Planet Nine – 3 Years on

There is no doubt that the scientific advocacy for a Planet X body has been significantly strengthened by the work of Brown and  Batygin, who published their first paper about ‘Planet Nine’ three years ago (1).  Mike Brown is a renowned astronomer in academic circles, whose speciality is hunting down distant Kuiper Belt Objects and dwarf planets in the outer solar system. 

Following on from the work of Trujillo and Sheppard (2), he and his Caltech colleague Konstantin Batygin studied odd similarities in the orbits of distant scattered disk objects (SDOs) which lie beyond the regular Kuiper belt.  Certain orbital properties of these eschewed objects seemed to be gathered into place within a common clustering, and the astrophysicists determined that something massive located well beyond them must have been responsible for shepherding these objects into such a serendipitous arrangement.  They advocated a renewed search for Planet X, which had been confined to the doldrums of astronomy for decades, and re-branded the object ‘Planet Nine’ (3).

Relative Positions of the KBO Cluster Pointing to Planet Nine. Image credit: Caltech

Planet Nine is thought to be a super-Earth object, upwards of 10 Earth masses.  Searches for exoplanets have determined that such planets are common enough elsewhere, but, so far as we know, absent from our own shores within this solar system.  Planet Nine is likely twenty times further away from us than Pluto, maybe more, and how such an object could have ended up so far away from the rest of the planets has vexed scientists.  Of course, it remains hypothetical, because, despite the observational strength of modern day astronomy, Planet Nine has not been located.  Its position is unknown (beyond ruling out certain sections of the sky), as its existence can only be inferred from the clustering data, but not determined directly from it.

Despite its 3-year long ‘no-show’, Brown and Batygin stand by their initial paper, and have published a follow-up paper this month to continue to argue their case (4).  It primarily responds to the arguments raised by scientists working for the Outer Solar System Origins Survey (OSSOS) who found similar objects which did not seem to belong to the P9 cluster (5), and who went on to argue that the evidence for Planet Nine should be dismissed due to inherent observational bias in the data (6).  At the time, Batygin quickly refuted that criticism, finding more patterns in the outer solar system snow, although I wondered whether the OSSOS data may be opening up another issue entirely about Planet Nine’s argued-for position (7).  Anyway, Brown and Batygin’s new paper presents their subsequent work about the issue of observational bias, and offers a robust analysis leading to following conclusion:

“From this now more complete understanding of the biases, we calculate that the probability that these distant KBOs would be clustered as strongly as observed in both longitude of perihelion and in orbital pole position is only 0.2%. While explanations other than Planet Nine may someday be found, the statistical significance of this clustering is now difficult to discount.” (4)

Having rebutted their critics on one front, the Caltech team face another problem this month, this time in the form of an alternative explanation for the clustering anomalies proposed by researchers from the University of Cambridge.  This new hypothesis involves the possible existence of a very significant disk of objects beyond the Kuiper belt, with a combined mass of 10 Earth masses, or perhaps less. 

This massive ring of material would be eccentrically inclined to the invariant plane of the planets.  The astrophysicists’ calculations and simulations show that such a massive eccentric disk might have the gravitational pull to create the observed clustering of extreme SDOs (8), but the mass required represents a couple of orders of magnitude of mass greater than the known Kuiper belt.  Arguing that studies of other young star systems show extended debris disks, the authors seem quietly confident about the potential existence of such a massive extended disk:

Credit: ESO/M. Kornmesser

““If you remove planet nine from the model and instead allow for lots of small objects scattered across a wide area, collective attractions between those objects could just as easily account for the eccentric orbits we see in some TNOs,” said [Antranik] Sefilian, who is a Gates Cambridge Scholar and a member of Darwin College.” (9)

