1 00:00:01,634 --> 00:00:04,236 [narrator] Uranus and Neptune. 2 00:00:04,270 --> 00:00:06,071 Two of the largest, 3 00:00:06,606 --> 00:00:08,173 farthest 4 00:00:08,207 --> 00:00:10,943 and strangest planets in our solar system. 5 00:00:12,278 --> 00:00:13,712 [Phil Plait] Of all the big planets, 6 00:00:13,747 --> 00:00:17,349 Uranus and Neptune remain the biggest enigmas. 7 00:00:17,384 --> 00:00:20,352 [narrator] Today, astronomers are finally unlocking 8 00:00:20,387 --> 00:00:24,456 why these supersized snowballs are so weird. 9 00:00:24,491 --> 00:00:26,392 [Grant Tremblay] In Uranus and Neptune, we have two planets 10 00:00:26,426 --> 00:00:29,461 that just don't like playing by the rules. 11 00:00:29,496 --> 00:00:31,330 [narrator] Can an ancient mega-collision 12 00:00:31,364 --> 00:00:34,266 reveal what knocked Uranus onto its side? 13 00:00:35,535 --> 00:00:38,070 Can a missing twin world 14 00:00:38,104 --> 00:00:41,740 unlock why one of Neptune's moons orbits the wrong way? 15 00:00:41,775 --> 00:00:43,242 [Hakeem Oluseyi] This is a large moon 16 00:00:43,276 --> 00:00:46,278 but yet it's orbiting the planet backwards. 17 00:00:46,312 --> 00:00:48,714 That's a mystery that needs explaining. 18 00:00:48,748 --> 00:00:51,683 [narrator] And what is the energy that drives the fastest winds 19 00:00:51,718 --> 00:00:54,353 in the solar system? 20 00:00:54,387 --> 00:00:58,457 It's a rare planet that has winds faster than the speed of sound. 21 00:00:58,491 --> 00:01:03,195 [narrator] To find out, we venture to these frozen planets 22 00:01:03,229 --> 00:01:05,330 explore their alien underworlds 23 00:01:06,566 --> 00:01:09,835 to reveal what makes these ice giants 24 00:01:09,869 --> 00:01:12,271 the weirdest worlds in the solar system. 25 00:01:26,486 --> 00:01:30,155 [narrator] At the limits of our solar system lie two huge planets. 26 00:01:33,259 --> 00:01:34,226 Uranus 27 00:01:35,295 --> 00:01:36,695 and Neptune. 28 00:01:38,031 --> 00:01:40,866 Both are over 14 times the mass of Earth. 29 00:01:42,435 --> 00:01:47,739 They orbit more than 2.8 billion miles away from the Sun. 30 00:01:47,774 --> 00:01:52,077 This makes them the farthest out, confirmed planets in the solar system. 31 00:01:52,112 --> 00:01:54,213 And the most mysterious. 32 00:01:54,781 --> 00:01:56,148 We have sent orbiters 33 00:01:56,182 --> 00:01:58,550 to every major planet in the solar system 34 00:01:58,585 --> 00:02:00,886 except Uranus and Neptune. 35 00:02:00,920 --> 00:02:04,123 Uranus and Neptune are really far away 36 00:02:04,157 --> 00:02:06,892 and sending a space probe out there is pretty hard. 37 00:02:10,130 --> 00:02:11,763 [beeping] 38 00:02:13,466 --> 00:02:15,134 [narrator] Only one NASA mission 39 00:02:15,168 --> 00:02:17,169 has ventured out for a closer look. 40 00:02:20,173 --> 00:02:23,842 Voyager 2 flies past Uranus in 1986 41 00:02:25,979 --> 00:02:28,447 and Neptune in 1989. 42 00:02:30,150 --> 00:02:33,452 The spacecraft reveals that these planets, 43 00:02:33,486 --> 00:02:36,355 sculpted from frozen water, methane and ammonium, 44 00:02:37,223 --> 00:02:40,459 are far from dead. 45 00:02:40,493 --> 00:02:43,061 [Michelle Thaller] The thing that's so poignant about the ice giant worlds, 46 00:02:43,096 --> 00:02:44,663 is that we've only visited them 47 00:02:44,697 --> 00:02:45,631 for a couple of days. 48 00:02:45,665 --> 00:02:48,567 The Voyager spacecraft just flew on by. 49 00:02:48,601 --> 00:02:50,335 And yet, in that brief encounter, 50 00:02:50,370 --> 00:02:51,904 we saw things we never expected. 51 00:02:51,938 --> 00:02:54,006 Instead of cold, boring worlds 52 00:02:54,040 --> 00:02:55,274 like we maybe thought they would be, 53 00:02:55,308 --> 00:02:57,776 they were exciting. They had weather. 54 00:02:57,810 --> 00:03:01,180 They turned out to be much more dynamic than we ever imagined. 55 00:03:01,214 --> 00:03:02,414 [beeping] 56 00:03:03,716 --> 00:03:06,285 [narrator] As Voyager 2 flies by, 57 00:03:06,319 --> 00:03:08,253 it captures detailed photographs, 58 00:03:08,288 --> 00:03:11,023 of the strange moons that orbit the ice giants. 59 00:03:13,159 --> 00:03:15,827 Uranus has 27 moons. 60 00:03:16,362 --> 00:03:18,297 Neptune has 14. 61 00:03:20,066 --> 00:03:21,233 [Tremblay] The ice giants have 62 00:03:21,267 --> 00:03:23,702 some of the most fascinating moons in the solar system. 63 00:03:23,736 --> 00:03:25,871 You have Naiad and Thalassa, 64 00:03:25,905 --> 00:03:27,639 two moons of Neptune that are locked 65 00:03:27,674 --> 00:03:29,541 in this gravitational dance with one another. 66 00:03:29,576 --> 00:03:31,109 And they zig-zag up and down 67 00:03:31,144 --> 00:03:33,345 unlike any other moons in the solar system. 68 00:03:34,147 --> 00:03:35,681 [Thaller] Uranus has Miranda, 69 00:03:35,715 --> 00:03:38,917 this moon that seems to have been cracked apart and reassembled, 70 00:03:38,952 --> 00:03:42,254 with vast craters and giant cliffs and fields of ice. 71 00:03:44,891 --> 00:03:47,092 [narrator] The weirdest moon of all is Triton. 72 00:03:48,928 --> 00:03:51,730 Triton is around half the size of Earth's moon. 73 00:03:53,566 --> 00:03:58,503 But it is 400 times more massive than Neptune's next largest moon, Proteus. 74 00:04:00,974 --> 00:04:04,643 [narrator] Two-thirds of Triton is rock and metal. 75 00:04:04,677 --> 00:04:07,846 The rest is a crust of water and nitrogen ice, 76 00:04:07,880 --> 00:04:13,352 frozen to a temperature of minus 390 degrees Fahrenheit. 77 00:04:13,386 --> 00:04:17,856 This ice world is unlike any other large moon in the solar system. 78 00:04:20,460 --> 00:04:21,994 [Tremblay] Triton is really unique 79 00:04:22,028 --> 00:04:24,096 in that it's the only large moon in the solar system 80 00:04:24,130 --> 00:04:25,731 with a retrograde orbit. 81 00:04:25,765 --> 00:04:29,268 That's an orbit that is opposite to the direction of the planet's spin. 82 00:04:30,737 --> 00:04:34,039 [narrator] Scientists think that most planets and their moons 83 00:04:34,073 --> 00:04:37,242 form from the same spinning cloud of gas and dust. 84 00:04:39,245 --> 00:04:40,779 The center of the spinning cloud 85 00:04:40,813 --> 00:04:44,116 collapses to form a rotating planet 86 00:04:44,150 --> 00:04:47,819 while the outer edge condenses into the planet's moons. 87 00:04:49,222 --> 00:04:51,456 Everything turns in the same direction 88 00:04:51,491 --> 00:04:52,991 as the original cloud. 89 00:04:54,227 --> 00:04:57,896 So, how does Triton end up orbiting Neptune in reverse? 90 00:05:01,768 --> 00:05:05,137 Today, new technologies and discoveries 91 00:05:05,171 --> 00:05:06,972 make it possible to finally unlock 92 00:05:07,006 --> 00:05:08,774 the mysteries of the ice giants. 