1 00:00:09,640 --> 00:00:12,360 There are some mysteries when we look around the solar system, 2 00:00:12,360 --> 00:00:15,440 where the theories really don't match what we see. 3 00:00:20,280 --> 00:00:23,440 Science fact can be a lot weirder than science fiction. 4 00:00:26,400 --> 00:00:27,680 We started finding planets 5 00:00:27,680 --> 00:00:31,360 in places we'd never thought could possibly form a planet. 6 00:00:31,360 --> 00:00:33,480 We had to go back to the drawing board. 7 00:00:36,040 --> 00:00:38,600 How do you make solar systems? How do you make planets? 8 00:00:40,400 --> 00:00:42,840 It's as if somebody took the solar system, 9 00:00:42,840 --> 00:00:45,320 picked it up and shook it real hard. 10 00:00:45,320 --> 00:00:47,160 Our planets might have moved. 11 00:00:47,160 --> 00:00:48,720 They might have moved a lot. 12 00:00:48,720 --> 00:00:50,680 All of a sudden, everything changed. 13 00:00:56,000 --> 00:00:57,720 It's changed the way we look at 14 00:00:57,720 --> 00:01:00,120 almost every process in the solar system. 15 00:01:02,360 --> 00:01:05,200 Sometimes the blood splattered on the wall 16 00:01:05,200 --> 00:01:07,160 can tell you more about what happened 17 00:01:07,160 --> 00:01:08,880 than the body lying on the floor. 18 00:01:29,600 --> 00:01:31,520 The Royal Observatory in Greenwich 19 00:01:31,520 --> 00:01:34,680 is the historical home of British astronomy. 20 00:01:37,520 --> 00:01:42,000 Discoveries have been made here and mysteries unravelled. 21 00:01:42,000 --> 00:01:46,320 It is also home to some unique astronomical treasures. 22 00:01:49,960 --> 00:01:51,480 This is an orrery, 23 00:01:51,480 --> 00:01:54,280 a clockwork model of the solar system, 24 00:01:54,280 --> 00:01:56,960 and for most of the last four centuries 25 00:01:56,960 --> 00:02:00,200 this has been the way we think about the planets in the solar system. 26 00:02:01,640 --> 00:02:04,320 Of course, the scale is all wrong. 27 00:02:04,320 --> 00:02:07,120 But it clearly shows the traditional view of the planets 28 00:02:07,120 --> 00:02:08,920 and their fixed orbits. 29 00:02:08,920 --> 00:02:11,720 In the centre we have the sun 30 00:02:11,720 --> 00:02:15,640 and then, around it, we have the four rocky planets, 31 00:02:15,640 --> 00:02:18,360 tiny Mercury rushing around in the middle, 32 00:02:18,360 --> 00:02:21,480 Venus, the earth with the moon going around it, 33 00:02:21,480 --> 00:02:23,200 and then Mars. 34 00:02:23,200 --> 00:02:25,400 And outside of the inner planets, 35 00:02:25,400 --> 00:02:27,040 we have the gas giants, 36 00:02:27,040 --> 00:02:29,040 Jupiter, the largest planet of all, 37 00:02:29,040 --> 00:02:32,080 and then Saturn with its beautiful ring system. 38 00:02:32,080 --> 00:02:35,320 And then the two outermost planets, Uranus and Neptune. 39 00:02:37,000 --> 00:02:40,240 Astronomers always thought that the planets have been fixed 40 00:02:40,240 --> 00:02:44,400 in these orbits since they formed, more than 4 billion years ago. 41 00:02:45,480 --> 00:02:49,280 Long enough for the earth to develop into the haven it is today 42 00:02:49,280 --> 00:02:51,360 for life to evolve. 43 00:02:51,360 --> 00:02:54,520 A mechanical model like this embodies an idea 44 00:02:54,520 --> 00:02:57,840 of the solar system in which the planets all keep 45 00:02:57,840 --> 00:03:00,520 to these very neat, orderly orbits, 46 00:03:00,520 --> 00:03:05,520 moving essentially in circles and at fixed distances from the sun. 47 00:03:05,520 --> 00:03:10,040 And the natural assumption to make is that everything we see now 48 00:03:10,040 --> 00:03:13,240 formed where it is and has stayed there ever since. 49 00:03:15,800 --> 00:03:19,200 The idea that the planets are fixed in their orbits 50 00:03:19,200 --> 00:03:23,560 has been the bedrock of our understanding for hundreds of years. 51 00:03:23,560 --> 00:03:26,280 But there are some mysteries about our solar system 52 00:03:26,280 --> 00:03:30,200 that mean we may have to rethink everything we thought we knew. 53 00:03:33,000 --> 00:03:36,240 It's time for a brand-new model. 54 00:03:36,240 --> 00:03:38,640 DRAMATIC DRUMBEAT 55 00:03:47,200 --> 00:03:50,360 Recently, astronomers have started to unravel 56 00:03:50,360 --> 00:03:52,960 the mystery of how the solar system came to be. 57 00:03:54,840 --> 00:03:56,600 And to explore it 58 00:03:56,600 --> 00:03:59,360 we first need a more accurate picture of our planets. 59 00:04:03,200 --> 00:04:07,600 We need to alter the scale to reflect the huge difference in size. 60 00:04:07,600 --> 00:04:11,000 For example, Jupiter, the largest planet, 61 00:04:11,000 --> 00:04:14,280 is 11 times the radius of the Earth, 62 00:04:14,280 --> 00:04:17,320 and if you look at the masses, the difference is even greater. 63 00:04:17,320 --> 00:04:21,200 Jupiter has about 300 times the mass of planet Earth. 64 00:04:23,840 --> 00:04:26,280 The sun we've left alone. 65 00:04:26,280 --> 00:04:31,000 If we scaled that up too it would fill half the room. 66 00:04:31,000 --> 00:04:34,200 And, of course, the planets are not all bunched up. 67 00:04:34,200 --> 00:04:36,840 MECHANICAL WHIRRING 68 00:04:39,480 --> 00:04:42,920 We need to push the gas and ice giants much further away. 69 00:04:47,000 --> 00:04:50,600 To be truly accurate, with the planets this size 70 00:04:50,600 --> 00:04:53,760 we'd have to make the orbits several thousand times bigger. 71 00:04:59,520 --> 00:05:02,160 However, exactly how we ended up with this neat 72 00:05:02,160 --> 00:05:04,640 and stable arrangement of planets 73 00:05:04,640 --> 00:05:07,720 is still one of the greatest mysteries in astronomy. 74 00:05:14,760 --> 00:05:16,800 In trying to solve this mystery, 75 00:05:16,800 --> 00:05:19,200 we may discover how the earth came to inhabit 76 00:05:19,200 --> 00:05:21,520 the perfect position for life to evolve. 77 00:05:23,480 --> 00:05:26,640 Getting an earth where we have our earth today 78 00:05:26,640 --> 00:05:29,960 was not a given when this whole solar system started. 79 00:05:32,000 --> 00:05:36,120 We may be able to understand the remarkable chain of events 80 00:05:36,120 --> 00:05:38,880 that created the biggest game of pinball in the galaxy... 81 00:05:39,840 --> 00:05:42,520 The solar system could have done a lot of different things, 82 00:05:42,520 --> 00:05:44,720 it could have evolved in a lot of different ways. 83 00:05:44,720 --> 00:05:47,400 We could have ended up with our Jupiter right next to the sun. 84 00:05:48,880 --> 00:05:50,960 And it looks like it was Jupiter 85 00:05:50,960 --> 00:05:53,360 that defined the fate of the solar system. 86 00:05:54,520 --> 00:05:59,000 The giant planets' story IS the story of our solar system. 87 00:05:59,000 --> 00:06:01,480 We like to think that the earth is really important, 88 00:06:01,480 --> 00:06:04,040 but the truth is that, if you were looking from afar, 89 00:06:04,040 --> 00:06:07,800 our solar system is mainly four big planets and some debris. 90 00:06:09,520 --> 00:06:11,720 Could our place in the universe 91 00:06:11,720 --> 00:06:15,000 really be nothing more than a lucky accident? 92 00:06:15,000 --> 00:06:16,760 The question that really arises 93 00:06:16,760 --> 00:06:20,160 is how common is a solar system like ours? 94 00:06:23,720 --> 00:06:27,440 The mystery of the birth of the solar system is set to unravel. 95 00:06:34,120 --> 00:06:37,640 As they try to work out how our solar system formed, 96 00:06:37,640 --> 00:06:40,840 astronomers have noticed some baffling puzzles. 97 00:06:40,840 --> 00:06:43,840 If we look at the solar system as it is today, 98 00:06:43,840 --> 00:06:46,160 it seems quite neat and simple. 99 00:06:46,160 --> 00:06:49,760 We have four small, rocky planets close to the sun 100 00:06:49,760 --> 00:06:53,200 and then four enormous giants further out. 101 00:06:53,200 --> 00:06:56,520 But when we try to model the formation of the solar system 102 00:06:56,520 --> 00:06:59,760 on a computer, something doesn't quite add up. 103 00:06:59,760 --> 00:07:01,800 It's really hard to get the model 104 00:07:01,800 --> 00:07:04,680 to make the planets in the places where we see them today. 105 00:07:09,760 --> 00:07:12,040 Take, for instance, the curious case 106 00:07:12,040 --> 00:07:14,760 of the undersized planet Mars. 107 00:07:14,760 --> 00:07:17,720 If we look at the rocky innermost planets, 108 00:07:17,720 --> 00:07:20,000 Venus and Earth have about the same mass, 109 00:07:20,000 --> 00:07:22,960 and we'd expect Mars to have a similar mass too, 110 00:07:22,960 --> 00:07:24,320 but it actually doesn't. 111 00:07:24,320 --> 00:07:27,560 It's only about one tenth the mass of the earth or Venus, 112 00:07:27,560 --> 00:07:30,200 and that's a mystery that's very hard to explain. 