A correspondent of mine had noted similarities between what I had been writing about and previous work by Paul LaViolette, who had written about the origins of the dust picked up by the Ulysses spacecraft:

"I would suggest that the dust originates from a circumsolar dust sheath that is concentrated toward the plane of the ecliptic in a fashion similar to the disk girdling the star Beta Pictoris and that is co-moving with the Sun. Infrared observations confirm the existence of dust sheaths around other stars in the solar neighborhood, leading to the conclusion that our Solar System is similarly shrouded." (3)

The 20 million year old star Beta Pictoris provides astronomers with the best example of a gas giant exoplanet found orbiting within an evolving proto-planetary disk, made all the more dramatic by its side-on view and the brightness of scattered light from the revolving disk:   

"In 1984 Beta Pictoris was the very first star discovered to host a bright disc of light-scattering circumstellar dust and debris. Ever since then Beta Pictoris has been an object of intensive scrutiny with Hubble and with ground-based telescopes. Hubble spectroscopic observations in 1991 found evidence for extrasolar comets frequently falling into the star." (4)

Such a theme is also explored, at last from the point of view of how the solar system might have appeared early in its history, by astrophysicist Dave Jewitt in his highly informative online article about the Edgeworth-Kuiper Belt:

"One suggestion is that the depletion of the Kuiper belt could have been caused by extensive collisional grinding of the objects into dust. The dust would have temporarily given the Solar system a ring-like appearance as observed in the debris disks of nearby stars (Moro-Martin et al. 2007, Wyatt 2008) but, over time, the dust would have been swept away by radiation forces. Alternatively, some suppose that the Kuiper belt was dynamically depleted, presumably by interactions with Neptune." (5)

Dr Jewitt describes how there is an 'armada' of in-falling dust and other debris from the Kuiper Belt created by the continuous risk of collisions among the literally trillions of objects which make up the Kuiper Belt.  This interplanetary dust has been detected by various spacecraft, and extends deeply back through the planetary zone of the solar system.  That said, the Kuiper Belt lies well within the heliosphere.  It is what is going on beyond which is of keen interest. 

My contention, as well as that of Paul LaViolette before me, is that beyond the heliopause such depletion by radiation forces does not take place, leaving an extant sheath of dust.  I am arguing, further, that this sheath is gradually swept up by planetary bodies lying beyond the heliopause to create dust disks around them, and that this sheath of interstellar dust is periodically rejuvenated when the solar system moves through areas of relatively dense dust clouds interspersed throughout the galaxy.

You might reasonably ask how such a sheath might have avoided direct detection?  In this context, it's interesting to note that the Kuiper Belt is also thought to have a debris cloud, but one that has not been directly detected because of the 'zodiacal light' reflected back at us from interplanetary dust in the foreground:

"In contrast to all other debris disks, where the dust can be seen via an infrared excess over the stellar photosphere, the dust emission of the Edgeworth-Kuiper belt (EKB) eludes remote detection because of the strong foreground emission of the zodiacal cloud." (6)

If that's the case for a debris dust disk thought to exist in the Kuiper Belt, then it most assuredly applies to any extensive dust disk lying beyond the heliosphere, which is located about twice the distance away.  Simply put, if such a sheath of dust exists beyond the heliopause, then the comparatively bright foreground light of the interplanetary dust in the inner solar system is effectively preventing its detection.  Only a spacecraft moving beyond the heliopause with dust-detecting equipment will know for sure. 

The only probes exploring beyond the heliopause at the moment are the Voyager spacecraft, with the Pioneers somewhat behind them.  The Voyager spacecraft are not equipped with actual dust detectors, but will record the electric effects of the plasma cloud generated during the high velocity impact between dust and spacecraft, and even potentially pick up evidence of charged particles passing between their electric antennae. 

However, interpreting this data is not necessarily very straightforward:

"On November 13,1980, the spacecraft Voyager 1 passed through Saturn's E ring - a faint dusty structure having a rather large radial extension and thickness. The Voyager spacecraft did not carry conventional dust detectors, but they carried two wave instruments designed for measuring radio and plasma waves. Both instruments recorded dust signals, but this was recognised much later because the plasma played a dirty trick: genuine dust signals were mixed with intense plasma waves, and that made them difficult to interpret." (9)

As interstellar medium, including dust, likely takes the form of diffuse plasma, then these spacecraft are still capable of sending back data about what they're encountering as they shoot through the heliosheath zone.  There are complications, though.  Larger particles of particulate matter will be mixed in with what is presumed to be a steady flow of interstellar plasma.  Furthermore, the rather archaic data return from the Voyager craft these days is necessarily limited by the reducing power output of their slowly dying batteries as their nuclear fuels are depleted.  Nonetheless, scientists were still able to ascertain that Voyager entered an unexpected, but immense cloud of interstellar plasma held together by a strong magnetic field (10):

Perhaps the issue here is more to do with the perceived density of this cloud, at least at a very local level.  The zones beyond the heliosphere may be awash with interstellar clouds; even flowing rivers of interstellar plasma.  But what about denser, heavier materials?  What about larger grained material which is not ionised into plasmas?

