Planet Nine

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
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]

    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[upper-alpha 1] 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 period with which it goes around the sun is a rational multiple of the periods of all the furthest Kuiper Belt objects.[11] 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][12][upper-alpha 2] The high eccentricity of Planet Nine's orbit could take it as far away as 1,200 AU at its aphelion.[13][14]

    The aphelion, or farthest point from the Sun, would be in the general direction of the constellations of Orion and Taurus,[15] 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.[16][17]

    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.[18]

    Size

    Planet Nine is hypothesized to be two to four times the diameter of Earth;[2][6] similar to the ice giants like Neptune.[19]

    The planet is estimated to have 10 times the mass[10][12] and two to four times the diameter of Earth.[6][20] 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.[21] 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.[22]

    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.[23] 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.[23] 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.[24]

    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]

    Hypotheses

    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:

    Trujillo and Sheppard (2014)

    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 extreme trans-Neptunian objects such as sednoids might be caused by a massive unknown planet at a few hundred astronomical units 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°[upper-alpha 3] so that their orbits cross the plane of the planet's orbit near perihelion and aphelion, at the farthest points from the planet.[39] 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 four known giant planets in the Solar System (Jupiter, Saturn, Uranus, and Neptune) should have left the perihelia of the twelve TNOs randomized, like in the rest of the trans-Neptunian region, unless there is something holding them in place, which they suggested as an unknown massive planet beyond 200 AU.[40][upper-alpha 4] In a later work announcing the discovery of several more distant objects Trujillo and Sheppard 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 degrees have arguments of perihelion between 280 - 360 degrees, and those with longitude of perihelion of 180 - 340 have argument of perihelion 0 - 40. They found a statistical significance of this correlation of 99.99%.[43]

    They numerically simulated a single body of 2–15 Earth masses in a circular low-inclination orbit between 200 AU and 300 AU as well as further simulations with a Neptune mass object in a high inclination orbit at 1500 AU to show 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.[40] 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.

    de la Fuente Marcos et al. (2014)

    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][44] 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.[45] 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.[upper-alpha 5] 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.[40][46] 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.[47] They also did not rule out other explanations and expected more clarity as researchers study orbits of more such distant objects.[48][49][50]

    Batygin and Brown (2016)

    Orbital correlations among six distant trans-Neptunian objects led to the hypothesis. (See: Final frame orbits)
    Including the original 6 and 4 new objects: 2013 FT28, 2014 FE72, 2014 SR349, and uo3L91, the first being aligned with P9 and the second being of extremely high eccentricity. Neptune's orbit is the small central circle around the sun.

    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 344°, 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 out of alignment with how the Kozai mechanism would align these orbits, at c. 0°.[1][upper-alpha 6] 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.

    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][52] They found that this would only occur with 0.007% probability by chance alone.[53]

    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][54] 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.[55] 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 numerical simulation was able to explain both the arguments of perihelion and the coincidence of orbital planes with mean-motion resonances caused by a hypothesized 10 M⊕ 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] Their 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.

    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.[56]

    Extreme Trans-Neptunian objects with perihelion greater than 30 AU and a semi-major axis greater than 250 AU[55]
    ObjectOrbitorbital planeBody
    Orbital
    period
    (years)    		Semimaj.
    (AU)    		Peri.
    (AU)     		Aphel.
    (AU)     		Curr.
    dist.
    from
    Sun
    (AU)    		Ecc.Arg.
    peri
    ω (°)    		incl.
    i (°)    		Long.
    asc
    ☊ or Ω (°)    		Long.
    peri
    ϖ
    =ω+Ω (°)    		HvCurrent
    mag.    		 Diam.
    (km)
    2012 VP113 4,212 260.8 80.27 441 83.5 0.69 292.8 24.1 90.8 23.6 4.0 23.3 600
    2013 RF98 6,549 350.0 36.10 664 36.8 0.90 311.8 29.6 67.6 19.4 8.7 24.4 70
    2004 VN112 5,661 317.7 47.32 588 47.7 0.85 327.1 25.6 66.0 33.1 6.5 23.3 200
    2007 TG422 10,630 483.5 35.57 931 37.3 0.93 285.7 18.6 112.9 38.6 6.2 22.0 200
    90377 Sedna 11,160 499.4 76.04 923 85.5 0.85 311.5 11.9 144.5 96.0 1.5 20.9 1,000
    2010 GB174 7,109 369.7 48.76 691 71.2 0.87 347.8 21.5 130.6 118.4 6.5 25.1 200
    2013 FT28 5,460 310.1 43.60 577 57.0 0.86 40.2 17.3 217.8 258.0 6.7 24.4 200
    2014 FE72 100,050 2,155.2 36.31 4,274 61.5 0.98 134.4 20.6 336.8 111.2 6.1 24.0 200
    2014 SR349 4,913 289.0 47.57 530 56.3 0.84 341.4 18.0 34.8 16.2 6.6 24.2 200
    uo3L91 20,000+ 740 50 1,430 0.93 4.2 61.6
    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

