1 00:00:02,903 --> 00:00:05,736 Narrator: It used to be the only planets we knew about 2 00:00:05,739 --> 00:00:07,434 were the ones that orbit our Sun. 3 00:00:07,441 --> 00:00:12,607 But now we've discovered rocky worlds 4 00:00:12,613 --> 00:00:16,709 and gas giants orbiting other stars. 5 00:00:16,717 --> 00:00:18,515 They tell an amazing story. 6 00:00:18,519 --> 00:00:21,079 Squyres: The early history of these planets 7 00:00:21,088 --> 00:00:24,251 would have been very, very violent. 8 00:00:24,258 --> 00:00:29,094 Narrator: Planets are made everywhere in the same way. 9 00:00:29,096 --> 00:00:31,428 They form from the dust and debris 10 00:00:31,432 --> 00:00:34,595 leftover from the birth of stars. 11 00:00:34,602 --> 00:00:38,766 So, if they're all made the same way, 12 00:00:38,772 --> 00:00:41,241 what makes them all so different? 13 00:00:56,323 --> 00:01:00,021 The universe is full of galaxies... 14 00:01:02,162 --> 00:01:06,224 ...gas clouds... 15 00:01:06,233 --> 00:01:09,225 stars." 16 00:01:09,236 --> 00:01:11,898 And planets, as it turns out. 17 00:01:11,906 --> 00:01:15,365 Our solar system has eight planets. 18 00:01:15,376 --> 00:01:18,368 But we now know they're a tiny group, 19 00:01:18,379 --> 00:01:21,838 compared to the huge cosmic family of planets 20 00:01:21,849 --> 00:01:24,011 across the galaxy. 21 00:01:24,018 --> 00:01:30,355 Marcy: It's an extraordinary moment in scientific history... 22 00:01:30,357 --> 00:01:32,689 To know for sure 23 00:01:32,693 --> 00:01:34,525 that there are other planetary systems out there. 24 00:01:34,528 --> 00:01:36,690 They're very common. 25 00:01:36,697 --> 00:01:41,191 And out of the 200 billion stars in our Milky Way galaxy, 26 00:01:41,201 --> 00:01:47,538 there are surely dozens of billions of planets out there. 27 00:01:47,541 --> 00:01:52,035 Narrator: In 2009, NASA launched the Kepler Space Telescope 28 00:01:52,046 --> 00:01:53,536 on a six-year mission 29 00:01:53,547 --> 00:01:57,074 to find new planets orbiting other stars. 30 00:02:00,354 --> 00:02:04,348 So far, astronomers have found over 400. 31 00:02:06,994 --> 00:02:09,986 Some are colossal balls of churning gas 32 00:02:09,997 --> 00:02:13,262 five times the size of Jupiter. 33 00:02:15,669 --> 00:02:20,607 Others are huge, rocky worlds many times larger than Earth. 34 00:02:22,676 --> 00:02:26,169 Some follow wild, erratic orbits, 35 00:02:26,180 --> 00:02:29,115 so close to a star they're burning up. 36 00:02:34,622 --> 00:02:40,459 One thing is clear -- no two planets are the same. 37 00:02:40,461 --> 00:02:44,625 Each is one of a kind. 38 00:02:44,632 --> 00:02:50,969 But most of these new planets are far away and hard to study. 39 00:02:50,971 --> 00:02:53,633 Most of what we know about how planets work 40 00:02:53,641 --> 00:02:57,976 comes from the eight that orbit our own star. 41 00:02:57,978 --> 00:03:00,208 Our own planets come in two main types. 42 00:03:00,214 --> 00:03:04,776 There are four rocky planets in the inner solar system -- 43 00:03:04,785 --> 00:03:06,719 Mercury... 44 00:03:06,720 --> 00:03:08,814 Venus... 45 00:03:08,822 --> 00:03:10,881 Earth... 46 00:03:10,891 --> 00:03:13,053 And Mars. 47 00:03:13,060 --> 00:03:14,892 And in the outer solar system, 48 00:03:14,895 --> 00:03:18,559 there are four giant gas planets -- 49 00:03:18,565 --> 00:03:20,067 Jupiter... 50 00:03:20,067 --> 00:03:22,934 Saturn... 51 00:03:22,936 --> 00:03:24,301 Uranus... 52 00:03:24,304 --> 00:03:26,796 And Neptune. 53 00:03:26,807 --> 00:03:33,907 Each of the eight planets is distinct and very different. 54 00:03:33,914 --> 00:03:38,408 Their unique personalities began to form 55 00:03:38,419 --> 00:03:43,152 at the birth of our solar system 4.6 billion years ago. 56 00:03:46,660 --> 00:03:49,152 When the Sun ignited, 57 00:03:49,163 --> 00:03:53,999 it left behind a huge cloud of gas and dust. 58 00:03:54,001 --> 00:03:56,163 All eight planets -- 59 00:03:56,170 --> 00:03:59,333 the inner rocky and the outer gas planets -- 60 00:03:59,339 --> 00:04:02,502 came from this cloud of cosmic debris. 61 00:04:02,509 --> 00:04:04,500 Squyres: The planets in our solar system 62 00:04:04,511 --> 00:04:06,843 are all made from the same stuff. 63 00:04:06,847 --> 00:04:09,839 They're made from the same cloud of gas and dust, 64 00:04:09,850 --> 00:04:12,342 but they formed under very different conditions. 65 00:04:12,352 --> 00:04:14,252 Some of them formed in close to the Sun, 66 00:04:14,254 --> 00:04:15,483 where it was much hotter, 67 00:04:15,489 --> 00:04:17,480 some much farther away, where it was much colder. 68 00:04:17,491 --> 00:04:21,826 And because the conditions were so different, the end result, 69 00:04:21,829 --> 00:04:26,824 the product of their formation, was different, as well. 70 00:04:26,834 --> 00:04:29,997 Alexander: So, you start the solar system, in my view, 71 00:04:30,003 --> 00:04:33,132 with a pretty homogeneous mix of silicates 72 00:04:33,140 --> 00:04:35,131 and water vapor and hydrogen, lots of hydrogen, 73 00:04:35,142 --> 00:04:37,474 and methane and other elements. 74 00:04:37,478 --> 00:04:39,469 Narrator: These elements in the dust cloud 75 00:04:39,480 --> 00:04:41,642 are like ingredients in a cake. 76 00:04:41,648 --> 00:04:44,481 They cook differently, depending on the combination 77 00:04:44,485 --> 00:04:47,318 of the ingredients and the temperature of the oven. 78 00:04:47,321 --> 00:04:50,655 Alexander: And just like with the cake, you'd mix the ingredients. 79 00:04:50,657 --> 00:04:52,489 And then you'd put it in the oven and bake it, 80 00:04:52,493 --> 00:04:54,518 and it would change. 81 00:04:54,528 --> 00:04:57,862 And so this is kind of what happened in the solar system. 82 00:04:57,865 --> 00:05:01,358 Overall, the planet cooks in a slightly different way, 83 00:05:01,368 --> 00:05:04,861 depending on how close it is to the Sun. 84 00:05:04,872 --> 00:05:07,204 Narrator: Close in, where it's hot, 85 00:05:07,207 --> 00:05:12,338 the Sun burns off gases and boils away water. 