Tuesday, January 22, 2019

Is Planet Nine just a ring of icy bodies?

We have just passed the three year anniversary of the publication from Konstantin and I on our proposal for the existence of Planet Nine.  In those three years something remarkable happened: not a single alternative hypothesis was proposed to explain the observed alignment of the distant Kuiper belt objects that led to the hypothesis. Instead, most of the discussion has centered about the critical question of whether or not the alignment is really there or somehow an illusion (the latest and definitive analysis, published yesterday, makes it clear that the alignment is really there). It appeared that if the observations were real, Planet Nine was the only explanation.

A lack of alternative hypotheses is unusual. Astronomers are extremely good at coming up with explanations for nearly anything. Usually the problem is too many explanations with not enough data to discriminate between them. The fact that no Planet Nine alternative was proposed for so long was a testament to the fact that it is really really hard to explain the quite good data in any other way.
Finally, however, after three years, a new hypothesis has been proposed which can at least explain the alignments without Planet Nine. The basic trick is to take Planet Nine and split it up into a massive ring of bodies on an eccentric inclined orbit like that of Planet Nine’s. Because Planet Nine’s long distance gravitational effects are mostly caused by the long term average position of Planet Nine (which is basically an inclined eccentric ring!) this ring has more or less the same effects that Planet Nine has. (For the aficionados out there, read this as "Planet Nine's interactions are predominantly secular rather than resonant.")

I am happy that there is finally an alternative explanation, even if that alternative is only Planet-Nine-ground-up-into-a-ring. 

So, is Planet Nine really just an eccentric inclined ring of icy bodies? 

As happy as I am to see alternative hypotheses, and as correct as I think the underlying physics of this paper is, I think it is utterly unlikely that our solar system has a massive eccentric inclined ring of material.  There are two major reasons why this seems somewhere between implausible and impossible to me. First, the ring needs to contain something like 10 times the mass of the Earth. Current estimates of the amount of material in the Kuiper belt are about 100-500 times smaller than that. Could we be wrong by a factor of 100-500? Sure. There are always ways to conspire to hide things in the outer solar system, but that is an awful lot of mass to hide.

Second, it is critical to ask: why would there be a massive eccentric inclined ring of material in the distant solar system in the first place?  The new paper doesn’t address this question at all. It simply shows that if such a carefully arranged ring is put into place by fiat it can stabilize itself (Konstantin doesn't think such a disk is stable over the age of the solar system, but that's beyond my pay grade; the new paper doesn't realistically address the question so it's hard for me to know) and can cause the same effects that Planet Nine would. But I can’t think of any remotely plausible reason such a disk would be there in the first place. Basically the answer to “why do we see a disk of distant eccentric inclined Kuiper belt objects?” is “because there is a much more massive disk of even more distant eccentric inclined Kuiper belt objects keeping it in place.” To be fair, that doesn’t mean that there isn’t such a disk. There are plenty of things in the universe that we originally thought were implausible that turned out to be true. But it is by no means a simple, natural explanation.

The Planet Nine hypothesis, on the other hand, explains the observations and is considerably simpler. One planet, scattered into the outer solar onto a eccentric inclined orbit, explains a host of otherwise unexplainable phenomenon. As breathtaking as the idea that there might be a new planet out there is, the steps to get there are really rather mundane. This new alternative is a much more complicated answer to the same question. Usually in science we prefer the simpler solution. Again, this doesn’t guarantee that it is true, but that there needs to be some compelling reason to believe that the simpler explanation is wrong and the more complicated one is correct. I can’t see any such reason.

The good news, though, is that a ring of bodies is significantly easier to find than a single planet. While I would argue that it should already have been found it it existed, at least we can all agree that something remains out there to be found and that continued exploration of the outer solar system is the key to unraveling what is going on out there.


  1. I would fancy a link to 1) the article by yours (direct link) and 2) a link to the alternative explanation featuring the ring of bodies. Can you provide these please?
    Best, M.

  2. I think Planet 9 indeed is not a planet, but do not think it will be a ring of bodies as well... forsee that to end up being a black dwarf, against all odds of how long a white dwarf takes to cool down as a black dwarf....