An anomalously high level of large dust grains has been observed by the Ulysses, Cassini and Galileo satellites, which moves with the ISM flow towards the inner solar system. This effect has yet to be satisfactorily explained, and implies that there is an unexpected source for these particles beyond the heliosphere (12). Calculations, based upon certain assumptions, indicate a possible source ranged some 500 AU away (13), well beyond the heliosheath zone.

Astrophysicists interested in debris disks in the outer solar system have done modelling on whether dust can clump together at the distance of the Kuiper Belt.  They have found from their calculations that dust which falls into a resonant orbit with Neptune can indeed clump, forming larger particles of debris matter which can statistically withstand the opposing mechanisms of collisional cascades (14). 

(Such collisional cascades are thought to account for an observed dust cloud surrounding a presumed giant exoplanet lying in the extensive debris disk orbiting the star Formalhaut (15)). 

In other words, dust in the Kuiper Belt can, under certain conditions, begin to accrete - at least hypothetically, anyway.  Such a mechanism has not been detected directly, for the same reasons described above - interplanetary Zodiacal dust blinds us to more subtle effects beyond, in the same way street lighting impedes stargazing.  What's important here is the shepherding influence of a major nearby planet which, in the case of the Kuiper Belt, is Neptune. 

So, one would assume that a similar catalyst would be required beyond the heliopause.  One requires the influence, then, of a Planet Nine (16), Planet X or Dark Star object.  Such a mechanism may explain the anomalous influx of materials from beyond the heliopause, and provide an on-going process of planet-building in interstellar space.

Written by Andy Lloyd,  8th-17th July 2016

iopscience.iop.org article

Does this object fit in with Mike Brown's analysis of the cluster of 6 (now 7) Sednoid objects which he argues (along with his dynamicist colleague Konstantin Batygin) point to the existence of a substantial planet beyond the Kuiper Belt (3)?  Given the vague data regarding the orbit of 2015 RR245, it is perhaps too early to say.  But other scientists are already citing the on-going discoveries of distant objects like 2015 RR245 as reasons to be cautious. 

In an informative on-line article, more nuanced than its title suggests, astrophysicist Ethan Siegel notes that the pattern of discovery of objects within and beyond the Kuiper Belt is subject to an observational bias favouring the closest objects.  This means that the unknown populations of objects yet to be discovered may eventually statistically overwhelm the small populations of extended scattered disk objects and Sednoids already discovered.  This, he argues, could bring Brown and Batygin's analysis of the cluster into question. 

Such an argument does not directly address the statistical fluke generated by the original clustering of six Sednoids, which were later joined in March 2016 by a seventh, uo3L91 (5).  Instead, Siegel's argument simply implies that this fluke could be overwhelmed by a much broader data set attained at a later point.    But this is irrelevant, because 2015 RR245 actually enjoys a 9:2 resonance relationship with Neptune, and so is not a member of the same set of objects which has caused this controversy (6).  Siegel's argument is contested here by Dr Bannister:

It does seem, though, that each new object discovered beyond the Edgeworth-Kuiper Belt is going to generate this kind of debate:  Does its presence help, hinder, or utterly fail to impact upon the potential existence of Planet Nine? 

Another aspect to consider here is that the Caltech team seem to consider the population of these anomalous clustered objects to be a dynamic one, rather than a static collection held forever in thrall of Planet Nine.  A newly-published on-line article by Konstantin Batygin traces the evolution of the thinking behind Planet Nine, and describes in some detail the rather odd dynamic patterns of resonance which the Sednoids have with his (and Dr Brown's) proposed distant planet (9).

It seems reasonable that larger-than-dwarf planets might be captured up in to extended scattered disk of Sednoids in this way, but what's interesting is that Brown and Batygin envision a fluctuating population of Sednoids where Kuiper Belt Objects are pulled up into the scattered disk for a while, before falling back once more into the regular disk, to be replaced by others which experience similar perturbations:

"Surprising and unforeseen results continued to accrue. Upon a cursory examination of the simulation data, we noticed that gravitational torques exerted onto the Kuiper belt by Planet Nine would induce long-period oscillations in the perihelion distances of the confined KBOs. This naturally generated detached orbits, such as those of Sedna and 2012 VP113. Suddenly, the origins of these objects became abundantly clear: they are regular KBOs that have been pulled away from their original locations by Planet Nine. Moreover, the evolutionary calculations suggested that if we were to revisit the Kuiper belt in a hundred million years, objects like Sedna and VP would once again look like conventional, garden-variety KBOs, while some of the more typical objects would now be in detached orbits." (9)

Again, this implies that the statistical criticism employed by Ethan Siegel is wide of the mark.  It is what is going on in the specific clustering of anomalous objects that is crucial, not any buffering effect attributed to the broader populations around them - even those populations which remain undiscovered.  And I'll bet that those more distant undiscovered populations are even more irregular still than the Sednoids.

