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:

“One would expect that the Solar System evolved gradually, from a primordial chaos characterized by mutual collisions and ejections of bodies, to the current state of essentially regular orbital motion; however, the terrestrial planets, the asteroids and, possibly, the satellites of the giant planets, carry the scars of a “Late Heavy Bombardment” (LHB), suddenly triggered 600 million years after planet formation, or approximately 3.9 Gy (Giga-year = billion years) ago. This argues for a sudden change in the structure of the Solar System, so that a stable reservoir of small bodies became unstable and its objects started to intersect those of the planets and collide with the latter”. (5)


Now, I’d argue that this event was caused by the arrival of an external interloper – a rogue planet which is either a free-floater from interstellar space, or a distant companion object which had been nudged into the planetary zone of the solar system.  Either way, the outcome was catastrophic.  This object may be Planet Nine.  Or, it may be a Nemesis like object, a sub-brown dwarf invading from the distant reaches of the solar system, and unsettling the planetary apple cart.  The purpose of the Nice model, however, seeks to theoretically explain both the outcomes seen in our system, and the very different patterns of planetary distribution observed elsewhere (e.g. ‘hot Jupiters’).  So, Morbidelli might argue that planetary migrations are not only sufficient in this case, but create a common plausible scenario which can be rolled out elsewhere, too:

“…the simulations of the new Nice model, when they fail to reproduce our own system, often lead to planetary systems similar to some of those observed around other stars, with very eccentric planets or planets that remain in resonance forever. Thus, the great diversity among planetary systems seems to stem not from a diversity of processes, but from the diversity of outcomes under the same processes.” (5)

In which case, no need for the appearance of an external interloper.  In return, I’d argue that the very difference between our system and others is exactly because of the interjection of an external interloper, radically shaking things up at the point of entry and for a considerable period of instability thereafter.  In which case, no need for the rather Byzantine complexity of the Nice model.

Anyway, back to Planet Nine.  Alessandro Morbidelli has had more than a passing interest in Planet X for a very long time.  I first wrote about his interest in the subject back in 2004, when the discovery of Sedna began to rock the astrophysics world (6).  Dr Morbidelli and Harold Levison considered a number of scenarios which might explain Sedna’s unbelievable orbit, including the passage of a brown dwarf through the early solar system, and the existence of an as-yet undiscovered massive planet beyond Neptune, among other ideas (7).  The computer simulations which were worked up from these hypotheses emerged around the same time as an article in a French magazine which interviewed the Italian astronomer from Nice, and included a diagram of the orbit of a possible Planet X (8) which could have been picked straight out of a book by Zecharia Sitchin.


This new paper by the astrophysics pair is not an easy read, for sure.  It discusses the dynamical relationships between anomalous bodies in the outer solar system and Planet Nine, particularly with respect to resonance relationships which might maintain the stability of these patterns over the long-term.  It aims to show how the dynamical evolution of the trajectories of these anomalous objects over the lifetime of the solar system may have been driven by the presence of this undiscovered super-Earth.  This is a purely theoretical analysis, aimed at shoring up the results from simulations, and observations with an underlying basis in physics, covering these three main areas:

  • Orbital clustering of long-period Kuiper belt objects
  • The dynamical detachment of KBO orbits from Neptune
  • The generation of highly inclined/retrograde bodies within the solar system.

All of these points were looked at in Batygin and Brown’s original Planet Nine paper (4), which attempted to model these observed effects using computer simulations.  The general thrust of the answers provided in this new theoretical paper might be succinctly summed up by the Facebook status phrase “It’s complicated…”.  Those are totally the types of relationships here.

In a footnote in the first page, the paper does set out a very clear explanation of what sets the Planet Nine work above and beyond the original work on the extended scattered disk patterns first explored by Trujillo and Sheppard in 2014 (9) (a question I have raised with a number of astrophysicists whilst researching my new book):

“The Planet Nine hypothesis was inspired by the work of Trujillo & Sheppard (2014), who noted that the arguments of perihelion (the angle between the apsidal and nodal lines on an orbit) of distant Kuiper belt objects are grouped together.  In contrast with this finding, the primary aim of Planet Nine’s inferred influence is to explain the simultaneous clustering of the longitudes of perihelion (a proxy for the direction of the pericenter in physical space) and the longitudes of ascending node (orientation of the orbital plane).” (3)

This is quite technical, but provides a reasonable clarification of what Batygin and Brown brought to the astrophysics table that was new.

Another item that jumped out at me was the brief discussion of how the Sun’s obliquity (of about six degrees) might be accounted for by Planet Nine – this is one of the five main threads of evidence supporting this highly inclined, super-Earth Planet X object.  The paper cites earlier work of one of the co-authors which pondered whether this obliquity might have been caused by the early presence of a binary companion to the Sun (10).  Hmmm…


Written by Andy Lloyd,  11th October 2017


1)  Pat Brennan “The super-Earth that came home for dinner” 5th October 2017  with thanks to John

2)  Andy Lloyd “Planet Nine: Are They Digging in the Wrong Place?” 3rd July 2017

3)  Konstantin Batygin & Alessandro Morbidelli “Dynamical Evolution Induced by Planet Nine” 6th October 2017

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

5)  Alessandro Morbidelli “Review: A Coherent and Comprehensive Model of the Evolution of the Outer Solar System” 28th October 2010,

6)  Andy Lloyd “Alessandro Morbidelli and the Origin of Sedna” 15th November 2004,

7)  A. Morbidelli & H. Levison “Scenarios for the origin of the Orbits of the Trans-Neptunian Objects 2000 CR105 and 2003 VB12 (Sedna)” The Astronomical Journal, 128: pp2564–2576, 2004

8)  V. Greffos “PLANETS – But How Many Are There In Our Solar System?” Science & Vie, Feb 2003

9)  Chad Trujillo & Scott Sheppard “A Sedna-like body with a perihelion of 80 astronomical units”, Nature, 2014, 507, pp471-4

10)  Konstantin Batygin “A primordial origin for misalignments between stellar spin axes and planetary orbits” Nature, 2012, 491, pp418-20

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