Planet Nine

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 proposed specific orbital parameters.^[1] The predicted planet could be a super-Earth, with an estimated mass of 10 Earths (approximately 5,000 times the mass of Pluto), a diameter two to four times that of Earth, and a highly elliptical orbit with an orbital period of approximately 15,000 years.^[6]

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

Batygin and Brown suggested Planet Nine was a primordial giant planet core that was ejected from its initial orbit by an encounter with Jupiter during the nebular epoch of the Solar System and was later perturbed into a stable orbit by a distant encounter with a passing star or by the gas nebula.^[1] Others have proposed that it was captured in a similar manner from another star,^[9] or that it formed on a very distant circular orbit and was perturbed into its current eccentric orbit during a distant encounter with another star.^[10][11]

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.^[12] It will likely be a name chosen from Roman or Greek mythology.^[13]

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".^[14] 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.^[16] 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 is estimated to be roughly 6999523598775598300♠30°±10°.^{[2][3][17][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.^[18][19]

The aphelion, or farthest point from the Sun, would be in the general direction of the constellation of Taurus,^[20] 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.^[21][22]

Brown thinks that if Planet Nine is confirmed to exist, a probe could fly by it in as little as 20 years, with a powered slingshot around the Sun.^[23]

Size and composition

Planet Nine is hypothesized to be two to four times the diameter of Earth;^[2][6] similar to the ice giants Uranus and Neptune.^[24]

The planet is estimated to have 10 times the mass^[15][17] and two to four times the diameter of Earth.^[6][25] 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.^[26] 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.^[27]

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.^[28] 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.^[28] 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.^[29]

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 object could have been an unknown planet on a distant orbit, a random star that passed near the Solar System, or a member of the Sun's birth cluster.^[30][31][32]
• In 2008 Tadashi Mukai and Patryk Sofia Lykawka suggested that a distant Mars- or Earth-sized minor planet currently in a highly eccentric orbit between 100 and 7013299195741400000♠200 AU and orbital period of 1000 years with an inclination of 20° to 40° was responsible for the structure of the Kuiper belt. They proposed that the perturbations of this planet excited the eccentricities and inclinations of the trans-Neptunian objects, truncated the planetesimal disk at 48 AU, and detached the orbits of some objects from Neptune. During Neptune's migration this planet is posited to have been captured in an outer resonance of Neptune and to have evolved into a higher perihelion orbit due to the Kozai mechanism leaving the remaining trans-Neptunian objects on stable orbits.^{[33][34][35][36]}
• In 2012, after analysing the orbits of a group of trans-Neptunian objects with highly elongated orbits, Rodney Gomes of the National Observatory of Brazil proposed that their orbits were due to the existence of an as yet undetected planet. This Neptune-massed planet would be on a distant orbit that would be too far away to influence the motions of the inner planets, yet close enough to cause the perihelia of scattered disc objects with semi-major axes greater than 300 AU to oscillate, delivering them into planet-crossing orbits similar to those of (308933) 2006 SQ372 and (87269) 2000 OO67 or detached orbits like that of Sedna. Alternatively the unusual orbits of these objects could be the result of a Mars-massed planet on an eccentric orbit that occasionally approached within 33 AU.^[37][38] 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.^[39] Later in 2015, Rodney Gomes, Jean Soares, and Ramon Brasser proposed that a distant planet was responsible for an excess of centaurs with large semi-major axes.^[40][41]
• The announcement in March 2014 of the discovery of a second sednoid, 2012 VP113, which shared some orbital characteristics with Sedna and other extreme trans-Neptunian objects, further raised the possibility of an unseen super-Earth in a large orbit.^[42][43]

Trujillo and Sheppard (2014)

The initial argument that the clustering of orbital elements of extreme trans-Neptunian objects such as sednoids might be caused by a massive unknown planet beyond Neptune was published in 2014 by astronomers Chad Trujillo and Scott S. Sheppard. Trujillo and Sheppard analyzed the orbits of twelve trans-Neptunian objects (TNOs) with perihelia greater than 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] In numerical simulations including only the known giant planets the arguments of perihelion of these objects circulated at varying rates which after billions of years would leave the perihelia of the twelve TNOs randomized, like in the rest of the trans-Neptunian region, unless there is something holding them in place.^[44] Trujillo and Sheppard suggested that the massive unknown planet at a few hundred astronomical units caused the arguments of perihelion of the extreme trans-Neptunian objects to librate about 0° or 180°^{[upper-alpha 3][upper-alpha 4]} via the Kozai mechanism so that their orbits cross the plane of the planet's orbit near perihelion and aphelion, at the closest and farthest points from the planet.^[47][4] Numerical simulations with a single body of 2–15 Earth masses in a circular low-inclination orbit between 7013299195741400000♠200 AU and 7013448793612100000♠300 AU indicated that the arguments of perihelia of Sedna and 2012 VP₁₁₃ would librate around 0° for billions of years (although the lower perihelion objects did not) and would undergo periods of libration with a Neptune mass object in a high inclination orbit at 1500 AU.^[4]

