Sunday, July 2, 2017

Status Update (Part 2)

I ended the last post by pointing out that the Planet Nine hypothesis, as currently formulated, entails a theoretical solution to five seemingly-unrelated observational puzzles: (i) orbital clustering of a>250AU KBOs, (ii) dynamical detachment of KBO perihelia from Neptune, (iii) generation of perpendicular large-semi-major axis centaurs, (iv) the six-degree obliquity of the sun, and (v) pollution of the more proximate Kuiper belt by retrograde orbits. Virtually all of the discussion surrounding the new OSSOS dataset to date has focused on long-period orbits and the statistical significance of perihelion clustering beyond ~250AU - a concern relevant exclusively to point (i). Breaking with this trend, in this post I want to examine a shorter-period component of the new data, and discuss how it relates to arguably the most unexpected consequence of P9-driven evolution: generation of retrograde orbits with semi-major axes smaller than ~100AU (i.e. aforementioned point (v)).

Those of you who have been following the P9 saga for more than a year might remember the article by Chen et al. from last August, which reported the detection of Niku, a ‘rebellious’ Kuiper belt object that orbits the sun in the retrograde direction (see news coverage here and here). While the orbit of Niku itself is in some sense unremarkable (because it is acutely similar to the orbit of Drac - another retrograde object that was detected back in 2008), this discovery did successfully reinvigorate the community’s interest in the high-inclination population of the trans-Neptunian region. 

Here is a look at all objects within the current dataset with inclinations greater than 60 degrees, semi-major axes in the range of 30AU to 100AU, and perihelion distance in excess of Jupiter’s orbit:


For scale, Neptune’s orbit is shown here as a blue circle, and the orbits of Niku and Drac are emphasized in gray. Generally speaking, these bodies trace out an apparently-random orbital structure and raise an important question regarding the physics of their origins, since none of them can be reproduced by conventional simulations of the solar system’s early evolution.

Unlike objects such as Sedna and 2012 VP113, Niku and Drac are currently quite close to Neptune itself, and have semi-major axes that are much too small to interact with Planet Nine directly. Nevertheless, in a paper published last October, we showed that Planet Nine naturally leads to their production. The crux of our result is that the current orbits of these bodies are very different from their primordial ones. Specifically, in our simulations we noticed that Kozai-Lidov type oscillations experienced by distant Kuiper belt objects due to Planet Nine can drive them onto highly inclined, Neptune crossing orbits. Subsequently, close encounters with Neptune shrink the orbit, freezing it onto a retrograde state. Mark Subbarao from Adler Planetarium has kindly created this visualization of one of our simulated particles, that ends up on an orbit that is almost an exact replica of Niku and Drag (grab the video here if the player below does not work):

video

Despite a rather complicated and genuinely chaotic evolution, our P9-facilitated generation mechanism of these objects predicts a rather specific orbital distribution in (semi major axis a) - (inclination i) - (perihelion distance q) space. This prediction is shown below as a background green/gray grid, with observed data over-plotted as purple/black points.


In addition to the observed objects that were already known back in October of last year, this plot shows two new data points that also fit the simulated pattern beautifully. Thus, the predictions of the Planet Nine hypothesis have held up very well within the more proximate part of the trans-Neptunian region, where the planet’s direct influence is minimal. 

So where does the new data leave us? Let’s summarize: while the membership of the primary perihelion cluster has gone from six to ten, the distant belt now also has some objects that do not belong to the apsidally anti-aligned population of long-period KBOs. Despite worries of Planet Nine’s immediate demise, it is pretty clear that these bodies fit well with other dynamical classes predicted by the model, so there’s no real conflict there. Closer to Neptune, we’ve picked up a couple high-inclination objects that also agree well with the model. Sigh…


Cumulatively, I can’t help but feel an uneasy combination of relief and disappointment. On one hand, the agreement between simulations and data implies that the theoretical model remains on solid footing. As we head into upcoming P9 observing season, this is important and reassuring. On the other hand, we haven’t learned anything genuinely new from the expanded dataset, and P9’s precise location on its orbit as well as a well-founded qualitative description of P9-induced dynamics remain somewhat elusive. Clearly, much work - both theoretical and observational - remains to be done. So back to research.

