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This article is about the hypothetical planet first suggested in 2014. For other uses, see Planet Nine (disambiguation).
Not to be confused with Planet X.
For doomsday speculation concerning Planet Nine, see Nibiru cataclysm § Planet Nine.
Planet Nine[1]
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
Aphelion1,200 AU (est.)[2]
Perihelion200 AU (est.)[3]
700 AU (est.)[1]
Eccentricity0.6 (est.)[3]
10,000 to 20,000 years[3]
Inclination30° to ecliptic (est.)[1]
150° (est.)[1]
Physical characteristics
13,000 to 26,000 km (8,000–16,000 mi)
2–4 R🜨 (est.)[3]
Mass6×1025 kg (est.)[3]
≥10 [[Earth mass|ME]] (est.)
>22.5 (est.)[2]

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]

In a 2014 letter to the journal Nature, 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 they proposed specific orbital parameters.[1] The predicted planet would 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]

On the basis of models of planet formation that might include planetary migration from the inner Solar System, such as the fifth giant planet hypothesis, Batygin and Brown suggest that it may be a primordial giant planet core that was ejected from its original orbit during the nebular epoch of the Solar System's evolution.[1]

Naming

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.[7] It will likely be a name chosen from Roman or Greek mythology.[8]

In their original paper, Batygin and Brown simply referred to the object as "perturber",[1] and only in later press releases did they use "Planet Nine".[9] 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 characteristics

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]

Orbit

Planet Nine is hypothesized to follow a highly elliptical orbit around the Sun, with an orbital period of 10,000–20,000 years. 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 estimated to be roughly 30°±10°.[2][3][11][B] The high eccentricity of Planet Nine's orbit could take it as far away as 1,200 AU at its aphelion.[12][13]

The aphelion, or farthest point from the Sun, would be in the general direction of the constellations of Orion and Taurus,[14] 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.[15][16]

Brown thinks that if Planet Nine is confirmed to exist, a probe could reach it in as little as 20 years, with a powered slingshot around the Sun.[17]

Size

Planet Nine is hypothesized to be two to four times the diameter of Earth;[2][6] similar to the ice giants like Neptune (pictured).[18]

The planet is estimated to have 10 times the mass[10][11] and two to four times the diameter of Earth.[6][19] 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.[20] 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.[21]

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.[22] 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.[22] 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.[23]

Composition

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]

Indirect detection

See also: Planets beyond Neptune § Orbits of distant objects

Early speculation

See also: Detached object § Orbits
  • 2004: Unusual orbit of Sedna: 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 could have been another planet; it could have been a star that came close to the Sun; or it could have been a lot of stars if the Sun had formed in a cluster.[24][25][26]
  • 2008: Inclined orbits of many trans-Neptunian objects: The large number of trans-Neptunian objects with large orbital eccentricities and inclinations led Tadashi Mukai and coworkers to suggest a distant Mars- or Earth-sized minor planet orbiting at an inclination of 20° to 40° in a highly eccentric orbit between 100 and 200 AU and orbital period of 1000 years.[27][28][29][30]
  • 2012: Distant and eccentric orbits of detached objects: 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 created models that demonstrated the possible existence of an as yet undetected planet (between the mass of Mars and Neptune) that would be too far away to influence the motions of the inner planets, yet close enough to the scattered disc objects to sway them into their elongated orbits.[31][32] 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.[33]
  • 2014: Similar orientation of sednoids: The announcement in March 2014 of the discovery of 2012 VP113, which shared a few odd orbital characteristics with Sedna and other extreme trans-Neptunian objects, further raised the possibility of an unseen super-Earth in a large orbit.[34][35]
  • 2014: Bunching of the perihelia of known objects beyond 150 AU: 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] Perturbations by the giant planets should have left their perihelia randomized, like in the rest of the trans-Neptunian region, unless there is something holding them in place.[36][C] Later, 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°.[39][40]

Case for a new planet

Trujillo and Sheppard

The initial argument for the existence of a planet beyond Neptune was published in 2014 by astronomers Chad Trujillo and Scott S. Sheppard, who suggested that the similar orbits of certain objects such as sednoids might be influenced by a massive unknown planet at the edge of the Solar System via the Kozai mechanism to explain the alignments.[4] In this arrangement the arguments of perihelion of the objects would librate about 0° or 180°[D] so that their orbits cross the plane of the planet's orbit near perihelion and aphelion, at the farthest points from the planet.[39]

They proposed a "single body of 2–15 Earth masses in a circular low-inclination orbit between 200 AU and 300 AU" to explain the pattern. It was not the only way to create the clustering of the orbital orientations, and they admitted the possibility that a Mars-mass object that is closer or a Neptune-mass planet that is farther away could also cause the bunching of the perihelia.[36] However, they could not formulate a model that successfully incorporated the planet.[1]

de la Fuente Marcos et al.

