1 00:00:02,240 --> 00:00:03,600 Our sun. 2 00:00:05,480 --> 00:00:07,760 The heart of the solar system. 3 00:00:07,760 --> 00:00:10,600 The giver of light, heat... 4 00:00:12,640 --> 00:00:14,720 ..and, of course, life. 5 00:00:15,880 --> 00:00:17,600 But what does its future hold? 6 00:00:20,560 --> 00:00:23,560 Scientists are looking to the stars to find out. 7 00:00:24,840 --> 00:00:28,320 Between these two stars is what's going to happen to our sun. 8 00:00:31,000 --> 00:00:34,920 Scientists today are almost like modern-day prophets. 9 00:00:34,920 --> 00:00:37,880 They foresee an apocalyptic future. 10 00:00:39,240 --> 00:00:41,200 Imagine the ball is Andromeda Galaxy 11 00:00:41,200 --> 00:00:44,000 on a head-on collision with the Milky Way Galaxy. 12 00:00:45,720 --> 00:00:49,280 The fate of the Earth hangs in the balance. 13 00:00:49,280 --> 00:00:50,720 Wow! Look at this! 14 00:00:50,720 --> 00:00:53,120 The temperature at the surface of the Earth 15 00:00:53,120 --> 00:00:55,120 will be enough to melt rock. 16 00:00:55,120 --> 00:00:58,480 Enough to melt the whole surface of the Earth. 17 00:00:58,480 --> 00:01:02,600 Unfortunately, nobody will be around to see it, which is a pity. 18 00:01:07,120 --> 00:01:10,000 This is the story of how our sun 19 00:01:10,000 --> 00:01:13,680 will transform the solar system it binds together. 20 00:01:16,960 --> 00:01:19,600 Before bringing it to a spectacular end. 21 00:01:44,040 --> 00:01:45,680 Peoria, Illinois. 22 00:01:48,080 --> 00:01:50,400 An average city in Midwest America. 23 00:01:53,320 --> 00:01:56,720 But it has one claim to fame that's out of this world. 24 00:02:00,560 --> 00:02:04,920 In the middle of the town, there's a 46-foot-wide mosaic of the sun. 25 00:02:08,680 --> 00:02:12,400 The centrepiece of a huge scale model of our solar system, 26 00:02:12,400 --> 00:02:15,320 created by local astronomer, Sheldon Schafer. 27 00:02:16,760 --> 00:02:19,360 And here we are at the sun. And, boy, is it hot! 28 00:02:19,360 --> 00:02:23,000 It's about 10,000 degrees here at the surface. 29 00:02:23,000 --> 00:02:26,320 And over a million Earths could fit inside of the sun. 30 00:02:28,280 --> 00:02:29,640 Peoria's solar system, 31 00:02:29,640 --> 00:02:32,760 99 million times smaller than the real thing, 32 00:02:32,760 --> 00:02:37,240 accurately reveals the relative sizes of our sun and its planets. 33 00:02:37,240 --> 00:02:39,200 OK, we're all together? 34 00:02:39,200 --> 00:02:41,280 And the distances between them. 35 00:02:44,440 --> 00:02:47,440 My job title is curator of the solar system. 36 00:02:47,440 --> 00:02:50,080 And we just went 33 million miles 37 00:02:50,080 --> 00:02:53,480 until we got to this tiny little two-inch Mercury. 38 00:02:53,480 --> 00:02:56,520 All right, so we're headed off to Venus! 39 00:02:58,040 --> 00:03:00,600 From Mercury, the inner planets are strung along 40 00:03:00,600 --> 00:03:05,040 a picturesque riverside park, all the way to Mars. 41 00:03:05,040 --> 00:03:08,400 These planets are relatively close together. 42 00:03:12,120 --> 00:03:15,240 The outer planets are much further away, 43 00:03:15,240 --> 00:03:17,320 in some bizarre locations. 44 00:03:20,560 --> 00:03:22,640 Five miles from the image of the sun, 45 00:03:22,640 --> 00:03:25,800 above the local airport's check-in desks, 46 00:03:25,800 --> 00:03:28,480 is five-foot-wide Jupiter. 47 00:03:29,800 --> 00:03:32,680 If you're going to have a planet, you may as well have the biggest! 48 00:03:32,680 --> 00:03:35,360 So it's fun to have Jupiter. 49 00:03:35,360 --> 00:03:37,840 Occasionally, we have birds that decorate, 50 00:03:37,840 --> 00:03:40,520 so we've had to clean it. But not very often. 51 00:03:42,920 --> 00:03:47,080 While the children's section of a neighbouring town's library 52 00:03:47,080 --> 00:03:48,760 is home to Saturn. 53 00:03:56,360 --> 00:03:59,400 Uranus is in Princeville, Illinois. 54 00:04:01,640 --> 00:04:05,280 From there, it's a 10-mile drive along Route 91, 55 00:04:05,280 --> 00:04:08,280 or almost a billion miles in cosmic terms, 56 00:04:08,280 --> 00:04:14,200 to the old railroad depot in Wyoming, Illinois, and Neptune. 57 00:04:19,680 --> 00:04:22,200 And finally, in a furniture store 58 00:04:22,200 --> 00:04:25,160 40 Earth miles away from the centre of the sun 59 00:04:25,160 --> 00:04:29,000 in Kewanee, Illinois is distant Pluto. 60 00:04:34,920 --> 00:04:39,640 Peoria's models are a perfect likeness of the solar system today. 61 00:04:39,640 --> 00:04:41,800 But it won't always be this way. 62 00:04:45,360 --> 00:04:49,520 Scientists know that one day, the sun will fundamentally change. 63 00:04:52,280 --> 00:04:54,160 And transform the planets. 64 00:04:57,760 --> 00:05:01,600 Imagine fast-forwarding through the next seven billion years 65 00:05:01,600 --> 00:05:05,120 to watch the end of the solar system. 