Artist's impression of Planet Nine as an ice giant eclipsing the central Milky Way, with the Sun in the distance. Neptune's orbit is shown as a small ellipse around the Sun.
| |
| Orbital characteristics | |
|---|---|
| Aphelion | 1,200 AU (est.) |
| Perihelion | 200 AU (est.) |
| 700 AU (est.) | |
| Eccentricity | 0.6 (est.) |
| 10,000 to 20,000 years | |
| Inclination | 30° to ecliptic (est.) |
| 150° (est.) | |
| Physical characteristics | |
Mean radius
| 13,000 to 26,000 km (8,000–16,000 mi) 2–4 R⊕ (est.) |
| Mass | 6×1025 kg (est.) ≥10 M⊕ (est.) |
| >22.5 (est.) | |
Planet Nine is a hypothetical planet in the outer region of the Solar System. Its gravitational influence could explain a statistical anomaly in the distribution of orbits for a group of distant trans-Neptunian objects (TNOs) found mostly beyond the Kuiper belt in the scattered disc region. This undiscovered super-Earth-sized planet would have an estimated mass of ten Earths, a diameter two to four times that of Earth, and an elongated orbit lasting 10,000 - 20,000 years. Batygin and Brown suggest that Planet Nine could be the core of a primordial giant planet that was ejected from its original orbit by Jupiter during the genesis of the Solar System. Others have proposed that the planet was captured from another star, is a captured rogue planet, or that it formed on a distant orbit and was scattered onto an eccentric orbit by a passing star. As of 2018, efforts have failed to directly observe Planet Nine.
History
Following the discovery of the planet Neptune
in 1846, there was considerable speculation that another planet might
exist beyond its orbit. The Planet Nine hypothesis predicts a specific
planet of a certain size and with certain orbital characteristics that
are different from past theories.
Attempts to detect planets beyond Neptune by indirect means such as orbital perturbation date back beyond the discovery of Pluto. George Forbes
was the first to postulate the existence of trans-Neptunian planets in
1880, and his work is considered similar to more recent Planet Nine
theories. In Forbes's model the planet had a semi-major axis of ∼300 AU (roughly three hundred times the distance from Earth to the Sun); locations were based on clustering of the aphelion distances of periodic comets.
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. 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.
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.
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. 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.
In 2014 astronomers Chad Trujillo and Scott S. Sheppard noted the similarities in the orbits of Sedna and 2012 VP113 and several other objects. In early 2016, Caltech's Konstantin Batygin and Michael E. Brown described how the similar orbits of six TNOs could be explained by Planet Nine and proposed a possible orbit for the planet. This hypothesis could also explain TNOs with orbits perpendicular to the inner planets and others with extreme inclinations, as well as the tilt of the Sun's axis.
Batygin was cautious in interpreting the results of the
simulation developed for his and Brown's research article, saying,
"Until Planet Nine is caught on camera it does not count as being real.
All we have now is an echo." Brown put the odds for the existence of Planet Nine at about 90%. Greg Laughlin, one of the few researchers who knew in advance about this article, gives an estimate of 68.3%.
Other skeptical scientists demand more data in terms of additional KBOs
to be analysed or final evidence through photographic confirmation. Brown, though conceding the skeptics' point, still thinks that there is enough data to mount a search for a new planet.
Predicted 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.
Orbit
Planet Nine is hypothesized to follow a highly elliptical orbit around the Sun with a period lasting 10,000–20,000 years. The planet's semi-major axis is estimated to be 700 AU,[A] roughly 20 times the distance from Neptune to the Sun, and its inclination to the ecliptic to be about 30°±10°. The high eccentricity of Planet Nine's orbit could bring it as close as 200 AU to the Sun at its perihelion and take it as far away as 1,200 AU at its aphelion.
The aphelion, or farthest point from the Sun, would be in the general direction of the constellation of Taurus, 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.
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.
Size and composition
Planet Nine is hypothesized to be two to four times the diameter of Earth; similar to the ice giants Uranus and Neptune.
The planet is estimated to have 10 times the mass and two to four times the diameter of Earth. An object with the same diameter as Neptune has not been excluded by previous surveys in visible light. Infrared surveys by the Wide-field Infrared Survey Explorer (WISE) may have the capabilities to detect Planet Nine, depending upon its location and characteristics. Past surveys by WISE have not excluded the existence of Planet Nine, and a new survey ongoing since 2017 may yet find it.
Brown thinks that if Planet Nine exists, its mass is sufficient to clear its orbit
of large bodies in 4.6 billion years (with possible exceptions for some
combinations of semi-major axis and mass) and that its gravity
dominates the outer edge of the Solar System, which is sufficient to
make it a planet by current definitions. Jean-Luc Margot has also stated that Planet Nine satisfies his criteria and would qualify as a planet if and when it is detected.
Brown speculates that the predicted planet is most probably an ejected ice giant, similar in composition to Uranus and Neptune: a mixture of rock and ice with a small envelope of gas. In fact, if it once orbited the region of the gas/ice giants, the planet probably acquired an atmosphere of hydrogen and helium.