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.[41] 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.[E] 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.[36][42] 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.[43] They also did not rule out other explanations and expected more clarity as researchers study orbits of more such distant objects.[44][45][46]

Batygin and Brown

Caltech's Konstantin Batygin and Michael E. Brown looked into refuting the mechanism proposed by Trujillo and Sheppard.[1] They showed that Trujillo and Sheppard's original formulation, which had identified a clustering of arguments of perihelion at 0°, was mostly under the effect of Neptune mean-motion resonances for many objects in their analysis set, and that, once filtered, the argument of perihelion for the remaining objects not affected by Neptune was at 318°±8°. This was grossly out of alignment with how the Kozai mechanism would align these orbits, at c. 0°.[why?][1][F] However, Batygin and Brown did find that the four remaining detached objects not affected by Neptune were approximately co-planar with the sednoids Sedna and 2012 VP113, as well as clustered around an argument of perihelion with them, and found that there was only a 0.007% likelihood that this was due to chance. They then pursued a numerical simulation, and found they were able to explain both results with mean-motion resonances caused by a hypothesized 10 ME massive object on a highly eccentric and moderately inclined orbit. The model generated a pattern of high-inclination objects that they speculated as resulting from a combination of mean-motion effect with the Kozai effect relative to the hypothetical planet,[1] and that they subsequently found in databases of minor objects in the Solar System. Their origin could previously not be explained well.

Their theoretical model explained three elusive aspects of the trans-Neptunian region in a single, unifying picture: The physical alignment of the distant orbits, the generation of detached objects well separated from the Kuiper belt such as Sedna, and the existence of a population of objects with high-inclination orbits.[9]

Planet Nine hypothesis

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

Batygin and Brown analyzed six extreme trans-Neptunian objects (ETNOs) in a stable configuration of orbits mostly outside the Kuiper belt (namely Sedna, 2012 VP113, 2007 TG422, 2004 VN112, 2013 RF98, 2010 GB174).[1] A closer look at the data showed that these six objects have orbits that are not just clustered in their arguments of perihelion, but are aligned in approximately the same direction in physical space, and lie in approximately the same plane.[9][48] They found that this would only occur with 0.007% probability by chance alone.[49]

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. And again, being the six most distant objects meant they were least likely to be disturbed by Neptune, which orbits 30 AU from the Sun.[10][50] Generally, TNOs with perihelia smaller than 36 AU experience strong encounters with Neptune.[1]

These six are 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.[51] All six objects are relatively small, but currently relatively bright because they are near their closest distance to the Sun in their elliptical orbits.

A seventh distant object, spotted by the Outer Solar System Origins Survey, was discussed by Michele Bannister at a recent lecture hosted by the SETI institute.[52] The perihelion of this object appears to be aligned with the other six distant objects but whether the plane of its orbit is also aligned is not known because this data has not been released.[53]

Within the Planet Nine hypothesis and depending on the actual values of the orbital parameters of the putative perturber, the ETNOs may be a primordial or a transient population.[54]

Extreme Trans-Neptunian objects with perihelion greater than 30 AU and a semi-major axis greater than 250 AU[51]
Object Orbit orbital plane Body
Orbital period
(years)		 Semimaj.
(AU)		 Peri.
(AU)		 Aphel.
(AU)		 Current
distance
from Sun
(AU)		 Eccen. Arg. peri
ω (°)		 inclination
i (°)		 Long. asc
☊ or Ω (°)		 Long. peri
ϖ=ω+Ω (°)		 Current
mag.		 Diam.
(km)
2012 VP113 4,300 263 80 450 83 0.70 292.9 24.0 90.8 23.7 23.4 600
2013 RF98 5,600 317 36 600 36.5 0.88 316.5 29.6 67.6 24.1 24.4 80
2004 VN112 5,850 327 47 607 48 0.85 327.2 25.6 66.0 33.2 23.3 200
2007 TG422 11,200 501 36 967 37 0.93 285.8 18.6 113.0 38.8 21.9 200
90377 Sedna 11,400 506 76 936 86 0.86 311.5 11.9 144.5 96.0 21.0 1,000
2010 GB174 6,600 351 48 650 71 0.87 347.7 21.5 130.6 118.3 25.2 200
Hypothesized Planet Nine ~15,000 ~700 ~200 ~1,200 ~1,000? ~0.6 ~150 ~30 91±15 241±15 >22 ~40,000