66 00:05:11,120 --> 00:05:14,400 Dr Eva Villaver can predict this future. 67 00:05:17,000 --> 00:05:19,840 Because everything that will happen to our sun 68 00:05:19,840 --> 00:05:22,760 is already happening to countless other stars. 69 00:05:25,680 --> 00:05:28,800 Some, known as solar twins, 70 00:05:28,800 --> 00:05:30,800 are remarkably similar to our own. 71 00:05:33,440 --> 00:05:36,120 The studies we are doing is because they are very important 72 00:05:36,120 --> 00:05:38,640 to understand not only the sun, 73 00:05:38,640 --> 00:05:42,880 but they tell us how the future of our own solar system will be. 74 00:05:44,480 --> 00:05:48,360 In 2013, a solar twin called CoRoT Sol 1 was discovered. 75 00:05:50,240 --> 00:05:54,120 CoRoT is over there, in the constellation of Monoceros. 76 00:05:54,120 --> 00:05:55,760 It's a star, like the sun, 77 00:05:55,760 --> 00:05:58,400 and has the same mass. Exactly the same mass. 78 00:05:58,400 --> 00:06:02,640 But astronomers found one particularly significant difference. 79 00:06:03,640 --> 00:06:06,840 It had a lower concentration of the element lithium, 80 00:06:06,840 --> 00:06:10,720 which helped them to accurately calculate its age. 81 00:06:10,720 --> 00:06:14,720 It's a star that is a little bit older than the sun. 82 00:06:14,720 --> 00:06:17,200 A few billion years older. 83 00:06:17,200 --> 00:06:19,920 And if we observe a star that is older than our sun, 84 00:06:19,920 --> 00:06:22,120 we know what will happen to the sun. 85 00:06:24,920 --> 00:06:28,840 This older version of our sun was giving out more radiation. 86 00:06:34,000 --> 00:06:37,000 So it helped us put the pieces together. 87 00:06:37,000 --> 00:06:39,240 As the sun will get older, 88 00:06:39,240 --> 00:06:41,640 it will become brighter. Much brighter. 89 00:06:47,480 --> 00:06:51,280 Our sun's luminosity is slowly increasing 90 00:06:51,280 --> 00:06:54,480 because of a change deep inside the sun. 91 00:06:58,080 --> 00:07:01,800 Where two opposing forces are in constant battle. 92 00:07:04,040 --> 00:07:07,120 Similar forces to those that act on a hot-air balloon. 93 00:07:12,520 --> 00:07:17,120 Pushing up and out is the immense pressure of hot gas. 94 00:07:17,120 --> 00:07:20,120 In the sun, this is created by nuclear fusion. 95 00:07:27,560 --> 00:07:30,360 The sun has been burning hydrogen into helium 96 00:07:30,360 --> 00:07:33,960 for thousands of millions of years now. 97 00:07:33,960 --> 00:07:36,280 This is like the propane bottles here. 98 00:07:36,280 --> 00:07:39,840 It's like generating heat that warms up the air 99 00:07:39,840 --> 00:07:41,480 that keeps the balloon going. 100 00:07:41,480 --> 00:07:43,560 That's what happens in the sun, too. 101 00:07:47,840 --> 00:07:50,960 But pulling down into the core of the sun 102 00:07:50,960 --> 00:07:54,320 is an equally powerful force - gravity. 103 00:07:54,320 --> 00:07:58,720 The life of the sun is nothing but a battle against gravity. 104 00:07:58,720 --> 00:08:01,400 We have the gravitational force trying to pull the stars, 105 00:08:01,400 --> 00:08:02,960 crush the stars together. 106 00:08:02,960 --> 00:08:06,000 I mean, like pushing it in, and then we have 107 00:08:06,000 --> 00:08:09,560 the thermal pressure of the gas pushing outwards. 108 00:08:09,560 --> 00:08:13,480 So the balance between the two forces is what keeps the sun stable. 109 00:08:18,680 --> 00:08:20,680 For 4.5 billion years, 110 00:08:20,680 --> 00:08:23,240 the two forces have been in perfect balance. 111 00:08:25,280 --> 00:08:28,360 But as time passes, this balance is shifting. 112 00:08:31,400 --> 00:08:33,360 As the sun fuses hydrogen, 113 00:08:33,360 --> 00:08:38,040 it produces around 600 million tonnes of helium every second, 114 00:08:38,040 --> 00:08:40,800 which is a denser gas. 115 00:08:42,520 --> 00:08:44,120 This change in density 116 00:08:44,120 --> 00:08:46,920 has a profound effect on the nuclear reactions. 117 00:08:49,600 --> 00:08:53,640 As the core gets denser, hydrogen is burned at a higher rate. 118 00:08:53,640 --> 00:08:55,480 It's like turning the burners up. 119 00:08:55,480 --> 00:08:57,920 I mean, we are increasing the energy 120 00:08:57,920 --> 00:09:00,520 that is coming out of the core at that point. 121 00:09:02,080 --> 00:09:07,200 As a result, our sun is getting 10% brighter every billion years. 122 00:09:11,920 --> 00:09:15,920 So the older it gets, the more it heats up the solar system. 123 00:09:18,760 --> 00:09:22,760 And scientists know that will one day have serious consequences... 124 00:09:26,160 --> 00:09:28,680 ..for Walter Kinsman's favourite planet. 125 00:09:30,640 --> 00:09:33,040 The Earth is my favourite planet to paint. 126 00:09:34,880 --> 00:09:36,400 I never get my fill of it. 127 00:09:38,080 --> 00:09:42,000 He's painted all the planets in the Peoria solar system model, 128 00:09:42,000 --> 00:09:45,680 except Jupiter, which was too big to fit in his house. 