These simulations showed the basic idea of how a single large planet can shepherd the smaller extreme trans-Neptunian objects into similar types of orbits. It was a basic proof of concept simulation that did not obtain a unique orbit for the planet as they state there are many possible orbital configurations the planet could have.^[44] 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°.^[47][48] 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.^[49] 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.^[44][50] 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.^[51] They also did not rule out other explanations and expected more clarity as researchers study orbits of more such distant objects.^[52][53][54] A later analysis of the distributions of the directions of perihelia and orbital poles of the ETNOs also suggests that one additional planet may not be sufficient to explain the observed clustering.^[55]

Batygin and Brown (2016)

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

Clustering

Caltech's Konstantin Batygin and Michael E. Brown looked into refuting the mechanism proposed by Trujillo and Sheppard.^[1] After eliminating the objects in Trujillo and Sheppard's original analysis that were unstable due to close approaches to Neptune or were affected by Neptune mean-motion resonances they determined that the arguments of perihelion for the remaining six objects (namely Sedna, 2012 VP113, 2004 VN₁₁₂, 2010 GB174, 2010 VZ₉₈, and 2000 CR₁₀₅) were 7000555014702134197♠318°±8°. This was out of alignment with how the Kozai mechanism would align these orbits, at c. 0° or 180°.^{[1][upper-alpha 6]}

Batygin and Brown also found that the orbits of the six objects with semimajor axes greater than 250 AU and perihelia beyond 30 AU (namely Sedna, 2012 VP113, 2007 TG422, 2004 VN₁₁₂, 2013 RF98, and 2010 GB174) were aligned in space with their perihelia in roughly the same direction and approximately co-planar. They determined that there was only a 0.007% likelihood that this was due to chance.^{[1][14][57][58]}

These six objects had been discovered by six different surveys on six different telescopes. That made it less likely that the clumping might be due to an observation bias such as pointing a telescope at a particular part of the sky. These six were the only minor planets known to have perihelia greater than 7012448793612100000♠30 AU and a semi-major axis greater than 7013373994676750000♠250 AU, as of January 2016.^[59] Most have perihelia significantly beyond Neptune, which orbits 7012448793612100000♠30 AU from the Sun.^[15][60] Generally, TNOs with perihelia smaller than 7012538552334520000♠36 AU experience strong encounters with Neptune.^[1] All six objects are relatively small, but currently relatively bright because they are near their closest distance to the Sun in their elliptical orbits.

In a companion paper the cluster was extended to include the seventh object 2000 CR₁₀₅ with a semi-major axis of 227 AU and the absence of objects with perihelia beyond 42 AU and semi-major axes between 100 AU and 200 AU was noted.^[61]

Extreme Trans-Neptunian objects with perihelion greater than 30 AU and a semi-major axis greater than 250 AU

Since early 2016, seven more extreme trans-Neptunian objects have been discovered with orbits. These are also included in the orbital diagrams and tables below.

The Extreme Trans-Neptunian object orbits

6 original and 7 new TNO object orbits with current positions near their perihelion in purple, with hypothetical Planet Nine orbit in green

Closer up view of the 13 TNO current positions

^{(*) longitude of perihelion, ϖ, outside expected range}

Simulation

Batygin and Brown simulated the effect of a planet of mass M = 10 M_⊕^{[upper-alpha 8]} 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 observed distribution of orbits.

• 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 9]}
• 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.^[14] This mechanism, known as phase protection, prevents the trans-Neptunian objects trapped in mean-motion resonance from having close encounters with Planet Nine, and keeps them aligned.^[17] The resonances include high-order resonances, for example 27:17, and are interconnected, yielding an orbital evolution that is fundamentally chaotic, causing their semi-major axes to vary unpredictably on million-year timescales.^[64] The simulations also revealed that Planet Nine raises the perihelia of large semi-major axis trans-Neptunian objects producing Sedna-like orbits. This effect is temporary, however, in half a billion years Sedna will be a more typical trans-Neptunian object while other scattered disk objects that have large semi-major axis orbits could be in Sedna-like orbits.^[65]

Further simulations (published later in a companion paper) showed:^[61]

• 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.
• poles of the orbits of anti-aligned objects roughly aligned with Planet Nine's.
• anti-alignment weakens as the inclination of Planet Nine is increased.
• no reproduction of the apparent alignment of arguments of perihelion.
• the tight clustering observed at that time was atypical even with Planet Nine. ^{[upper-alpha 10]}

