1 00:00:00,562 --> 00:00:05,697 Now we have the ingredients. We understand enough about the solar 2 00:00:05,697 --> 00:00:11,957 nebula to start following the process of how we got from a disk of gas and perhaps 3 00:00:11,957 --> 00:00:16,599 some dust to what we have today. And we'll start with the interior 4 00:00:16,599 --> 00:00:21,733 planets, the terrestrial planets. Remember that inside the snow line we 5 00:00:21,733 --> 00:00:27,220 have mostly hydrogen and helium class, but in addition, there are small amounts 6 00:00:27,220 --> 00:00:32,047 of solids that congeal. The solids in question are iron and 7 00:00:32,047 --> 00:00:35,812 nickel. Silicates, the kind of materials we find 8 00:00:35,812 --> 00:00:41,327 in meteorites and there's not a large quantity of those, because these elements 9 00:00:41,327 --> 00:00:44,313 are very rare in the nebula, but there's enough. 10 00:00:44,313 --> 00:00:49,396 And as the density increases, these dust grains that are in Keplerian orbits 11 00:00:49,396 --> 00:00:54,796 inside this massive cloud of dust collide with each other and they adhere, they 12 00:00:54,796 --> 00:01:00,133 adhere in the same way that grains of dust adhere to cleaning rag. They adhere 13 00:01:00,133 --> 00:01:02,929 electrostatically and then chemically bind. 14 00:01:02,929 --> 00:01:06,995 They're forming rocks. These rocks are held together by chemical 15 00:01:06,995 --> 00:01:10,814 forces. And slowly over time, the larger rocks 16 00:01:10,814 --> 00:01:15,998 have larger surface area. They collide with more of the dust. 17 00:01:15,998 --> 00:01:22,206 and they grow and eventually one forms objects of a size, order of magnitude of 18 00:01:22,206 --> 00:01:25,264 a kilometer. There will be about a billion of them in 19 00:01:25,264 --> 00:01:29,570 the inner solar system and these billion so-called planetesimals, 20 00:01:29,570 --> 00:01:33,606 microplanets, are reaching the size where as we'll see 21 00:01:33,606 --> 00:01:37,144 in the homework, when you reach a size of about one kilometer, 22 00:01:37,144 --> 00:01:39,580 an object is bound now by its gravitation. 23 00:01:39,580 --> 00:01:44,047 In other words, the gravitational force is enough to hold the object together. 24 00:01:44,047 --> 00:01:48,281 It's more gravitationally bound than chemically, and this is an important 25 00:01:48,281 --> 00:01:51,110 transition. Because now, these objects are heavy 26 00:01:51,110 --> 00:01:53,970 enough that they're moving in Keplerian orbits. 27 00:01:53,970 --> 00:01:58,656 They are no longer sustained by the pressure of gas, which is still enough to 28 00:01:58,656 --> 00:02:03,849 hold up, the dust is floating in the gas. But the larger, rocks are not floating in 29 00:02:03,849 --> 00:02:08,428 the gas, they're falling through it. So they're moving on Keplerian orbits, 30 00:02:08,428 --> 00:02:12,380 their velocities are higher than those of the gas and the dust. 31 00:02:12,380 --> 00:02:16,770 And so these large planetesimals are sweeping through the gla, the gas, 32 00:02:16,770 --> 00:02:21,475 and whatever grains of dust come close are immediately falling onto the 33 00:02:21,475 --> 00:02:24,548 planetesimals and being gravitationally attached. 34 00:02:24,548 --> 00:02:27,935 And so the gravitational attraction becomes important, 35 00:02:27,935 --> 00:02:32,640 and the larger objects grow faster. So there is this hierarchical growth, 36 00:02:32,640 --> 00:02:37,062 where the rate of, at which an object grows turns out to be proportional to its 37 00:02:37,062 --> 00:02:40,309 radius to the fourth. And this goes on for about a 100,000 38 00:02:40,309 --> 00:02:44,900 years in which these planetesimals, and certainly the larger among them, grow up 39 00:02:44,900 --> 00:02:51,742 to be rather large objects. And through these, at the end of this 100,000 years 40 00:02:51,742 --> 00:02:56,881 the largest of them have merged to form about a few hundreds of objects that are 41 00:02:56,881 --> 00:03:00,624 called