129 00:09:47,560 --> 00:09:51,360 He's now painting a spare Earth for the local museum. 130 00:09:51,360 --> 00:09:54,480 I'm in the process of painting a storm system 131 00:09:54,480 --> 00:09:57,240 in the southern Indian Ocean. 132 00:09:57,240 --> 00:09:59,560 The beautiful white clouds 133 00:09:59,560 --> 00:10:03,600 up against the blue oceans is breathtaking. 134 00:10:05,880 --> 00:10:08,440 The Earth only has its oceans and clouds 135 00:10:08,440 --> 00:10:12,920 because it orbits in a band around the sun called the habitable zone. 136 00:10:14,520 --> 00:10:19,080 Which means it's just the right temperature for liquid water. 137 00:10:19,080 --> 00:10:21,640 And that makes it the only planet in the solar system 138 00:10:21,640 --> 00:10:23,880 where we know life can thrive. 139 00:10:28,360 --> 00:10:30,800 But as the sun becomes more powerful, 140 00:10:30,800 --> 00:10:33,080 the habitable zone will move. 141 00:10:37,720 --> 00:10:40,640 For a vision of the Earth in two billion years' time, 142 00:10:40,640 --> 00:10:44,040 astrobiologist Professor Lynn Rothschild 143 00:10:44,040 --> 00:10:46,280 believes we should look to Venus. 144 00:10:54,560 --> 00:10:56,120 Venus is up in the sky there. 145 00:10:56,120 --> 00:10:59,080 It's the brightest object after the sun and the moon. 146 00:10:59,080 --> 00:11:01,560 It's right near Jupiter this morning. 147 00:11:01,560 --> 00:11:04,440 It's just an absolutely spectacular day to see it. 148 00:11:08,400 --> 00:11:11,320 Venus and the Earth formed out of the same materials. 149 00:11:11,320 --> 00:11:13,160 They're roughly the same size. 150 00:11:13,160 --> 00:11:16,160 The difference is that Venus is closer to the sun. 151 00:11:23,600 --> 00:11:27,280 The surface of Venus is the most hellish planetary surface 152 00:11:27,280 --> 00:11:28,880 in our entire solar system. 153 00:11:32,000 --> 00:11:35,680 The winds are ridiculous. They're 350 miles per hour. 154 00:11:35,680 --> 00:11:38,800 And then the temperature is unbelievably hot, 155 00:11:38,800 --> 00:11:42,320 about 900 degrees or so Fahrenheit. 156 00:11:42,320 --> 00:11:45,320 So this is not a place that you'd want to be. 157 00:11:48,200 --> 00:11:51,520 It's no surprise Venus is warmer than Earth, 158 00:11:51,520 --> 00:11:54,920 but strangely, Venus is even hotter than Mercury, 159 00:11:54,920 --> 00:11:57,440 despite being further from the sun. 160 00:12:08,000 --> 00:12:10,960 In 2006, the Venus Express probe 161 00:12:10,960 --> 00:12:13,440 launched towards our nearest planet 162 00:12:13,440 --> 00:12:18,040 to analyse the Venusian atmosphere in unprecedented detail. 163 00:12:21,560 --> 00:12:24,320 It found a vital clue among the clouds 164 00:12:24,320 --> 00:12:26,840 to how Venus became so hot. 165 00:12:28,560 --> 00:12:32,520 Venus Express allowed us to see that 166 00:12:32,520 --> 00:12:36,440 there was a lot of deuterium, which is a heavy form of hydrogen, left. 167 00:12:36,440 --> 00:12:40,160 And that's indicative of the fact that there was once water here. 168 00:12:41,400 --> 00:12:43,760 It soon became clear that in the past, 169 00:12:43,760 --> 00:12:45,960 Venus was a very different world. 170 00:12:48,920 --> 00:12:51,520 So here was this beautiful water world, 171 00:12:51,520 --> 00:12:55,280 not too dissimilar to maybe what the Earth is like today. 172 00:12:55,280 --> 00:12:58,640 There was liquid water and reasonable atmospheric pressure 173 00:12:58,640 --> 00:13:00,280 and organic compounds. 174 00:13:00,280 --> 00:13:03,440 There's no reason that there shouldn't have been life. 175 00:13:13,560 --> 00:13:17,680 The evidence suggests that Venus was once in the habitable zone. 176 00:13:20,040 --> 00:13:23,480 But, as the sun grew brighter three billion years ago, 177 00:13:23,480 --> 00:13:26,760 it would have had a dramatic effect on the planet's water. 178 00:13:29,360 --> 00:13:31,320 As the sun started to get hotter, 179 00:13:31,320 --> 00:13:33,560 the surface of Venus started to get hotter. 180 00:13:33,560 --> 00:13:36,240 And therefore, the water turns into steam. 181 00:13:36,240 --> 00:13:38,720 And steam is a greenhouse gas, 182 00:13:38,720 --> 00:13:41,800 so that means it traps the solar radiation. 183 00:13:41,800 --> 00:13:44,040 And therefore, just like a greenhouse, 184 00:13:44,040 --> 00:13:46,040 it starts to get hotter and hotter. 185 00:13:48,320 --> 00:13:50,640 It seems a runaway greenhouse effect 186 00:13:50,640 --> 00:13:54,200 caused Venus to become the hottest planet in the solar system. 187 00:13:58,320 --> 00:14:02,920 Mercury, although closer to the sun, has no atmosphere and no water. 188 00:14:05,640 --> 00:14:07,520 Earth has both. 189 00:14:07,520 --> 00:14:10,960 And as the brighter sun evaporates our oceans, 190 00:14:10,960 --> 00:14:14,160 the effect is likely to be far more intense 191 00:14:14,160 --> 00:14:16,880 than the man-made global warming we see today. 