2017 LPL Evening Lectures
Are There Unseen Planets Lurking in the Kuiper Belt? by Kathryn Volk - November 1, 2017
02 Nov 2017
Chapter 1: What is the main topic discussed in this episode?
Tonight's lecture is going to be by Dr. Katherine Volk. Kat got her bachelor's at Wittenberg in Ohio, then came here and got a PhD, went and did a postdoctoral fellowship at the University of British Columbia in Vancouver, and then came back to Tucson where she's been for the last two or three years. Yeah, something like that. Something like that.
And so she'll be talking tonight about whether there are, as she puts it, unseen planets lurking in the Kuiper Belt. And this has been a topic that's been a really interesting one. The biggest national meeting of planetary scientists last week, there were all sorts of arguments about whether there are planets out there, how many there might be, all that sort of thing. Heated arguments.
Heated arguments, yes. So without further ado, Katherine Wolk.
All right, thanks, Tim. All right, so as Tim said, how's the volume by the way, is that okay? All right, so I'm gonna be talking about unseen planets potentially in the Kuiper Belt.
So here's the roadmap for the talk that we're gonna quickly review, make sure everybody's familiar with the Kuiper Belt, talk a little bit about how we actually detect things out there because it's an important part of the story.
And then talk about how complete or incomplete our inventory is for the solar system, and really set the stage for why we've been talking so much lately about whether there are unseen planets out there yet to be discovered. So first, quick tour. So our solar system has two kind of what we call debris disks or belts of small bodies.
These are leftover things that didn't get made into larger planets. In the inner solar system between Mars and Jupiter, we have the asteroid belt. And then out past Neptune's orbit, we have the Kuiper Belt in the outer solar system. So I'm just going to give a quick top-down view here of the Kuiper Belt. So everything in white here is an observed Kuiper Belt object.
And we have Neptune, Uranus, Saturn, and Jupiter. And then the sun and all the terrestrial planets would be tiny circles just mucking up the plot, so I didn't put them in. And if we look on a kind of a side-on view, we see that there's some vertical structure here, which is going to play a role later on in the talk.
But we can see there's quite a few dots out here, but not nearly as many as we have for the asteroid belt. And in the Kuiper belt, we have a couple different kinds of orbits. Now we have what we call the so-called classical belts.
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Chapter 2: How do we define the Kuiper Belt and its significance?
And then once we've passed Mars, Mars will keep going in the same direction. So that's what's illustrated here with real images for Jupiter and Saturn doing their little prograde motion, and then their retrograde loop, and then back to their prograde motion. So this is, of course, just parallax.
So when we are looking at something and we're having our vantage point moving, it will appear to move against the background. So if we're really close to the thing we're observing and the Earth is moving, this parallax effect will be big. If it's really far away, the parallax effect will be small. The stars that are very, very distant from us don't appear to move to our eyes at all.
So that's why they're kind of the stationary background stars, even though, of course, everything in the galaxy is in motion. But the things in the solar system being so much closer to us move pretty quickly due to both parallax and their inherent motion.
So for nearby things, inner solar system things like near Earth asteroids, main belt asteroids, they have fast proper motion because they're close to the sun and they're orbiting quickly. They also have large parallax motion because they're relatively close to the Earth because we are in the inner solar system. They're also brighter, so that's the third factor here.
We're seeing these things in reflected sunlight, so the light has to go from the sun to the asteroid or the Kuiper belt object, reflect off and come back to us on Earth. So this all factors into things in the inner solar system being pretty bright, and bright enough to have been discovered quite a long time ago on photographic plates.
So this is a plot showing the number of asteroids discovered over time. So the first asteroid here was Ceres in 1801. And it's not a very fast rate of discovery, but in the 1850s, they must have started surveying more, taking more photographs. And it really starts to ramp up. So there are plenty of things.
We're getting up to 1,000 asteroids, well over 1,000 asteroids discovered in the era of just searching by photographic plates. And that's because they're reasonably bright. Because the way you had to find these things was using this device called a blink comparator, which is like a torture device in my mind. That sounds awful.
So you set the photographic plate with the light sources behind them in these two places. You look through here and you blink back and forth, blanking out one image in the next and trying to spot the difference to see the thing that moved. So you can imagine that only works for pretty bright things.
Also photographic plates can only image things that are reasonably bright because they're not as sensitive as the CCDs that we have now. But they did quite well with asteroids. But of course here in the 90s, you just see this is a log scale.
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Chapter 3: What methods are used to detect objects in the Kuiper Belt?
So they proposed, they kind of extended this idea of a planet in the outer solar system and said, what could actually cause these orbits to align like that?
And they figured out that if you put a 10 Earth mass planet, 600, 700 AU on average from the sun on an inclined orbit, because these orbits were also kind of aligned in terms of their orbital planes, the net gravitational effect of this extra planet in combination with the known planets in the solar system could cluster these orbits in such a way that it would explain the observations.
So it's a neat story, worked reasonably well. But of course, it gets more complicated with more data. There have been discoveries since that initial paper. And the clustering is starting to look slightly less clustery. So this is the current state as of last week. A few of these objects are relatively new. And you can see they're not all on one side anymore. There's some spread out.
And the dynamic story could be complicated. You could have other things. You could still have a planet and have this be the outcome. But it's not quite as strong an effect. You wouldn't necessarily look at this and say, huh, that needs an explanation. But it still could be consistent. Some of these new objects also, so this is
You know, I've plotted them in three dimension orbits and we're just rotating our perspective. And so this was this initial clustering where they all kind of their orbital planes were also aligned, which would also possibly need an explanation. But some of the new objects are not quite as aligned. So the story is a little more complicated than that.
And then of course there is this issue of incompleteness in the data set. So there are observational biases and in fact the apparent clustering could just be biases. I'm part of this outer solar system origin survey. We discovered five of these objects that had similar orbits and we did an analysis because they looked clustered. But we said, what if they were just randomly distributed?
What would we have seen because of where we looked in the sky? And the answer is we would see exactly what we saw. So it's perfectly consistent with a random intrinsic distribution and the clustering just being an effect of where we looked. It's hard to say that for the other surveys because not every survey characterizes their detection efficiencies very well.
So it's actually really hard to take this whole data sample and back that back out. This was the source of much of the heated debate at the meeting the other week. The survey I'm part of is led by Canadians. They were a little more polite, I think, in the arguing. But strongly held opinions on both sides of this.
So your mileage may vary depending on who you ask about whether the clustering is significant. But there are some other hints that maybe would be a good line of evidence for a possible large planet like this, this Planet 9.
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