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Planet Nine is a hypothetical large planet in the far outer Solar System, the gravitational effects of which would explain the improbable orbital configuration of a group of trans-Neptunian objects (TNOs) that orbit mostly beyond the Kuiper belt.[1][4][5]

Planet Nine[1]
Planet nine artistic plain.png
Artist's impression of Planet Nine as an ice giant eclipsing the central Milky Way, with the Sun in the distance.[2] Neptune's orbit is shown as a small ellipse around the Sun. (See labeled version.)
Orbital characteristics
Aphelion 1,200 AU (est.)[2]
Perihelion 200 AU (est.)[3]
700 AU (est.)[1]
Eccentricity 0.6 (est.)[3]
10,000 to 20,000 years[3]
Inclination 30° to ecliptic (est.)[1]
150° (est.)[1]
Physical characteristics
Mean radius
13,000 to 26,000 km (8,000–16,000 mi)
2–4 R (est.)[3]
Mass 6×1025 kg (est.)[3]
≥10 M (est.)
>22.5 (est.)[2]

Recent speculation about the alignment of extreme TNOs being due to a distant massive planet began with a 2014 letter to the journal Nature, in which astronomers Chad Trujillo and Scott S. Sheppard inferred the possible existence of a massive trans-Neptunian planet from similarities in the orbits of the distant trans-Neptunian objects Sedna and 2012 VP113.[4] On 20 January 2016, researchers Konstantin Batygin and Michael E. Brown at Caltech explained how a massive outer planet would be the likeliest explanation for the similarities in orbits of six distant objects, and proposed specific orbital parameters.[1] The predicted planet could be a super-Earth, with an estimated mass of 10 Earths (approximately 5,000 times the mass of Pluto), a diameter two to four times that of Earth, and a highly elliptical orbit with an orbital period of approximately 15,000 years.[6]

In addition to the clustering of the perihelia and the orbital poles of distant objects, Planet Nine offers explanations for the high perihelion of Sedna and 2012 VP113, for objects with orbits roughly perpendicular to those of the planets,[1] for high inclination TNOs with semi-major axes less than 100 AU,[7] and for the tilt of the Sun's rotation axis.[8] The objects it dynamically controls would form a cloud centered on its semi-major axis with a wide range of inclinations.[9]

Batygin and Brown suggested Planet Nine was a primordial giant planet core that was ejected from its initial orbit by an encounter with Jupiter during the nebular epoch of the Solar System and was later perturbed into a stable orbit by a distant encounter with a passing star or by the solar nebula.[1] Others have proposed that it was captured in a similar manner from another star,[10] or that it formed on a very distant circular orbit and was perturbed into its current eccentric orbit during a distant encounter with another star.[11][12]

Contents

NamingEdit

Planet Nine does not have an official name, and it will not receive one unless its existence is confirmed, typically through optical imaging. Once confirmed, the IAU will certify a name, with priority typically given to a name proposed by its discoverers.[13] It will likely be a name chosen from Roman or Greek mythology.[14]

In their original article, Batygin and Brown simply referred to the object as "perturber",[1] and only in later press releases did they use "Planet Nine".[15] They have also used the names "Jehoshaphat" and "George" for Planet Nine. Brown has stated: "We actually call it Phattie[A] when we're just talking to each other."[5]

Postulated characteristicsEdit

 
One hypothetical path through the sky of Planet Nine near aphelion crossing Orion west to east with about 2,000 years of motion. It is derived from that employed in the artistic conception on Brown's blog.[2]

OrbitEdit

Planet Nine is hypothesized to follow a highly elliptical orbit around the Sun, with an orbital period of 10,000–20,000 years. The period with which it goes around the Sun is a rational multiple of the periods of all the furthest Kuiper belt objects.[17] The planet's orbit would have a semi-major axis of approximately 700 AU, or about 20 times the distance from Neptune to the Sun, although it might come as close as 200 AU (30 billion km, 19 billion mi), and its inclination is estimated to be roughly 30°±10°.[2][3][18][B] The high eccentricity of Planet Nine's orbit could take it as far away as 1,200 AU at its aphelion.[19][20]

The aphelion, or farthest point from the Sun, would be in the general direction of the constellation of Taurus,[21] 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.[22][23]

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.[24]