Evidence
The gravitational influence of Planet Nine would explain five peculiarities of the Solar System:
- the clustering of the orbits of extreme trans-Neptunian objects (eTNOs);
- the high perihelia of objects like 90377 Sedna that are detached from Neptune's influence;
- the high inclinations of extreme trans-Neptunian objects with orbits roughly perpendicular to the orbits of the eight known planets,
- high-inclination trans-Neptunian objects with semi-major axis less than 100 AU;
- and the obliquity, or tilt, of the Sun's axis six degrees relative to the orbital planes of the major planets.
While other mechanisms have been offered for many of these
peculiarities, the gravitational influence of Planet Nine is the only
one that explains all five. The gravity of Planet Nine also excites the
inclinations of scattered disc objects, which in numerical simulations
leaves the short-period comets with a broader inclination distribution than is observed.
Observations: Orbital clustering amongst high perihelion objects
The
clustering of the orbits of extreme trans-Neptunian objects was first
described by Trujillo and Sheppard, who noted similarities between the
orbits of Sedna and 2012 VP113. Upon further analysis, they observed that the arguments of perihelion (which indicate the orientation of elliptical orbits within their orbital planes) of 12 eTNOs with perihelia greater than 30 AU and semi-major axes greater than 150 AU
were clustered near zero degrees. Trujillo and Sheppard proposed that
this alignment was caused by a massive unknown planet beyond Neptune via
the Kozai mechanism, which leads to a periodic exchange between eccentricity and inclination.
Batygin and Brown, looking to refute the mechanism proposed by
Trujillo and Sheppard, also examined the orbits of the extreme
trans-Neptunian objects.
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's mean-motion resonances, they determined that the arguments of perihelion for the remaining six objects (namely Sedna, 2012 VP113, 2004 VN112, 2010 GB174, 2000 CR105, and 2010 VZ98) were clustered around 318°±8°. This was out of alignment with how the Kozai mechanism would align these orbits, at c. 0° or 180°.
Orbital correlations among six distant trans-Neptunian objects led to the hypothesis.
Batygin and Brown also found that the orbits of the six objects with
semi-major axes greater than 250 AU and perihelia beyond 30 AU (namely
Sedna, 2012 VP113, 2004 VN112, 2010 GB174, 2007 TG422, and 2013 RF98) were aligned in space with their perihelia in roughly the same direction, resulting in a clustering of their longitudes of perihelion,
the directions where they make their closest approaches to the sun. The
orbits of the six objects were also tilted with respect to that of the ecliptic, the plane of the solar system, and approximately co-planar, producing a clustering of their longitudes of ascending nodes,
the directions where they each rise through the plane of the solar
system. They determined that there was only a 0.007% likelihood that
this combination of alignments was due to chance.
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. The observed clustering should be smeared
out by the object's varied precession rates in a few hundred million
years.
This indicates that it could not be due to an event in the distant
past, like a passing star, and is most likely being maintained by an
object orbiting the Sun.
In a later article Trujillo and Sheppard noted a correlation
between the longitude of perihelion and the argument of perihelion of
the eTNOs with semi-major axes greater than 150 AU. 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 suggested that the correlation is due to
the orbits of these objects avoiding close approaches to a massive
planet by passing above or below its orbit.
The extreme trans-Neptunian object orbits
6 original and 8 TNO object orbits with current positions near their perihelion in purple, with hypothetical Planet Nine orbit in green
Close up view of 13 TNO current positions
Simulations: Observed clustering reproduced, inference on Planet Nine orbit and mass
The
clustering of the orbits of extreme trans-Neptunian objects and raising
of their perihelia is reproduced in simulations that include Planet
Nine. In simulations conducted by Batygin and Brown, swarms of large
semi-major axis scattered disk objects that began with random orientations were sculpted into roughly collinear groups of spatially confined orbits by a massive distant planet in a highly eccentric orbit.
Batygin and Brown found that the distribution of the orbits of
the first six extreme trans-Neptunian objects was best reproduced in
simulations using a 10 M⊕ planet in the following orbit:
- semi-major axis a ≈ 700 AU (orbital period 7001.5=18,520 years)
- eccentricity e ≈ 0.6, (perihelion ≈ 280 AU, aphelion ≈ 1,120 AU)
- inclination i ≈ 30° to the ecliptic
- longitude of the ascending node Ω ≈ 94°.
- argument of perihelion ω ≈ 139° and longitude of perihelion ϖ = 235°±12°
This orbit results in strong anti-alignment, with objects having
perihelia opposite Planet Nine's perihelion, beyond 250 AU, weak
alignment between 150 AU and 250 AU, and little effect inside 150 AU. Simulations conducted by Becker et al. found a similar range for the
stability of eTNOs, semi-major axes ranging from 500 to 1200 AU and
eccentricities ranging from 0.3 to 0.6 with lower eccentricities being
favored at smaller semi-major axes. They noted that while stability was
favored with smaller eccentricities, anti-alignment was more likely at
higher eccentricities near the borders of stability.