93 00:05:10,877 --> 00:05:13,211 A clue about Triton's strange orbit 94 00:05:13,246 --> 00:05:17,382 lies in the data collected by one of NASA's most ambitious missions 95 00:05:17,417 --> 00:05:18,850 to the outer solar system. 96 00:05:20,687 --> 00:05:22,287 In 2015, 97 00:05:22,322 --> 00:05:26,124 NASA's New Horizons probe flies past Pluto. 98 00:05:27,727 --> 00:05:30,228 Pluto is a dwarf planet. 99 00:05:30,263 --> 00:05:33,632 It is the largest object in the Kuiper belt, 100 00:05:33,666 --> 00:05:36,201 a massive ring of billions of space rocks 101 00:05:36,235 --> 00:05:38,570 that lies beyond the orbit of Neptune, 102 00:05:38,604 --> 00:05:40,572 on the fringes of the solar system. 103 00:05:44,977 --> 00:05:46,945 New Horizons beams back to Earth 104 00:05:46,979 --> 00:05:49,381 breathtaking images of this frozen world. 105 00:05:51,050 --> 00:05:52,984 Pluto just blew me away. 106 00:05:53,019 --> 00:05:56,154 We saw glaciers made of nitrogen ice. 107 00:05:56,189 --> 00:05:58,890 And one of my favorite images from the past decade 108 00:05:58,925 --> 00:06:02,794 is sunlight glinting off of mountains of pure water ice. 109 00:06:04,630 --> 00:06:07,265 [narrator] The scientific instruments on board the probe 110 00:06:07,333 --> 00:06:10,135 analyzed Pluto's vital statistics 111 00:06:10,169 --> 00:06:13,705 to unlock what the dwarf planet is made from. 112 00:06:15,341 --> 00:06:16,819 The instruments revealed 113 00:06:16,843 --> 00:06:20,178 that Pluto and Triton are almost exactly the same. 114 00:06:21,614 --> 00:06:25,150 The bodies are roughly the same size. 115 00:06:25,184 --> 00:06:29,821 Both Pluto and Triton have cores made from rock and metal. 116 00:06:29,856 --> 00:06:35,227 And their crusts consist of layers of frozen nitrogen, water and methane. 117 00:06:36,696 --> 00:06:39,965 This evidence points to an incredible possibility. 118 00:06:39,999 --> 00:06:41,733 One that could solve the mystery 119 00:06:41,768 --> 00:06:44,970 of why Triton orbits Neptune backwards. 120 00:06:46,472 --> 00:06:47,706 [Tremblay] Triton and Pluto 121 00:06:47,740 --> 00:06:50,041 are in many ways geological twins. 122 00:06:50,076 --> 00:06:53,278 One of the simplest explanations that could account for that 123 00:06:53,312 --> 00:06:55,514 is that they originated from the same place. 124 00:06:56,916 --> 00:06:59,217 [narrator] That place is the Kuiper Belt. 125 00:07:00,420 --> 00:07:02,421 Why did Triton break out 126 00:07:02,455 --> 00:07:04,289 from this ring of rocks? 127 00:07:09,128 --> 00:07:11,096 Astronomers think that Neptune's size 128 00:07:11,130 --> 00:07:13,865 could reveal the answer. 129 00:07:13,933 --> 00:07:17,235 Neptune is 17 times more massive than Earth. 130 00:07:19,238 --> 00:07:20,806 This mass gives the planet 131 00:07:20,840 --> 00:07:23,542 a super-strong gravitational influence. 132 00:07:24,877 --> 00:07:26,545 It is possible that over time 133 00:07:26,579 --> 00:07:28,380 Neptune's huge gravity 134 00:07:28,414 --> 00:07:31,516 gradually pulls Triton away from this ring of rocks 135 00:07:33,586 --> 00:07:36,254 to eventually capture it as its moon. 136 00:07:38,024 --> 00:07:40,025 But there is one problem with this theory: 137 00:07:41,527 --> 00:07:45,730 the speed of the objects inside the Kuiper Belt. 138 00:07:45,765 --> 00:07:48,233 [Oluseyi] If you do the calculations what you will find is, 139 00:07:48,267 --> 00:07:52,571 by the time a solitary Triton passes by Neptune, 140 00:07:52,605 --> 00:07:54,706 it's moving too fast to be captured. 141 00:07:54,740 --> 00:07:57,108 Neptune's gravity would have deflected it, 142 00:07:57,143 --> 00:07:59,611 but it would have just kept on going. 143 00:07:59,645 --> 00:08:02,614 In order for Triton to end up in orbit around Neptune, 144 00:08:02,648 --> 00:08:06,218 something would have had to hit the brakes and slowed it down. 145 00:08:06,252 --> 00:08:08,720 [narrator] What slowed down Triton enough 146 00:08:08,754 --> 00:08:10,822 for Neptune to capture it? 147 00:08:11,557 --> 00:08:13,692 And how did this dwarf planet 148 00:08:13,726 --> 00:08:16,161 end up orbiting Neptune backwards? 149 00:08:26,138 --> 00:08:28,373 [narrator] The giant ice planet, Neptune, 150 00:08:28,407 --> 00:08:30,709 has a weird moon. 151 00:08:30,743 --> 00:08:33,345 Triton orbits in the opposite direction 152 00:08:33,379 --> 00:08:35,080 to Neptune's rotation. 153 00:08:36,082 --> 00:08:37,816 One explanation is that 154 00:08:37,850 --> 00:08:40,218 the planet pulled Triton from the Kuiper Belt. 155 00:08:41,721 --> 00:08:44,155 But what slowed down Triton enough 156 00:08:44,190 --> 00:08:45,857 to allow Neptune to capture it 157 00:08:45,892 --> 00:08:47,359 travelling the wrong way? 158 00:08:52,765 --> 00:08:55,433 Astronomers believe that one of Pluto's moons 159 00:08:55,468 --> 00:08:57,102 offers a clue. 160 00:08:59,238 --> 00:09:02,073 Charon is half the size of Pluto. 161 00:09:03,776 --> 00:09:05,310 This closeness in size 162 00:09:05,344 --> 00:09:07,679 makes the pair dance around each other 163 00:09:07,713 --> 00:09:09,214 as they orbit the sun. 164 00:09:11,117 --> 00:09:12,384 [Oluseyi] Pluto is not much bigger 165 00:09:12,418 --> 00:09:14,286 like all the other planets and moons. 166 00:09:14,320 --> 00:09:16,621 So instead of being like, a single large body 167 00:09:16,656 --> 00:09:18,256 with a smaller one orbiting it, 168 00:09:18,291 --> 00:09:19,624 it's more like a dumbbell, 169 00:09:19,659 --> 00:09:22,160 and this dumbbell orbits as one thing. 170 00:09:23,763 --> 00:09:27,165 [narrator] Astronomers call two orbiting bodies of a similar size 171 00:09:27,233 --> 00:09:28,867 a binary system. 172 00:09:31,671 --> 00:09:33,972 The strange orbit of binaries 173 00:09:34,006 --> 00:09:37,075 could unlock how Triton slows down enough 174 00:09:37,109 --> 00:09:40,445 for Neptune to capture it travelling the wrong way. 175 00:09:45,818 --> 00:09:49,454 When objects in the Kuiper Belt pick up a partner, 176 00:09:49,488 --> 00:09:52,023 they completely change the way they move 177 00:09:52,058 --> 00:09:53,825 through the cosmos. 178 00:09:58,130 --> 00:10:00,832 Once they've caught each other, 179 00:10:00,866 --> 00:10:04,436 they start revolving around their common center of gravity. 180 00:10:09,408 --> 00:10:11,576 This disrupts their trajectory, 181 00:10:11,611 --> 00:10:13,511 as on every turn, 182 00:10:13,546 --> 00:10:15,614 one partner must move backwards 183 00:10:15,648 --> 00:10:17,248 against their orbit. 