113 00:07:32,640 --> 00:07:35,360 This is the first of four key puzzles 114 00:07:35,360 --> 00:07:39,000 about the birth of the solar system that remain unsolved. 115 00:07:40,280 --> 00:07:42,720 And then, at the edge of the solar system, 116 00:07:42,720 --> 00:07:46,000 the two outermost planets, Uranus and Neptune, 117 00:07:46,000 --> 00:07:49,480 are much further away from the sun than we'd expect. 118 00:07:49,480 --> 00:07:52,440 It's very hard to explain how they could have formed 119 00:07:52,440 --> 00:07:56,400 and become so large at that great distance from the central star. 120 00:07:57,480 --> 00:08:00,480 If we go in and look at the asteroid belt, 121 00:08:00,480 --> 00:08:03,520 there are thousands of small, rocky objects there, 122 00:08:03,520 --> 00:08:05,360 but there are two broad types - 123 00:08:05,360 --> 00:08:09,560 some of them are very rocky and some have more of an icy content. 124 00:08:09,560 --> 00:08:11,880 And yet these two types are actually found 125 00:08:11,880 --> 00:08:13,560 relatively close together. 126 00:08:13,560 --> 00:08:16,240 It seems as though they formed under different circumstances 127 00:08:16,240 --> 00:08:19,200 but they've all ended up in roughly the same place. 128 00:08:19,200 --> 00:08:21,880 And, again, it's a mystery as to how that happened. 129 00:08:23,800 --> 00:08:25,520 And, closer to home, 130 00:08:25,520 --> 00:08:28,720 how to explain the rapid and massive bombardment 131 00:08:28,720 --> 00:08:30,920 that left the moon covered in craters. 132 00:08:33,000 --> 00:08:36,040 There are many mysteries in the solar system, 133 00:08:36,040 --> 00:08:38,080 but by unravelling these four - 134 00:08:38,080 --> 00:08:39,720 the size of Mars, 135 00:08:39,720 --> 00:08:42,400 the formation of the outer planets, 136 00:08:42,400 --> 00:08:44,400 the composition of the asteroid belt 137 00:08:44,400 --> 00:08:46,680 and the bombardment of the moon - 138 00:08:46,680 --> 00:08:49,720 we may be able to explain how our planet Earth 139 00:08:49,720 --> 00:08:53,320 found itself in a perfect position for life to evolve. 140 00:08:55,480 --> 00:08:57,960 And it all starts a long, 141 00:08:57,960 --> 00:08:59,400 long time ago. 142 00:09:01,000 --> 00:09:03,040 DRAMATIC DRUMBEAT 143 00:09:13,920 --> 00:09:16,280 Four and a half billion years ago, 144 00:09:16,280 --> 00:09:18,000 our sun burst into life 145 00:09:18,000 --> 00:09:21,240 from the collapse of a massive cloud of gas and dust. 146 00:09:23,080 --> 00:09:24,280 So, in the beginning, 147 00:09:24,280 --> 00:09:26,880 this is what you have in our early solar system. 148 00:09:26,880 --> 00:09:28,720 You have the young star just born, 149 00:09:28,720 --> 00:09:33,000 and the leftovers, just a cloud of gas, 150 00:09:33,000 --> 00:09:35,400 the nebula, the protoplanetary nebula, 151 00:09:35,400 --> 00:09:37,680 full of hydrogen and helium, 152 00:09:37,680 --> 00:09:40,160 dust and gas, 153 00:09:40,160 --> 00:09:42,280 and ice grains forming. 154 00:09:42,280 --> 00:09:46,280 And from this, eventually, you form the planets. 155 00:09:47,520 --> 00:09:51,840 We know surprisingly little about exactly how planets are formed. 156 00:09:51,840 --> 00:09:55,520 Most mysterious of all is the most important - 157 00:09:55,520 --> 00:09:58,640 the largest of all planets, Jupiter, 158 00:09:58,640 --> 00:10:00,640 which seems to have been made first. 159 00:10:01,840 --> 00:10:05,640 The first-born - giant, massive Jupiter. 160 00:10:05,640 --> 00:10:08,280 The meanest and largest of all the planets. 161 00:10:09,760 --> 00:10:12,920 It sucks up more than half of the existing nebula 162 00:10:12,920 --> 00:10:15,600 and becomes the king of the solar system. 163 00:10:17,560 --> 00:10:20,760 We know that Jupiter is made up almost entirely 164 00:10:20,760 --> 00:10:24,120 of the hydrogen and helium left over in this primordial cloud. 165 00:10:25,480 --> 00:10:29,680 Which means it must have formed incredibly quickly 166 00:10:29,680 --> 00:10:33,720 because, as the new sun heated up, it would have blasted the gas away. 167 00:10:36,000 --> 00:10:38,720 And so there's a time limit. 168 00:10:38,720 --> 00:10:42,080 Jupiter must have formed in the astronomical blink of an eye - 169 00:10:42,080 --> 00:10:44,200 just five million years. 170 00:10:46,240 --> 00:10:50,400 But exactly how it grew so fast and why it grew where it did 171 00:10:50,400 --> 00:10:52,520 remains shrouded in mystery. 172 00:11:01,400 --> 00:11:05,040 It's a mystery that Scott Bolton is hoping to shed some light on. 173 00:11:06,560 --> 00:11:08,680 He's sending a spaceship to Jupiter. 174 00:11:13,960 --> 00:11:17,120 At the Jet Propulsion Laboratory in Pasadena 175 00:11:17,120 --> 00:11:20,520 is NASA's deep space operations centre - 176 00:11:20,520 --> 00:11:24,720 control room for space flights to the moon and beyond... 177 00:11:28,160 --> 00:11:32,120 ..including Scott's mission to Jupiter, Juno. 178 00:11:32,120 --> 00:11:35,760 Four...three...two...one... 179 00:11:35,760 --> 00:11:41,040 Ignition and lift-off of the Atlas V with Juno 180 00:11:41,040 --> 00:11:43,320 on a trek to Jupiter. 181 00:11:46,200 --> 00:11:48,560 Juno launched in 2011 182 00:11:48,560 --> 00:11:51,240 and is currently more than 2 million miles away, 183 00:11:51,240 --> 00:11:55,640 speeding its way to Jupiter at about 150,000mph. 184 00:11:58,760 --> 00:12:01,440 Even at that speed, it's a five-year journey. 185 00:12:02,680 --> 00:12:06,160 In 2016, if all goes according to plan, 186 00:12:06,160 --> 00:12:08,560 the probe will reach Jupiter and go into orbit 187 00:12:08,560 --> 00:12:10,640 around the king of the solar system. 188 00:12:14,000 --> 00:12:18,120 Scott and his entire team will be in this room, watching intensely. 189 00:12:20,720 --> 00:12:23,120 On the day of the Jupiter orbit insertion, 190 00:12:23,120 --> 00:12:25,120 this room will be completely full. 191 00:12:26,560 --> 00:12:29,280 The spacecraft is approaching Jupiter. 192 00:12:29,280 --> 00:12:35,040 It's moving at an incredible speed - like, 150,000mph, or even faster. 193 00:12:35,040 --> 00:12:39,280 And when it gets to Jupiter we have to slow down enough 194 00:12:39,280 --> 00:12:41,840 that Jupiter's gravity field can grab us. 195 00:12:43,200 --> 00:12:44,960 So, we have a rocket on board, 196 00:12:44,960 --> 00:12:47,880 we point it forward and we fire it, 197 00:12:47,880 --> 00:12:51,080 and that rocket has to burn at just the right time 198 00:12:51,080 --> 00:12:53,520 for just the right amount of time 199 00:12:53,520 --> 00:12:58,400 for us to slow down the perfect amount for Jupiter to grab us, 200 00:12:58,400 --> 00:13:01,400 because if it misses we fly right past Jupiter. 201 00:13:03,000 --> 00:13:04,760 There'll be a huge cheer. 202 00:13:04,760 --> 00:13:09,080 Once we get the data down that shows us we're in orbit around Jupiter 203 00:13:09,080 --> 00:13:11,240 the room will explode. 204 00:13:11,240 --> 00:13:14,920 It's the same room where all NASA's critical events are controlled from, 205 00:13:14,920 --> 00:13:19,960 including the recent landing of the Curiosity rover on Mars. 206 00:13:19,960 --> 00:13:22,840 And here you see it's right on that screen. 207 00:13:26,520 --> 00:13:28,240 We'll celebrate just like that. 208 00:13:29,960 --> 00:13:31,200 I can't wait. 209 00:13:35,440 --> 00:13:39,920 Once in orbit, Juno will spend a year circling Jupiter, 210 00:13:39,920 --> 00:13:42,400 gathering vital clues about how it formed. 211 00:13:44,120 --> 00:13:46,200 Some of the most important data 212 00:13:46,200 --> 00:13:48,800 that we really want and can't wait to get 213 00:13:48,800 --> 00:13:52,600 is things that are tied to understanding the early solar system 214 00:13:52,600 --> 00:13:53,840 and how Jupiter formed. 215 00:13:53,840 --> 00:13:57,840 So, we want to know whether there's a core in the middle of Jupiter. 216 00:13:57,840 --> 00:13:59,280 Is there a core of heavy elements, 217 00:13:59,280 --> 00:14:01,800 a concentration of materials, down in the centre? 218 00:14:01,800 --> 00:14:05,120 Or is it the same hydrogen and helium and mixture of gases, 219 00:14:05,120 --> 00:14:06,600 just squeezed down? 220 00:14:08,800 --> 00:14:10,600 Knowing what's at the centre 221 00:14:10,600 --> 00:14:14,400 is a vital clue to understanding how Jupiter was built. 222 00:14:24,840 --> 00:14:28,240 Building planets, whether rocky or gassy, 223 00:14:28,240 --> 00:14:29,960 is a tricky business. 224 00:14:29,960 --> 00:14:32,600 But it's something that Juno will shed some light on. 225 00:14:34,120 --> 00:14:36,720 What Juno's about is actually trying to discover 226 00:14:36,720 --> 00:14:39,000 the recipe for the solar system. 227 00:14:39,000 --> 00:14:42,080 How do you make solar systems? How do you make planets? 