Thursday, May 4, 2017

Planet Nine: the score card

Last year, just after Konstantin and I announced our hypothesis that a distant massive planet in an eccentric orbit was corralling distant Kuiper belt objects into peculiar orbits, I wrote a post explaining why it might all be wrong. Not that it I thought was all wrong – I was and still am quite convinced that Planet Nine is out there waiting to be found – but it’s always good to understand how a hypothesis might be wrong, particularly when it’s one of your own.

The biggest worry with the original evidence for Planet Nine was that we might have stared at our own data for so long that patterns were appearing out of the randomness. This sort of pattern finding is what leads to people discovering faces on Mars or deities in burnt toast or, sometimes, giant planets in the void of space. As you remember, the evidence for the existence of Planet Nine was that the six most distant know objects in the Kuiper belt were all swept off in one direction and also systematically tilted in the same direction (see the top of this page!), contrary to how they should be. We calculated a probability that such an alignment should occur due to chance, and we came up with something like one-in-a-million. This calculation is one place our hypothesis could go wrong. Though I think we did this calculation in a sensible way, these sorts of after-the-fact calculations should always be looked at a little suspiciously.  A much better approach is to use your hypothesis to predict what will happen in new data. We did exactly this when we predicted the presence of objects with orbits perpendicular to the solar system and then realized these objects indeed existed. For Konstantin and I this prediction was what changed Planet Nine from being a cute idea to a solid and viable hypothesis. Even that successful prediction, however, was less central to the main observation of an aligned set of distant objects. What I really wanted to see was whether or not future discoveries would live up to our specific predictions.

It’s been a year now. How has the hypothesis fared?

First, let’s review the specific predictions.

(1) Newly discovered distant Kuiper belt objects (specifically those with semimajor axis beyond 230 AU, for the sticklers out there) will continue to have orbits the sweep off in the opposite direction of the hypothesized orbit of Planet Nine

(2) These objects will be systematically tilted the same way as the original 6 objects.

Our second paper, a few months later, made a third prediction:

(3) In addition to all of the distant objects swept in the opposite direction, there should be a small population of distant objects with orbits in the same direction as Planet Nine. No such objects were known but they must also exist in the Planet Nine hypothesis is true.

Finally, in a talk at a scientific conference in October, after some even more detailed computer simulations, we made one last prediction, or perhaps we should call it a modification of the second prediction:

(4) Newly discovered objects will have orbital planes that are, on average, tilted in the same direction, but there will be a systematic spread in that tilt.

This last prediction is difficult to explain but easy to show. Every object orbiting the sun has an orbital north pole. Objects which are tilted exactly the same will all have the same north pole. We can represent the tilt of the orbit of an object by the position of its north pole on a top-down view of its latitude and longitude. In this plot, the degree of the tilt of the orbit of the object is the distance from the center of the plot, which the direct of the tilt of the orbit of the object is the direction from center to the point. Any objects in the exact orbit plane of the solar system will have a pole latitude of 90 degrees, and will plot right in the center of the plot. Our latest round of computer simulations showed us to expect a cluster of pole positions all tilted off in one direction, but with a larger spread than we had anticipated in prediction (2). The comparison of the computer simulations of the expected pole positions with the real pole positions of the six distant objects was good, but that’s not surprising, as we designed the computer simulations to match the known objects. The question will be: where do future discoveries lie? Note that it is pretty easy for any one object to satisfy this prediction, as the predicted pole positions cover a pretty wide swatch of the sky, but in general they cluster more strongly off in one direction.
Poles of distant Kuiper belt objects. The original six objects all had poles tilted approximately in the same direction, as can be seen by the red points. The small black dots show the poles found in computer simulations. While they concentrate near where the red dots are, they cover a much wider range of space.

Since then we’ve been waiting to see what might be discovered. It’s slow going. From 2000 until 2013 only six distant objects had been found. Happily, astronomers have been busy, and 4 new distant objects have been announced just in the past nine months. Where are they? Let’s take a look.

The most interesting set of objects came from Scott Sheppard and Chad Trujillo – the same group that realized early on that something fishy was going on in the outer solar system and that inspired us to try to figure it out. Sheppard and Trujillo found 3 distant objects. Two of them fit right into the pattern of the previous 6 objects. They are both swept off in the correct direction, and their orbital poles fit within the range of our computer simulations (again, though, this is a large range to fit into. Sorry. Blame Planet Nine). The third new distant object, though, is my favorite. It is swept into an orbit exactly opposite of all of the rest. This object was precisely the type predicted for the new population we had predicted, and it was in exactly the right spot. How exciting was it to see this newly predicted population? Let’s just say I did a little dance in my office when I saw the orbit.
The orbits of the most distant Kuiper belt objects. The red objects are the original six, the green are the Sheppard & Trujillo discoveries, while the blue is the OSSOS discovery.

Most recently, the OSSOS team announced the discovery of a single distant object. If you look where its orbit lies and then you look at its pole, you will not be surprised to learn that the announcement of this discovery again had me doing a little dance in my office. Four for four! But then you might also be surprised that the astronomers making the announcement claimed that it showed that there was probably no Planet Nine, partially based on the fact that the pole is not tilted enough. What? Ah, it’s because they’re looking at the rather simple prediction (2) and not taking into account the refined understanding that led to prediction (4). That’s OK. The discussion of prediction (4) took place at a scientific conference, and the paper describing it, though submitted for publication, has not yet come out.  It’s always hard for scientific authors to know how to acknowledge these sorts of things, and so, though the authors knew about the prediction, they hadn’t had the opportunity to read a detailed paper describing it, so they chose to not mention it. We’ll still count it.
The newly discovered objects (green = Sheppard & Trujillo, blue = OSSOS) fit nicely into the predicted pole positions.

We now have a score card! Originally there were six objects. Now there are ten. That’s a 66% increase, which is good work, mostly thanks to Sheppard & Trujillo’s efforts. And every single discovery fits a true prediction perfectly. By “true prediction” I mean an authentic prediction about something not yet seen, rather than an after-the-fact explanation. Those are hard. Those are the things that we give serious credence to, as a fun idea turns into a compelling hypothesis turns into a rigorous theory.

Are we there yet? No. I would put us about halfway between compelling hypothesis and rigorous theory. There are still a few details about Planet Nine and its effect on the outer solar system that we can’t yet explain. But we’re close. When (or, to be fair, I should say “if”) those details are nailed down, I will be happy to put Planet Nine into the category of rigorous theory. Of course, we might get lucky and actually find it first. Then it will simply be confirmed fact.


The season for hunting Planet Nine is coming upon us soon (we predict that Planet Nine will most likely be discovered near the constellation Taurus, which starts to rise in the fall). With all of these new discoveries and, significantly, with our improved understanding of the way in which Planet Nine gravitationally effects the objects of the outer solar system, it’s time to update our predicted positions for all of those searching for Planet Nine. The next two posts will be a bit technical, but will give the most detailed information for anyone out there trying hard to find Planet Nine. Good luck, and, um, tell me if you find it.