Earth formation | Life on earth and in the universe | Cosmology & Astronomy | Khan Academy

By | December 7, 2019


What I’m going to attempt to do in the next two videos is really just give an overview of everything that’s happened to Earth since it came into existence. We’re going start really at the formation of Earth or the formation of our Solar system or the formation of the Sun, and our best sense of what actually happened is that there was a supernova in our vicinity of the galaxy, and this right here is a picture of a supernova remnant, actually, the remnant for Kepler’s supernova. The supernova in this picture actually happened four hundred years ago in 1604, so right at the center a star essentially exploded and for a few weeks was the brightest object in the night sky, and it was observed by Kepler and other people in 1604, and this is what it looks like now. What we see is kinda the shockwave that’s been traveling out for the past 400 years, so now it must be many light years across. It wasn’t, obviously, matter wasn’t traveling at the speed of light, but it must’ve been traveling pretty, pretty fast, at least relativistic speeds, a reasonable fraction of the speed of light. This has traveled a good bit out now, but what you can imagine is when you have the shockwave traveling out from a supernova, let’s say you had a cloud of molecules, a cloud of gas, that before the shockwave came by just wasn’t dense enough for gravity to take over, and for it to accrete, essentially, into a solar system. When the shockwave passes by it compresses all of this gas and all of this material and all of these molecules, so it now does have that critical density to form, to accrete into a star and a solar system. We think that’s what’s happened, and the reason why we feel pretty strongly that it must’ve been caused by a supernova is that the only way that the really heavy elements can form, or the only way we know that they can form is in kind of the heat of a supernova, and our uranium, the uranium that seems to be in our solar system on Earth, seems to have formed roughly at the time of the formation of Earth, at about four and a half billion years ago, and we’ll talk in a little bit more depth in future videos on exactly how people figure that out, but since the uranium seems about the same age as our solar system, it must’ve been formed at around the same time, and it must’ve been formed by a supernova, and it must be coming from a supernova, so a supernova shockwave must’ve passed through our part of the universe, and that’s a good reason for gas to get compressed and begin to accrete. So you fast-forward a few million years. That gas would’ve accreted into something like this. It would’ve reached the critical temperature, critical density and pressure at the center for ignition to occur, for fusion to start to happen, for hydrogen to start fusing into helium, and this right here is our early sun. Around the sun you have all of the gases and particles and molecules that had enough angular velocity to not fall into the sun, to go into orbit around the sun. They were actually supported by a little bit of pressure, too, because you can kinda view this as kind of a big cloud of gas, so they’re always bumping into each other, but for the most part it was their angular velocity, and over the next tens of millions of years they’ll slowly bump into each other and clump into each other. Even small particles have gravity, and they’re gonna slowly become rocks and asteroids and, eventually, what we would call “planetesimals,” which are, kinda view them as seeds of planets or early planets, and then those would have a reasonable amount of gravity and other things would be attracted to them and slowly clump up to them. This wasn’t like a simple process, you know, you could imagine you might have one planetesimal form, and then there’s another planetesimal formed, and instead of having a nice, gentle those two guys accreting into each other, they might have huge relative velocities and ram into each other, and then just, you know, shatter, so this wasn’t just a nice, gentle process of constant accretion. It would actually have been a very violent process, actually happened early in Earth’s history, and we actually think this is why the Moon formed, so at some point you fast-forward a little bit from this, Earth would have formed, I should say, the mass that eventually becomes our modern Earth would have been forming. Let me draw it over here. So, let’s say that that is our modern Earth, and what we think happened is that another proto-planet or another, it was actually a planet because it was roughly the size of Mars, ran into our, what it is eventually going to become our Earth. This is actually a picture of it. This is an artist’s depiction of that collision, where this planet right here is the size of Mars, and it ran into what would eventually become Earth. This we call Theia. This is Theia, and what we believe happened, and if you look up, if you go onto the Internet, you’ll see some simulations that talk about this, is that we think it was a glancing blow. It wasn’t a direct hit that would’ve just kinda shattered each of them and turned into one big molten ball. We think it was a glancing blow, something like this. This was essentially Earth. Obviously, Earth got changed dramatically once Theia ran into it, but Theia is right over here, and we think it was a glancing blow. It came and it hit Earth at kind of an angle, and then it obviously the combined energies from that interaction would’ve made both of them molten, and frankly they probably already were molten because you had a bunch of smaller collisions and accretion events and little things hitting the surface, so probably both of them during this entire period, but this would’ve had a glancing blow on Earth and essentially splashed a bunch of molten material out into orbit. It would’ve just come in, had a glancing blow on Earth, and then splashed a bunch of molten material, some of it would’ve been captured by Earth, so this is the before and the after, you can imagine, Earth is kind of this molten, super hot ball, and some of it just gets splashed into orbit from the collision. Let me just see if I can draw Theia here, so Theia has collided, and it is also molten now because huge energies, and it splashes some of it into orbit. If we fast-forward a little bit, this stuff that got splashed into orbit, it’s going in that direction, that becomes our Moon, and then the rest of this material eventually kind of condenses back into a spherical shape and is what we now call our Earth. So that’s how we actually think right now that the Moon actually formed. Even after this happened, the Earth still had a lot more, I guess, violence to experience. Just to get a sense of where we are in the history of Earth, we’re going to refer to this time clock a lot over the next few videos, this time clock starts right here at the formation of our solar system, 4.6 billion years ago, probably coinciding with some type of supernova, and as we go clockwise on this diagram, we’re moving forward in time, and we’re gonna go all the way forward to the present period, and just so you understand some of the terminology, “Ga” means “billions of years ago” ‘G’ for “Giga-” “Ma” means “millions of years ago” ‘M’ for “Mega-” So where we are right now, the Moon has formed, and we’re in what we call the Hadean period or actually I shouldn’t say “period.” It’s the Hadean eon of Earth. “Period” is actually another time period, so let me make this very clear. It’s the Hadean, we are in the Hadean eon, and an eon is kind of the largest period of time that we talk about, especially relative to Earth, and it’s roughly 500 million to a billion years is an eon, and what makes the Hadean eon distinctive, well, from a geological point of view what makes it distinctive is really we don’t have any rocks from the Hadean period. We don’t have any kind of macroscopic-scale rocks from the Hadean period, and that’s because at that time, we believe, the Earth was just this molten ball of kind of magma and lava, and it was molten because it was a product of all of these accretion events and all of these collisions and all this kinetic energy turning into heat. If you were to look at the surface of the Earth, if you were to be on the surface of the Earth during the Hadean eon, which you probably wouldn’t want to be because you might get hit by a falling meteorite or probably burned by some magma, whatever, it would look like this, and you wouldn’t be able to breathe anyway; this is what the surface of the Earth would look like. It would look like a big magma pool, and that’s why we don’t have any rocks from there because the rocks were just constantly being recycled, being dissolved and churned inside of this giant molten ball, and frankly the Earth still is a giant molten ball, it’s just we live on the super-thin, cooled crust of that molten ball. If you go right below that crust, and we’ll talk a little bit more about that in future videos, you will get magma, and if you go dig deeper, you’ll have liquid iron. I mean, it still is a molten ball. And this whole period is just a violent, not only was Earth itself a volcanic, molten ball, it began to harden as you get into the late Hadean eon, but we also had stuff falling from the sky and constantly colliding with Earth, and really just continuing to add to the heat of this molten ball. Anyway, I’ll leave you there, and, as you can imagine, at this point there was no, as far as we can tell, there was no life on Earth. Some people believe that maybe some life could’ve formed in the late Hadean eon, but for the most part this was just completely inhospitable for any life forming. I’ll leave you there, and where we take up the next video, we’ll talk a little bit about the Archean eon.

