floating planets being captured on orbits similar to that proposed for Planet Nine.

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, or the outward drift of solids in a dissipating disk forming a narrow ring from which the planet accreted over a billion years. If a planet formed at such a great distance while the Sun was in its birth cluster, the probability of it remaining bound to the Sun in a highly eccentric orbit is roughly 10%. A previous article reported that a massive disk extending beyond 80 AU would drive Kozai oscillations of objects scattered outward by Jupiter and Saturn, leaving some of them in high inclination (inc > 50°), low eccentricity orbits which have not been observed.

Ethan Siegel, who is deeply skeptical of the existence of an undiscovered planet in the Solar System, nevertheless speculates that at least one super-Earth, which have been commonly discovered in other planetary systems but have not been discovered in the Solar System, might have been ejected from the Solar System during a dynamical instability in the early Solar System. Hal Levison thinks that the chance of an ejected object ending up in the inner Oort cloud is only about 2%, and speculates that many objects must have been thrown past the Oort cloud if one has entered a stable orbit.

Astronomers expect that the discovery of Planet Nine would aid in understanding the processes behind the formation of the Solar System and other planetary systems, as well as how unusual the Solar System is, with a lack of planets with masses between that of Earth and that of Neptune, compared to other planetary systems.

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Alternate hypotheses

Temporary or coincidental nature of clustering

Simulations of 15 known extreme trans-Neptunian objects evolving under the influence of Planet Nine revealed a number of differences with observations. Cory Shankman et al. simulated clones (objects with similar orbits) of 15 objects with semi-major axis > 150 AU and perihelion > 30 AU under the influence of a 10 Earth-massed Planet Nine in Batygin and Brown's proposed orbit. While longitude of perihelion alignment of the objects with semi-major axis > 250 AU was observed in their simulations, the alignment of the arguments of perihelion was not. The simulations also revealed an increase in the inclinations of many objects, thereby predicting a larger reservoir of high-inclination TNOs that has not been observed. A previously published article concluded that current observations are insufficient to determine if this reservoir exists, however. The perihelia of the objects also rose and fell smoothly, in contrast with the observed absence of extreme TNOs with perihelia between 50 AU and 70 AU. Their perihelia also reached values where the objects would not be observed and, after declining, fell low enough for the objects to enter planet-crossing orbits leading to their ejection from the Solar System. These factors would require a population of Sednas significantly larger than current estimates, and inconsistent with current models of the early Solar System, to explain current observations. Based on these challenges Shankman et al. concluded that the existence of Planet Nine is unlikely and that the currently observed alignment of the existing TNOs is a temporary phenomenon that will disappear as more objects are detected.

The results of the Outer Solar System Survey (OSSOS) suggests that the observed clustering is the result of a combination of observing bias and small number statistics. OSSOS, a well-characterized survey of the outer Solar System with known biases, observed eight trans-Neptunian objects with semi-major axis > 150 AU with orbits oriented on a wide range of directions. After accounting for the known observational biases of the survey, no evidence for the arguments of perihelion (?) clustering identified by Trujillo and Sheppard was seen and the orientation of the orbits of the objects with the largest semi-major axis was statistically consistent with random. A previously released article by Mike Brown analyzed the discovery locations of eccentric trans-Neptunian objects. While identifying some biases he found that even with these biases the clustering of longitudes of perihelion of the known objects would be observed only 1.2% of the time if their actual distribution was uniform.

Inclination instability due to mass of undetected objects

Ann-Marie Madigan and Michael McCourt postulate that an inclination instability in a distant massive belt is responsible for the alignment of the arguments of perihelion of the ETNOs. The inclination instability occurs in a disk of particles in eccentric orbits around a massive object. The self-gravity of this disk causes 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. This process requires an extended time and significant mass of the disk, on the order of a billion years for a 1-10 Earth-mass disk. While an inclination instability can align the arguments of perihelion and raise perihelia, producing detached objects, it does not align the longitudes of perihelion. Mike Brown considers Planet Nine a more probable explanation, noting that current surveys do not support the existence of a scattered-disk region of sufficient mass to support this idea of "inclination instability". In Nice model simulations that included the self-gravity of the planetesimal disk an inclination instability did not occur due to a rapid precession of the objects' orbits and their being ejected on too short of a timescale.