Size and compositionEdit

 
Planet Nine is hypothesized to be two to four times the diameter of Earth;[2][6] similar to the ice giants Uranus and Neptune.[25]

The planet is estimated to have 10 times the mass[16][18] and two to four times the diameter of Earth.[6][26] An object with the same diameter as Neptune has not been excluded by previous surveys. An infrared survey by the Wide-field Infrared Survey Explorer (WISE) in 2009 allowed for a Neptune-sized object beyond 700 AU.[27] A similar study in 2014 focused on possible higher-mass bodies in the outer Solar System and ruled out Jupiter-mass objects out to 26,000 AU.[28]

Brown thinks that no matter where it is speculated to be, if Planet Nine exists, then its mass is higher than what is required to clear its feeding zone in 4.6 billion years, and thus that it dominates the outer edge of the Solar System, which is sufficient to make it a planet by current definitions.[29] Using a metric based on work by Jean-Luc Margot, Brown calculated that only at the smallest size and farthest distance was it on the border of being called a dwarf planet.[29] Margot himself says that Planet Nine satisfies the quantitative criterion for orbit-clearing developed by him in 2015, and that according to that criterion, Planet Nine will qualify as a planet—if and when it is detected.[30]

Brown speculates that the predicted planet is most likely an ejected ice giant, similar in composition to Uranus and Neptune: a mixture of rock and ice with a small envelope of gas.[2][6] Based on its size, the considerable gravitational pull of Planet Nine could theoretically promote life in subsurface oceans with liquid water in nearby moons. Subsurface oceans have been discovered on the moons Enceladus (water), Dione (possibly water), and subsurface water is postulated for Neptune's largest moon Triton, and possibly also found on Europa.[31]

Dynamical effectsEdit

The dynamical effects of Planet Nine would explain five aspects 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 plane of the Solar System, high inclination Kuiper belt objects with semi-major axis less than 100 AU, and the obliquity, or tilt, of the Sun's axis relative to the plane of the Solar System.[32] 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 dynamical effects of Planet Nine also excite the inclinations of scattering objects, which in numerical simulations leaves the inclination distribution of short-period comets with a broader distribution than is observed.[9]

Orbital clustering and high perihelion objectsEdit

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 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.[4] (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.[1] 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 argument of perihelion for the remaining six objects (namely Sedna, 2012 VP113, 2004 VN112, 2010 GB174, 2010 VZ98, and 2000 CR105) were 318°±. This was out of alignment with how the Kozai mechanism would align these orbits, at c. 0° or 180°.[1][C]

 
Orbital correlations among six distant trans-Neptunian objects led to the hypothesis. (See: Final frame orbits)

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, 2007 TG422, 2004 VN112, 2013 RF98, and 2010 GB174) 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.[1][15][34][35]

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. These six were the only minor planets known to have perihelia greater than 30 AU and a semi-major axis greater than 250 AU, as of January 2016.[36] Most have perihelia significantly beyond Neptune, which orbits 30 AU from the Sun.[16][37] Generally, TNOs with perihelia smaller than 36 AU experience strong encounters with Neptune.[1] All six objects are relatively small, but currently relatively bright because they are near their closest distance to the Sun in their elliptical orbits.

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.[38][39][40]

A companion article by Brown and Batygin extended the cluster to include a seventh object, 2000 CR105, with a semi-major axis of 227 AU; and noted the absence of objects with perihelia beyond 42 AU and semi-major axes between 100 AU and 200 AU.[41]

Extreme trans-Neptunian objectsEdit

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. These are also included in the orbital diagrams and tables below.

The Extreme Trans-Neptunian object orbits
 
6 original and 7 new TNO object orbits with current positions near their perihelion in purple, with hypothetical Planet Nine orbit in green
 
Closer up view of the 13 TNO current positions
Extreme Trans-Neptunian objects with perihelion greater than 30 AU and a semi-major axis greater than 250 AU[36][not in citation given]
Object Orbit orbital plane Body
Barycentric[D]
Orbital
period

(years)
Barycentric
Semimaj.
(AU)
Peri.
(AU)
Barycentric
Aphel.
(AU)
Curr.
dist.
from
Sun
(AU)
Ecc. Arg.
peri

