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Mongo
Free to view paper:

Evidence for a Distant Giant Planet in the Solar System

Konstantin Batygin and Michael E. Brown

QUOTE
Abstract

Recent analyses have shown that distant orbits within the scattered disk population of the Kuiper Belt exhibit an unexpected clustering in their respective arguments of perihelion. While several hypotheses have been put forward to explain this alignment, to date, a theoretical model that can successfully account for the observations remains elusive. In this work we show that the orbits of distant Kuiper Belt objects (KBOs) cluster not only in argument of perihelion, but also in physical space. We demonstrate that the perihelion positions and orbital planes of the objects are tightly confined and that such a clustering has only a probability of 0.007% to be due to chance, thus requiring a dynamical origin. We find that the observed orbital alignment can be maintained by a distant eccentric planet with mass gsim10 m⊕ whose orbit lies in approximately the same plane as those of the distant KBOs, but whose perihelion is 180° away from the perihelia of the minor bodies. In addition to accounting for the observed orbital alignment, the existence of such a planet naturally explains the presence of high-perihelion Sedna-like objects, as well as the known collection of high semimajor axis objects with inclinations between 60° and 150° whose origin was previously unclear. Continued analysis of both distant and highly inclined outer solar system objects provides the opportunity for testing our hypothesis as well as further constraining the orbital elements and mass of the distant planet.


QUOTE
6. SUMMARY

To date, the distinctive orbital alignment observed within the scattered disk population of the Kuiper Belt remains largely unexplained. Accordingly, the primary purpose of this study has been to identify a physical mechanism that can generate and maintain the peculiar clustering of orbital elements in the remote outskirts of the solar system. Here, we have proposed that the process of resonant coupling with a distant, planetary mass companion can explain the available data, and have outlined an observational test that can validate or refute our hypothesis.

We began our analysis with a re-examination of the available data. To this end, in addition to the previously known grouping of the arguments of perihelia (Trujillo & Sheppard 2014), we have identified ancillary clustering in the longitude of the ascending node of distant KBOs and showed that objects that are not actively scattering off of Neptune exhibit true orbital confinement in inertial space. The aim of subsequent calculations was then to establish whether gravitational perturbations arising from a yet-unidentified planetary-mass body that occupies an extended, but nevertheless bound, orbit can adequately explain the observational data.

The likely range of orbital properties of the distant perturber was motivated by analytic considerations, originating within the framework of octupole-order secular theory. By constructing secular phase-space portraits of a strictly planar system, we demonstrated that a highly eccentric distant perturber can drive significant modulation of particle eccentricities and libration of apsidal lines such that the perturber's orbit continuously encloses interior KBOs. Intriguingly, numerical reconstruction of the projected phase-space portraits revealed that, in addition to secular interactions, resonant coupling may strongly affect the dynamical evolution of KBOs residing within the relevant range of orbital parameters. More specifically, direct N-body calculations have shown that grossly overlapped, apsidally anti-aligned orbits can be maintained at nearly Neptune-crossing eccentricities by a highly elliptical perturber, resulting in persistent near-colinearity of KBO perihelia.

Having identified an illustrative set of orbital properties of the perturber in the planar case, we demonstrated that an inclined object with similar parameters can dynamically carve a population of particles that is confined both apsidally and nodally. Such sculpting leads to a family of orbits that is clustered in physical space, in agreement with the data. Although the model proposed herein is characterized by a multitude of quantities that are inherently degenerate with respect to one another, our calculations suggest that a perturber on an a' ~ 700 AU, e' ~ 0.6 orbit would have to be somewhat more massive (e.g., a factor of a few) than m' = 10 m⊕ to produce the desired effect.

A unique prediction that arises within the context of our resonant coupling model is that the perturber allows for the existence of an additional population of high-perihelion KBOs that do not exhibit the same type of orbital clustering as the identified objects. Observational efforts aimed at discovering such objects, as well as directly detecting the distant perturber itself constitute the best path toward testing our hypothesis.