Lawler et al. found that the population captured was smaller in
simulations with a planet in a circular orbit which also produced few
high inclination objects.
Investigations by Cáceres et al. showed that a planet with a lower
perihelion led to a narrower confinement of orbits of the eTNOs, with a
perihelion of 90 AU or higher being consistent with the distribution of
the classical Kuiper belt objects.
Dynamics: Effect on other objects in the Solar System
Planet
Nine modifies the orbits of extreme trans-Neptunian objects via a
combination of effects. On very long timescales exchanges of angular
momentum with Planet Nine causes the perihelia of anti-aligned objects
to rise until their precession reverses direction, maintaining their
anti-alignment, and later fall, returning them to their original orbits.
On shorter timescales mean-motion resonances with Planet Nine provides
phase protection, which stabilizes their orbits by slightly altering the
objects' semi-major axes, keeping their orbits synchronized with Planet
Nine's and preventing close approaches. The inclination of Planet
Nine's orbit weakens this protection, resulting in a chaotic variation
of semi-major axes as objects hop between resonances. The orbital poles
of the objects circle that of the Solar System's Laplace plane,
which at large semi-major axes is warped toward the plane of Planet
Nine's orbit, causing their poles to be clustered toward one side.
Apsidal anti-alignment
Long term evolution of eTNOs induced by Planet Nine for objects with semi-major axis of 250 AU. Blue:anti-aligned, Red:aligned, Green:metastable, Orange:circulating. Crossing orbits above black line.
The anti-alignment and the raising of the perihelia of extreme
trans-Neptunian objects with semi-major axes greater than 250 AU occur
on very long time scales relative to their orbital periods. At these
timescales the perturbations two objects exert on each other are the
average between all possible configurations. Effectively the
interactions become like those between two wires of varying thickness,
thicker where the objects spend more time, that are exerting torques on each other, causing exchanges of angular momentum but not energy. Therefore the eccentricities, inclinations and orientations of orbits are altered but not the semi-major axes.
Exchanges of angular momentum with Planet Nine cause the
perihelia of the anti-aligned objects to rise and fall while their
longitudes of perihelion librate, or oscillate
within a limited range of values. When the angle between an
anti-aligned object's perihelion and Planet Nine's (delta longitude of
perihelion on diagram) climbs beyond 180° the objects gains angular
momentum
from Planet Nine causing the eccentricity of its orbit to decline (see
blue curves on diagram) and its perihelion to rise away from Neptune's
orbit. The object's precession then slows and eventually reverses as its
eccentricity declines. After delta longitude of perihelion drops below
180° the object loses angular momentum to Planet Nine and its
eccentricity grows and perihelion falls. When the object's eccentricity
is once again large it precesses forward, returning the object to its
original orbit after several hundred million years.
The behavior of the orbits of other objects varies with their
initial orbits. Stable orbits exist for aligned objects with small
eccentricities. Although objects in these orbits have high perihelia and
have yet to be observed, they may have been captured at the same time
as Planet Nine due to perturbations from a passing star. Aligned objects with lower perihelia are only temporarily stable, their orbits precess until parts of the orbits are tangent to that of Planet Nine, leading to frequent close encounters.
After crossing this region the perihelia of their orbits decline,
causing them to encounter the other planets, leading to their ejection.
The curves the orbits follow vary with semi-major axis of the
object and if the object is in resonance. At smaller semi-major axes the
aligned and anti-aligned regions shrink and eventually disappear below
150 AU, leaving typical Kuiper belt objects unaffected by Planet Nine.
At larger semi-major axes the region with aligned orbits becomes
narrower and the region with anti-aligned orbits becomes wider.
These regions also shift to lower perihelia, with perihelia of 40 AU
becoming stable for anti-aligned objects at semi-major axes greater than
1000 AU.
The anti-alignment of resonant objects, for example if Sedna is in a
3:2 resonance with Planet Nine as proposed by Malhotra, Volk and Wang, is maintained by a similar evolution inside the mean-motion resonances.
The objects behavior is more complex if Planet Nine and the eTNOs are
in inclined orbits. Objects then undergo a chaotic evolution of their
orbits, but spend much of their time in the aligned or anti-aligned
orbits regions of relative stability associated with secular resonances.
Evolution and long-term stability of anti-aligned orbits
An example of phase-protection in a mean-motion resonance: The orbital resonances of Orcus and Pluto in a rotating frame with a period equal to Neptune's orbital period. (Neptune is held stationary.)
The long term stability of anti-aligned extreme trans-Neptunian
objects with orbits that intersect that of Planet Nine is due to their
being captured in mean-motion resonances. Objects in mean-motion resonances
with a massive planet are phase protected, preventing them from making
close approaches to the planet. When the orbit of a resonant object
drifts out of phase, causing it to make closer approaches to a massive planet, the gravity of the planet modifies its orbit,
altering its semi-major axis in the direction that reverses the drift.
This process repeats as the drift continues in the other direction
causing the orbit to appear to rock back and forth, or librate, about a
stable center when viewed in a rotating frame of reference.