228 00:14:42,080 --> 00:14:46,480 And the stage that we're at is we're collecting the ingredient list 229 00:14:46,480 --> 00:14:48,960 and that's really an important part of any recipe - 230 00:14:48,960 --> 00:14:51,800 first you gather up the ingredients, figure out what they are, 231 00:14:51,800 --> 00:14:53,720 then there's some process that you have to do 232 00:14:53,720 --> 00:14:55,240 in order to bake your cake. 233 00:14:56,800 --> 00:15:00,520 But the exact nature of that process is not entirely clear. 234 00:15:01,760 --> 00:15:07,280 The recipe for a rocky planet, like the earth or Mars, is a slow one. 235 00:15:07,280 --> 00:15:10,720 It can take up to 100 million years. 236 00:15:10,720 --> 00:15:13,600 But the ingredient list is simple - 237 00:15:13,600 --> 00:15:15,400 dust. 238 00:15:15,400 --> 00:15:18,120 Dust starts out in the early solar system 239 00:15:18,120 --> 00:15:21,040 in a very fine grain, like this - even finer. 240 00:15:21,040 --> 00:15:25,160 Then, eventually, they start to stick together through electrostatic forces 241 00:15:25,160 --> 00:15:27,640 and they build bigger and bigger pieces. 242 00:15:27,640 --> 00:15:30,720 Eventually, the rocks got big enough, they started to stick together 243 00:15:30,720 --> 00:15:33,560 to the point where they started to form their own gravity. 244 00:15:34,960 --> 00:15:37,760 But there's a big leap from dust grains 245 00:15:37,760 --> 00:15:40,200 to rocks that are large enough to clump together 246 00:15:40,200 --> 00:15:41,800 through their own gravity. 247 00:15:41,800 --> 00:15:44,920 These rocks, even being so large as they are 248 00:15:44,920 --> 00:15:46,640 that none of us could lift them, 249 00:15:46,640 --> 00:15:49,040 they still don't have important gravity. 250 00:15:50,040 --> 00:15:53,920 Even larger rocks are needed to really start to get enough gravity 251 00:15:53,920 --> 00:15:57,400 to start to attract the rest of the material for it to collapse 252 00:15:57,400 --> 00:16:01,160 and start to form a planet as large as Earth. 253 00:16:04,800 --> 00:16:06,800 It's a slow process, 254 00:16:06,800 --> 00:16:09,840 but there's no rush when it comes to building a rocky planet. 255 00:16:11,400 --> 00:16:14,400 Gas giants, on the other hand, are trickier. 256 00:16:14,400 --> 00:16:15,920 You have to make them fast. 257 00:16:18,480 --> 00:16:21,120 Because Jupiter and the other gas giants 258 00:16:21,120 --> 00:16:23,200 are mostly hydrogen and helium, 259 00:16:23,200 --> 00:16:26,280 and the sun is mostly hydrogen and helium, 260 00:16:26,280 --> 00:16:30,440 that tells us right away that those planets had to have formed 261 00:16:30,440 --> 00:16:33,920 while that nebula of hydrogen and helium was still around. 262 00:16:35,840 --> 00:16:40,840 There are two ways to build a gas giant like Jupiter that fast. 263 00:16:40,840 --> 00:16:44,000 We don't know exactly how Jupiter formed. 264 00:16:44,000 --> 00:16:45,360 The two main theories 265 00:16:45,360 --> 00:16:48,600 are either it has a direct gravitational collapse, 266 00:16:48,600 --> 00:16:51,200 like we think the sun had, from the nebula 267 00:16:51,200 --> 00:16:55,800 and sort of builds, um, from the outside in 268 00:16:55,800 --> 00:16:58,440 and formed Jupiter pretty quickly, 269 00:16:58,440 --> 00:17:01,160 or it starts to build from the inside out. 270 00:17:02,880 --> 00:17:05,080 If it collapsed from the cloud of gas, 271 00:17:05,080 --> 00:17:08,040 then it will be gas all the way through to the centre. 272 00:17:09,080 --> 00:17:13,160 But if the second theory is right, then it first built a rocky core 273 00:17:13,160 --> 00:17:15,800 up to ten times the mass of the Earth, 274 00:17:15,800 --> 00:17:17,800 which then drew in a blanket of gas. 275 00:17:18,960 --> 00:17:21,360 Either way, it had to happen fast. 276 00:17:23,480 --> 00:17:26,680 But if Jupiter was going to build a heavy core that quickly, 277 00:17:26,680 --> 00:17:28,680 it couldn't be done with dust alone. 278 00:17:29,880 --> 00:17:32,280 There was another crucial ingredient. 279 00:17:33,720 --> 00:17:35,000 Ice. 280 00:17:43,560 --> 00:17:45,680 Kevin Walsh is a planet builder. 281 00:17:47,240 --> 00:17:49,480 His job is to create theoretical models 282 00:17:49,480 --> 00:17:53,560 of how the planets in the solar system formed - 283 00:17:53,560 --> 00:17:57,440 models that can best explain the evidence and the clues. 284 00:17:57,440 --> 00:18:00,560 I think that the most likely way that Jupiter formed 285 00:18:00,560 --> 00:18:03,080 was by building a solid core of material 286 00:18:03,080 --> 00:18:05,560 and then hauling the gas down on top of it. 287 00:18:05,560 --> 00:18:07,440 Either way that you form Jupiter, 288 00:18:07,440 --> 00:18:09,640 either from accreting straight from the gas 289 00:18:09,640 --> 00:18:12,760 or building up a rocky core, it has to be done 290 00:18:12,760 --> 00:18:14,240 in four or five million years, 291 00:18:14,240 --> 00:18:17,440 before all of the gas is gone from the disc around the sun. 292 00:18:19,600 --> 00:18:21,800 That's a lot quicker than the time it took 293 00:18:21,800 --> 00:18:24,000 to build a rocky planet from dust alone. 294 00:18:26,000 --> 00:18:29,720 But Jupiter had the help of that extra icy ingredient. 295 00:18:32,160 --> 00:18:33,760 So, we think that the key ingredient 296 00:18:33,760 --> 00:18:36,120 that allowed Jupiter and Saturn to form so fast, 297 00:18:36,120 --> 00:18:37,640 compared to the rocky planets, 298 00:18:37,640 --> 00:18:40,240 is that they formed far enough from the sun 299 00:18:40,240 --> 00:18:43,880 that water could condense from the gas around the sun 300 00:18:43,880 --> 00:18:47,000 and form ice, and increase the density of material 301 00:18:47,000 --> 00:18:51,040 and give you more material to build a larger, rockier core faster. 302 00:18:54,200 --> 00:18:57,880 That could explain how Jupiter built a rocky core so quickly. 303 00:18:59,640 --> 00:19:03,000 But it doesn't explain why it grew where it did. 304 00:19:04,880 --> 00:19:06,320 It's not unreasonable to think 305 00:19:06,320 --> 00:19:09,480 it would form at the place with the most ice. 306 00:19:11,240 --> 00:19:14,360 That's a place called the ice line. 307 00:19:14,360 --> 00:19:16,440 But it's not where Jupiter is today. 308 00:19:18,520 --> 00:19:21,960 So, right now when we look at our solar system, we look at Jupiter 309 00:19:21,960 --> 00:19:25,360 and it's beyond the ice line by a fair bit, 310 00:19:25,360 --> 00:19:28,320 whereas we think it was really advantageous to form Jupiter 311 00:19:28,320 --> 00:19:30,440 right at the ice line. 312 00:19:30,440 --> 00:19:32,400 So already that's suspicious. 313 00:19:37,280 --> 00:19:40,440 If Jupiter was built from a collapsing cloud, 314 00:19:40,440 --> 00:19:42,320 we'd expect it to be further out. 315 00:19:44,040 --> 00:19:48,080 If, on the other hand, it was built from a rocky core, 316 00:19:48,080 --> 00:19:50,200 we'd expect it to be closer to the sun. 317 00:19:51,600 --> 00:19:54,400 But it's not in either of these two places. 318 00:19:54,400 --> 00:19:56,880 So the big question is, 319 00:19:56,880 --> 00:19:59,240 is Jupiter in the wrong place? 320 00:20:08,160 --> 00:20:12,080 To even ask that question has, until recently, been a heresy. 321 00:20:14,760 --> 00:20:19,280 At the historic Chamberlin Telescope in Denver, Colorado, 322 00:20:19,280 --> 00:20:21,560 Kevin Walsh is following in the footsteps 323 00:20:21,560 --> 00:20:23,200 of some famous astronomers. 324 00:20:29,840 --> 00:20:32,320 He's taking a closer look at Jupiter. 325 00:20:34,240 --> 00:20:36,400 You can see Jupiter with the naked eye, 326 00:20:36,400 --> 00:20:39,040 but looking at it through a telescope like this 327 00:20:39,040 --> 00:20:40,520 makes it a lot more fun. 328 00:20:41,880 --> 00:20:45,040 The bands of colour are really clear and crisp 329 00:20:45,040 --> 00:20:46,960 and the moons are real bright. 330 00:20:46,960 --> 00:20:50,600 It comes alive. It becomes a real planet when you look at it through a telescope. 331 00:20:52,760 --> 00:20:56,120 Galileo was the first astronomer to point a telescope at Jupiter, 332 00:20:56,120 --> 00:20:59,280 more than 400 years ago, 333 00:20:59,280 --> 00:21:02,400 and no-one ever questioned that Jupiter will always be, 334 00:21:02,400 --> 00:21:04,960 and has always been, in that same orbit. 335 00:21:06,440 --> 00:21:07,760 Jupiter, right now... 336 00:21:09,120 --> 00:21:11,600 ..looks the same as it would have looked for Galileo. 337 00:21:11,600 --> 00:21:14,400 It's a little bigger and brighter through this great telescope, 338 00:21:14,400 --> 00:21:15,760 but it's the same Jupiter, 339 00:21:15,760 --> 00:21:18,880 so if I came back tomorrow night, it'd look the same. 340 00:21:18,880 --> 00:21:20,280 So that's the view - 341 00:21:20,280 --> 00:21:23,960 the planets that we look at now seem like they never change. 