So about the size of Neptune, if their hypothesis is correct.
scalbers
This sounds familiar, perhaps written about recently in Sky and Telescope?

http://www.skyandtelescope.com/astronomy-n...int-ofplanet-x/

EDIT: I found the 2014 article above online - though I probably read a similar thing in the magazine.
Steve G
Help me out here on the math. If they are mentioning 700 AU (correct me if I'm wrong please) and Voyager 1 is currently 133.8 AU, and is considered to be in interstellar space, and this is 5 times more distant, is it really a planet or an interstellar companion?




Explorer1
scalbers: I recall that would have been about the 2014 results for the first KBO grouping; the second grouping being exactly where the models predict seals the deal as far as the paper is concerned. Exciting stuff.
It still needs to be actually spotted, of course (and then work can start on New Horizons 2 for real this time!).
JRehling
QUOTE (Steve G @ Jan 20 2016, 10:43 AM) *
Help me out here on the math. If they are mentioning 700 AU (correct me if I'm wrong please) and Voyager 1 is currently 133.8 AU, and is considered to be in interstellar space, and this is 5 times more distant, is it really a planet or an interstellar companion?


63,000 AU is one light year, and is approximately the radius of the Sun's Hill sphere. This is therefore only about 1% of the size of the largest orbits that are possible around the Sun, and only 0.25% of the distance to the nearest other star. So this doesn't seem very interstellar to me. On the interstellar scale, this object (if it exists) is right next to the Sun.

Perhaps more to the point, the observed dynamic, if it exists, depends upon the object having made many orbits around the Sun, not being something out there floating in its own orbit around the galaxy.
Gerald
That far out the Hill sphere of an object should be rather large. Therfore I'm wondering - provided the analysis isn't based on observational bias - whether there couldn't exist kind of a miniature globular cluster made of planetesimals, or a small version of a protoplanetary disk, of the same mass as the presumed planet, but without having formed an actual planet.
The simulations - as far as I understood - assume a certain point mass, but not necessarily united to one planet.
Or - if there is one population of KBOs - why not a second one, forming a Kozai-like resonance with the observed population.
stevesliva
QUOTE (scalbers @ Jan 20 2016, 12:29 PM) *
This sounds familiar, perhaps written about recently in Sky and Telescope?


Mike Brown's blog hinted that he was trying to explain Sedna's weird orbit. This would be that, but I don't think the details have been previously explained.
katodomo
QUOTE (Steve G @ Jan 20 2016, 06:43 PM) *
If they are mentioning 700 AU

700 AU is given as an example semi-major axis of a perturber, with from a cursory glance their possible range laying at 400 to 1500 AU semi-major axes. The minor planets cited as grouped by resonance with the hypothetical objext have semi-major axes of 150-600 AU.

What NASA used as definition for the solar system for Voyager is the heliosphere, i.e. the extend to which the stellar wind of the Sun interferes with the interstellar medium; an object with aphelion (such as most long-period comets) beyond basically dips in and out of this bubble. The IAU definition of a planet does not have any distance limitation.

The referred-to Hill Sphere of the Sun, being its gravitational influence zone, is probably the most appropriate measure when considering "interstellar companions" though.
Gladstoner
I recall reading somewhere about an observed (possible) clustering of long-period cometary orbits that could have resulted from a distant massive planet or even a brown dwarf (which has since been ruled out). It would be interesting to compare these with the findings of Batygin-Brown.
Holder of the Two Leashes
QUOTE (Mongo @ Jan 20 2016, 10:58 AM) *
"our calculations suggest that a perturber on an a' ~ 700 AU, e' ~ 0.6 orbit would have to be somewhat more massive (e.g., a factor of a few) than m' = 10 m⊕ to produce the desired effect."

So about the size of Neptune, if their hypothesis is correct.


Neptune has a mass about 17.15 earths, so a factor of "a few" time ten earths sounds to me like something bigger than Neptune, even if "a few" means three or four.

Neptune would have been visible to WISE out to 700 au. Saturn, at 95 earth masses, out to 10,000 au. This hypothesized planet somewhere in between. So... just how eccentric an orbit are we talking here?
0101Morpheus
Gladstoner.

