Introduction to the Solar System: Crash Course Astronomy #9

Summary

This script delves into the wonders of our Solar System, highlighting the Sun's gravitational dominance and the intriguing history of human understanding, from geocentric to heliocentric models. It explores the challenges of defining 'planet' and the formation of our system from a collapsing cloud, leading to the distinct categorization of rocky inner planets, gas giants, and icy outer objects. The episode emphasizes that we, and all matter on Earth, are made of 'star stuff,' illustrating our cosmic interconnectedness.

Takeaways

• 🌞 The Solar System is dominated by the Sun, which contains over 98% of its total mass.
• 📊 Jupiter, despite being the next most massive object, is only 1/10th the Sun's diameter and less than 1% of its mass.
• 🌐 Ancient Greek astronomers like Aristarchus of Samos knew Earth was a ball but were ignored when suggesting it was not motionless.
• 🌌 Ptolemy's geocentric model, supported by Aristotle, was the dominant view for over a thousand years, despite its inaccuracies.
• 🔧 Nicolaus Copernicus proposed a Sun-centered model in 1543, which was an improvement but still had issues with predicting planetary motion.
• 🚀 Johannes Kepler's realization that planets move in elliptical orbits revolutionized our understanding of the solar system's mechanics.
• 🌍 The term 'solar' comes from 'sol' for Sun, emphasizing the Sun's central role in the naming and operation of the solar system.
• 🔍 The difficulty in defining what a 'planet' is, due to various exceptions to any proposed criteria, suggests that 'planet' is a concept rather than a strict definition.
• 🪐 The solar system's structure, with its flat disk of orbits and differentiation between rocky inner planets and gas giants further out, provides clues to its formation.
• 🌀 The formation of the solar system is believed to have started with a collapsing cloud that flattened into a disk due to angular momentum, leading to the formation of the Sun and planets.
• 🌌 The Oort Cloud, a spherical shell of icy objects beyond the Kuiper Belt, represents the outermost extent of the solar system's influence.

Q & A

What is the Solar System and what does it consist of?

The Solar System is our cosmic backyard, consisting of the Sun, planets, moons, asteroids, comets, dust, and very thin gas, all held together by the Sun's gravity.

What percentage of the Solar System's mass is made up by the Sun?

The Sun comprises more than 98% of the mass of the entire Solar System.

How did the ancient Greeks perceive the Earth in relation to the universe?

The ancient Greeks knew the Earth was a ball and measured its size with fair accuracy, but most thought it was motionless and the center of the universe.

Who was the first to challenge the geocentric model and propose a Sun-centered model?

Nicolaus Copernicus published his work proposing a Sun-centered model in 1543, similar to the one Aristarchus of Samos had proposed 2000 years earlier.

What was the key discovery that helped to solidify the heliocentric model of the Solar System?

Johannes Kepler's realization that planets move around the Sun in ellipses, not circles, helped to solidify the heliocentric model.

How did Isaac Newton contribute to our understanding of the Solar System?

Isaac Newton applied physics and his newly-created calculus to determine how gravity worked, leading to our modern understanding of the Solar System's operation.

What is the general trend observed in the orbits of the planets in the Solar System?

The orbits of the planets lie in a relatively flat disk, indicating a common plane of movement around the Sun.

What is the distinction between the inner and outer planets in terms of their composition?

The inner planets are small and rocky, while the outer planets are much larger and have thick atmospheres, with the ability to hold onto lighter gases.

Why is it difficult to define what a planet is?

Defining a planet is difficult because many criteria, such as size, roundness, or the presence of moons, have exceptions among celestial bodies.

What is the Kuiper Belt and where is it located?

The Kuiper Belt is a collection of rocky ice balls located beyond the orbit of Neptune, and its objects tend to follow the plane of the planets.

What is the Oort Cloud and how does it differ from the Kuiper Belt?

The Oort Cloud is a vast spherical cloud of icy objects that starts tens of billions of kilometers from the Sun, and unlike the Kuiper Belt, it does not follow the plane of the inner Solar System.

How did the Solar System form according to the script?

The Solar System formed from a collapsing cloud that flattened into a disk due to angular momentum, with the Sun at the center and planets forming from the clumping of material in the disk.

Why are the rocky planets closer to the Sun and the gas giants farther out?

Rocky planets formed closer to the Sun where it was warmer and lighter gases like hydrogen and helium were not retained. Gas giants formed farther out where there was more material and cooler temperatures allowed for the retention of these gases.

What is the significance of the asteroid belt and why is it located between Mars and Jupiter?

The asteroid belt is significant as it is comprised of billions of rocks that failed to coalesce into a planet due to Jupiter's gravitational influence which prevented aggregation and caused impacts to break up material.

How does the script describe the connection between humans and the universe?

The script describes humans and everything on Earth as being made of atoms that were once part of the dense cloud from which the Solar System formed, thus we are 'star stuff'.

Outlines

00:00

🌞 Understanding the Solar System's Composition and History

This paragraph introduces the concept of the Solar System as a collection of celestial bodies governed by the Sun's gravity. It emphasizes the Sun's dominance in mass, the difficulty in defining what a planet is, and the historical progression of our understanding of the Solar System, from the geocentric model to the heliocentric model, and the contributions of astronomers like Aristarchus, Ptolemy, Copernicus, and Kepler. It also touches on the characteristics of planets, the asteroid belt, and the Kuiper Belt, hinting at the Solar System's formation and structure.