Object in lower-eccentricity orbit

Renu Malhotra, Kathryn Volk, and Xianyu Wang have proposed that the four detached objects with the longest orbital periods, those with perihelia beyond 40 AU and semi-major axes greater than 250 AU, are in n:1 or n:2 mean-motion resonances with a hypothetical planet. Two other objects with semi-major axes greater than 150 AU are also potentially in resonance with this planet. Their proposed planet could be on a lower eccentricity, low inclination orbit, with eccentricity e < 0.18 and inclination i ? 11°. The eccentricity is limited in this case by the requirement that close approaches of 2010 GB174 to the planet are avoided. If the ETNOs are in periodic orbits of the third kind, with their stability enhanced by the libration of their arguments of perihelion, the planet could be in a higher inclination orbit, with i ? 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.

Alignment due to the Kozai mechanism

Trujillo and Sheppard (2014)

Astronomers Chad Trujillo and Scott S. Sheppard argued in 2014 that a massive planet in a distant, circular orbit was responsible for the clustering of the arguments of perihelion of twelve extreme trans-Neptunian objects. Trujillo and Sheppard identified a clustering near zero degrees of the arguments of perihelion of the orbits of twelve trans-Neptunian objects (TNOs) with perihelia greater than 30 AU and semi-major axes greater than 150 AU. After numerical simulations showed that after billions of years the varied rates of precession should leave their perihelia randomized they suggested that a massive planet in a circular orbit at a few hundred astronomical units was responsible for this clustering. This massive planet would cause the arguments of perihelion of the eTNOs 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. In numerical simulations including a 2-15 Earth mass body in a circular low-inclination orbit between 200 AU and 300 AU the arguments of perihelia of Sedna and 2012 VP113 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. An additional process such as a passing star would be required to account for the absence of objects with arguments of perihelion near 180°.

These simulations showed the basic idea of how a single large planet can shepherd the smaller extreme trans-Neptunian objects into similar types of orbits. It was a basic proof of concept simulation that did not obtain a unique orbit for the planet as they state there are many possible orbital configurations the planet could have. Thus they did not fully formulate a model that successfully incorporated all the clustering of the extreme objects with an orbit for the planet. But they were the first to notice there was a clustering in the orbits of extremely distant objects 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.

de la Fuente Marcos et al. (2014)

Raúl and Carlos de la Fuente Marcos proposed a similar model but with two distant planets in resonance. An analysis by Carlos and Raúl de la Fuente Marcos with 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 200 AU from the Sun. Like Trujillo and Sheppard they theorized that the eTNOs are kept bunched together by a Kozai mechanism and compared their behavior to that of Comet 96P/Machholz under the influence of Jupiter. However, they also struggled to explain the orbital alignment using a model with only one unknown planet. They therefore suggested that this planet is itself in resonance with a more-massive world about 250 AU from the Sun. 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.

Previous models with additional planets

Attempts to detect planets beyond Neptune by indirect means such as orbital perturbation date back beyond the discovery of Pluto. A few observations were directly related to the Planet Nine hypothesis:

• George Forbes was the first to postulate the existence of trans-Neptunian planets in 1880, and his work is considered similar to more recent Planet Nine theories. In Forbes's model the planet had a semi-major axis of ~300 AU (roughly three hundred times the distance from Earth to the Sun); locations were based on clustering of the aphelion distances of periodic comets.
• The discovery of Sedna with its peculiar orbit in 2004 led to the conclusion that something beyond the known eight planets had perturbed Sedna away from the Kuiper belt. That object could have been an unknown planet on a distant orbit, a random star that passed near the Solar System, or a member of the Sun's birth cluster. The announcement in March 2014 of the discovery of a second sednoid, 2012 VP113, which shared some orbital characteristics with Sedna and other extreme trans-Neptunian objects, further raised the possibility of an unseen super-Earth in a large orbit.
• In 2008 Tadashi Mukai and Patryk Sofia Lykawka suggested that a distant Mars- or Earth-sized minor planet currently in a highly eccentric orbit between 100 and 200 AU and orbital period of 1000 years with an inclination of 20° to 40° was responsible for the structure of the Kuiper belt. They proposed that the perturbations of this planet excited the eccentricities and inclinations of the trans-Neptunian objects, truncated the planetesimal disk at 48 AU, and detached the orbits of objects like Sedna from Neptune. During Neptune's migration this planet is posited to have been captured in an outer resonance of Neptune and to have evolved into a higher perihelion orbit due to the Kozai mechanism leaving the remaining trans-Neptunian objects on stable orbits.
• In 2012, after analysing the orbits of a group of trans-Neptunian objects with highly elongated orbits, Rodney Gomes of the National Observatory of Brazil proposed that their orbits were due to the existence of an as yet undetected planet. This Neptune-massed planet would be on a distant orbit that would be too far away to influence the motions of the inner planets, yet close enough to cause the perihelia of scattered disc objects with semi-major axes greater than 300 AU to oscillate, delivering them into planet-crossing orbits similar to those of (308933) 2006 SQ372 and (87269) 2000 OO67 or detached orbits like that of Sedna. Alternatively the unusual orbits of these objects could be the result of a Mars-massed planet on an eccentric orbit that occasionally approached within 33 AU. Gomes argued that a new planet was the more probable of the possible explanations but others felt that he could not show real evidence that suggested a new planet. Later in 2015, Rodney Gomes, Jean Soares, and Ramon Brasser proposed that a distant planet was responsible for an excess of centaurs with large semi-major axes.