I was under the impression that the such ruled out any brown dwarf or massive planets currently around the sun. There could have been one out there once and was since lost. Or maybe an interstellar interloper made a close pass to the solar system and went on its way.
elakdawalla
Most of the answers to all of your questions can be found on Brown and Batygin's blog.
scalbers
QUOTE (stevesliva @ Jan 20 2016, 06:42 PM) *
Mike Brown's blog hinted that he was trying to explain Sedna's weird orbit. This would be that, but I don't think the details have been previously explained.


I believe what I had read about in Sky and Telescope magazine is the clustering of arguments of perihelion of KBO's. It was suggested in 2014 in an online S&T article these correlations of perihelia could be consistent with a distant planetary perturbation (a super-Earth around 250AU distant). I'm a bit unclear on whether what I'd read was in Emily's S&T print article from Feb 2014 or more likely another writeup a bit more recently. I added a relevant link into post #2. Explorer1 summarizes this also in post #4.
Phil Stooke
I'm waiting until I can make a map of it before I accept it as real.

Phil
tfisher
QUOTE (katodomo @ Jan 20 2016, 01:56 PM) *
700 AU is given as an example semi-major axis of a perturber, with from a cursory glance their possible range laying at 400 to 1500 AU semi-major axes.


Wow. If we launch a probe that goes at similar speed to voyager 1, it takes over 100 years to reach 400 AU. From the blog site, it sounds pretty likely this thing is sitting out near its aphelion. Seems like a multi-generation effort is needed to explore this.

HSchirmer
QUOTE (Mongo @ Jan 20 2016, 05:58 PM) *
Free to view paper:

Evidence for a Distant Giant Planet in the Solar System
Konstantin Batygin and Michael E. Brown

QUOTE

In this work we show that the orbits of distant Kuiper Belt objects (KBOs) cluster not only in argument of perihelion, but also in physical space.
We demonstrate that the perihelion positions and orbital planes of the objects are tightly confined and that such a clustering has only a probability of 0.007% to be due to chance, thus requiring a dynamical origin.
We find that the observed orbital alignment can be maintained by a distant eccentric planet with mass -10x earth- whose orbit lies in approximately the same plane as those of the distant KBOs, but whose perihelion is 180° away from the perihelia of the minor bodies.




Very interesting- basically a "balance argument"?
The lopsided grouping of known eccentric KBO orbits implies a counter balancing object in an eccentric orbit.
We likely can't see this object because it is currently hidden by the bright backdrop of the milky way.

Ok, what about a "null hypothesis" -
The lopsided grouping of known eccentric KBO orbits implies that KBOs are equally distributed around the solar system, but because of an observation bias, we have detected those that are against a dark sky, but have not detected those that are masked by the milky way?

Does the subset "known eccentric KBO orbits" imply
the set "unknown equally distributed eccentric KBO orbits" or does it imply "single large planet in eccentric orbit"?

Curious about the orbital mechanics, if we find eccentric KBO orbits with argument of perihelion going the opposite way, towards the milkly way, would that imply a Planet IX with a circular orbit?
alan
QUOTE
Ok, what about a "null hypothesis" -
The lopsided grouping of known eccentric KBO orbits implies that KBOs are equally distributed around the solar system, but because of an observation bias, we have detected those that are against a dark sky, but have not detected those that are masked by the milky way?

Figure 2 of the paper shows the distribution of perihelia, those used in the paper are distributed over 110 degrees of elliptic longitude, so I would expect at least some of those with perihelia in the opposite direction to miss the milky way.
ZLD
Wasn't the VLT paper just last month getting lambasted with many maintaining that there couldn't be a planet of this size left undiscovered in the solar system because surveys should have caught them? Why is this being lauded so highly if the same reasoning should still apply? Don't get me wrong, I find both intriguing and somewhat compelling and worthy of further research.
scalbers
QUOTE (tfisher @ Jan 21 2016, 06:46 AM) *
Wow. If we launch a probe that goes at similar speed to voyager 1, it takes over 100 years to reach 400 AU. From the blog site, it sounds pretty likely this thing is sitting out near its aphelion. Seems like a multi-generation effort is needed to explore this.