342 00:21:23,960 --> 00:21:25,480 And why would they change? 343 00:21:27,360 --> 00:21:30,120 This was the bedrock of our understanding - 344 00:21:30,120 --> 00:21:32,200 that the planets' orbits are fixed. 345 00:21:37,320 --> 00:21:41,000 The first hint of something odd came 35 years ago 346 00:21:41,000 --> 00:21:44,840 from astronomers trying to calculate how the solar system formed. 347 00:21:46,960 --> 00:21:49,960 They kept getting a strange result. 348 00:21:49,960 --> 00:21:53,280 Some of those calculations were suggesting that it was possible 349 00:21:53,280 --> 00:21:56,240 that a planet like Jupiter could have been moved around. 350 00:21:57,560 --> 00:22:01,320 It was a result so crazy that it was totally ignored. 351 00:22:01,320 --> 00:22:03,720 So if you built a model to try to understand 352 00:22:03,720 --> 00:22:05,640 some of the events of the early solar system 353 00:22:05,640 --> 00:22:09,120 and your model is telling you that planets could have migrated or moved, 354 00:22:09,120 --> 00:22:10,800 that Jupiter could have moved, 355 00:22:10,800 --> 00:22:13,800 then it was telling you that you probably made a mistake. 356 00:22:13,800 --> 00:22:17,400 So the idea of planet migration, it was just never possible. 357 00:22:17,400 --> 00:22:18,840 Just didn't seem possible. 358 00:22:23,800 --> 00:22:29,000 But in 1995, astronomers were forced to face up to the impossible. 359 00:22:31,440 --> 00:22:35,440 The mystery began to unravel when dramatic evidence was uncovered 360 00:22:35,440 --> 00:22:37,760 from somewhere completely unexpected. 361 00:22:54,560 --> 00:22:56,520 Astronomer Chris Watson 362 00:22:56,520 --> 00:22:59,400 is searching for the weirdest places in the galaxy. 363 00:23:01,320 --> 00:23:05,200 He's a planet hunter, one of a growing band of astronomers 364 00:23:05,200 --> 00:23:08,560 involved in the hunt for exoplanets - 365 00:23:08,560 --> 00:23:11,480 alien worlds circling around other stars. 366 00:23:13,280 --> 00:23:15,840 It's one of the hottest fields in astronomy. 367 00:23:17,480 --> 00:23:22,320 Up until now, or until recently, we've only had one planetary system 368 00:23:22,320 --> 00:23:24,960 that we could study, and that was the solar system, 369 00:23:24,960 --> 00:23:26,920 the planets around the sun. 370 00:23:26,920 --> 00:23:29,760 And there are about 100 billion stars in our galaxy 371 00:23:29,760 --> 00:23:32,400 and there are about 100 billion galaxies in the universe, 372 00:23:32,400 --> 00:23:34,320 and we could only study one. 373 00:23:36,000 --> 00:23:39,640 But the study of planets and planetary systems exploded 374 00:23:39,640 --> 00:23:42,480 with the amazing discovery in 1995 375 00:23:42,480 --> 00:23:45,600 of a planet orbiting around another star. 376 00:23:47,280 --> 00:23:50,320 Less than 20 years ago, we first found another planet 377 00:23:50,320 --> 00:23:52,760 around another star that's like our sun 378 00:23:52,760 --> 00:23:55,480 and that was a dramatic breakthrough. 379 00:23:55,480 --> 00:23:59,080 And we now know of over 1,000 planets. 380 00:23:59,080 --> 00:24:00,720 And they're very strange - 381 00:24:00,720 --> 00:24:03,160 these are nothing like our solar system 382 00:24:03,160 --> 00:24:06,640 and, in some cases, I think really science fact 383 00:24:06,640 --> 00:24:09,080 could be a lot weirder than science fiction. 384 00:24:11,680 --> 00:24:15,840 The planets they have been finding look stranger than anyone imagined. 385 00:24:21,680 --> 00:24:24,080 We've found planets around binary stars, 386 00:24:24,080 --> 00:24:27,240 where you actually have two stars orbiting each other, 387 00:24:27,240 --> 00:24:29,840 a bit like Tatooine off of Star Wars. 388 00:24:29,840 --> 00:24:33,600 That would be a magical world if you had a habitable planet there, 389 00:24:33,600 --> 00:24:35,280 because you would imagine 390 00:24:35,280 --> 00:24:37,600 you'd actually have two shadows on everything 391 00:24:37,600 --> 00:24:39,880 and two stars in the sky, as well. 392 00:24:46,960 --> 00:24:50,960 None of the planets found have been remotely like home. 393 00:24:52,640 --> 00:24:56,440 Recently, a planet was discovered a little bit bigger than Earth, 394 00:24:56,440 --> 00:24:58,960 but incredibly close to its star. 395 00:24:58,960 --> 00:25:02,160 It's probably a rocky world, but it's so hot 396 00:25:02,160 --> 00:25:04,160 it would actually be molten, 397 00:25:04,160 --> 00:25:07,520 so...it'd be this, but the actual lava on it. 398 00:25:08,840 --> 00:25:12,720 And most puzzling of all are the largest planets, 399 00:25:12,720 --> 00:25:14,880 so weird as to seem impossible. 400 00:25:16,280 --> 00:25:19,480 What was strange when we first discovered these planets 401 00:25:19,480 --> 00:25:21,480 is they were massive worlds, 402 00:25:21,480 --> 00:25:24,640 they were gas giants much like Jupiter. 403 00:25:24,640 --> 00:25:27,680 But these were much, much closer to their parent stars. 404 00:25:30,720 --> 00:25:35,320 Jupiter lives out in the cold outer reaches of our solar system, 405 00:25:35,320 --> 00:25:37,280 taking 12 years to orbit the sun. 406 00:25:38,440 --> 00:25:42,000 But these alien giants were found in the fiery heat, 407 00:25:42,000 --> 00:25:43,960 right next to their star, 408 00:25:43,960 --> 00:25:47,120 hurtling around in crazy orbits of just a few days. 409 00:25:48,800 --> 00:25:51,480 They were nicknamed "hot Jupiters". 410 00:25:55,240 --> 00:25:57,360 They're right up against the host star, 411 00:25:57,360 --> 00:25:59,400 and it's amazing, they really... 412 00:25:59,400 --> 00:26:01,520 At the time, we thought, "How did they get there? 413 00:26:01,520 --> 00:26:03,520 "They really shouldn't be there." 414 00:26:03,520 --> 00:26:06,160 Other scientists were thinking, "Well, you're a bit crazy, 415 00:26:06,160 --> 00:26:09,520 "these Jupiters should not be that close to the star." 416 00:26:12,360 --> 00:26:15,520 Everyone was baffled by the existence of hot Jupiters. 417 00:26:15,520 --> 00:26:19,440 They were planets that, quite simply, shouldn't exist. 418 00:26:22,520 --> 00:26:26,240 In theory, the only place you could build a gas giant 419 00:26:26,240 --> 00:26:30,280 would be out in the cold, far away from a star, 420 00:26:30,280 --> 00:26:32,040 because that's the only place 421 00:26:32,040 --> 00:26:34,440 you can find the necessary ingredients. 422 00:26:36,600 --> 00:26:39,840 What I'm holding in my hand is a lump of dry ice, 423 00:26:39,840 --> 00:26:42,600 and this represents the building blocks 424 00:26:42,600 --> 00:26:44,680 of planets like Jupiter. 425 00:26:44,680 --> 00:26:48,280 And this is fine - it's quite happy out here, 426 00:26:48,280 --> 00:26:51,880 far away from the fire that represents our sun 427 00:26:51,880 --> 00:26:55,800 or any other star that one of these gas giants might be forming round. 428 00:26:55,800 --> 00:27:01,040 But look at what happens when I bring it closer to the fire. 429 00:27:04,200 --> 00:27:07,560 Too close to the star and the ice just turns to gas. 430 00:27:08,680 --> 00:27:12,600 And without ice you can't build a gas giant. 431 00:27:12,600 --> 00:27:16,840 So there's very little left after just a few minutes. 432 00:27:16,840 --> 00:27:19,200 And what this means 433 00:27:19,200 --> 00:27:24,200 is that gas giants can't form close to the star. 434 00:27:24,200 --> 00:27:28,040 The building blocks just cannot exist that close. 435 00:27:28,040 --> 00:27:30,160 They have to have formed further away 436 00:27:30,160 --> 00:27:32,880 where the raw materials can exist. 437 00:27:35,400 --> 00:27:39,560 If these hot Jupiters couldn't have formed where we find them, 438 00:27:39,560 --> 00:27:41,960 it could only mean one thing. 439 00:27:41,960 --> 00:27:44,360 So we think that, in actual fact, 440 00:27:44,360 --> 00:27:48,000 these gas giants form further out, 441 00:27:48,000 --> 00:27:50,680 then they actually move towards the star, 442 00:27:50,680 --> 00:27:53,120 they actually migrate inwards. 443 00:27:56,920 --> 00:27:59,480 The planets are on the move. 444 00:28:02,960 --> 00:28:05,320 The discovery that planets could change orbit 445 00:28:05,320 --> 00:28:07,960 was a shocking revelation. 446 00:28:07,960 --> 00:28:11,000 It turned the world of planetary science on its head. 447 00:28:12,800 --> 00:28:15,280 The implications of a planet the size of Jupiter 448 00:28:15,280 --> 00:28:18,400 roaming freely around a planetary system 449 00:28:18,400 --> 00:28:20,080 could be devastating. 450 00:28:23,960 --> 00:28:28,120 Over recent years, the search for exoplanets has exploded. 