Such objects were expected to behave like comets, and outgas as they approach the Sun.  However, this object did not spray the solar system with its internal gases, leading astronomers to conclude that this object had originally been an asteroid which had been ejected from another star system.  However, recent observations and work on 1I/'Oumuamua's trajectory indicate that its motion is being affected by another factor beyond gravitational interactions - it is moving faster than it should (4).  This is thought to be due to outgassing after all, leading to the conclusion that this object is an interstellar comet (5).

"Such outgassing is a behaviour typical for comets and contradicts the previous classification of `Oumuamua as an interstellar asteroid. “We think this is a tiny, weird comet,” commented Marco Micheli. “We can see in the data that its boost is getting smaller the farther away it travels from the Sun, which is typical for comets.”

"“We did not see any dust, coma, or tail, which is unusual,” explained co-author Karen Meech of the University of Hawaii, USA. Meech led the discovery team’s characterisation of `Oumuamua in 2017. “We think that ‘Oumuamua may vent unusually large, coarse dust grains.”" (5)

This idea rests upon the erosion of the comet through interstellar space - the exact opposite of what I'm advocating.  From our local perspective, interstellar space begins beyond the heliopause, which means that all solar system objects lying outside the Sun's heliosphere are in interstellar space, including any potential Planet X body.  A heavy coating of space gloop on 1I/'Oumuamua would readily explain the lack of cometary tail: The main body of the body is so covered by a thick coating of grease, which would have toasted nicely during perihelion, that the volatiles were locked in.  The dark red surface of 1I/'Oumuamua shows variation in tone and colour (6), which is probably attributable to thick coverings of organic materials irradiated in interstellar space.  Such a covering could seal in the comet's volatiles, largely preventing the expected spectacular comet display at perihelion.  Rather like tarring a wooden boat to keep it watertight. 

This comet/asteroid has been colliding with 'space grease' and other materials for as long as it has been tumbling through interstellar space, the amount of which seem significantly higher than previously thought:

"Until now there has been uncertainty over how much carbon is drifting between the stars. About half is expected to be found in its pure form. The rest is chemically bound with hydrogen in either a grease-like form, known as aliphatic carbon, or as a gaseous version of naphthalene, the main chemical component of mothballs.

"Helen Fraser, a senior lecturer in astronomy at the Open University, said: “It remains a major question in astronomy how dust forms, evolves and is destroyed.” Fraser said the work suggests that are many more “grease-like” molecules in space than previously thought. “The consequence could be important in how such dust grains stick and form planets, or even ‘seed’ planetary surfaces with the ingredients for the origins of life,” she added." (1)

This raises the prospect of continuous accumulation of materials in interstellar space, as I have been arguing for a while (7).  Protoplanets ejected from young, chaotic planetary systems can continue to accumulate mass while floating freely in interstellar space, I argue.  This mops up interstellar materials, condensing their distribution into a more granular, i.e. dark planets.  We can't see them, because they are not illuminated by local stars.  I argue that such objects are more pervasive through the galaxy than previously though, adding to it overall mass.  This explains where all this space grease is, which remains a mystery in terms of direct observation:

"The resultant aliphatic carbon [a hydrocarbon with no aromatic ring] column densities are least five times higher than some values reported previously. Using the two ISDA [interstellar dust analogues] integrated absorption coefficients, we obtained an abundance range between ~ 54−135 ppm for aliphatic carbon in the ISM [Interstellar Medium]. This leaves a substantial proportion of the dust-bound carbon to be found in aromatic or olefinic structures." (8)

This space grease is surely an incredibly important component in planet-building, helping to explain why colliding materials don't simply blast away from each other at the kinds of speeds they are travelling relative to one another.  Instead, they stick together. 

The calculations showed that the broad early Kuiper belt generated a bimodal structure of scattered objects associated with Planet Nine - some aligned, some anti-aligned with its position in space (1).  The average distances from the Sun for these scattered objects is considerably greater than normal KBOs - making it difficult to find them using current observational technologies.