Unless we can resurrect the "Planet Imager" mission concept of a constellation of space interferometers. This next step to network several Terrestrial Planet Finder (TPF) systems was envisioned to image details on planets around other stars, though maybe it can be repurposed?

Also, maybe new rocket technology will come along at some point if we want to speculate on that.

Will a telescope like the LSST be able to locate this planet? And does this really clear out its orbit to qualify as a planet?
JRehling
QUOTE (Phil Stooke @ Jan 20 2016, 10:15 PM) *
I'm waiting until I can make a map of it before I accept it as real.


Here there be dragons.
stevesliva
QUOTE (ZLD @ Jan 21 2016, 11:35 AM) *
Wasn't the VLT paper just last month getting lambasted


I don't recall that, and I'm always interested in how people benchmark their skepticism as well. I do recall lambasting of the alpha centauri serendipitous discovery that wasn't.
Mongo
QUOTE (scalbers @ Jan 21 2016, 06:15 PM) *
Will a telescope like the LSST be able to locate this planet? And does this really clear out its orbit to qualify as a planet?


That definition uses very unfortunate wording. A planet does not literally have to clear away all other objects close to its orbit. If that were so there would be NO planets in the Solar System (except maybe Mercury or Venus). Another equally acceptable wording is that a planet must gravitationally dominate its neighborhood, which does apply to all eight known planets in the Solar System, but not to any of the known dwarf planets. It appears that this "Planet Nine" would gravitationally dominate its neighborhood (indeed, that was how its existence was theorized), so it would qualify as a full planet.
Explorer1
Margot's 2015 paper criteria ( http://arxiv.org/pdf/1507.06300.pdf ) is what Mike Brown relies upon for his nomenclature decision, posted on the new blog: http://www.findplanetnine.com/2016/01/is-p...ine-planet.html
Maybe that should be the last word on this specific topic, given the current forum rules.... wink.gif
JRehling
Two noteworthy sentences in the article:

"No obvious bias appears to cause the observed clustering."
"the precise range of perturber parameters required to satisfactorily reproduce the data is at present difficult to diagnose."

There's a lot being assumed there. I suppose all research has an implication like that behind it, but it leaves the conclusion in a vague realm: Of the things we've considered, one of them best explains the data.

There's a discussion of some biases, and it's noted that one aspect of the data could not be the result of observational bias. I'm not sure how much of the argument that applies to, and what parts of the argument that does not apply to. It seems clear that there's something unusual about the data that remains unexplained, but it's unclear how much the argument hinges on that powerful word "obvious" – the possibility that an un-obvious bias (i.e., any of the infinite number of possibilities that weren't considered) might explain it without the hypothesized ninth planet.

There's a lot of work left to do here, and I think the authors are clear about this. If someone can turn their analysis into a focused observational program looking for the ninth planet, that's great, but I think there's a lot more theorizing left to do, and the argument for a ninth planet might vanish by the time the pen-and-paper work is done.
Caotico09
QUOTE (ZLD @ Jan 21 2016, 10:35 AM) *
Wasn't the VLT paper just last month getting lambasted with many maintaining that there couldn't be a planet of this size left undiscovered in the solar system because surveys should have caught them? Why is this being lauded so highly if the same reasoning should still apply? Don't get me wrong, I find both intriguing and somewhat compelling and worthy of further research.


This Blog talks a little about surveys and what areas of Planet Nine's orbit have been looked at:

http://www.findplanetnine.com/p/blog-page.html
surbiton
QUOTE (Gerald @ Jan 20 2016, 06:35 PM) *
That far out the Hill sphere of an object should be rather large. Therfore I'm wondering - provided the analysis isn't based on observational bias - whether there couldn't exist kind of a miniature globular cluster made of planetesimals, or a small version of a protoplanetary disk, of the same mass as the presumed planet, but without having formed an actual planet.
The simulations - as far as I understood - assume a certain point mass, but not necessarily united to one planet.
Or - if there is one population of KBOs - why not a second one, forming a Kozai-like resonance with the observed population.