    Simulation

    Batygin and Brown simulated the effect of a planet of mass M = 10 M⊕[upper-alpha 7] 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:

    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. 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.[12] Further simulations(published later in a companion paper) showed that:[57]

    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.[60]

    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.

    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][61][62] 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.[63] When the elongated perpendicular centaurs get too close to a giant planet, orbits such as 2008 KV42 are created.[64][65]

    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] 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] 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.[66]

    High-inclination Trans-Neptunian objects with a semi-major axis greater than 250 AU[1]
    ObjectOrbitBody
    Perihelion
    (AU)
    Figure 9[1]    		Semimaj.
    (AU)
    Figure 9[1]    		Current
    distance
    from Sun
    (AU)    		inc
    (°)[63]     		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[67]

    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."[68]

    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.[54][69][70] Brown, though conceding the skeptics' point, still thinks that there is enough data to mount a search for a new planet.[71]

    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".[72]

    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.[68] 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".[73][74][upper-alpha 10]

    Object in lower-eccentricity orbit

    Renu Malhotra, Kathryn Volk, and Xianyu Wang have proposed that the four detached objects with the longest orbital periods, those with perihelia beyond 40 AU and semi-major axes greater than 250 AU, are in n:1 or n:2 mean-motion resonances with a hypothetical planet. 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.[75]

    Proposed resonances of distant Trans-Neptunian objects[75]
    BodyOrbital period
    Heliocentric
    (years)    		Orbital period
    Barycentric
    (years)    		Semimaj.
    (AU)    		Ratio
    2013 GP136 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

    Malhotra says she remains agnostic about Planet Nine, but notes 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 says. "To me, it's the most intriguing evidence for Planet Nine I've run across so far."[76]

    Temporary or co-incidental nature of clustering

    Cory Shankman et al examined the consequences of a distant massive perturber on the TNOs used to infer the planet's existence. While they observed longitude of perihelion alignment in their simulations, they failed to produce the alignments in argument of perihelion. Their simulations for the Planet Nine scenario also predicted a larger reservoir of high-inclination TNOs, that 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. Based on their simulations, they conclude that the observed alignment in argument of perihelion in existing TNOs is a temporary phenomenon that is due to a coincidence, and they expect this unusual alignment to disappear as more objects are detected.[76][77][78]

    Subsequent efforts toward indirect detection

    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 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 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.[79][80]

    Cassini measurements of perturbations of Saturn

    An analysis of Cassini data on Saturn's orbital residuals was inconsistent with Planet Nine being located with a true anomaly of −130° to −110° or −65° to 85°. The analysis, using Batygin and Brown's orbital parameters for Planet Nine, suggests that the lack of perturbations to Saturn's orbit is best explained if Planet Nine is located at a true anomaly of 117.8°+11°
    −10°    . At this location, Planet Nine would be approximately 630 AU from the Sun,[81] with right ascension close to 2h and declination close to −20°, in Cetus.[82] 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°.[83]

    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.[84]

    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."[85] Observations of Saturn's orbit neither prove nor disprove that Planet Nine exists. Rather, they suggest that Planet Nine could not be in certain sections of its proposed orbit because its gravity would cause a noticeable effect on Saturn's position, inconsistent with actual observations.