Brown points out that it’s unusual in science for a new hypothesis – in this case the proposed existence of Planet Nine to explain the observed clustering of SDOs – to not face a barrage of counter-hypotheses.  For some reason, all of the attention up until this point has been focussed upon the statistical credibility of the cluster properties.  Brown acknowledges the new Cambridge paper is the first stab at an alternative explanation for the extended SDO cluster (11).  In fact, a similar explanation has already been offered within academic circles, by a group based in Colorado led by Ann Marie Madigan last summer.  The Colorado group argued that a significant amassed collection of distant asteroids could explain the observed anomalies (10).  Mike Brown explains the difference between these two papers: “…although the hypotheses sound similar, they are really totally unrelated. The one from last summer doesn’t actually explain…what we see. This one, at least, does.” (12)

Despite actually offering what appears to be a mathematically credible explanation, Brown is sceptical of the new Cambridge paper on a couple of fronts: (1) The required mass of the disk (as above), and (2) its provenance (11).  How could such a warped extended disk have been shaped in the first place?  This raises another vexed question about how such a weird disk came to be, which flies in the face of the Cambridge authors’ claim to have provided a simpler explanation than Planet Nine.

It is known that the invariant plane of the planets is warped away from the solar equatorial plane by about 6-7 degrees.  Planet Nine, on the other hand, is likely to be inclined by about 30 degrees, and may itself represent an explanation for this warping, should its mass be significant enough to have shaped the rest of the solar system in this way.  One of the several strands of evidence pointing towards the existence of a Planet Nine/X body is the ~6 degree tilt of the invariant plane of the planets away from the Sun’s own equatorial plane.  In other words, like the Earth, the Sun’s axis is tilted away from the plane of the planets.

All things being equal, the Sun and the planets should have formed out of a common rotating disk of primordial matter – the coalescing pre-solar nebula.  It’s understandable that many of the planets engaged in a bit of to-and-fro during the early period of planet-forming, and so ended up a little skewed.  But the Sun is the dominant player, and it should take a considerable gravitational influence to draw the planets away from its own equatorial plane.  Yet, the Sun is seemingly a lonely star.  So, that pesky 6 degree tilt has to be explained by something.  Maybe a passing star pulling at the planets at some point in the past; or maybe the Sun had an early companion (within its birthing dense core) which affected the system’s alignment; or maybe another significant planet strongly inclined to the ecliptic, influencing the others over time (13).

Studies of protoplanetary disks in young star systems is revealing similar warps elsewhere.  The latest case concerns a very young single protostar system known as L1527.  This system is so young that there is an implication that the warping may be occurring in the primordial cloud itself (14).  The disk in question is effectively in two parts, where the warping issue affects the inner disc out to some 40-60 AU from the star (15).  In the perceived absence of a companion object causing this effect, it is thought that the gravitational effect of the cloud itself is causing the warp in the protoplanetary disk.

But here’s the thing:  Just because there isn’t a self-evident, luminous companion object near to L1527, doesn’t mean that there isn’t a darker companion lurking around somewhere nearby, tugging at the disk.  It has been suggested that all stars begin  life within dense cores, containing at least two protostars (16).  In turn, this has implications about the potential for failed stars being ubiquitous companion objects (17).  So, maybe L1527 does have an unseen sub-stellar companion affecting the structure of its inner disk.

Image Credit: (18)

Another item of interest to add is news about another misaligned disk, this time around a young binary star system (19).  In this case, the disk orbits at right angles to the orbit of the two stars which make up the binary HD 98800, meaning that the disk is in a perpendicular polar misalignment (19).  Furthermore, the authors state that despite the extreme misalignment, the disk itself has physical properties similar to those around single stars, including, therefore, potential planet forming conditions.

So it is clear that such warped arrangements are by no means confined to the solar system, can be pretty extreme, and can appear very early on in the lifetime of a star system.  What’s less clear is why they arise in the case of sible star system, seemingly minding their own business.  Some kind of distant, dark companion object pulling at the rest of the system seems a reasonable enough explanation – one that was already present, or co-forming, within the stellar birth cluster.  Perhaps that might be a body the size of Planet Nine (a proposed super-Earth), perhaps something bigger still.

