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.