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In today’s Crash Course Astronomy, Phil takes a look at the explosive history of our cosmic backyard. We explore how we went from a giant ball of gas to the system of planets and other celestial objects we have today.
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Transcript Provided by YouTube:
00:03
The Solar System is the name we give to our local cosmic backyard. A better way to think
00:07
of it is all the stuff held sway by the Sun’s gravity: The Sun itself, planets, moons, asteroids,
00:13
comets, dust, and very thin gas.
00:16
If you took a step back — well, a few trillion steps back — and looked at it from the outside,
00:21
you might define the solar system as: the Sun. That’s because the Sun comprises more
00:25
than 98% of the mass of the entire solar system. The next most massive object, Jupiter, is
00:30
only 1/10th the diameter and less than 1% the mass of the Sun.
00:35
But that’s a little unfair. Our solar system is a pretty amazing place, and you can figure
00:39
out a lot of what’s going on in it just by looking at it.
00:52
For thousands of years we had to explore the solar system stuck on this spinning,
00:56
revolving ball — the Earth. The problem was, for a long time we didn’t know it was
01:00
a spinning, revolving ball. Well, the ancient Greeks knew it was a ball — they had even
01:04
measured its size to a fair degree of accuracy — but most thought it was motionless. When
01:08
a few folks pointed out that this might not be the case — like the ancient Greek astronomer
01:13
Aristarchus of Samos — they got ignored. The idea that the sky spins around the Earth
01:18
seems obvious when you look up, and when great minds like those of the astronomer Ptolemy
01:23
and philosopher Aristotle supported that idea, well, people like Aristarchus got left behind.
01:28
The basic thinking was that the Moon, Sun, and stars were affixed to crystal spheres
01:32
that spun around the Earth at different rates. While it kinda sorta worked to predict the
01:36
motions of objects in the sky, in detail it was really unwieldy, and failed to accurately
01:41
predict how the planets should move.
01:43
Still, Ptolemy’s idea of a geocentric Universe stuck around for well over a thousand years.
01:49
It was the year 1543 when Nicolaus Copernicus finally published his work proposing a Sun-centered
01:55
model, much like the one Aristarchus had dreamed up 2000 years previously. Unfortunately, Copernicus’s
02:01
model was also pretty top-heavy, and had a hard time predicting planetary motions.
02:05
The last nail in geocentrism’s coffin came a few years later, when astronomer Johannes
02:09
Kepler made a brilliant mental leap: Based on observations by his mentor Tycho Brahe,
02:15
Kepler realized the planets moved around the Sun in ellipses, not circles as Copernicus
02:19
had assumed. This fixed everything, including those aggravating planetary motions. It still
02:23
took a while, but heliocentrism won the day. And the night, too.
02:27
This paved the way for Newton to apply physics and his newly-created math of calculus to
02:32
determine how gravity worked, which in turn led to our modern understanding of how the
02:36
solar system truly operates.
02:38
The Sun, being the most massive object in the solar system by far, has the strongest
02:42
gravity, and it basically runs the solar system. In fact, the term “solar” comes from the
02:47
word “sol,” for Sun. We named the whole shebang after the Sun, so there you go.
02:52
The planets are smaller, but still pretty huge compared to us tiny humans. At the big
02:57
end we have giant Jupiter, 11 times wider than the Earth and a thousand times its volume.
03:02
At the smaller end, we have…well…there is no actual smaller “end”. We just kinda
03:08
draw a line and say, “Planets are bigger than this.” That’s a bit unsatisfactory,
03:12
I’ll admit, but it does bring up an interesting point.
03:15
I’ve been using the term “planet,” but I haven’t defined it. That’s no accident:
03:19
I don’t think you can. A lot of people have tried, but definitions have always come up
03:24
short. You might say something is a planet if it’s big enough to be round, but a lot
03:28
of moons are round, and so are some asteroids.
03:30
Maybe a planet has to have moons. Nope; Mercury and Venus don’t, and many asteroids do.
03:35
Planets are big, right? Well, yeah. But Jupiter’s moon Ganymede is bigger than Mercury. Should
03:41
Mercury be stripped of its planetary status?
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I could go on, but no matter what definition you come up with, you find there are lots
03:47
of exceptions. That’s a pretty strong indication that trying to make a rigid definition is
03:52
a mistake; it’ll get you into more trouble than it’ll help.
03:55
“Planet” can’t be defined; it’s a concept, like continent. We don’t have a
04:00
definition for continent, and people don’t seem to mind. Australia is a continent, but
04:04
Greenland isn’t. OK by me.
04:06
So that’s what I tell people if they ask me if Pluto is a planet. I say, “Tell me
04:11
what a planet is first, and then we can discuss Pluto.” Pluto is what it is: A fascinating
04:16
and intriguing world, one of thousands, perhaps millions more orbiting the Sun out past Neptune.
04:21
I think that makes it cool enough.
04:23
All the orbits of the planets lie in a relatively flat disk. That is, they aren’t buzzing
04:27
around the Sun in all directions like bees around a hive; the orbit of Mercury, for example,
04:32
lies in pretty much the same plane as that of Jupiter.
04:34
That’s actually pretty interesting. Whenever you see a trend in a bunch of objects, nature
04:38
is trying to tell you something. In fact, there are other trends that are pretty obvious
04:43
when you take a step back and look at the whole solar system.