192 00:14:19,840 --> 00:14:23,760 Over the next two billion years, temperatures on Earth will rocket. 193 00:14:25,160 --> 00:14:30,040 Life here must adapt...or die. 194 00:14:48,240 --> 00:14:51,000 Yellowstone National Park in North America 195 00:14:51,000 --> 00:14:53,520 is a natural laboratory for Lynn to study 196 00:14:53,520 --> 00:14:56,480 how life can survive in extreme conditions. 197 00:14:59,840 --> 00:15:02,960 The reason it's so great is that we have the whole range, 198 00:15:02,960 --> 00:15:07,600 from the top predators, things like wolves and bears and so on, 199 00:15:07,600 --> 00:15:11,040 all the way down to the beavers and the herbivores 200 00:15:11,040 --> 00:15:13,440 and down to the very tiny organisms 201 00:15:13,440 --> 00:15:15,520 and even some incredible microbes. 202 00:15:19,440 --> 00:15:22,400 Life here is used to dealing with extremes. 203 00:15:23,800 --> 00:15:26,120 But in about half a billion years' time, 204 00:15:26,120 --> 00:15:29,360 these extremes will go in the opposite direction 205 00:15:29,360 --> 00:15:33,520 as temperatures could climb by up to 20 degrees in some places. 206 00:15:38,080 --> 00:15:42,280 By then, life as we know it will have evolved to be very different. 207 00:15:45,120 --> 00:15:47,240 But just as some of today's animals 208 00:15:47,240 --> 00:15:49,720 have adapted to survive harsh winters, 209 00:15:49,720 --> 00:15:53,160 in the future, they may use similar strategies 210 00:15:53,160 --> 00:15:55,880 to cope with scorching summers. 211 00:15:55,880 --> 00:15:58,840 As the sun gets hotter, you could imagine the winter 212 00:15:58,840 --> 00:16:00,560 as being the very pleasant season 213 00:16:00,560 --> 00:16:04,400 and the summers become unbearably hot. 214 00:16:04,400 --> 00:16:08,160 So if you're thinking about a bear that lives in an area like this 215 00:16:08,160 --> 00:16:10,840 that would normally hibernate in the winter, 216 00:16:10,840 --> 00:16:13,400 if you turn the thermostat on the Earth high enough, 217 00:16:13,400 --> 00:16:14,640 it might be the reverse. 218 00:16:14,640 --> 00:16:17,880 So that now, animals would be hibernating in the summer 219 00:16:17,880 --> 00:16:19,560 and be active in the winter. 220 00:16:19,560 --> 00:16:23,040 And grasses would be setting seed now, in the spring, 221 00:16:23,040 --> 00:16:26,520 the seeds would be what would carry the plant through this harsh summer, 222 00:16:26,520 --> 00:16:29,960 and then, as the rains started again in the autumn, 223 00:16:29,960 --> 00:16:33,800 they would germinate and you would get the lush green in the winter. 224 00:16:38,760 --> 00:16:41,280 In less than a billion years' time, 225 00:16:41,280 --> 00:16:43,960 the greenhouse effect is expected to take off. 226 00:16:45,600 --> 00:16:47,480 Sending temperatures soaring. 227 00:16:54,560 --> 00:16:56,680 As it gets hotter and hotter on the land, 228 00:16:56,680 --> 00:16:59,680 eventually, even the winters will be too hot 229 00:16:59,680 --> 00:17:02,000 for most organisms, certainly, to live. 230 00:17:04,040 --> 00:17:08,040 So if you have a large animal, like, say, a bison, 231 00:17:08,040 --> 00:17:10,920 that's also warm-blooded, as it gets hotter and hotter, 232 00:17:10,920 --> 00:17:14,080 it won't be able to cool down and it will eventually die. 233 00:17:16,440 --> 00:17:19,880 And so ultimately, large animals like that will go extinct. 234 00:17:21,400 --> 00:17:23,120 In just over a billion years from now, 235 00:17:23,120 --> 00:17:28,280 the land could be nothing but a parched desert, devoid of life. 236 00:17:29,920 --> 00:17:33,320 The air is going to heat up much more quickly than water will. 237 00:17:33,320 --> 00:17:37,720 And so I predict that, just like the ancestors of whales 238 00:17:37,720 --> 00:17:40,640 and dolphins and so on moved from the land to the water, 239 00:17:40,640 --> 00:17:43,920 so will the descendants of bison, if they want to survive. 240 00:17:45,400 --> 00:17:48,120 But models suggest that in two billion years' time, 241 00:17:48,120 --> 00:17:50,360 even the water will have gone. 242 00:17:52,200 --> 00:17:57,080 As it boils away, the Earth would increasingly resemble Venus today. 243 00:18:01,600 --> 00:18:04,600 For those of us who are interested in the future of planet Earth, 244 00:18:04,600 --> 00:18:08,280 Venus is a really good model system. 245 00:18:10,360 --> 00:18:13,640 As the sun heats up and the oceans turn into steam, 246 00:18:13,640 --> 00:18:16,080 we will have a world that's not too dissimilar 247 00:18:16,080 --> 00:18:17,920 from what you see behind me in Yellowstone, 248 00:18:17,920 --> 00:18:21,000 where you see the hot water coming up to the surface 249 00:18:21,000 --> 00:18:24,240 and then turning into steam and going away. 250 00:18:24,240 --> 00:18:26,880 In less than three billion years' time, 251 00:18:26,880 --> 00:18:30,560 it's thought that the searing sun and a runaway greenhouse effect 252 00:18:30,560 --> 00:18:33,880 will have wiped out virtually all life on Earth. 