93 thoughts on “Earth formation | Life on earth and in the universe | Cosmology & Astronomy | Khan Academy

  1. dedknd Post author

    that was a good and open-minded thought when you corrected to "the only way we know" 😉

    Reply
  2. OhMyGoddlessGamer Post author

    @reik2006 wow….there is no god you piece of antimatter 😀

    Reply
  3. dedknd Post author

    @OhMyGoddlessGamer ohhh no, thats not what I meant. It was more about the fact, that theories are true as long as nobody proved them wrong (falsification) and therefore one shoudn't sound to absolutistic about how the world works out. That's the open-minded thought I was talking about 😉

    Reply
  4. mars Cubed Post author

    @rinwhr "a video on Earth Destruction!"
    Just play this video backwards.

    Reply
  5. dedknd Post author

    @MarvelsofaLifetime I admit you're right. The theories are not true, they are just valid. People can look for elves on Neptune (it's a finite place) and tell you there aren't any (.. at least that's what I expect to happen). If you say on the other hand, that there are elves somewhere out there, it's neither a valid theory and of course nor a true one (you have to look anywhere, which is impossible). Thank you for pointing out my careless use of words and the contradictions.

    Reply
  6. Vegan Phobic Post author

    The earth started with god and he made animals and trees and nappy time and unicorns and rainbows and 4 leaf clovers

    duh

    Reply
  7. Vegan Phobic Post author

    @bevon17 i read it in a book so its true

    dont question it

    Reply
  8. Magic and Mundane Post author

    So did Theia's material merge with the "proto-earth" or keep going?