    Analysis of Pluto's orbit

    An analysis of Pluto's orbit, 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; this could help explain the Kuiper cliff. There could be a planet more massive or closer to the Sun instead of the planet predicted by Batygin and Brown.[84]

    Search for additional extreme trans-Neptunian objects

    Finding more objects would allow astronomers to make more accurate predictions about the orbit of the hypothesized planet.[86] 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.[69]

    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]

    A systematic exploration (performed by Carlos de la Fuente Marcos & team, July-2016) 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.[87] 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.[83]

    Simulations (by Samantha Lawler & team, May-2016) indicate that the number of objects in high-perihelion and moderate-semimajor-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. These objects also have a wider inclination distribution, with a significant fraction having inclinations greater than 60°, if the distant planet has 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.[77] Newly discovered Extreme Trans-Neptunian objects include:

    Effect on Oort cloud

    An unpublished preprint[99] by Technion doctoral student Erez Michaely and Harvard astronomy professor Avi Loeb has suggested that Planet Nine would lead to the formation of a spheroidal structure within the Oort cloud at approximately 1200 AU, which could be a source of comets and would differ from structure produced by a passing star. They suggested that the Large Synoptic Survey Telescope or Breakthrough Initiatives could detect this structure, if it exists.

    Direct detection

    Location

    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.[12] 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.[16]

    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.[100] 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,[101] and a search could be conducted by cosmic microwave background experiments operating at mm wavelengths.[102][103][upper-alpha 11] 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.[72]

    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][upper-alpha 12]

    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.[38] 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][108]

    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.[16]

    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.[109] DES observes about 105 nights per season, lasting from August to February. Brown and Batygin initially 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.[110] Subsequent refinements by Batygin and Brown have reduced the search space to 600-800 square degrees of sky.[111] David Gerdes who helped develop the camera for USDOE 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 - a new software developed recently and used to identify objects such as 2014 UZ224 can help to find it.[112]

    Origin

    A number of possible origins for Planet Nine have been examined including its ejection from the neighborhood of the current giant planets, capture from another star, and in situ formation.

    In their initial 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,[113] then reduced the eccentricity of its orbit, raising its perihelion, leaving it on a very wide but stable orbit.[66][114] 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.[115] 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,[116] which would then require another explanation.[117] These ejections, however, are likely to have been two events well separated in time.[118]

    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.[119] 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.[120][121]

    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.[122] 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%.[120]

    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.[62][123] 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.[124]

    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.[125]

    See also

    Notes

    1. ↑ Most news outlets reported the name as Phattie (a slang term for "cool" or "awesome"; also, a marijuana cigarette)[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 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. ↑ 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°".
    4. ↑ 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.[41][42]
    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:[51]
      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 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⊕.
    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 a paper submitted to The Astronomical Journal on aug/sep-2016, Scott S. Sheppard & Chad Trujillo suggest that a new object 2013 FT28 falls under this category of objects.[43][58][59]
    10. ↑ 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."
    11. ↑ 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.[104]
    12. ↑ The 8-meter Subaru telescope has achieved a 27.7 magnitude photographic limit with a ten-hour exposure,[105] 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.[106] 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.[107]