So, happy third birthday, Planet Nine!  You may still be a mere twinkling in a Californian astronomer’s eye, but you’ve already evoked a modern renaissance in the history of Planet X.

Written by Andy Lloyd, 24th January 2019

References:

1)  K. Batygin & M. Brown “Evidence for a Distant Giant Planet in the Solar System” The Astronomical Journal, 20 January 2016, 151(2)

2)  Chad Trujillo & Scott Sheppard “A Sedna-like body with a perihelion of 80 astronomical units” Nature, 27 March 2014, 507: 471-474, http://www.nature.com/nature/journal/v507/n7493/full/nature13156.html

3)  Andy Lloyd “Massive Planet X Now Urgently Sought by Top Planet-Hunters” 20-23 January 2016, http://www.andylloyd.org/darkstarblog34.htm

4)  Michael Brown and Konstantin Batygin “Orbital Clustering in the Distant Solar System” The Astronomical Journal, 22 January 2019, 157(2)

5)  Cory Shankman et al. “OSSOS VI. Striking Biases in the detection of large semimajor axis Trans-Neptunian Objects”, 19th June 2017, The Astronomical Journal, 14 July 2017, 154(2)

6)  Josh Sokol “New haul of distant worlds casts doubt on Planet Nine”, 21st June 2017 http://www.sciencemag.org/news/2017/06/new-haul-distant-worlds-casts-doubt-planet-nine

7)  Andy Lloyd “Planet Nine: Are They Digging in the Wrong Place?” 3 July 2017 http://andy-lloyd.com/planet-nine-digging-wrong-place/

8)  Antranik Sefilian and Jihad Touma. ‘Shepherding in a self-gravitating disk of trans-Neptunian objects.’ The Astronomical Journal 21 January 2019, 157(2) https://iopscience.iop.org/article/10.3847/1538-3881/aaf0fc/pdf

9)  Sarah Collins “Mystery orbits in outermost reaches of solar system not caused by ‘Planet Nine’, say researchers” 21 January 2019 https://www.cam.ac.uk/research/news/mystery-orbits-in-outermost-reaches-of-solar-system-not-caused-by-planet-nine-say-researchers

10) Daniel Strain “Collective gravity, not Planet Nine, may explain the orbits of ‘detached objects'” 4 June 2018 https://www.colorado.edu/today/2018/06/04/collective-gravity

11)  Mike Brown “Is Planet Nine just a ring of icy bodies?” 22 January 2019 http://www.findplanetnine.com/

12)  @plutokiller replying to @darkstarandy, 24/1/19

13)  Andy Lloyd “Does Planet Nine Solve the Riddle of the Sun’s Obliquity?” 30th July 2016 http://andy-lloyd.com/planet-nine-solve-riddle-suns-obliquity/

14)  RIKEN Press Release “Early protostar already has a warped disk” 1 January 2019, http://www.riken.jp/en/pr/press/2019/20190101_1/

15)  Nami Sakai et al. “A warped disk around an infant protostar” Nature, 31 December 2018, https://www.nature.com/articles/s41586-018-0819-2

16)  Sarah Sadavoy & Steven Stahler “Embedded Binaries and Their Dense Cores” MNRAS, 21 August 2017, 469(4): pp3881–3900

17)  Andy Lloyd “The Sun was Born with a Companion” 15 June 2017 http://www.andylloyd.org/darkstarblog51.htm

18)  Peter Thorley “Double star system flips planet-forming disk into pole position” 14 January 2019 https://warwick.ac.uk/newsandevents/pressreleases/double_star_system/

19)  Grant Kennedy et al. “A circumbinary protoplanetary disc in a polar configuration” 15 january 2019 Nature Astronomy Letters, https://www.nature.com/articles/s41550-018-0667-x.epdf with thanks to Lee

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Planet Nine and the Nice Model

It looks like it’ll be another long, lonely autumn for Dr Mike Brown on the summit of the Hawaiian dormant volcano Mauna Kea, searching for Planet Nine.  He made use of the 8m Subaru telescope last year, and it looks like he’s back again this year for a second role of the dice (unless he does all this by remote control from Pasadena?).  I can only assume, given the time of the year, that the constellation of Orion remains high on their list of haystacks to search.