    Reply
  9. Jason Dietz Post author

    @mindofmine2 no, i dont think so, he's a bloody genius! =)

    Reply
  10. KingPtolemyIII Post author

    This should go into a new Geology playlist as well as the Cosmology and Biology playlists.

    Reply
  11. Purav Patel Post author

    @KingPtolemyIII Actually, it fits into the Biology playlist, since these topics are covered in AP Biology and college Biology.

    Reply
  12. xESOTERlC Post author

    I freely admit my ignorance… Watching this video, random thought came to mind… in free fall, does acceleration occur due to an increase in gravitational force the closer you get to the earth/center.. :S I guess I can do quick bit of browsing.. cuz that would mean you weighed less the higher you got, and that's absurd.. *facepalm*

    Reply
  13. Naylob Swisher Post author

    @xESOTERlC You accelerate faster as you fall toward the earth, but not the center of it. If you could be at the direct center, you would be apparently weightless. The more you go inside the earth, the less force due to gravity you experience.

    Reply
  14. xESOTERlC Post author

    @oexnorth No no… I realize that ^^ I attached "center" so it would be understood that I didn't just think that gravity was pulling me towards the ground. Now, you are saying, though, that if I dug a hole 1/2/10/20 miles deep that I would at least accelerate less? o.O Also, that's supposing you could hollow out the center of the earth instantaneously/magically so that I could float there..
    Anyway, neither here nor there 😛 Doesn't correlate with my initial inquiry

    Reply
  15. LAnonHubbard Post author

    Very very interesting video. You certainly have a way with presenting information Mr Khan!

    Reply
  16. Armash Kamal Post author

    To be Islamic or Religious, Quran has 1400 years back told in itself how the Earth came into being. Anyhow, the try of science to find out how it came into being is still appreciable but not true. =) Hope you people were Muslim and you would get to know the truth and agree with me… !!!

    Reply
  17. Naylob Swisher Post author

    @xESOTERlC Force between two objects is Force=G*(mass of object 1)*(massof object 2)/(distance between two objects squared) G is a constant, so the only way to lessen the gravitational force between two objects is to either take mass away from one object or the other, or two put more distance between the two objects. Also remember, Force=(mass)*(acceleration), or (acceleration)=(Force)/(mass). If the force remains constant and mass is increases, acceleration becomes less.

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  18. xESOTERlC Post author

    @oexnorth Hm. So my first question was right? Acceleration in freefall occurs due to an increase in gravitational force?
    Your last part is skewed. if F=MA, then yes–A=F/M.. That is NOT saying that if(magically) the mass of something increases during freefall, the acceleration will decrease. Instead, it is just showing the correlation between the 3. So that if you know the force of an impact of freefall, you can equate its acceleration based on its mass

    Reply
  19. Xlaxsauce Post author

    @xESOTERlC

    f=ma is a general representation of newtons 2 law… it represents much more than what you see

    Reply
  20. xESOTERlC Post author

    @Xlaxsauce o.O
    I'm saying that the assertion "if the force remains constant and mass is increased, acceleration becomes less" is not an accurate one based on f=ma ~ a=f/m
    Or will you tell me that if you jump out of a plane with someone else.. and halfway towards the earth you join together.. that your acceleration will decrease just because the mass just increased?

    Reply
  21. Naylob Swisher Post author

    @xESOTERlC Actually, that's exactly what A=F/M is saying. If you divide a constant by a larger and larger number, what happens to the value? It gets smaller and smaller. Divide 1/10, then divide 1/100, what happens, the value gets smaller. Same thing here.

    Reply
  22. xESOTERlC Post author

    @oexnorth well, it's more complicated than that.. the force WOULDN'T stay the same because you'd be adding mass. A falling object would not decelerate because you added mass to it

    Reply
  23. Naylob Swisher Post author

    @xESOTERlC I'm just speaking in strictly theoretical terms. If F remains constant and mass increases, then yes, acceleration would decrease. Of course, in actual circumstances, if you added mass, then you would change the force due to gravity, and then acceleration would actually increase because in the gravitational equation mass is in the numerator. Again, I was just saying IF force could remain constant.

    Reply
  24. Naylob Swisher Post author

    @xESOTERlC No. Wrong, sorry. F=(G*m1*m2)/r^2 m is your mass. It's in the numerator.

    Reply
  25. xESOTERlC Post author

    @oexnorth I thought you were referring to acceleration, not force

    Reply
  26. Kaia Stoesz Post author

    You are just amazing to listen to! Thank you so much!