253 00:18:35,320 --> 00:18:38,640 But intelligent life may just find a way out. 254 00:18:40,440 --> 00:18:43,760 We have something that the other organisms out there don't have. 255 00:18:43,760 --> 00:18:45,840 And that is we have technology. 256 00:18:45,840 --> 00:18:48,840 And we're going to have the option of going to other planets. 257 00:18:48,840 --> 00:18:52,960 As it gets too hot for the Earth, Mars will start to warm up. 258 00:18:52,960 --> 00:18:56,440 And so that means that it's just possible 259 00:18:56,440 --> 00:18:58,760 Mars will become a better place for life. 260 00:18:58,760 --> 00:19:01,760 Who knows? I have great faith in our descendants. 261 00:19:01,760 --> 00:19:05,000 By then, Mars is expected to be in the habitable zone. 262 00:19:06,200 --> 00:19:09,000 So it could provide a refuge. 263 00:19:09,000 --> 00:19:11,280 But not for ever. 264 00:19:11,280 --> 00:19:15,200 Because the next threat will be to the entire solar system. 265 00:19:20,400 --> 00:19:24,280 From 100 billion stars racing towards us. 266 00:19:27,680 --> 00:19:29,680 The Andromeda Galaxy. 267 00:19:33,240 --> 00:19:35,120 Scientists have long suspected 268 00:19:35,120 --> 00:19:39,000 it will one day crash into our galaxy, the Milky Way. 269 00:19:43,240 --> 00:19:47,120 But until recently, no-one had been able to say for sure. 270 00:19:51,720 --> 00:19:55,800 In 2012, Dr Tony Sohn stepped up to the plate. 271 00:19:57,040 --> 00:20:00,680 He and his team set out to precisely measure Andromeda's path 272 00:20:00,680 --> 00:20:04,400 and discover if it would be a near miss, 273 00:20:04,400 --> 00:20:06,000 a glancing blow... 274 00:20:07,320 --> 00:20:08,800 ..or a head-on hit. 275 00:20:10,840 --> 00:20:12,960 To predict the outcome, he used a technique 276 00:20:12,960 --> 00:20:15,080 familiar to baseball players. 277 00:20:15,080 --> 00:20:18,200 I ran a experiment that can help explain how we measure 278 00:20:18,200 --> 00:20:19,720 the motion of Andromeda. 279 00:20:19,720 --> 00:20:21,680 Imagine a game of baseball. 280 00:20:21,680 --> 00:20:25,200 The batter is waiting for the ball thrown by the pitcher. 281 00:20:27,200 --> 00:20:29,360 To work out if the ball is on target, 282 00:20:29,360 --> 00:20:33,000 the batter needs to see whether it's drifting to the side or not. 283 00:20:34,080 --> 00:20:37,400 So they instinctively compare the motion of the ball 284 00:20:37,400 --> 00:20:39,600 against the background. 285 00:20:41,040 --> 00:20:43,200 Tony needed to apply the same principle 286 00:20:43,200 --> 00:20:46,000 to discover if Andromeda was heading towards us. 287 00:20:47,880 --> 00:20:50,360 But in order to measure the galaxy's motion, 288 00:20:50,360 --> 00:20:54,600 he had to find fixed points behind Andromeda to compare it to. 289 00:20:56,160 --> 00:20:59,040 A daunting task. 290 00:20:59,040 --> 00:21:02,720 Most of the stars we see in the sky are in our galaxy, 291 00:21:02,720 --> 00:21:06,480 so they cannot be used as background objects. 292 00:21:07,680 --> 00:21:10,480 Instead, Tony had to search for distant galaxies 293 00:21:10,480 --> 00:21:13,040 hundreds of millions of light-years away. 294 00:21:14,280 --> 00:21:16,800 Only one telescope was up to the job. 295 00:21:18,280 --> 00:21:21,200 We used the Hubble Space Telescope to do this project 296 00:21:21,200 --> 00:21:24,000 because we needed a very stable instrument 297 00:21:24,000 --> 00:21:26,880 and we needed to be above the Earth's atmosphere 298 00:21:26,880 --> 00:21:29,120 to get very high resolution of the image. 299 00:21:29,120 --> 00:21:30,960 With data from Hubble, 300 00:21:30,960 --> 00:21:34,880 Tony painstakingly tracked stars in Andromeda 301 00:21:34,880 --> 00:21:36,520 against distant galaxies. 302 00:21:38,360 --> 00:21:40,320 Just like a batter tracks a ball. 303 00:21:41,760 --> 00:21:43,800 Imagine the ball is Andromeda Galaxy 304 00:21:43,800 --> 00:21:47,200 and the fence behind that are background galaxies. 305 00:21:47,200 --> 00:21:50,760 And what we did was we compared the position 306 00:21:50,760 --> 00:21:54,320 of the Andromeda galaxies against the background galaxies over time. 307 00:21:54,320 --> 00:21:57,240 And that's how we measure the sideways motion. 308 00:21:57,240 --> 00:21:59,520 The results were conclusive. 309 00:22:01,880 --> 00:22:05,680 The sideways speed of Andromeda we measured was effectively zero. 310 00:22:05,680 --> 00:22:08,840 So we can say with certainty that Andromeda 311 00:22:08,840 --> 00:22:11,720 is on a head-on collision with the Milky Way Galaxy. 312 00:22:13,320 --> 00:22:16,800 Tony's team confirmed that over 100 billion stars 313 00:22:16,800 --> 00:22:18,760 are on course for a strike 314 00:22:18,760 --> 00:22:22,920 at 2,000 times the speed of a fastball. 