Artist's
impression of Planet Nine 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 | |
|---|---|
| 400–800 AU | |
| Eccentricity | 0.2–0.5 |
| Inclination | 15°–25° |
| 150° (est.) | |
| Physical characteristics | |
| Mass | 5–10 M⊕ (est.) |
| >22.5 (est.) | |
Planet Nine is a hypothetical planet in the outer region of the Solar System. Its gravitational effects could explain the unusual clustering of orbits for a group of extreme trans-Neptunian objects (eTNOs), bodies beyond Neptune that orbit the Sun at distances averaging more than 250 times that of the Earth. These eTNOs tend to make their closest approaches to the Sun in one sector, and their orbits are similarly tilted. These improbable alignments suggest that an undiscovered planet may be shepherding the orbits of the most distant known Solar System objects.
This undiscovered super-Earth-sized planet would have a predicted mass of five to ten times that of the Earth, and an elongated orbit 400 to 800 times as far from the Sun as the Earth. Konstantin Batygin and Michael E. Brown suggest that Planet Nine could be the core of a giant planet that was ejected from its original orbit by Jupiter during the genesis of the Solar System. Others propose that the planet was captured from another star, was once a rogue planet, or that it formed on a distant orbit and was pulled into an eccentric orbit by a passing star.
As of the end of 2018, no observation of Planet Nine had been announced. While sky surveys such as Wide-field Infrared Survey Explorer (WISE) and Pan-STARRS did not detect Planet Nine, they have not ruled out the existence of a Neptune-diameter object in the outer Solar System. The ability of these past sky surveys to detect Planet Nine were dependent on its location and characteristics. Further surveys of the remaining regions are ongoing using NEOWISE and the 8-meter Subaru Telescope. Unless Planet Nine is observed, its existence is purely conjectural. Several alternative theories have been proposed to explain the observed clustering of TNOs.
History
Following the discovery of Neptune in 1846, there was considerable speculation that another planet might exist beyond its orbit. The best-known of these theories predicted the existence of a distant planet that was influencing the orbits of Uranus and Neptune. After extensive calculations Percival Lowell predicted the possible orbit and location of the hypothetical trans-Neptunian planet and began an extensive search for it in 1906. He called the hypothetical object Planet X, a name previously used by Gabriel Dallet. Clyde Tombaugh continued Lowell's search and in 1930 discovered Pluto, but it was soon determined to be too small to qualify as Lowell's Planet X. After Voyager 2's flyby of Neptune in 1989, the difference between Uranus' predicted and observed orbit was determined to have been due to the use of a previously inaccurate mass of Neptune.Attempts to detect planets beyond Neptune by indirect means such as orbital perturbation date back to before the discovery of Pluto. Among the first was George Forbes who postulated the existence of two trans-Neptunian planets in 1880. One would have an average distance from the Sun, or semi-major axis, of 100 astronomical units (AU), 100 times that of the Earth. The second would have a semimajor axis of 300 AU. His work is considered similar to more recent Planet Nine theories in that the planets would be responsible for a clustering of the orbits of several objects, in this case the aphelion distances of periodic comets similar to that of the Jupiter-family comets.
The discovery of Sedna's peculiar orbit in 2004 led to speculation that it had encountered a massive body other than one of the eight known planets. Sedna's orbit is detached, with a perihelion distance of 76 AU that is too large to be due to gravitational interactions with Neptune. Several authors proposed that Sedna entered this orbit after encountering an unknown planet on a distant orbit, a member of the open cluster that formed with the Sun, or another star that later passed near the Solar System. The announcement in March 2014 of the discovery of a second sednoid with a perihelion distance of 80 AU, 2012 VP113, in a similar orbit led to renewed speculation that an unknown super-Earth remained in the distant Solar System.
At a conference in 2012, Rodney Gomes proposed that an undetected planet was responsible for the orbits of some eTNOs with detached orbits and the large semi-major axis Centaurs, small Solar System bodies that cross the orbits of the giant planets. The proposed Neptune-massed planet would be in a distant (1500 AU), eccentric (eccentricity 0.4), and inclined (inclination 40°) orbit. Like Planet Nine it would cause the perihelia of objects with semi-major axes greater than 300 AU to oscillate, delivering some into planet-crossing orbits and others into detached orbits like that of Sedna. An article by Gomes, Soares, and Brasser was published in 2015, detailing their arguments.
