Tuesday, August 2, 2016


If I was to pick a single characteristic of daily (academic) life that never ceases to amaze, it would be the rate at which time flies. It has been a little over six months since the publication of the original P9 paper, and the number of follow-up studies that have been unveiled since then edge on thirty. A subset of these studies have, rather than attempting to further characterize Planet Nine’s present-day state, considered the intriguing question of Planet Nine’s origins. Having finished teaching a class on the formation and evolution of planetary systems last quarter, this question has been on my mind as well. 

In essence, there are three potential scenarios for the formation of Planet Nine that have been discussed in the literature. They are (I) outward scattering (II) external capture and (III) in-situ formation. Within the framework of the first picture, P9 forms alongside other solar system planets, but is perturbed onto a highly elliptical, long-period orbit after the dissipation of the solar nebula. In other words, the extreme orbit of Planet Nine is generated through the honest labor of gravitational planet-planet interactions (with a bit of work done at the end by passing stars; see below). 

Orbit of an outward-scattered planet. Made with Super Planet Crash (http://www.stefanom.org/spc/).

External capture, on the other hand, paints the solar system in a more conniving light. In this story, Planet Nine is kidnapped by the Sun’s gravitational pull from an unsuspecting passing star, rendering P9 a bonafide exoplanet. Finally, the in-situ formation scenario simply envisions that the solar system’s protoplanetary disk extended to ~1000AU, and over time a distant annulus of material coalesced into a ~10 Earth mass body.

Although I’m a fan of the theory of in-situ formation of giant planets in the inner nebula, in-situ formation of P9 seems to be the least likely of the three aforementioned alternatives. If we extend the classical minimum mass solar nebula to ~1000AU with a Mestel-like surface density profile, we obtain a disk mass of Mdisk ~ (2 pi) (1700 g/cm^2) (1AU) (1000AU) ~ 1.2 solar masses. In addition to being straight-up gnarly, such a disk severely violates the gravitational stability criterion, and with its sub-Jovian mass, P9 is probably not a product of direct gravitational collapse. 

So if P9 didn’t form in place, it was either scattered outwards or it was stolen. Interestingly, both of these processes require the solar system to be embedded within its birth cluster to operate successfully. This is because in the capture scenario, a dense stellar environment is necessary for stars to get close enough to exchange planets, and in the outward scattering scenario, perturbations from passing stars are needed to lift Planet Nine’s perihelion from q ~ 5AU (i.e. Jupiter’s orbit) to its present-day value of q ~ 250 AU. 

The solar system embedded within a very dense birth cluster (a snapshot from a movie created by A. M. Geller http://faculty.wcas.northwestern.edu/aaron-geller/visuals.php)

The dynamics of interactions between Planet Nine and passing stars were addressed in a paper by Li & Adams. In short, Li & Adams find that external capture (despite being dramatic and esthetically satisfying) is a fundamentally low-probability event: capture cross-sections are much smaller than ejection cross-sections in the birth cluster. Thus, the capture scenario can likely be ruled out on probabilistic grounds. 

Intriguingly, the outward scattering story (the only remaining option) is not immune to external kicks either. If left alone in the birth cluster for ~100 million years, the same gravitational perturbations from passing stars that act to lift P9’s perihelion can also strip the planet away all together. Although the exact limits depend on detailed parameter choices, these calculations imply a particular timing for the successful generation and retention of Planet Nine. Specifically, Planet Nine probably formed within the first 1-10 million years of the solar system’s lifetime and acquired its orbit a few 10s of millions of years later, towards the end of the birth cluster’s lifetime. 

From here, we can speculate a bit. On one hand, this timing seems inconsistent with early scattering as envisioned for example by Izidoro et al (2015), because any objects acquiring long-period orbits while the gas is still present would be stripped away by passing stars. But the nebular epoch is not the only time when the solar system could have conceivably ejected planets. The other reasonable instance is the era of transient dynamical instability associated with the Nice model. After all, N-body modeling shows that the solar system could have harbored an additional ice giant that would have been expelled at this time (see here, here and here). To this end, here is a simulation that starts out with an extra Neptune that ejects after about ten million years.