In the example at right, when the orbit of a plutino drifts backward it
loses angular momentum when it makes closer approaches ahead of
Neptune, causing its semi-major axis and period to shrink, reversing the drift.
In a simplified model where all objects orbit in the same plane and the giant planets are represented by rings,
objects captured in strong resonances with Planet Nine could remain in
them for the lifetime of the Solar System. At large semi-major axes,
beyond a 3:1 resonance with Planet Nine, most of these objects would be
in anti-aligned orbits. At smaller semi-major axes the longitudes of
perihelia of an increasing number of objects could circulate, passing
through all values ranging from 0° to 360°, without being ejected, reducing the fraction of objects that are anti-aligned. 2015 GT50 may be in one of these circulating orbits.
If this model is modified with Planet Nine and the eTNOs in
inclined orbits the objects alternate between extended periods in stable
resonances and periods of chaotic diffusion of their semi-major axes.
The distance of the closest approaches varies with the inclinations and
orientations of the orbits, in some cases weakening the phase protection
and allowing close encounters. The close encounters can then alter the
eTNO's orbit, producing stochastic jumps in its semi-major axis as it hops between resonances, including higher order resonances. This results in a chaotic diffusion
of an object's semi-major axis until it is captured in a new stable
resonance and the secular effects of Planet Nine shift its orbit to a
more stable region.
The chaotic diffusion reduces the range of longitudes of perihelion
that anti-aligned objects can reach while remaining in stable orbits.
Neptune's gravity can also drive a chaotic diffusion of semi-major axes when all objects are in the same plane.
Distant encounters with Neptune can alter the orbits of the eTNOs,
causing their semi-major axes to vary significantly on million year
timescales.
These perturbations can cause the semi-major axes of the anti-aligned
objects to diffuse chaotically while occasionally sticking in resonances
with Planet Nine. At semi-major axes larger than Planet Nine's, where
the objects spend more time, anti-alignment may be due to the secular
effects outside mean-motion resonances.
The phase protection of Planet Nine's resonances stabilizes the
orbits of objects that interact with Neptune via its resonances, for
example 2013 FT28, or by close encounters for objects with low perihelia like 2007 TG422 and 2013 RF98.[59]
Instead of being ejected following a series of encounters these objects
can hop between resonances with Planet Nine and evolve into orbits no
longer interacting with Neptune.
A shift in the position of Planet Nine in simulations from the location
favored by an analysis of Cassini data to a position near aphelion has
been shown to increase the stability of some of the observed objects,
possibly due to this shifting the phases of their orbits to a stable
range.
Clustering of orbital poles (nodal alignment)
Tilting of Laplace Plane by Planet Nine
The clustering of the orbital poles, which produces an apparent
clustering of the longitude of the ascending nodes and arguments of
perihelion of the extreme TNOs, is the result of a warping of the
Laplace plane of the Solar System toward that of Planet Nine's orbit.
The Laplace plane defines the center around which the pole of an
object's orbit precesses
with time. At larger semi-major axes the angular momentum of Planet
Nine causes the Laplace plane to be warped toward that of its orbit. As a result, when the poles of the eTNO orbit precess around
the Laplace plane's pole they tend to remain on one side of the
ecliptic pole. For objects with small inclination relative to Planet
Nine, which were found to be more stable in simulations, this off-center
precession produces a libration of the longitudes of ascending nodes
with respect to the ecliptic making them appear clustered.
In simulations the precession is broken into short arcs by encounters
with Planet Nine and the positions of the poles are clustered in an
off-center elliptical region.
In combination with the anti-alignment of the longitudes of perihelion
this can also produce clustering of the arguments of perihelion.
Objects in perpendicular orbits with large semi-major axis
The
orbits of the five objects with high-inclination orbits (nearly
perpendicular to the ecliptic) are shown here as cyan ellipses with the
hypothetical Planet Nine in orange. Those of four are towards the left
in this view, and that of one (2012 DR30) is towards the right, with an aphelion over 2,000 AU.
Planet Nine can deliver extreme trans-Neptunian objects into orbits roughly perpendicular to the plane of the Solar System. Several objects with high inclinations, greater than 50°, and large semi-major axes, above 250 AU, have been observed. Their high inclination orbits can be generated by a high order secular resonance with Planet Nine involving a linear combination
of the orbit's arguments and longitudes of perihelion: Δϖ - 2ω. Low
inclination eTNOs can enter this resonance after first reaching low
eccentricity orbits. The resonance causes their eccentricities and
inclinations to increase, delivering them into perpendicular orbits with
low perihelia where they are more readily observed. The orbits then
evolve into retrograde
orbits with lower eccentricities after which they pass through a second
phase of high eccentricity perpendicular orbits before returning to low
eccentricity, low inclination orbits. Unlike the Kozai mechanism this
resonance causes objects to reach their maximum eccentricities when in
nearly perpendicular orbits. In simulations conducted by Batygin and
Brown this evolution was relatively common, with 38% of stable objects
undergoing it at least once.