protoplanets. These are objects with a radius on the 42 00:03:00,624 --> 00:03:04,811 order of 1,000 kilometers. so slightly smaller than the current 43 00:03:04,811 --> 00:03:07,539 moon. And here another important transition 44 00:03:07,539 --> 00:03:12,678 happens. When you reach radius or size of about 1,000 kilometers, the object is 45 00:03:12,678 --> 00:03:17,310 certainly gravitationally bound. What has been holding up the rock so far, 46 00:03:17,310 --> 00:03:21,348 the reason a rock doesn't. con, doesn't collapse under its own 47 00:03:21,348 --> 00:03:24,732 gravity, is basically that a rock is rigid material. 48 00:03:24,732 --> 00:03:28,049 Chemical bonds hold it up against its own gravity. 49 00:03:28,049 --> 00:03:32,561 When an object reaches 1,000 kilometers, two important things happen. 50 00:03:32,561 --> 00:03:37,338 One is, the heat of the collisions and the Kelvin-Helmholtz heating from 51 00:03:37,338 --> 00:03:42,779 converting gravitational potential energy as objects collide into heat, along with 52 00:03:42,779 --> 00:03:46,041 the radioactivity. The heat released by the radioactive 53 00:03:46,041 --> 00:03:48,908 elements that are trapped inside, melt a proto-planet. 54 00:03:48,908 --> 00:03:52,100 When you reach a size of about a 1,000 kilometers, you melt. 55 00:03:52,100 --> 00:03:55,819 this is very important. Things that melt are going to be 56 00:03:55,819 --> 00:03:58,562 spherical. Why are they going to be spherical? 57 00:03:58,562 --> 00:04:02,365 Well, because gravity flattens them out to the form of a sphere. 58 00:04:02,365 --> 00:04:07,254 Imagine a molten liquid Earth well, we have parts of the Earth that are liquid, 59 00:04:07,254 --> 00:04:10,634 they are the oceans. There are no mountains in the ocean, 60 00:04:10,634 --> 00:04:15,584 because a mountain in the ocean would be leveled by gravity when the Earth was 61 00:04:15,584 --> 00:04:18,240 molten. And when a protoplanet is melted and 62 00:04:18,240 --> 00:04:22,586 liquid, it will naturally form into a symmetric spherical shape if it is 63 00:04:22,586 --> 00:04:25,423 twisting. It'll be, if it is spinning, it will be 64 00:04:25,423 --> 00:04:29,347 slightly oblate, slightly fatter at the equator than at the poles, 65 00:04:29,347 --> 00:04:33,414 because of, again, the centrifugal barrier that we talked about. 66 00:04:33,414 --> 00:04:38,187 But as long as it's not spinning too rapidly, it will be approximately 67 00:04:38,187 --> 00:04:41,403 spherical. This is one important process that takes 68 00:04:41,403 --> 00:04:44,184 place. All of these protoplanets are assuming 69 00:04:44,184 --> 00:04:47,644 spherical shape. And then, the other important thing that 70 00:04:47,644 --> 00:04:52,217 happens at 1,000 kilometers when things melt is something called chemical 71 00:04:52,217 --> 00:04:55,925 differentiation. these initially roughly uniform or 72 00:04:55,925 --> 00:05:00,621 objects where various elements were scattered haphazardly, depending on when 73 00:05:00,621 --> 00:05:04,329 they were accreted throughout the object, becomes segregated. 74 00:05:04,329 --> 00:05:08,470 Because once an object melts, then the heavier elements, iron, nickel, 75 00:05:08,470 --> 00:05:13,128 sink through the lighter silicates say, down towards the center. 76 00:05:13,128 --> 00:05:18,986 And one forms a chemically differentiated object where there be, there forms a core 77 00:05:18,986 --> 00:05:23,503 with, which is rich in the heavier elements, iron, nickel, etcetera. 78 00:05:23,503 --> 00:05:27,597 Along with an external envelope, which is the silicates. 79 00:05:27,597 --> 00:05:32,130 And. much cooler crust as the core as, as in 80 00:05:32,130 --> 00:05:37,130 falling matter falls in, we get additional heat at the crust. 81 00:05:37,130 --> 00:05:43,238 In addition, once an object is fluid, it's no longer held up by the chemical 82 00:05:43,238 --> 00:05:48,325 bonds, by rigidity. What holds the earth, or a proto planet up against collapse 83 00:05:48,325 --> 00:05:51,496 under it's own gravity is hydro-static pressure. 