    References

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    2. 1 2 3 4 5 6 7 8 9 10 11 "Where is Planet Nine?". The Search for Planet Nine. 20 January 2016. Archived from the original on 30 January 2016.
    3. 1 2 3 4 5 6 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. 1 2 3 4 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.
    8. ↑ Totten, Sanden (22 January 2016). "Planet 9: What should its name be if it's found?". 89.3 KPCC. Retrieved 7 February 2016. 'We like to be consistent,' said Rosaly Lopes, a senior research scientist at NASA's Jet Propulsion Laboratory and a member of the IAU's Working Group for Planetary System Nomenclature. ... For a planet in our solar system, being consistent means sticking to the theme of giving them names from Greek and Roman mythology.
    9. 1 2 3 4 5 Batygin, Konstantin (19 January 2016). "Search for Planet 9 – Premonition". The Search for Planet Nine. Archived from the original on 30 January 2016.
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    11. ↑ Caltech Observatory (29 January 2016). "Planet 9 Explained and Explored with Astronomer Konstantin Batygin". YouTube. 2:22.
    12. 1 2 3 4 Fesenmaier, Kimm (20 January 2016). "Caltech Researchers Find Evidence of a Real Ninth Planet". California Institute of Technology. Retrieved 20 January 2016.
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    14. ↑ Plait, Phil (21 January 2016). "More (and Best Yet) Evidence That Another Planet Lurks in the Dark Depths of Our Solar System". Bad Astronomy. Retrieved 22 January 2016. The best fit was for a planet that never got closer than 200 AU from the Sun (... Neptune is roughly 30 AU out from the Sun). The orbit of this planet would be highly elliptical, but it's not clear how elliptical; that's less well constrained. At its most distant, it could be between 500 and 1,200 AU out ... The most likely mass of the planet is about 10 times Earth's mass ..., though it could be somewhat less or more.
    15. ↑ Griffith Observatory (29 January 2016). "Mike Brown (@plutokiller) visits Griffith Observatory to discuss Planet 9". YouTube. 4:40.
    16. 1 2 3 See RA/Dec chart at Batygin, Konstantin; Brown, Mike (20 January 2016). "Where is Planet Nine?". The Search for Planet Nine. Archived from the original on 30 January 2016. Retrieved 24 January 2016.
    17. ↑ See embedded video simulation at Lemonick, Michael D. (20 January 2016). "Strong Evidence Suggests a Super Earth Lies beyond Pluto". Scientific American. Retrieved 22 January 2015.
    18. ↑ Becker, Adam; Grossman, Lisa; Aron, Jacob (22 January 2016). "How Planet Nine may have been exiled to solar system's edge". New Scientist. Retrieved 25 January 2016.
    19. ↑ Williams, David R. (18 November 2015). "Planetary Fact Sheet - Ratio to Earth". NASA/Goddard Space Flight Center. Retrieved 2016-07-18.
    20. ↑ Watson, Traci (20 January 2016). "Researchers find evidence of ninth planet in solar system". USA Today. Retrieved 2016-07-18.
    21. ↑ Lakdawalla, Emily (27 August 2009). "The Planetary Society Blog: "WISE Guys"". The Planetary Society. Retrieved 26 December 2009.
    22. ↑ Luhman, Kevin L. (2014). "A search for a distant companion to the Sun with the Wide-Field Infrared Survey Explorer". The Astrophysical Journal. 781 (4): 4. Bibcode:2014ApJ...781....4L. doi:10.1088/0004-637X/781/1/4.
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    24. ↑ Margot, Jean-Luc (22 January 2016). "Would Planet Nine Pass the Planet Test?". University of California at Los Angeles. Retrieved 2016-07-18.
    25. ↑ Wall, Mike (24 August 2011). "A Conversation With Pluto's Killer: Q & A With Astronomer Mike Brown". Space.com. Retrieved 7 February 2016.
    26. ↑ Brown, Micheal E.; Trujillo, Chadwick; Rabinowitz, David (2004). "Discovery of a Candidate Inner Oort Cloud Planetoid". The Astrophysical Journal. 617 (1): 645–649. arXiv:astro-ph/0404456Freely accessible. Bibcode:2004ApJ...617..645B. doi:10.1086/422095.