451 00:28:30,200 --> 00:28:33,960 So here we are, nearly 2,400 metres up 452 00:28:33,960 --> 00:28:37,360 on the volcanic island of La Palma. 453 00:28:37,360 --> 00:28:42,080 What you can see before you are suites of professional telescopes 454 00:28:42,080 --> 00:28:46,000 and what we're going to do is we're going to use one of these 455 00:28:46,000 --> 00:28:50,360 telescopes to actually look at planets orbiting another star. 456 00:28:51,760 --> 00:28:54,600 So, this cloud can be a bit of a problem. 457 00:28:54,600 --> 00:28:57,840 Normally it's not so cloudy, but we are in the depths of winter 458 00:28:57,840 --> 00:28:59,880 and this is actually quite local cloud. 459 00:29:01,280 --> 00:29:06,520 20 years ago, all these telescopes were busy looking at stars. 460 00:29:06,520 --> 00:29:10,440 Now, increasingly, many are focusing on planets. 461 00:29:11,600 --> 00:29:16,400 There's quite a few telescopes here, and probably every night 462 00:29:16,400 --> 00:29:20,480 there's some project related to extrasolar planets going on. 463 00:29:20,480 --> 00:29:25,000 It is a really rich, blossoming field of astronomy. 464 00:29:29,960 --> 00:29:32,240 And, provided the clouds clear, 465 00:29:32,240 --> 00:29:35,040 tonight Chris will be pointing his telescope 466 00:29:35,040 --> 00:29:37,520 at an exoplanet called WASP-84 b. 467 00:29:39,840 --> 00:29:41,280 These clouds will clear. 468 00:29:47,400 --> 00:29:49,440 But, even with clear skies, 469 00:29:49,440 --> 00:29:52,280 spotting alien planets is no easy matter. 470 00:30:04,320 --> 00:30:09,160 To see other planets in our solar system from Earth is pretty easy. 471 00:30:09,160 --> 00:30:11,520 So this candle represents our sun 472 00:30:11,520 --> 00:30:15,080 and if I pop down this little rock, representing a planet, 473 00:30:15,080 --> 00:30:17,800 you can clearly see the reflected sunlight. 474 00:30:19,040 --> 00:30:22,040 But even the nearest stars are so far away 475 00:30:22,040 --> 00:30:25,440 that the reflected light from the planets gets completely lost. 476 00:30:27,760 --> 00:30:30,480 So, now we have our star, much further away, 477 00:30:30,480 --> 00:30:32,960 and if I put my planet down, 478 00:30:32,960 --> 00:30:35,440 while it's still reflecting the starlight, 479 00:30:35,440 --> 00:30:36,920 because you're so far away, 480 00:30:36,920 --> 00:30:39,440 the reflected light is actually drowned out 481 00:30:39,440 --> 00:30:41,240 in the glare of the star itself. 482 00:30:45,200 --> 00:30:47,200 Because the planets are so hard to see, 483 00:30:47,200 --> 00:30:50,520 astronomers have found other ways to detect them. 484 00:30:50,520 --> 00:30:54,640 One of the best ways is actually to watch and see 485 00:30:54,640 --> 00:30:58,440 if the planet actually crosses in front of the star. 486 00:30:58,440 --> 00:31:03,360 So, if we were an alien civilisation looking back at our solar system, 487 00:31:03,360 --> 00:31:07,960 we happen to catch Jupiter transiting the face of our sun, 488 00:31:07,960 --> 00:31:11,280 we would see a 1% dip in the sunlight. 489 00:31:11,280 --> 00:31:15,920 For a planet a lot smaller, like the Earth, 490 00:31:15,920 --> 00:31:19,680 that dip is much, much smaller - it's minuscule. 491 00:31:19,680 --> 00:31:22,720 And that's why it's so, so difficult to detect these. 492 00:31:28,000 --> 00:31:30,720 But techniques have improved dramatically 493 00:31:30,720 --> 00:31:33,720 and now, for astronomers like Chris Watson, 494 00:31:33,720 --> 00:31:36,720 planet-hunting is all part of a night's work. 495 00:31:44,760 --> 00:31:48,640 This is our telescope, Telescopio Nazionale Galileo, 496 00:31:48,640 --> 00:31:50,840 and this will be our baby for the night. 497 00:31:54,760 --> 00:31:56,840 The skies are clearing beautifully, 498 00:31:56,840 --> 00:31:59,600 so I think we're in for a really nice night ahead. 499 00:32:01,840 --> 00:32:04,040 Thanks to ground-based telescopes like this, 500 00:32:04,040 --> 00:32:07,920 as well as space telescopes like NASA's Kepler mission, 501 00:32:07,920 --> 00:32:10,320 thousands of planets have now been found. 502 00:32:12,120 --> 00:32:15,800 And not just planets, but entire planetary systems. 503 00:32:23,160 --> 00:32:25,080 So this is the Kepler Orrery, 504 00:32:25,080 --> 00:32:29,960 which shows the orbits and the sizes of planets. 505 00:32:29,960 --> 00:32:33,640 So these are candidates that the Kepler space mission has found. 506 00:32:33,640 --> 00:32:36,520 So these are transiting planets. 507 00:32:36,520 --> 00:32:39,840 They don't, however, look much like we'd expect. 508 00:32:39,840 --> 00:32:41,480 And up there, on the top left, 509 00:32:41,480 --> 00:32:47,440 you can see the orbits of the four innermost planets of our solar system 510 00:32:47,440 --> 00:32:49,760 from Mercury out to Mars. 511 00:32:49,760 --> 00:32:54,120 What you can see is the huge diversity 512 00:32:54,120 --> 00:32:56,360 of all the different planetary systems. 513 00:32:59,400 --> 00:33:03,120 Each set of rings shows a different planetary system 514 00:33:03,120 --> 00:33:06,640 and each blob, a different planet, with its size and orbit. 515 00:33:08,360 --> 00:33:10,600 They break every rule in the book 516 00:33:10,600 --> 00:33:12,920 and make us look like the odd one out. 517 00:33:14,520 --> 00:33:18,560 So we have large gas giant planets in there, 518 00:33:18,560 --> 00:33:22,360 and then you can see the really short period, 519 00:33:22,360 --> 00:33:25,000 really weird solar systems. 520 00:33:26,320 --> 00:33:29,480 They really don't look anything like our own solar system. 521 00:33:29,480 --> 00:33:35,120 Some of these planets actually have orbits of just a few hours. 522 00:33:36,560 --> 00:33:40,720 There's even systems spiralling around multiple planets in here. 523 00:33:43,320 --> 00:33:45,800 That one's weird. What's going on here? 524 00:33:57,640 --> 00:34:01,880 Who knows what we might discover in this rich smorgasbord of planets? 525 00:34:04,120 --> 00:34:05,760 It is ridiculous, actually. 526 00:34:05,760 --> 00:34:07,680 HE CHUCKLES 527 00:34:07,680 --> 00:34:09,200 What is going on with that? 528 00:34:12,000 --> 00:34:13,400 Extraordinary worlds. 529 00:34:16,240 --> 00:34:18,720 Some may host life. 530 00:34:21,520 --> 00:34:23,720 Our exploration of these alien worlds 531 00:34:23,720 --> 00:34:26,280 is only just beginning, 532 00:34:26,280 --> 00:34:29,880 but already they're revealing some incredible secrets. 533 00:34:36,200 --> 00:34:39,720 Tonight, Chris and his team are training the telescope 534 00:34:39,720 --> 00:34:42,520 on a star they known has a hot Jupiter orbiting it. 535 00:34:42,520 --> 00:34:45,520 INDISTINCT CONVERSATION 536 00:34:48,240 --> 00:34:51,080 They hope to reveal just how devastating 537 00:34:51,080 --> 00:34:53,080 a migrating gas giant could be. 538 00:34:55,160 --> 00:34:58,320 So this star that we're looking at, WASP-84, 539 00:34:58,320 --> 00:35:01,600 was actually discovered to have a transiting planet around it. 540 00:35:01,600 --> 00:35:04,040 That transiting planet, we know at the moment, 541 00:35:04,040 --> 00:35:08,280 is about a little bit less massive than Jupiter. 542 00:35:08,280 --> 00:35:10,440 And we know its orbital period, 543 00:35:10,440 --> 00:35:12,720 so its year is about eight-and-a-half days, 544 00:35:12,720 --> 00:35:16,680 and we're going to follow it as it transits the star. 545 00:35:16,680 --> 00:35:19,240 A planet the size of Jupiter 546 00:35:19,240 --> 00:35:22,000 orbiting its star once every eight days 547 00:35:22,000 --> 00:35:24,080 is already pretty weird. 548 00:35:24,080 --> 00:35:28,960 But some of these alien worlds have even weirder orbits than that. 549 00:35:28,960 --> 00:35:32,320 What's the air mass with that, then? About 1.25? 550 00:35:32,320 --> 00:35:35,760 'We would expect the planet and the star' 551 00:35:35,760 --> 00:35:37,680 to be spinning in the same way. 552 00:35:37,680 --> 00:35:41,400 But we see quite a few systems where that is just not the case. 553 00:35:42,560 --> 00:35:46,000 Some of these planets are going completely the wrong way. 554 00:35:47,120 --> 00:35:49,440 If the star is spinning clockwise, 555 00:35:49,440 --> 00:35:52,040 the planet is spinning anti-clockwise. 556 00:35:52,040 --> 00:35:55,240 169, we're talking about. Yeah, it's about... 557 00:35:55,240 --> 00:35:57,520 A planet orbiting in the wrong direction 558 00:35:57,520 --> 00:35:59,920 is a sign of some truly cataclysmic event. 559 00:36:01,120 --> 00:36:04,680 And tonight, as it passes in front of its star, 560 00:36:04,680 --> 00:36:07,640 Chris will be able to analyse the orbit of WASP-84 b. 561 00:36:08,640 --> 00:36:11,160 'The purpose of these observations is actually' 562 00:36:11,160 --> 00:36:15,000 to see whether we have a nicely aligned system - 563 00:36:15,000 --> 00:36:17,800 a bit like the planets we have in our solar system, 564 00:36:17,800 --> 00:36:22,920 where the star spins in the same direction as the planet orbits. 