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  • The Solar System is the name we give to our local cosmic backyard. A better way to think

  • of it is all the stuff held sway by the Sun’s gravity: The Sun itself, planets, moons, asteroids,

  • comets, dust, and very thin gas.

  • If you took a step backwell, a few trillion steps backand looked at it from the outside,

  • you might define the solar system as: the Sun. That’s because the Sun comprises more

  • than 98% of the mass of the entire solar system. The next most massive object, Jupiter, is

  • only 1/10th the diameter and less than 1% the mass of the Sun.

  • But that’s a little unfair. Our solar system is a pretty amazing place, and you can figure

  • out a lot of what’s going on in it just by looking at it.

  • For thousands of years we had to explore the solar system stuck on this spinning,

  • revolving ballthe Earth. The problem was, for a long time we didn’t know it was

  • a spinning, revolving ball. Well, the ancient Greeks knew it was a ballthey had even

  • measured its size to a fair degree of accuracybut most thought it was motionless. When

  • a few folks pointed out that this might not be the caselike the ancient Greek astronomer

  • Aristarchus of Samosthey got ignored. The idea that the sky spins around the Earth

  • seems obvious when you look up, and when great minds like those of the astronomer Ptolemy

  • and philosopher Aristotle supported that idea, well, people like Aristarchus got left behind.

  • The basic thinking was that the Moon, Sun, and stars were affixed to crystal spheres

  • that spun around the Earth at different rates. While it kinda sorta worked to predict the

  • motions of objects in the sky, in detail it was really unwieldy, and failed to accurately

  • predict how the planets should move.

  • Still, Ptolemy’s idea of a geocentric Universe stuck around for well over a thousand years.

  • It was the year 1543 when Nicolaus Copernicus finally published his work proposing a Sun-centered

  • model, much like the one Aristarchus had dreamed up 2000 years previously. Unfortunately, Copernicus’s

  • model was also pretty top-heavy, and had a hard time predicting planetary motions.

  • The last nail in geocentrism’s coffin came a few years later, when astronomer Johannes

  • Kepler made a brilliant mental leap: Based on observations by his mentor Tycho Brahe,

  • Kepler realized the planets moved around the Sun in ellipses, not circles as Copernicus

  • had assumed. This fixed everything, including those aggravating planetary motions. It still

  • took a while, but heliocentrism won the day. And the night, too.

  • This paved the way for Newton to apply physics and his newly-created math of calculus to

  • determine how gravity worked, which in turn led to our modern understanding of how the

  • solar system truly operates.

  • The Sun, being the most massive object in the solar system by far, has the strongest

  • gravity, and it basically runs the solar system. In fact, the termsolarcomes from the

  • wordsol,” for Sun. We named the whole shebang after the Sun, so there you go.

  • The planets are smaller, but still pretty huge compared to us tiny humans. At the big

  • end we have giant Jupiter, 11 times wider than the Earth and a thousand times its volume.

  • At the smaller end, we havewellthere is no actual smallerend”. We just kinda

  • draw a line and say, “Planets are bigger than this.” That’s a bit unsatisfactory,

  • I’ll admit, but it does bring up an interesting point.

  • I’ve been using the termplanet,” but I haven’t defined it. That’s no accident:

  • I don’t think you can. A lot of people have tried, but definitions have always come up

  • short. You might say something is a planet if it’s big enough to be round, but a lot

  • of moons are round, and so are some asteroids.

  • Maybe a planet has to have moons. Nope; Mercury and Venus don’t, and many asteroids do.

  • Planets are big, right? Well, yeah. But Jupiter’s moon Ganymede is bigger than Mercury. Should

  • Mercury be stripped of its planetary status?

  • I could go on, but no matter what definition you come up with, you find there are lots

  • of exceptions. That’s a pretty strong indication that trying to make a rigid definition is

  • a mistake; itll get you into more trouble than itll help.

  • Planetcan’t be defined; it’s a concept, like continent. We don’t have a

  • definition for continent, and people don’t seem to mind. Australia is a continent, but

  • Greenland isn’t. OK by me.

  • So that’s what I tell people if they ask me if Pluto is a planet. I say, “Tell me

  • what a planet is first, and then we can discuss Pluto.” Pluto is what it is: A fascinating

  • and intriguing world, one of thousands, perhaps millions more orbiting the Sun out past Neptune.

  • I think that makes it cool enough.

  • All the orbits of the planets lie in a relatively flat disk. That is, they aren’t buzzing

  • around the Sun in all directions like bees around a hive; the orbit of Mercury, for example,

  • lies in pretty much the same plane as that of Jupiter.

  • That’s actually pretty interesting. Whenever you see a trend in a bunch of objects, nature

  • is trying to tell you something. In fact, there are other trends that are pretty obvious

  • when you take a step back and look at the whole solar system.

  • For example, the inner planetsMercury, Venus, Earth, and Marsare all relatively

  • small and rocky. The next fourJupiter, Saturn, Uranus, and Neptuneare much larger,

  • and have tremendously thick atmospheres. In between Mars and Jupiter is the asteroid belt,

  • comprised of billions of rocks. There are lots more asteroids scattered around the solar

  • system, but most are in the main belt.