In 2014, astronomers Chad Trujillo and Scott S. Sheppard noted the similarities in the orbits of Sedna and 2012 VP113 and several other eTNOs. They proposed that an unknown planet in a circular orbit between 200 and 300 AU was perturbing their orbits. Later, in 2015, Raúl and Carlos de la Fuente Marco argued that two massive planets in orbital resonance were necessary to produce the similarities of so many orbits.
Batygin and Brown hypothesis
In early 2016, California Institute of Technology's
Batygin and Brown described how the similar orbits of six eTNOs could
be explained by Planet Nine and proposed a possible orbit for the
planet. This hypothesis could also explain eTNOs with orbits perpendicular to the inner planets and others with extreme inclinations, and had been offered as an explanation of the tilt of the Sun's axis.
Orbit
Planet Nine is hypothesized to follow an elliptical orbit around the Sun with an eccentricity of 0.2 to 0.5. The planet's semi-major axis is estimated to be 400 AU to 800 AU, roughly 13 to 26 times the distance from Neptune to the Sun. Its inclination to the ecliptic, the plane of the Earth's orbit, is projected to be 15° to 25°. 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 reach it in as little as 20 years by using a powered slingshot trajectory around the Sun.
Mass and radius
The planet is estimated to have 5 to 10 times the mass of Earth and a radius of 2 to 4 times Earth's. Brown thinks that if Planet Nine exists, its mass is sufficient to clear its orbit
of large bodies in 4.6 billion years, the age of the Solar System, and
that its gravity dominates the outer edge of the Solar System, which is
sufficient to make it a planet by current definitions. Astronomer Jean-Luc Margot has also stated that Planet Nine satisfies his criteria and would qualify as a planet if and when it is detected.
Origin
Several possible origins for Planet Nine have been examined including
its ejection from the neighborhood of the known 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. The gravity of a nearby star, or drag from the gaseous remnants of the Solar nebula,
then reduced the eccentricity of its orbit. This raised its perihelion,
leaving it in a very wide but stable orbit beyond the influence of the
other planets. 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 than Uranus or Neptune.
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 as the gas was cleared from the outer parts of the
proto-planetary disk.
As Planet Nine passed through this disk its gravity would alter the
paths of the individual objects in a way that reduced Planet Nine's
velocity relative to it. This would lower the eccentricity of Planet
Nine and stabilize its orbit. If this 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,
with the observed clustering more likely if the inner edge is at 200 AU.
Unlike the gas nebula, the planetesimal disk is likely to have been
long lived, potentially allowing a later capture.
Planet Nine could have been captured from outside the Solar
System during a close encounter between the Sun and another star. If a
planet was in a distant orbit around this star, three-body
interactions during the encounter could alter the planet's path,
leaving it in a stable orbit around the Sun. A planet originating in a
system without Jupiter-massed planets could remain in a distant
eccentric orbit for a longer time, increasing its chances of capture. The wider range of possible orbits would reduce the odds of its capture in a relatively low inclination orbit to 1–2 percent.
This process could also occur with rogue planets, but the likelihood of
their capture is much smaller, with only 0.05–0.10% being captured in
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
original cluster, the probability of it remaining bound to the Sun in a
highly eccentric orbit is roughly 10%.
A previous article reported that if the massive disk extended beyond 80
AU some objects scattered outward by Jupiter and Saturn would have been
left in high inclination (inc > 50°), low eccentricity orbits which
have not been observed.
An extended disk would also have been subject to gravitational
disruption by passing stars and by mass loss due to photoevaporation
while the Sun remained in the open cluster where it formed.
Evidence
The gravitational influence of Planet Nine would explain four peculiarities of the Solar System:
- The clustering of the orbits of eTNOs;
- The high perihelia of objects like 90377 Sedna that are detached from Neptune's influence;
- The high inclinations of eTNOs with orbits roughly perpendicular to the orbits of the eight known planets;
- High-inclination trans-Neptunian objects (TNOs) with semi-major axis less than 100 AU.