Simulation

Batygin and Brown simulated the effect of a planet of mass M = 10 ME[G] with a semi-major axis ranging from 200 to 2,000 AU and an eccentricity varying from 0.1 to 0.9 on these extreme TNOs and inner Oort cloud objects. The capture of Kuiper belt objects (KBOs) into long-lived apsidally anti-aligned orbital configurations occurs, with variable success, across a significant range of companion parameters (semi-major axis a ≈ 400–1,500 AU, eccentricity e ≈ 0.5–0.8).

They found that orbital parameters centered around the following values produced the best fit, for the kind of distribution of orbits observed.

Upon running the simulations, Batygin and Brown found that their hypothetical planet produced a number of effects on the orbits of distant minor planets, some of which were later confirmed by observation:

  • Generation and grouping of detached objects: The simulations showed that planetesimal swarms could be sculpted into collinear groups of spatially confined orbits by Planet Nine if it is substantially more massive than Earth and on a highly eccentric orbit.
  • Physical alignment of distant orbits: The confined orbits would cluster in a configuration where the long axes of their orbits are anti-aligned with respect to Planet Nine, signalling that the dynamical mechanism at play is resonant in nature.[9] This mechanism, known as mean-motion resonance, prevents trapped trans-Neptunian objects from colliding with Planet Nine, and keeps them aligned.[11]

The resonances at play are exotic and interconnected, yielding orbital evolution that is fundamentally chaotic. In other words, perturbed by Planet Nine, the distant orbits of the Kuiper belt remain approximately aligned, while changing their shape unpredictably on million-year timescales.[55]

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.
  • Objects with high-inclination orbital trajectories: The results of their simulations also predicted there should be a population of objects with a perpendicular orbital inclination (relative to the first set of TNOs considered and the Solar System in general) and they realized that objects such as 2008 KV42 and 2012 DR30 fit this prediction of the model.[1][56][57] These objects would have a high semi-major axis and an inclination greater than 60°.[1] These objects may be created by the Kozai effect inside the mean-motion resonances.[9] The only TNOs known with a semi-major axis greater than 250 AU, an inclination greater than 40°, and perihelion beyond Jupiter are: (336756) 2010 NV1, (418993) 2009 MS9, 2010 BK118, 2012 DR30, and 2013 BL76.[58] When the elongated perpendicular centaurs get too close to a giant planet, orbits such as 2008 KV42 are created.[59]
  • Unaffected nearby objects: Simulations have shown that objects with a semi-major axis less than 150 AU are largely unaffected by the presence of Planet Nine, because they have a very low chance of coming in its vicinity.[1]
  • Farther objects in circular orbits: The simulation also predicts a yet-to-be-discovered population of distant objects that have semi-major axes greater than 250 AU, but lower eccentricities and orbits that would be aligned with Planet Nine.[1]
  • Temporary nature of displacement: The simulations showed that Planet Nine sweeps up the trans-Neptunian objects and places them only temporarily in highly elliptical orbits. In half a billion years Sedna will be a more typical trans-Neptunian object and now typical trans-Neptunian objects will have been scattered into Sedna-like orbits.[60]
High-inclination Trans-Neptunian objects with a semi-major axis greater than 250 AU[1]
Object Orbit Body
Perihelion
(AU)
Figure 9[1]		 Semimaj.
(AU)
Figure 9[1]		 Current
distance
from Sun
(AU)		 inc
(°)[58]		 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[61]

Inference

Batygin was cautious in interpreting the results, saying, "Until Planet Nine is caught on camera it does not count as being real. All we have now is an echo."[62]

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 paper, 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.[50][63][64] Brown, though conceding the skeptics' point, still thinks that there is enough data to mount a search for a new planet.[65]

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".[66]