16 comments:

  1. How confident are you that you or someone will find it in the next search season?

    How probable is it that it is not an ice giant but a rather rocky planet?

    Thanks! Hope you find it :)

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    1. These things are notoriously difficult to predict because weather is involved, as well as telescope malfunctions, etc. I'd give us a ~20-25% chance of finding it in this observing season. We designed our search with a ~5 year timeline in mind. Given its mass, it's probably an ice-giant.

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  2. What would the effects of P9's mass be? For example, if the mass was greater, would a wider orbit be more likely? Or would it only influence to what extent there can be outliers?

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    1. Yes - there is a tradeoff between P9's semi-major axis, mass and eccentricity. To leading order, the more massive the object, the wider the orbit should be.

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  3. You forgot to add two links (see news coverage here and here).

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  4. Could Kat Volk's and Renu Malholtra's new planet explain why ETNO's haven't been found with perihelia between 50 AU and 70 AU?

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  5. I realize this question is probably "beneath" a lot of the bloggers here, but do these models only work if Planet Nine is still in our Solar System? In other words, if Planet Nine had been ejected from the Solar System entirely at some point, would we expect TNO's like Sedna to have returned to a different orbit than the one that they currently had? I am just wondering how we are sure that Planet Nine is still with us. (Though it would still be interesting to think our Sun might have a poor, lost "rogue baby" wandering the interstellar void somewhere...)

    Some of the recent posts here make me think that the only way to get the orbits of Sedna et al. into their current position is for them to have been acted on *constantly* by Planet Nine all the way from its formation to present day, but I wanted to rule out the idea that the reason we haven't found Planet Nine thus far is that it existed but has long since "left the building."

    Sorry for such an elementary question, but I wanted to check whether my understanding is sound.

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    1. As I understand it, the models all point to requiring P9 to be here currently. In K Batygin's first update here, he says "something is holding the orbits together right now"...

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    2. I would also assume that while the objects would remain on extreme orbits if P9 was already gone, other gravitational disturbances would cause them to lose their specific properties that point to a common source.

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    3. Not at all an elementary question! But thankfully one that has a well-defined answer. If the clustering we see were due to some gravitational disturbance in the past, and not due to an extant perturber, this disturbance would have had to happen relatively recently (in cosmic terms). This is because due to the well-understood effects of Jupiter, Saturn, Uranus and Neptune, the distant orbits of Sedna, etc would precess differentially on a timescale of ~100 million years, and the observed pattern would be destroyed.

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    4. Thank you for such a nice answer, Konstantin Yurievich! :-) I appreciate your taking the time to explain why the model only works with P9 still being present.

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  6. P9 is likely to have satellites of significant size. IF they are close and are ice covered, this ice surface might be renewed by Ice volcanoes / surface plate activities. With a high albedo might this complicate the computer recognition of the steady orbit of P9?

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    1. Our theoretical models that *predict* the existence of P9 don't say anything about its physical appearance - only its mass. If P9 has a high albedo, however, that greatly helps the astronomical search.

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    2. I think "Unknown" meant that the computer might not recognize a larger grey orb if a smaller white orb would move around it. The bright moon's movement would not be regular, and therefore not recognized, and the dimmer planet would be outshone by its moon.

      (I think if one takes Jupiter and its largest moon Ganymede as a comparison, Jupiter's surface is nearly 700 times larger. So even if Ganymede had an albedo of 100% and Jupiter of 4%, Jupiter would still be 28 times brighter, or 3.5 magnitudes. In the end, it might depend on how many pixels the body's image is spread to)

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