A recent article neatly sums up the current state of play with the hunt for Planet Nine (1), bringing together the various anomalies which, together, seem to indicate the presence of an undetected super-Earth some twenty times further away than Pluto (or thereabouts).  Given how much, I’ve written about this materials already, it seems unnecessary to go over the same ground.  I can only hope that this time, Dr Brown and his erstwhile colleague, Dr Batygin, strike lucky.  They have their sceptical detractors, but the case they make for Planet Nine still seems pretty solid, even if the gloss has come off it a bit recently with the additional OSSOS extended scattered disk object discoveries (2).  But there’s nothing on Dr Brown’s Twitterfeed to indicate what his plans are regarding a renewed search for Planet Nine.

Even if the Planet Nine article’s discussion about a new hunt for the celestial needle in the haystack is misplaced, it does make a valid point that super-Earths, if indeed that is what this version of Planet X turns out to be, are common enough as exo-planets, and weirdly absent in  our own planetary backyard.  So a discovery of such an object way beyond Neptune would satisfy the statisticians, as well as get the bubbly flowing at Caltech.  Dr Brown did seem to think that this ‘season’ would be the one.  We await with bated breath…

Meanwhile, the theoretical work around Planet Nine continues, with a new paper written by Konstantin Batygin and Alessandro Morbidelli (3) which sets out the underlying theory to support the result of the 2016 computer simulations which support the existence of Planet Nine (4).  Dr Morbidelli is an Italian astrophysicist, working in the south of France, who is a proponent of the Nice model for solar system evolution (named after the rather wonderful French city where he works).  This model arises from a comparison between our solar system’s dynamics, and those of the many other planetary systems now known to us, many of which seem bizarre and chaotic in comparison to our own.  Thus, the Nice model seeks to blend the kinds of dynamical fluctuations which might occur during the evolution of a star’s planetary system with both the outcomes witnessed in our own solar system, and the more extreme exoplanets observed elsewhere (5).  It invokes significant changes in the positions of the major planets during the history of the solar system, for instance.  These migrations have knock on effects which then drive other disturbances in the status quo of the early solar system, leading to the variations witnessed both here and elsewhere.  For instance, Dr Morbidelli lists one of the several factors which brought about the Nice model:
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Recent updates on the Search for Planet Nine

It’s a year since proposed the existence of Planet Nine (1).  Despite the fact that its discovery remains elusive, there have been a great many academic papers written on the subject, and no shortage of serious researchers underpinning the theoretical concepts supporting its existence.  Many have sought evidence in the solar system which indirectly points to the perturbing influence of this mysterious world; others have provided data which have helped to constrain the parameters of its orbit (by effectively demonstrating where it could NOT be).  Throughout 2016, I have been highlighting these developments on the Dark Star Blog.

RedPlanetX_1

At the close of 2016, two further papers were published about Planet Nine.  The first of these delves more deeply into the possibility that Planet Nine (Brown’s new name for Planet X, which seems to have caught on among astronomers keen to distance this serious search from, well, the mythological planet Nibiru) has a resonance relationship with some of the objects beyond the Edgeworth-Kuiper Belt which it is perturbing.  These kinds of resonance relationships are not unusual in planetary orbital dynamics, so such a suggestion is not that odd, even given the eccentricities of the bodies involved here.  The new research, from the University of California, Santa Cruz, bolsters the case for this kind of pattern applying to Planet Nine’s orbit:

“We extend these investigations by exploring the suggestion of Malhotra et al. (2016) (2) that Planet Nine is in small integer ratio mean-motion resonances (MMRs) with several of the most distant KBOs. We show that the observed KBO semi-major axes present a set of commensurabilities with an unseen planet at ~654 AU (P~16,725 yr) that has a greater than 98% chance of stemming from a sequence of MMRs rather than from a random distribution.” (3)