86 00:05:12,346 --> 00:05:16,510 Only materials that stay solid at high temperatures, 87 00:05:16,517 --> 00:05:19,919 like metals and rock, can survive, 88 00:05:19,920 --> 00:05:26,622 which is why only rocky planets form close to the Sun. 89 00:05:26,627 --> 00:05:29,289 Move farther away from the heat of the Sun, 90 00:05:29,296 --> 00:05:32,960 and you get different kinds of planets cooking. 91 00:05:32,966 --> 00:05:35,628 But it's the ingredients in the cloud 92 00:05:35,636 --> 00:05:39,766 that determine precisely what kinds of planets will form. 93 00:05:39,773 --> 00:05:44,142 Well, depending on the type of cloud a solar system forms in, 94 00:05:44,144 --> 00:05:47,478 you could have solar systems that don't have rocky planets 95 00:05:47,481 --> 00:05:49,813 because it was just too poor in the materials 96 00:05:49,817 --> 00:05:52,149 to build something like the Earth, 97 00:05:52,152 --> 00:05:55,144 and instead you could end up with more gas giants 98 00:05:55,155 --> 00:05:59,490 and no rocky planets at all. 99 00:05:59,493 --> 00:06:02,485 Narrator: If you want rocky planets, 100 00:06:02,496 --> 00:06:05,932 you need a cloud full of metals and rock. 101 00:06:08,502 --> 00:06:12,996 Next step -- turn the heat down. 102 00:06:13,006 --> 00:06:16,340 As it cools down, some of the elements in there 103 00:06:16,343 --> 00:06:21,213 that have a high boiling point start to condense out as solids. 104 00:06:21,215 --> 00:06:26,381 And you can get these very tiny little mineral grains forming. 105 00:06:26,386 --> 00:06:28,548 Narrator: These tiny mineral grains 106 00:06:28,555 --> 00:06:31,490 are the seeds of a new rocky planet. 107 00:06:31,491 --> 00:06:34,654 Over time, they start to stick together. 108 00:06:34,661 --> 00:06:37,494 You would have one dust molecule and another dust molecule, 109 00:06:37,497 --> 00:06:39,329 and they would basically slam into each other 110 00:06:39,333 --> 00:06:41,768 and become one slightly bigger dust molecule. 111 00:06:41,768 --> 00:06:44,260 And they would pick up more and more and more. 112 00:06:44,271 --> 00:06:46,433 This process is called accretion. 113 00:06:46,440 --> 00:06:51,207 As these things got bigger, they became basically rocks. 114 00:06:53,447 --> 00:06:58,044 Narrator: Then rocks slam into other rocks and form boulders. 115 00:07:01,288 --> 00:07:07,125 Boulders smash together to form bigger boulders. 116 00:07:07,127 --> 00:07:10,119 Eventually, you've got something big enough 117 00:07:10,130 --> 00:07:12,064 that it's gravity was strong enough 118 00:07:12,065 --> 00:07:14,193 that it could start drawing material in. 119 00:07:14,201 --> 00:07:15,862 So, instead of just slamming into material 120 00:07:15,869 --> 00:07:17,530 and gaining mass that way, 121 00:07:17,537 --> 00:07:20,802 it was actually actively pulling material in. 122 00:07:20,807 --> 00:07:23,401 Narrator: In our own solar system, 123 00:07:23,410 --> 00:07:29,577 there were many growing infant planets at first -- maybe 100. 124 00:07:29,583 --> 00:07:33,577 Most of them didn't make it. 125 00:07:33,587 --> 00:07:35,248 If you go to the Asteroid Belt 126 00:07:35,255 --> 00:07:39,453 and look at the asteroid 4 Vesta, 127 00:07:39,459 --> 00:07:45,296 that is a good indicator of how big a rocky planet has to be 128 00:07:45,299 --> 00:07:47,631 before it can pull itself into a spherical shape. 129 00:07:47,634 --> 00:07:52,970 Narrator: Vesta is only 329 miles across, 130 00:07:52,973 --> 00:07:57,103 not quite big enough to become a sphere. 131 00:07:57,110 --> 00:07:59,602 For a growing planet to become round, 132 00:07:59,613 --> 00:08:02,275 it has to reach 500 miles across. 133 00:08:02,282 --> 00:08:07,118 Then it has enough gravity to crush it into a sphere. 134 00:08:07,120 --> 00:08:11,956 Any smaller, and it stays an irregular shape. 135 00:08:11,959 --> 00:08:16,123 As round infant planets keep eating up stuff, 136 00:08:16,129 --> 00:08:19,463 each collision makes them hotter and hotter, 137 00:08:19,466 --> 00:08:22,299 until they start to melt. 138 00:08:22,302 --> 00:08:27,502 Now gravity begins to separate the heavy stuff from the light. 139 00:08:27,507 --> 00:08:32,604 Lathrop: Lighter materials tend to float up into crusty film, 140 00:08:32,612 --> 00:08:35,479 and the heavier materials -- many of the metals -- 141 00:08:35,482 --> 00:08:36,916 falling down and forming 142 00:08:36,917 --> 00:08:40,012 a much denser core at the center of the planet. 143 00:08:44,057 --> 00:08:45,559 Narrator: The young planets 144 00:08:45,559 --> 00:08:48,153 are finally beginning to look like planets. 145 00:08:51,365 --> 00:08:54,391 But now they have to survive a period 146 00:08:54,401 --> 00:08:57,496 of violence and destruction... 147 00:09:01,475 --> 00:09:03,136 ...a brutal phase that determines 148 00:09:03,143 --> 00:09:08,013 which planets will live and which planets will die. 149 00:09:10,717 --> 00:09:12,481 Narrator: After the birth of the Sun, 150 00:09:12,486 --> 00:09:14,648 our eight planets all evolved 151 00:09:14,654 --> 00:09:17,123 from the same cloud of dust and gas, 152 00:09:17,124 --> 00:09:21,789 and yet they ended up completely different. 153 00:09:21,795 --> 00:09:26,631 There was no real blueprint for each of the newborn planets. 154 00:09:26,633 --> 00:09:29,796 They did obey the laws of physics and chemistry, 155 00:09:29,803 --> 00:09:33,706 but the most important things happened by pure chance. 156 00:09:33,707 --> 00:09:37,541 4.5 billion years ago, 157 00:09:37,544 --> 00:09:41,811 around 100 baby planets circled our Sun. 158 00:09:45,585 --> 00:09:48,418 It turned into a demolition derby. 159 00:09:48,422 --> 00:09:53,258 Planet hit planet. Most were destroyed. 160 00:09:57,297 --> 00:09:59,459 The early history of these planets 161 00:09:59,466 --> 00:10:01,935 would have been very, very violent, 162 00:10:01,935 --> 00:10:03,926 with lots of these impacts taking place 163 00:10:03,937 --> 00:10:06,838 in the final stages of the growth of each planet. 164 00:10:06,840 --> 00:10:12,745 As these impacts took place, as objects ran into each other, 165 00:10:12,746 --> 00:10:14,737 certain objects began to grow 166 00:10:14,748 --> 00:10:18,082 at the expense of all the others in this swarm of planetesimals. 