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Searches for Planet Nine

Visibility and location

Due to its extreme distance from the Sun, Planet Nine would reflect little sunlight, potentially evading telescope sightings. It is expected to have an apparent magnitude fainter than 22, making it at least 600 times fainter than Pluto. If Planet Nine exists and is close to its perihelion, astronomers could identify it based on existing images. For its aphelion, the largest telescopes would be required. However, if the planet is currently located in between, many observatories could spot Planet Nine. Statistically, the planet is more likely to be closer to its aphelion at a distance greater than 500 AU. This is because objects move more slowly when near their aphelion, in accordance with Kepler's second law.

Searches of existing data

The search in databases of stellar objects performed by Batygin and Brown has already excluded much of the sky the predicted planet could be in, save the direction of its aphelion, or in the difficult to spot backgrounds where the orbit crosses the plane of the Milky Way, where most stars lie. This search included the archival data from the Catalina Sky Survey to magnitude c. 19, Pan-STARRS to magnitude 21.5, and infrared data from WISE.

David Gerdes who helped develop the camera used in the Dark Energy Survey claims that it is quite possible that one of the images taken for his galaxy map may actually contain a picture of Planet Nine, and if so, purpose-built software, which was used to identify objects such as 2014 UZ224, can help to find it.

Michael Medford and Danny Goldstein, graduate students at the University of California, Berkeley, are also examining archived data using a technique that combines multiple 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.

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 ?m wavelength used by WISE) and is estimated to be able to detect a 10 Earth mass object out to 800-900 AU.

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 aperture large enough to see faint objects and a wide field of view to shorten the search. Two teams of astronomers--Batygin and Brown, as well as Trujillo and Sheppard--are undertaking this search together, and both teams cooperatively expect the search to take up to five years. Brown and Batygin initially narrowed the search for Planet Nine down to roughly 2,000 square degrees of sky near Orion, a swath of space, that in Batygin's opinion, could be covered in about 20 nights by the Subaru Telescope. Subsequent refinements by Batygin and Brown have reduced the search space to 600-800 square degrees of sky.

A zone around the constellation Cetus, where Cassini data suggest Planet Nine may be located, is being searched as of 2016 by the Dark Energy Survey--a project in the Southern Hemisphere designed to probe the acceleration of the Universe. DES observes about 105 nights per season, lasting from August to February.

Radiation

Although a distant planet such as Planet Nine would reflect little light, it would still be radiating the heat from its formation as it cools due to its large mass. At its estimated temperature of 47 K (-226.2 °C), the peak of its emissions would be at infrared wavelengths. This radiation signature could be detected by Earth-based infrared telescopes, such as ALMA, and a search could be conducted by cosmic microwave background experiments operating at mm wavelengths. Additionally, Jim Green of NASA is optimistic that it could be observed by the James Webb Space Telescope, the successor to the Hubble Space Telescope, that is expected to be launched in 2020.

Citizen science

Zooniverse Backyard Worlds: Planet 9 project

The Zooniverse Backyard Worlds project, started in February 2017, is using archival data from the WISE spacecraft to search for Planet Nine. The project will additionally search for substellar objects like brown dwarfs in the neighborhood of the Solar System. 32,000 animations of four images each, which constitute 3 per cent of the WISE data has been uploaded to the Backyard World's website. By looking for moving objects in the animations, citizen scientists could find Planet Nine.

Zooniverse SkyMapper project

In April 2017, using data from the SkyMapper telescope at Siding Spring Observatory, citizen scientists on the Zooniverse platform reported four candidates for Planet Nine. These candidates will be followed up on by astronomers to determine their viability. The project, which started on 28 March, completed their goals in less than three days with around five million classifications by more than 60,000 individuals.