Planet Nine was initially proposed to explain the clustering of
orbits, via a mechanism that would also explain the high perihelia of
objects like Sedna. The evolution of some of these objects into
perpendicular orbits was unexpected, but found to match objects
previously observed. The orbits of some objects with perpendicular
orbits were later found to evolve toward smaller semimajor axes when the
other planets were included in simulations. Although other mechanisms
have been offered for many of these peculiarities, the gravitational
influence of Planet Nine is the only one that explains all four. The
gravity of Planet Nine could also increase the inclinations of other
objects that cross its orbit, however, leaving the short-period comets with a broader inclination distribution than is observed.
Previously Planet Nine was hypothesized to be responsible for the 6
degree tilt of the Sun's axis relative to the orbits of the planets. Recent updates to its predicted orbit and mass limit this shift to ~1 degree.
Observations: Orbital clustering of high perihelion objects
Diagram illustrating the true anomaly, argument of periapsis, longitude of ascending node, and inclination of a celestial body.
The clustering of the orbits of TNOs with large semi-major axes was
first described by Trujillo and Sheppard, who noted similarities between
the orbits of Sedna and 2012 VP113.
Without the presence of Planet Nine, these orbits should be distributed
randomly, without preference for any direction. Upon further analysis,
Trujillo and Sheppard observed that the arguments of perihelion of 12 TNOs with perihelia greater than 30 AU and semi-major axes greater than 150 AU
were clustered near zero degrees, meaning that they rise through the
ecliptic when they are closest to the sun. Trujillo and Sheppard
proposed that this alignment was caused by a massive unknown planet
beyond Neptune via the Kozai mechanism.
For objects with similar semi-major axes the Kozai mechanism would
confine their arguments of perihelion to near 0 or 180 degrees. This
confinement allows objects with eccentric and inclined orbits to avoid
close approaches to the planet because they would cross the plane of the
planet's orbit at their closest and farthest points from the Sun, and
cross the planet's orbit when they are well above or below its orbit.
Trujillo and Sheppard's hypothesis about how the objects would be
aligned by the Kozai mechanism has been supplanted by further analysis
and evidence.
Batygin and Brown, looking to refute the mechanism proposed by
Trujillo and Sheppard, also examined the orbits of the TNOs with large
semi-major axes.
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, Batygin and Brown determined that the arguments of perihelion for the remaining six objects (Sedna, 2012 VP113, 2004 VN112, 2010 GB174, 2000 CR105, and 2010 VZ98) were clustered around 318°±8°. This finding did not agree with how the Kozai mechanism would tend to align orbits with arguments of perihelion at 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 eTNOs with
semi-major axes greater than 250 AU and perihelia beyond 30 AU (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 location 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 and approximately coplanar, producing a clustering of their longitudes of ascending nodes,
the directions where they each rise through the ecliptic. 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 in a few hundred million years due to the locations of the perihelia
and the ascending nodes changing, or precessing, at differing rates due to their varied semi-major axes and eccentricities.
This indicates that the clustering could not be due to an event in the
distant past, such as a passing star, and is most likely maintained by
the gravitational field of an object orbiting the Sun.
Two of the six objects (2013 RF98 and 2004 VN112) also have very similar orbits and spectra. This has led to the suggestion that they were a binary object
disrupted near aphelion during an encounter with a distant object. The
disruption of a binary would require a relatively close encounter, which
becomes less likely at large distances from the Sun.
In a later article Trujillo and Sheppard noted a correlation
between the longitude of perihelion and the argument of perihelion of
the TNOs 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 between 180° and 340°
have arguments of perihelion between 0° and 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.
A 2017 article by Carlos and Raul de la Fuente Marcos noted that
distribution of the distances to the ascending nodes of the eTNOs, and
those of centaurs and comets with large semi-major axes, may be bimodal. They suggest it is due to the eTNOs avoiding close approaches to a planet with a semi-major axis of 300–400 AU.
The extreme trans-Neptunian object orbits
Six original and eight additional eTNO objects orbits with current positions near their perihelion in purple, with hypothetical Planet Nine orbit in green
Close up view of 13 eTNO current positions
Simulations: Observed clustering reproduced
The clustering of the orbits of eTNOs and raising of their perihelia
is reproduced in simulations that include Planet Nine. In simulations
conducted by Batygin and Brown, swarms of objects with large semi-major
axes that began with random orientations were sculpted into roughly collinear
and coplanar groups of spatially confined orbits by a massive distant
planet in a highly eccentric orbit. The objects' perihelia tended to
point in the same direction and the objects' orbits tended to align in
the same plane. Many of these objects entered high-perihelion orbits
like Sedna and, unexpectedly, some entered perpendicular orbits that
Batygin and Brown later noticed had been previously observed.