167 00:10:18,085 --> 00:10:20,918 And these planets, these things that would become planets, 168 00:10:20,921 --> 00:10:22,912 grew and grew, and as they got bigger, 169 00:10:22,923 --> 00:10:27,326 they swept up all the smaller planetesimals around them, 170 00:10:27,327 --> 00:10:29,489 the consequence on the surface of that protoplanet 171 00:10:29,496 --> 00:10:32,830 being an enormous amount of bombardment 172 00:10:32,833 --> 00:10:35,097 by debris from space. 173 00:10:39,139 --> 00:10:41,471 Narrator: When it was over, 174 00:10:41,475 --> 00:10:46,743 all that was left were four very different rocky planets. 175 00:10:49,049 --> 00:10:53,213 Love: Each planet's impact history left its stamp, 176 00:10:53,220 --> 00:10:55,052 and that's why they're all so different from each other. 177 00:10:55,055 --> 00:11:00,824 Narrator: Mars is a frozen wasteland. 178 00:11:00,827 --> 00:11:04,388 Earth flows with liquid water. 179 00:11:04,397 --> 00:11:07,492 Venus is a volcanic hellhole. 180 00:11:10,237 --> 00:11:15,300 And Mercury is tiny, bleak, and super hot, 181 00:11:15,308 --> 00:11:18,073 the result of a monster collision. 182 00:11:28,188 --> 00:11:30,520 Plait: Mercury, for example, 183 00:11:30,524 --> 00:11:32,686 is extremely dense and has a very thin crust. 184 00:11:32,692 --> 00:11:35,821 So, it's possible it started off as a bigger planet. 185 00:11:35,829 --> 00:11:39,493 And then something hit it at an angle, 186 00:11:39,499 --> 00:11:42,332 and it sheared off the lighter-weight crust, 187 00:11:42,335 --> 00:11:44,599 leaving only the dense core. 188 00:11:52,512 --> 00:11:58,178 Narrator: The young Earth also took a big hit. 189 00:11:58,185 --> 00:12:01,348 Squyres: Sometime late in its development, 190 00:12:01,354 --> 00:12:06,190 the Earth was impacted by another object 191 00:12:06,193 --> 00:12:09,754 that ripped debris out of the Earth's mantle... 192 00:12:17,003 --> 00:12:19,995 ...which then went into orbit around the Earth 193 00:12:20,006 --> 00:12:23,135 and re-accumulated to form what is now the Moon. 194 00:12:34,888 --> 00:12:36,219 Narrator: There's also evidence 195 00:12:36,223 --> 00:12:40,217 that something crashed into Mars. 196 00:12:40,227 --> 00:12:44,323 The northern hemisphere has a thinner crust than the southern. 197 00:12:47,267 --> 00:12:50,601 A theory that has emerged for how this happened 198 00:12:50,604 --> 00:12:53,437 is that early in the planet's history, 199 00:12:53,440 --> 00:12:56,774 the northern hemisphere of Mars was whacked by some object 200 00:12:56,776 --> 00:12:59,711 that blasted a lot of the crust off of it. 201 00:13:10,991 --> 00:13:14,120 And that crust re-accumulated on the southern half of Mars. 202 00:13:14,127 --> 00:13:18,121 Narrator: All these collisions did two things. 203 00:13:18,131 --> 00:13:22,034 They cut down the number of surviving infant planets. 204 00:13:24,271 --> 00:13:28,265 And they brought more ingredients to the survivors. 205 00:13:28,275 --> 00:13:30,266 Lathrop: If you had a collision 206 00:13:30,277 --> 00:13:33,110 with something that was metal-rich, 207 00:13:33,113 --> 00:13:35,707 those chunks would tend to descend down 208 00:13:35,715 --> 00:13:38,480 into what was becoming the core... 209 00:13:42,455 --> 00:13:46,187 ...where if you collided with something light or icy, 210 00:13:46,192 --> 00:13:48,024 they would tend to just float about 211 00:13:48,028 --> 00:13:50,963 and form part of the crust instead. 212 00:13:54,100 --> 00:13:56,762 Narrator: The four rocky planets close to the Sun 213 00:13:56,770 --> 00:13:58,761 were almost complete. 214 00:13:58,772 --> 00:14:01,764 They had a solid, hot-iron core 215 00:14:01,775 --> 00:14:04,733 surrounded by a layer of liquid iron, 216 00:14:04,744 --> 00:14:08,180 all wrapped in a jacket of molten rock. 217 00:14:11,151 --> 00:14:15,816 Above that, an outer surface crust. 218 00:14:15,822 --> 00:14:19,816 These rocky planets all formed in the same basic way, 219 00:14:19,826 --> 00:14:22,420 from the same basic stuff. 220 00:14:24,931 --> 00:14:29,596 But each of them was very different... 221 00:14:29,602 --> 00:14:35,234 Different sizes and very different destinies. 222 00:14:41,748 --> 00:14:45,912 Narrator: Space may look empty, but it's not. 223 00:14:45,919 --> 00:14:49,913 It's full of stuff blown out of the Sun. 224 00:14:49,923 --> 00:14:53,587 The Sun generates powerful magnetic fields 225 00:14:53,593 --> 00:14:58,929 that rise above the surface in giant loops. 226 00:14:58,932 --> 00:15:03,096 When they clash, it triggers a storm of super hot, 227 00:15:03,103 --> 00:15:07,199 highly charged particles blasting out into space. 228 00:15:11,211 --> 00:15:15,637 It's called the solar wind. 229 00:15:17,784 --> 00:15:22,449 Astronauts in space can see it... 230 00:15:22,455 --> 00:15:26,016 But only when they close their eyes. 231 00:15:26,025 --> 00:15:29,017 Occasionally, you see a little flash with your eyes shut. 232 00:15:29,028 --> 00:15:30,757 And that is an energetic particle 233 00:15:30,764 --> 00:15:33,256 coming through your head 234 00:15:33,266 --> 00:15:35,598 and interacting with the fluid inside your eye, 235 00:15:35,602 --> 00:15:37,468 and it makes a little light flash. 236 00:15:37,470 --> 00:15:40,235 And you see these every couple of minutes or so 237 00:15:40,240 --> 00:15:42,504 that you're awake with your eyes shut. 238 00:15:42,509 --> 00:15:46,036 Narrator: If the astronauts were exposed 239 00:15:46,045 --> 00:15:48,878 to a lot more of the solar wind, it could be a killer. 240 00:15:48,882 --> 00:15:52,216 Love: During the Apollo program, 241 00:15:52,218 --> 00:15:55,244 in between two of the Moon missions, 242 00:15:55,255 --> 00:15:56,814 there was an outburst on the Sun 243 00:15:56,823 --> 00:15:58,348 that would have killed the astronauts 244 00:15:58,358 --> 00:16:01,191 if they had been there. 245 00:16:01,194 --> 00:16:04,858 So, space radiation is a serious business. 