Saillenfest et al. also observed this behavior in their study of the
secular dynamics of eTNOs and noted that it caused the perihelia to fall
below 30 AU for objects with semi-major axes greater than 300 AU, and
with Planet Nine in an inclined orbit it could occur for objects with
semi-major axes as small as 150 AU.
In simulations the arguments of perihelion of the objects with roughly
perpendicular orbits and reaching low perihelia are clustered near or
opposite Planet Nine's and their longitudes of ascending node are
clustered around 90° in either direction from Planet Nine's.
This is in rough agreement with observations with the differences
attributed to distant encounters with the known giant planets.
Seven high inclination objects with semi-major axes greater than 250 AU
and perihelia beyond Jupiter's orbit are currently known:
| Object | Orbit | |||||||
|---|---|---|---|---|---|---|---|---|
| Perihelion (AU) Figure 9 |
Semimaj. (AU) Figure 9 |
Current distance from Sun (AU) |
inc (°) |
Eccen. | Arg. peri ω (°) |
Mag. | Diam. (km) | |
| (336756) 2010 NV1 | 9.4 | 323 | 14 | 141 | 0.97 | 133 | 22 | 20–45 |
| (418993) 2009 MS9 | 11.1 | 348 | 12 | 68 | 0.97 | 129 | 21 | 30–60 |
| 2010 BK118 | 6.3 | 484 | 11 | 144 | 0.99 | 179 | 21 | 20–50 |
| 2013 BL76 | 8.5 | 1,213 | 11 | 99 | 0.99 | 166 | 21.6 | 15–40 |
| 2012 DR30 | 14 | 1,404 | 17 | 78 | 0.99 | 195 | 19.6 | 185 |
| 2014 LM28 | 16.8 | 268 | 17 | 85 | 0.94 | 38 | 22 | 46 |
| 2015 BP519 | 35.3 | 449 | 53 | 54 | 0.92 | 348 | 21.5 | 550 |
High inclination TNOs with smaller semi-major axis orbits
A
population of high inclination trans-Neptunian objects with semi-major
axes less than 100 AU may be generated by the combined effects of Planet
Nine and the other giant planets. The extreme trans-Neptunian objects
that enter perpendicular orbits have perihelia low enough for their
orbits to intersect those of Neptune or the other giant planets.
Encounters with one of these planets can lower their semi-major axes to
below 100 AU where their evolution would no longer be controlled by
Planet Nine, leaving them on orbits like 2008 KV42.
The orbital distribution of the longest lived of these objects is
nonuniform. Most objects have orbits with perihelia ranging from 5 AU to
35 AU and inclinations below 110 degree, beyond a gap with few objects
are others with inclinations near 150 degrees and perihelia near 10 AU. Previously it was proposed that these objects originated in the Oort cloud.
Solar obliquity
Analyses
conducted contemporarily and independently by Bailey, Batygin and
Brown; by Gomes, Deienno and Morbidelli; and later by Lai suggest that
Planet Nine could be responsible for inducing the spin–orbit
misalignment of the Solar System. The Sun's axis of rotation is tilted
approximately six degrees from the orbital plane of the giant planets.
The exact reason for this discrepancy remains an open question in
astronomy. The analyses used analytical models and computer simulations
to show that both the magnitude and direction of tilt can be explained
by the gravitational torques exerted by Planet Nine. The torques would
cause the orbits of the other planets to precess, similar to but slower
than the eTNOs, covering short arcs over the lifetime of the Solar
System. These observations are consistent with the Planet Nine
hypothesis but do not prove that Planet Nine exists as there are other
potential explanations for the spin–orbit misalignment of the Solar System.
Oort cloud and comets
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. This reduction of objects captured in the Oort cloud also occurred in simulations with the giant planets on their current orbits.
The inclination distribution of Jupiter-family (or ecliptic) comets would become broader under the influence of Planet Nine. Jupiter-family comets
originate primarily from the scattering objects, trans-Neptunian
objects with semi-major axes that vary over time due to distant
encounters with Neptune. In a model including Planet Nine, the
scattering objects that reach large semi-major axes dynamically interact
with Planet Nine, increasing their inclinations. As a result, the
population of the scattering objects, and the population of comets
derived from it, is left with a broader inclination distribution. This
inclination distribution is broader than is observed, in contrast to a
five-planet Nice model without a Planet Nine that can closely match the
observed inclination distribution.
In a model including Planet Nine, part of the population of Halley-type comets
is derived from the cloud of objects that Planet Nine dynamically
controls. This Planet Nine cloud is made up of objects with semi-major
axes centered on that of Planet Nine that have had their perihelia
raised by the gravitational influence of Planet Nine. The continued
dynamical effects of Planet Nine drive oscillations of the perihelia of
these objects, delivering some of them into planet-crossing orbits.
Encounters with the other planets can then alter their orbits, placing
them in low-perihelion orbits where they are observed as comets. 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.
Reception
Brown is supported by Jim Green, director of NASA's Planetary Science Division, who said, "the evidence is stronger now than it's been before". But Green also cautioned about the possibility of other explanations for the observed motion of distant TNOs and, quoting Carl Sagan, he said, "extraordinary claims require extraordinary evidence."