In their original analysis Batygin and Brown found that the
distribution of the orbits of the first six eTNOs was best reproduced in
simulations using a 10 Earth mass 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 ≈ 141° and longitude of perihelion ϖ = 235°±12°
These parameters for Planet Nine produce different simulated effects
on TNOs. Objects with semi-major axis greater than 250 AU are strongly
anti-aligned with Planet Nine, with perihelia opposite Planet Nine's
perihelion. Objects with semi-major axes between 150 AU and 250 AU are
weakly aligned with Planet Nine, with perihelia in the same direction as
Planet Nine's perihelion. Little effect is found on objects with
semi-major axes less than 150 AU. The simulations also revealed that objects with semimajor axis greater than 250 AU could have stable, aligned orbits if they had lower eccentricities. These objects have yet to be observed.
Other possible orbits for Planet Nine were also examined, with semi-major axes between 400 AU and 1500 AU,
eccentricites up to 0.8, and a wide range of inclinations. These orbits
yield varied results. Batygin and Brown found that orbits of the eTNOs
were more likely to have similar tilts if Planet Nine had a higher
inclination, but anti-alignment also decreased.
Simulations by Becker et al. showed that their orbits were more stable
if Planet Nine had a smaller eccentricity, but that anti-alignment was
more likely at higher eccentricities.
Lawler et al. found that the population captured in orbital resonances
with Planet Nine was smaller if it had a circular orbit, and that fewer
objects reached high inclination orbits.
Investigations by Cáceres et al. showed that the orbits of the eTNOs
were better aligned if Planet Nine had a lower perihelion orbit, but its
perihelion would need to be higher than 90 AU. Later investigations by Batygin et al. found that higher eccentricity orbits reduced the average tilts of the eTNOs orbits.
While there are many possible combinations of orbital parameters and
masses for Planet Nine, none of the alternative simulations have been
better at predicting the observed alignment of objects in the Solar
System. The discovery of additional distant Solar System objects would
allow astronomers to make more accurate predictions about the orbit of
the hypothesized planet. These may also provide further support for, or
refutation of, the Planet Nine hypothesis.
Dynamics: How Planet Nine modifies the orbits of eTNOs
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.
Planet Nine modifies the orbits of eTNOs via a combination of effects. On very long timescales Planet Nine exerts a torque on the orbits of the eTNOs that varies with the alignment of their orbits with Planet Nine's. The resulting exchanges of angular momentum
cause the perihelia to rise, placing them in Sedna-like orbits, and
later fall, returning them to their original orbits after several
hundred million years. The motion of their directions of perihelion also
reverses when their eccentricities are small, keeping the objects
anti-aligned, see blue curves on diagram, or aligned, red curves. 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 gravity of Neptune and the
other giant planets, and the inclination of Planet Nine's orbit, weaken
this protection. This results in a chaotic
variation of semi-major axes as objects hop between resonances,
including high order resonances such as 27:17, on million-year
timescales. The mean-motion resonances may not be necessary for the survival of eTNOs if they and Planet Nine are both on inclined orbits. The orbital poles of the objects precess around, or circle, the pole of the Solar System's Laplace plane.
At large semi-major axes the Laplace plane is warped toward the plane
of Planet Nine's orbit. This causes orbital poles of the eTNOs on
average to be tilted toward one side and their longitudes of ascending
nodes to be clustered.
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.
Planet Nine can deliver eTNOs into orbits roughly perpendicular to the ecliptic. Several objects with high inclinations, greater than 50°, and large semi-major axes, above 250 AU, have been observed. These orbits are produced when some low inclination eTNOs enter a secular resonance
with Planet Nine upon reaching low eccentricity orbits. The resonance
causes their eccentricities and inclinations to increase, delivering the
eTNOs into perpendicular orbits with low perihelia where they are more
readily observed. The eTNOs 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 and inclination orbits. The secular resonance with
Planet Nine involves a linear combination
of the orbit's arguments and longitudes of perihelion: Δϖ – 2ω. Unlike
the Kozai mechanism this resonance causes objects to reach their maximum
eccentricities when in nearly perpendicular orbits. In simulations
conducted by Batygin and Morbidelli this evolution was relatively
common, with 38% of stable objects undergoing it at least once.