184 00:10:20,252 --> 00:10:22,187 Once every pirouette, 185 00:10:22,221 --> 00:10:24,189 they can slow down so much 186 00:10:24,223 --> 00:10:26,791 that they become vulnerable to capture. 187 00:10:30,029 --> 00:10:31,663 Some astronomers believe that 188 00:10:31,697 --> 00:10:35,400 Triton once had its own binary partner, 189 00:10:35,434 --> 00:10:37,702 just like Pluto has Charon today. 190 00:10:38,804 --> 00:10:40,639 They think that this missing binary 191 00:10:40,673 --> 00:10:44,309 is the key that unlocks how Neptune captures Triton. 192 00:10:45,144 --> 00:10:46,511 [Plait] The discovery that 193 00:10:46,545 --> 00:10:49,547 ten to fifteen percent of Kuiper Belt objects are binary 194 00:10:49,582 --> 00:10:51,483 may solve the Triton problem. 195 00:10:51,517 --> 00:10:53,551 If Triton itself was a binary, 196 00:10:53,586 --> 00:10:55,220 it had a large companion 197 00:10:55,254 --> 00:10:57,022 when it passed by Neptune. 198 00:10:57,056 --> 00:10:58,623 That could have put the brakes on it 199 00:10:58,658 --> 00:11:00,525 and allowed it to orbit Neptune. 200 00:11:05,831 --> 00:11:07,432 [narrator] Triton and its companion 201 00:11:07,466 --> 00:11:09,734 orbit each other inside the Kuiper Belt. 202 00:11:11,303 --> 00:11:12,470 Over time, 203 00:11:12,505 --> 00:11:14,439 Neptune's gravity gradually pulls 204 00:11:14,473 --> 00:11:16,441 the pair away from the Belt. 205 00:11:17,343 --> 00:11:19,711 They fall towards the giant planet 206 00:11:19,745 --> 00:11:21,579 and slingshot around it. 207 00:11:22,348 --> 00:11:24,482 Thanks to their binary dance, 208 00:11:24,517 --> 00:11:27,252 there is a point when Triton moves slowly enough 209 00:11:27,987 --> 00:11:29,988 for Neptune to capture it. 210 00:11:31,557 --> 00:11:33,258 This breaks the gravitational bond 211 00:11:33,292 --> 00:11:36,394 between Triton and its partner 212 00:11:36,429 --> 00:11:40,165 and hurls its companion across the solar system. 213 00:11:41,100 --> 00:11:42,701 Triton is left alone 214 00:11:42,735 --> 00:11:44,869 orbiting Neptune the wrong way. 215 00:11:48,374 --> 00:11:50,842 This companion was kind of a sacrificial lamb 216 00:11:50,876 --> 00:11:53,078 that got kicked out into the depths of space 217 00:11:53,112 --> 00:11:56,381 in order for Triton itself to be captured into Neptune's orbit. 218 00:12:01,787 --> 00:12:03,088 [narrator] Triton's strange orbit 219 00:12:03,122 --> 00:12:06,124 is just one of this weird moon's mysteries. 220 00:12:09,261 --> 00:12:10,495 Voyager 2 221 00:12:10,529 --> 00:12:13,932 photographs Triton up-close in 1989. 222 00:12:17,303 --> 00:12:20,305 The camera on board captures something weird 223 00:12:20,339 --> 00:12:22,974 on the surface of this strange world. 224 00:12:25,077 --> 00:12:26,911 Hidden in this grainy photograph 225 00:12:26,979 --> 00:12:28,613 is a mysterious plume 226 00:12:28,647 --> 00:12:30,782 that rises from the surface of Triton. 227 00:12:32,351 --> 00:12:35,386 The plume is a geyser of frozen nitrogen 228 00:12:35,421 --> 00:12:36,955 five miles high. 229 00:12:38,958 --> 00:12:41,760 Winds in the moon's thin upper atmosphere 230 00:12:41,794 --> 00:12:43,995 blow the vapor into trails 231 00:12:44,029 --> 00:12:46,464 that are hundreds of miles long. 232 00:12:46,499 --> 00:12:47,999 When Voyager flew past Triton, 233 00:12:48,033 --> 00:12:49,834 it saw the unexpected. 234 00:12:49,869 --> 00:12:52,504 There were jets of gas shooting out from the moon's surface. 235 00:12:53,105 --> 00:12:54,305 No one expected that. 236 00:12:54,340 --> 00:12:56,875 It's out at the far reaches of the solar system. 237 00:12:56,909 --> 00:13:00,178 We expected to find a cold, dead moon. 238 00:13:00,212 --> 00:13:02,647 What the heck is going on? 239 00:13:02,681 --> 00:13:06,551 [narrator] Why do geysers blast from the surface of Triton? 240 00:13:06,585 --> 00:13:09,287 And what is the power source that drives them? 241 00:13:12,491 --> 00:13:14,492 In 2019, 242 00:13:14,527 --> 00:13:17,896 investigators used the Gemini South telescope in Chile 243 00:13:17,930 --> 00:13:20,098 to study Triton's icy crust. 244 00:13:23,302 --> 00:13:24,803 Researchers discovered that 245 00:13:24,837 --> 00:13:26,771 the ice on the surface of Triton 246 00:13:26,806 --> 00:13:28,673 is a special cocktail 247 00:13:28,707 --> 00:13:31,576 of carbon monoxide and frozen nitrogen. 248 00:13:31,610 --> 00:13:33,278 [Thaller] In some ways you can think of Triton 249 00:13:33,312 --> 00:13:34,979 as having a frozen atmosphere. 250 00:13:35,014 --> 00:13:37,148 Our atmosphere on Earth is mostly nitrogen 251 00:13:37,183 --> 00:13:38,750 and that's true on Triton too. 252 00:13:38,784 --> 00:13:41,252 But the nitrogen's frozen into a layer of ice. 253 00:13:42,555 --> 00:13:45,256 That ice is actually transparent to light. 254 00:13:45,291 --> 00:13:47,992 So, sunlight can get through that transparent ice 255 00:13:48,027 --> 00:13:49,627 and heat up little pockets in it. 256 00:13:51,797 --> 00:13:53,097 [narrator] Some types of ice 257 00:13:53,132 --> 00:13:55,900 transform directly from a solid into gas, 258 00:13:55,935 --> 00:13:58,036 when they heat up inside an atmosphere 259 00:13:58,070 --> 00:13:59,571 that is super thin. 260 00:14:01,740 --> 00:14:05,210 Astronomers believe that this volatility of Triton's ice 261 00:14:05,244 --> 00:14:08,046 unlocks they mystery of its mega geysers. 262 00:14:11,851 --> 00:14:13,685 At a point in Neptune's year, 263 00:14:13,719 --> 00:14:16,621 Triton's north pole leans towards the Sun. 264 00:14:18,824 --> 00:14:20,892 This gives one hemisphere of the moon 265 00:14:20,926 --> 00:14:23,928 a summer that lasts 41 Earth years. 266 00:14:25,531 --> 00:14:27,465 The rays from the Sun are weak 267 00:14:28,667 --> 00:14:31,636 but they can pass through Triton's nitrogen ice. 268 00:14:34,106 --> 00:14:36,274 Here, they hit a dark layer of dust 269 00:14:37,543 --> 00:14:39,644 which heats up by just a few degrees. 270 00:14:40,613 --> 00:14:43,348 Enough to turn the ice above into gas. 271 00:14:43,382 --> 00:14:45,416 [ice cracks] 272 00:14:45,451 --> 00:14:48,519 The pressure rises as the gas builds up, 273 00:14:48,554 --> 00:14:51,823 until it erupts as powerful geysers of nitrogen. 274 00:14:55,628 --> 00:14:58,997 [faint beeping] 275 00:14:59,031 --> 00:15:03,301 Voyager 2 shows that ice giants contain many secrets. 276 00:15:05,170 --> 00:15:07,472 The data it beams back to Earth 277 00:15:07,506 --> 00:15:10,541 reveals another mystery for astronomers to solve. 278 00:15:11,777 --> 00:15:14,312 The magnetometer on board the probe 279 00:15:14,346 --> 00:15:16,547 detects that the magnetic field of Uranus 280 00:15:16,582 --> 00:15:17,982 is super strange. 