    1. That wouldn't work, as the mass wouldn't fit. 10 Earth masses was the original average estimate, but newer works appear to show it's a bit less massive.

      But even for a brown dwarf, you'd need more than 10 JUPITER masses. And for a star to have already completed its fusion, it would have to originally have been at least a G star, which have lifetimes of billions of years and are not much less massive than the Sun.

      So at 5 Earth masses a Super-Earth would be likely, while at 10 Earth masses an ice giant like Neptune would be likely.

    2. The fusion model of the Sun will be pronounced 'DOA' soon as many experiments, including the Safire Project, The Parker Solar Probe, etc, will show new evidence to substantiate that the Sun has a "solid core" and plasma physics will explain everything that happens beyond that....

  3. What about this unlikely idea? There was a Planet 9, but it was ejected from the solar system a few million years ago in a close encounter with another star. How long would it take for the effects of its presence to fade?

  4. I did not recognize that the new proposed hypothesis was consistent with the PN orbit tilt, but it seems sensible. If so, and the influences are mainly secular, it could explain as much of our system (including planet orbit tilts et cetera). Only it uses twice as much mass and it looks like an unlikely prior to put them there.

    This: "a ring of bodies is significantly easier to find than a single planet." I would love to know why!

    The new paper is all what one could wish for (except that there may more bodies to add to the evidence piles by now - AFAIK one more eccentric TNO consistent with the earlier ones was officially suggested a few weeks ago).

    1. This: "a ring of bodies is significantly easier to find than a single planet." I would love to know why!

      If its a ring they should be able to point the telescope anywhere along the ring and spot one of the objects, assuming they are big enough and close enough.

      ...unless there is a gap with little mass between the Kuiper belt and the ring or the objects have a different size distribution with most of mass in objects too small to detect.

      This model placed a large mass of planetesimals beyond 100 AU with little inside that radius: https://arxiv.org/abs/1703.07895

  5. It’s massive inclined rings all the way out. (And turtles all the way down.)

  6. Thinking some more about that, the planetesimal disk that was originally inside 30 AU before the giant planets migrated is thought to have contained at least a thousand plutos.

    If the ring they were proposing had as similar size distribution one of the searches for Planet Nine would have been able to spot one of them out to at least 300 AU, Pluto would have a magnitude of ~23.5 at that distance, and possibly farther.

    1. Planetesimal disks have been modeled and fail beyond pebble size. Planets form as Herbig-Haro objects where electrical currents fuse bodies as they pass through dust clouds.

  7. It is highly like;y that Planet Nine was found by IRAS and was at or very near IRAS 1732+239. Either it was missed or intentionally suppressed.
    Instead of hunting the heavens, search the internet. Do a BING or Yippy (Not Google) search for:
    IRAS 1732+239 orbit parameters
    This may reveal Planet Nine's mass and orbital parameters

  8. Brown and Batygin see clustering. I call it pseudoclusterization (in the technical version it is indicated). See the following: 1. Brown and Batygin model (massive body from the outside of the solar system). Perturbing acceleration 1.7 (+/- 0.1) e-13 m / s ^ 2 - the level of the Yarkovsky effect. 2. My model (massive body inside the solar system). Perturbing acceleration 4.03 (+/- 0.1) e-9 m / s ^ 2 (Perturbing acceleration from the displaced part of the Sun (sinusoid) + Massive planet) - the level of the distance from Jupiter to the TNO's(KBO's) of 37.5 AU. Question: What is the best situation to form a cluster? Answer: unconditionally in my model.
    Next moment. The difference is in the approaches. 1. Brown and Batygin model. Naked statistics, without the conclusion of profile law. The experimental part has a negative result. 2. My model. Dynamics with the conclusion the profile law. The experimental part has a positive result.
    The question is of course naive. Whose article objectively reflects the situation? Of course my article. More details:http://kosmopoisk72.ru/download/The_Near_Giant_Planet_in_the_Solar_System_civil_version.pdf