The arguments of perihelion of these objects 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 when they
reach low perihelia.
This is in rough agreement with observations with the differences
attributed to distant encounters with the known giant planets.
Orbits of high-inclination objects
A population of high-inclination TNOs with semi-major axes less than
100 AU may be generated by the combined effects of Planet Nine and the
other giant planets. The eTNOs that enter perpendicular orbits have
perihelia low enough for their orbits to intersect those of Neptune or
the other giant planets. An encounter with one of these planets can
lower an eTNO's semi-major axis to below 100 AU, where the object's
orbits is no longer controlled by Planet Nine, leaving it in an orbit
like 2008 KV42.
The predicted orbital distribution of the longest lived of these
objects is nonuniform. Most would have orbits with perihelia ranging
from 5 AU to 35 AU and inclinations below 110 degree; beyond a gap with
few objects are would be others with inclinations near 150 degrees and
perihelia near 10 AU. Previously it was proposed that these objects originated in the Oort Cloud, a theoretical cloud of icy planetesimals surrounding the Sun at distances of 2,000 to 200,000 AU.
Oort cloud and comets
Planet Nine would alter the source regions and the inclination
distribution of comets. In simulations of the migration of the giant
planets described by the Nice model fewer objects are captured in the Oort cloud
when Planet Nine is included. Other objects would be captured in a
cloud of objects dynamically controlled by Planet Nine. This Planet Nine
cloud, made up of the eTNOs and the perpendicular objects, would extend
from semimajor axes of 200 AU to 3000 AU and contain roughly 0.3–0.4
Earth masses.
When the perihelia of objects in the Planet Nine cloud drop low enough
for them to encounter the other planets some would be scattered into
orbits that enter the inner Solar System where they could be observed as
comets. If Planet Nine exists these would make up roughly one third of
the Halley-type comets. Planet Nine would also alter the orbits of the scattering disk objects,
those with semi-major axes greater than 50 AU and perihelia near
Neptune's orbit, that cross its orbit, increasing their inclinations.
This would increase the inclinations of the Jupiter-family comets derived from that population leaving them with a broader inclination distribution than is observed. Recent estimates of a smaller mass and eccentricity for Planet Nine would reduce its effect on these inclinations.
Reception
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 analyzed 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.
The Planet Nine hypothesis is supported by several astronomers and academics. Jim Green, director of NASA's Science Mission Directorate, 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 eTNOs and, quoting Carl Sagan, he said, "extraordinary claims require extraordinary evidence." Massachusetts Institute of Technology Professor 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. Astronomer 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."
Astronomer Renu Malhotra
remains agnostic about Planet Nine, but noted that she and her
colleagues have found that the orbits of eTNOs 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."
Other authorities have varying degrees of skepticism. American astrophysicist Ethan Siegel,
who previously speculated that planets may have been ejected from the
Solar System during an early dynamical instability, is skeptical of the
existence of an undiscovered planet in the Solar System.
In a 2018 article discussing a survey that did not find evidence of
clustering of the eTNOs' orbits he suggests the previously observed
clustering could have been the result of observing bias and claims most
scientists think Planet Nine does not exist. 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.
Alternative hypotheses
Temporary or coincidental 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 objects with
semi-major axis > 150 AU with orbits oriented on a wide range of
directions. After accounting for the 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.
This result differed from an analysis of discovery biases in the
previously observed eTNOs by Mike Brown. He found that after observing
biases were accounted for the clustering of longitudes of perihelion of
10 known eTNOs would be observed only 1.2% of the time if their actual
distribution was uniform. When combined with the odds of the observed
clustering of the arguments of perihelion the probability was 0.025%.
A later analysis of the discovery biases of 14 eTNOs by Brown and
Batygin found a probability of the observed clustering of the longitudes
of perihelion and the orbital pole locations of 0.2%.