315 00:22:26,360 --> 00:22:30,080 But since it's so far away, the galaxies won't collide 316 00:22:30,080 --> 00:22:33,000 until nearly four billion years from now. 317 00:22:36,880 --> 00:22:39,720 Tony's precise measurements allow him to predict 318 00:22:39,720 --> 00:22:42,560 how this clash of the titans will look. 319 00:22:44,800 --> 00:22:48,880 To anyone on Earth, it would be a spectacular sight. 320 00:22:50,040 --> 00:22:54,320 We'll see the Andromeda Galaxy getting bigger and bigger on the sky 321 00:22:54,320 --> 00:22:57,080 and then eventually, in about four billion years from now, 322 00:22:57,080 --> 00:22:59,640 we'll see the collision of the two galaxies. 323 00:23:06,480 --> 00:23:09,960 On impact, clouds of dust will be crushed together. 324 00:23:11,400 --> 00:23:13,160 With sensational results. 325 00:23:16,240 --> 00:23:18,840 What we'll see is a lot of stars getting formed, 326 00:23:18,840 --> 00:23:22,480 and this will look something like stellar fireworks on the sky. 327 00:23:30,320 --> 00:23:33,760 Tony can even calculate the odds that our solar system 328 00:23:33,760 --> 00:23:36,840 will crash into one of Andromeda's billions of stars 329 00:23:36,840 --> 00:23:38,760 during the collision. 330 00:23:40,080 --> 00:23:44,480 Perhaps surprisingly, the prognosis is good. 331 00:23:44,480 --> 00:23:47,240 Galaxies are essentially empty space. 332 00:23:47,240 --> 00:23:51,880 So the chance of stars colliding with another star is very slim 333 00:23:51,880 --> 00:23:55,000 because this distance between the stars is vast. 334 00:23:55,000 --> 00:23:57,080 So when the collision happens, the solar system 335 00:23:57,080 --> 00:24:00,040 will pass through an empty space between the stars. 336 00:24:04,920 --> 00:24:07,120 After passing like ghosts in the night, 337 00:24:07,120 --> 00:24:10,960 the irresistible pull of gravity will draw them back together 338 00:24:10,960 --> 00:24:13,200 over the next two billion years. 339 00:24:14,520 --> 00:24:20,280 To finally settle as a new super-galaxy, nicknamed Milkomeda. 340 00:24:24,720 --> 00:24:27,360 Our galaxy will no longer exist. 341 00:24:27,360 --> 00:24:32,080 Yet calculations suggest the solar system will survive. 342 00:24:34,760 --> 00:24:36,720 It will merge into one big galaxy 343 00:24:36,720 --> 00:24:39,200 and it will look like a giant ball on the sky. 344 00:24:41,480 --> 00:24:44,520 Sadly, it's unlikely anyone will be on Earth 345 00:24:44,520 --> 00:24:47,400 to witness this colossal galactic collision. 346 00:24:52,080 --> 00:24:55,600 But there's a slim chance an extreme form of life 347 00:24:55,600 --> 00:24:59,400 could be clinging on as the two galaxies meet, 348 00:24:59,400 --> 00:25:03,200 despite the searing heat from the ageing sun. 349 00:25:08,920 --> 00:25:12,560 In Yellowstone, Professor Lynn Rothschild has found evidence 350 00:25:12,560 --> 00:25:16,080 of what those last remaining Earthlings might be like. 351 00:25:18,320 --> 00:25:22,560 This area of Yellowstone is extremely acidic, and it's also hot. 352 00:25:22,560 --> 00:25:24,240 You can see the steam rising. 353 00:25:24,240 --> 00:25:27,640 So in other words, it's sort of like boiling battery acid. 354 00:25:27,640 --> 00:25:32,240 Very few living things can actually live at this high temperature. 355 00:25:32,240 --> 00:25:35,480 But there are a couple of organisms that are very well adapted for it 356 00:25:35,480 --> 00:25:38,240 and you can see the beautiful colours behind me. 357 00:25:40,480 --> 00:25:43,200 The kaleidoscopic colours of Yellowstone springs 358 00:25:43,200 --> 00:25:45,720 are caused by heat-loving microbes. 359 00:25:53,320 --> 00:25:55,880 We can pretty much use these as a thermometer. 360 00:25:55,880 --> 00:25:57,760 Anything that is green means 361 00:25:57,760 --> 00:26:00,800 that it's got chlorophyll, just like plants. 362 00:26:00,800 --> 00:26:05,280 And once they get to a temperature above about 73 degrees or so, 363 00:26:05,280 --> 00:26:07,280 their chlorophyll breaks down. 364 00:26:07,280 --> 00:26:10,080 And so when you start getting warmer than that, 365 00:26:10,080 --> 00:26:12,640 you start to move into other sorts of organisms. 366 00:26:12,640 --> 00:26:15,240 Organisms that, for example, eat iron. 367 00:26:15,240 --> 00:26:18,520 And then you see these beautiful orange colours. 368 00:26:18,520 --> 00:26:21,120 Once all the water on Earth has turned to steam, 369 00:26:21,120 --> 00:26:24,880 it's possible that heat-loving microbes could continue to live. 370 00:26:26,360 --> 00:26:28,520 In the clouds. 371 00:26:29,640 --> 00:26:33,320 We know some of the earliest organisms on the Earth were thermophiles. 372 00:26:33,320 --> 00:26:35,440 Organisms that lived at high temperature. 373 00:26:35,440 --> 00:26:38,160 And so at some point, it may be organisms like this 374 00:26:38,160 --> 00:26:40,160 that once again inherit the Earth. 