    27. ↑ Brown, Michael E. (28 October 2010). "There's something out there – part 2". Mike Brown's Planets. Retrieved 2016-07-18.
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    30. ↑ Than, Ker (18 June 2008). "Large 'Planet X' May Lurk Beyond Pluto". Space.com. Retrieved 2016-07-18.
    31. ↑ Hasegawa, Kyoko (28 February 2008). "Japanese scientists eye mysterious 'Planet X'". BibliotecaPleyades.net. Retrieved 2016-07-18.
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    34. ↑ Gomes, Rodney (2015). "The observation of large semi-major axis Centaurs: Testing for the signature of a planetary-mass solar companion". Icarus. 258: 37–49. Bibcode:2015Icar..258...37G. doi:10.1016/j.icarus.2015.06.020.
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    38. 1 2 Mortillaro, Nicole (9 February 2016). "Meet Mike Brown: Pluto killer and the man who brought us Planet 9". Global News. 'It was that search for more objects like Sedna ... led to the realization ... that they're all being pulled off in one direction by something. And that's what finally led us down the hole that there must be a big planet out there.' – Mike Brown
    39. 1 2 de la Fuente Marcos, Carlos; de la Fuente Marcos, Raúl (2014). "Extreme trans-Neptunian objects and the Kozai mechanism: signalling the presence of trans-Plutonian planets". Monthly Notices of the Royal Astronomical Society Letters. 443 (1): L59–L63. arXiv:1406.0715Freely accessible. Bibcode:2014MNRAS.443L..59D. doi:10.1093/mnrasl/slu084.
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    52. ↑ McDonald, Bob (24 January 2016). "How did we miss Planet 9?". CBC News. Retrieved 2016-07-18. It's like seeing a disturbance on the surface of water but not knowing what caused it. Perhaps it was a jumping fish, a whale or a seal. Even though you didn't actually see it, you could make an informed guess about the size of the object and its location by the nature of the ripples in the water.
    53. ↑ Lakdawalla, Emily (20 January 2016). "Theoretical evidence for an undiscovered super-Earth at the edge of our solar system". The Planetary Society. Retrieved 2016-07-18.
    54. 1 2 Grush, Loren (20 January 2016). "Our solar system may have a ninth planet after all — but not all evidence is in (We still haven't seen it yet)". The Verge. Retrieved 2016-07-18. The statistics do sound promising, at first. The researchers say there's a 1 in 15,000 chance that the movements of these objects are coincidental and don't indicate a planetary presence at all. ... 'When we usually consider something as clinched and air tight, it usually has odds with a much lower probability of failure than what they have,' says Sara Seager, a planetary scientist at MIT. For a study to be a slam dunk, the odds of failure are usually 1 in 1,744,278. ... But researchers often publish before they get the slam-dunk odds, in order to avoid getting scooped by a competing team, Seager says. Most outside experts agree that the researchers' models are strong. And Neptune was originally detected in a similar fashion — by researching observed anomalies in the movement of Uranus. Additionally, the idea of a large planet at such a distance from the Sun isn't actually that unlikely, according to Bruce Macintosh, a planetary scientist at Stanford University.
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    59. ↑ "Hunt for ninth planet reveals new extremely distant Solar System objects". 2013 FT28 shows similar clustering in some of these parameters (its semi-major axis, eccentricity, inclination, and argument of perihelion angle, for angle enthusiasts out there) but one of these parameters, an angle called the longitude of perihelion, is different from that of the other extreme objects, which makes that particular clustering trend less strong
