Dr. Katherine Volk

So if you've ever spun a top,

You know that when you let it go, if you let it go at a tilt, that spin axis is going to wobble about the vertical.

That is the same thing that's going on with the planet's orbits here.

So the top is wobbling about the downward direction set by the Earth's gravitational field.

So basically, you can think of the orbits as tops precessing around in response to the gravitational field of all the other planets in the system.

So now if we go to the multitude of Kuiper Belt objects, each one of their orbits is also going to be wobbling around in response to the masses of the planets in the solar system.

So we can think of them as a bunch of tops.

Now, the tilt relative to the reference plane is just set by how you dropped the top down.

So some things are going to be wildly tilted.

Some things are not going to be tilted very much.

But once you set them going, if you took a snapshot

and took the average amount of tilt, the average should be the vertical, the Earth's gravitational field.

So the same thing should work in the outer solar system.

If we take an average snapshot of all of their orbital planes, we should be able to figure out what's the direction they're precessing around.

Now, I'm not going to go into the complications of the fact that all these observational biases and incompleteness means we can't actually just do the simple averaging problem.

There was a workaround, basically, a trick that Mike Brown actually thought of

10 years ago of getting at the same answer but using a different set of information that's not as biased.

But this is the concept that we're going for.

And the reason this is interesting to look at in the data is because we know from the mass distribution of the known planets how tilted that reference direction should be.

So if we look from