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][67][68] 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.^[14] 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.^[69] When the elongated perpendicular centaurs get too close to a giant planet, orbits such as that of 2008 KV₄₂ are created.^[70][71]

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 high perihelion objects that have semi-major axes greater than 7013373994676750000♠250 AU, and orbits that would be aligned with Planet Nine. Although they may include high eccentricity objects the most stable of these objects would have lower eccentricities.^{[1][upper-alpha 11]}

Alternate hypotheses

Inclination instability due to mass of undetected objects

Ann-Marie Madigan and Michael McCourt postulate that an inclination instability in a distant massive belt is responsible for the alignment of the arguments of perihelion of the ETNOs. The inclination instability occurs in a disk of particles in eccentric orbits around a massive object. The self-gravity of this disk causes its spontaneous organization, increasing the inclinations of the objects and aligning the arguments of perihelion, forming it into a cone above or below the original plane. This process requires an extended time and significant mass of the disk, on the order of a billion years for a 1–10 Earth-mass disk.^{[76][upper-alpha 12]} While an inclination instability can align the arguments of perihelion and raise perihelia, producing detached objects, it does not align the longitudes of perihelion.^[77] Mike Brown considers Planet Nine a more probable explanation, noting that current surveys do not support the existence of a scattered-disk region of sufficient mass to support this idea of "inclination instability".^[78][79]

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. Two other objects with semi-major axes greater than 7013224396806050000♠150 AU are also potentially in resonance with this planet. Their proposed planet could be on a lower eccentricity, low inclination orbit, with eccentricity e < 0.18 and inclination i ≈ 11°. The eccentricity is limited in this case by the requirement that close approaches of 2010 GB174 to the planet are avoided. If the ETNOs are in periodic orbits of the third kind, with their stability enhanced by the libration of their arguments of perihelion, the planet could be in a higher inclination orbit, with i ≈ 48°. Unlike Batygin and Brown, Malhotra, Volk and Wang do not specify that most of the distant detached objects would have orbits anti-aligned with the massive planet.^[80][81]

Temporary or coincidental 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. They simulated clones (objects with similar orbits) of 15 objects with semi-major axis > 150 AU and perihelion > 30 AU under the influence of a 10 Earth-massed Planet Nine in Batygin and Brown's best fit orbit.^{[upper-alpha 13]} While longitude of perihelion alignment of the objects with semi-major axis > 250 AU was observed in their simulations, the alignment of the arguments of perihelion was not. The simulations also revealed an increase in the inclinations of many objects, thereby predicting a larger reservoir of high-inclination TNOs.^{[upper-alpha 14]} These objects should have been detected in existing surveys but are still unseen so far—suggesting there is a currently missing or unseen signature of Planet Nine. A previously published paper concluded that current observations are insufficient to identify this signature, however.^[82] The perihelia of many of the objects also rose and fell smoothly, inconsistent with the current absence of extreme TNOs with perihelia between 50 AU and 70 AU. Their perihelia also reached values where the objects would not be observed and, after declining, fell low enough for the objects to enter planet-crossing orbits leading to their ejection from the Solar System. These factors would require a population of Sednas significantly larger than current estimates, and inconsistent with current models of the early Solar System, to explain current observations. Based on these challenges Shankman et al. concluded that the existence of Planet Nine is unlikely and that the currently observed alignment of the existing TNOs is a temporary phenomenon that will disappear as more objects are detected.^[83][84]

The results of the Outer Solar System Survey (OSSOS) suggests that the observed clustering is the result of a combination of observing bias and small number statistics. OSSOS, a well-characterized survey of the outer Solar System with known biases, observed eight trans-Neptunian objects with semi-major axis > 150 AU with orbits oriented on a wide range of directions. After accounting for the known observational biases of the survey, no evidence for the arguments of perihelion (ω) clustering^{[upper-alpha 15]} was seen and the orientation of the orbits of the objects with the largest semi-major axis was statistically consistent with random.^[85][86] A previously released paper by Mike Brown analyzed the discovery locations of eccentric trans-Neptunian objects. While identifying some biases he found that even with these biases the clustering of longitudes of perihelion of the known objects would be observed only 1.2% of the time if their actual distribution was uniform.^[77]

Subsequent efforts toward indirect detection

Cassini measurements of perturbations of Saturn

An analysis of Cassini data on Saturn's orbital residuals was inconsistent with Planet Nine being located with a true anomaly of −130° to −110° or −65° to 85°. The analysis, using Batygin and Brown's orbital parameters for Planet Nine, suggests that the lack of perturbations to Saturn's orbit is best explained if Planet Nine is located at a true anomaly of 7000205599785884932♠117.8°+11°
−10°. At this location, Planet Nine would be approximately 7013942466585410000♠630 AU from the Sun,^[87] with right ascension close to 2^h and declination close to −20°, in Cetus.^[88] 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°.^[55]

