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	7014179517444840000♠1,200 AU (est.)[2]
Perihelion	7013299195741400000♠200 AU (est.)[3]
Semi-major axis	7014104718509490000♠700 AU (est.)[1]
Eccentricity	6999600000000000000♠0.6 (est.)[3]
Orbital period	10,000 to 20,000 years[3]
Inclination	6999523598775598300♠30° to ecliptic (est.)[1]
Argument of perihelion	7000261799387799150♠150° (est.)[1]
Physical characteristics
Mean radius	13,000 to 26,000 km (8,000–16,000 mi) 2–4 R⊕ (est.)[3]
Mass	7025600000000000000♠6×1025 kg (est.)[3] ≥10 M⊕ (est.)
Apparent magnitude	>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 7014104718509490000♠700 AU, or about 20 times the distance from Neptune to the Sun, although it might come as close as 7013299195741400000♠200 AU (30 billion km, 19 billion mi), and its inclination estimated to be roughly 6999523598775598300♠30°±10°.^{[2][3][12][upper-alpha 2]} The high eccentricity of Planet Nine's orbit could take it as far away as 7014179517444840000♠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:

• 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.^[25][26][27]
• The large number of trans-Neptunian objects with large orbital eccentricities and inclinations led Tadashi Mukai and coworkers to suggest in 2008 a distant Mars- or Earth-sized minor planet orbiting at an inclination of 20° to 40° in a highly eccentric orbit between 100 and 7013299195741400000♠200 AU and orbital period of 1000 years.^{[28][29][30][31]}
• 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.^[32][33] Later in 2015, they also proposed that the Centaurs with large semi-major axis are also being pushed around by the putative Planet.^[34][35] 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.^[36]
• 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.^[37][38]

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 7012448793612100000♠30 AU and semi-major axes greater than 7013224396806050000♠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 7013299195741400000♠200 AU and 7013448793612100000♠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 7013299195741400000♠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 7013373994676750000♠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 7000555014702134197♠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 7012448793612100000♠30 AU from the Sun.^[10][54] Generally, TNOs with perihelia smaller than 7012538552334520000♠36 AU experience strong encounters with Neptune.^[1]

These six are the only minor planets known to have perihelia greater than 7012448793612100000♠30 AU and a semi-major axis greater than 7013373994676750000♠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]

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 7014299195741400000♠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 ≈ 400–7014224396806050000♠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.

• aphelion ≈ 7014179517444840000♠1,200 AU, perihelion ≈ 7013299195741400000♠200 AU, semi-major axis a ≈ 7014104718509490000♠700 AU,
• eccentricity e ≈ 0.6, orbital period 10,000 to 20,000 years,
• an inclination i ≈ 30° to the ecliptic,^{[upper-alpha 8]}
• an argument of perihelion ω ≈ 150° and longitude of perihelion ϖ = 7000420624349730634♠241°±15° (both the longitude and argument of perihelion are about 180° away from the 6 clustered extreme TNOs).^[1]

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]

• • appearance of strong anti-alignment for objects with semi-major axis beyond 250 AU.
  • preference for slight alignment for objects with semi-major axis ranging between 100-250 AU.^{[upper-alpha 9]}

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 7014299195741400000♠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 7013373994676750000♠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 7013224396806050000♠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 7013373994676750000♠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]

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 7012598391482800000♠40 AU and semi-major axes greater than 7013373994676750000♠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 7013224396806050000♠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]

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 7000205599785884932♠117.8°+11°
−10°. At this location, Planet Nine would be approximately 7013942466585410000♠630 AU from the Sun,^[81] with right ascension close to 2^h 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 3.0^h to 5.5^h 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 60–7013149597870700000♠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 7013373994676750000♠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 > 7012553512121590000♠37 AU, 50 < a < 7013747989353500000♠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:

• uo3L91, another distant object spotted by the Outer Solar System Origins Survey, discussed by Michele Bannister at a March 2016 lecture hosted by the SETI Institute and the same was reported in October 2016 AAS conference.^[88][89][90] 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.^[91][92] "L91" – as it has been dubbed – looks as if it might fit in with the antialigned group, but astrophysicist Michele Bannister of Queen's University Belfast, who described the object at the meeting, cited modeling that suggests maybe it does not have anything to do with Planet Nine.^[93][94]
• 2013 FT28, located on the opposite side of the sky (Longitude of perihelion aligned with Planet Nine) – but well within the proposed orbit of Planet Nine, where computer modeling suggests it would be safe from gravitational kicks.^[93]
• 2014 SR349, falling right in line with the earlier six objects.^[93]
• 2014 FE72, an object with an orbit so extreme that it reaches about 3000 AU from the Sun in a massively-elongated ellipse – at this distance its orbit is influenced by the galactic tide and other stars.^{[95][96][97][98]}

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

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

Notes

References

External links

Wikimedia Commons has media related to Planet Nine.

• A New Planet in our Solar System? NASA Takes a Look (NASA video, 21 January 2016)
• A new 9th planet for the solar system? (Science Magazine video, 20 January 2016)
• The Search for Planet Nine – blog by study authors
• A summary of the history behind the search & claims for a ninth planet
• Could You Live on Planet Nine? – Science article by Rhett Allain
• 'Planet Nine': Facts About the Mysterious Solar System World (space.com infographic)
• Planet Nine May Help Us Slingshot Our Way to the Stars
• Is Planet Nine truly "discovered"? A brief history of discoveries before & leading-up to the predicted planet.

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2016 in space
Spacecraft launches	ExoMars Trace Gas Orbiter (Mars orbiter) Schiaparelli EDM lander (Mars lander) Hitomi (X-ray observatory) OSIRIS-REx (mission to asteroid)
Selected NEOs	Asteroid close approaches 2013 TX68 252P/LINEAR 2016 EU85 (469219) 2016 HO3 2016 PQ
Planets	Exoplanets HD 19467 b 2MASS J2126-8140 BD+20594b Pr 0211-c TRAPPIST-1b TRAPPIST-1c TRAPPIST-1d Kepler-1229b Kepler-1520b Kepler-1638b Kepler-1647b Kepler-20g V830 Tau b Qatar-3b Qatar-4b Qatar-5b K2-33b HIP 41378 b HIP 41378 c HIP 41378 d HIP 41378 e HIP 41378 f HD 131399 Ab HD 164922 c KELT-11b K2-72b K2-72c K2-72d K2-72e Proxima Centauri b 38 Virginis b TW Hydrae b OGLE-2007-BLG-349(AB)b Gliese 536 b Solar Planet Nine (hypothesized) Rogue 2MASS J1119–1137
Space exploration	Akatsuki: entered Venus orbit Juno: entered Jupiter orbit Rosetta / Philae: end of mission to comet 67P
Miscellaneous	BOSS Great Wall GN-z11 Crater 2 2011 KT19 Virgo I
Category:2015 in space — Category:2016 in space — Category:2017 in space