That sounds like the most plausible explanation. I also read that it is supposed to be rocky. I am not sure how that can be known.
A "mass" can be gaseous too, however, this is in the belt Surely, one "planet" would have been detected by now ?
surbiton
QUOTE (Mongo @ Jan 21 2016, 06:06 PM) *
That definition uses very unfortunate wording. A planet does not literally have to clear away all other objects close to its orbit. If that were so there would be NO planets in the Solar System (except maybe Mercury or Venus). Another equally acceptable wording is that a planet must gravitationally dominate its neighborhood, which does apply to all eight known planets in the Solar System, but not to any of the known dwarf planets. It appears that this "Planet Nine" would gravitationally dominate its neighborhood (indeed, that was how its existence was theorized), so it would qualify as a full planet.


Would that mean dear Pluto is back as a planet ? Whooopie !
JRehling
Gravity follows an inverse square law. If a collection of independent objects had enough mass to influence objects far (>1 AU) away, it would certainly have enough mass to congeal quite rapidly in geological time. It would be pulling itself together much more than it would be exerting subtle tugs on other objects.

The presumption that the object is rocky is simply a comment, I think, on whether it has enough mass to have collected a gaseous component like Neptune or not. There isn't any direct evidence as the composition (e.g., rock vs. metal vs. ice). Nor, in fact, any direct evidence about this body's actual existence.
Gerald
Globuar clusters are billions of years old and of considerable mass. So I'm not sure, whether it's straightforward, that clusters of debris need to congeal rapidly, if sufficiently distant from the Sun.
Kozai oscillations happen in small steps over many orbits.

Edit: That far out this could imply, that despite the whole cluster being, e.g. of 10 Earth masses, the largest object may still be considerably lighter.
As a consequence it may not be able keep a helium/hydrogen atmosphere. Other volatiles would be frozen, leading to a potentially comet-nucleus-like low albedo surface, hence reduced visual brightness.
Julius
QUOTE (surbiton @ Jan 22 2016, 04:36 AM) *
Would that mean dear Pluto is back as a planet ? Whooopie !

It would mean that the solar system has got 2 belts, the asteroid belt between Mars and Jupiter and the Kuiper belt between Neptune and what lies beyond such as planet 9.
Mongo
QUOTE (surbiton @ Jan 22 2016, 04:36 AM) *
Would that mean dear Pluto is back as a planet ? Whooopie !


Nope. Pluto is far too small to have any significant gravitational effect on other objects in similar orbits. It's merely one of numerous bodies of similar or slightly smaller size in its region of the Solar System, none of which are gravitationally significant. The dominant mass in its region is Neptune, which does control which orbits around it are occupied and which are empty, through resonances. The possible "Planet Nine", on the other hand, would count as a full planet because of its huge gravitational impact on everything in its region of the Solar System.
scalbers
On the other hand, planet 9 would be near the borderline of planethood, based on the graph we've seen in the 2015 Margot paper and in this blog:

http://www.findplanetnine.com/2016/01/is-p...ine-planet.html
HSchirmer
QUOTE (scalbers @ Jan 22 2016, 06:38 PM) *
On the other hand, planet 9 would be near the borderline of planethood, based on the graph we've seen in the 2015 Margot paper and in this blog:

http://www.findplanetnine.com/2016/01/is-p...ine-planet.html


The graph mentioned above illustrates an interesting point, " a planet clearing its orbit" IS actually a function of time AND mass. So the smaller bodies just take longer to BECOME planets.

It seems like Planet-9's current location would be the result of Jupiter or Saturn clearing IT out of THEIR orbit.
INSTABILITY-DRIVEN DYNAMICAL EVOLUTION MODEL OF A PRIMORDIALLY 5 PLANET OUTER
SOLAR SYSTEM
http://arxiv.org/pdf/1111.3682v1.pdf

Several runs of Nice Model simulations suggest that an ice giant between Saturn and Uranus would be the
most likely planet to be ejected.
Rather odd, then, that it WAS NOT a planet when it was being ejected from the orbital path of a gas giant,
then it becomes a planet when it clears out the area where it has been ejected to.
JRehling
This paper, Trujillo and Sheppard (2014), is so important to the discussion, that it should be read by anyone trying to understand the new work:

http://home.dtm.ciw.edu/users/sheppard/pub...heppard2014.pdf

It gives details of the key observation, that among minor planets with q>30 AU, the 12 objects with a>150 AU show common orbital characteristics that are not seen among the larger population of objects with q>30 AU and a<150 AU. 12 is sufficient to show statistical significance, so, simply put, there is something here that needs to be explained.