ω (°)
incl.
i (°)
Long.
asc

☊ or Ω (°)
Long.
peri

ϖ=ω+Ω (°)
Hv Current
mag.
Diam.
(km)
2012 VP113 4,300 266 80.27 441 83.5 0.69 292.8 24.1 90.8 23.6 4.0 23.3 600
2013 RF98 6,900 364 36.10 690 36.8 0.90 311.8 29.6 67.6 19.4 8.7 24.4 70
2004 VN112 5,900 327 47.32 607 47.7 0.85 327.1 25.6 66.0 33.1 6.5 23.3 200
2007 TG422 11,300 503 35.57 970 37.3 0.93 285.7 18.6 112.9 38.6 6.2 22.0 200
90377 Sedna 11,400 507 76.04 936 85.5 0.85 311.5 11.9 144.5 96.0 1.5 20.9 1,000
2010 GB174 6,600 351 48.76 654 71.2 0.87 347.8 21.5 130.6 118.4 6.5 25.1 200
2013 FT28 5,050 295 43.60 546 57.0 0.86 40.2 17.3 217.8 258.0 (*) 6.7 24.4 200
2014 FE72 58,000 1,500 36.31 2,960 61.5 0.98 134.4 20.6 336.8 111.2 6.1 24.0 200
2014 SR349 5,160 299 47.57 549 56.3 0.84 341.4 18.0 34.8 16.2 6.6 24.2 200
2013 SY99 19,700 730 49.91 1,410 60.3 0.93 32.4 4.2 29.5 61.9 6.7 24.5 250
2015 GT50 5510 310 38.45 580 41.7 0.89 129.2 8.8 46.1 175.3 (*) 8.5 24.9 80
2015 RX245 8,920 430 45.48 815 61.4 0.89 65.4 12.2 8.6 74.0 6.2 24.2 250
2015 KG163 17,730 680 40.51 1,320 40.8 0.95 32.0 14.0 219.1 251.1 (*) 8.1 24.3 100
Ideal range for ETNOs
under the hypothesis
>250 >30 >0.5 10~30 2~120
Hypothesized
Planet Nine
~15,000 ~700 ~200 ~1,200 ~1,000? ~0.6 ~150 ~30 91±15 241±15 >22 ~40,000

Numerical simulations of extreme TNOsEdit

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 planetesimals that began with large semi-major axis orbits at random orientations and initially interacting with Neptune were sculpted into collinear groups of spatially confined orbits by a distant planet if it is substantially more massive than Earth and on a highly eccentric orbit. These surviving objects 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.[1] 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.[44] Some of the eTNOs also evolved into orbits perpendicular to the plane of the Solar System.[1]

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[E] planet in the following orbit:

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.[41] 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.[46]

Numerical simulations including a distant massive planet were also conducted by other groups. Lawler et al. found that the dynamical effect of Planet Nine would generate a cloud of objects centered around Planet Nine's semi-major axis, with most at greater semi-major axes. A massive planet in a circular orbit at 250 AU increased the number of high perihelion objects threefold; one at 500 AU with an eccentricity of 0.5 increased these tenfold, and left a significant fraction with inclinations greater than 60°.[47] Simulations conducted by Nesvorny et al. of the giant planet migration described in the Nice model that included Planet Nine in its nominal orbit yield similar results, with roughly 0.3–0.4 Earth masses of an initial 20 Earth mass planetesimal disk remaining in what they called the Planet Nine cloud at the end of a simulation of the last 4 billion years.[9]

Dynamics of extreme TNOsEdit

Planet Nine modifies the orbits of extreme trans-Neptunian objects via a combination of secular and resonant dynamics. The secular dynamics acts on very long timescales via exchanges of angular momentum and causes the distances of the perihelia of anti-aligned objects to rise and fall while the directions of their perihelia oscillate. The resonant dynamics acts on shorter timescales and 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.[48]

Secular dynamicsEdit

 
Secular evolution of eTNOs induced by Planet Nine for objects with semi-major axis of 250 AU.[49][48] Blue:anti-aligned, Red:aligned, Green:metastable, Orange:circulating. Crossing orbits above black line.