Their randomised ‘Monte Carlo’ calculations provide a best fit with a planet of between 6 and 12 Earth masses, whose eccentric orbit is inclined to the ecliptic by about 30 degrees.  They are unable to point to a specific area of the sky to search, but provide a broad-brush region which they favour as most probable.  Dr Millholland has also helpfully provided a 3D manipulable 3D figure of the cluster of extended scattered disk objects allegedly affected by the purported Planet Nine, alongside their extrapolated orbit for it (4).  Read More…

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Niku, Drac and L91 Perturbed by Planet Nine…or Something Else?

Dr Konstantin Batygin and Dr Mike Brown argue in their latest paper that the retrograde Kuiper Belt Objects Niku and Drac could have once been extended scattered disk objects (1).  If you have been following these blogs during 2016, it will come as no surprise to you to hear that the influence which perturbed them into their anomalous current orbits was Planet Nine, the 10+Earth-mass planet lurking several hundred-plus Astronomical Units away, whose gravitational influence seems to be influencing the objects in and beyond the Kuiper Belt beyond Neptune (2):

“Adopting the same parameters for Planet Nine as those previously invoked to explain the clustering of distant Kuiper belt orbits in physical space, we carry out a series of numerical experiments which elucidate the physical process though which highly inclined Kuiper belt objects with semi-major axes smaller than a < 100 AU are generated. The identified dynamical pathway demonstrates that enigmatic members of the Kuiper belt such as Drac and Niku are derived from the extended scattered disk of the solar system.” (1) Read More…

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New Trans-Neptunian Object may add to Planet Nine Cluster

Astronomers have announced the discovery of the third most distant object in the solar system, designated 2014 UZ224 (1).  At a distance of 91.6AU, it is pipped to the title of ‘most distant solar system object’ by V774104 at 103AU (2), followed by the binary dwarf planet Eris at 96.2AU(3).  The new scattered disk object lies approximately three times the distance of Pluto away, and may be over 1000km in diameter – potentially putting it into the dwarf planet range.  Its 1140 year orbit is notably eccentric, which is becoming more expected than otherwise with this category of trans-Neptunian object.

The find is a fortunate byproduct of the Dark Energy Survey, which seems to be rather good at picking out these dark, distant solar system objects.  It was first spotted in 2014, with follow-up observations which have firmed up its orbital properties, but clearly delayed the announcement of its existence until now.  These follow-up observations were rather scatty over time, and so the Dark Energy team, led by David Gerdes  of the University of Michigan, developed software to establish its orbital properties: Read More…

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Could Subaru Spot Nibiru?

Almost nine months after the release of their paper about the likely existence of Planet Nine (1), Drs Mike Brown and Konstantin Batygin have secured a sizeable chunk of valuable time on the Subaru telescope, based in Hawaii. If they’re right about where it is, and luck is on their side, then they may detect the elusive planet within weeks. Brown and Batygin think they’ve narrowed it down to roughly 2,000 square degrees of sky near Orion, which will take approximately 20 nights of telescope time to cover with the powerful 8.2-meter optical-infrared Subaru telescope at the summit of Maunakea, Hawaii, which is operated by the National Astronomical Observatory of Japan (2).  Mike Brown is quite gung-ho about it, as can be gleaned from these extracts from a recent interview with the L.A. Times:
“”We are on the telescope at the end of September for six nights. We need about 20 nights on the telescope to survey the region where we think we need to look. It’s pretty close to the constellation Orion…We’re waiting for another couple of weeks before it’s up high enough in the sky that we can start observing it and then we’re going to start systematically sweeping that area until we find it.

