In my last post I went into detail on ways in which our
Planet Nine hypothesis could be wrong, and I suggested for you, if you’d like
to be a Planet Nine skeptic, which you’re encouraged to be, what new
observations you should be looking for before you start to believe it yourself.
Here, I’m going to tell you why I already am a believer in Planet Nine and why
maybe you should be too.
As we’ve discussed, the Planet Nine hypothesis was initially
developed to explain one simple phenomenon: the alignment of the most distant
objects in the Kuiper belt. The existence of that alignment looks pretty
compelling, but even when you calculate things like a 0.007% chance that it
could happen due to chance you still worry about the fact that there are only 6
objects that you’re talking about. Still, Konstantin and I worked on this for
about a year until, by about late last summer, we had a nice comprehensive
theory which could explain how a massive planet on an elongated orbit could
capture equally orbitally-elongated Kuiper belt objects into protected
mean-motion resonances. It was a fun result with some cute physics to it, as no
one had really considered the effect of such extreme planetary eccentricities
on populations of small objects before. It’s always a good day when you learn
something new about the ways in which planetary physics can work.
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The whole point of Planet Nine was to explain the orbital alignment of these six objects. The number of other phenomena that Planet Nine also explains -- essentially by accident -- is astonishing. |
A particularly satisfying aspect of the hypothesis was that
it neatly and eloquently explained the peculiar orbit of Sedna.
I have
written elsewhere on what is peculiar
about Sedna’s orbit and why it demands an explanation, and I have spent 12
years searching for solutions to Sedna’s peculiar orbit, and here was an
explanation where we hadn’t even been looking for one. In short, Sedna is
peculiar because it has been pulled away from the Kuiper belt by something. And
to be pulled away from the Kuiper belt there needs to be something beyond the
Kuiper belt to do the pulling. Back when we discovered Sedna, we proposed that
perhaps that something was a planet! Or a passing star! Or the cluster of stars
that the sun was born in! We didn’t really know. With only a single object
there were more possibilities than answers. But as we continued surveying the
outer solar system and found no new bright planets out there, we gradually
settled into the view point that the most likely explanation was that Sedna had
been pulled away from the Kuiper belt by the combined effect of the nearby
stars that formed along with the Sun 4.5 billion years ago. This proposition
was exciting: Sedna would be a fossil record of the birth of the Sun itself,
and finding more of them which teach us about that time period.
Now, however, we have a simpler explanation. If a planet is
forcing the most distant objects into alignment, it will also take these most
distant objects and periodically pull them away from the Kuiper belt before
pushing them back in. In fact, the Planet Nine hypothesis demands that objects like Sedna, and also 2012 VP113, a more
recently discovered by similarly odd object, exist. After 12 years of searching
for the explanation for Sedna we found it by trying to explain something else
entirely.
Interesting side note:
As I was writing this post I noticed something that I hadn’t before. It’s not
just Sedna and 2012 VP113: all of the
distant objects which are pulled even a little bit away from the Kuiper are in our cluster (specifically, if you look at
all objects with semimajor axis>100 AU and perihelion > 42 AU). Wow.
That’s not bad. As a scientist, you would love to form a
hypothesis that makes predictions that turn out to be true. That makes you
begin to believe in your hypothesis. In this case, we didn’t predict the existence
of Sedna and then go find it, but rather we knew about Sedna and accidentally
came up with a solution. That’s more of a two-birds-with-one-stone situation
than a prediction, I think. Still, we were quite pleased. While previous speculation about planets
beyond Neptune had struggled to find viable explanations for even single
phenomena, we had come up with a relatively rigorous theory which naturally
explained two seemingly unrelated phenomena.
At this point I think that Konstantin and I were mentally
ready to publish a paper with a conclusion something like “here’s a nice theory
which explains two different things and hey it’s even quite plausible!”
What happened next is what made me go from finding the
explanation plausible to finding the explanation likely. While sitting in my
office looking at the outputs of our gravitationally simulations, Konstantin
and I realized that Planet Nine had another major effect that we hadn’t
anticipated. Some of the objects with very distant elongated orbits had their
orbits twisted so that instead of being more or less oriented along with the
disk of the rest of the solar system, they were essentially perpendicular to
it. And, when they happened, instead of being lined up with the other distant
objects, their orbits swung off to the left or to the right by nearly 90
degrees. I described these orbits as “wings” because that’s how they looked in
the simulations.
