Where Are You?

September 25, 2021

We here at Kristuzac apologize for a few technical glitches with some some of the visual images. We will either upgrade the images or remove them entirely.

I have struggled a little on how to publish this paper. It is quite lengthy coming in at over 11,000 words. I had originally intended to break it down in to smaller bite sized chunks, however, in the end; we would all be better informed if the document as a whole is published in its entirety.

The paper below is based solely in science.  Its premise is the search for planet nine, which we call Nibiru. Nibiru is the supposed home planet of our creators, the Annunaki. The paper itself makes no mention of these aliens, nor the possibility of life on that distant world.

It is thought that Nibiru was an exoplanet that was drifting through the early solar system; it was then caught by the gravity of the outer planets, it approached us from the opposite orbital direction at an inclined angle, smacked in to earth where both earth’s and Nibiru’s orbital speed was slowed dramatically. The reduction in Nibiru’s velocity caused it to be caught by the sun’s gravity causing it to become the twelfth member of our solar system. Nibiru’s planetary mass and speed vs gravity assured that the planet’s orbit would encompass our entire solar system and would influence Kuiper belt objects.

Full credit is of course given to the paper’s author(s). Any errors or omissions in the data is theirs.

Draft version August 30, 2021

Typeset using LATEX preprint2 style in AASTeX63

The Orbit of Planet Nine

Michael E. Brown1 and Konstantin Batygin1

1Division of Geological and Planetary Sciences California Institute of Technology

Pasadena, CA 91125, USA

(Accepted 22 Aug 2021)

Submitted to AJ


The existence of a giant planet beyond Neptune – referred to as Planet Nine (P9)

  • has been inferred from the clustering of longitude of perihelion and pole position of distant eccentric Kuiper belt objects (KBOs). After updating calculations of observa- tional biases, we find that the clustering remains significant at the 99.6% confidence level. We thus use these observations to determine orbital elements of P9. A suite of numerical simulations shows that the orbital distribution of the distant KBOs is strongly influenced by the mass and orbital elements of P9 and thus can be used to infer these parameters. Combining the biases with these numerical simulations, we cal- culate likelihood values for discrete set of P9 parameters, which we then use as input into a Gaussian Process emulator that allows a likelihood computation for arbitrary values of all parameters. We use this emulator in a Markov Chain Monte Carlo analysis to estimate parameters of P9.  We find a P9 mass of 6.2+2.2  Earth masses,  semimajor


  • −1.3


axis of 380−80  AU, inclination of 16 ± 5  and perihelion of 300−60 AU. Using samples

of the orbital elements and estimates of the radius and albedo of such a planet, we cal- culate the probability distribution function of the on-sky position of Planet Nine and of its brightness. For many reasonable assumptions, Planet Nine is closer and brighter than initially expected, though the probability distribution includes a long tail to larger distances, and uncertainties in the radius and albedo of Planet Nine could yield fainter objects.

  1. Text Box: arXiv:2108.09868v2 [astro-ph.EP] 26 Aug 2021INTRODUCTION

Hints of the possibility of a massive planet well beyond the orbit of Neptune have been emerg- ing for nearly twenty years. The first clues came from the discovery of a population of distant ec- centric Kuiper belt objects (KBOs) decoupled

Corresponding author: Michael Brown mbrown@caltech.edu

from interactions with Neptune (Gladman et al. 2002; Emel’yanenko et al. 2003; Gomes et al. 2006), suggesting some sort of additional grav- itational perturbation. While the first such de- coupled objects were only marginally removed from Neptune’s influence and suggestions were later made that chaotic diffusion could create similar orbits (Bannister et al. 2017), the dis- covery of Sedna, with a perihelion far removed from Neptune, clearly required the presence of

a past or current external perturber (Brown et al. 2004). Though the orbit of Sedna was widely believed to be the product of pertur- bation by passing stars within the solar birth cluster (Morbidelli & Levison 2004; Schwamb et al. 2010; Brasser et al. 2012), the possibility of an external planetary perturber was also noted (Brown et al. 2004; Morbidelli & Levison 2004; Gomes et al. 2006). More recently, Gomes et al. (2015) examined the distribution of objects with very large semimajor axes but with perihelia in- side of the planetary regime and concluded that their overabundance can best be explained by the presence of an external planet of mass ∼10 Me (where Me is the mass of the Earth) at a distance of approximately 1000 AU. Simultane- ously, Trujillo & Sheppard (2014) noted that distant eccentric KBOs with semimajor axis a > 150 AU all appeared to come to perihelion approximately at the ecliptic and always travel- ling from north-to-south (that is, the argument of perihelion, ω, is clustered around zero), a sit- uation that they speculated could be caused by Kozai interactions with a giant planet, though detailed modeling found no planetary configu- ration that could explain the observations.

These disparate observations were finally uni- fied with the realization by Batygin & Brown (2016) that distant eccentric KBOs which are not under the gravitational influence of Nep- tune are largely clustered in longitude of peri- helion, meaning that their orbital axes are ap- proximately aligned, and simultaneously clus- tered in the orbital plane, meaning that their angular momentum vectors are approximately aligned (that is, they share similar values of in- clination, i, and longitude of ascending node, Ω). Such a clustering is most simply explained by a giant planet on an inclined eccentric or- bit with its perihelion location approximately

180 degrees removed from those of the clus- tered KBOs. Such a giant planet would not only explain the alignment of the axes and or-

bital planes of the distant KBOs, but it would also naturally explain the large perihelion dis- tances of objects like Sedna, the overabundance of large semimajor axis low perihelion objects, the existence of a population of objects with orbits perpendicular to the ecliptic, and the ap- parent trend for distant KBOs to cluster about ω = 0 (the clustering near ω = 0 is a coin- cidental consequence of the fact that objects sharing the same orbital alignment and orbital plane will naturally come to perihelion at ap- proximately the same place in their orbit and, in the current configuration of the outer solar system, this location is approximately centered at ω ∼ −40). The hypothesis that a giant planet on an inclined eccentric orbit keeps the axes and planes of distant KBOs aligned was called the Planet Nine hypothesis.

