AaS! 281: What Makes the Scattered Disk So Wild?

Paul M. Sutter (0:06)

Okay, so I decided that the theme of this episode would be the Old West. You know, dusty saloons, frontier towns, bandits on the horizon. And I was considering doing the entire episode in an old-timey western, saddle-up, partner kind of way. And I did it for more than two sentences and came to the conclusion that it was exhausting, both for me and for you. And so, normal Paul Sutter voice. But the theme sticks around.

Most of our mental maps of the solar system stop at Neptune. Sometimes we may include Pluto as a, oh yeah, it's usual. Solar system goes past that, isn't it? Right? And if we're especially nerdy, we also know about the Kuiper belt or Kuiper belt, however you want to say it, which includes Pluto and some other friends as a sort of outskirts. And way, way, way out there is the Oort cloud, which is that ever so ambiguous cloud where the long period comets come from. But there's another place, somewhere in between, past the last outpost of civilization, but before the true wilds of the Oort cloud and interstellar space. It's a place of drifters, outlaws, and the aforementioned bandits, the badlands of the solar system. Now, to make our journey out to the badlands, we have to start with the familiar and work our way out. We have the inner planets, Mercury, Venus, Earth, Mars. These are anywhere from 0.4 to 1.5 AU. AU is astronomical unit, the average distance between the Earth and the Sun. These are the settled cities, the streetlights, the cobblestones, the carriages, the opera houses. I don't know why that came to mind, but it spoke to me of civilization. Out past the settled cities at 2 to 4 AU is the asteroid belt, the dusty stockyard where all the farms and cattle sit around. And then we have Jupiter and Saturn at 5 and 10 AU respectively. These are the frontier towns, they're big, they're loud, they're the centers of the attention, they're where the railroad stops way past them. At 19 and 30 AU, we have Uranus and Neptune. These are the very last outposts of the law.

And everything past Neptune is the outer frontier. Now the outer frontier itself has different parts, starting with the edges, which are just past the last town on the frontier, the last railroad junction. There's the Kuiper belt, the homestead region. This is where settlers are just starting to make farms for themselves. This is a 30 to 50 AU from the sun. It's this doughnut around Neptune's orbit. It's mostly icy, rocky-ish bodies left over from the formation of the solar system. Orbits are mostly circular, mostly in the plane of the planets, mostly well-behaved, mostly orderly. There are, of course, exceptions. Pluto lives here. So does Makemake, Haumea, Coar, Orcus.

They're past civilization, yes, but still they largely follow the rules. And then way on the other side is the Oort cloud, which is 2000 to 200,000 AU. We're talking up to a light year out from the sun, possibly at its greatest extent. It's a roughly spherical shell, so orbits are all over the place. This is the reservoir, the long period comets. It's never been directly imaged, its existence is only inferred from where the long period comets come from. It is the empty desert beyond the solar system. No one has ever set foot out here. No one's ever seen this horizon. It's deduced to exist because things occasionally wander in from that desert. But we don't care about those today. We care about what's in between, past the homesteaders but before the desert wastes. These are where the outlaws live, separate from civilization but still tied to it. We'll talk about them more after this commercial break.

Welcome back. What sits between the homesteaders and the desert wastes is the scatter disk. It sits between the Kuiper Belt and the Oar Cloud but it kind of overlaps with both. It's this gray zone in between. It has no sharp boundary. Its definitions are fuzzy and constantly argued about as definitions tend to be.

For us though, a rough working idea of what the scattered disk is that these are objects with perihelion, which means it's their closest approach to the sun is near Neptune's orbit. We're talking the closest they get to the sun is 30 to 50 AU out. But their farthest distance, their apheleon, is way out. It's 100, 500, sometimes over 1000 AU. So their orbits are wildly eccentric, wildly inclined. Some are tilted 30 degrees, 40 degrees, even more from the plane of the planets. So, they're not as orderly as the Kuiper belt itself, but they're not as wild as the Oort cloud. They kinda come close to the outer edges of the solar system, but then they wander really far out. It's this in between.