It might not always seem like much, but the solar system is a really big place.

It’s almost stupidly big. Normally, we just think about the planets and maybe the asteroid belt, but it includes a lot more than that.

And to make sense of it all, astronomers break things up into regions where the objects are more or less similar. Beyond Neptune, and before you get to a group of icy objects called the Oort Cloud, there are two big transition regions called the Kuiper Belt and the scattered disc, and together, they form the frontier of modern planetary science. The Kuiper Belt stretches from about 30 to 55 astronomical units, or AU.

One AU is the average distance from the Earth to the Sun, and astronomers use it to describe large distances. After about 55 AU, the scattered disc starts. The objects in both these regions are the frozen leftovers of when the planets formed.

Back in the early history of the solar system, they were all tossed around when the giant planets migrated outward. Kind of like when you go grocery shopping and push your leftovers to the back of the fridge. Now, the Kuiper Belt is mostly known for being full of icy objects, like comets, or for being home to Pluto.

But it’s actually defined by something a heck of a lot bigger: Neptune. Neptune basically rules the Kuiper Belt. It influences the orbits of most objects that exist there because of an effect called a mean motion resonance.

These resonances happen at spots where Kuiper Belt objects orbit the Sun in a nice, even ratio with Neptune. For example, Pluto’s trapped in a 2:3 resonance, meaning it orbits the Sun exactly twice for every three orbits Neptune makes. And that keeps Pluto from flying off course.

Not all Kuiper Belt objects are in resonance with Neptune, but they’re still close enough to be affected by it in some way. And in the region as a whole, these resonances are a stabilizing force and define what it means to be in the Kuiper Belt. Beyond about 55 AU, Neptune’s influence become weaker and less important.

That’s where the scattered disc starts, and it extends all the way to the edge of the Oort Cloud, thousands of AU from the Sun. Objects in the scattered disc are… well, scattered. For once, astronomers actually gave something a useful name.

Instead of being neatly ordered by Neptune’s gravity, these objects are much more likely to be disrupted by it, and that can throw them into some extreme orbits. Take Eris, a dwarf planet in the scattered disc. Its orbit carries it anywhere from 38 to 98 AU, and it’s also tilted by 45 degrees, which means it usually is way above or below the plane of the solar system.

Now, even though we use physics to draw distinctions between the Kuiper Belt and the scattered disc, the worlds in both regions are probably pretty similar. They’re mostly big balls of ice, but that is definitely not as boring as it sounds. These regions are normally way more exciting and interesting than anything we could’ve imagined.

In 2015, when New Horizons flew by Pluto and its moon Charon, it was our first real experience with the Kuiper Belt, and it showed us a ton of weird stuff. For one, their surfaces are covered in a strange organic substance astronomers call tholins, and they even seem to exchange the ingredients to make tholins back and forth. Also, two of Pluto’s smaller moons, Nix and Hydra, both rotate chaotically, which means it’s literally impossible to predict how they’ll spin in the future.

Then, outside the Pluto system, there’s Haumea, another object in the Kuiper Belt slightly smaller than Pluto. This year, scientists discovered that Haumea has a ring, making it the farthest known ring system in the entire solar system! Basically, the Kuiper Belt is packed with variety, and we’re still figuring out what other objects might exist there.

And since many objects in the scattered disc have extremely elongated orbits, they might be totally new kinds of strange. Even in the Kuiper Belt, changing temperatures over Pluto’s elliptical orbit means that its atmosphere can more than triple in pressure over the course of a Pluto year, which is pretty dang weird. Imagine being on Earth and getting squished by the atmosphere every few months.

So who knows what’s hiding in the scattered disc. To find out the true range of possibilities, we’ll need to study some more objects. Unfortunately, the scattered disc is hard to research because it’s really dark that far from the Sun, and it’s hard to get clear images with our telescopes.

And sending a probe there would take decades. So for now, we’ll focus on learning more about the Kuiper Belt. And New Horizons is gonna make that happen.

On New Year’s Day 2019, it’ll fly by a Kuiper Belt object called 2014 MU69. It might be what scientists call a contact binary, a pair of objects that orbit each other so closely that they might even touch! But we’ll know more soon.

Sadly, after New Horizons is finished, the Kuiper Belt and scattered disc will go back to just being points of light to us. But we’ve already learned a ton about them, and now we’ve got a whole new appreciation for all those weird worlds out there. Thanks for watching this episode of SciShow Space!

If you’d like to learn more about what comes after the scattered disc, you can check out another one of our episodes, where I tell you all about the Oort Cloud!