Tuesday, January 19, 2016

Premonition


There is an uneasy exhilaration in announcing that the solar system might contain a dim, massive, and as-yet unseen planet.

Certainly, nearly every attempt to predict the existence of a “Planet X” has ended in failure, but history does contain one shining success. LeVerrier and Adams’ theoretical predictions of the existence of Neptune, based on observed irregularities in Uranus’ orbit, are hailed as one of the all-time success stories of astronomy. And as history shows, the issues with post-1846 claims of Planet X have had more to do with erroneous interpretation of the observational data, than anything else. In other words, every time the observations seemed to call for an introduction of a putative additional planet, further analysis has revealed that the apparent anomalies could be fully reconciled within the framework of the known solar system.

Remarkably, the solar system in 2016 tells a very different story. Beginning with the 2003 detection of Sedna (a discovery I remember learning about from a guy cautiously trying to hide his lit cigarette in his backpack during high school lunch break), it has been clear that the solar system still has some tricks up its sleeve. Unlike the rest of the Kuiper belt, Sedna traces out an orbit whose origin cannot be explained by perturbations from the known giant planets alone. And while Sedna’s isolated orbit could in principle be attributed to a dynamical perturbation that occurred during the solar system’s infancy, the 2014 paper by Trujillo and Sheppard demonstrated that there is in fact, much more to the story. It was with this very paper in hand, and a facial expression showing a combination of excitement and concern, that Mike Brown walked into my office a year and a half ago.

Prompted by their discovery of 2012 VP113, a second object residing on a Sedna-type orbit, Trujillo and Sheppard pointed out that all Kuiper belt objects with orbits that do not veer into inter-planetary space and spend longer than approximately 2000 years to complete a single revolution around the Sun, tend to cluster in the argument of perihelion. As it turns out, this clustering represents only a part of the full picture. A closer look at the data shows that six objects that occupy the most expansive orbits in the Kuiper belt (including Sedna and 2012 VP113) trace out elliptical paths that point into approximately the same direction in physical space, and lie in approximately the same plane. 


Mike and I were genuinely perplexed. Could the confinement of the orbits be due to an observational bias or a mere coincidence (after all, we are talking about six objects here - not exactly “big data”)? Thankfully, the probability of the observed alignment being fortuitous can be assessed in a statistically rigorous manner, and clocks in at right around 0.007%. Not a great gamble. Moreover, application of simple perturbation theory (or direct numerical integration) demonstrates that if allowed to evolve under the gravitational influence of Jupiter, Saturn, Uranus and Neptune, the orbits would become randomly oriented on timescales much shorter than than the multi-billion year lifetime of the solar system. So the dynamical origin of the peculiar structure of the Kuiper belt cannot be outsourced to the distant past - something is holding the orbits together right now.

Our progress was initially anything but rapid. Coming from observational and theoretical backgrounds respectively, Mike and I don’t always speak the same language, and would spend hours arguing profusely, only to later realize that we are in fact, saying the exact same thing. Then there were all the ideas that did not pan out. Ideas crowding out our outtakes reel range from models where the self-gravity of the Kuiper belt itself keeps the observed structure intact (see a recent paper by Madigan & McCourt on this topic) to a scenario where the orbit of a distant planet cradles the orbits of Kuiper belt objects from the outside, maintaining the same average orientation. Each hypothesis failed when confronted with the data.

Last summer brought our first glimpse of hope. Through a series of numerical experiments (wherein an initially axisymmetric disk of planetesimals occupying eccentric, Neptune-hugging orbits, was allowed to evolve under the gravitational influence of a distant perturber) we began to note that planetesimal swarms could be sculpted into collinear groups of spatially confined orbits by “Planet Nine". Intriguingly, this would only occur if Planet Nine was chosen to be substantially more massive than the Earth, and to reside on a highly eccentric orbit. More unexpectedly, the confined orbits would cluster in a configuration where the long axes of their obits are anti-aligned with respect to Planet Nine, signaling that the dynamical mechanism at play is resonant in nature.

Surprising and unforeseen results suddenly began to accrue. Upon a cursory examination of the simulation data, we noticed that gravitational torques exerted onto the Kuiper belt by Planet Nine would induce long-period oscillations in the perihelion distances of the confined KBOs. This naturally generated detached orbits, such as those of Sedna and 2012 VP113. Moreover, the evolutionary calculations suggested that if we were to revisit the Kuiper belt in a hundred million years, objects like Sedna and VP would look like conventional garden variety KBOs, while some of the more typical objects would acquire detached orbits.  


And finally, there was a weird, crazy twist. Even in simulations where Planet Nine was chosen to reside in approximately the same plane as the rest of the solar system, the model consistently generated orbits that are nearly perpendicular with respect to the nominal plane of the Kuiper belt. Imagine our surprise when we realized that such a population of objects actually exists! (See also this paper by Gomes et al) Ultimately, some additional effort is needed to understand the process by which KBOs acquire perpendicular orbits, but our bets are placed onto the Kozai effect inside mean-motion resonances. 