With one of the key lines of evidence for Planet Nine being the orbital clustering, much emphasis has been placed on trying to assess whether or not such clustering is statistically significant or could be a product of observa- tional bias. In analyses of all available contem- porary data and their biases, Brown (2017) and Brown & Batygin (2019, hereafter BB19) find only a 0.2% chance that the orbits of the dis- tant Kuiper belt objects (KBOs) are consistent with a uniform distribution of objects. Thus the initial indications of clustering from the origi- nal analysis appear robust when an expanded data set that includes observations taken over widely dispersed areas of the sky are considered. In contrast, Shankman et al. (2017), Bernar- dinelli et al. (2020), and Napier et al. (2021) using more limited – and much more biased – data sets, were unable to distinguish between clustering and a uniform population. Such dis- crepant results are not surprising: BB19 showed that the data from the highly biased OSSOS survey, which only examined the sky in two dis- tinct directions, do not have the sensitivity to detect the clustering already measured for the

full data set. Bernardinelli et al. (2020) recog- nize that the sensitivity limitations of the even- more-biased DES survey, which only examined the sky in a single direction, precluded them from being able to constrain clustering. It ap- pears that Napier et al., whose data set is dom- inated by the combination of the highly-biased OSSOS and DES surveys, suffers from similar lack of sensitivity, though Napier et al. do not provide sensitivity calculations that would allow this conclusion to be confirmed. Below, we up- date the calculations of BB19 and demonstrate that the additional data now available contin- ues to support the statistical significance of the clustering. We thus continue to suggest that the Planet Nine hypothesis remains the most viable explanation for the variety of anomolous behaviour seen in the outer solar system, and we work towards determining orbital parameters of Planet Nine.

Shortly after the introduction of the Planet

Nine hypothesis, attempts were made to con- strain various of the orbital elements of the planet. Brown & Batygin (2016) compared the observations to some early simulations of the ef- fects of Planet Nine on the outer solar system and showed that the data were consistent with a Planet Nine with a mass between 5 and 20 Earth masses, a semimajor axis between 380 and 980 AU, and a perihelion distance between 150 and 350 AU. Others sought to use the possibility that the observed objects were in resonances to determine parameters (Malhotra et al. 2016), though Bailey et al. (2018) eventually showed that this route is not feasible. Millholland & Laughlin (2017) invoked simple metrics to com- pare simulations and observations, and Batygin et al. (2019) developed a series of heuristic met- rics to compare to a large suite of simulations and provided the best constraints on the orbital elements of Planet Nine to date.

Two problems plague all of these attempts at

deriving parameters of Planet Nine. First, the

metrics used to compare models and observa- tions, while potentially useful in a general sense, are ad hoc and difficult to justify statistically. As importantly, none of these previous meth- ods has attempted to take into account the ob- servational biases of the data. While we will demonstrate here that the clustering of orbital parameters in the distant Kuiper belt is un- likely a product of observational bias, observa- tional bias does affect the orbital distribution of distant KBOs which have been discovered. Ignoring these effects can potentially bias any attempts to discern orbital properties of Planet Nine.

Here, we perform the first rigorous statisti- cal assessment of the orbital elements of Planet Nine. We use a large suite of Planet Nine sim- ulations, the observed orbital elements of the distant Kuiper belt, as well as the observa- tional biases in their discoveries, to develop a detailed likelihood model to compare the obser- vations and simulations. Combining the likeli- hood models from all of the simulations, we cal- culate probability density functions for all or- bital parameters as well as their correlations, providing a map to aid in the search for Planet Nine.

  1. 1.      DATA SELECTION

The existence of a massive, inclined, and ec- centric planet beyond ∼250 AU has been shown to be able to cause multiple dynamical effects, notably including a clustering of longitude of perihelion, w, and of pole position (a combi- nation of longitude of ascending node, Ω, and inclination, i) for distant eccentric KBOs. Criti- cally, this clustering is only strong in sufficiently distant objects whose orbits are not strongly af- fected by interactions with Neptune (Batygin & Brown 2016; Batygin et al. 2019). Objects with perihelia closer to the semimajor axis of Nep- tune, in what is sometimes referred to as the “scattering disk,” for example, have the strong clustering effects of Planet Nine disrupted and

are more uniformly situated (i.e. Lawler et al. 2017). In order to not dilute the effects of Planet Nine with random scattering caused by Neptune, we thus follow the original formula- tion of the Planet Nine hypothesis and restrict our analysis to only the population not inter- acting with Neptune. In Batygin et al. (2019) we use numerical integration to examine the or- bital history of each known distant object and classify them as stable, meta-stable, or unsta- ble, based on the speed of their semimajor axis diffusion. In that analysis, all objects with q < 42 AU are unstable with respect to peri- helion diffusion, while all objects with q > 42 AU are stable or meta-stable. Interestingly, 11 of the 12 known KBOs with a > 150 AU and q > 42 AU have longitude of perihelion clus- tered between 7 < w < 118, while only 8 of 21 with 30 < q < 42 AU are clustered in this re- gion, consistent with the expectations from the Planet Nine hypothesis. We thus settle on se- lecting all objects at a > 150 AU with perihe- lion distance, q > 42 AU for analysis for both the data and for the simulations below.

A second phenomenon could also dilute the clustering caused by Planet Nine. Objects which are scattered inward from the inner Oort cloud also appear less clustered than the longer- term stable objects (Batygin & Brown 2021). These objects are more difficult to exclude with a simple metric than the Neptune-scattered ob- jects, though excluding objects with extreme semimajor axes could be a profitable approach. Adopting our philosophy from the previous sec- tion, we exclude the one known object in the sample with a > 1000 AU as possible contam- ination from the inner Oort cloud. While we again cannot know for sure if this object is in- deed from the inner Oort cloud, removing the object can only have the effect of decreasing our sample size and thus increasing the uncertain- ties in our final orbit determination, for the po-

Table 1. Orbital elements of all reported ob- jects with 150 < a < 1000 and q > 42 AU.

namea AUei deg.Ω deg.  deg.