References:

In addition, the presence of a distant, sizeable Planet X object, whose closest approach to the Sun is argued to be 250 Astronomical Units away (3), could be affecting the tilt of the entire planetary system orbiting the Sun. 

"Using an analytic model for secular interactions between Planet Nine and the remaining giant planets, here we show that a planet with similar parameters can naturally generate the observed obliquity as well as the specific pole position of the sun's spin axis, from a nearly aligned initial state. Thus, Planet Nine offers a testable explanation for the otherwise mysterious spin-orbit misalignment of the solar system." (3)

The key to this quite dramatic effect is the high inclination of Planet Nine's projected orbital plane with respect to the rest of the solar system, rather than the object's inherent mass.  This point was made by Alessandro Morbidelli (2) who, along with his colleagues, has also published similar work at the same time as the Caltech team.  Their work goes further, providing a series of scenarios for the orbit of Planet Nine, and thus offering further constraints upon its present position:

"Under some specific configurations for planet 9, this precession [of plane of the planets relative to the total angular momentum vector of the Solar System] can explain the current tilt of approximately 6 degrees between the invariant plane of the giant planets and the solar equator. ...Some of the planet 9 configurations that allow explaining the current solar tilt are compatible with those proposed to explain the orbital confinement of the most distant Kuiper belt objects. Thus, this work on the one hand gives an elegant explanation for the current tilt between the invariant plane of the inner giant planets and the solar equator and, on the other hand, adds new constraints to the orbital elements of planet 9." (4)

Over very long time periods, the computer simulations independently deployed by both team shows that Planet Nine might generate the observed solar system anomalies naturally.  Significantly, this would imply that Planet Nine would have to have taken its place within the ranks of the planets quite early on in the lifetime of the solar system.  This, in turn, implies that its orbit is a stable feature of the solar system, rather than the result of a relatively recent capture of an exoplanet.

Of course, this is all conjecture based in turn upon hypothesis.  It should be pointed out that there are other possible explanations for the tilt of the rotational axis of the Sun, and the relative obliquity of the Sun's equatorial plane compared to the invariant plane of the ecliptic (2).   But the existence of a Planet X object at a high inclination could readily explain these effects, as well as the anomalies of the extended scattered disk beyond the Edgeworth-Kuiper Belt. 

Among the findings is increasingly strong evidence for the existence of clouds of water, or water ice, in its atmosphere (2), adding to previous similar findings (3).   Water ice?  This really is a cold 'star', whose frosty atmosphere exhibits sub-zero temperatures thought to range between −48 and −13 °C, similar to that of our North Pole.   These are the first water/water ice clouds detected outside our solar system, and occur in a cold brown dwarf whose colouring is, rather counter-intuitively for such a cold object, remarkably red (5).

The triple star system is indeed curious.  The two minor stars (B and C) orbit the main star A at a distance of about 300 Astronomical Units, all the time twirling around each other at approximately Saturn's distance from the Sun.  The newly discovered exoplanet, HD131399ab, also orbits around the main star A in a wide orbit "about twice as large as Pluto's if compared to our solar system, and brings the planet to about one-third of the separation of the stars [B & C] themselves." (2). 

The massive planet's orbit around its parent star is by far the widest known orbit within a multi-star system.

"The location of HD 131399Ab on a wide orbit in a triple system demonstrates that massive planets may be found on long and possibly unstable orbits in multi-star systems. HD 131399Ab is one of the lowest mass (4 ± 1 MJup) and coldest (850 ± 50 K) exoplanets to have been directly imaged." (3)

According to the University of Arizona team's calculations, this set-up is barely stable over the long-term, and could very easily result in the ejection of HD131399ab if any of the system's variables were to alter.  Future data should firm up a more accurate model of this star system (2), and perhaps provide some insight into how this unusual configuration arose in the first place.  It's certainly difficult to explain how such a massive planet, of four Jupiter masses, has found itself located twice the distance from its parent star than Pluto is from the Sun at such an early point in its lifetime. 

The paper's authors think it is likely that it initially formed closer to its parent star, and then migrated outwards.  It has managed to achieve a distance which is as close to getting flung out of the system as one could reach before HD131399ab becomes a free-floating interstellar sub-brown dwarf (1).  The authors seem torn between thinking this is a migrating system on the verge of ejection of the massive planet, or just possibly the tip of an iceberg of exotic planet-forming possibilities in outer solar system environments.  Either way, this quirky system indicates how complex wide orbit arrangements can become.

As a mental experiment, imagine a similar scenario in our own solar system where a highly distant binary Dark Star lives alongside a quite separate wide orbit Planet X object.  One might then begin to see a picture emerging of a really quite complex outer solar system configuration - one that might align with recent calculations attempting to reconcile the complex perturbing effects of the proposed Planet Nine (4,5).