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.[50][51]

Without the influence of Planet Nine the perihelia of eTNOs would precess at rates that varied with their semi-major axes and eccentricities.[G] The directions, or the longitudes of perihelia would circulate, passing through all values ranging from 0° to 360°, and any initial alignment would be smeared out by their varied precession rates. The distance of their perihelia and their eccentricities, in contrast, would remain unchanged.[1]

Planet Nine alters this behavior, causing the longitudes of perihelion of anti-aligned objects to librate, or oscillate within a limited range of values, while their perihelia rise and fall. As 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 an increasingly positive average torque on the object. This torque increases the object's angular momentum,[H] 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 slows and eventually reverses as its eccentricity declines, causing delta longitude of perihelion to descend. After delta longitude of perihelion drops below 180° the object begins to feel a negative average torque causing its angular momentum to decrease, and its eccentricity grows and perihelion falls. When the object's eccentricity is again large, its precession rate increases, causing delta longitude of perihelion to climb toward 180°, returning the object to its original orbit after several hundred million years.[48][50]

The behavior of the orbits of other objects varies with their initial orbits. Stable orbits exist for aligned objects with small eccentricities. Objects in these orbits have high perihelia and have yet to be observed, however, and an additional perturbation would have been required to be captured in these orbits. 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.[51][48] The curves the orbits follow varies with the semi-major axis of the object, with the region where aligned orbits are possible shrinking for larger semi-major axes.[49] Including the resonant dynamics, for example if Sedna is in a 3:2 resonance with Planet Nine as proposed by Malhotra, Volk and Wang,[52] also alters these curves and can shift the centers of the libration islands away from anti-alignment.[49] The dynamics is more complex for inclined objects, 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.[53]

Resonant dynamicsEdit

 
An example of phase-protection in a mean-motion resonance: The orbital resonances of Orcus and Pluto in a rotating frame with a period equal to Neptune's orbital period. (Neptune is held stationary.)

The long term stability of anti-aligned extreme trans-Neptunian objects with orbits that intersect that of Planet Nine is the result of their resonant dynamics. 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,[I] causing it to make closer approaches to a massive planet, the changes in the perturbations the planet exerts on the object alters 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.[54][48] 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,[J] causing its semi-major axis and period to shrink, reversing the drift.[55]

If Planet Nine and the extreme trans-Neptunian objects orbited in the plane of the other planets 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 perihelia of an increasing number of objects could circulate without being ejected,[K] reducing the fraction of objects that are anti-aligned.[48][51] 2015 GT50 may be in one of these circulating orbits.[56]

The dynamics are more complex if Planet Nine and the eTNOs are in inclined orbits, combining extended periods in stable resonances with 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.[48][51]

The phase protection of Planet Nine's resonances also stabilizes the orbits of objects that interact with Neptune, either through its resonances that can affect orbits with semi-major axis as large as 350 AU,[57] for example 2013 FT28;[58] 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.[46] 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.[59][60]

Laplace planeEdit

 
Tilting of Laplace Plane by Planet Nine

The bending of the Laplace plane of the Solar System toward that of Planet Nine's orbit can produce the apparent clustering of the longitude of the ascending nodes and arguments of perihelion of the extreme TNOs. The Laplace plane defines the center about 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 bend toward that of its orbit.[L] As a result when the poles of the eTNO orbit precess about the Laplace plane 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.[48] 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.[61] In combination with the anti-alignment of the longitudes of perihelion this can also produce clustering of the arguments of perihelion.[48]

Objects with perpendicular orbitsEdit

 
The orbits of the five objects with high-inclination orbits (nearly perpendicular to the ecliptic) are shown here as cyan ellipses with the hypothetical Planet Nine in orange. Those of four are towards the left in this view, and that of one (2012 DR30) is towards the right, with an aphelion over 2,000 AU.

Planet Nine can deliver extreme trans-Neptunian objects into orbits roughly perpendicular to the plane of the Solar System.[62][63] Several objects with high inclinations, greater than 60°, and large semi-major axes, above 250 AU, have been observed.[64] 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: Δϖ - 2ω. 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.[48] 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.[53] 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.[1] Six objects with semi-major axes greater than 250 AU and perihelia beyond Jupiter's orbit are currently known:

High-inclination Trans-Neptunian objects with a semi-major axis greater than 250 AU[1][48][65]
Object Orbit Body
Perihelion
(AU)
Figure 9[1]
Semimaj.
(AU)
Figure 9[1]
Current
distance
from Sun
(AU)
inc
(°)[64]
Eccen. Arg. peri ω
(°)
Mag. Diam.
(km)
(336756) 2010 NV1 9.4 323 14 141 0.97 133 22 20–45
(418993) 2009 MS9 11.1 348 12 68 0.97 129 21 30–60
2010 BK118 6.3 484 11 144 0.99 179 21 20–50
2013 BL76 8.5 1,213 11 99 0.99 166 21.6 15–40
2012 DR30 14 1,404 17 78 0.99 195 19.6 185[66]
2014 LM28 16.8 268 17 85 0.94 38 22 46

High inclination TNOsEdit

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. Extreme trans-Neptunian objects that reach high inclination orbits while in the higher order secular resonance can have their perihelia lowered sufficiently for their orbits to intersect that of Neptune or the other giant planets. Encounters with one of these planets can then 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.[67][7][68] Previously it was proposed that these objects originated in the Oort cloud.[69]

Solar obliquityEdit

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 analysis used 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 could be some other reason, or more than one reason, for the spin–orbit misalignment of the Solar System.[8][70][71][72]

Oort cloud and cometsEdit

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.[9] This reduction of objects captured in the Oort cloud also occurred in simulations with the giant planets on their current orbits.[47]

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.[9][73]

In a model including Planet Nine, part of the population of Halley-type comets is derived from the Planet Nine cloud. The dynamical effects of the planet drive oscillations of the perihelia of the objects in the Planet Nine cloud, delivering some of them into planet-crossing orbits. Encounters with the other planets can then alter their orbits placing them on low-perihelion orbits. 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.[9]

OriginEdit

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.[1] Gravitational interactions with nearby stars in the Sun's birth cluster, or dynamical friction from the gaseous remnants of the Solar nebula,[74] then reduced the eccentricity of its orbit, raising its perihelion, leaving it on a very wide but stable orbit.[44][75] 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.[6] 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.[76] 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.[16] 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,[77] which would then require another explanation.[78] These ejections, however, are likely to have been two events well separated in time.[79]

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 gas disk.[80] 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.[81]

Close encounters between the Sun and other stars in its birth cluster could have resulted in the capture of a planet from beyond the Solar System. Three-body interactions during these encounters can perturb the path of planets on distant orbits around another star, or free-floating planets, in a process similar to the capture of irregular satellites around the giant planets, leaving one in a stable orbit around the Sun. A planet that originated in a system with a number of Neptune-massed planets and without Jupiter-massed planets, could be scattered into a more long-lasting distant eccentric orbit, increasing its chances of capture by another star.[10] Although this increases the odds of the Sun capturing another planet from another star, a wide 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.[12] In a simulation where the capture takes place between co-planar systems 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.[82] 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.[83]

A planet could also be perturbed from a distant circular orbit into an eccentric orbit by an encounter with another star. The in situ formation of a planet at this distance would require a very massive and extensive disk,[1] or the outward drift of solids in a dissipating disk forming a narrow ring from which the planet accreted over a billion years.[11] 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%.[12] 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.[84]

Ethan Siegel, who is deeply skeptical of the existence of an undiscovered new 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.[63][85] 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.[86]

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.[87]

Alternate hypothesesEdit

Temporary or coincidental nature of clusteringEdit

Cory Shankman et al. examined the consequences of a distant massive perturber on the TNOs used to infer the planet's existence. They 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 best fit orbit.[M] 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.[N] These objects should have been detected in existing surveys but are still unseen so far—suggesting there is a currently missing or unseen signature of Planet Nine. A previously published article concluded that current observations are insufficient to identify this signature, however.[47] The perihelia of many of the objects also rose and fell smoothly, inconsistent with the current 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.[88][89]

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 (ω) clustering identified by Trujillo and Sheppard was seen[O] and the orientation of the orbits of the objects with the largest semi-major axis was statistically consistent with random.[91][90] 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.[92]

Inclination instability due to mass of undetected objectsEdit

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.[93][P] While an inclination instability can align the arguments of perihelion and raise perihelia, producing detached objects, it does not align the longitudes of perihelion.[92] 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".[94][95]

Object in lower-eccentricity orbitEdit

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.[52][96]