    Reply
  27. ShadowKitten Post author

    @ChenJunHung lol check out the why do people laugh at creationists serious and you will find out =)

    Reply
  28. ShadowKitten Post author

    @ChenJunHung the first 5 or 10 are the ones with hovind in them lol(trust me they are worth it to watch though if you have time)

    Reply
  29. Rulazm Post author

    @bevon17 i dnt believe that god made the earth, i dnt believe that he brought animals here, i dnt think all that is bullshit.. i just think that is not true……

    Reply
  30. Shiva Kumar Post author

    i think the thoery that u explained at the start is not true to the extend of my knowledge because i believe in the formation of solar system due to crash between shooting star and sun

    Reply
  31. Shiva Kumar Post author

    i think the thoery that u explained at the start is not true to the extend of my knowledge because i believe in the formation of solar system due to crash between shooting star and sun.

    Reply
  32. emergencylipstick Post author

    don't think the bible was meant to be taken literally o_O i mean, to each their own interpretation, I guess

    Reply
  33. Big1Poe Post author

    Where did the matter come from for the "big bang" to occur?
    It cannot 'just be there'.

    Reply
  34. Pricee123 Post author

    this has no relation to anything that i am doing in school, I watched because it is damn interesting

    Reply
  35. vicksoma Post author

    @Big1Poe When you're asking questions about things like the big bang it's important to remember that physics is a very strange and unintuitive field of study. There are many things which completely defy common sense, and yet they are verifiable by experiment.

    At the moment of the big bang matter-energy spontaneously created itself, possibly from virtual particles. Virtual particles are known to pop in and out of existence as demonstrated in lab experiments.

    Reply
  36. Fred suder Post author

    The man on the video is basing everything on assuming….Why does he not just say hew I do not know any more than anyone else

    Reply
  37. Rakib Kamal Post author

    he is super smart. but i have a question. is he religious or a beliver of science

    Reply
  38. ndog37 Post author

    This video is a nothing more than science fiction the product of a high school fantasy gone too far. If our universe is really 13billion years old surely there would be still stars forming? Ah no. There is not a single star observed to be forming.

    Reply
  39. ndog37 Post author

    Also how do you explain how all the water got from space to the planet? It magically floated here? Missing the sun? Which is only 97% of the solar system mass. Man you believe some funky stuff

    Reply
  40. Against Nihilism Post author

    @kibdara He doesn't answer what his religious views are and I think it's best that way. If he were in fact an atheist I feel that many Americans would be offended by this and stop watching his videos just because America is extremely religious when compared to the rest of the developed world.

    Reply
  41. Against Nihilism Post author

    @ndog37 Water is two parts hydrogen and one part oxygen it did not "magically float into the planet". If you don't have a basic understanding of chemistry then please don't try and act like you know what you are talking about.

    Reply
  42. ndog37 Post author

    @batistaker123 – so give me your "basic" reasoning of how water appeared/formed/got there. It is a joke for water to have come from space, which we can agree on. However to state that some funky chemical reaction made it happen.. you sure have a LOT of belief/faith in some strange ideas

    Reply
  43. MrBpigz Post author

    @ndog37 "Funky" chemical reaction? Holy hell bud – do some research on a subject before making assertions about something you obviously know nothing about. Throw in an introductory chemistry course while you're at it.

    Reply
  44. MrBpigz Post author

    @ndog37 If the world was 6k-10k years old, light from very distant starts that we know of would not be visible, due to the amount of time it takes light to reach a given point.

    Reply
  45. ndog37 Post author

    @MrBpigz – the red shift problem refutes the universe age theory problem. Also I see no demonstrative proof or explanation to explain why does earth have water, why don't other planets have it? Hell if comets hit the earth with ice, then the moon and every other planet should be pretty wet. I don't oppose this idea I just struggle to find proof.

    Reply
  46. Against Nihilism Post author

    @ndog37 Earth is close enough so that the water won't freeze and far enough so that it won't be a be completely dry. Again this is information that you can learn in an elementary school science class. Also your explanation for a young 6000 year old earth has been refuted in an article title "Debunking Robert Gentry's
    "New Redshift Interpretation" Cosmology". Google it and you can easily find it online. If you don't understand science then read an actual science book rather than arguing here.