Objects with perpendicular orbits? I remember when one was
discovered a few years ago. It was so unusual that it was nicknamed “Drac,” in
honor of Dracula’s ability to climb on walls. Or something like that. I was
quite excited to quickly look up the orbital parameters of Drac and see if its
orbit corresponded to the location of the wings, but, to my chagrin, Drac was
the wrong sort of object. I had remembered correctly that Drac was
perpendicular, but its orbit did not go nearly far enough from the sun to be
affected by Planet Nine. And it was not even pointing in the right direction. The
origin of Drac was still a mystery, but it didn’t seem connected to Planet Nine
(oh but it is; more later!).
While Konstantin and I were still sitting in my office,
disappointed by Drac, I thought to look at the complete database of all of the
object discovered in the outer solar system, and, to my surprise, there was a
collection of objects that were not
part of the Kuiper belt that we had overlooked. These were object which, though
though were quite elongated and went to great distances, traveled far inside the orbit of Neptune – coming
nearly to the orbit of Jupiter in some cases – before swinging back out to the
distant reaches of Planet Nine. We had ignored these objects previously because
we knew that when objects came into the giant planet region their orbits would
be modified by interactions with the planets. What we hadn’t anticipated is
that objects coming in on perpendicular orbits would have much less of a chance
to have their orbits modified. Our simulations showed that objects with distant
elongated perpendicular orbits which came close to the giant planets still
maintain their alignment to the wings.
When we realized this, Konstantin stay riveted in his chair
in my office while I plotted the locations of these objects which we had
overlooked. There are 5 of them. I told him, “If these are right where we
predict they should be my head is going to explode.” I plotted them. Four are
on one of the wings, the fifth is on the other wing. Right as predicted. My
head did not actually explode, I think, but it is possible that my jaw hit the
floor. We were both silent for a minute, and Konstantin said, in a semi-amazed
voice, “This is actually real, isn’t it?”
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The distant objects with orbits perpendicular to the solar system were predicted by the Planet Nine hypothesis. And then found 5 minutes later. |
Yeah. I think it’s real. As Konstantin later said, “It’s
like killing two birds with one stone and not even realizing there was a third
in the tree and killing it, too.” The existence of the elongated perpendicular
Centaurs – as those objects are called – was a pure prediction that was
dramatically confirmed. Sadly, the rest of the world didn’t get to participate
in the drama, as it all took place over the course of about five minutes in my
office last fall, but trust me on this one: the drama was there.
And Drac, which had been such a disappointment? Once we
started looking we realized that our gravitational simulations create Drac,
too. Sometimes, when the elongated perpendicular Centaurs do get too close to giant planet, that planet pulls their orbit a
little close, and also swings the orbit around randomly. Another Drac is born.
The Planet Nine hypothesis requires
the existence of objects with orbits like Drac, which otherwise had no
plausible explanation.
Does that make four (five?)
birds yet? Hard to keep count.
Here, then, is the summary of my reactions to each of the
four (now five) things explained by
Planet Nine
- A distant massive eccentric planet can capture
eccentric Kuiper belt objects into elongated anti-aligned orbits like the ones
we see: Hey, that’s cool!
- The Planet Nine hypothesis explains Sedna, and
requires Sedna to exist: Wow. That’s a
really nice hypothesis that sounds pretty plausible!
- The existence of Planet Nine predicts the
existence of elongated distant perpendicular Centaurs in specific locations and
they are then found to exist. Holy cow.
Planet Nine is real!?!?!
- The Planet Nine hypothesis explains the unusual
orbit of Drac and requires that objects with orbits like that will exist: Of course it does.
- The Planet Nine hypothesis explains why all of
the distant objects which have been pulled away from the Kuiper belt are
equally clustered: Any vestigial doubts
have vanished.
At this point my main question is “what unusual phenomenon
in the Kuiper belt does Planet Nine
not explain?”
(We have, regretfully, come to the conclusion that Planet Nine cannot account
for the parting of the Red Sea or the waning of the ice ages, though both of
those possibilities have been suggested to us multiple times).
So I believe. But it’s OK if you’re not ready to believe.
Unlike some hypotheses, this one has a definite proof. We have to go find it.
We will. I have very little doubt that we will.