I think we need to see more work considering alternative explanations before a planetary perturber stands out as the only good explanation. Some things the two papers note:

1) Observational biases may exist, but can't explain all of the orbital similarities.
2) An origin based solely in the initial conditions of the outer solar system would not survive gigayear exposure to torques caused by the known outer planets.

Remaining models consist of various combinations of one or more perturbing objects orbiting the Sun combined possibly with some close stellar interaction in the past.

A difficulty is that the number of possible combinations of those models is wildly unconstrained. Finding a model that matches the observations pretty well isn't going to eliminate the infinite number of possible explanations that weren't considered. So, I think we're a long way from being able to duplicate the success of Neptune's discovery, where careful analysis gives astronomers a pinpoint location in the sky where the unseen object must exist.
JRehling
Funny, yet thought-provoking:

Possible Undiscovered Planets

http://xkcd.com/1633/
vossinakis
QUOTE (Gladstoner @ Jan 21 2016, 12:54 AM) *
I recall reading somewhere about an observed (possible) clustering of long-period cometary orbits that could have resulted from a distant massive planet or even a brown dwarf (which has since been ruled out). It would be interesting to compare these with the findings of Batygin-Brown.


Are you referring to this??? Arguments for the presence of a distant large undiscovered Solar system
planet http://astro.u-szeged.hu/ismeret/murray.pdf
Floyd
The latest Scientific American has an article on the search for planet X---by Michael Lemonick. Article good as background for this thread.
TheAnt
Oh yes I did run into that SciAm article also, before reading it I considered this to be one interesting idea but one that might have other explanations.
However when seeing this graphic with orbits of KBO's that are far from the ecliptic I began to understand why they find this such an tantalizing possibility.

Still when considering a planet like Neptune it should have an atmosphere that's gaseous even in the cold realm so far from the Sun. One such should show an IR excess for one or another reason, latent heat released when gas turning liquid and rain down for example. Now actually finding it might depend where this putative planet is located in its orbit, if it's anywhere of the furthest part of the orbit right now, the distance and very small proper motion might make it very difficult to detect. So yes, perhaps, they might be onto something here - yet, saying 'evidence' is stretching the meaning of the word a little bit to far yet. biggrin.gif
jasedm
I agree. Evidence there isn't (as yet)

fredk
Agreed. Possibly "indirect evidence" or "circumstantial evidence", with the caveat already mentioned here that an "unobvious bias" may also explain the observations. "Theoretical evidence" also doesn't make sense, since theories aren't observations and so can't provide evidence!

In my business, we'd probably call it a "hint".
Habukaz
It's easy for a definition of the word evidence to be circular:

a) evidence is something which shows that something exists
b) we know that something exists because we have evidence for its existence

So in order to know if something counts as evidence towards something, we first have to know if that something is real, and in order to know if something is real, we need evidence..


I think I'd rather put it this way: evidence is an observation consistent with a hypothesis, regardless of whether we assume the hypothesis to be true or false.

(now, you could say that evidence is part of what convinces you that something is real, but the subjectivity of this would make the definition problematic)

So I'd finally say that there is evidence - "inconclusive" evidence - for a fifth giant planet in the solar system. The consensus would also appear to be that not (good) enough evidence has been presented yet for this case in order to get really excited about it.
Floyd
We have the original data in the orbital parameters of several KBOs. Trying to explain the unusual clustering of these orbits, the idea of a 9th planet seemed a possibility. Much experimentation with computer modeling found a planet 9 mass and general orbit that could cause such clustering. Now there is a hypothesis. To test the hypothesis requires new data(KBOs) or spotting planet 9. The original data for a hypothesis can't be used as evidence for that hypothesis--must get new data.
HSchirmer
QUOTE (Habukaz @ Jan 27 2016, 11:00 PM) *
It's easy for a definition of the word evidence to be circular:
...
I think I'd rather put it this way: evidence is an observation consistent with a hypothesis, regardless of whether we assume the hypothesis to be true or false.
...