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Searches for additional extreme trans-Neptunian objects

Finding more objects would allow astronomers to make more accurate predictions about the orbit of the hypothesized planet. The Large Synoptic Survey Telescope, when it is completed in 2023, will be able to map the entire sky in just a few nights, providing more data on distant Kuiper belt objects that could both bolster evidence for Planet Nine and help pinpoint its current location.

Extreme trans-Neptunian objects discovered by Trujillo and Sheppard include:

• 2013 FT28, located on the opposite side of the sky (Longitude of perihelion aligned with Planet Nine) - but well within the proposed orbit of Planet Nine, where computer modeling suggests it would be safe from gravitational kicks.
• 2014 SR349, falling right in line with the earlier six objects.
• 2014 FE72, an object with an orbit so extreme that it reaches about 3,000 AU from the Sun in a massively-elongated ellipse - at this distance its orbit is influenced by the galactic tide and other stars.

Other extreme trans-Neptunian objects discovered by the Outer Solar System Origins Survey include:

• 2013 SY99, which has a lower inclination than many of the objects, and which was discussed by Michele Bannister at a March 2016 lecture hosted by the SETI Institute and later at an October 2016 AAS conference.
• 2015 KG163, which has an orientation similar to 2013 FT₂₈ but has a larger semi-major axis that may result in its orbit crossing Planet Nine's.
• 2015 RX245, which fits with the other anti-aligned objects.
• 2015 GT50, which is in neither the anti-aligned nor the aligned groups; instead, its orbit's orientation is at a right angle to that of the proposed Planet Nine. Its argument of perihelion is also outside the cluster of arguments of perihelion.

Batygin and Brown also predict a yet-to-be-discovered population of distant objects. These objects would have semi-major axes greater than 250 AU, but they would have lower eccentricities and orbits that would be aligned with that of Planet Nine. The larger perihelia of these objects would make them fainter and more difficult to detect than the anti-aligned objects.

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Efforts toward indirect detection

Cassini measurements of perturbations of Saturn

An analysis of Cassini data on Saturn's orbital residuals was inconsistent with Planet Nine being located with a true anomaly 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 117.8°+11°
-10°. At this location, Planet Nine would be approximately 630 AU from the Sun, with right ascension close to 2^h and declination close to -20°, in Cetus. In contrast, if the putative planet is near aphelion it could be moving projected towards the area of the sky with boundaries: right ascension 3.0^h to 5.5^h and declination -1° to 6°.

An improved mathematical 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.

The Jet Propulsion Laboratory has stated that according to their mission managers and orbit determination experts, the Cassini spacecraft is not experiencing unexplained deviations in its orbit around Saturn. William Folkner, a planetary scientist at JPL stated, "An undiscovered planet outside the orbit of Neptune, 10 times the mass of Earth, would affect the orbit of Saturn, not Cassini ... This could produce a signature in the measurements of Cassini while in orbit about Saturn if the planet was close enough to the Sun. But we do not see any unexplained signature above the level of the measurement noise in Cassini data taken from 2004 to 2016." Observations of Saturn's orbit neither prove nor disprove that Planet Nine exists. Rather, they 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, inconsistent with actual observations.

Analysis of Pluto's orbit

An analysis of Pluto's orbit by Matthew J. Holman and Matthew J. 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 60-100 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.

Optimal orbit if objects are in strong resonances

An analysis by Sarah Millholland and Gregory Laughlin indicates that the commensurabilities (period ratios consistent with pairs of objects in resonance with each other) of the extreme TNOs are most likely to occur if Planet Nine has a semi-major axis of 654 AU. They used 11 then-known extreme TNOs with their semi-major axis over 200, and perihelion over 30 AU [1], with five bodies close to four simple ratios (5:1, 4:1, 3:1, 3:2) with a 654 AU distance: 2002 GB32, 2000 CR105 (5:1), 2001 FP185 (5:1), 2012 VP113 (4:1), 2014 SR349, 2013 FT28, 2004 VN112 (3:1), 2013 RF98, 2010 GB174, 2007 TG422, and (90377) Sedna (3:2). Beginning with this semi-major axis they determine that Planet Nine best maintains the anti-alignment of their orbits and a strong clustering of arguments of perihelion if it is near aphelion and has an eccentricity e ? 0.5, inclination i ? 30°, argument of perihelion ? ? 150°, and longitude of ascending node ? ? 50° (the last differs from Brown and Batygin's value of 90°). The favored location of Planet Nine is a right ascension of 30° to 50° and a declination of -20° to 20°. They also note that in their simulations the clustering of arguments of perihelion is almost always smaller than has been observed.

