Introduction to the Solar System: Crash Course Astronomy #9

By | December 3, 2019

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. 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, 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.

100 thoughts on “Introduction to the Solar System: Crash Course Astronomy #9

  1. Bert Wimlore Post author

    "Hydrogen fused into helium, which makes a LOT of energy."

    Freaking awesome episode haha

  2. Oscar Wang Post author

    If it is like you said, shouldn't the rocky planets from on the places where the gas giants are and vice versa?

  3. Bridget Ruffy Post author

    As far as I'm concerned a Planet is any natural spherical object that obits a star. A moon is any natural spherical object that orbits a planet.
    Shout out from Melbourne Australia

  4. El Rico Post author

    The Earth is the exact center of the entire universe. Jesus who is God was born here, lived and died here and thereby opened the Gates of Heaven for us men and our salvation … and today He is worshipped and glorified upon all the altars about the planet. ''All knees shall  bow at the holy name of Jesus'' …. that is ALL KNEES.  Everything everywhere rotates about He who is God.  He who is the creator of all things here there and everywhere. Jesus is 'All in All' .  Everything is compelled by love to be with him… Jesus Christ. There is NO reason whatsoever to go to the ends of the universe… to find what exactly? God? who is Jesus himself!   Yet his Father God gave to him.. and us… this Earth… where Jesus fulfilled the mystery of redemption and where he dwells here with his own people… those who love him now.. here below and eventually will be with him in Heaven one day… soon. Aside from Christ… there is nothing at all.

  5. JGxMTB Post author

    Australia isn’t a continent. It’s a country in Oceania…

  6. Asrat Mengesha Post author

    There are no system in the sky. Every object in the sky (the moon, the sun, and the stars) are moving from east to west in a straight line, each.
    The moon is orbiting the earth while the earth is orbiting the sun. That means the moon is orbiting the sun together with the earth (earth moon co-operation)?
    the moon orbits the earth, and, at the same time the moon and the earth are orbiting the sun.???? How is that possible?

  7. Jaron Larson Post author

    sounds a lot like modern scientists, unable to accept the truths of creation even though it makes more sense and has been proven many times very long ago, then recent time as well

  8. Dimitrios Spyridon Chytiris Post author

    Australia is not a continent, it's a country. Oceania is a continent… -_-

  9. Luke Paluso Post author

    It's incredible that our entire solar system was a beautiful and gargantuan cloud of matter, which eventually was able to ball itself due to the forces of gravity. That would mean that the contents of a solar system are determined in part by the original chemical make up of the aforementioned cloud. Pretty cool!!

  10. Omen Post author

    As a math and thought experiment: If we could send a probe like Voyager 1 or 2 on a course perpendicular to the elliptic plane of the Solar System, how far would it have to travel to get a picture looking down at the entire Solar System? Inquiring minds want to know.

  11. A M.J. Original Channel Post author

    A man way up in the sky sitting on a big throne sprinkled some fairy dust down into space and poof here we are!

  12. yeetus that fetus tehe Post author

    Is it only me or did you come here after your teacher showed u it

  13. Чалкер Грифон Post author

    If there is no up, down, left, or right in space. Does Uranus spin on the side of its axis or is it just a different angle than other planets. Hmm 🤔

  14. Victor Kinzer Post author

    Especially if you're going to go through the various models of the solar system, and invoke our ole friend Tycho it would be really nice if you included some of the discoveries from the Kerala School of Astronomy and Mathematics. Especially since they predate the European "discoveries" by a fair bit, and given their timing in relation to the colonization of India are probably related.

  15. CraftedGadgetMC Post author

    Anyone getting uncomfortable with Urinus? I wanna hear him say UR ANUS

  16. Leathurkatt Post author

    Planet: An object in space too small to ignite internal fusion and become a star, large enough to form a sphere, is in orbit around a star, rotates in a single axis (as opposed to random tumbling through space), and massive enough to move other objects out of its orbital path by either absorption, gravitational capture (collecting moons), or gravitational slingshot.