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. Alessandro Morbidelli, who reviewed the research article for The Astronomical Journal, concurred, saying, "I don't see any alternative explanation to that offered by Batygin and Brown."
Malhotra remains agnostic about Planet Nine, but noted that she
and her colleagues have found that the orbits of extremely distant KBOs
seem tilted in a way that is difficult to otherwise explain. "The amount
of warp we see is just crazy," she said. "To me, it's the most
intriguing evidence for Planet Nine I've run across so far."
American astrophysicist Ethan Siegel,
who is deeply skeptical of the existence of an undiscovered 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. Planetary scientist 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.
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.
Alternate hypotheses
Temporary or coincidental nature of clustering
The
results of the Outer Solar System Survey (OSSOS) suggest that the
observed clustering is the result of a combination of observing bias and
small number statistics. OSSOS, a well-characterized survey of the
outer Solar System with known biases, observed eight trans-Neptunian
objects with semi-major axis > 150 AU with orbits oriented on a wide
range of directions. After accounting for the known observational biases
of the survey, no evidence for the arguments of perihelion (ω)
clustering identified by Trujillo and Sheppard was seen and the orientation of the orbits of the objects with the largest semi-major axis was statistically consistent with random.
A previously released article by Mike Brown analyzed the discovery
locations of eccentric trans-Neptunian objects. While identifying some
biases he found that even with these biases the clustering of longitudes
of perihelion of the known objects would be observed only 1.2% of the
time if their actual distribution was uniform.
Simulations of 15 known extreme trans-Neptunian objects evolving
under the influence of Planet Nine also revealed a number of differences
with observations. Cory Shankman et al. 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 proposed orbit.
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 that had not yet been
observed. A previously published article concluded that observations were insufficient to determine if this reservoir exists, however.
The perihelia of the objects rose and fell smoothly, in contrast with
the observed absence of extreme TNOs with perihelia between 50 AU and 70
AU. Their perihelia also reached values where the objects would not be
observed, requiring a population of Sednas significantly larger than
current estimates. They then, after declining, fell to values low enough
for the objects to enter planet-crossing orbits leading to their
ejection from the Solar System, requiring an initial population
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.
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.
While an inclination instability can align the arguments of perihelion
and raise perihelia, producing detached objects, it does not align the
longitudes of perihelion.
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".
In Nice model simulations that included the self-gravity of the
planetesimal disk an inclination instability did not occur due to a
rapid precession of the objects' orbits and their being ejected on too
short of a timescale.
Object in lower-eccentricity orbit
Renu Malhotra,
Kathryn Volk, and Xianyu Wang have proposed that the four detached
objects with the longest orbital periods, those with perihelia beyond 40 AU and semi-major axes greater than 250 AU, are in n:1 or n:2 mean-motion resonances with a hypothetical planet. Two other objects with semi-major axes greater than 150 AU
are also potentially in resonance with this planet. Their proposed
planet could be on a lower eccentricity, low inclination orbit, with
eccentricity e < 0.18 and inclination i ≈ 11°. The eccentricity is limited in this case by the requirement that close approaches of 2010 GB174 to the planet are avoided. If the eTNOs are in periodic orbits of the third kind,
with their stability enhanced by the libration of their arguments of
perihelion, the planet could be in a higher inclination orbit, with i
≈ 48°. Unlike Batygin and Brown, Malhotra, Volk and Wang do not specify
that most of the distant detached objects would have orbits
anti-aligned with the massive planet.
| Body | Orbital period Heliocentric (years) |
Orbital period Barycentric (years) |
Semimaj. (AU) |
Ratio |
|---|---|---|---|---|
| 2013 GP136 | 1,830 | 151.8 | 9:1 | |
| 2000 CR105 | 3,304 | 221.59±0.16 | 5:1 | |
| 2012 VP113 | 4268±179 | 4,300 | 265.8±3.3 | 4:1 |
| 2004 VN112 | 5845±30 | 5,900 | 319.6±6.0 | 3:1 |
| 2010 GB174 | 7150±827 | 6,600 | 350.7±4.7 | 5:2 |
| 90377 Sedna | ≈ 11,400 | 506.84±0.51 | 3:2 | |
| Hypothetical planet | ≈ 17,000 | ≈ 665 | 1:1 |
Alignment due to the Kozai mechanism
Trujillo and Sheppard (2014)
Trujillo
Sheppard argued in 2014 that a massive planet in a distant, circular
orbit was responsible for the clustering of the arguments of perihelion
of twelve extreme trans-Neptunian objects. Trujillo and Sheppard
identified a clustering near zero degrees of the arguments of perihelion
of the orbits of twelve trans-Neptunian objects with perihelia greater
than 30 AU and semi-major axes greater than 150 AU.
After numerical simulations showed that after billions of years the
varied rates of precession should leave their perihelia randomized they
suggested that a massive planet in a circular orbit at a few hundred
astronomical units was responsible for this clustering.