246 00:16:04,864 --> 00:16:06,195 Narrator: But here on Earth, 247 00:16:06,199 --> 00:16:08,691 the solar wind isn't much of a threat 248 00:16:08,701 --> 00:16:12,035 because we have an invisible protective shield, 249 00:16:12,038 --> 00:16:16,305 a magnetic field generated by the planet's core. 250 00:16:21,014 --> 00:16:25,611 The very center of the Earth is the solid inner core. 251 00:16:29,355 --> 00:16:33,349 It's a hard, iron, crystalline ball. 252 00:16:33,359 --> 00:16:36,522 Then there's a thick layer of liquid iron, 253 00:16:36,529 --> 00:16:39,362 which is convecting churning motions, 254 00:16:39,365 --> 00:16:43,700 which give rise to the magnetic field. 255 00:16:43,703 --> 00:16:47,196 Narrator: Well, that's the theory. 256 00:16:49,642 --> 00:16:54,512 To prove that an iron core can generate a magnetic shield, 257 00:16:54,514 --> 00:16:59,509 scientists built their own planet in a lab. 258 00:16:59,519 --> 00:17:02,489 This 10-foot, 26-ton sphere 259 00:17:02,489 --> 00:17:07,017 simulates conditions deep inside the Earth. 260 00:17:07,026 --> 00:17:11,964 A metal ball in the center acts as the planet's inner core. 261 00:17:13,900 --> 00:17:18,235 Liquid sodium spins around it at 90 miles an hour, 262 00:17:18,238 --> 00:17:19,899 imitating the effects of molten metal 263 00:17:19,906 --> 00:17:22,341 spinning around the Earth's core. 264 00:17:26,079 --> 00:17:30,073 We built this experiment to try to generate a magnetic field 265 00:17:30,083 --> 00:17:34,748 to attempt to understand why the Earth has a magnetic field 266 00:17:34,754 --> 00:17:38,588 and why other planets do not have magnetic fields. 267 00:17:38,591 --> 00:17:41,253 Narrator: It works like the generator in your car, 268 00:17:41,261 --> 00:17:43,992 where rotating coils of wire produce electricity. 269 00:17:46,900 --> 00:17:52,066 In the experiment, liquid sodium churns around the core 270 00:17:52,071 --> 00:17:56,633 and generates a magnetic field. 271 00:17:56,643 --> 00:17:59,510 Lathrop: It's very much like an electrical generator. 272 00:17:59,512 --> 00:18:02,675 You have motion that is able to 273 00:18:02,682 --> 00:18:05,344 generate magnetic fields by turning the energy, the motion, 274 00:18:05,351 --> 00:18:09,345 into magnetic energy. 275 00:18:09,355 --> 00:18:13,451 Narrator: The same thing happens deep inside the Earth. 276 00:18:13,459 --> 00:18:17,123 As the Earth spins, the hot liquid metal 277 00:18:17,130 --> 00:18:20,532 flows around the solid core, transforming its energy 278 00:18:20,533 --> 00:18:25,130 into a magnetic field that emerges from the poles. 279 00:18:27,974 --> 00:18:32,241 It protects the planet's atmosphere from the solar wind. 280 00:18:34,414 --> 00:18:37,748 And if the planet has a magnetic field, 281 00:18:37,750 --> 00:18:39,411 that solar wind will be diverted 282 00:18:39,419 --> 00:18:42,719 around the planet by the magnetic field. 283 00:18:42,722 --> 00:18:45,054 Narrator: The magnetic field 284 00:18:45,058 --> 00:18:48,050 deflects the solar wind around the planet, 285 00:18:48,061 --> 00:18:52,396 protecting the atmosphere and everything on Earth's surface. 286 00:18:52,398 --> 00:18:55,390 Sometimes big storms of solar radiation 287 00:18:55,401 --> 00:18:58,393 will mix it up with the magnetic field. 288 00:18:58,404 --> 00:19:03,342 Then we get big light shows over the poles -- the auroras. 289 00:19:09,482 --> 00:19:12,144 Without a magnetic force field, 290 00:19:12,151 --> 00:19:17,521 the solar wind would blast away Earth's atmosphere and water... 291 00:19:17,523 --> 00:19:23,587 Leaving a dead, arid planet... 292 00:19:23,596 --> 00:19:26,531 A lot like Mars. 293 00:19:29,869 --> 00:19:31,701 Mars formed just like Earth. 294 00:19:31,704 --> 00:19:36,301 But today it's cold and dry, with little atmosphere. 295 00:19:39,879 --> 00:19:43,645 So, why are the two planets now so different? 296 00:19:46,386 --> 00:19:52,484 In 2004, NASA sent two robot explorers to Mars to find out. 297 00:19:54,060 --> 00:19:57,394 The rovers, named Spirit and Opportunity, 298 00:19:57,397 --> 00:20:00,560 explored miles of the Martian surface. 299 00:20:00,566 --> 00:20:04,298 They confirmed that Mars is a dry and hostile desert, 300 00:20:04,303 --> 00:20:07,773 with only 1% the atmosphere of Earth. 301 00:20:07,774 --> 00:20:12,610 But they did find evidence of water in the past. 302 00:20:12,612 --> 00:20:16,276 Mars was not always a desert. 303 00:20:16,282 --> 00:20:19,274 We have found compelling evidence 304 00:20:19,285 --> 00:20:23,119 that water was once beneath the surface, came to the surface, 305 00:20:23,122 --> 00:20:24,385 and evaporated away. 306 00:20:27,026 --> 00:20:30,519 We also see in a few places ripples preserved, 307 00:20:30,530 --> 00:20:35,024 of the sort that are formed when water flows over sand. 308 00:20:35,034 --> 00:20:36,866 So, not only did water exist below the surface. 309 00:20:36,869 --> 00:20:38,963 It had flowed across the surface. 310 00:20:40,907 --> 00:20:43,399 Narrator: If Mars had water once, 311 00:20:43,409 --> 00:20:45,741 it probably also had a thick atmosphere. 312 00:20:45,745 --> 00:20:47,907 So what happened? 313 00:20:47,914 --> 00:20:51,578 We can see that Mars once had active volcanoes. 314 00:20:51,584 --> 00:20:54,576 So, it had a hot interior at some point. 315 00:20:54,587 --> 00:20:57,420 And because it was made of the same stuff as Earth, 316 00:20:57,423 --> 00:21:00,358 it would have had a hot-iron core, 317 00:21:00,359 --> 00:21:03,590 surrounded by liquid metal at its center. 318 00:21:03,596 --> 00:21:06,759 So, it should have had a magnetic field, too. 319 00:21:06,766 --> 00:21:10,930 The question is -- where did it go? 320 00:21:10,937 --> 00:21:12,598 Squyres: Early in the planet's history, 321 00:21:12,605 --> 00:21:15,540 Mars apparently had a strong magnetic field. 322 00:21:17,376 --> 00:21:20,209 And it was probably caused in the same way as it is on Earth. 323 00:21:22,348 --> 00:21:24,680 But Mars is a smaller planet than Earth. 324 00:21:24,684 --> 00:21:29,019 It's gonna lose its heat more rapidly as a consequence. 