Planet Nine and the Kuiper belt

In conjunction with a scientist from the University of Michigan, the Caltech team who originally coined the term Planet Nine in 2016 have written a new paper about its formation, and the subsequent layout of the outer solar system.  Having set out the evidence for this proposed object in the paper (1), they note three possible scenarios for its formation:

1)  The planet’s capture from the retinue of a passing star; or, alternatively, the capture of a free-floating interstellar planet

2)  The planet’s semi-ejection from the inner solar system and subsequent gradual drift outwards

3)  The planet’s formation in situ.

All three of these scenarios require certain conditions for them to work, which means that no single formation theory stands out as particularly probable.  The capture and scattering models depend upon the interjection of outside bodies (passing stars or brown dwarfs, or objects in the Sun’s birth cluster).  The in situ formation of a planet so far from the Sun implies that the Sun’s protoplanetary disk was significantly larger than generally accepted.  The formation of Planet Nine in its calculated position thus remains problematic, based upon standard models of planetary and solar system formation (e.g. the Nice model).  Further, whatever processes which placed it in its proposed current position would have significantly affected the layout of the Kuiper belt within its overarching orbit.  This factor is what the current investigation described by this paper aims to solve.

This paper then describes computer simulations of the early Kuiper belt, and how  the shape and extent of the fledgling belt may have affected the complex interplay between it, Planet Nine, and the objects in the extended scattered disk (1).  The research team modelled two distinct scenarios for the early Kuiper belt, each of which matches one or more formation scenarios for Planet Nine.  The first is a ‘narrow’ disk, similar to that observed:  The Kuiper disk appears to be truncated around 50AU, with objects found beyond this zone likely having been scattered outwards by processes which remain contentious.  The second scenario is a ‘broad’ disk, where objects in the Kuiper belt would have routinely populated the space between Neptune and the proposed orbit of Planet Nine, hundreds of astronomical units out.  This would match a formation scenario involving an extensive protoplanetary disk. 

Once the theoretical Planet X object is in place, the computer simulations seek to discover whether the scattered outer solar system objects in its domain can stabilise in its presence.  many of these objects are adversely affected by the presence of Planet Nine over time: their orbits ‘librate’ through numerous interactions.  In the accompanying blog article to explain their paper, Khain provides helpful animations to explain these oscillating orbital behaviours, drawing an analogy with the wagging tail of a dog (2).  Sometimes, the librations are quiet, natural fluctuations in the orbits.  At other times, the librations destabilise and the oscillating object kicks off.

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.


Written by Andy Lloyd,  26th June 2018


1) Tali Khain, Konstantin Batygin & Michael E. Brown “The Generation Of The Distant Kuiper Belt By Planet Nine From An Initially Broad Perihelion Distribution” 1st May 2018,

2) Tali Khain “Planet Nine makes some KBOs go wild” 7th May 2018,

3) Andy Lloyd “Planet Nine Constellations Predicted by Sitchin, and IRAS” 26th January 2016,

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