"We compute that the median final perihelion distance of the detectable objects in the narrow distribution is 38.13 AU for the anti-aligned KBOs and 33.07 AU for the aligned KBOs, and in the broad distribution, 44.16 AU for the anti-aligned KBOs, and 100.29 AU for the aligned KBOs." (1)

The researchers note that the averaged distances for the anti-aligned objects is similar in both Kuiper belt scenarios (the broader anti-aligned objects seem to be perturbed back down into the solar system, becoming centaurs).  The real difference is seen in the aligned objects, whose distributions vary greatly with initial Kuiper belt conditions. 

"So, what’s the bottom line? Not all anti-aligned objects are stable! And not all aligned objects are stable. And it all depends on their perihelion distance, which is closely tied with their librations in longitude of perihelion.  Moreover, it turns out that what kind of objects we find surviving through the end of our simulations depends on the initial conditions we put in. What do I mean by initial conditions? Well, for example, we expect that different scenarios of Planet Nine’s formation would have affected the initial configuration of the Kuiper belt in different ways." (2)

The team also ran simulations with Planet Nine in the plane of the planets, and inclined to the ecliptic, finding little difference in their results.  This is important because Planet Nine is thought to be highly inclined to the ecliptic. 

The shape and configuration of the Kuiper belt depends upon which of the above formation scenarios is correct for Planet Nine.  Certain configurations appear to shape the cluster in different ways, allowing astronomers to potentially work backwards from future discoveries to determine how Planet Nine came to be.  The team draw this conclusion:

"Thus, the presence of high-q [large semi-major axis] aligned objects in our solar system would not only lend support for the existence of Planet Nine but would indicate that our current Kuiper Belt stems from an initially wide q-distribution. This, in turn, would signal that Planet Nine’s formation likely involved stellar encounters, as this is the most likely mechanism that would create such a spread out Kuiper Belt." (1)

What I'm taking from this is that the aligned objects are no longer considered to be in an unstable situation.  Potentially, then, Planet Nine could be oriented with this cluster in physical space.  If so, then Planet Nine could be in the opposite side of the sky to that indicated by the Caltech team in 2016. 

In reviewing the clustering effects of the extended trans-Neptunian objects (ETNOs), there appears to be a shift in emphasis on whether the objects which are aligned or anti-aligned to Planet Nine are stable.  I detect movement on this issue, which may have a significant impact upon the projected location of the planet itself.  Regrettably, my questions to Mike Brown on this issue have gone unanswered.  Ever since the first paper about Planet Nine was published, I have argued that their analysis about the planet's position is diametrically opposite to a previous body of evidence about the location of Planet X (3).  I suggest that the aligned/anti-aligned bodies in the initial Planet Nine cluster are the wrong way around, and that this will become increasingly apparent as more of these scattered objects are discovered.

If the broad early Kuiper belt scenario is correct, then there may be a substantial population of scattered objects awaiting discovery, whose average distances spread out significantly.  The current cluster of ETNOs could be the tip of the proverbial iceberg.  If this potential population is discovered as the ability to detect such distant objects improves, then it could have repercussions on our understanding of the Sun's birth environment within an early stellar cluster.  This paper provides such a theoretical model, assuming, of course, the presence of Planet Nine.

But not everyone agrees, and other explanations have been offered to explain this strange orbits.  High on the list are the action of passing stars during the lifetime of the solar system.  Again, such an explanation relies upon the temporary incursion of a massive objects, which draws out some of the Kuiper Belt objects.  Another possibility is that the objects in the extended scattered disk (ETNOs) originated within the comet clouds - more specifically, the inner Oort cloud, of Hills cloud.  They may be transitioning comets which will eventually become Halley-like comets closer in to the Sun.

A new explanation, offered by scientists working at Boulder, Colorado is that these objects aren't really so weird after all.  They have set up some computer simulations which suggest that they orbital paths may become more eccentric and extended simply as a result of gravitational interactions from within the Kuiper Belt itself (1).  If true, then there is no need to seek out Planet Nine (which is proving evasive, despite the near certainty with which it is being claimed).  Although a series of small-scale perturbations may provide a model to explain anomalies, such an explanation does not seem to offer an explanation for the patterning of the ETNOs, however.  One would expect such a series of small-scale events to randomise the effect.  It may also not explain the highly inclined nature of some of these objects (2).

I suspect Mike Brown et al. will shortly respond and point out their analysis of the common 'argument of perihelion' of these objects.  This is a technical issue, but an important one.  A series of small-scale adjustments over time would randomise the effect, and not create such a definitive clustering.

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