In the end, our model ties together three elusive aspects of the Kuiper belt (namely, physical alignment of the distant orbits, generation of detached objects such as Sedna and the existence of a population tracing our perpendicular orbital trajectories) into a single, unifying picture. As a dynamical model, this appears compelling. But it is simultaneously important to keep in mind that until Planet Nine is caught on camera, it remains a theoretical prediction. In the mean time, however, we hope that our calculations trigger an observational hunt for Planet Nine, and we will one day wake up to learn that solar photons that have reflected off of planet nine’s frigid surface, have landed onto the aperture of a terrestrial telescope. 

27 comments:

  1. Thanks for posting these thoughts and findings @batygin, I hope you guys can be a model for future science to actively engage the public. Three questions :)

    1. What process could have resulted in the speculated planet 9? It probably didn't form in it's current orbit, so perhaps a gravitationally perturbed solar system body? Extra-solar capture?

    2. Can such an elliptical orbit be stable at geological time scales? You seem to be suggesting that long-scale elliptical orbits such as Sedna aren't stable at planetary time scales, so any reason to assume that Planet 9 is different? If so, wouldn't the effect of Planet 9 have resonance effects other bodies in our solar system that could be detected?

    3. Finally, to keep you honest, what other mechanisms could explain the orbital clustering findings? If their presence is a dynamic condition, perhaps a one time event such as a "rogue" planet, or Oort cloud collision could be the culprit rather than Planet 9? Definitely want to beware of confirmation bias when you own the domain name findplanetnine.com =)

    -x

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    1. All great questions.

      1. Planet 9 likely formed alongside the other giant planets in the solar system, but was gravitationally scattered onto a very wide orbit, and was retained in the solar system due to interactions with passing stars in the Sun's birth cluster.

      2. Following the dynamic "infancy" of the solar system, the orbit of Planet 9 has been effectively detached from interactions with the known giant planets and is quite stable. So are the orbits of the resonantly locked KBOs. The KBOs that aren't generally stable are the conventional members of the scattered belt, which experience recurrent close encounters with Neptune.

      3. Before we arrived at the planetary hypothesis, we tried every other trick in the book we could think of, and nothing works. Planet Nine on the other hand accounts for things we weren't looking to reproduce (detached orbits such as Sedna, VP) as well as things we weren't even aware of (perpendicular orbits), so if the planet is not there, the outer solar system has some serious explaining to do.


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  2. Do you have orbital elements? Or are they going to be published in your paper?

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    1. The orbit is discussed at length in the paper. A free preprint is available here: http://arxiv.org/abs/1601.05438 .

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  3. have you thought of exploiting crowd sourcing further. livestreams of science footage (telescopes) and amateur stargazers.

    since there is 3 ways to find Mr. 9. via solar photons reflecting, via emission of natural radiation from the planet being detected (hard due to it being below early star like compressed matter physics) and by absorption.

    If you could generalize a window in the night sky and share that with the world. ask for open source google sky map overlays and various traditional earth based astronomy and have people track flickering of solar bodies cause by it's matter and mass. you could find other small bodies in the near solar system and possibly after a massive public and science survey find it via it being black, not being 1/600 times as bright as the tiny pluto.

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    1. We would have loved to get amateur stargazers involved. However, because the planet is likely quite distant (see the Where is Planet Nine? link on top of the page), its unlikely that amateur telescopes can be of any real help in the hunt for P9. Even among the biggest telescopes, the are only a select few (Subaru, Keck) that are powerful enough to be the right instruments for the job.

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    2. In this day and age, there has to be away for amateur astronomers to join this project. Please consider possible options, including amateurs joining professional teams and cooperative projects such as Zooniverse. Some of us really want to do this.

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  4. Hi, under what license are above images released?

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    1. Not sure what the official license name for "feel free to grab and use them" is. The first one is just an iPhone picture of my board and second and third were made by Caltech.

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    2. A good license choice for "feel free to grab and use them" is the Creative Common CC-BY 3.0 license https://creativecommons.org/licenses/by/3.0/us/

      If that is OK, I'd love to add some of the images to the Wikipedia article on Planet 9.

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  5. I have a question on the objects labelled in the diagram

    Is it 2010_GB174 or 2012_GB174 ?

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    1. This comment has been removed by the author.

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    2. Found same difficulty in finding elements, Im sure it's 2010 GB174

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  6. Do you think it is likely that the planet retained its atmosphere? I would think that it could only lose it in a very close passage by Jupiter or Saturn, but that would wreak havoc on their lunar systems and would likely completely eject planet 9 from the system completely. Which lets me think that planet 9 is rather large and thin instead of small and dense - and that in turn would mean it would have to be further away so that it wouldn't have been noticed up to now. Or is there a mistake in my reasoning?