Proposed resonances of distant Trans-Neptunian objects[52]
Body Orbital period
Heliocentric
(years)
Orbital period
Barycentric
(years)
Semimaj.
(AU)
Ratio
2013 GP136 1,830 151.8 9:1
2000 CR105 3,304 221.59±0.16 5:1
2012 VP113 4268±179 4,300 265.8±3.3 4:1
2004 VN112 5845±30 5,900 319.6±6.0 3:1
2010 GB174 7150±827 6,600 350.7±4.7 5:2
90377 Sedna ≈ 11,400 506.84±0.51 3:2
Hypothetical planet ≈ 17,000 ≈ 665 1:1

Alignment due to the Kozai mechanismEdit

Trujillo and Sheppard (2014)Edit

The initial argument that the clustering of orbital elements of extreme trans-Neptunian objects such as sednoids might be caused by a massive unknown planet beyond Neptune was published in 2014 by astronomers Chad Trujillo and Scott S. Sheppard. Trujillo and Sheppard analyzed the orbits of twelve trans-Neptunian objects (TNOs) with perihelia greater than 30 AU and semi-major axes greater than 150 AU, and found they had a clustering of orbital characteristics, particularly their arguments of perihelion (which indicates the orientation of elliptical orbits within their orbital planes).[1][4] In numerical simulations including only the known giant planets the arguments of perihelion of these objects circulated at varying rates which after billions of years would leave the perihelia of the twelve TNOs randomized, like in the rest of the trans-Neptunian region, unless there is something holding them in place.[97] Trujillo and Sheppard suggested that the massive unknown planet at a few hundred astronomical units caused the arguments of perihelion of the extreme trans-Neptunian objects to librate about 0° or 180°[Q][R] via the Kozai mechanism so that their orbits cross the plane of the planet's orbit near perihelion and aphelion, at the closest and farthest points from the planet.[100][4] Numerical simulations with a single body of 2–15 Earth masses in a circular low-inclination orbit between 200 AU and 300 AU indicated that the arguments of perihelia of Sedna and 2012 VP113 would librate around 0° for billions of years (although the lower perihelion objects did not) and would undergo periods of libration with a Neptune mass object in a high inclination orbit at 1500 AU.[4]

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.[97] Thus they did not fully formulate a model that successfully incorporated all the clustering of the extreme objects with an orbit for the planet.[1] 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.[101]

de la Fuente Marcos et al. (2014)Edit

In June 2014, Raúl and Carlos de la Fuente Marcos included a thirteenth minor planet and noted that all have their argument of perihelion close to 0°.[100][102] In a further analysis, Carlos and Raúl de la Fuente Marcos with Sverre J. Aarseth confirmed that the only known way that the observed alignment of the arguments of perihelion can be explained is by an undetected planet. They also theorized that a set of extreme trans-Neptunian objects (ETNOs) are kept bunched together by a Kozai mechanism similar to the one between Comet 96P/Machholz and Jupiter.[103] They speculated that it would have a mass between that of Mars and Saturn and would orbit at some 200 AU from the Sun. However, they also struggled to explain the orbital alignment using a model with only one unknown planet.[S] They therefore suggested that this planet is itself in resonance with a more-massive world about 250 AU from the Sun, just like the one predicted in the work by Trujillo and Sheppard.[97][104] They also did not rule out the possibility that the planet could have to be much farther away but much more massive in order to have the same effect and admitted the hypothesis needed more work.[105] They also did not rule out other explanations and expected more clarity as researchers study orbits of more such distant objects.[106][107][108] A later analysis of the distributions of the directions of perihelia and orbital poles of the ETNOs also suggests that one additional planet may not be sufficient to explain the observed clustering.[109]

Previous models with additional planetsEdit

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.[110]
  • 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.[38][39][40] 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.[111][112]
  • 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.[113][114][115][116]
  • 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.[117][118] 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.[119] 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.[120][121]

Searches for Planet NineEdit

Visibility and locationEdit

Due to its extreme distance from the Sun, Planet Nine would reflect little sunlight, potentially evading telescope sightings.[6] It is expected to have an apparent magnitude fainter than 22, making it at least 600 times fainter than Pluto.[2][T] 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.[18] Statistically, the planet is more likely to be closer to its aphelion at a distance greater than 500 AU.[2] This is because objects move more slowly when near their aphelion, in accordance with Kepler's second law.