325 00:21:29,021 --> 00:21:33,686 And what that means is that liquid core can freeze solid. 326 00:21:33,693 --> 00:21:37,527 Freeze the core solid, the convection will stop. 327 00:21:37,530 --> 00:21:40,693 The convection stops, the magnetic field goes away. 328 00:21:40,700 --> 00:21:43,192 Narrator: As the magnetic shield died, 329 00:21:43,202 --> 00:21:46,194 the solar wind blasted away the atmosphere, 330 00:21:46,205 --> 00:21:48,435 and the water evaporated. 331 00:21:48,441 --> 00:21:51,672 Mars became a cold, barren planet. 332 00:21:51,677 --> 00:21:57,673 Mars, Earth, Venus, and Mercury -- the rocky planets -- 333 00:21:57,683 --> 00:22:02,018 all formed within 150 million miles of the Sun. 334 00:22:02,021 --> 00:22:04,513 But four times farther out, 335 00:22:04,524 --> 00:22:08,859 the Sun baked a very different kind of planet. 336 00:22:08,861 --> 00:22:12,297 They're gigantic, they're made of gas, 337 00:22:12,298 --> 00:22:17,600 and these monsters have no solid surfaces at all. 338 00:22:22,175 --> 00:22:24,303 Narrator: So far, astronomers have discovered 339 00:22:24,310 --> 00:22:29,578 over 400 new planets orbiting in far-off solar systems. 340 00:22:31,851 --> 00:22:37,119 Nearly all of them are gigantic and made of gas. 341 00:22:38,891 --> 00:22:42,555 We have four of these so-called gas giants 342 00:22:42,562 --> 00:22:44,656 in our own solar system. 343 00:22:49,502 --> 00:22:54,497 Lathrop: Jupiter, Saturn, Uranus, Neptune... 344 00:22:54,507 --> 00:22:57,238 Which all have these very thick, 345 00:22:57,243 --> 00:23:00,008 very soupy atmospheres, lots of hydrogen, 346 00:23:00,012 --> 00:23:03,983 lots of helium, lots of methane. 347 00:23:03,983 --> 00:23:06,475 Narrator: Why are these outer four made of gas 348 00:23:06,485 --> 00:23:08,579 when the inner ones are rocky? 349 00:23:10,656 --> 00:23:14,320 It all has to do with location. 350 00:23:14,327 --> 00:23:19,925 Out here, 500 million miles from the Sun, it's very cold. 351 00:23:22,001 --> 00:23:25,665 At the start of the solar system, there was some dust, 352 00:23:25,671 --> 00:23:31,838 but mostly gas and water, frozen in ice grains. 353 00:23:31,844 --> 00:23:35,007 Love: Where the giant planets started to form, 354 00:23:35,014 --> 00:23:38,678 it was cold enough to get solid snow. 355 00:23:38,684 --> 00:23:43,349 And we think we were able to make ice snowflakes, 356 00:23:43,356 --> 00:23:44,846 and these things were able to clump together 357 00:23:44,857 --> 00:23:46,848 to form the cores of the giant planets. 358 00:23:46,859 --> 00:23:47,985 And we think that's maybe 359 00:23:47,994 --> 00:23:49,723 why the giant planets got to be so big. 360 00:23:49,729 --> 00:23:54,724 Narrator: There was so much ice and gas their cores grew huge, 361 00:23:54,734 --> 00:23:57,999 around 10 times larger than the Earth. 362 00:24:00,339 --> 00:24:04,242 These giant cores generated a lot of gravity. 363 00:24:04,243 --> 00:24:06,234 They had so much pulling power, 364 00:24:06,245 --> 00:24:09,078 they sucked in all the surrounding gas 365 00:24:09,081 --> 00:24:12,073 and built up thick, soupy atmospheres 366 00:24:12,084 --> 00:24:16,749 tens of thousands of miles deep. 367 00:24:16,756 --> 00:24:21,091 The larger they got, the more gravity they generated. 368 00:24:21,093 --> 00:24:23,425 More and more dust and debris 369 00:24:23,429 --> 00:24:25,921 got pulled in towards the planets, 370 00:24:25,932 --> 00:24:29,698 and this became the building blocks of their moons. 371 00:24:35,374 --> 00:24:40,312 Jupiter and Saturn have over 60 moons each. 372 00:24:43,783 --> 00:24:50,120 The gas planets have another special feature -- rings. 373 00:24:50,122 --> 00:24:52,454 Plait: Saturn is unique among the planets 374 00:24:52,458 --> 00:24:54,790 in that it has this gorgeous ring system. 375 00:24:54,794 --> 00:24:56,956 It turns out Jupiter and Uranus and Neptune -- 376 00:24:56,963 --> 00:24:58,226 they have ring systems, as well, 377 00:24:58,230 --> 00:24:59,789 but they're really weak and pathetic 378 00:24:59,799 --> 00:25:00,823 and extremely hard to detect. 379 00:25:03,002 --> 00:25:05,334 Narrator: But they are there. 380 00:25:05,338 --> 00:25:08,672 All four of the gas giants have rings, 381 00:25:08,674 --> 00:25:12,008 but Saturn's are the most obvious. 382 00:25:12,011 --> 00:25:17,006 From a distance, Saturn's rings look like a single flat disk. 383 00:25:17,016 --> 00:25:20,680 However, they're actually thousands of separate ringlets, 384 00:25:20,686 --> 00:25:23,348 each only a few miles wide. 385 00:25:23,356 --> 00:25:25,518 When the Cassini Probe flew past, 386 00:25:25,524 --> 00:25:29,188 it detected billions of pieces of ice and cosmic rubble 387 00:25:29,195 --> 00:25:31,186 orbiting inside the rings 388 00:25:31,197 --> 00:25:34,189 at speeds of up to 50,000 miles an hour. 389 00:25:34,200 --> 00:25:36,191 These bits of ice and rock 390 00:25:36,202 --> 00:25:38,694 constantly crash into each other. 391 00:25:38,704 --> 00:25:44,040 Some grow into tiny moons. Others smash apart. 392 00:25:44,043 --> 00:25:47,035 But they never form into larger moons 393 00:25:47,046 --> 00:25:50,812 because Saturn's immense gravity tears them apart. 394 00:25:53,386 --> 00:25:56,879 Scientists are only just beginning to figure out 395 00:25:56,889 --> 00:26:01,224 how the rings formed in the first place. 396 00:26:01,227 --> 00:26:05,221 The theory goes like this -- 397 00:26:05,231 --> 00:26:09,896 a comet smashed into a moon and knocked it out of its orbit 398 00:26:09,902 --> 00:26:12,166 and closer to the planet. 399 00:26:18,411 --> 00:26:22,348 Saturn's gravity tore it to pieces. 400 00:26:24,583 --> 00:26:26,915 And all of that debris 401 00:26:26,919 --> 00:26:30,514 got trapped in rings around the planet. 402 00:26:33,692 --> 00:26:38,357 But the real mysteries of the gas giants lie deep inside them, 403 00:26:38,364 --> 00:26:41,493 tens of thousands of miles beneath the clouds. 404 00:26:43,702 --> 00:26:49,038 This is where the real action is. 405 00:26:49,041 --> 00:26:54,480 It's a place so extreme it challenges the laws of nature. 