    Reply
  47. ndog37 Post author

    You have a valid point about the position of the earth in relation to the sun however it is not sufficient enough considering the sun makes up more than 97% of the mass in the solar system. As for the age of the earth do you believe in the magnetic reversals theory which supposes that 100% of the magnetic field did not lose energy. I'm just wondering how far you have to stretch the imagination to fit in your your science fiction beliefs

    Reply
  48. Daniel Johnson Post author

    @justwanttocomment13 You say people don't accept God as if it is something they "should" do. I disagree with that. There are so many issues with the whole idea of God, and merely the fact that Christianity still teaches the Earth was created in a week by some invisible being the no one can see, yet we should worship unconditionally? The fact that people still believe that causes so much frustration for people who want a mentally advanced society. There is simply no evidence to support you

    Reply
  49. Daniel Johnson Post author

    @justwanttocomment13 "Why is it so hard to accept that God made all these incredible things happen?" You used God with a capital G. Christianity's god. What you said connotated to Christianity. Your target was not to people who don't believe in evolution, and if it was, you did an awful job portraying that. And I merely stated I disagree with what you had to say. I apologize for thinking that I could actually have an intellectual conversation with someone over the internet.

    Reply
  50. ndog37 Post author

    @justwanttocomment13 – thanks for your comments. The problem with this theory also lies with other planets in our solar system eg Jupiter. Jupiter is 1300 x bigger than earth but is mainly gas intact it may have no core. Any theory using model would have any 'collected' gas get sucked into the sun.

    Reply
  51. ndog37 Post author

    Coming back to the water problem. Life started off in muddy ponds of boiling water as far as evolution is concerned. If the planet was hotter and ice comets did hit the plant why wouldn't they evaporate on entry? Also how much water would it take and at what rate would it take to cool the planet. Important trying to believe this theory but i have to use my imagination

    Reply
  52. Daniel Johnson Post author

    @justwanttocomment13 Try learning more about religion before you start spouting out about it, hm?

    Reply
  53. Rachel Maselli Post author

    can u talk about like differentiation and accretion

    Reply
  54. LokyNoKey Post author

    USA isn't the only ridiculously religious country. Trust me. I'm from Europe.

    Reply
  55. LokyNoKey Post author

    Our solar system formed from a supernova. Heavy elements like oxygen are formed from supernovae. Hydrogen is the most abundant element. Put those two together and you get H2O.

    Reply
  56. LokyNoKey Post author

    If I know history right then Japan has a mix of many religions.

    Reply
  57. Lexo Cee Post author

    Great video. I'd just like to assert that the force that initiated the accretion of the planets was electromagnetism. Yes, all matter has gravity that is directly related to its mass, but tiny particles would not have enough gravity to attract each other in the beginning. Some of the collisions between the particles would have shifted their electrons into a more stable position and accretion could continue until the gravity became strong enough to attract more matter. Electromagnetism > Gravity

    Reply
  58. Lexo Cee Post author

    There is water all over the solar system. In the polar ice caps of Mars, in the rings of Saturn and its moons like Enceladus, the moons of Jupiter like Europa, and in vast numbers of comets and dwarf planets in the Kuiper belt like Pluto. Earth probably got most of its water from icy comets and planetesimals that bombarded the young planet back in the Hadean Eon.

    Reply
  59. Anomalocaria Post author

    I'm honestly just watching these videos for fun at this point.

    Reply
  60. Kevin Holland Post author

    The above explanation is all science fiction.  Maybe these events happened billions of years ago.  Mostly likely these events did not occur.  Hard to know what happened billions of years ago.  These are somewhat informed guesses based on very limited information nothing more.  The above explantions are not facts.  This should be emphasized.  There are millions of other possibilities for the formation of the solar system.  More emphasis should be placed on the fact that the above explanations are nothing more than possible events that happened billions of years ago.   More truth less pontificating for our students.

    Reply
  61. Ralph Latham Post author

    Looking around and seeing hundreds of billions of similar stars in this galaxy and a hundred billion other galaxies- this supernova shock wave hypothesis, were it true, is so thin it would require its own shock wave to get it started.

    Reply
  62. Gash Awx Post author

    Ah! This is perfect but i need it in Spanish

    Reply
  63. Cesar Miguel Pimentel Post author

    Are you really giving a scientific explanation when you say "we believe", "we imagine", "we feel", "probably", "maybe", etc.? Sounds like faith to me.

    Reply
  64. Manit Manil Post author

    According to the text the earth was made by the explosion of sun but if all planets were made by the explosion why only in earth there is atomsphere and other things which are needed for humans to survive olz answer

    Reply
  65. Happy Kids Post author

    Let me get this straight. Will-be Earth was chilling ready to have life and then Thea collides with it. All the stuff coming out of Earth formed our one moon. What about Jupiter or Saturn or even Mars? They have more that one moon but they have moons. Did the same thing happened to them? If no, how did they form?
    – Thanks! Sania

    Reply

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