An improved mathematical analysis of Cassini data by astrophysicists Matthew Holman and Matthew Payne tightened the constraints on possible locations of Planet Nine. Holman and Payne developed a more efficient model that allowed them to explore a broader range of parameters than the previous analysis. The parameters identified using this technique to analyze the Cassini data was then intersected with Batygin and Brown's dynamical constraints on Planet Nine's orbit. Holman and Payne concluded that Planet Nine is most likely to be located within 20° of RA = 40°, Dec = −15°, in an area of the sky near the constellation Cetus.^[89][90]

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."^[91] Observations of Saturn's orbit neither prove nor disprove that Planet Nine exists. Rather, they suggest that Planet Nine could not be in certain sections of its proposed orbit because its gravity would cause a noticeable effect on Saturn's position, inconsistent with actual observations.

Analysis of Pluto's orbit

An analysis of Pluto's orbit by Matthew J. Holman and Matthew J. Payne found perturbations much larger than predicted by Batygin and Brown's proposed orbit for Planet Nine. Holman and Payne suggested three possible explanations: systematic errors in the measurements of Pluto's orbit; an unmodeled mass in the Solar System, such as a small planet in the range of 60–7013149597870700000♠100 AU (potentially explaining the Kuiper cliff); or a planet more massive or closer to the Sun instead of the planet predicted by Batygin and Brown.^[92][83]

Optimal orbit if objects are in strong resonances

An analysis by Sarah Millholland and Gregory Laughlin indicates that the commensurabilities (period ratios consistent with pairs of objects in resonance with each other) of the extreme TNOs are most likely to occur if Planet Nine has a semi-major axis of 654 AU. They used 11 then-known extreme TNOs with their semimajor axis over 200, and perihelion over 30 AU , with five bodies close to four simple ratios (5:1, 4:1, 3:1, 3:2) with a 654 AU distance: 2002 GB32, 2000 CR105 (5:1), 2001 FP185 (5:1), 2012 VP113 (4:1), 2014 SR349, 2013 FT28, 2004 VN112 (3:1), 2013 RF98, 2010 GB174, 2007 TG422, and (90377) Sedna (3:2). Beginning with this semi-major axis they determine that Planet Nine best maintains the anti-alignment of their orbits and a strong clustering of arguments of perihelion if it is near aphelion and has an eccentricity e ≈ 0.5, inclination i ≈ 30°, argument of perihelion ω ≈ 150°, and longitude of ascending node Ω ≈ 50° (the last differs from Brown and Batygin's value of 90°).^{[upper-alpha 16]} The favored location of Planet Nine is a right ascension of 30° to 50° and a declination of −20° to 20°. They also note that in their simulations the clustering of arguments of perihelion is almost always smaller than has been observed.^[94]

A previous analysis by Carlos and Raul de la Fuente Marcos of commensurabilities among the known ETNOs using Monte Carlo techniques revealed a pattern similar to that of the Kuiper belt, where accidental commensurabilities occur due to objects in resonances with Neptune. They find that that this pattern would be best explained if the ETNOs were in resonance with an additional planetary-sized object beyond Pluto and note that a number of these objects may be in 5:3 and 3:1 resonances if that object had semi-major axis of ≈700 AU.^[95]

Dynamical stability of distant objects

Juliette Becker and colleagues, noting the orbits must remain stable for an extended period for anti-alignment to occur, analyzed the stability of eight extreme trans-Neptunian objects under the influence of Planet Nine and Neptune. They found that many of these objects would remain stable, with semi-major axes varying by less than 100 AU, if Planet Nine's orbital elements fell in a broad region with semi-major axes ranging from 500 to 1200 AU and eccentricities ranging from 0.3 to 0.6 with lower eccentricities being favored at smaller semi-major axes. A Planet Nine with a semi-major axis of 700 AU and eccentricity of 0.4 provided the greatest stability, which contrasts with Batygin and Brown's best-fit orbit with an eccentricity of 0.6. They also determined that anti-alignment became more likely if Planet Nine had a higher eccentricity, with some overlap between favored regions of stability and anti-alignment indicating that the objects may be on the borders of instability. Some objects like 2007 TG422 and 2013 RF98, which were unstable under the influence of Neptune alone, became more stable when Planet Nine was added, although their semi-major axes tended to migrate. A number of objects spent significant periods near a resonance with Planet Nine with some hopping between resonances.^[96]