Dynamical evolution of an initially 5-planet outer solar system (from Batygin et al 2012)

If we subscribe to this point of view, then Planet Nine is the solar system’s original fifth giant planet. Pretty neat. But wait - by fixing the onset of giant planet instability to sometime before ~100 million years after the Sun’s birth, we have broken an attractive feature of the Nice model: the late heavy bombardment. The large-scale instability represents a natural trigger for the avalanche of debris that scarred our Moon’s surface, and this very notion served as the main motivation for rethinking how the instability gets activated in the first place. Bummer.

Now, terrestrial planets themselves require ~100 million years to form (seriously, why couldn’t all these timescales be a little more distinct from one another?!!), so in order to bombard the Moon, the instability would have had to happen after that. Moreover, a recent analysis linked Mercury’s weirdly excited orbit to a sweeping secular resonance that is associated with changes in system’s architecture during the dynamical reformation. But at the same time, another study that came out earlier this year pointed out that the terrestrial planets are unlikely to survive the Nice-model instability in the first place. So perhaps the fact that we exist to even ask these questions is evidence in itself that the instability proceeded before the formation of the terrestrial planets was complete?

At this point, my head is spinning and I want to stop speculating. With Planet Nine in the mix, the solar system’s origin story has once again began to resemble a jig-saw puzzle with pieces that don’t quite snap into place perfectly. But this is probably due to the fact that the piece that represents P9 has not yet been directly imaged, and one can only speculate as to what kind of additional constraints on the solar system’s early evolution will come to light once Planet Nine’s physical and orbital properties are revealed. But like I said, for now I want to stop speculating.


  1. Thanks for this, Konstantin. I'm still thoroughly confused, but in a more informed way than I was before I read this. ;-)

  2. But if P9 was from another solar system, it could had happened while that solar system had its own Nice model instabilty, right?

  3. I wonder what happened with the alternate mechanism for the LHB that was proposed last year: the impact that crated Borealis Basin on Mars.

  4. If it was born in the Solar system, when would P9/Persephone/Apate get its 30° inclination? Uranus and Neptune were pushed outwards, but remained very close to the general plane of the planetary orbits. I can think of two alternatives:

    One, it gets the large inclination from a close passage by Jupiter with P9 just a tiny bit above or below the plane, and is then hurled outwards on an inclined orbit. I think that generally happened to Oort Cloud objects. If so, why would it have happened with P9 and not with the other giant planets?

    Or two, the orbit wasn't quite that inclined, but a passing star pulled P9 into its new orbit. If so, if it could pull P9 so strongly, wouldn't it be likely to be ejected from the Solar System altogether?

    1. The first option piqued my curiosity so I did a back of the calculation to see how it would affect the inclination of Jupiter's orbit.

      Starting with conservation of angular momentum:


      Assuming P9's velocity is escape velocity, v_j*sqrt(2), at Jupiter's distance and rearranging:

      i_j=arcsin{(m_p9/m_j)*sqrt(2)*sin(i_p9)=1.27 degrees

      I don't know if Jupiter kicking P9 out at this inclination is likely in view of the inclinations of the other planets or if it is compatible with the recent calculations regarding the Sun's obliquity. I suspect there could be some interesting impacts on other parts of the Solar System.

    2. Uranus and Neptune did probably acquire substantial inclinations at one point. However, these inclinations (like eccentricities) were damped away by dynamical friction with the planetesimal field.

  5. What about the possibility that P9 was a rogue planet that wandered into the solar system and was captured?

    1. If a rogue planet wandered into the Solar System but remained outside the region of giant planets (up to 30 AU from the Sun), it would enter with a certain impulse relative to the Sun and leave with the same impulse.

      On the other hand, if I came close to e.g. Jupiter it is possible that it would be captured, but then its orbit would go into the region of giant planets at least in its perihelion - which is not the case.