565 00:36:22,920 --> 00:36:26,400 Or do we have something that would be the smoking gun 566 00:36:26,400 --> 00:36:29,240 of a really violent interaction 567 00:36:29,240 --> 00:36:34,280 which has maybe scattered that planet into one of these weird orbits? 568 00:36:34,280 --> 00:36:35,920 So, perhaps over the poles, 569 00:36:35,920 --> 00:36:40,160 or actually spinning in the opposite direction to that of the star. 570 00:36:41,920 --> 00:36:43,840 But the big question is 571 00:36:43,840 --> 00:36:46,840 what could be the cause of such planetary upheaval? 572 00:36:46,840 --> 00:36:48,000 Whoa! 573 00:36:52,280 --> 00:36:54,760 After following the transit through the night, 574 00:36:54,760 --> 00:36:58,800 Chris has the verdict on Planet WASP-84 b. 575 00:37:00,400 --> 00:37:04,960 So, the transit's finished. We've had a quick look at the data 576 00:37:04,960 --> 00:37:09,360 and what we've found has actually taken us a bit by surprise. 577 00:37:09,360 --> 00:37:12,640 We thought that this planet system would be misaligned. 578 00:37:12,640 --> 00:37:14,560 Now that we've had a look at the data, 579 00:37:14,560 --> 00:37:16,440 it looks as though it's actually aligned. 580 00:37:18,000 --> 00:37:21,800 WASP-84 b turns out to be orbiting the right way. 581 00:37:23,280 --> 00:37:25,880 But Chris has found many of these hot Jupiters 582 00:37:25,880 --> 00:37:28,640 that are travelling in completely the wrong direction. 583 00:37:30,320 --> 00:37:34,720 It's evidence of how, in migrating, they must have caused havoc. 584 00:37:36,320 --> 00:37:38,200 With these very strange orbits, 585 00:37:38,200 --> 00:37:42,320 it looks as though it's been a very violent process. 586 00:37:42,320 --> 00:37:44,960 To actually take one of these planets 587 00:37:44,960 --> 00:37:48,160 and just chuck it into a different orbit, 588 00:37:48,160 --> 00:37:50,000 that's very violent. 589 00:37:50,000 --> 00:37:54,720 One of the easiest ways to do that is to have a collision. 590 00:37:54,720 --> 00:37:58,640 Take two planets, interaction between them, 591 00:37:58,640 --> 00:38:00,760 and you can eject one planet 592 00:38:00,760 --> 00:38:04,320 and fling the other planet really close in to the star. 593 00:38:09,960 --> 00:38:13,200 These giant gas planets are the bully of the playground. 594 00:38:14,440 --> 00:38:17,160 They have the power to throw other planets around 595 00:38:17,160 --> 00:38:19,040 like a game of cosmic pinball. 596 00:38:23,080 --> 00:38:26,280 Beasts the size of Jupiter are so vast 597 00:38:26,280 --> 00:38:28,960 they can eject entire planets from the system. 598 00:38:32,960 --> 00:38:35,960 They can launch them into crazy polar orbits. 599 00:38:39,200 --> 00:38:42,320 They even have the power to destroy entire worlds. 600 00:38:49,320 --> 00:38:53,120 A planet like Jupiter, the mass of Jupiter, the size of it, 601 00:38:53,120 --> 00:38:55,560 just dominates planetary systems, 602 00:38:55,560 --> 00:38:58,600 and it's got the power to really decide 603 00:38:58,600 --> 00:39:01,040 the fate of the other planets. 604 00:39:03,880 --> 00:39:07,880 I think we'd be quite glad there's not a hot Jupiter in our system. 605 00:39:07,880 --> 00:39:09,520 We wouldn't be seeing this. 606 00:39:18,160 --> 00:39:21,360 We've discovered other systems where planets migrate 607 00:39:21,360 --> 00:39:23,280 and hot Jupiters cause havoc. 608 00:39:24,880 --> 00:39:26,960 But what about our own solar system? 609 00:39:28,440 --> 00:39:33,160 Our planets certainly seem fixed in their rigid, clockwork orbits. 610 00:39:43,880 --> 00:39:49,000 Our earth has been the same distance from the sun for 4.5 billion years. 611 00:39:50,000 --> 00:39:52,120 Long enough to create an atmosphere, 612 00:39:52,120 --> 00:39:54,080 build mountains, 613 00:39:54,080 --> 00:39:55,560 and for life to evolve. 614 00:39:57,560 --> 00:40:01,040 But the evidence from other planetary systems now means 615 00:40:01,040 --> 00:40:05,880 a complete rethink on how and where our planets formed. 616 00:40:05,880 --> 00:40:09,080 When we started discovering planets around other stars, 617 00:40:09,080 --> 00:40:12,760 we started finding planets in completely unexpected places, 618 00:40:12,760 --> 00:40:15,520 places we never thought could possibly form a planet. 619 00:40:15,520 --> 00:40:18,120 We had to go back to the drawing board and say, 620 00:40:18,120 --> 00:40:19,560 "Wow, planets can move. 621 00:40:19,560 --> 00:40:22,520 "Planets can really move. Maybe that happened here." 622 00:40:26,520 --> 00:40:29,160 It's a big leap, and to make that leap 623 00:40:29,160 --> 00:40:33,040 and say things might have been completely unstable, 624 00:40:33,040 --> 00:40:38,200 totally chaotic for a time period, that's really hard to imagine. 625 00:40:38,200 --> 00:40:41,080 But that's the leap that we need to take. 626 00:40:43,880 --> 00:40:47,600 The crazy results that suggest Jupiter might have changed orbit 627 00:40:47,600 --> 00:40:50,400 might not be mistakes after all. 628 00:40:51,960 --> 00:40:55,440 Instead, migration could be the key that unlocks 629 00:40:55,440 --> 00:40:58,680 many of the mysteries of how our solar system came to be. 630 00:40:58,680 --> 00:41:01,640 Now that we've taken this tool of planetary migration that we started 631 00:41:01,640 --> 00:41:04,240 to understand by looking at planets around other stars, 632 00:41:04,240 --> 00:41:07,400 we've realised that it's absolutely critical to understand 633 00:41:07,400 --> 00:41:10,200 how our solar system formed and evolved. 634 00:41:10,200 --> 00:41:13,640 And central to it all is mighty Jupiter. 635 00:41:13,640 --> 00:41:16,560 Certainly in our planetary system, Jupiter is the key. 636 00:41:16,560 --> 00:41:19,600 It's over three hundred times more massive than the Earth, 637 00:41:19,600 --> 00:41:22,040 so Jupiter wins. 638 00:41:22,040 --> 00:41:23,840 Jupiter decides what happens. 639 00:41:26,560 --> 00:41:31,000 The inescapable truth seems to be that planets move. 640 00:41:31,000 --> 00:41:34,360 And, if it can happen in exoplanetary systems, 641 00:41:34,360 --> 00:41:36,360 it can happen in ours. 642 00:41:36,360 --> 00:41:38,520 If we want to make a model that explains 643 00:41:38,520 --> 00:41:40,560 how our solar system came to be, 644 00:41:40,560 --> 00:41:44,200 we have to break the brass rods and set the planets free. 645 00:41:48,880 --> 00:41:53,000 Once we accept the idea that the planets can move, 646 00:41:53,000 --> 00:41:57,360 we can begin to explain some of the unsolved mysteries of the solar system. 647 00:42:02,880 --> 00:42:06,400 In particular, why Mars is so small 648 00:42:06,400 --> 00:42:09,720 and the curious composition of the asteroid belt. 649 00:42:28,600 --> 00:42:32,000 Kevin Walsh has developed a model of the early solar system 650 00:42:32,000 --> 00:42:34,520 that involves a wild dance of the planets. 651 00:42:38,280 --> 00:42:40,880 It's an intricate and chaotic dance, 652 00:42:40,880 --> 00:42:43,240 and if it had gone slightly differently 653 00:42:43,240 --> 00:42:46,560 it could have stopped our developing solar system in its tracks. 654 00:42:48,760 --> 00:42:52,640 In his model, Jupiter takes a wild ride through the solar system. 655 00:42:58,520 --> 00:43:01,000 It takes us right back to the moment of birth, 656 00:43:01,000 --> 00:43:04,760 when Jupiter had just formed from the cloud of gas. 657 00:43:06,120 --> 00:43:08,320 The key is that, though Jupiter is really big, 658 00:43:08,320 --> 00:43:10,040 it's 300 times the mass of the earth, 659 00:43:10,040 --> 00:43:13,080 the gas disc around the sun was much more massive, 660 00:43:13,080 --> 00:43:16,240 so the gas can actually push Jupiter in towards the sun. 661 00:43:17,520 --> 00:43:22,040 As soon as it was born, Jupiter began to migrate inwards. 662 00:43:22,040 --> 00:43:25,160 Over the course of half a million years, 663 00:43:25,160 --> 00:43:27,160 it spiralled in towards the sun. 664 00:43:28,880 --> 00:43:31,280 It was on its way to becoming a hot Jupiter. 665 00:43:33,360 --> 00:43:36,160 So, the idea that you could form something as big as Jupiter 666 00:43:36,160 --> 00:43:38,360 and have it pushed inward by the gas disc 667 00:43:38,360 --> 00:43:40,400 actually makes a fair amount of sense, 668 00:43:40,400 --> 00:43:42,400 because we see it, we see it all over. 669 00:43:46,120 --> 00:43:49,640 But something stopped Jupiter from crashing into the sun 670 00:43:49,640 --> 00:43:51,520 or ending up as a hot Jupiter. 