281 00:15:19,785 --> 00:15:23,254 The field lines continually twist and shift. 282 00:15:23,289 --> 00:15:25,189 The magnetic poles are offset 283 00:15:25,224 --> 00:15:27,258 from the planet's axis of rotation 284 00:15:27,293 --> 00:15:29,093 by a massive 60 degrees. 285 00:15:30,896 --> 00:15:32,897 Voyager 2 discovers later 286 00:15:32,932 --> 00:15:34,999 that the magnetic field of Neptune 287 00:15:35,034 --> 00:15:38,102 is also irregular and misaligned. 288 00:15:38,137 --> 00:15:40,571 When you think about the magnetic of a planet, 289 00:15:40,606 --> 00:15:43,441 sometimes you get something very simple, like the Earth. 290 00:15:43,475 --> 00:15:45,176 It's almost as if there's a giant bar magnet 291 00:15:45,210 --> 00:15:46,945 going right through the core of our planet 292 00:15:46,979 --> 00:15:49,213 with a very clear north pole and south pole. 293 00:15:49,748 --> 00:15:50,748 With the ice giant, 294 00:15:50,783 --> 00:15:53,184 something altogether different is going on. 295 00:15:53,218 --> 00:15:55,153 The magnetic fields are chaotic. 296 00:15:55,187 --> 00:15:57,622 The magnetic field lines twist and turn 297 00:15:57,656 --> 00:15:59,657 like a bag of writing snakes. 298 00:15:59,692 --> 00:16:01,859 And the south pole of the planet 299 00:16:01,894 --> 00:16:04,562 is actually along the equator. 300 00:16:04,596 --> 00:16:08,166 [narrator] Why are the magnetic fields of the ice giants twisted? 301 00:16:10,135 --> 00:16:12,904 Can a strange material deep inside Uranus 302 00:16:12,938 --> 00:16:14,439 unlock the mystery? 303 00:16:25,818 --> 00:16:28,686 [narrator] Uranus and Neptune 304 00:16:28,721 --> 00:16:31,823 lie at the farthest reaches of our solar system. 305 00:16:33,192 --> 00:16:35,860 Voyager 2 remains the only mission 306 00:16:35,894 --> 00:16:38,363 to visit these ice giant planets. 307 00:16:39,898 --> 00:16:43,234 The spacecraft discovers that they have strange moons 308 00:16:45,104 --> 00:16:47,939 and bizarre, twisted magnetic fields. 309 00:16:49,307 --> 00:16:53,511 Why are the magnetic fields of Uranus and Neptune so chaotic? 310 00:16:58,650 --> 00:17:00,785 Scientists believe the answer lies 311 00:17:00,819 --> 00:17:03,654 in discovering what inside the ice giants 312 00:17:03,689 --> 00:17:05,656 generates their magnetic fields? 313 00:17:08,961 --> 00:17:11,696 A mass of molten iron and nickel 314 00:17:11,730 --> 00:17:14,732 sits at the center of a rocky planet like Earth. 315 00:17:15,734 --> 00:17:17,568 This super dense liquid metal 316 00:17:17,603 --> 00:17:19,771 forms a thick and compact shell 317 00:17:19,805 --> 00:17:21,472 wrapped around the planet's core. 318 00:17:23,509 --> 00:17:26,711 Heat drives powerful currents inside this concentration 319 00:17:26,745 --> 00:17:28,212 of molten material. 320 00:17:29,348 --> 00:17:32,016 These deep currents generate a magnetic field. 321 00:17:33,552 --> 00:17:36,687 The rotation of Earth twists the currents 322 00:17:36,722 --> 00:17:40,058 into orderly streams that move in the same direction 323 00:17:40,092 --> 00:17:41,793 as Earth turns. 324 00:17:43,228 --> 00:17:46,898 The result is that the axis of Earth's magnetic field 325 00:17:46,932 --> 00:17:50,902 almost matches the axis of the planet's rotation. 326 00:17:50,936 --> 00:17:52,870 Well, the magnetic field is generated 327 00:17:52,905 --> 00:17:55,673 by rotating liquids inside the planet 328 00:17:55,707 --> 00:17:57,308 and those liquids are going to rotate 329 00:17:57,342 --> 00:17:58,910 along with the planet itself. 330 00:17:58,944 --> 00:18:00,378 So, it makes sense that 331 00:18:00,412 --> 00:18:02,847 the planet's geographic north and south pole 332 00:18:02,881 --> 00:18:07,151 align with the magnetic north and south pole. 333 00:18:07,186 --> 00:18:10,822 However, with Uranus, we don't see that. 334 00:18:10,856 --> 00:18:13,624 [narrator] An ice giant contains no moving metal 335 00:18:13,659 --> 00:18:15,960 at its core. 336 00:18:15,994 --> 00:18:18,463 Scientists believe the cores of these planets 337 00:18:18,497 --> 00:18:20,031 are made from a mixture of 338 00:18:20,065 --> 00:18:23,868 rock, water, methane and ammonia. 339 00:18:23,902 --> 00:18:26,737 So, what happens at the center of ice giants? 340 00:18:28,140 --> 00:18:31,709 Scientists at the University of Rochester in New York 341 00:18:31,743 --> 00:18:34,612 believe that the huge pressure at the core of ice giants 342 00:18:34,646 --> 00:18:36,347 helps unlock the mystery. 343 00:18:36,381 --> 00:18:37,548 [beeping] 344 00:18:38,650 --> 00:18:40,118 In 2018, 345 00:18:40,152 --> 00:18:42,820 they test how this pressure transforms water 346 00:18:42,855 --> 00:18:46,190 into weird states of matter with strange properties. 347 00:18:51,330 --> 00:18:54,098 They use an anvil made of diamonds 348 00:18:54,133 --> 00:18:55,833 to compress a sample of water 349 00:18:55,868 --> 00:18:59,270 to a pressure similar to that at the core of an ice giant. 350 00:19:03,475 --> 00:19:06,844 The crust water transforms at the molecular level 351 00:19:06,879 --> 00:19:10,548 and turns into a super-hot jet black material 352 00:19:10,582 --> 00:19:12,416 called superionic ice. 353 00:19:15,687 --> 00:19:18,422 Its oxygen atoms form a rigid lattice 354 00:19:18,457 --> 00:19:20,925 that hydrogen atoms can float through, 355 00:19:20,959 --> 00:19:23,928 meaning that it can become electrically charged. 356 00:19:26,865 --> 00:19:29,867 But this weird ice is rigid. 357 00:19:29,902 --> 00:19:32,503 So if it is at the core of the ice giants 358 00:19:32,538 --> 00:19:34,705 it creates a fundamental problem. 359 00:19:38,777 --> 00:19:41,179 Only moving material inside a planet 360 00:19:41,213 --> 00:19:44,682 can act like a dynamo and generate a magnetic field. 361 00:19:46,952 --> 00:19:48,452 The Rochester experiment 362 00:19:48,487 --> 00:19:51,489 shows that an ice giant's core could be static. 363 00:19:52,824 --> 00:19:55,493 This means that the planet's magnetic field 364 00:19:55,527 --> 00:19:57,128 can't come from there. 365 00:19:57,162 --> 00:19:59,630 Water ice under the right conditions 366 00:19:59,665 --> 00:20:01,632 very compressed to the core of a planet 367 00:20:01,667 --> 00:20:03,935 is actually stopping the magnetic field 368 00:20:03,969 --> 00:20:06,304 going through the very center of the planet. 369 00:20:07,506 --> 00:20:09,040 So there's gonna be no magnetic field 370 00:20:09,074 --> 00:20:10,508 based in the core of Uranus. 