Searches of existing dataEdit

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.[22] 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.[2][2][22]

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, new software developed recently and used to identify objects such as 2014 UZ224 can help to find it.[125]

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.[73]

A search combining multiple images collected by WISE and NEOWISE data has also been conducted. The initial search which covered the region of the sky identified by Holman and Payne using Cassini range data is being expanded to cover other regions away from the galactic plane.[126]

Ongoing searchesEdit

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.[112] 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.[16][127] 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.[128] Subsequent refinements by Batygin and Brown have reduced the search space to 600–800 square degrees of sky.[129]

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.[130] DES observes about 105 nights per season, lasting from August to February.

RadiationEdit

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, the peak of its emissions would be at infrared wavelengths.[131] This radiation signature could be detected by Earth-based infrared telescopes, such as ALMA,[132] and a search could be conducted by cosmic microwave background experiments operating at mm wavelengths.[133][134][U] 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 2019.[136]

Citizen scienceEdit

Zooniverse Backyard Worlds: Planet 9 projectEdit

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.[137][138]

Zooniverse SkyMapper projectEdit

In April 2017,[139] 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.[140] 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.[140]

Efforts toward indirect detectionEdit

Cassini measurements of perturbations of SaturnEdit

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,[141] with right ascension close to 2h and declination close to −20°, in Cetus.[142] In contrast, if the putative planet is near aphelion it could be moving projected towards the area of the sky with boundaries: right ascension 3.0h to 5.5h and declination −1° to 6°.[109]

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.[143][130]

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."[144] 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 orbitEdit

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.[145][88]

Optimal orbit if objects are in strong resonancesEdit

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 ω ≈ 150°, and longitude of ascending node Ω ≈ 50° (the last differs from Brown and Batygin's value of 90°).[V] 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.[110]

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.[147]

Correlation of arguments and longitudes of perihelionEdit

Trujillo and Sheppard in an article announcing the discovery of several more distant objects noted a correlation between the longitude of perihelion and the argument of perihelion of these objects. 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 suggest that the correlation is due to the orbits of these objects avoiding close approaches to Planet Nine by passing above or below its orbit. Trujillo and Sheppard also noted that the arguments of perihelion of ETNOs with perihelion less than 35 AU are opposite those with perihelion greater than 35 AU.[58][W]

Ascending nodes of large semi-major axis objectsEdit

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.[148][149]

Orbits of nearly parabolic cometsEdit

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°.[150] Scott Sheppard, who is skeptical of this analysis, notes that many different forces influence the orbits of comets.[88]

Possible disrupted binaryEdit

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.[151][152] The disruption of a binary would require a relatively close encounter with Planet Nine,[153] however, which becomes less likely at large distances from the Sun.

Search for additional extreme trans-Neptunian objectsEdit

Finding more objects would allow astronomers to make more accurate predictions about the orbit of the hypothesized planet.[154] 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.[155]

New 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.[156]
  • 2014 SR349, falling right in line with the earlier six objects.[156]
  • 2014 FE72, an object with an orbit so extreme that it reaches about 3000 AU from the Sun in a massively-elongated ellipse – at this distance its orbit is influenced by the galactic tide and other stars.[157][158][159][160]

Other new extreme trans-Neptunian objects discovered by the Outer Solar System Origins Survey include:[161]

  • 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.[162][163]
  • 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.[1]

CommentaryEdit

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."[164] 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%.[6] Greg Laughlin, one of the few researchers who knew in advance about this article, gives an estimate of 68.3%.[5] Other skeptical scientists demand more data in terms of additional KBOs to be analysed or final evidence through photographic confirmation.[37][155][165] Brown, though conceding the skeptics' point, still thinks that there is enough data to mount a search for a new planet.[166]

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".[136]

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.[164] 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."[5][6]

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."[88]

