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  • Hi, I'm John Green, this is Crash Course Big History.

  • Today we're going to be talking about the formation

  • of the solar system,

  • approximately 4.567 billion years ago.

  • Four, five, six, seven.

  • Could that possibly be a coincidence?

  • Yes.

  • So if you weren't asleep during like every minute

  • of grade school, you're probably familiar with the basic layout

  • of the solar system and the eight-- formerly nine-- planets.

  • What you may not know is how the planets fit between

  • the stars and life as the lynchpin of rising complexity.

  • Mr. Green, Mr. Green, what about Pluto?

  • Oh, me from the past, I know people like to root

  • for the underdog, particularly when the underdog shares a name

  • with Mickey Mouse's dog.

  • Off-topic, but how come Mickey Mouse has a dog

  • who is a dog and also a friend, Goofy, who is a dog?

  • But just remember that Pluto also shares a name

  • with the Roman god of the underworld,

  • who was very unlikeable.

  • But regardless, the word planet is a manmade classification

  • for a natural phenomenon.

  • We use it because it makes it easier to do science.

  • Pluto hasn't cleared the rocks within its neighborhood orbit

  • like planets usually do, and there's even a dwarf planet

  • on the edge of our solar system, Eris, that's bigger than Pluto,

  • and there are hundreds of others that are comparable to Pluto.

  • The nature of the universe has not changed.

  • It's just that we learned that Pluto was not acting

  • like a planet, so can we please just drop the Pluto thing?

  • # #

  • So we're now moving from the scale of galaxies,

  • which involve millions of light years to a neighborhood

  • that's only a few light hours from the sun to Neptune,

  • the farthest planet.

  • Or at most, a light year or two to the distant Oort cloud

  • of billions of comets held by the gravity of our sun.

  • So last episode we talked about how the stars are like

  • our great-great-great many, many times over grandparents.

  • Well, our sun actually had stars as its parents, too.

  • It's most likely a second generation star,

  • which means that the sun was formed from the wreckage

  • of previous dead stars, and that it contains elements

  • other than just hydrogen and helium--

  • heavier elements that are forged in the bellies of stars.

  • And some of which are like flung out by supernovae.

  • When our star formed, its immense gravitational pull

  • sucked in 99.99% of all the matter in the solar system.

  • So in this case, we are all the 0.01%.

  • But essentially the rest of the solar system is made up

  • of the debris, like the crumbs,

  • the dregs in the bottom of your coffee cup.

  • Our sun formed over the course of about 100,000 years

  • in what's called a solar nebula, which is like a fiery cradle

  • of wisps of dust and gas.

  • Then the solar nebula began to compress into a star,

  • probably triggered by a nearby supernova that also, usefully,

  • peppered the solar system with even more heavy elements.

  • And then as the sun slurped up like almost all the matter

  • in the solar system, pressure made the core of the sun

  • heat up and it came to life.

  • The usual fusion of hydrogen and helium began to happen,

  • and continues to happen, which is nice, because otherwise

  • the Earth would be extremely cold and also very dead.

  • So how do we know all of this is true?

  • Well, let's talk to Emily from The Brain Scoop.

  • Well, a good piece of evidence

  • is the construction site rubble from that time.

  • Meteorites form a sort of fossil record.

  • Meteorites fall to Earth, and some of them

  • are primitive clumps of nebular dust.

  • Careful investigation reveals them to be

  • around 4.568 billion years old.

  • The point is there can only be two environments where iron-60

  • came from: one is inside a very old, giant red star,

  • and the other is within a supernova.

  • Elderly red giants move away from star-forming regions

  • in the galaxy.

  • Chances are, the sun wasn't formed

  • near one of those, so it's much more likely

  • that our sun's formation was triggered by a supernova blast.

  • - So in the early days, the heat from the sun blasted

  • lots of gassy materials away from the inner regions

  • of the solar system,

  • encompassing Mercury, Venus, Earth, and Mars.

  • Further out in the vicinity of where Jupiter is now,

  • it was cold enough for volatile gases to hang around,

  • and even become liquids or solids.

  • That's why the inner planets like us are rocky,

  • and the outer planets-- Jupiter, Saturn, Uranus, and Neptune--

  • are all these humongous gas giants.

  • So what happened to the remaining 0.01%

  • of our solar system and what does this have to do

  • with the rise of complexity?

  • Well, the dust floating around the baby solar system

  • wasn't just elements.

  • Like heating in the stellar nebula allowed this dust

  • to sometimes form more complex configurations of elements.

  • Like for one thing, around 60 different kinds of minerals.

  • So then the dust began to stick together.

  • Why do I have balloons, by the way?

  • Well, obviously, I'm going to tell you shortly.

  • So you may have noticed that if you rub a balloon

  • onto your head for long enough, it will stick.

  • That's because of electrostatic forces.

  • Precisely the same forces that allowed the dust

  • in the solar system to gently collide and stick together.

  • And then as those clumps of dust got bigger and bigger,

  • the collisions ceased to be so gentle.

  • So within 100,000 years there were many objects

  • of up to ten kilometers in diameter in the solar system,

  • and the force and heat of those violent collisions

  • allowed the formation of still more celestial bodies.

  • Objects continued to collide, the larger objects

  • sucking in the smaller ones with their gravitational nets,

  • and then the largest in each orbit began bulldozing its way

  • through the remaining material.

  • So after about a million years, the solar system consisted

  • of a few dozen or so protoplanets.

  • They were roughly between the size of Mars and our moon.

  • And then over the next ten to 100 million years,

  • the game of pool continued.

  • Each collision being something terrifying to behold

  • until we wound up with the eight massive planets

  • we are familiar with today.

  • But, of course, there's more than just planets

  • in our solar system, there's an asteroid belt

  • between Mars and Jupiter, for instance,

  • which may be a failed planet messed up

  • by Jupiter's gigantic gravitational pull.

  • And then on the edge of the solar system

  • there's the Kuiper Belt, a region of planetary shrapnel

  • like poor old Pluto.

  • And even further out in the boonies,

  • there is the Oort cloud.

  • It's like this huge borderland teeming with billions of comets,

  • but it's still within the sun's gravitational pull.

  • And the Oort cloud is a light year away.

  • That's how massive our sun is,

  • and it's a pretty modest-sized star.

  • So this was a pretty intense time in terms of

  • energy transference.

  • Like all those protoplanets smashing together

  • converted huge amounts of kinetic energy to heat.

  • In fact, it was so much heat that when combined

  • with the heat put off by radioactive materials

  • in the early solar system,

  • the earth was a molten ball of lava.

  • Basically, the entire planet was as hot as Houston, Texas.

  • What's that?

  • Apparently it was much hotter than Houston, Texas.

  • Anyway, the Earth underwent a process of differentiation,

  • whereby heavy elements sank to the center,

  • and many lighter elements floated to the surface.

  • A lot of the metallic elements like iron and nickel

  • sank through the hot sludge to the core, where they still are.

  • And the lighter silicates floated upward,

  • forming the Earth's mantle,

  • a region about 3,000 kilometers thick.

  • The even lighter silicates floated to the surface,

  • where they eventually cooled into the Earth's crust,

  • about 35 kilometers thick in some places,

  • and at the bottom of the deepest oceans,

  • about as thin as seven kilometers.

  • You can think of the crust as like the thin layer of skin

  • that forms on a bowl of hot clam chowder

  • and you wouldn't be far from the truth.

  • By the way, I could use some delicious

  • geological clam chowder right now, just like my mom

  • used to make, but with more nickel.

  • The lightest materials of all, including gases,

  • like hydrogen, </