An analysis by Carlos and Raul de la Fuente Marcos revealed that if Planet Nine were on its nominal orbit and in the direction favored by the analysis of Cassini data, some objects like Sedna would remain on stable orbits while the orbits of others would become unstable leading to their ejection from the Solar System. Other possible orbits, however, could leave all six of the objects identified by Batygin and Brown on stable orbits that maintain anti-alignment.^[97][98]

Correlation of arguments and longitudes of perihelion

Trujillo and Sheppard in a paper announcing the discovery of several more distant objects noted a correlation between the longitude of perihelion and the argument of perihelion of these objects. Those with a longitude of perihelion of 0–120° have arguments of perihelion between 280–360°, and those with longitude of perihelion of 180–340° have argument of perihelion 0–40°. The statistical significance of this correlation was 99.99%. They suggest that the correlation is due to the orbits of these objects avoiding close approaches to Planet Nine by passing above or below its orbit. Trujillo and Sheppard also noted that the arguments of perihelion of ETNOs with perihelion less than 35 AU are opposite those with perihelion greater than 35 AU.^{[72][upper-alpha 17]}

Ascending nodes of large semi-major axis objects

In a paper by Carlos and Raul de la Fuente Marcos evidence is shown for a possible bimodal distribution of the nodal distances of the ETNOs. This correlation is unlikely to be the result of observational bias since it also appears in the nodal distribution of large semi-major axis centaurs and comets. If it is due to the extreme TNOs experiencing close approaches to Planet Nine, it is consistent with a planet with a semi-major axis of 300–400 AU.^[99][100]

Secular dynamics of extreme TNOs

Hervé Beust numerically calculated the Hamiltonian describing the secular dynamics of objects perturbed by Planet Nine. Plots of eccentricity vs. longitude of perihelion using these results formed closed curves, or libration islands for aligned and anti-aligned objects. These resemble the plots of Batygin and Brown's original paper that showed the evolution of the orbital elements of ETNOs in simulations under the influence of Planet Nine. Beust also produced similar plots for objects in resonance with a Planet Nine at a semi-major axis of 665 AU, for example Sedna in a 3:2 resonance, as proposed by Malhotra, Volk and Wang.^[80] The libration islands in some of these cases included locations other than alignment or anti-alignment. Beust notes that while the phase protection of resonant objects provides additional protection, bodies in the anti-aligned population need not be in resonance with Planet Nine to remain in long-term stable orbits.^[101]

Saillenfest, Fouchard, Tommei, and Valsecchi expanded on Beust's work examining secular dynamics for the non-planar case including the effects of the eccentric Kozai mechanism,^{[upper-alpha 18]} that can result in the flipping of orbits from prograde to retrograde. For a Planet Nine in the plane of the other planets they found that the aligned and anti-aligned configurations identified by Beust extended to inclined objects with semi-major axis under 300 AU. They also identified stable configurations where inclinations oscillated around 90°, that may be related to perpendicular objects observed in simulations by Batygin and Brown, for semi-major axes of 200 to 300 AU. At higher semi-major axes the orbits of these perpendicular objects crossed those of the other planets. Beyond 300 AU the appearance of alignment and anti-alignment was more the result of sticky chaos rather than confinement: orbits wandered everywhere but spent more time in regions of relative stability associated with secular resonances. With an inclined Planet Nine its influence extended as low as 70 AU, generating some orbital chaos, and beyond 150 AU was strong enough to potentially generate flips from prograde to retrograde orbits. Unlike Beust their analysis did not extend to mean-motion resonances.^[103] In a previous paper they found that objects objects in resonance with Neptune with semi-major axes as large as 350 AU could undergo significant increases in their perihelia and inclinations due to the Kozai mechanism if there is no Planet Nine.^[104] Trujillo and Sheppard noted that in simulations 2013 FT₂₈ had significant variations in its orbit and suggested this was due to interactions with Neptune's resonances.^[72]

Orbits of nearly parabolic comets

An analysis of the orbits of comets with nearly parabolic orbits identifies five new comets with hyperbolic orbits that approach the nominal orbit of Planet Nine described in Batygin and Brown's initial paper. If these orbits are hyperbolic due to close encounters with Planet Nine the analysis estimates that Planet Nine is currently near aphelion with a right ascension of 83°–90° and a declination of 8°–10°.^[105] Scott Sheppard, who is skeptical of this analysis, notes that a lot of forces influence the orbits of comets.^[83]

Possible disrupted binary

Similarities between the orbits of 2013 RF98 and (474640) 2004 VN112 have led to the suggestion that they were a binary object disrupted near aphelion during an encounter with a distant object. The visible spectra of (474640) 2004 VN₁₁₂ and 2013 RF₉₈ are also similar but very different from that of 90377 Sedna. The value of their spectral slopes suggests that the surfaces of (474640) 2004 VN₁₁₂ and 2013 RF₉₈ can have pure methane ices (like in the case of Pluto) and highly processed carbons, including some amorphous silicates.^[106][107] The disruption of a binary would require a relatively close encounter with Planet Nine, however, which becomes less likely with its large semi-major axis.