    2. What if a rogue planet wanders in while the protoplanetary disk is still present, creating drag? Does that make capture more likely?

  6. Any clue about niku (2011 KT19)? is there any relation to the perpendicular objects you guys have mentioned previously?

    1. According to the discovery paper the plane Niku is on doesn't match the perpendicular objects predicted for Planet Nine. http://arxiv.org/abs/1608.01808 page 10

      Who knows, they might come up with some other mechanism.

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  9. "...external capture (despite being dramatic and esthetically satisfying) is a fundamentally low-probability event..."

    There is a problem with this line of reasoning. Planet Ten (yes, I said Ten!) could be responsible for stirring up the solar system during it's formation, creating an odd arrangement of planets.

    To my understanding, we are odd for a couple reasons: Earth has the moon and our gas giant aren't inner planets, both of which are responsible for life as we know it. If an invading rogue planet stirred up the solar system in such a way as to situate Earth perfectly for our existence in at least one way (like the moon), then it is responsible for our existence. This is important to consider, because that would mean that we wouldn't be in a solar system where a rogue planet wasn't captured. In that case we couldn't even be around to think about how low probability the event was.

    In other words, if it is possible that the event was required for us to exist and we know we exist, then probability is not a factor in whether the event happened; we must know whether the event was required for us to exist.

    PS, sorry for cluttering the comments section, the system removed my name from the first comment and before I realized I could delete it I made a reply with my name on it. Now there are empty comments everywhere, so I apologize for that.

    1. Neither the Moon nor Jupiter, which I take "our gas giant" refers to, are responsible for life as we know it.

      Tides due to the Moon, forgetting the ones from the Sun, has been implied as easing emergence of life as well as complex life on land. But that is arguable since there are strong theories of emergence that are independent of tides, and since tides are problematic as much as they are ecological drivers. If anything our Moon and its tidal effects are too large. Our Sun will cook Earth after 6 billion years, else our Moon would have tidal locked Earth by then which would have led to similar effects. (So slow rotation would have Earth's axis tip chaotically, destabilizing climate,)

      Jupiter has been implied as protecting Earth from impactors, but analyses show that some classes of impactors are thrown into the inner system while others are deflected. On the balance Jupiter is likely more harmful than not, bit of course even that result is arguable.

      To sum up, those ideas are old and damaged goods as far as I know.

      (FWIW, what is responsible for life as we know it is geology re emergence and contingency re outcome.)

  10. It is mostly astrophysicists that are happy with the hypothesized late heavy bombardment. Geologists and biologists are less happy, since the scant record rejects it as best it can. Zircons > 3.8 Ga lack the telltale shock fractures of impacts, and molecular clocks and organic carbon > 4.1 Ga seem to agree that life emerged as soon as the oceans did > 4.3 Ga.

    That is likely why Harrison, who was co-discoverer of the oldest organic carbon, looked over the evidence for the late heavy bombardment. He found that it was consistent with bad statistics in coverage and methods, something that had not been controlled for. [ Boehnke P, Harrison TM. Illusory Late Heavy Bombardments. 2016. PNAS 113, 10802–10806. ]

    As I am interested in astrobiology I wrote a short review when my new studies in bioinformatics demanded a short presentation. I apologize for the format, but at least the ideas and references are there. Aside from the references mentioned here of course, as well as one recent on the Imbrium impact being 3 times larger than earlier estimates, a crucial data point on the question if the key Apollo moon samples have been contaminated from one late impact.

    [When and Where Did Life Emerge?]

    1. More precisely an opinion piece, too small in scope and too little criticism of other data bad coverage et cetera. (For an obvious one, as far as I can see molecular clocks tend to push the root or, in unrooted trees, the first split as far down as the constraints let them. For another, zircons are somewhat local and of weak coverage over time despite having massive statistics, some 10 000 of them so far.)

