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.