84 00:05:51,496 --> 00:05:56,714 The same kind of hydro-static pressure balance that held our slinky in balance, 85 00:05:56,714 --> 00:06:00,462 or the water in our cup. Pressure is largest in the middle, 86 00:06:00,462 --> 00:06:03,197 decreases as you move up through the Earth. 87 00:06:03,197 --> 00:06:07,650 Each layer is in hydro-static equilibrium, it's weight balanced by the 88 00:06:07,650 --> 00:06:12,394 extra pressure below it compared to the pressure above it, and so we obtain high 89 00:06:12,394 --> 00:06:17,317 pressure and high temperature in the core with decreasing pressure and temperature 90 00:06:17,317 --> 00:06:20,520 towards the outside. This is the situation on a planet. 91 00:06:20,520 --> 00:06:23,426 This happens when you create a protoplanet. 92 00:06:23,426 --> 00:06:27,815 So the transition from plan, well planitesimal, an object which is bound by 93 00:06:27,815 --> 00:06:30,893 gravity. But where gravity is not yet dominant, to 94 00:06:30,893 --> 00:06:35,527 a protoplanet, which melts, become spherical and chemically differentiates 95 00:06:35,527 --> 00:06:40,416 is the transition that we go through. After about a hundred thousand years, we 96 00:06:40,416 --> 00:06:45,241 have generated, protoplanets, a few hundreds of protoplanets orbiting the sun 97 00:06:45,241 --> 00:06:49,940 in this region that will become the inner solar system, within about 98 00:06:49,940 --> 00:06:52,940 two or three astronomical units from the Sun. 99 00:06:52,940 --> 00:06:57,875 how do we know about protoplanets? Well, we are fortunate enough to have 100 00:06:57,875 --> 00:07:02,009 some of them left. What we see in this beautiful video clip 101 00:07:02,009 --> 00:07:08,570 is an image of an actual proto-planet. This is the asteroid Vesta. 102 00:07:08,570 --> 00:07:12,950 As imaged, as it says, by the Dawn spacecraft. 103 00:07:12,950 --> 00:07:18,037 In 2011, there, the space craft was orbiting the asteroid. 104 00:07:18,037 --> 00:07:21,205 And so it got to take a 360 degree panoramic image. 105 00:07:21,205 --> 00:07:25,865 We see the roughly spherical shape. We see, the impact craters, we have to 106 00:07:25,865 --> 00:07:29,965 talk about what created those. This is an object that melted as it 107 00:07:29,965 --> 00:07:33,257 contracted gravitationally, so it's roughly spherical. 108 00:07:33,257 --> 00:07:36,612 The asteroid Vesta is an interesting object in itself. 109 00:07:36,612 --> 00:07:40,277 It, will be visible this winter. It will be in opposition. 110 00:07:40,277 --> 00:07:44,999 And if you get a chance to go out and take a look with a sufficiently large 111 00:07:44,999 --> 00:07:49,037 telescope, you're invited to view Vesta. You won't get a view like this, 112 00:07:49,037 --> 00:07:50,290 though. . 113 00:07:50,290 --> 00:07:57,620 Carrying on with our plan of producing planets, we now have these 114 00:07:57,620 --> 00:08:01,925 proto-planets orbiting. They rapidly, gravitationally accrete the 115 00:08:01,925 --> 00:08:05,692 remaining planetesimals. So now what goes on is that the 116 00:08:05,692 --> 00:08:10,132 gravitational force of these proto-planets begins to be important. 117 00:08:10,132 --> 00:08:14,774 And their interactions with the planetesimals are gravitational, they 118 00:08:14,774 --> 00:08:20,626 either accrete the planetesimals or, if they collide with them, they might, blow 119 00:08:20,626 --> 00:08:24,057 them up and eject them from the inner solar system. 120 00:08:24,057 --> 00:08:29,506 The result of this process is that you end up with about 100 objects the size of 121 00:08:29,506 --> 00:08:33,692 the moon, Mars, so smaller than the Earth. these are the 122 00:08:33,692 --> 00:08:39,626 protoplanets, and they have cleared gaps in the disc, so where a protoplanet is 123 00:08:39,626 --> 00:08:43,201 orbiting there will be no more planetesimals. 124 00:08:43,201 --> 00:08:47,689 Planetesimals will still be orbiting in the gaps between. 