406 00:27:00,453 --> 00:27:01,352 Narrator: Most of the new planets 407 00:27:01,353 --> 00:27:05,347 we're finding around distant stars are gas giants. 408 00:27:05,357 --> 00:27:09,351 They're so huge they make Jupiter look small. 409 00:27:09,361 --> 00:27:12,854 But what goes on inside all gas giant planets, 410 00:27:12,865 --> 00:27:19,100 both in our solar system and way out there, is a mystery. 411 00:27:19,105 --> 00:27:21,972 We know Jupiter's dense atmosphere 412 00:27:21,974 --> 00:27:23,806 is 40,000 miles deep, 413 00:27:23,809 --> 00:27:26,972 and we can see high-speed bands of gas 414 00:27:26,979 --> 00:27:30,973 creating violent storms that rage across its surface. 415 00:27:30,983 --> 00:27:35,147 But what we don't know is what's going on deep inside, 416 00:27:35,154 --> 00:27:38,818 far beneath the storms. 417 00:27:38,824 --> 00:27:43,660 To find out, NASA launched the spacecraft Galileo 418 00:27:43,662 --> 00:27:45,994 on a 14-year mission to Jupiter. 419 00:27:45,998 --> 00:27:48,831 Woman: ...2, 1. 420 00:27:48,834 --> 00:27:51,667 We have ignition and lift-off of Atlantis 421 00:27:51,670 --> 00:27:56,403 and the Galileo spacecraft bound for Jupiter. 422 00:28:01,981 --> 00:28:06,145 Narrator: December 7, 1995. 423 00:28:06,152 --> 00:28:08,814 Galileo dropped a probe that dove 424 00:28:08,821 --> 00:28:14,089 into Jupiter's atmosphere at 160,000 miles an hour. 425 00:28:19,498 --> 00:28:22,160 Parachutes slowed it down 426 00:28:22,168 --> 00:28:24,830 as it dropped through the thick atmosphere. 427 00:28:24,837 --> 00:28:28,330 It detected lightning in the clouds 428 00:28:28,340 --> 00:28:33,073 and winds of 450 miles an hour. 429 00:28:33,078 --> 00:28:39,552 The probe transmitted data back to Earth for 58 minutes. 430 00:28:39,552 --> 00:28:40,951 So, people have asked me, 431 00:28:40,953 --> 00:28:42,887 "What happened to the Galileo probe that we dropped in?" 432 00:28:42,888 --> 00:28:45,050 It didn't hit anything. 433 00:28:45,057 --> 00:28:49,961 It just fell continually into the Jupiter environment, 434 00:28:49,962 --> 00:28:53,626 and the pressure increased and increased and increased. 435 00:28:53,632 --> 00:28:56,966 Narrator: As it descended, it recorded pressures 436 00:28:56,969 --> 00:28:59,939 23 times greater than on Earth 437 00:28:59,939 --> 00:29:03,432 and temperatures of over 300 degrees. 438 00:29:05,544 --> 00:29:07,046 When you're in the gas-giant environment 439 00:29:07,046 --> 00:29:11,040 and you go deeper and deeper into this hydrogen soup 440 00:29:11,050 --> 00:29:13,075 that has no solid surface, 441 00:29:13,085 --> 00:29:16,077 it nevertheless can have a tremendous weight. 442 00:29:16,088 --> 00:29:18,750 And so eventually you would be crushed 443 00:29:18,757 --> 00:29:22,591 by the overlying weight of the material that's there. 444 00:29:22,595 --> 00:29:26,429 Narrator: Even though the probe descended for only 124 miles 445 00:29:26,432 --> 00:29:28,594 before it was crushed, 446 00:29:28,601 --> 00:29:32,560 it gave scientists a glimpse of Jupiter's interior. 447 00:29:35,574 --> 00:29:40,512 But the dark heart of the planet still remains a mystery. 448 00:29:43,449 --> 00:29:46,282 Like some rocky planets, 449 00:29:46,285 --> 00:29:49,619 the gas giants have a magnetic field, too. 450 00:29:49,622 --> 00:29:52,455 But these are off the charts. 451 00:29:52,458 --> 00:29:54,290 Jupiter's magnetic field 452 00:29:54,293 --> 00:29:58,958 is 20,000 times more powerful than Earth's 453 00:29:58,964 --> 00:30:02,958 and so huge it extends all the way to Saturn, 454 00:30:02,968 --> 00:30:07,462 more than 400 million miles away. 455 00:30:07,473 --> 00:30:11,637 Like on Earth, the magnetic field deflects the solar wind 456 00:30:11,644 --> 00:30:15,376 and protects Jupiter's atmosphere. 457 00:30:15,381 --> 00:30:19,045 When scientists studied Jupiter's magnetic field, 458 00:30:19,051 --> 00:30:22,112 they discovered it was affecting Jupiter's moons. 459 00:30:26,959 --> 00:30:33,399 The volcanic moon lo orbits only 217,000 miles from the planet. 460 00:30:35,801 --> 00:30:39,635 Lo's volcanoes blast a ton of gas and dust 461 00:30:39,638 --> 00:30:42,573 into space every second. 462 00:30:46,211 --> 00:30:49,841 And Jupiter's magnetic field supercharges it, 463 00:30:49,848 --> 00:30:52,840 creating powerful belts of radiation. 464 00:30:55,087 --> 00:30:57,579 Lathrop: And that makes the vicinity of Jupiter 465 00:30:57,589 --> 00:31:00,354 very active in many different ways. 466 00:31:00,359 --> 00:31:03,090 If you point a radio antenna at Jupiter, 467 00:31:03,095 --> 00:31:05,587 one can hear all sorts of interactions 468 00:31:05,597 --> 00:31:09,761 happening between the planets and the magnetic field. 469 00:31:09,768 --> 00:31:14,763 Narrator: This is the sound of Jupiter's magnetic field. 470 00:31:14,773 --> 00:31:17,208 [high-pitched chirping] 471 00:31:24,149 --> 00:31:29,485 Jupiter and Saturn don't need the solar wind to make auroras. 472 00:31:29,488 --> 00:31:34,517 They have huge magnetic fields that create their own. 473 00:31:34,526 --> 00:31:37,689 The Chandra Space Telescope 474 00:31:37,696 --> 00:31:41,792 took these images of Jupiter's auroras. 475 00:31:41,800 --> 00:31:43,632 And NASA's Cassini Probe 476 00:31:43,635 --> 00:31:48,971 took these beautiful pictures of auroras on Saturn. 477 00:31:48,974 --> 00:31:50,305 These auroras are proof 478 00:31:50,309 --> 00:31:53,472 that gas planets have magnetic fields, too. 479 00:31:55,581 --> 00:31:59,415 But how do gas planets generate magnetic fields? 480 00:31:59,418 --> 00:32:02,410 On Earth, a super-hot liquid metal 481 00:32:02,421 --> 00:32:05,914 spinning around the planet's solid-iron core does the job. 482 00:32:05,924 --> 00:32:11,795 Gas planets probably do roughly the same thing. 483 00:32:11,797 --> 00:32:17,793 But gas planets don't have hot-iron cores. 484 00:32:17,803 --> 00:32:22,468 They formed around frozen cores of dust and ice. 485 00:32:22,474 --> 00:32:28,311 So, exactly what's going on deep inside is a mystery. 