375 00:26:43,520 --> 00:26:46,000 The microbes will have their day. 376 00:26:46,000 --> 00:26:48,960 But their reign will inevitably be cut short. 377 00:26:52,560 --> 00:26:55,760 Because when the sun is twice the age it is now, 378 00:26:55,760 --> 00:27:01,320 astronomers foresee a turbulent new phase...written in the stars. 379 00:27:05,280 --> 00:27:07,520 On a clear night, many of the stars 380 00:27:07,520 --> 00:27:09,920 you can see with your naked eye today 381 00:27:09,920 --> 00:27:12,520 are going through this phase. 382 00:27:12,520 --> 00:27:16,600 You can tell which ones they are because of their colour. 383 00:27:16,600 --> 00:27:18,720 They're known as red giants. 384 00:27:22,080 --> 00:27:25,440 It's very easy to see red giant stars because they are very bright. 385 00:27:25,440 --> 00:27:27,400 They are giant and they are bright. 386 00:27:27,400 --> 00:27:30,560 So they are everywhere in the sky. 387 00:27:30,560 --> 00:27:32,440 Some red giants are so large, 388 00:27:32,440 --> 00:27:36,880 you could fit our own sun inside them - millions of times over. 389 00:27:38,800 --> 00:27:42,400 Yet astronomers are confident our sun will one day grow 390 00:27:42,400 --> 00:27:44,960 to become one itself. 391 00:27:44,960 --> 00:27:48,080 So these stars are a glimpse of our future. 392 00:27:52,000 --> 00:27:55,320 If we study the stars that grow in size, we can tell the fate 393 00:27:55,320 --> 00:27:58,200 of the planetary systems that are orbiting in them. 394 00:27:58,200 --> 00:28:00,280 Stars like that give us already clues 395 00:28:00,280 --> 00:28:04,320 about what will be the future fate of our own solar system. 396 00:28:08,480 --> 00:28:11,520 The transformation of our sun into a red giant 397 00:28:11,520 --> 00:28:14,240 will begin deep below its surface, 398 00:28:14,240 --> 00:28:17,760 where all the heat is generated. 399 00:28:20,000 --> 00:28:25,240 The burning core is the only place hot enough for hydrogen to fuse. 400 00:28:25,240 --> 00:28:28,920 And yet it makes up less than 2% of the sun's total volume. 401 00:28:31,760 --> 00:28:33,640 For the next five billion years, 402 00:28:33,640 --> 00:28:36,880 it's thought the core will be stable, 403 00:28:36,880 --> 00:28:40,840 finely balanced between two phenomenal opposing forces. 404 00:28:42,920 --> 00:28:44,800 The crushing pull of gravity... 405 00:28:46,880 --> 00:28:50,880 ..and the explosive push of nuclear-heated gas. 406 00:28:54,240 --> 00:28:55,960 But, like a hot-air balloon, 407 00:28:55,960 --> 00:28:58,640 the core will eventually run out of fuel. 408 00:29:00,240 --> 00:29:03,560 Just as gravity pulls the spent balloon down, 409 00:29:03,560 --> 00:29:08,760 in the sun, gravity will pull on the core, unopposed. 410 00:29:10,680 --> 00:29:12,600 When the balance is broken, 411 00:29:12,600 --> 00:29:14,760 because the hydrogen runs out in the core, 412 00:29:14,760 --> 00:29:16,800 the dominant force will be gravity. 413 00:29:16,800 --> 00:29:18,720 It will try to squeeze the core. 414 00:29:20,000 --> 00:29:22,720 But the sun will be far from spent. 415 00:29:22,720 --> 00:29:24,520 As gravity crushes the core, 416 00:29:24,520 --> 00:29:27,920 it will trigger a transformation in the rest of the sun. 417 00:29:29,200 --> 00:29:33,240 For the first time, the hydrogen gas surrounding the core 418 00:29:33,240 --> 00:29:35,880 will begin to fuse, 419 00:29:35,880 --> 00:29:39,800 giving the sun access to far more fuel than it's already burnt. 420 00:29:41,560 --> 00:29:43,960 We ran out already of one bottle of propane, 421 00:29:43,960 --> 00:29:46,000 but we have three more. 422 00:29:47,560 --> 00:29:49,560 So...it's like the sun. 423 00:29:53,680 --> 00:29:57,440 The burning shell of hydrogen releases so much heat 424 00:29:57,440 --> 00:29:59,240 that gravity is overwhelmed. 425 00:30:00,760 --> 00:30:03,640 Tipping the balance in favour of rapid expansion. 426 00:30:04,920 --> 00:30:07,480 Gravity is not winning the battle, 427 00:30:07,480 --> 00:30:10,200 so the star expands as a red giant. 428 00:30:14,760 --> 00:30:18,280 Astronomers predict that in about five billion years, 429 00:30:18,280 --> 00:30:22,560 the sun will start to grow into a vast, seething ball of fire. 430 00:30:26,320 --> 00:30:28,440 A red giant. 431 00:30:29,840 --> 00:30:33,080 Sending temperatures soaring across the solar system. 432 00:30:36,600 --> 00:30:40,520 The inner planets will become far too hot to support any kind of life. 433 00:30:41,960 --> 00:30:44,120 But the distant outer planets 434 00:30:44,120 --> 00:30:48,000 will bask in the warm glow of the sun for the first time. 435 00:30:51,760 --> 00:30:56,640 The habitable zone, where life can exist, will sweep out. 436 00:31:01,440 --> 00:31:06,040 In Peoria's solar system model, it would mean the habitable zone 437 00:31:06,040 --> 00:31:09,320 would leave town and head for the outskirts. 