There are a few other definitions, the one I'm familiar with is logic/legal.

Evidence is anything that helps to prove or disprove an ultimate fact.

In much the same way that you have equations and variables, you also have law (hypothesis) and facts.
Facts are usually variables, unknowns that can be established or measured.
A series of facts, combined with an argument, eventually gets you to a proof.
nprev
MOD NOTE: Enough with the semantics, please.
Gerald
Citing this article:
QUOTE
The 0.007% chance that the clustering of the six objects is coincidental gives the planet claim a statistical significance of 3.8 sigma—beyond the 3-sigma threshold typically required to be taken seriously, but short of the 5 sigma that is sometimes used in fields like particle physics.

The range between 3 and 5 sigma is usually called "evident", greater or equal 5 sigma is called "definitive". 3 < 3.8 < 5. That's all.

The question is essentially, whether the numerical experimental settings leading to these 3.8 sigma confidence level are consistent with the way astronomers would have looked for the KBOs, hence whether the observational bias is considered appropriately.

One issue might be, that the same arguments preventing the direct observation of a possible planet 9 prevented observations of KBOs, e.g. the densely crowded Milky Way background.
Another issue might be an adjustment of the observation and detection methods to the first observed KBO of the presumed cluster. This disturbs the independence of the individual finds, as assumed in most randomized statistical tests, hence modifies inferred probabilities, and eventually the confidence level.

Edit: Another example: The probability of six objects randomly found in the same predefined 0.203 fraction of the sky is 7e-5 (the 3.8 sigma); the probability of six objects randomly found in the same predefined 0.379 fraction of the sky is 3e-3 (3.0 sigma). Hence another uncertainty is the size of the region of the sky the six observations are assigned to.
JRehling
Part of the background of the complexity here is the high number of possible scenarios to model.

The possible scenarios involving a single large planet as the explanation allow for variation in several variables. The planet's mass is one variable and its orbit entails about three more. Ideally, modeling work could investigate a large number of the possible permutations, iterating over the range of possible values in a fine-grained way. To model the evolution of the outer solar system over eons in each of thousands of different scenarios is a feat requiring sheer CPU time, and that work certainly ought to be done. Then, we might learn if there are any combination of parameters that explain the observations, and which cannot do so. Examining only one, or even only 100, of the possibilities is just scratching the surface of the possibilities. Nobody, novice or professional, is going to work this out with mere deep thought.
fredk
QUOTE (JRehling @ Jan 28 2016, 12:09 PM) *
its orbit entails about three more

The general orbit requires six parameters to describe it. If you assume it's orbital plane is close to the ecliptic that's still four parameters.

Of course your point is well taken - it's going to be really hard to thoroughly explore that parameter space.
Anders
There was a SETI talk by dr Ann-Marie Madigan. She had a simulation that could explain the Sedna objects strange orbits with smaller object (but still with 1-10 total earth masses):

http://www.seti.org/weeky-lecture/bizarre-...-beyond-neptune

Available on Youtube.

Can't see if she written any paper about it.

OK, paper has been up for a while,http://arxiv.org/abs/1509.08920 - Brown/Konstantin references it.

Only news is that a nice talk about it became available this week.
0101Morpheus
Even if the inclination instability model was correct, there could still be one or more objects with planetary mass in the outer solar system. Ceres contains a third of the mass in the asteroid belt and while the case may not be so severe in a large disk, when dealing with a disk of one or more earth masses, you will start reaching planetary masses rather quickly. There could be subjects ranging from Mercury to Mars and greater. I've read before that if Ganymede and Titan would be considered planets if they orbited the sun. That was then, now if we find larger objects out there, wouldn't they still be called planets?
JRehling
FWIW, Ganymede and Titan have larger diameters than Mercury but less than half its mass.

I'm not sure of any way in which considering a thing a planet or not is relevant except in the circular sense of its own sake.

For something to be producing the effects that have been discussed here, it would almost certainly have to be much more massive than Ganymede; probably more massive than Earth, which itself is dozens of times the mass of Ganymede.
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