Note—As of 20 August 2021.

tential gain of decreasing any biases in our final results.

The sample with which we will compare our observations thus includes all known multi- opposition KBOs with 150 < a < 1000 AU and q > 42 AU reported as of 20 August 2021. Even after half of a decade of intensive search for dis- tant objects in the Kuiper belt, only 11 fit this stringent criteria for comparison with models. The observed orbital elements of these 11 are shown in Table 1. These objects are strongly clustered in w and pole position, though obser- vational biases certainly can affect this observed clustering.

  1. 1.      BIAS

All telescopic surveys contain observational biases. Correctly understanding and imple- menting these biases into our modeling is crit- ical to correctly using the observations to ex- tract orbital parameters of Planet Nine. BB19 developed a method to use the ensemble of all known KBO detections to estimate a full geo- metric observational bias for individual distant

figure 1

Figure 1. Semimajor axes versus     for the 11 KBOs of our sample (green points). The points are plotted as ∆  , defined as   − 9, where here we plot the points for an assumed value of   9 = 254. For each known distant KBO we show a one-dimensional projection of the bias with respect to (blue). While consistent bias exists, the    cluster is approximately 90 removed from the direction of bias. We also show the probability density of versus semimajor axis in the maximum likelihood model with m9 = 5 Me,  a9  =  300  AU,  e9  =  0.15  and  i9  =  16 (red).   The  density  plot  is  normalized  at  every  semimajor  axis to better show the longitudinal structure. Note that this comparison is simply for visualization; the full maximum-likelihood model compares the full set of orbit elements of each object to the simulations and also incorporates the observational biases on each observed objects.

KBOs. For each of the distant KBOs, they cre- ate the function


B(a,e,H)j [(i, w, Ω)|U ],                  (1)


where, for our case, j represents one of the 11 distant KBOs of the sample and B(a,e,H)j is the probability that distant KBO j, with semima- jor axis, eccentricity, and absolute magnitude (a, e, H)j would be detected with orbital angles i, w, and Ω, if the population were uniformly distributed in the sky, given U , the ensemble of all known KBO detections. The details of the method are explained in BB19, but, in short, it relies on the insight that every detection of ev-

ery KBO can be thought of (with appropriate caveats) as an observation at that position in the sky that could have detected an equivalent object j with (a, e, H)j if, given the required or- bital angles (i, w, Ω) to put object j at that po- sition in the sky, the object would be predicted to be as bright as or brighter at that sky position than the detected KBO. For each sample object j with (a, e, H)j, the ensemble of all KBO de- tections can thus be used to estimate all of the orbital angles at which the object could have been detected. This collection of orbital angles at which an object with (a, e, H)j could have been detected represents the bias in (i, w, Ω)

figure 2

Figure 2. A comparison of the projection of the pole position of the distant detached KBOs (the green points show (sin i cos ∆Ω, sin i sin ∆Ω), where ∆Ω is the difference between the longitude of ascending node of the observed object and of the modeled Planet Nine, assumed to be 108 here) and a density plot of their expected values in the maximum likelihood model (red background). In blue we show an average of the two-dimensional projection of the pole position bias of all of the objects. While strong bias in pole position exists, no preferential direction is apparent. White circles indicate 30 and 60 degree inclinations.

for object j. While biases calculated with this method are strictly discrete, we smooth to one degree resolution in all parameters for later ap- plication to our dynamical simulations.

Note that this method differs from bias calcu- lations using full survey simulators. It does not rely on knowledge of the survey details of the detections, but rather just the fact of the de- tection itself. Comparison of these bias calcula- tions with the bias calculated from a full survey

similar for the OSSOS survey shows comparable results (BB19).

Of the objects in our sample, all were included in the BB19 calculations with the exception of 2013 RA109, which had not been announced at the time of the original publication. We repro- duce the algorithm of BB19 to calculate a bias probability function for this object.

While the bias is a separate 3-dimensional function for each object, we attempt to give an approximate visual representation of these

biases in Figure 1, which collapses the bias of each object in w into a single dimension. As can be seen, a strong observational bias in w exists, but the observed clustering is approxi- mately 90 removed from the position of this bias. Figure 2 shows the bias in pole position. While, again, each object has an individual bias, the pole position biases are sufficiently similar that we simply show the sum of all of the biases, collapsed to two dimensions. Strong pole posi- tion biases exist, but none which appear capable of preferentially biasing the pole in any partic- ular direction.


With the bias function now available, we re- examine the statistical significance of the an- gular clustering of the distant KBOs by up- dating the analysis of BB19 for the objects in our current analysis set. As in that analysis, we perform 106 iterations where we randomly chose (i, w, Ω) for the 11 objects of our sam- ple assuming uniform distributions in w and Ω and a sin i exp( i2/2σ2) distribution with σ = 16 for i, and project these to the four- dimensional   space  of   the   canonical   Poincar´e variables (x, y, p, q), corresponding roughly to longitude of perihelion (x, y) and pole position (p, q) (see BB19 for details). For each of the iter- ations we compute the average four-dimensional position of the 11 simulated sample objects and note whether or not this average position is more distant than the average position of the real sample. This analysis finds that the real data are more extreme than 99.6% of the simu- lated data, suggesting only a 0.4% chance that these data are selected from a random sample. Examination of Figures 1 and 2 give a good vi- sual impression of why this probability is so low. The data are distributed very differently from the overall bias, contrary to expectations for a uniform sample.

The significance of the clustering retrieved here is slightly worse than that calculated by BB19. While one new distant object has been

added to the sample, the main reason for the change in the significance is that, after the Baty- gin et al. (2019) analysis, we now understand much better which objects should most be ex- pected to be clustered by Planet Nine, thus our total number of objects in our sample is smaller. Though this smaller sample leads to a slightly lower clustering significance, we nonetheless rec- ommend the choice of 150 < a < 1000 AU and q > 42 AU for any analyses going forward in- cluding newly discovered objects.