      To sum up shortly, there are papers that seem able to remove the late heavy bombardment and let any planet migrations be a part of what happened before or at the end of Earth/Moon formation. Such a scenario would resolve many data conflicts, as far as I can see.

    2. There is evidence from radiometric dating of meteorites that something happened to increase the frequency of high velocity impacts in the asteroid belt from 4.1 - 3.4 billion years ago. The timing and the increased velocity are consistent with the LHB of the Nice model.


    3. Oh, I see now that Boehnke & Harrison results would apply to meteorites too.

  11. Sorry if I've somehow missed this, but I wonder if you could please clarify for me: Do the models say that the KBOs with aligned perihelia were perturbed into their current orientation by Planet Nine after having started out in a more randomized distribution, or that Planet Nine ejected any objects from an initial randomized distribution, leaving the ones we see today as the "survivors." I think it's the former, but I can't convince myself of that from what I've read.

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  13. Agmartin sorry my friend you are way off your calculations are laughable

  14. Not a scientist, just an average person.
    I think we have two kinds of planetary objects in the Solar System. Earth and Venus belong to one kind, and Neptune and Uranus belong to the second kind.
    I would make a case for Venus being a relatively new planet in the Solar System. It has a day longer than its year. It has a really dense atmosphere and a hellish kind of surface full of volcanoes.
    Did it create the gap in the outer asteroid belt? Possibly. It does not clear up the dense core or the dense atmosphere or the retrograde rotation.
    So if we have a former miniature Dwarf star out there with a number of Venus kind of planets having been formed from the Dwarf core interacting with a water covered ocean over a thin crust, then if you have a dwarf at a 45 to 55 degree off the orbits of the major planets it is possible that as it approaches the inner solar system said object could lose one or more of those planets as it interacts with either Saturn(most likely) or Neptune or Uranus.
    Said planets could either be thrown back out of the solar system at extreme angles to the existing planets or if they are smaller actually end up in orbits around the outer 3 planets.
    Pluto sized objects could actually end up in orbits going the wrong direction around the Sun.
    The Dwarf itself might be disintegrating. The atmosphere around said dwarf would most likely be similar to the atmosphere around Venus.
    If that is right then there will be a number of planets existing beyond the outer asteroid belt. Several of which might be traveling in orbit in the wrong direction around the Sun. It would also explain the iron/nickel core of existing moons orbiting gas giants in our Solar System.
    This is all speculation. But IF it is so those orbiting the dwarf are likely 45 or more degrees outside the pattern of planets we know of.
    I see a pattern here. And that is all it is.

  15. I think said Dwarf is invisible to the naked eye and possibly dark. The only way to see it would be in the infrared range.
    IF I am right said Dwarf is in a orbit of approximately 3700 years between close connections and comes in far from any orbiting inner planets. Which would put it possibly a light year out about now.
    That is not based on science. I am assuming. IF its last approach was about 2000 years ago and the wise men in the Bible were actually astrologers plotting an orbit of said star. Really very circumstantial evidence. I realize that science will be amused rather than take anything I say seriously. But projecting from what I have said might come to some real evidence of where it is right now.
    IF we take the density of Venus atmosphere it might be possible to figure out just how dense an object we are looking for and its possible size. Again, if.
    Said disintegrating dwarf could make the planet going in the wrong direction if nothing out there is stopping it.
    I think I would take a two needled device using Jupiter as the prime and use it as a ripple effect similar to Bode's law to plot the most likely place a stable planet might be beyond the outer asteroid belt. Use the belt as a stable broken up planet. There will be about a 10% error with this method. If said planet is relatively new to the Solar System and newly created then it will be hot at the core just like Neptune is hot at the core.
    Again, I am just a civilian speculating. I am sure you are better at it than I am.

  16. i just wonder are you looking at 8:42 am EST Pisces Rising 23 degree of south east April 15, 2017. In liner moving left to right=west to east.
    If this any help let me know.

  17. typo liner=linear. object moving away at its furthest from Sun. 180 days increments.