486 00:32:28,313 --> 00:32:31,977 At the very deepest interior of Jupiter, 487 00:32:31,984 --> 00:32:34,476 we really don't understand 488 00:32:34,486 --> 00:32:37,148 what composes those deep interior states. 489 00:32:37,156 --> 00:32:41,582 So, it could be that the very center of Jupiter 490 00:32:41,593 --> 00:32:45,086 has a solid core. 491 00:32:45,097 --> 00:32:50,365 Or it could actually just be still fluid. 492 00:32:54,706 --> 00:32:56,697 Narrator: We may never find out. 493 00:32:56,708 --> 00:32:59,973 No probe could ever make the 44,000-mile journey 494 00:32:59,978 --> 00:33:02,811 to the planet's center to investigate. 495 00:33:04,950 --> 00:33:06,281 Galileo was crushed 496 00:33:06,285 --> 00:33:09,380 before it got anywhere near the planet's core. 497 00:33:11,523 --> 00:33:15,289 So, now scientists are recreating Jupiter's interior 498 00:33:15,294 --> 00:33:19,060 right here in a lab on Earth. 499 00:33:19,064 --> 00:33:21,965 Here at the National Ignition Facility 500 00:33:21,967 --> 00:33:23,958 in Livermore, California, 501 00:33:23,969 --> 00:33:25,960 they're simulating Jupiter's core 502 00:33:25,971 --> 00:33:30,431 using the world's most powerful laser. 503 00:33:30,442 --> 00:33:33,707 This facility is really designed 504 00:33:33,712 --> 00:33:38,149 to compress hydrogen to extreme densities and temperatures. 505 00:33:40,152 --> 00:33:44,487 Narrator: Inside Jupiter, extreme pressures are created 506 00:33:44,490 --> 00:33:48,984 by the weight of 40,000 miles of hydrogen gas 507 00:33:48,994 --> 00:33:50,758 crushing down on the core. 508 00:33:52,898 --> 00:33:58,064 In the lab, it's done by focusing 192 laser beams 509 00:33:58,070 --> 00:34:00,596 on a tiny sample of hydrogen. 510 00:34:02,641 --> 00:34:04,735 As the pressure in the sample 511 00:34:04,743 --> 00:34:08,805 reaches over a million times the surface pressure on Earth, 512 00:34:08,814 --> 00:34:11,840 the hydrogen turns into a liquid. 513 00:34:11,850 --> 00:34:13,716 But when it reaches tens of millions 514 00:34:13,719 --> 00:34:18,213 of times the pressure -- more like at Jupiter's core -- 515 00:34:18,223 --> 00:34:21,215 something really weird happens to the hydrogen. 516 00:34:23,495 --> 00:34:25,156 The pressure is so great 517 00:34:25,163 --> 00:34:27,996 that it actually re-arranges the hydrogen, 518 00:34:28,000 --> 00:34:32,995 which is a very basic molecule, until it is able to conduct. 519 00:34:33,005 --> 00:34:38,842 So it changes the structure of H2 into a metallic form. 520 00:34:38,844 --> 00:34:40,334 Narrator: Scientists think 521 00:34:40,345 --> 00:34:43,838 this is what's happening inside Jupiter -- 522 00:34:43,849 --> 00:34:45,749 pressure and heat 523 00:34:45,751 --> 00:34:50,348 have transformed the planet's core into metallic hydrogen. 524 00:34:52,424 --> 00:34:57,089 Jupiter's metallic core works like the iron core in the Earth. 525 00:34:57,095 --> 00:35:02,864 It generates the gas planet's gigantic magnetic field. 526 00:35:06,271 --> 00:35:09,935 Gravity and heat shape how planets evolve, 527 00:35:09,942 --> 00:35:13,105 from their inner cores to their outer atmospheres. 528 00:35:13,111 --> 00:35:17,548 They're the great creative forces in planet building. 529 00:35:20,185 --> 00:35:22,847 But there's another ingredient 530 00:35:22,854 --> 00:35:26,882 that has a lot to do with how planets turn out. 531 00:35:26,892 --> 00:35:31,125 And that ingredient is water. 532 00:35:36,868 --> 00:35:40,805 Narrator: Planets may seem fixed and unchanging, 533 00:35:40,806 --> 00:35:42,968 but they never stop evolving. 534 00:35:42,975 --> 00:35:44,465 In our own solar system, 535 00:35:44,476 --> 00:35:50,506 one lost its atmosphere and became a barren wasteland. 536 00:35:50,515 --> 00:35:54,611 Another heated up and became the planet from hell. 537 00:35:56,688 --> 00:35:59,521 Planet Earth has changed, as well, 538 00:35:59,524 --> 00:36:03,461 and the game changer... was water. 539 00:36:05,731 --> 00:36:11,067 When you look at Earth from space, you see a lot of water. 540 00:36:11,069 --> 00:36:13,060 We are the Blue Planet, after all. 541 00:36:13,071 --> 00:36:16,837 So, it must be really wet, right? 542 00:36:18,877 --> 00:36:21,710 It looks at first glance that our Earth -- 543 00:36:21,713 --> 00:36:25,377 of course, covered 3/4 by oceans -- 544 00:36:25,384 --> 00:36:27,045 it's a very water-rich world. 545 00:36:27,052 --> 00:36:28,542 Not true. 546 00:36:28,553 --> 00:36:35,391 The Earth, by mass, is only 0.06% water. 547 00:36:35,394 --> 00:36:38,056 There's some water on the surface in the form of oceans, 548 00:36:38,063 --> 00:36:40,054 some water trapped in the mantle. 549 00:36:40,065 --> 00:36:43,660 But actually, the Earth is a relatively dry rock. 550 00:36:45,804 --> 00:36:48,466 Narrator: All of the inner rocky planets 551 00:36:48,473 --> 00:36:53,741 formed very close to the Sun, so they started off dry. 552 00:36:56,515 --> 00:37:00,281 Any water they might have had evaporated away 553 00:37:00,285 --> 00:37:02,777 or was blown away by impacts. 554 00:37:04,923 --> 00:37:07,915 Marcy: These massive collisions that formed the Earth 555 00:37:07,926 --> 00:37:11,590 were so energetic... 556 00:37:11,596 --> 00:37:14,588 That any water that had been here 557 00:37:14,599 --> 00:37:20,038 would have been vaporized and lost from the Earth. 558 00:37:23,175 --> 00:37:24,836 Narrator: So, where did Earth 559 00:37:24,843 --> 00:37:28,336 get all the new water we have today? 560 00:37:28,346 --> 00:37:32,010 It moved here. 561 00:37:32,017 --> 00:37:34,884 Marcy: When you look farther out 562 00:37:34,886 --> 00:37:37,878 and you look at Jupiter, Saturn, Uranus, and Neptune, 563 00:37:37,889 --> 00:37:41,382 those planets have enormous amounts of water 564 00:37:41,393 --> 00:37:42,656 locked up inside them. 565 00:37:45,597 --> 00:37:49,090 And even more dramatically are the moons. 566 00:37:49,101 --> 00:37:53,436 The moons of Jupiter, Saturn, Uranus, and Neptune 567 00:37:53,438 --> 00:37:56,897 are at least 50% water. 568 00:37:56,908 --> 00:38:00,344 Narrator: There was a lot of water out there. 