With the reassurance that the clustering is in- deed robust, we now turn to using the biases to help determine the orbital parameters of Planet Nine.



To estimate orbital parameters of Planet Nine, we require a likelihood model for a set of orbital parameters given the data on the observed dis- tant KBOs. In practice, because of the struc- ture of our bias calculations, which only account for on-sky geometric biases and do not attempt to explore biases in semimajor axis or perihe- lion, we reformulate this likelihood to be that of finding the observed value of (i, w, Ω)j given the specific value of (a, e)j for each distant ob- ject j. Conceptually, this can be thought of as calculating the probability that an object with a measured value of a and a measured value of q would be found to have the measured values of i, w, and Ω for a given set of Planet Nine orbital parameters.

The random variables for this model are the

mass of the planet, m9, the semimajor axis, a9, the eccentricity, e9, the inclination, i9, the lon- gitude of perihelion, w9, and the longitude of ascending node, Ω9. As the effects of Planet Nine are now understood to be mainly secular (Beust 2016), the position of Planet Nine within its orbit (the mean anomaly, M9) does not af- fect the outcome, so it is unused. We thus write the likelihood function of the jth KBO in our

data set as:


Lj       [(m9, a9, e9, i9, w9, Ω9)|(i, w, Ω)j],        (2)

where the (a, e)j superscript refers to the fixed value of a and e for object j. The full likelihood, LP9 is the product of the individual object like- lihoods. The likelihood of observing (i, w, Ω)j given a set of Planet Nine parameters depends on both the physics of Planet Nine and the ob- servational biases.

  • 4.1.       Simulations

While LP9 is presumably a continuous func- tion of the orbital parameters, we must calculate the value at discrete locations using numerical simulations. We perform 121 of these simula- tions at manually chosen values of m9, a9, e9, and i9, as detailed below. The two angular pa- rameters, w9 and Ω9, yield results that are rota- tionally symmetric so we need not individually simulate these results but rather rotate our ref- erence frame to vary these parameters later. We set M9 = 0 for the starting position of all sim- ulations as this parameters does not affect the final orbital distributions.

To save computational time, previous Planet Nine simulations have often included only the effects of Neptune plus a J2 term to simulate the combined orbit-averaged torque of the three inner gas giants. While this approach captures the relevant processes at the qualitative level, here, as we are interested in a detailed com- parison with observations, we fully include all four inner giant planets. For each independent simulation a set of between 16,800 and 64,900 test particles is initially distributed with semi- major axis between 150 and 500 AU, perihelion between 30 and 50 AU, inclination between 0 and 25, and all other orbital angles randomly distributed. The orbits of the 5 giant planets and test particles are integrated using the mer- cury6 gravitational dynamics software package (Chambers 1999). To carry out the integrations

we used the hybrid symplectic/Bulirsch-Stoer algorithm of the package, using a time step of 300 days which is adaptively reduced to directly resolve close encounters. Objects that collide with planets or reach r < 4.5 or r > 10000 AU are removed from the simulation for con- venience. The orbital elements of all objects are defined in a plane in which the angles are referenced to the initial plane of the four inte- rior giant planets. As Planet Nine precesses the plane of the planets, however, the fixed refer- ence coordinate system no longer corresponds to the plane of the planets. Thus, after the sim- ulations are completed, we recompute the time series of ecliptic-referenced angles by simply ro- tating to a coordinate system aligned with the orbital pole of Jupiter. In this rotation we keep the longitude zero-point fixed so that nodal pre- cession of test particles and Planet Nine can be tracked.

A total of 121 simulations was performed,

varying the mass (m9), semimajor axis (a9), ec- centricity (e9), and inclination (i9). Parame- ters for Planet Nine were chosen by hand in an attempt to explore a wide range of parameter space and find the region of maximum likeli- hood. The full set of parameters explored can be seen in Table 2. Examination of the initial results from these simulations confirms the con- clusions of Batygin et al. (2019): varying the or- bital parameters of Planet Nine produces large effects on the distant Kuiper belt (Fig. 3). We see, for example, that fixing all parameters but increasing m9 smoothly narrows the spread of the distant cluster (the feature labeled “clus- ter width” in Figure 3). Increasing i9 smoothly moves the orbital plane of the clustered objects to follow the orbital plane of Planet Nine, until, at a values above i9 > 30, the increased incli- nation of Planet Nine tends to break the cluster- ing entirely (Figure 4). Increasing m9 also leads to a decrease in the distance to the transition between unclustered and clustered objects (the

feature labeled the “wall” in Figure 3), while in- creasing the perihelion distance of Planet Nine (q9) increases the distance to the wall. Many other more subtle effects can be seen in the full data set. While we point out all of these phenomena, our point is not to parameterize or make use of any of them, but rather to make the simple case that the specific orbital parameters of Planet Nine cause measurable effects on the distributions of objects in the distant Kuiper belt. Thus, we should be able to use the mea- sured distributions to extract information about the orbital parameters of Planet Nine. We will accomplish this task through our full likelihood model.

  • 4.1.       Kernel density estimation

Each numerical simulation contains snapshots of the orbital distribution of the outer solar sys- tem for a finite number of particles. We use ker- nel density estimation to estimate a continuous function for the probability distribution func- tion (PDF) from these discrete results of each simulation, that is, we seek the probability of observing an object at (i, w, Ω)j given (a, e)j for each simulation. The early times of the simu- lations contain a transient state that appears to reach something like a steady-state in orbital distribution after ∼ 1 Gyr. We thus discard these initial time steps and only include the fi- nal 3 Gyr in our analysis. In all simulations the number of surviving objects continues to de- crease with time, with a wide range in variation of the ejection rate that depends most strongly on P9 mass and perihelion distance.