371 00:20:12,077 --> 00:20:13,477 [narrator] If the super pressurized 372 00:20:13,512 --> 00:20:15,146 ice core of an ice giant 373 00:20:15,180 --> 00:20:17,515 doesn't generate the planet's magnetic field, 374 00:20:18,550 --> 00:20:20,051 then what does? 375 00:20:25,557 --> 00:20:28,593 One theory is that a liquid ocean of compressed water 376 00:20:28,627 --> 00:20:30,294 exists in a thin shell 377 00:20:30,329 --> 00:20:32,863 around the solid core of an ice giant. 378 00:20:34,266 --> 00:20:35,766 [Thaller] Farther up from the core, 379 00:20:35,801 --> 00:20:37,702 there's this sort of a Goldilocks Zone. 380 00:20:37,736 --> 00:20:40,304 Not too compressed, the water can flow around 381 00:20:40,339 --> 00:20:41,939 yet still be electrically charged, 382 00:20:41,974 --> 00:20:43,641 and there's your magnetic field. 383 00:20:45,110 --> 00:20:47,478 [narrator] How this layer of pressurized liquid 384 00:20:47,512 --> 00:20:51,048 moves around this solid superionic ice core, 385 00:20:51,083 --> 00:20:53,117 could unlock why the magnetic fields 386 00:20:53,151 --> 00:20:54,952 of ice giants are chaotic. 387 00:20:58,056 --> 00:20:59,557 Deep inside Uranus... 388 00:20:59,591 --> 00:21:01,292 [rumbling] 389 00:21:01,326 --> 00:21:05,062 ...lies a compressed superionic ice core 390 00:21:05,097 --> 00:21:08,132 that is a scorching 9,000 degrees Fahrenheit. 391 00:21:10,302 --> 00:21:12,603 Somewhere in between this rigid ball 392 00:21:12,638 --> 00:21:14,338 and the surface of the planet 393 00:21:14,373 --> 00:21:18,075 is a shell of highly pressurized water that can flow. 394 00:21:20,812 --> 00:21:23,614 But the disconnected currents inside this layer 395 00:21:24,983 --> 00:21:27,518 do not generate a magnetic field that matches 396 00:21:27,552 --> 00:21:29,787 the direction of the planet's rotation. 397 00:21:33,625 --> 00:21:36,627 So that's how the magnetic fields of the ice giants 398 00:21:36,662 --> 00:21:39,430 can slip and become misaligned. 399 00:21:45,504 --> 00:21:48,372 Weird water unlocks why the ice giants 400 00:21:48,407 --> 00:21:50,508 have super strange magnetic fields. 401 00:21:52,110 --> 00:21:54,679 But these mega planets contain more mysteries 402 00:21:54,713 --> 00:21:56,647 for astronomers to solve. 403 00:21:56,682 --> 00:21:58,149 [beeping] 404 00:21:58,183 --> 00:22:03,020 Voyager 2 sees up close, arguably, the single strangest characteristic 405 00:22:03,055 --> 00:22:05,122 of either of these distant planets. 406 00:22:06,692 --> 00:22:08,693 [Plait] There's a lot of weird stuff about Uranus 407 00:22:08,760 --> 00:22:14,799 but maybe the weirdest is that it orbits the Sun on its side. 408 00:22:14,833 --> 00:22:17,968 [narrator] Uranus leans over 98 degrees from the vertical. 409 00:22:19,371 --> 00:22:21,839 It is the only planet in the solar system 410 00:22:21,873 --> 00:22:23,341 that is orientated this way. 411 00:22:25,344 --> 00:22:28,145 Why does this ice giant roll around the Sun 412 00:22:28,180 --> 00:22:29,780 like a huge beach ball? 413 00:22:34,986 --> 00:22:37,154 Astronomers think that planets form 414 00:22:37,189 --> 00:22:38,956 from a disc of gas and dust... 415 00:22:40,492 --> 00:22:43,594 orbiting a young star. 416 00:22:43,628 --> 00:22:47,598 These young worlds inherit both their direction and axes of spin 417 00:22:47,632 --> 00:22:49,033 from the angle of the disc. 418 00:22:53,305 --> 00:22:55,272 This means that all of the planets 419 00:22:55,307 --> 00:22:57,074 should be born with a spin axis 420 00:22:57,109 --> 00:22:59,477 that is roughly 90 degrees to the disc. 421 00:23:01,380 --> 00:23:05,249 This model predicts that Uranus starts life upright, 422 00:23:05,283 --> 00:23:09,353 just like all the other planets in the solar system. 423 00:23:09,388 --> 00:23:12,790 So what tips Uranus onto its side after it forms? 424 00:23:17,829 --> 00:23:22,266 Scientists believe that a clue lies in our solar system's violent past. 425 00:23:22,768 --> 00:23:24,635 [loud crashing] 426 00:23:25,570 --> 00:23:27,204 The early days of the solar system 427 00:23:27,239 --> 00:23:29,540 were a chaotic maelstrom of smash-and-grabs. 428 00:23:31,576 --> 00:23:34,512 We see evidence of this, of course, in the form of craters 429 00:23:34,546 --> 00:23:36,380 on many bodies in the solar system. 430 00:23:38,383 --> 00:23:42,052 [narrator] Does an ancient mega-collision knock Uranus onto its side? 431 00:23:44,189 --> 00:23:48,192 And what are the mysterious spots in the atmosphere of Neptune? 432 00:23:59,538 --> 00:24:04,208 [narrator] Uranus leans over so much that it orbits the Sun on its side. 433 00:24:05,844 --> 00:24:08,946 One explanation is that Uranus is involved 434 00:24:08,980 --> 00:24:12,082 in a mega-collision billions of years ago. 435 00:24:13,618 --> 00:24:16,120 But what kind of cosmic crash 436 00:24:16,154 --> 00:24:18,422 can knock over an entire planet? 437 00:24:22,294 --> 00:24:26,130 Researchers at the University of Durham in the United Kingdom 438 00:24:26,164 --> 00:24:29,733 simulate impacts between Uranus and large protoplanets. 439 00:24:30,836 --> 00:24:33,337 They build a virtual Uranus 440 00:24:33,371 --> 00:24:37,174 and fire space objects at it from different angles. 441 00:24:39,478 --> 00:24:42,079 These simulations show that a head-on collision 442 00:24:42,113 --> 00:24:43,747 blows apart the virtual Uranus. 443 00:24:45,917 --> 00:24:49,620 But a sideswipe generates just the right amount of force 444 00:24:49,654 --> 00:24:52,189 to leave Uranus spinning on its side 445 00:24:52,224 --> 00:24:54,358 without destroying the ice giant. 446 00:24:57,996 --> 00:25:00,397 Around four billion years ago, 447 00:25:00,432 --> 00:25:02,967 icy protoplanets surround a young Uranus. 448 00:25:04,569 --> 00:25:09,173 One ice ball planet, twice the size of Earth, wanders too close. 449 00:25:11,109 --> 00:25:13,177 The ice giant's immense gravity 450 00:25:13,211 --> 00:25:15,346 locks the planet into a collision course. 451 00:25:17,249 --> 00:25:19,250 The impact knocks Uranus backwards 452 00:25:20,652 --> 00:25:23,454 The smaller planet disintegrates in a shower 453 00:25:23,488 --> 00:25:25,189 of water and methane ice. 454 00:25:26,291 --> 00:25:29,093 The crash leaves Uranus tilted over permanently. 455 00:25:30,462 --> 00:25:33,430 The icy debris comes together over time 456 00:25:33,465 --> 00:25:36,300 to form the ice giant's 27 moons, 457 00:25:36,334 --> 00:25:41,105 and a spectacular ring system, similar to the one around Saturn. 458 00:25:41,139 --> 00:25:43,674 When you think about this violent destructive collision 459 00:25:43,708 --> 00:25:45,709 that tilted the whole world on its side, 460 00:25:45,744 --> 00:25:48,579 there was also a creative flipside to it, as well. 461 00:25:48,613 --> 00:25:51,382 It generated a whole new system of moons and rings. 