125 00:08:47,689 --> 00:08:54,155 These will be about 100 equally spaced orbits between the interior of the disc 126 00:08:54,155 --> 00:08:59,100 just outside the Sun and say two or three astronomical units out. 127 00:08:59,100 --> 00:09:04,153 And in this region will be a hundred proto planet orbiting in gap, in, in, in 128 00:09:04,153 --> 00:09:08,600 clear gap and in between them, there will still be planettissimals. 129 00:09:08,600 --> 00:09:11,181 Now. These, the gravitation, no interactions 130 00:09:11,181 --> 00:09:14,465 between proto planets themselves now become important. 131 00:09:14,465 --> 00:09:19,149 We'll talk about that in the next clip. And this distorts their orbit, so that 132 00:09:19,149 --> 00:09:23,798 they start moving through these gaps, ejecting or accreting the remaining 133 00:09:23,798 --> 00:09:27,529 planetesimals. These distorted orbits now lead to actual 134 00:09:27,529 --> 00:09:32,126 collisions between the protoplanets. And these collisions are massive 135 00:09:32,126 --> 00:09:35,391 collisions. These are moon sized objects crashing 136 00:09:35,391 --> 00:09:40,988 into each other with polarian velocities. this can lead to a complete destruction 137 00:09:40,988 --> 00:09:44,586 of some of the objects. The collisions are now violent. 138 00:09:44,586 --> 00:09:49,450 and there're suitable conditions. The two objects partially remelt, and 139 00:09:49,450 --> 00:09:52,499 merge, and this leads from these 100 or so 140 00:09:52,499 --> 00:09:56,565 moon-sized objects to a few large ones like Venus and Earth. 141 00:09:56,565 --> 00:10:01,377 This is where the larger planets come from, from these mergers of these 142 00:10:01,377 --> 00:10:05,104 protoplanets. one of such collision we think left 143 00:10:05,104 --> 00:10:10,051 Mercury stripped down to its core. We'll see that Mercury is a very dense 144 00:10:10,051 --> 00:10:12,966 planet. The explanation is that a collision 145 00:10:12,966 --> 00:10:18,085 evaporated and blew off into space its envelope, leaving just essentially the 146 00:10:18,085 --> 00:10:21,103 core, Mars never grows beyond a Mars-sized object. 147 00:10:21,103 --> 00:10:26,030 It's smaller, it's about the size of the moon, it's much lighter than earth, We'll 148 00:10:26,030 --> 00:10:30,834 see what it is that limits, or stunts the growth of Mars, and within ten to 100 149 00:10:30,834 --> 00:10:34,900 million years, remember ten million years is when the dust is gone, 150 00:10:34,900 --> 00:10:39,198 and the gas is gone. The T tauri winds clean out the system, 151 00:10:39,198 --> 00:10:44,794 and the orbits which have been perturbed, settle down by mutual friction, down into 152 00:10:44,794 --> 00:10:49,912 the near circular orbits that we see. So within ten to 100 million years, we 153 00:10:49,912 --> 00:10:53,596 have pretty much the inner solar system as we know it. 154 00:10:53,596 --> 00:10:59,738 We see this in this beautiful video. we start with, the orbiting, lift clouds 155 00:10:59,738 --> 00:11:04,654 of planetesimals. We see the gaps where larger proto-planets are forming. 156 00:11:04,654 --> 00:11:08,614 In a minute, we'll be able to see the actual proto-planets. 157 00:11:08,614 --> 00:11:12,574 Here's a proto-planet. And, the planets, as they orbit, clear 158 00:11:12,574 --> 00:11:18,104 away, lanes inside the planetesimals, and slowly their influence cleans out the 159 00:11:18,104 --> 00:11:21,996 leftover planetesimals, either accreting or ejecting them. 160 00:11:21,996 --> 00:11:27,458 So that, after about ten million years, we are left with roughly the solar system 161 00:11:27,458 --> 00:11:33,569 as we know it, and the sun blows away the remaining gas and dust, and we find the 162 00:11:33,569 --> 00:11:38,028 interior, interior solar system, the inner solar system looking pretty 163 00:11:38,028 --> 00:11:42,248 much the way we see it today. What happens beyond is going to be the 164 00:11:42,248 --> 00:11:43,843 subject of the next clip.