For each numerical model, k, and each ob- served KBO, j, we repeat the following steps. First, we collect all modeled objects that pass within a defined smoothing range of aj and qj, the parameters of the observed KBO. Because of our finite number of particles, smoothing is re- quired to overcome the shot noise which would otherwise dominate the results. Based on our observation that the behaviour of the modeled

KBOs changes rapidly with changing semima- jor axis around the transition region (we do not know this transition region a priori but it is within 200-400 AU in all the simulations; see Figures 1 and 3, for example) but changes little at large semimajor axes, we define the smooth- ing range in a as a constant value of 5% for aj < 230 AU, but, because the number of par- ticles in the simulations declines with increas- ing semimajor axis, we allow the smoothing dis- tance to linearly rise to 30% by aj = 730 AU. For perihelia beyond 42 AU, we observe little change in behaviour as a function of q, so we define a simple smoothing length of qj ± 10 AU with a lower limit of 42 AU (which is the limit we imposed on the observed KBOs). The main effect of these two smoothing parameters will be to slightly soften the sharp transition region (“the wall”) which, in practice, will contribute to the uncertainties in our derived mass, eccen- tricity, and, semimajor axis.

We select all of the modeled KBOs at times af- ter the initial Gyr of simulation that pass within these a and q limits at any time step, and we weight them with two Gaussian kernels, each with a σ equal to half of the smoothing distances defined above. The selected objects now all con- tain similar semimajor axis and perihelion dis- tance as the jth observed KBO, and their nor- malized distribution gives the probability that such an observed KBO would have a given in- clination, longitude of perihelion, and longitude of ascending node. At this point the simulated values of w and Ω are all relative to w9 and Ω9, rather than in an absolute coordinate sys- tem. We refer to these relative values as ∆w and ∆Ω.

We create the three-dimension probability dis- tribution function of (i, w, ∆Ω) by select- ing a value of ∆w and then constructing a probability-distribution function of the pole po- sition (sin i cos Ω, sin i sin Ω), again using kernel density estimation now using a Gaussian ker-

Figure 3

Figure 3. Probability density of a versus ∆    for a variety of simulated Planets Nine, in the same format as Fig.1. At the lowest masses the cluster appears double-peaked as the clustered objects librate and spend greater amounts of time at their inflection points. The dashed line labeled ”wall” shows the transition between the nearby uniform population and the more distant clustered population. This transition distance decrease  with  increasing  m9  and  decreasing  a9  and  q9.  The  width  of  the  cluster  decreases  with  increasing m9. Systematic changes such as these demonstrate that the orbital distribution of the distant KBOs is strongly influenced by the orbital parameters of Planet Nine. Figure 4. The probability distribution of the pole positions of KBOs with a > 150 AU and q > 42 AU for  four  Planet  Nine  models.  All  models  have  m9  = 7  Mearth,  a9  = 500  AU,  and  e9  = 0.33  and  the  effect of changing i9 can be seen. The green dot shows the pole position of Planet Nine in the simulations. The white circle show 30 and 60 inclinations. For small inclinations the distant KBOs track the inclination of Planet Nine. As i9 increases, however, the overall longitudinal clustering diminishes and the pole positions cluster less tightly around that of Planet Nine.

nel with σ = 2 in great-circle distance from the pole position and σ = 10 in longitudinal distance from ∆w and multiplying by the a, q weighting from above. In practice we grid our pole position distribution as a HEALPIX1 map (with NSIDE=32, for an approximately 1.8 de- gree resolution) and we calculate separate pole position distributions for each value of ∆w at one degree spacings. This three-dimensional function is the probability that an unbiased sur- vey that found a KBO with aj and qj would have found that object with (i, w, ∆Ω)j in the kth simulation, or

j,k                                  j         9    9    9    9 k  

P (a,e)j [(i, w, ∆Ω) |(m , a , e , i ) ].        (3)

For arbitrary values of w9 and Ω9, this probabil- ity distribution can be translated to an absolute frame of reference with simple rotations to give


P (a,e)j [(i, w, Ω) |(m , a , e , i ) , w , Ω ].             (4)

  • 4.1.       Likelihood

With functions now specified for the probabil- ity of detecting object j at (i, w, Ω)j and also for the probability of detecting object j at (i, w, Ω)j assuming a uniform distribution across the sky, we can calculate our biased probability distribu- tion for object j in simulation k, Pjl,k, by simple multiplication:


P l(a,e)j [(i, w, Ω) |(m , a , e , i ) , w , Ω ] =

where X represents the full set of orbital ele- ments of the distant KBOs from Table 2. The likelihood is discretely sampled by the numeri- cal models in the first four parameters and con- tinuously sampled analytically in the two angu- lar parameters.

The likelihoods sparsely sample a seven- dimensional, highly-correlated parameter space. With even a cursory examination of the like- lihoods, however, several trends are apparent (Figures 5 and 6). First, the model with the maximum likelihood, M9 = 5 Mearth, a9 = 300

AU, i9  =  17,  e9 =  0.15,  w9 =  254,  and

Ω9 = 108, is nearly a local peak in every di- mension. Semimajor axes inside of ∼300 AU lead to low likelihoods, but more distant Plan-


ets Nine are viable (particularly if they are more massive), even if at reduced likelihood. The in- clination appears quite well confined to regions near 15, and strong peaks near w9 = 250 and Ω = 100 are evident.

  • 4.2.      Gaussian process emulation

To further explore the orbital parameters, their correlations, and their uncertainties, we require a continuous, rather than dis- cretely sampled, likelihood function. To esti- mate this likelihood at an arbitrary value of (m9, a9, i9, e9, w9, Ω9) we perform the following steps. First, because the likelihoods as func- tions of w9 and Ω9 are densely sampled for

Figure 4

P     × B           [(i, w, Ω) |U ]  (5)

polation to obtain an estimated likelihood for

j,k          j


each simulation at the specific desired value

where the arguments for Pj,k are omitted for simplicity. We rewrite this probability as our likelihood function in the form of Equation (2) and take the product of the individual j like- lihoods to form the overall likelihood for each model k at the values of w9 and Ω9:

Lk[(m9, a9, e9, i9)k, w9, Ω9|X],             (6)

1 https://healpix.jpl.nasa.gov/html/idl.htm

of w9 and Ω9. We next take the 121 simu- lations with their now-interpolated likelihoods and use these to create a computationally inex- pensive Gaussian Process model as an emulator for the likelihoods. The behaviour of the like- lihoods is extremely asymmetric, in particular in m9 and a9, with likelihood falling rapidly at small values of m9 but dropping only slowly at higher values. Likewise, the likelihoods change rapidly for small values of a9, while changing Figure 5. One dimensional plots of the log(likelihood) values as function of semimajor axis (a9), inclination (i9) and perihelion distance (q9), at the maximum likelihood in Ω9 and  9 for each simulation.  The points are colored by the mass of Planet Nine (m9) as shown in the legend. The one-dimensional plots show the general behavior but do not show the significant correlations between parameters

also add a constant kernel. Beyond the domain of the simulations we add artificial points with low likelihood to prevent unsupported extrapo- lation. The model is implemented using scikit- learn in Python (Pedregosa et al. 2011).