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.[62] Alessandro Morbidelli, who reviewed the paper for The Astronomical Journal, concurred, saying, "I don't see any alternative explanation to that offered by Batygin and Brown."[5][6]

Alternate hypotheses

Inclination instability due to mass of undetected objects

Ann-Marie Madigan and Michael McCourt postulate that a yet to be discovered belt of objects far more massive than the current-day Kuiper belt, at larger distances, and preferentially tilted from the plane of the planets could be shaping the orbits of these ETNOs based on self-organization. But Brown remains confident that his and Batygin's hypothesis is correct, and regards the prospect of Planet Nine as more probable, because surveys do not suggest/support the existence of a scattered-disk region of sufficient mass to support this idea of "inclination instability".[67][68][I]

Undiscovered mass in addition to Planet Nine

An analysis of Pluto's orbit, in a preprint 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. The data regarding Pluto's orbit could have significant systematic errors. There could be unmodeled mass in the solar system, such as an undiscovered small planet in the range of 60–100 AU, in addition to Planet Nine. There could be a planet more massive or closer to the Sun instead of the planet predicted by Batygin and Brown.[69]

Object in lower-eccentric 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. There are two more objects with semi-major axes greater than 150 AU. Their proposed planet could be on a lower eccentricity orbit, with e < 0.18. Unlike Batygin and Brown, they do not specify that most of the distant detached objects would have orbits anti-aligned with the massive planet.[70]

Proposed resonances of distant Trans-Neptunian objects[70]
Body Orbital period
Heliocentric
(years)		 Orbital period
Barycentric
(years)		 Semimaj.
(AU)		 Ratio
2013 GP136[71] 1870 151.8 9:1
2000 CR105 3420 221.59±0.16 5:1
2012 VP113 4268±179 4300 265.8±3.3 4:1
2004 VN112 5845±30 5900 319.6±6.0 3:1
2010 GB174 7150±827 6600 350.7±4.7 5:2
90377 Sedna ≈ 11400 506.84±0.51 3:2
Hypothetical planet ≈ 17000 ≈ 665 1:1

Direct detection

Location

If the planet 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.[11] 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. 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.[15]

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,[72] with right ascension close to 2h and declination close to −20°, in Cetus.[73] 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 -1º to 6º.[74]

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 intersected their preferred regions, based on Saturn's position measurements, 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 in an area of the sky near the constellation Cetus, at (RA, Dec) = (40°, -15°), and extending c. 20° in all directions. They recommend this area as high priority for an efficient observational campaign.[69]

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."[75] Observations of Saturn's orbit do not prove that Planet Nine exists. Rather, they suggest that Planet Nine could not be in certain locations of its proposed orbit because its gravity would cause a noticeable effect on Saturn's position, inconsistent with actual observations.

Solar obliquity

The analyses conducted contemporarily and independently by Bailey, Batygin and Brown and Gomes, Deienno and Morbidelli 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 6 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 on the other planets 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.[76]

Radiation

A distant planet such as this would reflect little light, but, because it is hypothesized to be a large body, it would still be cooling from its formation with an estimated temperature of 47 K.[77] At this temperature the peak of its emissions would be at infrared wavelengths. This radiation signature could be detected by Earth-based infrared telescopes, such as ALMA,[78] and a search could be conducted by cosmic microwave background experiments operating at mm wavelengths.[79][80][J] Additionally, Jim Green of NASA is optimistic that the James Webb Space Telescope, which will be the successor to the Hubble Space Telescope and is expected to launch in 2018, could find it.[66]

Visibility

Telescopes are searching for the object, which, due to its extreme distance from the Sun, would reflect little sunlight and potentially evade telescope sightings.[6] It is expected to have an apparent magnitude fainter than 22, making it at least 600 times fainter than Pluto.[2][K]

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.[35] Two teams of astronomers—Batygin and Brown, as well as Trujillo and Sheppard—are undertaking this search together, and both teams cooperatively expect this search to take up to five years.[10][85]

A preliminary search of the archival data from the Catalina Sky Survey to magnitude c. 19, Pan-STARRS to magnitude 21.5, and WISE has not identified Planet Nine.[2] The remaining areas to search are near aphelion, which is located close to the galactic plane of the Milky Way.[2] This aphelion direction is where the predicted planet would be faintest and has a complicated star-rich field of view in which to spot it.[15]

