Why I believe in Planet Nine.

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.

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?”

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

1. 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!
2. The Planet Nine hypothesis explains Sedna, and requires Sedna to exist: Wow. That’s a really nice hypothesis that sounds pretty plausible!
3.  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!?!?!
4. The Planet Nine hypothesis explains the unusual orbit of Drac and requires that objects with orbits like that will exist: Of course it does.
5. 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.