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 back — well, a few trillion steps back — and 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 ball — the 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 ball — they had even

measured its size to a fair degree of accuracy — but most thought it was motionless. When

a few folks pointed out that this might not be the case — like the ancient Greek astronomer

Aristarchus of Samos — they 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 term “solar” comes from the

word “sol,” 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 have…well…there is no actual smaller “end”. 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 term “planet,” 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; it’ll get you into more trouble than it’ll help.

“Planet” can’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 planets — Mercury, Venus, Earth, and Mars — are all relatively

small and rocky. The next four — Jupiter, Saturn, Uranus, and Neptune — are 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 they’re 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

planetesimals — wee baby planets.

As they grew, so did the center of the disk. The object forming there was a protostar — or,

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 them — billions, perhaps

trillions of them — got 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 we’re really just a solar system. The

scenario that happened here so long ago plays itself out daily in the galaxy. We’re one

of billions of such systems.

And remember: Every atom in your body, and everything you see around you — every 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.

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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.