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.^[108] 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.^[109]

New extreme trans-Neptunian objects discovered by Trujillo and Sheppard include:

• 2013 FT28, located on the opposite side of the sky (Longitude of perihelion aligned with Planet Nine) – but well within the proposed orbit of Planet Nine, where computer modeling suggests it would be safe from gravitational kicks.^[110]
• 2014 SR349, falling right in line with the earlier six objects.^[110]
• 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.^{[111][112][113][114]}

Other new extreme trans-Neptunian objects discovered by the Outer Solar System Origins Survey include:^[115]

• 2013 SY99, which has a lower inclination than many of the objects, and which was discussed by Michele Bannister at a March 2016 lecture hosted by the SETI Institute and later at an October 2016 AAS conference.^[116][117]
• 2015 KG163, which has an orientation similar to 2013 FT₂₈ but has a larger semi-major axis that may result in its orbit crossing Planet Nine's.
• 2015 RX245, which fits with the other anti-aligned objects.
• 2015 GT50, which is in neither the anti-aligned nor the aligned groups; instead, its orbit's orientation is at a right angle to that of the proposed Planet Nine. Its argument of perihelion is also outside the cluster of arguments of perihelion.

Batygin and Brown also predict a yet-to-be-discovered population of distant objects. These objects would have semi-major axes greater than 7013373994676750000♠250 AU, but they would have lower eccentricities and orbits that would be aligned with that of Planet Nine. The larger perihelia of these objects would make them fainter and more difficult to detect than the anti-aligned objects.^[1]

Other impacts

Solar obliquity

Analyses conducted contemporarily and independently by Bailey, Batygin and Brown, and by Gomes, Deienno and Morbidelli, and later by Lai suggest that Planet Nine could be responsible for inducing the spin–orbit misalignment of the Solar System. The Sun's axis of rotation is tilted approximately six degrees from the orbital plane of the giant planets. The exact reason for this discrepancy remains an open question in astronomy. The analysis used computer simulations to show that both the magnitude and direction of tilt can be explained by the gravitational torques exerted by Planet Nine 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.^{[118][119][7][120]}

High inclination TNOs

A population of high inclination trans-Neptunian objects with semi-major axes less than 100 AU is generated via the combined effects of Planet Nine and the other giant planets. Large semi-major axis objects that reach high inclination orbits due to the Kozai mechanism can have their perihelia lowered during this process bringing them into orbits that intersect that of Neptune. Encounters with Neptune can then lower their semi-major axes to below 100 AU where their evolution is no longer controlled by Planet Nine. The orbital distribution of the longest lived of these objects is nonuniform. Most objects have orbits with perihelia ranging from 5 AU to 35 AU and inclinations below 110 degree, beyond a gap with few objects are others with inclinations near 150 degrees and perihelia near 10 AU.^[71]

Planet Nine cloud

The dynamical effect of Planet Nine generates a cloud of objects with semi-major axes centered around Planet Nine's semi-major axis. In simulations with the planets in their current orbits the number of objects in high-perihelion and moderate semi-major axis orbits (q > 7012553512121590000♠37 AU, 50 < a < 7013747989353500000♠500 AU) is increased threefold with a distant planet in a circular orbit at 250 AU and tenfold if it is in an eccentric orbit with a semi-major axis of 500 AU. The cloud of objects has 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.^[82] Simulations of the giant planet migration described in the Nice model with Planet Nine in its nominal orbit yield similar results. Most objects in the Planet Nine cloud have semi-major axes greater than Planet Nine's and their inclinations extend up to 180 degrees. Roughly 0.3–0.4 Earth masses of an initial 20 Earth mass planetesimal disk remain in the cloud at the end of the simulation of the last 4 billion years.^[8]

Effect on Oort cloud

An unpublished preprint by Technion doctoral student Erez Michaely and Harvard astronomy professor Avi Loeb finds that the perturbations of a Planet Nine orbiting within an inner Oort cloud with initially circular co-planar orbits^{[upper-alpha 19]} would produce a spheroidal structure at approximately 1200 AU, an inclined disk between 1500 AU and 3000 AU, and a warped disk beyond this extending to 5000 AU. Objects escaping from the spherical structure would be a potential source of comets. The structure differs from that produced by a passing star and they suggested that it could be detected by the Large Synoptic Survey Telescope or Breakthrough Initiatives.^[121] The spherical structure resembles the Planet Nine cloud captured from objects scattered outward by the other planets in other models.