Simulations of 15 known objects evolving under the influence of
Planet Nine also revealed differences with observations. Cory Shankman
and his colleagues included Planet Nine in a simulation of many clones
(objects with similar orbits) of 15 objects with semi-major axis more than
150 AU and perihelion more than 30 AU.
While they observed alignment of the orbits opposite that of Planet
Nine's for the objects with semi-major axis greater than 250 AU,
clustering of the arguments of perihelion was not seen. Their
simulations also showed that the perihelia of the eTNOs rose and fell
smoothly, leaving many with perihelion distances between 50 AU and 70 AU
where none had been observed, and predicted that there would be many
other unobserved objects.
These included a large reservoir of high-inclination objects that would
have been missed due to most observations being at small inclinations,
and a large population of objects with perihelia so distant that they
would be too faint to observe. Many of the objects were also ejected
from the Solar System after encountering the other giant planets. The
large unobserved populations and the loss of many objects led Shankman
et al. to estimate that the mass of the original population was tens of
Earth masses, requiring that a much larger mass had been ejected during
the early Solar System.[L]
Shankman et al. concluded that the existence of Planet Nine is unlikely
and that the currently observed alignment of the existing eTNOs is a
temporary phenomenon that will disappear as more objects are detected.
Inclination instability in a massive disk
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.
An inclination instability could occur in a disk of particles with high
eccentricity orbits (e>0.6) around a central body, such as the Sun.
The self-gravity of this disk would cause 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 would require 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 could align the arguments of
perihelion and raise perihelia, producing detached objects, it would not
align the longitudes of perihelion.
Mike Brown considers Planet Nine a more probable explanation, noting
that current surveys have not revealed a large enough scattered-disk to
produce an "inclination instability".
In Nice model simulations of the Solar System that included the
self-gravity of the planetesimal disk an inclination instability did not
occur. Instead, the simulation produced a rapid precession of the
objects' orbits and most of the objects were ejected on too short of a
timescale for an inclination instability to occur.
Shepherding by a massive disk
Antranik Sefilian and Jihad Touma propose that a massive disk of
moderately eccentric TNOs is responsible for the clustering of the
longitudes of perihelion of the eTNOs. Their predicted 10 Earth-mass
disk would contain TNOs with aligned orbits and eccentricities that
increased with their semimajor axes ranging from zero to 0.165. The
gravitational effects of this disk would offset the forward precession
driven by the giant planets so that the orbital orientations of its
individual objects are maintained. The orbits of objects with high
eccentricities, such as the observed eTNOs, would be stable and have
roughly fixed orientations, or longitudes of perihelion, if their orbits
were anti-aligned with this disk.
Although Brown thinks the proposed disk could explain the observed
clustering of the eTNOs, he finds it implausible that the disk could
survive over the age of the Solar System.
Batygin thinks that there is insufficient mass in the Kuiper belt to
explain the formation of the disk and asks "why would the protoplanetary
disk end near 30 AU and restart beyond 100 AU?"
Planet in lower eccentricity orbit
The Planet Nine hypothesis includes a set of predictions about the
mass and orbit of the planet. An alternative theory predicts a planet
with different orbital parameters. 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 less than 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.
Alignment due to the Kozai mechanism
Trujillo and Sheppard argued in 2014 that a massive planet in a circular orbit with an average distance between 200 AU and 300 AU
was responsible for the clustering of the arguments of perihelion of
twelve TNOs with large semi-major axes. Trujillo and Sheppard identified
a clustering near zero degrees of the arguments of perihelion of the
orbits of twelve TNOs with perihelia greater than 30 AU and semi-major axes greater than 150 AU.
After numerical simulations showed that the arguments of perihelion
should circulate at varying rates, leaving them randomized after
billions of years, 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 TNOs
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.
Another 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 TNOs into similar types of orbits. They
were basic proof of concept simulations 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 eTNOs with an orbit for the
planet.
But they were the first to notice there was a clustering in the orbits
of TNOs 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.
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 TNOs are kept bunched
together by a Kozai mechanism and compared their behavior to that of Comet 96P/Machholz under the influence of Jupiter.
They also struggled to explain the orbital alignment using a model with
only one unknown planet, and 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, but 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 600 AU. This is because objects move more slowly when near their aphelion, in accordance with Kepler's second law.
Planet Nine is predicted to be brighter, magnitude 21–22, if it is near
the small end of its estimated mass range because it would need to be
closer to have the same dynamical effect.