A previous analysis by Carlos and Raul de la Fuente Marcos of commensurabilities among the known ETNOs using Monte Carlo techniques revealed a pattern similar to that of the Kuiper belt, where accidental commensurabilities occur due to objects in resonances with Neptune. They find that this pattern would be best explained if the ETNOs were in resonance with an additional planetary-sized object beyond Pluto and note that a number of these objects may be in 5:3 and 3:1 resonances if that object had semi-major axis of ?700 AU.

Ascending nodes of large semi-major axis objects

In an article by Carlos and Raul de la Fuente Marcos evidence is shown for a possible bimodal distribution of the distances to the ascending nodes of the ETNOs. This correlation is unlikely to be the result of observational bias since it also appears in the nodal distribution of large semi-major axis centaurs and comets. If it is due to the extreme TNOs experiencing close approaches to Planet Nine, it is consistent with a planet with a semi-major axis of 300-400 AU.

Orbits of nearly parabolic comets

An analysis of the orbits of comets with nearly parabolic orbits identifies five new comets with hyperbolic orbits 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°. Scott Sheppard, who is skeptical of this analysis, notes that many different forces influence the orbits of comets.

Possible disrupted binary

Similarities between the orbits of 2013 RF98 and (474640) 2004 VN112 have led to the suggestion that they were a binary object disrupted near aphelion during an encounter with a distant object. The visible spectra of (474640) 2004 VN₁₁₂ and 2013 RF₉₈ are also similar but very different from that of 90377 Sedna. The value of their spectral slopes suggests that the surfaces of (474640) 2004 VN₁₁₂ and 2013 RF₉₈ can have pure methane ices (like in the case of Pluto) and highly processed carbons, including some amorphous silicates. The disruption of a binary would require a relatively close encounter with Planet Nine, however, which becomes less likely at large distances from the Sun.

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Commentary

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." Batygin's and Brown's work is similar to how Urbain Le Verrier predicted the position of Neptune based on Alexis Bouvard's observations and theory of Uranus' peculiar motion.

Brown put the odds for the existence of Planet Nine at about 90%. Greg Laughlin, one of the few researchers who knew in advance about this article, gives an estimate of 68.3%. Other skeptical scientists demand more data in terms of additional KBOs to be analysed or final evidence through photographic confirmation. Brown, though conceding the skeptics' point, still thinks that there is enough data to mount a search for a new planet.

Brown is supported by Jim Green, director of NASA's Planetary Science Division, who said that "the evidence is stronger now than it's ever been before".

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. 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."

Malhotra remains agnostic about Planet Nine, but noted that she and her colleagues have found that the orbits of extremely distant KBOs 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."

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See also

• History of Solar System formation and evolution hypotheses
• Hypothetical planets of the Solar System
• Kuiper belt
• (471325) 2011 KT19
• Nemesis (hypothetical star)
• Planets beyond Neptune
• Trans-Neptunian object
• Tyche (hypothetical planet)
• Discovery of Neptune

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Notes

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References

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Further reading

• Calandrelli, Emily; Escher, Anna (16 December 2016). "The top 15 events that happened in space in 2016". TechCrunch. Astronomers find evidence for Planet 9. Retrieved 16 December 2016. 

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External links

• A New Planet in our Solar System? NASA Takes a Look (NASA video, 21 January 2016)
• A new 9th planet for the solar system? (Science Magazine video, 20 January 2016)
• The Search for Planet Nine - blog by study authors
• A summary of the history behind the search and claims for a ninth planet
• Could You Live on Planet Nine? - Science article by Rhett Allain
• 'Planet Nine': Facts About the Mysterious Solar System World (space.com infographic)
• Planet Nine May Help Us Slingshot Our Way to the Stars
• Is Planet Nine truly "discovered"? A brief history of discoveries before and leading-up to the predicted planet
• Solar System Survey Casts Doubt on Mysterious "Planet Nine" Nature 23 June 2017
• Is There a Giant Planet Lurking Beyond Pluto? A race is on to discover Planet Nine using classical astronomy and new computational techniques IEEE Spectrum 31 July 2017 - a good summary as of publication date

Source of the article : Wikipedia