The emulator produces a likelihood value at arbitrary values of (M9, a9, i9, e9, w9, Ω9), and appears to do a reasonable job of reproducing the likelihoods of the numerical models, inter- polating between these models, and smoothly

extending the models over the full range of inter- est. Figure 7 gives an example of the correspon- dence between individual measured likelihoods

and the emulator in the rescaled variable al.

Figure 5a
Figure 5b
Figure 5c

Figure 6. The longitude of ascending node (Ω9)

and the longitude of perihelion ( 9) at which the

maximum likelihood occurs in each of the simula- tions. The points are colored by the mass of Planet Nine (m9).

more slowly at higher a9. To better repre- sent this behaviour, we rescale the variables that we use in our Gaussian Process model- ing. We use al = (a9/m9)0.5 and we replace e9 with a similarly-scaled function of perihelion distance, ql = {a9 ∗ (1 − e9)/m9}−0.5. These scalings cause the likelihoods to appear approx- imately symmetric about their peak values and to peak at similar values of al and ql for all masses (Figure 6). To enforce the smoothness and symmetry in the Gaussian Process model, we  choose  a  Mate´rn  kernel,  which  allows  for  a freely adjustable smoothness parameter, ν. We chose a value of ν = 1.5, corresponding to a once-differentiable function, and which appears to adequately reproduce the expected behavior of our likelihood models. We force the length scales  of  the  Mat´ern  kernel  to  be  within  the bounds (0.5, 2.0), (0.02, 0.05), (1.0, 10.0), and

(1.0,100.0) for our 4 parameters and for units of earth masses, AU, and degrees, correspond- ing to the approximate correlation length scales that we see in the likelihood simulations. We multiply this kernel by a constant kernel and

Viewed in the rescaled variables, the likelihoods and the emulator are relatively regular, sym- metric, and well-behaved. Similar results are seen for i9 and q9. While the emulator does not perfectly reproduce the simulation likelihoods, the large-scale behavior is captured with suffi- cient fidelity to allow us to use these results for interpolation between the discrete simulations.

  • 4.1.       MCMC

We use this Gaussian Process emulator to pro- duce a Markov Chain Monte Carlo (MCMC) model of the mass and orbital parameters of Planet Nine. We use the Python package em- cee (Foreman-Mackey et al. 2013) which im- plements the Goodman & Weare (2010) affine- invariant MCMC ensemble sampler. We con- sider two different priors for the semimajor axis distribution. The Planet Nine hypothesis is agnostic to a formation mechanism for Planet Nine, thus a uniform prior in semimajor axis seems appropriate. Nonetheless, different for- mation mechanisms produce different semima- jor axis distributions. Of the Planet Nine for- mation mechanisms, ejection from the Jupiter- Saturn region followed by cluster-induced peri- helion raising is the most consistent with known solar system constraints (Batygin et al. 2019). In Batygin & Brown (2021) we consider this process and find a distribution of expected semi-

Figure 7

Figure 7. The discrete log(likelihoods) from the simulation versus the continuous results from the emulator. Results from different values of m9 are shown as different colors and labeled in the legend. The top panel shows the rescaled parameter al = (a9/m9)−0.5. In this rescaled variable the likelihoods are approximately symmetric and vary smoothly as a function of mass different masses. The lines show the output from the Gaussian Process emulator with a fixed value of i9 = 15, maximum likelihood values of Ω9 and 9 and an iterative search to find the maximum likelihood value for e9.  The emulator result shown thus represents the highest possible maximum likelihood at i9 = 15 for each value of al for each m9. The middle panel shows emulator results versus inclination for the maximum likelihood values of all parameters, while the bottom panel shows emulator results versus perihelion at a fixed inclination of i9 = 15 and maximum likelihood values of the other parameters. In all cases, the emulator output follows the upper envelope of the likelihoods, as expected.


major axes that smoothly rises from about 300 AU to a peak at about 900 AU before slowly de- clining. The distribution from these simulations can be empirically fit by a Fr´echet distribution of the form p(a) = (a µ)(α+1) exp(−((a µ))    ) with α = 1.2, β   = 1570 AU, and µ = −70 AU. We consider both this and the uniform prior and discuss both below. Addi- tionally, we assume priors of sin(i9) in inclina- tion and e9 in eccentricity to account for phase- space volume. Priors in the other parameters are uniform. We sample parameter space using 100 separate chains (“walkers”) with which we obtain 20890 samples each. We use the emcee package to calculate the autocorrelation scales of these chains and find that maximum is 130 steps, which is 160 times smaller than the length of the chain, ensuring that the chains have con- verged. We discard the initial 260 steps of each chain as burn-in and sample each every 42 steps to obtain 49100 uncorrelated samples.