462 00:25:55,220 --> 00:25:57,121 [beeping] 463 00:25:57,155 --> 00:26:00,724 [narrator] Voyager 2 arrives at Neptune in 1989. 464 00:26:02,761 --> 00:26:05,596 Here it uncovers another ice giant enigma. 465 00:26:08,567 --> 00:26:12,636 The spacecraft captures this image of a mysterious cyclone 466 00:26:12,671 --> 00:26:14,471 in Neptune's southern hemisphere. 467 00:26:17,375 --> 00:26:21,145 It is almost 8,000 miles wide. 468 00:26:21,179 --> 00:26:23,180 [Thaller] There was a beautiful surprise waiting for us 469 00:26:23,214 --> 00:26:25,449 when Voyager finally got out to Neptune. 470 00:26:25,483 --> 00:26:28,953 There was a giant storm churning away in the atmosphere. 471 00:26:28,987 --> 00:26:30,654 This storm was about the size of the Earth 472 00:26:30,689 --> 00:26:32,189 and we called it the Dark Spot. 473 00:26:33,658 --> 00:26:36,226 [narrator] The instruments on board Voyager 2 474 00:26:36,261 --> 00:26:39,029 measured the wind speed inside this mega-storm 475 00:26:39,064 --> 00:26:42,499 to be in excess of 1,500 miles per hour. 476 00:26:43,735 --> 00:26:45,803 The winds are moving so fast, 477 00:26:45,837 --> 00:26:48,339 they're moving twice the speed of a commercial airliner. 478 00:26:49,908 --> 00:26:52,176 [Plait] When you look at pictures of Uranus and Neptune, 479 00:26:52,210 --> 00:26:55,579 they're just kinda green and just kinda blue. 480 00:26:55,614 --> 00:26:59,149 But, in fact, these atmospheres are incredibly dynamic 481 00:26:59,184 --> 00:27:02,086 with enormous storms erupting all the time. 482 00:27:03,888 --> 00:27:07,057 [Thaller] The very workings of Neptune's atmosphere are very mysterious. 483 00:27:07,092 --> 00:27:10,194 How do you form a huge Earth-size storm 484 00:27:10,228 --> 00:27:13,197 and winds that blow that fast at that cold temperature? 485 00:27:14,966 --> 00:27:18,335 [narrator] The Sun powers the weather on Earth. 486 00:27:18,370 --> 00:27:21,105 Its radiation heats the surface of our planet. 487 00:27:22,774 --> 00:27:25,275 This heat creates currents in the atmosphere 488 00:27:25,310 --> 00:27:27,211 that move to make wind. 489 00:27:30,315 --> 00:27:33,984 Neptune is almost three billion miles away from the Sun. 490 00:27:35,487 --> 00:27:38,222 The energy that Neptune receives from this star 491 00:27:38,256 --> 00:27:43,027 is too weak to drive its weather in the same way as on Earth. 492 00:27:43,061 --> 00:27:46,664 So where does the massive energy that drives these storms come from? 493 00:27:52,270 --> 00:27:55,005 The Hubble Space Telescope finds a clue 494 00:27:55,040 --> 00:27:57,675 almost 30 years after Voyager 2 495 00:27:57,709 --> 00:27:59,743 discovers the Great Dark Spot. 496 00:28:01,646 --> 00:28:03,147 In 2018, 497 00:28:03,181 --> 00:28:06,884 the telescope captures this image of a new dark spot. 498 00:28:08,920 --> 00:28:11,455 This one is in the planet's northern hemisphere. 499 00:28:13,458 --> 00:28:14,892 [Thaller] The amazing thing about Neptune 500 00:28:14,926 --> 00:28:16,727 is how dynamic it is. 501 00:28:16,761 --> 00:28:20,030 We see evidence of new dark spots forming. 502 00:28:20,065 --> 00:28:22,066 So this world is very much alive. 503 00:28:22,100 --> 00:28:24,268 It may be cold, but it's not dead. 504 00:28:27,172 --> 00:28:31,141 [narrator] Astronomers scour other images of Neptune taken by Hubble 505 00:28:31,176 --> 00:28:34,411 for clues that can unlock the genesis of this new spot. 506 00:28:37,449 --> 00:28:42,519 Hubble photographs the surface of the ice giants every year. 507 00:28:42,554 --> 00:28:48,659 It creates these rolled out two-dimensional images of the surface of Neptune. 508 00:28:48,693 --> 00:28:52,896 Astronomers use these images to monitor changes in Neptune's atmosphere. 509 00:28:55,667 --> 00:29:00,237 They see something unusual in this photograph from 2017. 510 00:29:01,472 --> 00:29:04,074 Mysterious clouds of methane cover the area 511 00:29:04,109 --> 00:29:07,244 where Hubble sees the dark spot a year later. 512 00:29:09,280 --> 00:29:13,851 [Plait] On Earth, storms form when warm air rises carrying water vapor with it 513 00:29:13,885 --> 00:29:15,853 and that forms bright clouds. 514 00:29:15,887 --> 00:29:18,856 Well, we think a similar thing is happening on Neptune. 515 00:29:18,890 --> 00:29:21,058 Methane is coming up through the atmosphere, 516 00:29:21,092 --> 00:29:25,162 rises up and cools and forms these bright white clouds we see on Neptune, 517 00:29:25,196 --> 00:29:28,832 right before the big storm erupts. 518 00:29:28,867 --> 00:29:31,702 [narrator] These clouds are evidence that it is an energy source 519 00:29:31,736 --> 00:29:35,405 inside Neptune that drives the dark spot storms. 520 00:29:39,377 --> 00:29:42,146 What is this mysterious source of energy? 521 00:29:43,948 --> 00:29:47,885 And how does it generate the fastest winds in the solar system? 522 00:29:59,030 --> 00:30:00,998 [narrator] The storm winds on Neptune 523 00:30:01,032 --> 00:30:03,667 exceed 1500 miles per hour. 524 00:30:05,470 --> 00:30:08,238 But how can a giant planet made of ice, 525 00:30:08,273 --> 00:30:10,440 far away from the Sun, 526 00:30:10,475 --> 00:30:13,377 have the energy to drive these mega-hurricanes? 527 00:30:19,818 --> 00:30:24,354 The very large telescope in Chile takes infrared images of Neptune 528 00:30:24,389 --> 00:30:25,989 to reveal its temperature. 529 00:30:27,492 --> 00:30:31,228 This heat map shows that the upper atmosphere of Neptune 530 00:30:31,262 --> 00:30:34,264 is -330 degrees Fahrenheit. 531 00:30:35,967 --> 00:30:38,936 But this frozen ice world has a core temperature 532 00:30:38,970 --> 00:30:42,639 of 12600 degrees Fahrenheit. 533 00:30:42,674 --> 00:30:45,242 That is hotter than the surface of the Sun. 534 00:30:47,145 --> 00:30:51,415 Neptune's high core temperature is the result of the planet's formation 535 00:30:51,449 --> 00:30:52,883 billions of years ago. 536 00:30:54,485 --> 00:30:59,189 The early solar system is a massive fast-moving disc of gas and dust. 537 00:31:00,825 --> 00:31:04,127 Gravity collapses clouds of this gas into protoplanets. 538 00:31:05,897 --> 00:31:08,031 The temperature inside a planet 539 00:31:08,066 --> 00:31:11,134 rises as more material compresses to grow its mass. 540 00:31:13,471 --> 00:31:18,609 [Tremblay] Neptune formed from the collapse of gas and dust under its own gravity. 541 00:31:18,643 --> 00:31:22,579 And that collapse can lock in a lot of that internal energy 542 00:31:22,614 --> 00:31:25,749 left over from the early days of the solar system 543 00:31:25,783 --> 00:31:28,719 that can last within the planet for billions of years. 