Numerical simulations of the migration of the giant planets show that the number of objects captured in the Oort cloud is reduced if Planet Nine was in its predicted orbit at that time.^[8] This reduction of objects captured in the Oort cloud also occurred in simulations with the giant planets on their current orbits.^[82]

Comets

The inclination distribution of Jupiter-family (or ecliptic) comets becomes broader under the influence of Planet Nine. Jupiter-family comets originate primarily from the scattering objects, trans-Neptunian objects with semi-major axes that vary over time due to distant encounters with Neptune. In a model including Planet Nine the scattering objects that reach large semi-major axes dynamically interact with Planet Nine increasing their inclinations. As a result the population of the scattering objects, and the population of comets derived from it, is left with a broader inclination distribution. This inclination distribution is broader than is observed, in contrast to a five-planet Nice model without a Planet Nine that can closely match the observed inclination distribution.^[8][122]

In a model including Planet Nine part of the population of Halley-type comets is derived from the Planet Nine cloud. The dynamical effects of the planet drive oscillations of the perihelia of the objects in the Planet Nine cloud, delivering some of them into planet-crossing orbits. Encounters with the other planets can then alter their orbits placing them on low-perihelion orbits. The first step of this process is slow, requiring more than 100 million years, compared to comets from the Oort cloud, which can be dropped into low-perihelion orbits in one period. The Planet Nine cloud contributes roughly one-third of the total population of comets, which is similar to that without Planet Nine due to a reduced number of Oort cloud comets.^[8]

Direct detection

Visibility and location

Due to its extreme distance from the Sun, Planet Nine would reflect little sunlight, potentially evading telescope sightings.^[6] It is expected to have an apparent magnitude fainter than 22, making it at least 600 times fainter than Pluto.^{[2][upper-alpha 20]} 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.^[17] Statistically, the planet is more likely to be closer to its aphelion at a distance greater than 500 AU.^[2] This is because objects move more slowly when near their aphelion, in accordance with Kepler's second law.

Searches of existing data

The search in databases of stellar objects performed by Batygin and Brown has already excluded much of the sky the predicted planet could be in, save the direction of its aphelion, or in the difficult to spot backgrounds where the orbit crosses the plane of the Milky Way, where most stars lie.^[21] This search included the archival data from the Catalina Sky Survey to magnitude c. 19, Pan-STARRS to magnitude 21.5, and infrared data from WISE.^[2][2][21]

David Gerdes who helped develop the camera used in the dark Energy Survey claims that it is quite possible that one of the images taken for his galaxy map may actually contain a picture of Planet Nine, and if so, new software developed recently and used to identify objects such as 2014 UZ224 can help to find it.^[126]

Michael Medford and Danny Goldstein, graduate students at the University of California, Berkeley, are also examining archived data using a technique that combines multiple images, taken at different times. Using a supercomputer they will offset the images to account for the calculated motion of Planet Nine, allowing many faint images of a faint moving object to be combined to produce a brighter image.^[122]

A search combining multiple images collected by WISE and NEOWISE data has also been conducted. The initial search which covered the region of the sky identified by Holman and Payne using Cassini range data is being expanded to cover other regions away from the galactic plane.^[127]

Ongoing searches

Because the planet is predicted to be visible in the Northern Hemisphere, the primary search is expected to be carried out using the Subaru telescope, which has both an aperture large enough to see faint objects and a wide field of view to shorten the search.^[43] Two teams of astronomers—Batygin and Brown, as well as Trujillo and Sheppard—are undertaking this search together, and both teams cooperatively expect the search to take up to five years.^[15][128] Brown and Batygin initially narrowed the search for Planet Nine down to roughly 2,000 square degrees of sky near Orion, a swath of space, that in Batygin's opinion, could be covered in about 20 nights by the Subaru telescope.^[129] Subsequent refinements by Batygin and Brown have reduced the search space to 600–800 square degrees of sky.^[130]

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.^[90] DES observes about 105 nights per season, lasting from August to February.