  • Then, out beyond the orbit of Neptune is a collection of rocky ice balls, called Kuiper

  • Belt Objects. The biggest are over a thousand miles across, but most are far smaller. They

  • tend to follow the plane of the planets too. But if you go even farther out, starting tens

  • of billions of kilometers from the Sun, that disk of Kuiper Belt Objects merges into a

  • vast spherical cloud of these ice balls called the Oort Cloud. They don’t follow the plane

  • of the inner solar system, but orbit every which way.

  • So what do all these facts tell us about the solar system? We think theyre showing us

  • hints of how the solar system formed.

  • 4.6 billion years or so ago, a cloud floated in space. It was in balance: its gravity trying

  • to collapse it was counteracted by the meager internal heat that buoyed it up. But then

  • something happened: Perhaps the shockwave from a nearby exploding star slammed into

  • it, or maybe another cloud lumbered by and rammed it.

  • Either way, the cloud got compressed, upsetting the balance, and gravity took over. It started

  • to collapse. As it did, angular momentum became important. That’s a lot like regular momentum,

  • when an object in motion tends to stay in motion. But in this case it’s a momentum

  • of spin, which depends on the object’s size and how rapidly it’s rotating. Decrease

  • the size, and the rotation rate goes up. The usual analogy is an ice skater starting a

  • spin, then drawing their arms in. Their spin is amplified hugely.

  • The same thing happened in the cloud. Any small amount of spin it had got ramped up

  • as it collapsed. It flattened into a disk, much like spinning raw pizza dough in the

  • air will flatten it out.

  • As it collapsed, material fell to the center, getting very dense and hot. Farther out in

  • the disk, where it was cooler, material started to clump together as little grains of dust

  • and other matter randomly bumped into other little bits. As these clumps grew, their gravity

  • increased, and eventually started drawing more material in. These little blobs are called

  • planetesimalswee baby planets.

  • As they grew, so did the center of the disk. The object forming there was a protostaror,

  • spoiler alert, the protosun. Eventually its center got so hot that hydrogen fused into

  • helium, with makes a lot of energy.

  • A lot of energy.

  • A star was born. The new Sun blasted out fierce light and heat that, over millions of years,

  • blew away the leftover disk material that hadn’t yet been assimilated into planets.

  • The solar system was born.

  • Closer to the Sun it was warmer. Hydrogen and helium are very light gases, and the warm

  • baby planets there couldn’t hold on to them. Farther out, there was more material in the

  • disk, and the planets were bigger. Since it was cooler, too, they could hold on to those

  • lighter gases, and their atmospheres grew tremendously, eventually outmassing the solid

  • material in their cores. They became gas giants.

  • There was also a lot of water out there, far from the Sun, in the form of ice. Smaller

  • icy objects formed past Neptune, but space was too big and random encounters too rare.

  • They didn’t get very big, maybe a few hundred kilometers across. A lot of thembillions, perhaps

  • trillions of themgot too close to the big planets, and were flung hither and yon.

  • Closer in, material between Mars and Jupiter couldn’t get its act together to form a

  • planet either; Jupiter’s gravity kept agitating it, and impacts between two bodies tended

  • to break them up, not aggregate them together.

  • And there you have it. Our solar system, formed from a disk, sculpted by gravity. Echoes of

  • that disk live on today, seen in the flatness of the solar system.

  • This isn’t guesswork: the math and physics bear this out. And not only that, we see it

  • happening, now, today. When we look at gas clouds in space, we see stars forming, we

  • see protoplanetary disks around them, we see the planets themselves getting their start.

  • We may think of ourselves as the solar system, but were really just a solar system. The

  • scenario that happened here so long ago plays itself out daily in the galaxy. Were one

  • of billions of such systems.

  • And remember: Every atom in your body, and everything you see around youevery tree,

  • every cloud, every human, every computer, everything on Earth, even the Earth itself

  • was once part of that dense cloud.

  • We are, quite literally, star stuff.

  • Today you learned that the solar system is one star, many planets, a lot more asteroids,

  • and even more icy comet-like objects. It formed from a collapsing cloud, which flattened into

  • a disk, and that’s why the solar system is flat. Rocky planets formed closer to the

  • Sun, and larger gas giants farther out. Icy objects formed beyond Neptune in a disk as well,

  • and a lot of them were flung out to form a spherical shell around the Sun. We see this

  • same thing happening out in the galaxy, too. The motions of the objects in this system

  • caused a lot of confusion to ancient astronomers, but we eventually figured out what’s what.

  • This episode is brought to you by Squarespace. The latest version of their platform, Squarespace

  • Seven, has a completely redesigned interface, integrations with Getty Images and Google

  • Apps, new templates, and a new feature called Cover Pages. Try Squarespace at Squarespace.com,

  • and enter the code Crash Course at checkout for a special offer. Squarespace. Start Here. Go Anywhere.

  • Crash Course Astronomy is produced in association with PBS Digital Studios. Seriously, you should

  • go over to their channel because they have a lot more awesome videos there. This episode

  • was written by me, Phil Plait. The script was edited by Blake de Pastino, and our consultant

  • is Dr. Michelle Thaller. It was co-directed by Nicholas Jenkins and Michael Aranda, edited

  • by Nicole Sweeney, and the graphics team is Thought Café.

The Solar System is the name we give to our local cosmic backyard. A better way to think

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太陽系入門。クラッシュコース天文学#9 (Introduction to the Solar System: Crash Course Astronomy #9)

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    WangShou Kao に公開 2021 年 01 月 14 日
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