This massive planet would cause the arguments of perihelion of the
eTNOs to librate about 0° or 180° via the Kozai mechanism so that their
orbits crossed the plane of the planet's orbit near perihelion and
aphelion, the closest and farthest points from the planet. In numerical simulations including a 2–15 Earth mass body in a circular low-inclination orbit between 200 AU and 300 AU the arguments of perihelia of Sedna and 2012 VP113
librated around 0° for billions of years (although the lower perihelion
objects did not) and underwent periods of libration with a Neptune mass
object in a high inclination orbit at 1,500 AU.
An additional process such as a passing star would be required to
account for the absence of objects with arguments of perihelion near
180°.
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.
Thus they did not fully formulate a model that successfully
incorporated all the clustering of the extreme objects with an orbit for
the planet.
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)
Raúl and Carlos de la Fuente Marcos proposed a similar model but with two distant planets in resonance. An analysis by Carlos and Raúl de la Fuente Marcos with Sverre J. Aarseth
confirmed that the observed alignment of the arguments of perihelion
could not be due to observational bias. They speculated that instead it
was caused by an object with a mass between that of Mars and Saturn that
orbited at some 200 AU from the Sun. Like
Trujillo and Sheppard they theorized that the eTNOs are kept bunched
together by a Kozai mechanism and compared their behavior to that of Comet 96P/Machholz under the influence of Jupiter.
However, they also struggled to explain the orbital alignment using a
model with only one unknown planet. They therefore suggested that this
planet is itself in resonance with a more-massive world about 250 AU from the Sun.
In their article, Brown and Batygin noted that alignment of arguments
of perihelion near 0° or 180° via the Kozai mechanism requires a ratio
of the semi-major axes nearly equal to one, indicating that multiple
planets with orbits tuned to the data set would be required, making this
explanation too unwieldy.
Detection attempts
Visibility and location
Due to its extreme distance from the Sun, Planet Nine would reflect little sunlight, potentially evading telescope sightings. It is expected to have an apparent magnitude fainter than 22, making it at least 600 times fainter than Pluto.
If Planet Nine exists and is close to perihelion, astronomers could
identify it based on existing images. At aphelion, the largest
telescopes would be required. However, if the planet is currently
located in between, many observatories could spot Planet Nine. Statistically, the planet is more likely to be closer to its aphelion at a distance greater than 500 AU. 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. 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 the WISE satellite.
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,
purpose-built software, which was used to identify objects such as 2014 UZ224, can help to find it.
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.
A search combining multiple images collected by WISE and NEOWISE
data has also been conducted without detecting Planet Nine. This search
covered regions of the sky away from the galactic plane at the "W1"
wavelength (the 3.4 μm wavelength used by WISE) and is estimated to be
able to detect a 10 Earth mass object out to 800–900 AU.
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.
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. 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. Subsequent refinements by Batygin and Brown have reduced the search space to 600–800 square degrees of sky.
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. 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 (−226.2 °C), the
peak of its emissions would be at infrared wavelengths. This radiation signature could be detected by Earth-based submillimeter telescopes, such as ALMA, and a search could be conducted by cosmic microwave background experiments operating at mm wavelengths. Additionally, Jim Green of NASA's Science Mission Directorate 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 2021.
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.
32,000 animations of four images each, which constitute 3 per cent of
the WISE data has been uploaded to the Backyard World's website. By
looking for moving objects in the animations, citizen scientists could
find Planet Nine.
Zooniverse SkyMapper project
In April 2017, 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.
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.
Attempts to gather additional evidence
Search for additional extreme trans-Neptunian objects
Finding more eTNO's would allow astronomers to make more accurate predictions about the orbit of the hypothesized planet. 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.
Batygin and Brown also predict a yet-to-be-discovered population
of distant objects. These objects would have semi-major axes greater
than 250 AU, but they would have lower
eccentricities and orbits that would be aligned with that of Planet
Nine. The larger perihelia of these objects would make them fainter and
more difficult to detect than the anti-aligned objects.
Cassini measurements of perturbations of Saturn
An analysis of Cassini data on Saturn's orbital residuals was inconsistent with Planet Nine being located with a true anomaly
of −130° to −110° or −65° to 85°. The analysis, using Batygin and
Brown's orbital parameters for Planet Nine, suggests that the lack of
perturbations to Saturn's orbit is best explained if Planet Nine is
located at a true anomaly of 117.8°+11°
−10°. At this location, Planet Nine would be approximately 630 AU from the Sun, with right ascension close to 2h and declination close to −20°, in Cetus. In contrast, if the putative planet is near aphelion it could be moving projected towards the area of the sky with boundaries: right ascension 3.0h to 5.5h and declination −1° to 6°.
−10°. At this location, Planet Nine would be approximately 630 AU from the Sun, with right ascension close to 2h and declination close to −20°, in Cetus. In contrast, if the putative planet is near aphelion it could be moving projected towards the area of the sky with boundaries: right ascension 3.0h to 5.5h and declination −1° to 6°.
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.
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."
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 60–100 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.
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 semi-major 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 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°).