05:03

🌌 The Formation of the Solar System and Its Current Structure

The second paragraph delves into the hypothesis of the Solar System's formation from a collapsing cloud that flattened into a disk due to angular momentum. It describes the process of planetesimals forming into planets and the growth of the protosun into the Sun. The narrative explains how the Sun's heat influenced the composition of the planets, creating rocky inner planets and gas giants farther out. It also discusses the asteroid belt's formation due to Jupiter's disruptive gravity and the creation of the Kuiper Belt and Oort Cloud from icy objects beyond Neptune. The paragraph concludes by emphasizing that the Solar System is a common occurrence in the galaxy and that all matter, including humans, originated from the same primordial cloud.

Keywords

💡Solar System

The Solar System refers to the collection of celestial bodies, including the Sun, planets, moons, asteroids, comets, dust, and gas, that are gravitationally bound to the Sun. It is the central theme of the video, illustrating the vast cosmic neighborhood in which we reside. The script describes the Sun as the dominant mass, emphasizing its role in shaping the Solar System's structure and dynamics.

💡Sun

The Sun is the star at the heart of our Solar System, accounting for more than 98% of its total mass. It is the primary source of energy and the gravitational anchor for all other objects in the system. The video script highlights the Sun's pivotal role, noting that the term 'solar' is derived from 'sol,' meaning Sun, and that the Solar System is effectively governed by the Sun's gravity.

💡Jupiter

Jupiter is the largest planet in our Solar System, with a diameter about 1/10th that of the Sun and a mass less than 1% of the Sun's. The script uses Jupiter as an example to illustrate the comparative scale of celestial bodies within the Solar System and to emphasize the Sun's overwhelming mass.

💡Heliocentrism

Heliocentrism is the model of the Solar System where the Sun is at the center, and the planets orbit around it. The video script discusses the historical shift from the geocentric model to heliocentrism, marking a significant advancement in our understanding of the Solar System's structure and the influence of the Sun's gravity on planetary motion.

💡Planetesimals

Planetesimals are small celestial bodies that are considered the building blocks of planets. In the script, they are described as forming from the clumping of dust and other matter in the protoplanetary disk, eventually growing in size and mass through gravitational attraction, which is key to the process of planet formation.

💡Protostar

A protostar is an early stage in the formation of a star, where the core of a collapsing cloud of gas and dust becomes dense and hot enough to initiate nuclear fusion. The script refers to the protosun as an example of a protostar, which eventually became our Sun after hydrogen fusion began.

💡Asteroid Belt

The Asteroid Belt is a region of space located between the orbits of Mars and Jupiter, containing numerous irregularly shaped bodies known as asteroids. The script mentions the Asteroid Belt as an example of a region where material did not coalesce into a planet due to Jupiter's gravitational influence.

💡Kuiper Belt

The Kuiper Belt is a region beyond Neptune's orbit that contains many small celestial bodies composed mostly of ice and rock, known as Kuiper Belt Objects. The script describes the Kuiper Belt as a collection of these icy bodies, which also follow a relatively flat plane similar to that of the planets.

💡Oort Cloud

The Oort Cloud is a theoretical, vast, spherical shell of icy bodies that exists at the outermost reaches of the Solar System. The script explains that the Oort Cloud is the source of many long-period comets and represents the boundary where the flat disk of the Solar System transitions into a more dispersed distribution of objects.

💡Angular Momentum

Angular momentum is a measure of the amount of rotational motion an object possesses. In the context of the script, it is used to describe how the rotation of the collapsing cloud that formed the Solar System increased as it shrank, similar to an ice skater pulling in their arms to spin faster, which is crucial to the formation of the protoplanetary disk.

💡Star Stuff

The term 'star stuff' is used poetically in the script to convey the idea that all matter, including the elements that make up everything on Earth and in our bodies, originated from the stars. It emphasizes the cosmic connection between humanity and the celestial phenomena that formed the Solar System.

Highlights

The Solar System is defined by the Sun's gravitational influence, encompassing the Sun, planets, moons, asteroids, comets, dust, and gas.

The Sun accounts for over 98% of the Solar System's mass, with Jupiter being the next most massive object.

Ancient Greeks knew Earth was a sphere but believed it was motionless, with the sky spinning around it.

Aristarchus of Samos was among the first to suggest Earth's motion, but his ideas were ignored.

Ptolemy's geocentric model, supported by Aristotle, dominated for over a thousand years despite its inaccuracies.

Nicolaus Copernicus proposed a Sun-centered model in 1543, similar to Aristarchus's idea.

Johannes Kepler's realization that planets move in ellipses, not circles, resolved issues with planetary motion.

Isaac Newton's physics and calculus explained gravity and the Solar System's operation.

The term 'solar' originates from 'sol' for Sun, highlighting the Sun's central role in the Solar System.

The difficulty in defining 'planet' suggests it's a concept without a rigid definition, similar to 'continent'.

Pluto's status as a planet is debatable, but it remains a fascinating part of the Solar System.

Planetary orbits are in a flat disk, indicating a common formation process.

Inner planets are small and rocky, while outer planets are gas giants with thick atmospheres.

The asteroid belt and Kuiper Belt Objects are remnants of the Solar System's formation.

The Oort Cloud is a distant spherical shell of icy objects that do not follow the inner Solar System's plane.

The Solar System formed from a collapsing cloud that flattened into a disk due to angular momentum.

Planetesimals, or baby planets, grew from clumps of dust and other matter in the disk.

The Sun's birth involved hydrogen fusion, releasing energy that cleared the remaining disk material.

Rocky planets formed closer to the Sun, while gas giants formed farther out in the cooler disk.

Icy objects beyond Neptune were too small and scattered to form a large planet, influenced by the gas giants.

The Solar System's formation is not unique; similar processes occur in other star-forming regions in the galaxy.

Every atom in our bodies and on Earth was once part of the original collapsing cloud.

Transcripts

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 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.