544 00:31:30,355 --> 00:31:32,789 [narrator] Astronomers think that this difference in temperature 545 00:31:32,824 --> 00:31:36,193 between Neptune's hot core and cold atmosphere 546 00:31:36,227 --> 00:31:38,829 is what triggers the planet's superstorms. 547 00:31:44,269 --> 00:31:48,105 The extreme heat that radiates from Neptune's core 548 00:31:48,139 --> 00:31:51,742 vaporizes the frozen gas close to the planet's surface. 549 00:31:55,914 --> 00:31:57,881 This forces methane gas 550 00:31:57,916 --> 00:32:00,717 high into the coldest parts of the atmosphere, 551 00:32:00,752 --> 00:32:03,787 where it crystalizes into wispy clouds. 552 00:32:08,493 --> 00:32:12,396 The hot rising gas beneath finally erupts. 553 00:32:12,430 --> 00:32:14,865 It punches a hole in the upper atmosphere 554 00:32:14,899 --> 00:32:17,301 that appears as a Giant Dark Spot. 555 00:32:19,404 --> 00:32:22,506 The extreme temperature difference between the gases 556 00:32:22,540 --> 00:32:24,908 generates mega convection currents 557 00:32:24,943 --> 00:32:28,178 that whip the spot into a supersonic vortex. 558 00:32:31,215 --> 00:32:35,152 The driving force behind Neptune's mysterious mega winds 559 00:32:35,186 --> 00:32:38,121 is not the final mystery the ice giants pose. 560 00:32:39,791 --> 00:32:43,026 Uranus is 14 times more massive than Earth. 561 00:32:44,429 --> 00:32:48,632 Neptune is a whopping 17 times more massive. 562 00:32:48,666 --> 00:32:53,203 These huge worlds prowl the farthest reaches of the solar system 563 00:32:53,237 --> 00:32:58,208 between 1.8 and 3 billion miles from the Sun. 564 00:32:58,242 --> 00:33:02,379 This strange combination of supersize and distant position 565 00:33:02,413 --> 00:33:06,016 presents a big puzzle for astronomers to solve. 566 00:33:06,050 --> 00:33:08,051 [Oluseyi] One of the mysteries of Uranus and Neptune 567 00:33:08,086 --> 00:33:10,454 is their location in the solar system. 568 00:33:10,488 --> 00:33:12,389 They're so far away from the Sun 569 00:33:12,423 --> 00:33:14,891 that there would not have been enough material 570 00:33:14,926 --> 00:33:19,296 in the early solar system to build large planets of that size. 571 00:33:19,330 --> 00:33:22,666 It shouldn't be possible for these guys to exist where they do. 572 00:33:22,700 --> 00:33:23,934 But they do. 573 00:33:29,440 --> 00:33:34,878 [narrator] The early solar system is a swirling disc of gas and dust. 574 00:33:34,912 --> 00:33:38,548 When the Sun ignites, its heat blows away the lighter elements. 575 00:33:40,618 --> 00:33:43,086 Over 400 million miles out, 576 00:33:43,121 --> 00:33:47,524 it is cold enough for these elements to condense as gas and ice, 577 00:33:47,558 --> 00:33:49,893 and settle in a band around the Sun. 578 00:33:53,264 --> 00:33:59,036 This material binds together to form the beginnings of Jupiter and Saturn. 579 00:33:59,070 --> 00:34:03,340 These planets grow larger as they absorb more and more gas and dust. 580 00:34:05,977 --> 00:34:08,412 But where Neptune and Uranus orbit, 581 00:34:08,446 --> 00:34:11,148 the raw materials to build giant planets 582 00:34:11,182 --> 00:34:13,316 are in much shorter supply. 583 00:34:16,287 --> 00:34:20,023 So how is it possible for the ice giants to form this far out? 584 00:34:23,694 --> 00:34:26,029 [Tremblay] To build a planet, especially a giant one, 585 00:34:26,064 --> 00:34:29,166 we need countless tiny rocky and icy bodies 586 00:34:29,200 --> 00:34:32,102 to come together, to grow over time. 587 00:34:32,136 --> 00:34:34,738 And we think that where we see these planets nowadays, 588 00:34:34,772 --> 00:34:36,807 in the far outskirts of our solar system, 589 00:34:36,841 --> 00:34:38,909 this material was just too scarce 590 00:34:38,943 --> 00:34:40,577 to grow planets that large. 591 00:34:42,313 --> 00:34:45,126 [narrator] Astronomers arrive at an astonishing conclusion 592 00:34:45,150 --> 00:34:46,716 to explain this anomaly. 593 00:34:48,219 --> 00:34:51,721 The ice giants come from somewhere else in the solar system. 594 00:34:53,324 --> 00:34:54,958 [Plait] One possible solution is that 595 00:34:54,992 --> 00:34:58,228 Uranus and Neptune didn't form where they are now. 596 00:34:58,262 --> 00:35:00,297 If they formed closer into the Sun, 597 00:35:00,331 --> 00:35:02,099 there would have been enough material 598 00:35:02,133 --> 00:35:03,934 to create such large planets. 599 00:35:03,968 --> 00:35:08,839 And then somehow, Uranus and Neptune moved out to where they are today. 600 00:35:08,873 --> 00:35:11,041 [narrator] Why did these ice giants migrate 601 00:35:11,075 --> 00:35:14,744 to the farthest reaches of the solar system? 602 00:35:14,779 --> 00:35:19,749 And what was the role of these planets in shaping the solar system we see today? 603 00:35:30,728 --> 00:35:33,396 Astronomers think that Neptune and Uranus 604 00:35:33,431 --> 00:35:36,566 were not born in the far reaches of the solar system, 605 00:35:36,601 --> 00:35:37,801 where they are today. 606 00:35:41,038 --> 00:35:43,874 Instead, they formed closer to the Sun 607 00:35:43,908 --> 00:35:46,309 and then moved farther and farther out. 608 00:35:49,046 --> 00:35:52,349 What triggered this epic migration of the ice giants? 609 00:35:57,421 --> 00:36:02,859 Scientists believe the answer is locked inside meteorites. 610 00:36:02,894 --> 00:36:06,630 Meteorites are the parts of ancient asteroids that collide with Earth. 611 00:36:12,703 --> 00:36:16,139 They contain tiny quantities of radioactive elements. 612 00:36:18,176 --> 00:36:21,478 These elements allow scientists to date the meteorites, 613 00:36:21,512 --> 00:36:24,848 to discover the exact age of the asteroids they come from. 614 00:36:25,917 --> 00:36:27,617 [Plait] Meteorites that fall to Earth 615 00:36:27,652 --> 00:36:30,654 have elements like uranium, which is radioactive. 616 00:36:30,688 --> 00:36:33,757 And this radioactive decay that uranium undergoes 617 00:36:33,791 --> 00:36:35,158 is like a ticking clock 618 00:36:35,193 --> 00:36:37,294 that allows us to get the age of the meteorite. 619 00:36:39,063 --> 00:36:41,498 [narrator] The radioactive clock starts ticking 620 00:36:41,532 --> 00:36:46,102 when the space rock first forms at the birth of the solar system. 621 00:36:46,137 --> 00:36:50,907 That means all meteorites should be around 4.6 billion years old. 622 00:36:52,043 --> 00:36:54,878 But scientists make an extraordinary discovery 623 00:36:54,912 --> 00:36:58,582 when they date the space rocks that fall to Earth. 624 00:36:58,616 --> 00:37:01,718 One that can unlock the mystery of why the ice giants 625 00:37:01,752 --> 00:37:04,654 moved farther out across the solar system. 