569 00:38:00,345 --> 00:38:03,178 So, how did some of it get to planet Earth? 570 00:38:05,751 --> 00:38:08,413 And the answer almost certainly is 571 00:38:08,420 --> 00:38:11,412 that left farther out in our solar system 572 00:38:11,423 --> 00:38:14,916 were some asteroids and some comets, 573 00:38:14,926 --> 00:38:19,363 far enough from the Sun that they could retain their water. 574 00:38:21,500 --> 00:38:25,164 Narrator: Millions of these watery comets and asteroids 575 00:38:25,170 --> 00:38:29,141 came flying into the inner solar system. 576 00:38:29,141 --> 00:38:34,136 And some of them smashed into Earth. 577 00:38:34,146 --> 00:38:39,482 Over the eons, the Earth acquired the water 578 00:38:39,484 --> 00:38:42,977 that had been a part of the asteroids, 579 00:38:42,988 --> 00:38:47,152 and that indeed makes up the mass of water 580 00:38:47,159 --> 00:38:50,652 that nearly covers the Earth today. 581 00:38:56,234 --> 00:38:58,896 Narrator: But the amount of water that was delivered? 582 00:38:58,904 --> 00:39:01,396 That was the luck of the draw. 583 00:39:01,406 --> 00:39:04,569 Marcy: Couldn't it have been the case that the Earth 584 00:39:04,576 --> 00:39:08,240 would have acquired maybe half as much water as it did? 585 00:39:08,246 --> 00:39:12,080 If so, the Earth would be nearly dry on its surface, 586 00:39:12,083 --> 00:39:14,745 if not completely dry, the sponge of the interior 587 00:39:14,753 --> 00:39:18,883 soaking up the rest of the water. 588 00:39:18,890 --> 00:39:22,451 Narrator: No surface water would have meant no life. 589 00:39:22,460 --> 00:39:26,294 And what about too much water? 590 00:39:26,298 --> 00:39:32,135 We would be a water world, the oceans much deeper, 591 00:39:32,137 --> 00:39:35,505 covering the continents, even Mt. Everest. 592 00:39:35,507 --> 00:39:40,206 And so you can ask, then, "if the Earth were covered by water, 593 00:39:40,212 --> 00:39:42,544 only having twice as much as it currently has, 594 00:39:42,547 --> 00:39:44,811 would we have had a planet 595 00:39:44,816 --> 00:39:49,253 that was suitable for technological life?" 596 00:39:51,323 --> 00:39:55,988 Technology requires dry land. 597 00:39:55,994 --> 00:40:01,160 And it's quite likely that the precise amount of water 598 00:40:01,166 --> 00:40:03,828 that the Earth just happens to have 599 00:40:03,835 --> 00:40:08,170 has allowed a technological species like we homo sapiens 600 00:40:08,173 --> 00:40:09,504 to spring forth. 601 00:40:12,711 --> 00:40:14,201 Narrator: The world as we know it 602 00:40:14,212 --> 00:40:17,705 exists because a blizzard of comets and asteroids 603 00:40:17,716 --> 00:40:20,310 delivered just the right amount of water 604 00:40:20,318 --> 00:40:23,083 about four billion years ago. 605 00:40:26,191 --> 00:40:29,855 And just maybe the same thing is happening right now 606 00:40:29,861 --> 00:40:33,627 somewhere else in the universe. 607 00:40:35,901 --> 00:40:41,237 One thing's for sure -- there is plenty of water out there. 608 00:40:41,239 --> 00:40:45,073 Marcy: Hydrogen, the most common atom in the universe, 609 00:40:45,076 --> 00:40:47,977 and oxygen, one of the next most common atoms in the universe -- 610 00:40:47,979 --> 00:40:53,315 H20 is certainly going to be a very popular molecule -- 611 00:40:53,318 --> 00:40:55,286 and indeed it is -- within our universe. 612 00:40:55,287 --> 00:40:58,951 Narrator: So, water is everywhere in the universe, 613 00:40:58,957 --> 00:41:01,619 and we're discovering that planets are, too. 614 00:41:01,626 --> 00:41:03,287 But we still haven't found 615 00:41:03,295 --> 00:41:06,128 another planet with liquid water. 616 00:41:06,131 --> 00:41:10,295 Scientists have discovered more than 400 new planets. 617 00:41:10,302 --> 00:41:14,296 None of them look like our world. 618 00:41:14,306 --> 00:41:17,799 Marcy: What we have not yet found is a planet 619 00:41:17,809 --> 00:41:20,471 that is about the same size and mass 620 00:41:20,478 --> 00:41:23,004 and chemical composition as the Earth, 621 00:41:23,014 --> 00:41:25,813 orbiting another star. 622 00:41:25,817 --> 00:41:30,516 So, it remains an extraordinary holy grail for humanity 623 00:41:30,522 --> 00:41:35,358 to find other abodes that remind us of home. 624 00:41:35,360 --> 00:41:38,022 Narrator: But we'll keep looking. 625 00:41:38,029 --> 00:41:40,862 We know that there are around 200 billion stars 626 00:41:40,865 --> 00:41:44,699 in our galaxy alone. 627 00:41:44,703 --> 00:41:49,800 And as many as 40 billion of them could have planets. 628 00:41:53,378 --> 00:41:55,369 Alexander: We're still hopeful 629 00:41:55,380 --> 00:41:57,576 that when we discover terrestrial-style planets 630 00:41:57,582 --> 00:41:59,778 that will help us tremendously in understanding 631 00:41:59,784 --> 00:42:04,221 how our own inner-solar-system planets and the Earth 632 00:42:04,222 --> 00:42:08,489 evolved in comparison to the outer-solar-system planets. 633 00:42:10,595 --> 00:42:14,498 We are entering into what is gonna be thought of 634 00:42:14,499 --> 00:42:18,800 in the future as the Golden Age of planetary discovery. 635 00:42:20,939 --> 00:42:24,933 We will really for the first time begin to truly understand 636 00:42:24,943 --> 00:42:26,934 the actual diversity that lies out there. 637 00:42:26,945 --> 00:42:30,006 I think it's gonna be a fantastically exciting time. 638 00:42:32,751 --> 00:42:34,241 Narrator: Planets form 639 00:42:34,252 --> 00:42:37,415 according to the laws of physics and chemistry. 640 00:42:37,422 --> 00:42:41,757 What they become -- that has a lot more to do with luck. 641 00:42:41,760 --> 00:42:45,924 Many scientists believe it's only a matter of time 642 00:42:45,930 --> 00:42:49,423 before we find another planet like Earth, 643 00:42:49,434 --> 00:42:52,597 one that formed from the same ingredients, 644 00:42:52,604 --> 00:42:57,269 in the right place, with just the right amount of water. 645 00:42:57,275 --> 00:42:58,936 One thing's for sure -- 646 00:42:58,943 --> 00:43:01,105 there are billions of planets out there 647 00:43:01,112 --> 00:43:04,878 waiting to be discovered.