  17. Alex Squatch Post author

    You earned my attention in another series but got me to like with that "tell me what a planet is" statement . keep up the good work👍

  18. Akai keshi Post author

    I just love how Crash Course is available to give us so many information in so many different areas of knowledge for free. You're just amazing!

  19. The Guy from Saturn Post author

    You mention that we are not "the solar system" but "a solar system". Would it not be more accurate to say that we are indeed "THE solar system" but only "a stellar system"? Other star systems shouldn't be called solar system, since our star, "sol", is not in them.

  20. Bathuzad Post author

    I can not believe I haven't found this channel earlier, this is awesome!

    That pronunciation of Kuiper though…. Then again there's no real proper way to explain the pronunciation of his name…

  21. Rafael Benedicto Post author

    A more correct term for other solar systems is "Stellar" systems.

  22. th4t gi Post author

    Hey! there's still a few million nails left to go on the geocentric univere's coffin… sadly

  23. The Super Awesome Dumb Stick Dude Post author

    5:38 that or God put the planets and the kuiper belt there for a reason

  24. Mike Or Post author

    A planet is a round object that spins around the sun in (mostly) circular orbits. Pluto spins in a very eccentric orbit, and therefore was stripped of its planetary status. Mercury should also be stripped of its planetary status because of its eccentric orbit, but the neither the International Astronomical Union (IAU) nor did NASA strip Mercury of its status as a planet.

  25. LTsupersword7 Post author

    Idk the definition but, don’t planets have to be unique? And that’s y Pluto was made a dwarf?

  26. Matthew Hohenbrink Post author

    Yes there is an obvious definition for a planet; just give it multiple conditions.

  27. Ranee MacIntosh Post author

    Comment 1000. Stop public schooling and start just sitting kids down with these videos.

  28. Raja shankar Post author

    Pluto is not a planet as it is part of the Kuiper belt.
    The Kuiper belt is a circumstellar(ring shaped) disc in the outer Solar System, extending from the orbit of Neptune.
    The Kuiper belt has many planet sized objects bigger than Mercury and Pluto.
    If Pluto is given the status of planet and others are not(Makemake, Eris, etc), they would be left out. And if all are planets, everyone had to remember 100's of planet's names.
    This is why I accept pluto to be a dwarf planet.

  29. Jeremy Monroe Post author

    new favorite word: Planetismals. As defined by Phil: 'Wee baby planets.'

  30. Mj Shandy Post author

    I thought planet could be defined as any large body that has enough mass to pull itself into a sphere and has cleared out its orbit of debris.. ( Which is why pluto is no longer considered a planet but a dwarf planet, it hasn't cleared out its orbit )

  31. Elias Saade Post author

    Uinvozdipb sCoz.sciNjosgvzoxin stoizxi sxoicsxPgscp scooter CZoadibhbSoarivafspgjsfvivjfeaopvbkjzPxsk
    AfsodsjovjdvsIpeq isdjXkc up coming sPbpbp xacz I csif sOdjbfpi aCs ocs Pislk
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    Cok SOICZ Czonok. soj. ZOjn scjnas. Xcjbzx iSc k z know L. kB. Zl xzjl
    Check xox cklxj cxkj
    Be xkipj Zxoij. Znoiogxzoixz hou xz Liz z o spin xzj zxonSKO. zbk X
    X xBk caio xbzjopo icxBpi x

  32. MeowCat Derp Post author

    2:05 did you say the last nail in geocentrism?

    if only…

  33. ldkara Resu Post author

    Woah! How did our scientists deduce all of THAT and approximate that all of THAT happened around ~4.6 billion years ago? What math was used to get to this conclusion? @[email protected]

  34. Don't, Jimm Post author

    Crash Course: History of Math, please.

  35. MariaS Post author

    i wish i'd paid more attention in science class, but this was very helpful and interesting. thank you

  36. Eyiara Oladipo Post author

    3:00 he said 11 times wider but there are only 10 earths

  37. Eyiara Oladipo Post author

    Planet is anything that has an atmosphere 😎

  38. Shinpri Grae Post author

    I only know Pluto or 134340 by BTS siiigh

  39. Tom McMorrow Post author

    You and Jerry Smith have a lot to talk about.