Searches of existing data
The search of databases of stellar objects
by Batygin and Brown has already excluded much of the sky along Planet
Nine's predicted orbit. The remaining regions include the direction of
its aphelion, where it would be too faint to be spotted by these
surveys, and near the plane of the Milky Way, where it would be difficult to distinguish from the numerous stars. 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 Wide-field Infrared Survey Explorer (WISE) satellite.
Other researchers have been conducting searches of existing data. David Gerdes, who helped develop the camera used in the Dark Energy Survey, claims that software designed to identify distant Solar System objects such as 2014 UZ224 could find Planet Nine if it was imaged as part of that survey, which covered a quarter of the southern sky. Michael Medford and Danny Goldstein, graduate students at the University of California, Berkeley, are also examining archived data using a technique that combines 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 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 Batygin thinks 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. In December 2018, they spent 4 half–nights and 3 full nights observing with the Subaru Telescope.
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. 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
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 also 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, have been uploaded to the Backyard Worlds website. By
looking for moving objects in the animations, citizen scientists might
find Planet Nine.
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 2017, completed their goals in
less than three days with around five million classifications by more
than 60,000 individuals.
Attempts to predict location
Cassini measurements of Saturn's orbit
Precise observations of Saturn's orbit using data from Cassini
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. This data neither proves nor disproves that Planet
Nine exists.
An initial analysis by Fienga, Laskar, Manche, and Gastineau
using Cassini data to search for Saturn's orbital residuals, small
differences with its predicted orbit due to the Sun and the known
planets, was inconsistent with Planet Nine being located with a true anomaly,
the location along its orbit relative to perihelion, 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 would be located near 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 would be located near right ascension 3.0h to 5.5h and declination −1° to 6°.
A later 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.
William Folkner, a planetary scientist at the Jet Propulsion Laboratory (JPL), has stated that the Cassini
spacecraft is not experiencing unexplained deviations in its orbit
around Saturn. An undiscovered planet would affect the orbit of Saturn,
not Cassini. This could produce a signature in the measurements of Cassini, but JPL has seen no unexplained signatures in Cassini data.
Analysis of Pluto's orbit
An analysis in 2016 of Pluto's orbit by Holman and 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.
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.
Attempts to predict semimajor axis
An analysis by Sarah Millholland and Gregory Laughlin identified a pattern of commensurabilities
(ratios between orbital periods of pairs of objects consistent with
both being in resonance with another object) of the eTNOs. They identify
five objects that would be near resonances with Planet Nine if had a
semi-major axis of 654 AU: Sedna (3:2), 2004 VN112 (3:1), 2012 VP113 (4:1), 2000 CR105 (5:1), and 2001 FP185 (5:1). They identify this planet as Planet Nine but propose a different orbit with 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°).
Carlos and Raul de la Fuente Marcos also note commensurabilities
among the known eTNOs similar to that of the Kuiper belt, where
accidental commensurabilities occur due to objects in resonances with
Neptune. They find that some of these objects would be in 5:3 and 3:1
resonances with a planet that had a semi-major axis of ≈700 AU.
Three objects with smaller semi-major axes near 172 AU (2013 UH15, 2016 QV89 and 2016 QU89)
have also been proposed to be in resonance with Planet Nine. These
objects would be in resonance and anti-aligned with Planet Nine if it
had a semi-major axis of 315 AU, below the range proposed by Batygin and
Brown. Alternatively, they could be in resonance with Planet Nine, but
have orbital orientations that circulate instead of being confined by
Planet Nine if it had a semi-major axis of 505 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 has an eccentric orbit.
Naming
Planet Nine does not have an official name and will not receive one
unless its existence is confirmed via imaging. Only two planets, Uranus
and Neptune, have been discovered in recorded history. However, a great
many minor planets, including dwarf planets, asteroids, and comets have been discovered and named. Consequently, there is a well-established process for naming newly discovered solar system objects. If Planet Nine is observed, 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 and it's
frankly insulting", suggesting the name Planet X until its discovery.
He signed a statement with 34 other 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." According to Brown, "'Planet X'
is not a generic reference to some unknown planet, but a specific
prediction of Lowell's which led to the (accidental) discovery of Pluto.
Our prediction is not related to this prediction."