Examining the two different choices of prior for a9 we see that the posterior distributions of the angular parameters, i9, Ω9, and w9, are un- changed by this choice. The parameters m9, a9, and e9 are, however, affected. This effect can best be seen in the posterior distributions of a9 for the two different priors. The uniform prior has 16th, 50th, and 84th percentile values of a9 = 300, 380, and 520 AU (380+140 AU) ver-


sus a9 = 360, 460, and 640 AU (460+180 AU)

for the cluster scattering prior. While the two posterior distributions agree within 1σ, the dif- ferences are sufficiently large that predictions of expected magnitude, for example, could be af- fected. Here we will retain the uniform prior for continued analysis, but we keep in mind be- low the effects of a semimajor axis distribution with values approximately 20% larger. For this uniform prior, the marginalized perihelion and

aphelion  distances  of  Planet  Nine  are  300+85

Figure 8 shows a corner plot illustrating the full two-dimensional correlation between the posterior distribution of pairs of parameters for the cluster scattering prior in a9. We see the clear expected correlations related to a9, m9, and e9. No strong covariances exist between the other parameters. The posterior distribu- tions for i9 and Ω9 are among the most tightly confined, suggesting that the data strongly con- fine the pole position – and thus orbital path through the sky – of Planet Nine.

Examination of Fig. 1 helps to explain why low values of m9 and a9 are preferred. The mass is directly related to the width of the cluster, and masses greater than 6 Mearth lead to nar- rower clusters than those observed. Likewise, a low m9 planet requires a small semimajor axis to have a distance to the wall of only ∼200 AU as the data appear to support. It is possible, of course, that the two KBOs with a ∼ 200 AU are only coincidentally situated within the clus- ter and the real wall, and thus a9 is more dis- tant, but the likelihood analysis correctly ac- counts for this possibility.


With distributions for the mass and orbital elements of Planet Nine now estimated, we are capable of determining the probability distribu- tion of the on-sky location, the heliocentric dis- tance, and the predicted brightness of Planet Nine. We first use the full set of samples from the MCMC to determine the probability distri- bution function of the sky position and heliocen- tric distance of Planet Nine. To do so we calcu- late the heliocentric position of an object with the orbital parameters of each MCMC sample at one degrees spacings in mean anomaly, M9. The sky density of these positions is shown in Fig- ure 9. Appropriately normalized, this sky plane

and 460+200 AU, respectively.



density represents the probability distribution function of finding Planet Nine at any heliocen- tric position in the sky. Approximately 95% of

Figure 8

Figure 8. A corner plot showing the histograms and covariances of the parameters. The dashed lines on the histograms show the median and 16th and 84th percentiles of each marginalized distribution. The two dimensional histograms include the 1, 2, and 3σ contour lines.

Text Box: i9Text Box: e9Text Box: a9Text Box: 91000750500250403020100. probability is within a swath of the sky that is ±12 in declination from an orbit with an in- clination of 16◦ and an ascending node of97, the median marginalized values of these param- eters.

To estimate the magnitude of Planet Nine we need not just the mass, but also the diameter and the albedo, neither of which we directly con- strain. We thus model what we consider to be reasonable ranges for these parameters.

For masses between 4-20 Mearth we assume that the most likely planetary composition is that of a sub-Neptune, composed of an icy- rocky core with a H-He rich envelope (we discuss alternatives below). We assume a simple mass-diameter relationship of r9 = (m9/3Mearth) Rearth based on fits to (admittedly much warmer) planets in this radius and mass range by Wu & Lithwick (2013). The albedo of such an object has been modeled by Fortney et al. (2016), who find that all absorbers are

Figure 9

Figure 9. The sky plane density, heliocentric distance, and predicted R magnitude for our Planet Nine samples. The top panel is a Mollweide equal area projection centered at an RA of 180 and declination of 0. Thick lines show the celestial equator, the ecliptic, and 20 from the galactic plane. Thin lines show every 45 of RA and declination. The middle panel shows the probability distribution of heliocentric distance as a function of RA. The lines show the 16th, 50th, and 84th percentiles of the distribution. The bottom panel shows modeled R-band magnitudes again with the 16th, 50th, and 84th percentile distributions.The peak in probability density at R.A.∼ 45 corresponds to the predicted aphelion position of P9 where an eccentric object spends more of its time.

condensed out of the atmosphere and the planet should have a purely Rayleigh-scattering albedo of ∼0.75. We conservatively assume a full range of albedos from 0.2 – half that of Neptune – to 0.75. With these diameters and albedos we can use the modeled distances to determine the brightness of Planet Nine for each of the sam- ples. Figure 8 shows the predicted magnitudes of Planet Nine. At the brightest end, Planet Nine could already have been detected in mul- tiple surveys, while at the faintest it will require dedicated searches on 8-10 meter telescopes.

  • 6.     CAVEATS

Both the maximum likelihood and the fully marginalized MCMC posterior distributions suggest that Planet Nine might be closer and potentially brighter than previously expected. The original analysis of Batygin & Brown (2016) was a simple proof-of-concept that an inclined eccentric massive planet could cause outer solar system clustering, so the choice of m9=10 Mearth, e9=0.7, and i9 = 30 was merely notional. Brown & Batygin (2016) showed that a wide range of masses and semimajor axes were acceptable with the constraints available at the time, while Batygin et al. (2019) showed hints of a preference for lower mass and semimajor axis. As previously discussed, one of the strongest drivers for the lower mass and semimajor axis of Planet Nine is the width of the longitude of perihelion cluster. With longitudes of perihelion ranging from 7 to 118, this 111 wide cluster is best matched by low masses, which necessitates low semimajor axes to bring the wall in as close as 200 AU.

One possibility for artificially widening the longitude of perihelion cluster is contamination by objects recently scattered into the 150 < a < 1000 AU, q > 42 AU region. It is plau- sible that 2013FT28, the major outlier outside of the cluster, is one such recently Neptune- scattered object. While integration of the or- bit of 2013FT28 shows that it is currently

metastable, with a semimajor axis that diffuses on ∼Gyr timescales, and while we attempted to exclude all recent Neptune-scattered objects by requiring q > 42 AU, we nonetheless note that within the 200 Myr of our simulations ∼20% of the objects that start as typical scattering ob- jects with 30 < q < 36 AU and a < 150AU have diffused to the q > 42 AU, a > 150 AU region. These diffusing objects are broadly clustered around ∆w ∼ 0◦ instead of around

w ∼ 180 like the stable cluster. 2013FT28

is such a strong outlier, however, that whether it is a contaminant from this route or not, its presence has little affect on our final retrieved orbital parameters. No Planet Nine simulations are capable of bringing it into a region of high likelihood.

