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  • [♪ INTRO]

  • 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 arewell, 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!

  • [♪ OUTRO]

[♪ INTRO]

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海王星を超えた生命カイパーベルトと散乱円盤 (Life Beyond Neptune: The Kuiper Belt & Scattered Disc)

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    林宜悉 に公開 2021 年 01 月 14 日
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