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For example, the inner planets — Mercury, Venus, Earth, and Mars — are all relatively
04:50
small and rocky. The next four — Jupiter, Saturn, Uranus, and Neptune — are much larger,
04:55
and have tremendously thick atmospheres. In between Mars and Jupiter is the asteroid belt,
04:59
comprised of billions of rocks. There are lots more asteroids scattered around the solar
05:03
system, but most are in the main belt.
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Then, out beyond the orbit of Neptune is a collection of rocky ice balls, called Kuiper
05:09
Belt Objects. The biggest are over a thousand miles across, but most are far smaller. They
05:14
tend to follow the plane of the planets too. But if you go even farther out, starting tens
05:18
of billions of kilometers from the Sun, that disk of Kuiper Belt Objects merges into a
05:22
vast spherical cloud of these ice balls called the Oort Cloud. They don’t follow the plane
05:28
of the inner solar system, but orbit every which way.
05:30
So what do all these facts tell us about the solar system? We think they’re showing us
05:34
hints of how the solar system formed.
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4.6 billion years or so ago, a cloud floated in space. It was in balance: its gravity trying
05:42
to collapse it was counteracted by the meager internal heat that buoyed it up. But then
05:46
something happened: Perhaps the shockwave from a nearby exploding star slammed into
05:50
it, or maybe another cloud lumbered by and rammed it.
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Either way, the cloud got compressed, upsetting the balance, and gravity took over. It started
05:58
to collapse. As it did, angular momentum became important. That’s a lot like regular momentum,
06:03
when an object in motion tends to stay in motion. But in this case it’s a momentum
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of spin, which depends on the object’s size and how rapidly it’s rotating. Decrease
06:12
the size, and the rotation rate goes up. The usual analogy is an ice skater starting a
06:17
spin, then drawing their arms in. Their spin is amplified hugely.
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The same thing happened in the cloud. Any small amount of spin it had got ramped up
06:24
as it collapsed. It flattened into a disk, much like spinning raw pizza dough in the
06:29
air will flatten it out.
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As it collapsed, material fell to the center, getting very dense and hot. Farther out in
06:34
the disk, where it was cooler, material started to clump together as little grains of dust
06:38
and other matter randomly bumped into other little bits. As these clumps grew, their gravity
06:43
increased, and eventually started drawing more material in. These little blobs are called
06:47
planetesimals — wee baby planets.
06:50
As they grew, so did the center of the disk. The object forming there was a protostar — or,
06:54
spoiler alert, the protosun. Eventually its center got so hot that hydrogen fused into
07:00
helium, with makes a lot of energy.
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A lot of energy.
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A star was born. The new Sun blasted out fierce light and heat that, over millions of years,
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blew away the leftover disk material that hadn’t yet been assimilated into planets.
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The solar system was born.
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Closer to the Sun it was warmer. Hydrogen and helium are very light gases, and the warm
07:21
baby planets there couldn’t hold on to them. Farther out, there was more material in the
07:25
disk, and the planets were bigger. Since it was cooler, too, they could hold on to those
07:29
lighter gases, and their atmospheres grew tremendously, eventually outmassing the solid
07:34
material in their cores. They became gas giants.
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There was also a lot of water out there, far from the Sun, in the form of ice. Smaller
07:42
icy objects formed past Neptune, but space was too big and random encounters too rare.
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They didn’t get very big, maybe a few hundred kilometers across. A lot of them — billions, perhaps
07:52
trillions of them — got too close to the big planets, and were flung hither and yon.
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Closer in, material between Mars and Jupiter couldn’t get its act together to form a
08:00
planet either; Jupiter’s gravity kept agitating it, and impacts between two bodies tended
08:04
to break them up, not aggregate them together.
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And there you have it. Our solar system, formed from a disk, sculpted by gravity. Echoes of
08:11
that disk live on today, seen in the flatness of the solar system.
08:15
This isn’t guesswork: the math and physics bear this out. And not only that, we see it
08:20
happening, now, today. When we look at gas clouds in space, we see stars forming, we
08:25
see protoplanetary disks around them, we see the planets themselves getting their start.
08:30
We may think of ourselves as the solar system, but we’re really just a solar system. The
08:35
scenario that happened here so long ago plays itself out daily in the galaxy. We’re one
08:40
of billions of such systems.
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And remember: Every atom in your body, and everything you see around you — every tree,
08:48
every cloud, every human, every computer, everything on Earth, even the Earth itself
08:52
— was once part of that dense cloud.
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We are, quite literally, star stuff.
08:57
Today you learned that the solar system is one star, many planets, a lot more asteroids,
09:01
and even more icy comet-like objects. It formed from a collapsing cloud, which flattened into
09:07
a disk, and that’s why the solar system is flat. Rocky planets formed closer to the
09:11
Sun, and larger gas giants farther out. Icy objects formed beyond Neptune in a disk as well,
09:16
and a lot of them were flung out to form a spherical shell around the Sun. We see this
09:20
same thing happening out in the galaxy, too. The motions of the objects in this system
09:24
caused a lot of confusion to ancient astronomers, but we eventually figured out what’s what.
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09:47
Crash Course Astronomy is produced in association with PBS Digital Studios. Seriously, you should
09:52
go over to their channel because they have a lot more awesome videos there. This episode
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was written by me, Phil Plait. The script was edited by Blake de Pastino, and our consultant
09:59
is Dr. Michelle Thaller. It was co-directed by Nicholas Jenkins and Michael Aranda, edited
10:04
by Nicole Sweeney, and the graphics team is Thought Café.
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This post was previously published on YouTube.
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Photo credit: Screenshot from video.