A more worrisome possibility for inflating the width of the longitude of perihelion clustering is the scattering inward of objects from the inner Oort cloud (Batygin & Brown 2021). As noted earlier, we have no clear way to discriminate against these objects, and while the most dis- tant objects are more likely to have originated from this exterior source, such objects can be pulled down to small semimajor axes, too. We have no understanding of the potential magni- tude – if any – of this potential contaminating source, so we assess the maximum magnitude of the effect by systematically examining the exclusion of objects from the data set. Lim- iting the number of objects under consideration will necessarily raise the uncertainties in the ex- tracted parameters, but we instead here simply look at how it changes the maximum likelihood simulation.

We recalculate the maximum likelihood val-

ues of each simulation after exclusion of the ob- ject most distant from the average w position of the cluster (with the exception of 2013FT29, which we always retain). Even after excluding the 6 most extreme objects in the cluster and re- taining only 4, the maximum likelihood changes

only from m9  =  5 to m9  =  6 M and from a9 = 300 to a9 = 310 AU. The orbital angles do not change substantially.

We conclude that the preference for smaller values of mass and semimajor axis is robust, and that the orbital angles (i9, Ω9, w9) are largely unaffected by any contamination. While the posterior distributions for m9 and a9 have large tails towards larger values, the possibility of a closer brighter Planet Nine needs to be seriously considered.

An additional uncertainty worth considering is the diameter and albedo of Planet Nine. We have assumed values appropriate for a gas-rich sub-Neptune which, a priori, seems the most likely state for such a distant body. Given our overall ignorance of the range of possibilities in the outer solar system, we cannot exclude the possibility of an icy body resembling, for ex- ample, a super-Eris. Such an icy/rocky body

could be ∼50% smaller than an equivalent sub- Neptune in this mass range (Lopez & Fortney

2014), and while the large KBOs like Eris have high albedos, much of this elevated albedo could be driven by frost covering of darker irradiated materials as the objects move through very dif- ferent temperature regimes on very eccentric or- bits. An object at the distance of Planet Nine

– which stays below the condensation tempera- ture of most volatiles at all times – could well lack such volatile recycling and could have an

albedo closer to the ∼10% of the large but not

volatile-covered KBOs (Brown 2008). Overall the effect of a smaller diameter and smaller albedo could make Planet Nine ∼ 3 magni-

tudes dimmer. Such a situation would make

the search for Planet Nine considerably more difficult. While the possibility of a dark super- Eris Planet Nine seems unlikely, it cannot be excluded.

Finally, we recall the affect of the choice of the prior on a9.  A prior assuming formation in a cluster would put Planet Nine more distant

than shown here, though it would also predict higher masses. Combining those effects we find that the magnitude distribution seen in Figure 8 would shift fainter by about a magnitude near aphelion but would change little close to peri- helion.

While all of these caveats affect the distance, mass, and brightness of Planet Nine, they have no affect on the sky plane position shown in Figure 8. To a high level of confidence, Planet Nine should be found along this delineated path.

  • 6.     CONCLUSION

We have presented the first estimate of Planet Nine’s mass and orbital elements using a full statistical treatment of the likelihood of detec- tion of the 11 objects with 150 < a < 1000 AU and q > 42 AU as well as the observa- tional biases associated with these detections. We find that the median expected Planet Nine semimajor axis is significantly closer than previ- ously understood, though the range of potential distances remains large. At its brightest pre- dicted magnitude, Planet Nine could well be in range of the large number of sky surveys being performed with modest telescope, so we expect that the current lack of detection suggests that it is not as the brightest end of the distribution, though few detailed analysis of these surveys has yet been published.

Much of the predicted magnitude range of

Planet Nine is within the single-image detec- tion limit of the LSST survey of the Vera Rubin telescope, r ∼ 24.3, though the current survey plan does not extend as far north as the full pre- dicted path of Planet Nine. On the faint end of the distribution, or if Planet Nine is unexpect- edly small and dark, detection will still require imaging with 10-m class telescopes or larger.

Despite recent discussions, statistical evidence for clustering in the outer solar system remains strong, and a massive planet on a distant in- clined eccentric orbit remains the simplest hy- pothesis. Detection of Planet Nine will usher ina new understanding of the outermost part of our solar system and allow detailed study of a fifth giant planet with mass common through- out the galaxy.


This manuscript owes a substantial debt to the participants at the MATH + X Sympo- sium on Inverse Problems and Deep Learning in Space Exploration held at Rice University in Jan 2019 with whom we discussed the issue of inverting the observations of KBOs to solve for Planet Nine. We would also like to thank two anonymous reviewers of a previous paper whose excellent suggestions ended up being incorpo- rated into this paper and @Snippy X and @si- welwerd on Twitter for advice on notation for our likelihood functions.

Software: HEALPix (Gorski et al. 2005), as- tropy (Astropy Collaboration et al. 2013), scikit- learn (Pedregosa et al. 2011), emcee (Foreman- Mackey et al. 2013), corner (Foreman-Mackey 2016)

Table 2.

m9 (Mearth)a9 (AU)i9 (deg)e9 9 (deg)Ω9 (deg)  num. particles

Table 2 continued

m9 (Mearth)a9 (AU)i9 (deg)e9 9 (deg)Ω9 (deg)  num. particles

Table 2 continued

Table 2 (continued)

m9 (Mearth)a9 (AU)i9 (deg)e9 9 (deg)Ω9 (deg)  num. particles

Table 2 continued

m9 (Mearth)a9 (AU)i9 (deg)e9 9 (deg)Ω9 (deg)  num. particles

Note—Parameters used in the numerical simulations on the effects of Planet Nine (m9,  a9,  i9,  e9)  and  the  maximum  ln(likelihood),   ,  which  occurs  at  the  listed value of 9 and Ω9. ∆ gives the difference in ln(likelihood) from the maximum value, which occurs at m9 = 5, a9 = 310, i9 = 15, and e9 = 0.10.


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