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    83. 1 2 de la Fuente Marcos, Carlos; de la Fuente Marcos, Raúl (2016). "Finding Planet Nine: apsidal anti-alignment Monte Carlo results". Monthly Notices of the Royal Astronomical Society. 462 (2): 1972–1977. arXiv:1607.05633Freely accessible. Bibcode:2016MNRAS.462.1972D. doi:10.1093/mnras/stw1778.
    84. 1 2 Holman, Matthew J.; Payne, Matthew J. (30 March 2016). "Observational Constraints on Planet Nine: Astrometry of Pluto and Other Trans-Neptunian Objects". arXiv:1603.09008Freely accessible [astro-ph.EP].
    85. ↑ "Saturn Spacecraft Not Affected by Hypothetical Planet 9". NASA/Jet Propulsion Laboratory. 8 April 2016. Retrieved April 20, 2016.
    86. ↑ Scharping, Nathaniel (20 January 2016). "Planet Nine: A New Addition to the Solar System?". Discover Magazine. Retrieved 2016-07-18.
    87. ↑ de la Fuente Marcos, Carlos; de la Fuente Marcos, Raúl (2016). "Commensurabilities between ETNOs: a Monte Carlo survey". Monthly Notices of the Royal Astronomical Society Letters. 460 (1): L64–L68. arXiv:1604.05881Freely accessible. Bibcode:2016MNRAS.460L..64D. doi:10.1093/mnrasl/slw077.
    88. ↑ SETI Institute (18 March 2016). "Exploring the outer Solar System: now in vivid colour - Michele Bannister (SETI Talks)". YouTube. 28:17. Retrieved 2016-07-18.
    89. ↑ "A new high-perihelion a ~ 700 AU object in the distant Solar System".
    90. ↑ "Abstract book - DPS48/EPSC11, 16-21 October 2016, Pasadena, CA" (PDF).
    91. ↑ Hall, Shannon (30 March 2016). "More evidence for Planet Nine as odd celestial alignment emerges". New Scientist. Retrieved 1 April 2016.
    92. ↑ "Astronomers spot distant world in Solar System's far reaches". Konstantin Batygin argues, Planet Nine might be shepherding L91's orbit more simply and directly. Bannister counters that L91 travels in an orbit that is almost within the plane of the Solar System, rather than being tilted at high angles (as might be expected if it were being battered around by a Planet Nine).
    93. 1 2 3 "Objects beyond Neptune provide fresh evidence for Planet Nine". The new evidence leaves astronomer Scott Sheppard of the Carnegie Institution for Science in Washington, D.C., "probably 90% sure there's a planet out there." But others say the clues are sparse and unconvincing. "I give it about a 1% chance of turning out to be real," says astronomer JJ Kavelaars, of the Dominion Astrophysical Observatory in Victoria, Canada.
    94. ↑ "This newly discovered icy world could be the key to finding Planet Nine". The team that found L91 thinks it(its orbit) could be due to the historical effects of Neptune's pull on the minor planet – perhaps paired with gravitational interactions with a passing star. But not everybody is convinced with this explanation. "It's a story that's not implausible, but I also think it's not needed," says planetary scientist Konstantin Batygin.
    95. ↑ "PLANET 9 SEARCH TURNING UP WEALTH OF NEW OBJECTS".
    96. ↑ "Extreme New Objects Found At The Edge of The Solar System".
    97. ↑ "The Search for Planet Nine: New Finds Boost Case for Distant World".
    98. ↑ "HUNT FOR NINTH PLANET REVEALS NEW EXTREMELY DISTANT SOLAR SYSTEM OBJECTS".
    99. ↑ Erez Michaely, Abraham Loeb (2016-09-27). "Shaping of the inner Oort cloud by Planet Nine". Retrieved 2016-10-13.
    100. ↑ Linder, Esther F.; Mordasini, Christoph (2016). "Evolution and Magnitudes of Candidate Planet Nine". Astronomy & Astrophysics. 589: A134. arXiv:1602.07465Freely accessible. Bibcode:2016A&A...589A.134L. doi:10.1051/0004-6361/201628350.
    101. ↑ Powel, Corey S. (22 January 2016). "A Little Perspective on the New "9th Planet" (and the 10th, and the 11th)". Discover Magazine. Retrieved 2016-07-18.
    102. ↑ Cowan, Nicolas B.; Holder, Gil; Kaib, Nathan A. (2016). "Cosmologists in Search of Planet Nine: the Case for CMB Experiments". The Astrophysical Journal Letters. 822 (1): L2. arXiv:1602.05963Freely accessible. Bibcode:2016ApJ...822L...2C. doi:10.3847/2041-8205/822/1/L2.
    103. ↑ Aron, Jacob (24 February 2016). "Planet Nine hunters enlist big bang telescopes and Saturn probe". New Scientist. Retrieved 27 February 2016.