Radiation

Although a distant planet such as Planet Nine would reflect little light, it would still be radiating the heat from its formation as it cools due to its large mass. At its estimated temperature of 47 K, the peak of its emissions would be at infrared wavelengths.^[131] This radiation signature could be detected by Earth-based infrared telescopes, such as ALMA,^[132] and a search could be conducted by cosmic microwave background experiments operating at mm wavelengths.^{[133][134][upper-alpha 21]} Additionally, Jim Green of NASA is optimistic that it could be observed by the James Webb Space Telescope, the successor to the Hubble Space Telescope, that is expected to be launched in 2018.^[136]

Citizen science

Zooniverse Backyard Worlds: Planet 9 project

The Zooniverse Backyard Worlds project, started in February 2017, is using archival data from the WISE spacecraft to search for Planet Nine. The project will additionally search for substellar objects like brown dwarfs in the neighborhood of the Solar System.^[137][138]

Zooniverse SkyMapper project

In April 2017,^[139] using data from the SkyMapper telescope at Siding Spring Observatory, citizen scientists on the Zooniverse platform reported four candidates for Planet Nine. These candidates will be followed up on by astronomers to determine their viability.^[140] The project, which started on 28 March, completed their goals in less than three days with around five million classifications by more than 60,000 individuals.^[140]

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 into a distant eccentric orbit following a close encounter with Jupiter or Saturn during the nebular epoch.^[1] Gravitational interactions with nearby stars in the Sun's birth cluster, or dynamical friction from the gaseous remnants of the solar nebula,^[141] then reduced the eccentricity of its orbit, raising its perihelion, leaving it on a very wide but stable orbit.^[65][142] 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.^[143] 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.^[15] 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,^[144] which would then require another explanation.^[145] These ejections, however, are likely to have been two events well separated in time.^[146]

Close encounters between the Sun and other stars in its birth cluster could have resulted in the capture of a planet from beyond the Solar System. Three-body interactions during these encounters can perturb the path of planets on distant orbits around another star, or free-floating planets, in a process similar to the capture of irregular satellites around the giant planets, leaving one in a stable orbit around the Sun. A planet that originated in a system with a number of Neptune-massed planets and without Jupiter-massed planets, could be scattered into a more long-lasting distant eccentric orbit, increasing its chances of capture by another star.^[9] 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.^[11] In a simulation where the capture takes place between co-planar systems a large number of other objects are also captured into orbits aligned with the planet, potentially allowing this capture scenario to be distinguished from others.^[147]

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.^[10] 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%.^[11] A previous paper found that a massive disk extending beyond 80 AU would drive Kozai oscillations of objects scattered outward by Jupiter and Saturn leaving some of them in high inclination (inc > 50°), low eccentricity orbits which have not been observed.^[148]

Ethan Siegel, who is deeply skeptical of the existence of an undiscovered new planet in the Solar System, nevertheless speculates that at least one super-Earth, which have been commonly discovered in other planetary systems but have not been discovered in the Solar System, might have been ejected from the Solar System during a dynamical instability in the early Solar System.^[68][149] 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.^[150]

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.^[151]

Commentary

Batygin was cautious in interpreting the results of the simulation developed for his and Brown's paper, saying, "Until Planet Nine is caught on camera it does not count as being real. All we have now is an echo."^[152] Batygin's and Brown's work is similar to how Urbain Le Verrier predicted the position of Neptune based on Alexis Bouvard's observations and theory of Uranus' peculiar motion.

Brown put the odds for the existence of Planet Nine at about 90%.^[6] Greg Laughlin, one of the few researchers who knew in advance about this 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.^{[60][109][153]} Brown, though conceding the skeptics' point, still thinks that there is enough data to mount a search for a new planet.^[154]

Brown is supported by Jim Green, director of NASA's Planetary Science Division, who said that "the evidence is stronger now than it's ever been before".^[136]

Tom Levenson concluded that, for now, Planet Nine seems the only satisfactory explanation for everything now known about the outer regions of the Solar System.^[152] 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]

Malhotra remains agnostic about Planet Nine, but noted that she and her colleagues have found that the orbits of extremely distant KBOs seem tilted in a way that is difficult to otherwise explain. "The amount of warp we see is just crazy," she says. "To me, it's the most intriguing evidence for Planet Nine I've run across so far."^[83]

See also

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

Notes

References

Further reading

• Calandrelli, Emily; Escher, Anna (16 December 2016). "The top 15 events that happened in space in 2016". TechCrunch. Astronomers find evidence for Planet 9. Retrieved 2016-12-16. 

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 and claims for a ninth planet
• Could You Live on Planet Nine? – Science article by Rhett Allain
• 'Planet Nine': Facts About the Mysterious Solar System World (space.com infographic)
• Planet Nine May Help Us Slingshot Our Way to the Stars
• Is Planet Nine truly "discovered"? A brief history of discoveries before and leading-up to the predicted planet
• Solar System Survey Casts Doubt on Mysterious “Planet Nine” Nature June 23, 2017
• Is There a Giant Planet Lurking Beyond Pluto? A race is on to discover Planet Nine using classical astronomy and new computational techniques IEEE Spectrum July 31, 2017 – a good summary as of publication date

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