See alsoEdit

NotesEdit

  1. ^ Most news outlets reported the name as Phattie (a slang term for "cool" or "awesome"; also, a marijuana cigarette)[16] 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.
  2. ^ 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.[5]"
  3. ^ Two types of protection mechanisms are possible:[33]
    1. For bodies whose values of a and 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 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).
    2. Another mechanism is viable when at low inclinations when 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.
  4. ^ Given the orbital eccentricity of these objects, different epochs can generate quite different heliocentric unperturbed two-body best-fit solutions to the semi-major axis and orbital period. For objects at such high eccentricity, the Sun's barycenter is more stable than heliocentric values. Barycentric values better account for the changing position of Jupiter over Jupiter's 12 year orbit. As an example, 2007 TG422 has an epoch 2012 heliocentric period of ~13,500 years,[42] yet an epoch 2017 heliocentric period of ~10,400 years.[43] The barycentric solution is a much more stable ~11,300 years.
  5. ^ Batygin and Brown provide an order of magnitude estimate for the mass.
    • If M were equal to 0.1 M, 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 M were equal to 1 M, 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 M greater than 10 M would imply a longer semi-major axis.
    Hence they estimate that the mass of the object is likely in the range of 5 M to 15 M.
  6. ^ The average of longitude of the ascending node for the 6 objects is about 102°. In a blog published later, Batygin and Brown constrained their estimate of the longitude of the ascending node to 94°.
  7. ^ The precession rate   where   are the mass and semi-major axes of the planets Jupiter through Neptune
  8. ^ Angular momentum in an elliptical orbit  
  9. ^ Formally this is defined by the resonant angle:   where k and l are integers, λ and λP} are the mean longitudes of the object and the planet, and ϖ is the longitude of perihelion.[48]
  10. ^ In a normal reference frame the plutino's orbit does not rock back and forth, instead when its period is greater than 3/2 that of Neptune it arrives later at perihelion when Neptune is closer.
  11. ^ The resonant angle for the circulating objects is   i.e.  . As the resonant angle contains   the resonant angle can librate while the object's perihelion circulates.
  12. ^ At smaller semi-major axes the Laplace Plane is close to the invariable plane so the precession of the poles of typical Kuiper belt objects is unaffected by Planet Nine.
  13. ^ A link to the plots of the orbital evolution of all 15 is included in the arxiv version of the article.
  14. ^ This process was slower (or not observed) for the Sedna clones with smaller (or zero) inclinations of Planet Nine.
  15. ^ Of the eight objects with semi-major axis > 150 AU OSSOS found three with arguments of perihelion (ω) outside the cluster previously identified by Trujillo and Sheppard (2014):[4] 2015 GT50, 2015 KH163, and 2013 UT15.[90]
  16. ^ In their article, Brown and Batygin note that "the vast majority of this (primordial planetesimal disk) material was ejected from the system by close encounters with the giant planets during, and immediately following, the transient dynamical instability that shaped the Kuiper Belt in the first place. The characteristic timescale for depletion of the primordial disk is likely to be short compared with the timescale for the onset of the inclination instability (Nesvorný 2015), calling into question whether the inclination instability could have actually proceeded in the outer solar system."
  17. ^ In their article, Brown and Batygin note that "the lack of ω ~ 180 objects suggests that some additional process (such as encounter with a passing star) is required to account for the missing objects at 180°".
  18. ^ 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 (ETNOs with q > 30 and 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.[98][99]
  19. ^ In their article, Brown and Batygin note that the fact that the ratio of the perturbed object to perturber semi-major axis is nearly equal to one "means that trapping all of the distant objects within the known range of semimajor axes into Kozai resonances likely requires multiple planets, finely tuned to explain the particular data set". Hence, they do not favor this theory, as they view it too unwieldy.
  20. ^ The 8-meter Subaru Telescope has achieved a 27.7 magnitude photographic limit with a ten-hour exposure,[122] which is about 100 times dimmer than Planet Nine is expected to be. For comparison, the Hubble Space Telescope has detected objects as faint as 31st magnitude with an exposure of about 2 million seconds (555 hours) during Hubble Ultra Deep Field photography.[123] However, Hubble's field of view is very narrow, as is the Keck Observatory Large Binocular Telescope.[16] Brown hopes to make a request for use of the Hubble Space Telescope the day the planet is spotted.[124]
  21. ^ It is estimated that to find Planet Nine, telescopes that can resolve a 30 mJy point source are needed, and that can also resolve an annual parallax motion of ~5 arcminutes per year.[135]
  22. ^ A 3-D version of this orbit and those of several ETNOs is available.[146]
  23. ^ Five of the eight objects with perihelion < 35 AU are the perpendicular objects noted by Batygin and Brown

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