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.
A previous analysis by Carlos and Raul de la Fuente Marcos of
commensurabilities among the known eTNOs using Monte Carlo techniques
revealed a pattern similar to that of the Kuiper belt, where accidental
commensurabilities occur due to objects in resonances with Neptune. They
find that this pattern would be best explained if the eTNOs were in
resonance with an additional planetary-sized object beyond Pluto and
note that a number of these objects may be in 5:3 and 3:1 resonances if
that object had semi-major axis of ≈700 AU.
A later analysis by Elizabeth Bailey, Michael Brown and
Konstantin Batygin found that if Planet Nine is in an eccentric and
inclined orbit the capture of many of the eTNOs in higher order
resonances and their chaotic transfer between resonances prevent the
identification of Planet Nine's semi-major axis using current
observations. They also determined that the odds of the first six
objects observed being in N/1 or N/2 period ratios with Planet Nine are
less than 5% if it is in an eccentric orbit.
Ascending nodes of large semi-major axis objects
In an article by Carlos and Raul de la Fuente Marcos evidence is shown for a possible bimodal
distribution of the distances to the ascending nodes of the extreme
TNOs. 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.
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 article. If these orbits are hyperbolic due to close
encounters with Planet Nine the analysis estimates that Planet Nine is
currently near aphelion with a right ascension of 83°–90° and a
declination of 8°–10°. Scott Sheppard, who is skeptical of this analysis, notes that many different forces influence the orbits of comets.
Possible disrupted binary
Similarities between the orbits of 2013 RF98 and 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 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. The disruption of a binary would require a relatively close encounter with Planet Nine, however, which becomes less likely at large distances from the Sun.
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 article, Batygin and Brown proposed that Planet
Nine formed closer to the Sun and was ejected into a distant eccentric
orbit following a close encounter with Jupiter or Saturn during the nebular epoch. Gravitational interactions with nearby stars in the Sun's birth cluster, or dynamical friction from the gaseous remnants of the Solar nebula, then reduced the eccentricity of its orbit, raising its perihelion, leaving it on a very wide but stable orbit. 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.
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.
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. 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, which would then require another explanation. These ejections, however, are likely to have been two events well separated in time.
Dynamical friction from a massive belt of planetesimals
could also enable Planet Nine's capture in a stable orbit. Recent
models propose that a 60–130 Earth mass disk of planetesimals could have
formed via streaming instabilities following the photoevaporation of the outer parts of the proto-planetary disk.
If the disk had a distant inner edge, 100–200 AU, a planet encountering
Neptune would have a 20% chance of being captured in an orbit similar
to that proposed for Planet Nine. The observed clustering is more likely
if the inner edge is at 200 AU. Unlike the gas nebula the planetesimal
disk is likely to be long lived, potentially allowing a later capture.
Planet Nine could have been captured from beyond the Solar System
during a close encounter between the Sun and another star in its birth
cluster. Three-body interactions during these encounters can perturb the
path of planets on distant orbits around another star, or rogue planet,
leaving one in a stable orbit around the Sun via a process similar to
the capture of irregular satellites
around the giant planets. If the planet originated in a system with a
number of Neptune-massed planets, and without Jupiter-massed planets, it
could be scattered into a more long-lasting distant eccentric orbit,
increasing its chances of capture.
Although the odds of the Sun capturing another planet from another star
can be higher, a wider 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.
In simulations where the planets orbiting the Sun and the other star
are in the same plane 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.
The likelihood of the capture of a rogue planet is much smaller, with
only 0.05% - 0.10% of 10,000 simulated rogue planets being captured on
orbits similar to that proposed for Planet Nine.
An encounter with another star could also alter the orbit of a
distant planet, shifting it from a circular to an eccentric orbit. The in situ formation of a planet at this distance would require a very massive and extensive disk, or the outward drift of solids in a dissipating disk forming a narrow ring from which the planet accreted over a billion years.
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%.
A previous article reported that a massive disk extending beyond 80 AU
would drive Kozai oscillations, a periodic exchange between eccentricity
and inclination, 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.
Naming
Planet
Nine does not have an official name and will not receive one unless its
existence is confirmed, typically through optical imaging. Once
confirmed, the International Astronomical Union will certify a name, with priority usually given to a name proposed by its discoverers. It is likely to be a name chosen from Roman or Greek mythology.
In their original article, Batygin and Brown simply referred to the object as "perturber", and only in later press releases did they use "Planet Nine". They have also used the names "Jehoshaphat" and "George" for Planet Nine. Brown has stated: "We actually call it Phattie when we're just talking to each other."
In 2018, planetary scientist Alan Stern objected to the name Planet Nine, saying, "It is an effort to erase Clyde Tombaugh's legacy [the discovery of Pluto] and it's frankly insulting", suggesting the name Planet X until its discovery.
A statement was signed by 35 scientists saying, "We further believe the
use of this term [Planet Nine] should be discontinued in favor of
culturally and taxonomically neutral terms for such planets, such as
Planet X, Planet Next, or Giant Planet Five."