671 00:43:52,760 --> 00:43:55,400 So, if it formed and started migrating inwards, 672 00:43:55,400 --> 00:43:57,440 there must have been a mechanism to stop it 673 00:43:57,440 --> 00:44:00,120 and bring it back out to the outer part of the solar system. 674 00:44:00,120 --> 00:44:03,560 We think the key to stop its inward migration, to keep it 675 00:44:03,560 --> 00:44:08,200 from going all the way in towards the sun is the presence of Saturn. 676 00:44:08,200 --> 00:44:12,160 While Jupiter was on its wild ride, Saturn was born. 677 00:44:12,160 --> 00:44:16,160 Saturn is also growing. It's going through the same process Jupiter did. 678 00:44:16,160 --> 00:44:19,240 It's building a big core and it's getting really massive, 679 00:44:19,240 --> 00:44:23,440 and once it gets really massive as well it can move in the disc also. 680 00:44:23,440 --> 00:44:26,960 And it too began spiralling in towards the sun. 681 00:44:26,960 --> 00:44:30,280 So as Saturn is racing inwards, it gets very close to Jupiter 682 00:44:30,280 --> 00:44:33,280 and they actually get close enough that they get locked in a resonance 683 00:44:33,280 --> 00:44:35,760 where their orbital periods are closely aligned 684 00:44:35,760 --> 00:44:38,960 and they interact very closely gravitationally. 685 00:44:38,960 --> 00:44:41,200 Now, when these two get really close 686 00:44:41,200 --> 00:44:44,240 it actually stops Jupiter's inward migration. 687 00:44:44,240 --> 00:44:48,480 The two planets were involved in a kind of gravitational dance. 688 00:44:48,480 --> 00:44:52,360 And, as they came close, Jupiter changed direction 689 00:44:52,360 --> 00:44:54,800 and was flung back to the outer solar system... 690 00:44:56,360 --> 00:44:59,760 ..just like a sailing ship changing course in a grand tack. 691 00:45:03,680 --> 00:45:06,560 So this theory called the grand tack is called that 692 00:45:06,560 --> 00:45:09,560 because our planets are moving inwards 693 00:45:09,560 --> 00:45:12,400 and they get really close and they stop and they turn and they go back 694 00:45:12,400 --> 00:45:15,240 outwards, and it's kind of like a sailboat tacking across the wind. 695 00:45:15,240 --> 00:45:19,080 Jupiter's wild ride could explain two key mysteries - 696 00:45:19,080 --> 00:45:22,400 first, why Mars is so small. 697 00:45:22,400 --> 00:45:25,440 So, when Jupiter migrates inwards 698 00:45:25,440 --> 00:45:29,000 it kind of snowploughs all the rocky material it sees, 699 00:45:29,000 --> 00:45:31,000 it snowploughs it and pushes it inwards. 700 00:45:32,680 --> 00:45:34,920 Much of the dust and rocky debris 701 00:45:34,920 --> 00:45:38,600 that would have gone on to build Mars got pushed out of the way. 702 00:45:39,840 --> 00:45:42,640 So, by Jupiter coming in and clearing out all of this 703 00:45:42,640 --> 00:45:45,760 material on its way, it kind of reduces the total 704 00:45:45,760 --> 00:45:49,440 amount of material that Mars can feed on to grow, 705 00:45:49,440 --> 00:45:52,400 and so Mars ends up kind of being starved of rocky material 706 00:45:52,400 --> 00:45:55,240 and only grows to be a tenth the mass of the earth. 707 00:45:57,640 --> 00:46:02,280 And this explains why Mars is the planetary runt we see today. 708 00:46:06,480 --> 00:46:10,840 The theory also explains why the asteroid belt has an icy ring 709 00:46:10,840 --> 00:46:13,000 and a rocky ring so close together. 710 00:46:22,200 --> 00:46:25,840 During its travels, Jupiter scattered everything in its path. 711 00:46:28,200 --> 00:46:31,920 It threw rocks from the inner part of the solar system outwards... 712 00:46:35,640 --> 00:46:38,600 ..and ice from the outer reaches inwards... 713 00:46:40,240 --> 00:46:42,880 ..leaving the two distinct bands we see today. 714 00:46:44,400 --> 00:46:47,080 That's how we end up with two different types of material 715 00:46:47,080 --> 00:46:49,960 sitting on top of each other in the middle of the asteroid belt 716 00:46:49,960 --> 00:46:51,440 in a very small region. 717 00:46:51,440 --> 00:46:55,320 So Jupiter's wild ride could explain two key mysteries - 718 00:46:55,320 --> 00:46:58,840 the size of Mars and the composition of the asteroid belt. 719 00:47:00,840 --> 00:47:03,800 And if it had travelled any further in 720 00:47:03,800 --> 00:47:07,080 the earth itself may have become a very different type of planet. 721 00:47:14,640 --> 00:47:16,440 But the birth of the solar system 722 00:47:16,440 --> 00:47:19,200 wasn't the only turbulent time in its history. 723 00:47:21,400 --> 00:47:25,280 About 500 million years later, 4 billion years ago, 724 00:47:25,280 --> 00:47:28,760 the solar system entered its teenage years - 725 00:47:28,760 --> 00:47:32,960 an intense period of trouble, chaos and uncertainty. 726 00:47:34,960 --> 00:47:37,520 It's a period of turbulence that could explain 727 00:47:37,520 --> 00:47:39,000 two further mysteries... 728 00:47:41,320 --> 00:47:43,360 ..the craters on the moon 729 00:47:43,360 --> 00:47:45,880 and the birth of Uranus and Neptune. 730 00:47:51,360 --> 00:47:54,480 There are some mysteries when we look around the solar system, 731 00:47:54,480 --> 00:47:57,880 where the theories really don't match what we see. 732 00:47:59,440 --> 00:48:02,280 If we just take a bunch of small icy objects 733 00:48:02,280 --> 00:48:05,080 from which Uranus and Neptune were made, 734 00:48:05,080 --> 00:48:08,760 put them out in the outer part of the solar system in our computer 735 00:48:08,760 --> 00:48:13,280 models and watch how they grow, it turns out they can't grow at all. 736 00:48:15,600 --> 00:48:19,800 A couple of billion miles away from these ice giants, on the moon, 737 00:48:19,800 --> 00:48:21,920 there's a clue that Hal Levison believes 738 00:48:21,920 --> 00:48:24,160 could help solve the puzzle. 739 00:48:24,160 --> 00:48:27,720 In fact, the moon is covered in clues. 740 00:48:28,920 --> 00:48:30,480 So, when you look at the moon, 741 00:48:30,480 --> 00:48:34,760 some of these biggest crater impact basins, like here and here, 742 00:48:34,760 --> 00:48:37,280 all formed in a very short period of time 743 00:48:37,280 --> 00:48:40,520 that we call the Late Heavy Bombardment. 744 00:48:40,520 --> 00:48:45,120 It came roughly 500 million years after the birth of the solar system, 745 00:48:45,120 --> 00:48:47,240 when all the planets had long formed. 746 00:48:49,000 --> 00:48:51,800 And all of a sudden, out of the blue, 747 00:48:51,800 --> 00:48:57,360 the moon got clobbered by big objects coming in and hitting it. 748 00:48:57,360 --> 00:49:02,280 And that indicates that you had this very violent upheaval 749 00:49:02,280 --> 00:49:07,840 and the only way that we can form an influx like this, stuff raining 750 00:49:07,840 --> 00:49:14,160 down in onto the moon, is through changing the orbits of the planets. 751 00:49:15,200 --> 00:49:17,480 To account for this violent upheaval, 752 00:49:17,480 --> 00:49:20,080 Hal and some colleagues devised a new model. 753 00:49:21,520 --> 00:49:24,320 It explains why we now see Uranus and Neptune 754 00:49:24,320 --> 00:49:27,080 in places they can't possibly have formed. 755 00:49:28,520 --> 00:49:31,680 We think what happened is they formed closer to the sun 756 00:49:31,680 --> 00:49:34,920 and got delivered to where we see them today. 757 00:49:36,240 --> 00:49:40,160 Uranus and Neptune must have formed much closer in, 758 00:49:40,160 --> 00:49:42,720 where there was plenty of icy material, 759 00:49:42,720 --> 00:49:46,800 and beyond them was a neat disc of icy, comet-like objects. 760 00:49:48,760 --> 00:49:51,600 But this was not a stable system, 761 00:49:51,600 --> 00:49:56,560 and a series of small changes led to a period of utter chaos. 762 00:49:56,560 --> 00:49:59,120 These objects leak out of this disc, 763 00:49:59,120 --> 00:50:03,120 get gravitationally scattered by all these planets, 764 00:50:03,120 --> 00:50:05,040 like billiard balls going around, 765 00:50:05,040 --> 00:50:08,760 and get eventually ejected to interstellar space by Jupiter. 766 00:50:12,960 --> 00:50:16,760 That causes the planets' orbits to slightly spread over time 767 00:50:16,760 --> 00:50:19,680 and what we think happened is that Jupiter 768 00:50:19,680 --> 00:50:24,400 and Saturn got to the point where Jupiter goes around the sun 769 00:50:24,400 --> 00:50:28,520 exactly twice for every time Saturn moves around the sun. 770 00:50:28,520 --> 00:50:33,240 And that allows their tugs on one another to become much stronger 771 00:50:33,240 --> 00:50:37,080 and as a result, Jupiter and Saturn get a little excited, 772 00:50:37,080 --> 00:50:40,080 their orbits become less circular and more inclined 773 00:50:40,080 --> 00:50:43,240 and they start getting... sort of tugging on one another. 