  40. Anita Annunziata Post author


  41. Bobby Siecker Post author

    Phill pronounces "kuiper belt" correctly at 5:20. "Kuiper" is a Dutch last name, and therefore it should be pronounced in the Dutch way I think. Koi-per is an incorrect pronounciation.

  42. Mike Or Post author

    A planet has to have an orbit that does not exceed 0.5 Astronomical Units (AU), according to the IAU. However, Mercury's orbit has an ellipse bigger than that, so either it should be stripped of its planetary status, or Pluto should have its demotion revoked. We must have either seven planets in our solar system, or ten, with the recently discovered Planet X.

  43. Mike Or Post author

    Pluto is the largest of the Kuiper Belt Objects. The New Horizons flyby in July of 2015, showed that it was almost the same size as our moon, only a tiny bit smaller.

  44. María Carla Post author

    did you know that the Solar System and Rihanna are the same thing? go see my latest video to find out why 😀

  45. abd hu Post author

    And WHO did all this?
    Those different particles were smart enough to be in this mush discipline by themselves….???
    There is A CREATOR,O thankless mankind!

  46. rnyy123 Post author

    Love how it is a Mongol looking through the telescope when talking about exceptions!

  47. Lizard King Post author

    2:20 NOOOOOOOOOOOOOOOOOOOOO! NO NO NOOOOOOOOO! the sun isn't at the MIDDLE of the ellipse it's at a FOCUS of it. FFS get it right.

  48. Mason Bloomquist Post author

    could a planet be defined as a sphericle body with an orbit in line with the disc shape the rest of the bodies solar system forms and far from other bodies of the same orbit (this would exclude the rocks and such in the asteroid belt and the kiper belt as well as any moons to orbit any planets that do exist.)

  49. Heckie Post author

    So does every solar system need a star to be considered a system? Like couldn’t there be a system in which there is 1 gigantic planet with enough gravity to pull in other planets into its angular momentum?

  50. tawkinhedz Post author

    Came here for knowledge, got a lot of talk about blobs and how skaters spin, and what a planet is? Overall, not great content.

  51. kok fah chong Post author

    The solar system is  not formed by coalescing a lump of gas.  Defined gas? Gas has no boundary and all gaseous atoms go their separate way. Generally stars generate heat and light through fission on their vast deposit of radioisotopes. Fusion process is a bluff. Really? Einstein's famous equation, E=mc^2 is wrong otherwise garbage also can be used to make nuclear bombs as long as it is matter or it has mass. Energy and matter can't interchange one another according to Einstein's famous equation. One must have photons before one can emit out photons. Photons are particles and they have mass. All forms of EMWs including light and heat are dynamic photons per volume per time in different saturations. Fusion process required more input of energy by saturating larger number of nucleons with more photons in order to strengthen their angular momentum to resist stronger repulsive forces among more number of protons within the nucleus. Thus, fusion process will not dissipate out energy instead required more input of energy. In conclusion, stars like sun generate heat and light must through fission on their vast deposit of radioisotopes. If you are interested in real discoveries, I would recommend you to read my book,, The Unification Theory – Volume One and you will be amazed with lots of new, interesting discoveries. In God I trust.

  52. JohnIsKey Post author

    A planet is a big enough mass that can sustain a gravitational pull and have to have a surface that’s round

  53. complete channel Post author

    A planet is a object when seen through earth's atmosphere DOESNOT TWINKLE

    That's my own definition anyone liked it? ??

  54. Andrew Baughman Post author

    Could someone point me to the evidence for the stars and solar systems forming. He said we’ve seen this happening, but I’d like to read something about it if possible.

  55. Baxter Bastard Post author

    "We are, quite literally, star stuff"

    I don't know about you but that statement makes me happy.

  56. Garret Lemaster Post author

    I like your videos man but I have a better explanation the one no one understands so they rule it out…………….GOD yep you probly didn't see that coming

  57. Lone Wolf Post author

    I have a doubt
    Why is it written in textbooks as "the solar system" it should be "a solar system" or "our solar system"😁

  58. Andrew Decker Post author

    The sun makes up about 99.8% of our solid systems mass and more than half of what's left is Jupiter.


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