626 00:37:11,862 --> 00:37:14,931 [Plait] When we get the ages of meteorites through radioactive decay, 627 00:37:14,966 --> 00:37:18,735 we find that most of them are about the same age as the solar system itself. 628 00:37:18,769 --> 00:37:22,505 However, a lot of meteorites have a significantly younger age. 629 00:37:24,075 --> 00:37:26,276 [narrator] The clocks of these younger asteroids 630 00:37:26,310 --> 00:37:30,547 start ticking around 80 million years after the solar system forms. 631 00:37:32,516 --> 00:37:35,986 Super-heating and melting the minerals inside asteroids 632 00:37:36,020 --> 00:37:38,388 resets their radioactive clocks. 633 00:37:39,557 --> 00:37:41,191 Scientists believe that 634 00:37:41,225 --> 00:37:44,894 only a super-violent solar system-wide catastrophe 635 00:37:44,929 --> 00:37:47,264 could generate temperatures high enough 636 00:37:47,298 --> 00:37:49,065 to melt down so much rock. 637 00:37:49,800 --> 00:37:51,234 We're talking destruction 638 00:37:51,269 --> 00:37:53,370 on a truly unprecedented scale. 639 00:37:53,404 --> 00:37:56,673 Entire worlds colliding together and getting obliterated, 640 00:37:56,707 --> 00:37:59,442 asteroids flying every which way you look. 641 00:37:59,477 --> 00:38:02,479 If you were to actually look up in the sky during this time, 642 00:38:02,513 --> 00:38:05,482 you'd have seen this destructive light show unlike any other. 643 00:38:07,184 --> 00:38:08,785 [Plait] Objects were streaming around, 644 00:38:08,819 --> 00:38:10,987 smashing into each other, and remelting. 645 00:38:11,856 --> 00:38:13,556 And that's why these meteorites 646 00:38:13,591 --> 00:38:16,092 seem to be younger than the solar system itself. 647 00:38:17,895 --> 00:38:19,863 [narrator] Scientists think this disaster 648 00:38:19,897 --> 00:38:22,332 impacts all the planets in the solar system... 649 00:38:23,067 --> 00:38:25,435 including the ice giants. 650 00:38:26,704 --> 00:38:29,472 But what triggers this mega violent event? 651 00:38:30,508 --> 00:38:33,243 And how does it banish Uranus and Neptune 652 00:38:33,277 --> 00:38:36,846 to a region of space billions of miles from the Sun? 653 00:38:42,153 --> 00:38:45,221 Astronomers believed that the proximity of the ice giants 654 00:38:45,256 --> 00:38:48,658 to their gas giant neighbors, Jupiter and Saturn, 655 00:38:48,693 --> 00:38:51,261 unlocks the mystery. 656 00:38:51,295 --> 00:38:53,797 [Thaller] Probably all four major worlds of our solar system, 657 00:38:53,831 --> 00:38:55,899 Jupiter, Saturn, Uranus and Neptune 658 00:38:55,933 --> 00:38:58,168 were competing for planet-building materials, 659 00:38:58,202 --> 00:39:00,970 jockeying for position in a young solar system. 660 00:39:01,005 --> 00:39:03,840 All of a sudden, you're kind of playing planetary pinball. 661 00:39:05,109 --> 00:39:07,944 [narrator] Around 4.6 billion years ago, 662 00:39:07,978 --> 00:39:11,614 Jupiter, Saturn, Neptune and Uranus 663 00:39:11,649 --> 00:39:15,719 orbit much closer to each other than they do today. 664 00:39:15,753 --> 00:39:19,789 Back then, their proximity causes gravitational chaos. 665 00:39:21,392 --> 00:39:24,794 The huge gravity of the larger Jupiter and Saturn 666 00:39:24,829 --> 00:39:27,664 forces out the smaller ice giants. 667 00:39:27,698 --> 00:39:32,502 And they begin an epic journey across the solar system. 668 00:39:32,536 --> 00:39:36,506 One theory is that they even swap places along the way. 669 00:39:38,876 --> 00:39:42,145 Uranus and Neptune travel billions of miles 670 00:39:42,179 --> 00:39:46,216 to end up in the sparse outer reaches of the solar system. 671 00:39:49,453 --> 00:39:51,154 [Thaller] The truth of the matter is that 672 00:39:51,188 --> 00:39:53,623 if you're jostling with Jupiter and Saturn for real estate, 673 00:39:53,657 --> 00:39:54,724 even an ice giant 674 00:39:54,759 --> 00:39:56,226 really doesn't stand much of a chance. 675 00:39:56,260 --> 00:39:57,727 It's out to the suburbs. 676 00:39:58,996 --> 00:40:00,397 [narrator] This gravitational battle 677 00:40:00,431 --> 00:40:03,500 between Jupiter, Saturn and the ice giants 678 00:40:03,534 --> 00:40:07,337 triggers an intense asteroid bombardment on the solar system. 679 00:40:09,306 --> 00:40:13,320 Space rocks smash together in mega impacts 680 00:40:13,344 --> 00:40:16,913 in the most violent period of the solar system's history. 681 00:40:18,149 --> 00:40:21,418 This stress melts and reforms the asteroids, 682 00:40:21,485 --> 00:40:24,521 and resets their radioactive clocks. 683 00:40:24,555 --> 00:40:26,790 Over time, many of these rocks 684 00:40:26,824 --> 00:40:29,225 drift into the inner solar system 685 00:40:29,260 --> 00:40:31,895 and crashed to Earth as meteorites. 686 00:40:37,468 --> 00:40:39,836 The migration of Uranus and Neptune 687 00:40:39,870 --> 00:40:44,707 explains why the ice giants orbit so far from the Sun. 688 00:40:44,742 --> 00:40:48,077 Their movement reshuffles the entire solar system. 689 00:40:48,112 --> 00:40:51,381 And locks it into the configuration we know today. 690 00:40:57,621 --> 00:41:00,890 The ice giants are the weirdest worlds in the solar system. 691 00:41:02,159 --> 00:41:05,028 From Neptune's superfast winds... 692 00:41:05,062 --> 00:41:08,698 and the giant geysers on its moon, Triton... 693 00:41:08,732 --> 00:41:12,168 to the bizarre sideways rotation of Uranus... 694 00:41:12,203 --> 00:41:14,170 and it's strange magnetic field 695 00:41:14,205 --> 00:41:16,439 powered by an exotic form of water... 696 00:41:17,908 --> 00:41:21,544 these two planets break all the rules. 697 00:41:21,579 --> 00:41:24,681 Uranus and Neptune are dynamic, interesting, 698 00:41:24,715 --> 00:41:27,617 and above all else, weird places. 699 00:41:28,953 --> 00:41:30,587 [Thaller] The ice giants prove to us that 700 00:41:30,621 --> 00:41:33,122 there's no such thing as a normal planet. 701 00:41:33,157 --> 00:41:35,525 They work in an entirely different way 702 00:41:35,559 --> 00:41:38,561 than any other type of planet that we know of. 703 00:41:38,596 --> 00:41:40,797 [Oluseyi] They just completely blow away our ideas 704 00:41:40,831 --> 00:41:42,765 of how planets are formed. 705 00:41:42,833 --> 00:41:45,602 So far, we only had a glimpse of these ice giant planets. 706 00:41:45,636 --> 00:41:47,403 And the next probes that go there, 707 00:41:47,438 --> 00:41:49,172 might find they're even weirder 708 00:41:49,206 --> 00:41:50,206 than we think now. 709 00:41:51,642 --> 00:41:52,775 [narrator] Our solar system 710 00:41:52,810 --> 00:41:54,878 would be a very different place, 711 00:41:54,912 --> 00:41:58,381 if it were not for these strange oddball worlds.