    104. ↑ Kohler, Susanna (25 April 2016). "Can CMB Experiments Find Planet Nine?". AAS Nova. Retrieved 29 April 2016.
    105. ↑ "What is the faintest object imaged by ground-based telescopes?". Sky & Telescope. 24 July 2006. Retrieved 2016-07-18.
    106. ↑ Illingworth, G.; Magee, D.; Oesch, P.; Bouwens, R. (25 September 2012). "Hubble goes to the eXtreme to assemble the deepest ever view of the Universe". SpaceTelescope.org. Retrieved 7 February 2016.
    107. ↑ Deep Astronomy (19 February 2016). "Ninth Planet Beyond Neptune?". YouTube. 46:57.
    108. ↑ Wall, Mike (21 January 2016). "How Astronomers Could Actually See 'Planet Nine'". Space.com. Retrieved 24 January 2016.
    109. ↑ Hall, Shannon (20 April 2016). "We are closing in on possible whereabouts of Planet Nine". New Scientist. Retrieved 2016-07-18.
    110. ↑ Crockett, Christopher (5 July 2016). "New clues in search for Planet Nine". Science News. Retrieved 6 July 2016.
    111. ↑ "Closing in on a Giant Ghost Planet". scientificamerican.com. October 25, 2016. Retrieved October 26, 2016.
    112. ↑ "A Friend For Pluto: Astronomers Find New Dwarf Planet In Our Solar System".
    113. ↑ Bromley, Benjamin C.; Kenyon, Scott J. (25 March 2016). "Making Planet Nine: A Scattered Giant in the Outer Solar System". arXiv:1603.08010Freely accessible [astro-ph.EP].
    114. ↑ Totten, Sanden (20 January 2016). "Caltech researchers answer skeptics' questions about Planet 9". 89.3 KPCC. Retrieved 2016-07-18.
    115. ↑ Raymond, Sean (4 February 2016). "Planet Nine: Solar System renegade?". PlanetPlanet.net. Retrieved 2016-07-18.
    116. ↑ Gomes, R.; Levison, H. F.; Tsiganis, K.; Morbidelli, A. (2005). "Origin of the cataclysmic Late Heavy Bombardment period of the terrestrial planets" (PDF). Nature. 435 (7041): 466–469. Bibcode:2005Natur.435..466G. doi:10.1038/nature03676. PMID 15917802.
    117. ↑ Raymond, Sean (2 February 2016). "Planet Nine: kicked out by the moody young Solar System?". PlanetPlanet.net. Retrieved 2016-07-18.
    118. ↑ Drake, Nadia (22 January 2016). "How Can We Find Planet Nine? (And Other Burning Questions)". National Geographic: Phenomena. Retrieved 23 January 2016. 'These two events are both ejections, but they are well separated in epoch,' Batygin says. 'The planet that was ejected during the giant instability of the solar system—which, by the way, coincided with the formation of the Kuiper Belt—that planet, if it was there, it was just ejected. It doesn't get to stick around.'
    119. ↑ Raymond, Sean (30 March 2016). "Planet Nine from Outer Space!". PlanetPlanet.net. Retrieved 30 March 2016.
    120. 1 2 Li, Gongjie; Adams, Fred C. (2016). "Interaction Cross Sections and Survival Rates for Proposed Solar System Member Planet Nine". The Astrophysical Journal Letters. 823 (1): L3. arXiv:1602.08496Freely accessible. Bibcode:2016ApJ...823L...3L. doi:10.3847/2041-8205/823/1/L3.
    121. ↑ Mustill, Alexander J.; Raymond, Sean N.; Davies, Melvyn B. (21 July 2016). "Is there an exoplanet in the Solar System?". Monthly Notices of the Royal Astronomical Society: Letters. 460 (1): L109–L113. arXiv:1603.07247Freely accessible. Bibcode:2016MNRAS.460L.109M. doi:10.1093/mnrasl/slw075.
    122. ↑ Kenyon, Scott J.; Bromley, Benjamin C. (2016). "Making Planet Nine: Pebble Accretion at 250–750 AU in a Gravitationally Unstable Ring". The Astrophysical Journal. 825 (1): 33. arXiv:1603.08008Freely accessible. Bibcode:2016ApJ...825...33K. doi:10.3847/0004-637X/825/1/33.
    123. ↑ Siegel, Ethan (3 November 2015). "Jupiter May Have Ejected A Planet From Our Solar System". Forbes. Retrieved 22 January 2016.
    124. ↑ Beatty, Kelly (26 March 2014). "New Object Offers Hint of "Planet X"". Sky & Telescope. Retrieved 2016-07-18.
    125. ↑ Anderson, Paul Scott (22 January 2016). "Has 'Planet X' finally been found? A cautionary tale". Planetaria.ca. Retrieved 2016-07-18.

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