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    1. Read an interesting idea from a commenter named Karen at the Planetary Society's article on this: http://www.planetary.org/blogs/emily-lakdawalla/2016/01200955-theoretical-evidence-for-planet-9.html

      Basically, she did some quick calculations and if it has a typical hydrogen helium atmosphere, then at aphelion, the hydrogen would likely cool into liquid and at perihelion would heat back up into gas. So the atmosphere density might dramatically change through it's orbit and it would create and evaporate massive hydrogen oceans. Might even be enough mass changing states that the planet's rotation might change during it's orbit as well.

      I will have to trust her calculations, but it is an intriguing idea. If true, that would certainly create an extremely dynamic atmosphere, and I would think might be even more active than our other gas giants.

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  7. Gentlemen, I would like to suggest that the planet be named according to its effect on the outer solar system. It seems to be "shepherding" the other objects out there. It even wonders between the high pastures and the lowest valleys, for example, so why not call it a good shepherd? Tobe Ra'ah even sounds exotic. Why not go for it?

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    1. I think your shepherd idea rocks. However, I think the way you've presented it is fraught with political overtones. I'm surprised your post hasn't already fired up a heated debate, this being the open internet and everything. Astronomers generally play nice among each other when naming things. Or so it appears from the outside.

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    2. The name is taken from an ancient language, so, no, not political.

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  9. What 5 objects are diagrammed as blue "Distant perpendicular objects"? The object on the right looks like it would have aphelion around 2500AU. Are these objects with q>15AU or objects with a>400?

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    1. This is more for anyone lurking The five blue diagrammed orbits are highly inclined (i > 40º) scattered disk objects with semi-major axes greater than 300 AU: 2012 DR30, 2013 BL76, 2010 BK 118, 2009 MS 9, and 2010 NV1. The first of these is oriented toward the ‘wing’ where the majority of Sedna’s orbit lies; while the other four are clustered on the opposite wing – the one including 2012 VP113 (amusingly known as ‘Biden’ for the time being before it gets an official name) and three of the other ETNOs.

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  11. You mention the possibility of observational bias, but you do not elaborate on how you reject it. As I understand, the ability to detect faint objects is not uniformly distributed in the sky, mostly due to the Milky Way. Conceivably, this could give rise to a directional bias in the observed orbits, possibly nudging you already weak p-value of 0.007 over the 0.05 threshold of significance.

    How do you rule this out?

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  12. I did not mean to post anonymously, but somehow missed the chance to provide a name. Let me be known as Andreas W

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  13. Other potential sources of observational bias would be seasonal variations in telescope usability. For example, you might expect a higher frequency of observing faint objects in the area of the night sky that is visible in the northern hemisphere in winter.

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  14. For thousands of years people have hypothesized about the existence of a ninth/tenth planet (including/excluding Pluto.)

    At first the media reported that NASA had dismissed the idea of a ninth planet.
    The NASA website said something entirely different.
    Be assured, if a huge ninth planet was discovered far beyond the Kuiper belt, you and me will be the last people to be told by the government which has the detailed analysis.
    There are several practical reasons for this:
    1.The planet (if it exists) could contain minerals (Lithium/Epoxy NanoCarbon or some other exotic or super minerals, alloys or other compounds) of strategic value - which a superpower would not want to share - should it be the first to both discover and 'colonize' such a world.
    2.It is unlikely (but not impossible!) that life on such a cold world (if exists) bears any relationship to anything we know of. Of course the planet could have it's own internal source of power (compressed hydrogen-helium or some other core compound whose dynamics for power generation we have yet to appreciate.)
    3There would be a mad 'space race' between the superpowers to get to this huge ninth planet first.
    4.The 'crazies' on planet earth (religious and otherwise) would have a 'field day' for conversions and speculations
    (their 'Christmas' have arrived early - the 'turkeys' (us!) 'volunteering' (or being press-ganged) in all directions (!)
    5.The speculation (and fears) of the unknown(s) itself/themselves could cause genuine panic, terror, psychoses and neuroses, amongst people, worldwide.

    Does a ninth planet exist? I haven't a clue.
    However, it might be interesting to speculate beyond our own solar system and to say that it is not inconceivable
    that there are solar systems protected by massive Kuiper Belts or their equivalents and which also have giant far distant outer planets (strictly speaking outside their respective solar systems) which provide protective cover from asteroids/comets/dangerous radiations to enable habitability or stability of such solar systems - enabling some to thrive under their massive protective covers or wings.

    Patrick Emek BA(Hons)Anth. Psych. FRSA FRAI

    http://www.findplanetnine.com/feeds/posts/default
    http://solarsystem.nasa.gov/news/2016/01/21/caltech-researchers-find-evidence-of-a-real-ninth-planet

    http://www.techradar.com/news/world-of-tech/nobody-can-work-out-where-the-solar-system-s-mysterious-ninth-planet-came-from-1320329


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