774 00:50:43,240 --> 00:50:47,320 Uranus and Neptune, which are much smaller than Jupiter and Saturn, feel 775 00:50:47,320 --> 00:50:52,280 that fight, feel that tension and as a result, their orbits just go nuts. 776 00:50:54,520 --> 00:50:57,400 In a sudden period of chaos, Uranus and Neptune 777 00:50:57,400 --> 00:51:00,080 were flung out into the orbits we see today. 778 00:51:02,040 --> 00:51:05,480 The ice giants, Uranus and Neptune, get scattered into this disc 779 00:51:05,480 --> 00:51:08,960 that existed outside their orbits, and that thing went kaplooie. 780 00:51:13,920 --> 00:51:17,320 Vast lumps of ice were scattered everywhere, 781 00:51:17,320 --> 00:51:19,320 raining into the inner solar system 782 00:51:19,320 --> 00:51:21,520 and bombarding the earth and the moon. 783 00:51:25,440 --> 00:51:29,200 Every square inch of the earth at one time got hit 784 00:51:29,200 --> 00:51:33,560 due to this instability, so it was not a very safe place to be. 785 00:51:35,760 --> 00:51:39,080 We had this view that the solar system was this nice clock 786 00:51:39,080 --> 00:51:42,000 and things just moved around in nice regular ways. 787 00:51:43,080 --> 00:51:47,400 What this new model shows is a real paradigm shift. 788 00:51:47,400 --> 00:51:53,200 It says that the solar system is not this nice, safe, quiescent place, 789 00:51:53,200 --> 00:51:56,040 but can go through periods of intense violence. 790 00:51:58,040 --> 00:52:02,520 This new model of the solar system is now dynamic and turbulent. 791 00:52:04,560 --> 00:52:09,120 The prime mover in all this upheaval is our playground bully, Jupiter. 792 00:52:16,960 --> 00:52:21,200 Such a bully, in fact, that David Nesvorny believes that Jupiter 793 00:52:21,200 --> 00:52:24,920 may have been responsible for the ultimate planetary crime. 794 00:52:26,840 --> 00:52:29,080 He ran the new model over and over again 795 00:52:29,080 --> 00:52:31,880 with slightly different starting conditions. 796 00:52:33,760 --> 00:52:38,520 I ran about 3,000, 4,000 models like this, 797 00:52:38,520 --> 00:52:41,240 just playing with the initial state 798 00:52:41,240 --> 00:52:44,360 and at a time, I considered the standard theory, 799 00:52:44,360 --> 00:52:48,160 which was that the outer solar system had four planets. 800 00:52:49,760 --> 00:52:52,480 His results were alarming - 801 00:52:52,480 --> 00:52:55,120 change the starting conditions even slightly 802 00:52:55,120 --> 00:52:57,600 and the solar system looks very different. 803 00:52:59,520 --> 00:53:02,600 Frequently, what happened in my simulation was that Jupiter 804 00:53:02,600 --> 00:53:05,960 just slingshots Uranus and Neptune from the solar system 805 00:53:05,960 --> 00:53:09,280 and they ended somewhere in interstellar space. 806 00:53:09,280 --> 00:53:11,520 So that wasn't right. 807 00:53:11,520 --> 00:53:13,320 Obviously not right. 808 00:53:15,600 --> 00:53:18,240 Then David had a radical idea. 809 00:53:18,240 --> 00:53:21,960 If Neptune and Uranus didn't get flung out of the solar system, 810 00:53:21,960 --> 00:53:23,680 maybe something else did. 811 00:53:25,520 --> 00:53:29,360 I couldn't quite fit the solar system, how it looks like today. 812 00:53:30,480 --> 00:53:34,720 So I was thinking and thinking and thinking, and then I thought, 813 00:53:34,720 --> 00:53:37,840 "How about if the solar system had an extra planet?" 814 00:53:41,880 --> 00:53:44,720 He started investigating the possibility 815 00:53:44,720 --> 00:53:47,520 that an entire planet might have gone missing. 816 00:53:49,760 --> 00:53:53,160 As ever, the prime suspect was Jupiter. 817 00:53:56,280 --> 00:53:59,040 So, now I am pointing at Jupiter, 818 00:53:59,040 --> 00:54:02,600 so I can see the disc of Jupiter, 819 00:54:02,600 --> 00:54:07,160 and then, nicely aligned, four giant moons. 820 00:54:08,560 --> 00:54:10,120 It has a huge influence... 821 00:54:11,480 --> 00:54:14,920 ..and could have had an even bigger influence in the past. 822 00:54:19,160 --> 00:54:22,560 It may even have been able to eject an entire planet 823 00:54:22,560 --> 00:54:24,400 from our solar system. 824 00:54:25,560 --> 00:54:27,520 This is the solar system. 825 00:54:27,520 --> 00:54:29,360 The sun is in the middle... 826 00:54:30,480 --> 00:54:32,800 ..then we have the terrestrial planets. 827 00:54:32,800 --> 00:54:36,560 Then there's the asteroid belt and the outer planets. 828 00:54:37,640 --> 00:54:41,000 To get the arrangement of planets we see today, David thinks 829 00:54:41,000 --> 00:54:45,360 we once had an extra ice giant but it was thrown out by Jupiter. 830 00:54:46,920 --> 00:54:53,360 I start playing with the possibility that we had an additional planet. 831 00:54:53,360 --> 00:54:57,520 So the best case I have found was when I placed 832 00:54:57,520 --> 00:55:02,680 this third ice giant between Saturn and Uranus initially, 833 00:55:02,680 --> 00:55:04,000 somewhere here. 834 00:55:04,000 --> 00:55:07,960 What happens in this case is that during the instability, 835 00:55:07,960 --> 00:55:11,280 this planet evolves, has close encounters with Jupiter 836 00:55:11,280 --> 00:55:15,600 and Saturn and gets ejected from the solar system. 837 00:55:27,640 --> 00:55:31,400 The ejected planet may have been a sacrificial lamb 838 00:55:31,400 --> 00:55:34,680 that saved us from Jupiter's destructive powers 839 00:55:34,680 --> 00:55:37,800 and allowed our planets to settle in the pattern we see today. 840 00:55:40,840 --> 00:55:43,400 So, what became of our missing lonely planet? 841 00:55:44,680 --> 00:55:49,680 In the simulations I have, the planet is ejected from the solar system 842 00:55:49,680 --> 00:55:52,960 with a speed of about 1km per second. 843 00:55:54,120 --> 00:55:59,720 But this happened about four billion years ago, so do your math. 844 00:56:01,440 --> 00:56:03,960 It will end up very far from the solar system, 845 00:56:03,960 --> 00:56:07,720 so today it can be almost anywhere in the galaxy. 846 00:56:20,680 --> 00:56:24,680 20 years ago, the mystery of the solar system began to unravel. 847 00:56:26,600 --> 00:56:29,960 Evidence from alien worlds shattered the long-held view 848 00:56:29,960 --> 00:56:32,040 that our planets have fixed orbits. 849 00:56:34,040 --> 00:56:37,920 It led to a whole new understanding of a turbulent and dynamic past... 850 00:56:39,800 --> 00:56:43,600 ..which makes us wonder, might things have turned out differently? 851 00:56:43,600 --> 00:56:46,120 The solar system could have done a lot of different things, 852 00:56:46,120 --> 00:56:49,160 it could have evolved in a lot of different ways. 853 00:56:49,160 --> 00:56:51,880 What we see in our own solar system 854 00:56:51,880 --> 00:56:57,640 is the result of a lot of unlikely or random events, 855 00:56:57,640 --> 00:56:59,440 and so our solar system is unique. 856 00:57:03,240 --> 00:57:07,080 Ending up with a stable system of planets was just a fluke, 857 00:57:07,080 --> 00:57:09,000 a lucky roll of the dice. 858 00:57:10,440 --> 00:57:12,920 It's amazing we survived at all. 859 00:57:12,920 --> 00:57:17,000 Getting an earth where we have our earth today was not a given 860 00:57:17,000 --> 00:57:19,800 when this whole solar system started. 861 00:57:19,800 --> 00:57:22,680 It took all these series of events to get a rocky planet 862 00:57:22,680 --> 00:57:25,840 of this size at this distance with this amount of water 863 00:57:25,840 --> 00:57:28,120 to build the earth that we live on today. 864 00:57:30,440 --> 00:57:34,440 The fate of the entire solar system, including the earth, 865 00:57:34,440 --> 00:57:38,160 was defined above all by the movements of our gas giant, Jupiter. 866 00:57:40,640 --> 00:57:43,320 If Jupiter's orbit moved differently, 867 00:57:43,320 --> 00:57:46,400 if Jupiter moved into the inner solar system, 868 00:57:46,400 --> 00:57:49,480 then it's unlikely that the earth would be here. 869 00:57:51,720 --> 00:57:54,800 Of all the planetary systems so far discovered, 870 00:57:54,800 --> 00:57:58,080 it seems we are the only one with the lucky roll of the dice. 871 00:57:59,360 --> 00:58:02,760 You might think that maybe the solar system that we have here 872 00:58:02,760 --> 00:58:05,960 is actually the oddball and that the natural order 873 00:58:05,960 --> 00:58:09,440 of things are these other systems that we think of as weird. 874 00:58:11,200 --> 00:58:13,360 And, if we are so unusual, 875 00:58:13,360 --> 00:58:17,960 will we ever find anywhere else in the universe so welcoming to life? 876 00:58:19,040 --> 00:58:23,720 Even though our solar system might be...let's say one in a million - 877 00:58:23,720 --> 00:58:26,280 that may seem like a really small number - 878 00:58:26,280 --> 00:58:29,000 there are 100 billion stars in the galaxy. 879 00:58:29,000 --> 00:58:32,720 So even something as unlikely as our solar system, 880 00:58:32,720 --> 00:58:35,160 there may be lots of them around.