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.[86] DES observes about 105 nights per season, lasting from August to February. Brown and Batygin think that they have narrowed it 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.[87]

More predicted objects

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 Planet Nine. The larger perihelia of these objects would make them fainter and more difficult to detect than the anti-aligned objects.[1]

Finding more such objects would allow astronomers to make more accurate predictions about the orbit of the hypothesized planet.[88] 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.[63]

A systematic exploration on the existence of commensurabilities between the known ETNOs using their heliocentric and barycentric semi-major axes, their uncertainties, and Monte Carlo techniques has shown that additional planetary-sized objects beyond Pluto are likely.[89] The analysis of the distributions of the directions of perihelia and orbital poles of the ETNOs also suggest the presence of more than one trans-Plutonian planet.[74]

Simulations indicate that the number of objects in high-perihelion and moderate-semi-major-axis orbits (q > 37 AU, 50 < a < 500 AU) is increased threefold if there is a distant planet in a circular orbit and tenfold if it is in an eccentric orbit. However, because such distant objects are difficult to detect with current instruments and often not relocated, current surveys are as yet unable to distinguish between these possibilities.[90]

Origin

See also: Nice model, Nice 2 model, and Planetary migration

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 paper Batygin and Brown proposed that Planet Nine formed closer to the Sun and was ejected onto 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,[91] then reduced the eccentricity of its orbit, raising its perihelion, leaving it on a very wide but stable orbit.[60][92] 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.[93] 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.[10] 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,[94] which would then require another explanation.[95] These ejections, however, are likely to have been two events well separated in time.[96]

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 onto a more long-lasting distant eccentric orbit, increasing its chances of capture by another star.[97] 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.[98][99]

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.[100] 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%.[98]

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 inner Solar System due to the inward migration of Jupiter in the early Solar System.[57][101] Hal Levinson 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.[102]

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.[103]

See also

Notes

  1. ^ Most news outlets reported the name as Phattie (a slang term for "cool" or "awesome")[10] but The New Yorker quote cited above uses "fatty" in what appears to be a nearly unique variation. The apparently correct spelling has been subbed.
  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. ^ Assuming that the orbital elements of these objects have not changed, Jilkova & co 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.[37][38]
  4. ^ In their paper Brown & 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°".
  5. ^ In their paper Brown & Batygin note that the fact that the ratio of the perturbed object to perturber semimajor 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 too unwieldy.
  6. ^ Two types of protection mechanisms are possible:[47]
    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 semimajor 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.
  7. ^ Batygin & Brown provide an order of magnitude estimate for the mass.
    • If M were equal to 0.1 ME, 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 ME, 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 ME would imply a longer semi-major axis.
    Hence they estimate that the mass of the object is likely in the range of 5 ME to 15 ME.
  8. ^ Fixing of the orbital plane requires a combination of two parameters: inclination and longitude of the ascending node. The average of longitude of the ascending node for the 6 objects is about 102°. In a paper published later, Batygin and Brown constrained their estimate of the longitude of the ascending node to 91°±15°.
  9. ^ In their paper Brown & 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."
  10. ^ 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.[81]
  11. ^ The 8-meter Subaru telescope has achieved a 27.7 magnitude photographic limit with a ten-hour exposure,[82] 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.[83] However, Hubble's field of view is very narrow, as is the Keck Observatory Large Binocular Telescope.[10] Brown hopes to make a request for use of the Hubble Space Telescope the day the planet is spotted.[84]

References

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  2. ^ a b c d e f g h i j k "Where is Planet Nine?". The Search for Planet Nine. 20 January 2016. Archived from the original on 30 January 2016. {{cite web}}: Unknown parameter |dead-url= ignored (|url-status= suggested) (help)
  3. ^ a b c d e f Witze, Alexandra (2016). "Evidence grows for giant planet on fringes of Solar System". Nature. 529 (7586): 266–7. Bibcode:2016Natur.529..266W. doi:10.1038/529266a. PMID 26791699.
  4. ^ a b c d Trujillo, Chadwick A.; Sheppard, Scott S. (2014). "A Sedna-like body with a perihelion of 80 astronomical units" (PDF). Nature. 507 (7493): 471. Bibcode:2014Natur.507..471T. doi:10.1038/nature13156. PMID 24670765.
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  7. ^ "Naming of Astronomical Objects". International Astronomical Union. Retrieved 25 February 2016.
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