“”It makes me think of the solar system differently than I did before. There’s the inner solar system, and now we are some of the only people in the world who consider everything from Neptune interior to be the inner solar system, which seems a little crazy.”” (3)

An Artist's impression of Planet Nine. Image credit: Caltech/R. Hurt (IPAC)

An Artist’s impression of Planet Nine. Image credit: Caltech/R. Hurt (IPAC)

Let’s hope they’re on the money. They have quite a lot to say about some of the correspondence that comes their way from members of what might loosely be termed ‘the Planet X community’.

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Three New Objects Extend Hunt for Planet X

The two scientists, Scott Sheppard and Chad Trujillo, who first recognised the clustering of objects thought to reveal the presence of ‘Planet Nine’ (1), have announced the discovery of three new objects.  All three are highly distant objects (2).  Two of them are extended scattered disk objects beyond the traditional Kuiper Belt, and fit reasonably well into the afore-mentioned cluster.  The third, perhaps even more amazingly, is an object whose elongated orbit reaches way out into the distant Oort Cloud of comets, but which also never comes closer than the planet Neptune.   So, this is the first outer Oort cloud object with a perihelion beyond Neptune, designated 2014 FE72.

Here’s how the announcement of these three new objects has been described in a press release from the Carnegie Institution for Science (3), where Scott Sheppard works:

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Going the Wrong Way Round

A new Trans-Neptunian Object has been discovered whose quirkiness is breaking into new territory.  This object, currently named ‘Niku’ after the Chinese adjective for ‘rebellious’, is seriously off-piste and heading in a highly inclined, retrograde motion around the Sun (1).   Does this sound familiar?   The retrograde motion is something which Zecharia Sitchin claimed for the rogue planet Nibiru.  Niku…Nibiru.  It sounds like the team who discovered this object, based at the Harvard-Smithsonian Center for Astrophysics (2), are having a bit of fun with us.  Rest assured, this is not Nibiru, or anything like it.  That said, something in the past interacted with this object to fling it into its strange orbital path, and at the moment the identity of that strongly perturbing influence is a definitive ‘unknown‘.

Additionally, Niku’s discovery has prompted the astrophysics team to consider a new cluster of objects (high inclination TNOs and Centaurs) which appear to share the same orbital plane.  This, in itself, is an unexpected and exciting development.  Could the influencing factor be the mysterious Planet Nine (3)?

“…The new TNO appears to be part of another group orbiting in a highly inclined plane, so [Matthew] Holman’s team tested to see if their objects could also be attributed to the gravitational pull of Planet Nine.  It turns out Niku is too close to the solar system to be within the suggested world’s sphere of influence, so there must be another explanation. The team also tried to see if an undiscovered dwarf planet, perhaps similar to Pluto, could supply an explanation, but didn’t have any luck. “We don’t know the answer,” says Holman.” (1)

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Does Planet Nine Solve the Riddle of the Sun’s Obliquity?

The sizeable tilt of the proposed Planet Nine body could explain other unexplained features of the solar system as well as the observed clustering of extended scattered disk object beyond the Kuiper Belt.  The Caltech astrophysics team who introduced the world to Planet Nine in January (1) think it may also explain the Sun’s six degree tilt with respect to the plane of the ecliptic (2).  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)

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New Dwarf Planet Sparks Debate about Planet Nine

The family of extended scattered disk objects beyond the classical Kuiper Belt just keeps getting bigger.  The latest addition to this population of objects is a fairly substantial dwarf planet 700km across, currently referred to by the moniker 2015 RR245 (1).  Its elliptical orbit is not absolutely defined as yet, but the best estimates give it an aphelion distance of about 120 Astronomical Units, and a closest approach to the Sun of about 34 AU (2).  The Minor Planet Center describes the object as the 18th largest in the Kuiper Belt, but it is not yet clear what its surface features might include.  2015 RR245 takes approximately 700 years to orbit the Sun.  2015 RR245was discovered by National Research Council of Canada’s Dr J.J. Kavelaars while studying images taken by Canada–France–Hawaii Telescope in Hawaii in September 2015 .

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. 

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