438 00:31:12,880 --> 00:31:17,280 Here, at the airport, is Jupiter. 439 00:31:17,280 --> 00:31:19,040 You got your bag sheet? 440 00:31:19,040 --> 00:31:21,880 You're all good, you're going to go to gate number ten. 441 00:31:21,880 --> 00:31:25,640 - And it is delayed until 1:30? - Yes, ma'am. 442 00:31:27,040 --> 00:31:30,080 My favourite planet, I would say, is Earth, 443 00:31:30,080 --> 00:31:32,560 but Jupiter's second, for sure. 444 00:31:32,560 --> 00:31:34,400 It's very cool. It's very cool. 445 00:31:39,040 --> 00:31:42,760 When the sun grows, Jupiter will come in from the cold. 446 00:31:45,800 --> 00:31:47,520 And although life as we know it 447 00:31:47,520 --> 00:31:49,760 could never survive on gassy Jupiter, 448 00:31:49,760 --> 00:31:54,000 the solar system's biggest planet has several icy moons. 449 00:31:57,760 --> 00:32:01,840 These are likely to melt and become cosmic watering holes 450 00:32:01,840 --> 00:32:05,800 for any refugees fleeing the parched inner solar system. 451 00:32:07,120 --> 00:32:10,840 Astronomers have speculated that Jupiter could change colour. 452 00:32:14,880 --> 00:32:17,360 As clouds of ammonia vaporise, 453 00:32:17,360 --> 00:32:20,000 it might turn a deep shade of blue. 454 00:32:25,320 --> 00:32:28,800 After Jupiter, astronomers expect the habitable zone 455 00:32:28,800 --> 00:32:31,120 to move swiftly towards Saturn. 456 00:32:33,720 --> 00:32:37,000 On Saturdays, we'll have families making that interplanetary trip 457 00:32:37,000 --> 00:32:39,920 from one planet to the other in our area. 458 00:32:39,920 --> 00:32:43,480 I think Saturn is more interesting because of the rings. 459 00:32:46,120 --> 00:32:49,120 If Saturn still has its icy rings by then, 460 00:32:49,120 --> 00:32:52,640 they're forecast to vaporise and disappear. 461 00:32:59,280 --> 00:33:04,360 But, like Jupiter, Saturn's icy moons could melt 462 00:33:04,360 --> 00:33:06,080 and be safe havens for life. 463 00:33:13,200 --> 00:33:16,040 Then, models predict the habitable zone 464 00:33:16,040 --> 00:33:18,040 will sweep out faster and faster, 465 00:33:18,040 --> 00:33:22,040 past the solar system's most distant planets and their moons. 466 00:33:23,720 --> 00:33:25,800 First, Uranus. 467 00:33:32,440 --> 00:33:34,800 Then, deep-blue Neptune. 468 00:33:36,920 --> 00:33:39,920 Astronomers think they, too, will be transformed. 469 00:33:39,920 --> 00:33:43,520 But exactly how they'll look in the future is still a mystery. 470 00:33:48,600 --> 00:33:52,160 Eventually, the habitable zone is forecast to pass beyond 471 00:33:52,160 --> 00:33:54,400 all the planets and their moons. 472 00:33:58,360 --> 00:34:00,720 But although Neptune's the final planet, 473 00:34:00,720 --> 00:34:03,040 the solar system doesn't finish there. 474 00:34:07,520 --> 00:34:10,560 At the Good's furniture store in Kewanee, Illinois... 475 00:34:12,760 --> 00:34:16,920 ..adjacent to a wide selection of cabinets and coffee tables, 476 00:34:16,920 --> 00:34:19,920 is ex-planet Pluto 477 00:34:19,920 --> 00:34:22,240 and its large moon, Charon. 478 00:34:24,120 --> 00:34:28,960 In 2006, Pluto was downgraded to a dwarf planet. 479 00:34:30,240 --> 00:34:32,760 They say planet Pluto is no longer a planet, 480 00:34:32,760 --> 00:34:34,840 but to us, it will always be a planet. 481 00:34:36,440 --> 00:34:39,200 People are so amazed at how small planet Pluto is. 482 00:34:39,200 --> 00:34:41,120 They get up real close with their camera, 483 00:34:41,120 --> 00:34:43,920 just a couple of inches away to snap a really close shot. 484 00:34:46,680 --> 00:34:49,600 Astronomers have tried to predict what will happen 485 00:34:49,600 --> 00:34:52,800 as this distant outpost of the solar system warms. 486 00:34:55,640 --> 00:34:58,120 But because it's so small and remote, 487 00:34:58,120 --> 00:35:01,080 this world was shrouded in mystery... 488 00:35:03,080 --> 00:35:04,320 ..until recently. 489 00:35:10,680 --> 00:35:14,680 In July 2015, the New Horizons' mission 490 00:35:14,680 --> 00:35:17,480 finally revealed Pluto's secrets. 491 00:35:23,200 --> 00:35:27,040 The first clear images ever captured of the dwarf planet 492 00:35:27,040 --> 00:35:29,040 revealed some startling terrain. 493 00:35:31,160 --> 00:35:34,680 Strange troughs, cliffs... 494 00:35:34,680 --> 00:35:36,720 and even dunes. 495 00:35:43,120 --> 00:35:46,240 Professor Lynn Rothschild is fascinated 496 00:35:46,240 --> 00:35:50,280 by this tiny world and its potential for life. 497 00:35:52,400 --> 00:35:54,120 With the New Horizons' mission, 498 00:35:54,120 --> 00:35:57,480 we really knew almost nothing about Pluto. 499 00:35:57,480 --> 00:36:02,040 And our knowledge of Pluto has just blossomed enormously. 500 00:36:02,040 --> 00:36:04,560 In fact, it's not blossomed, it's exploded. 501 00:36:11,720 --> 00:36:15,920 One of the most unexpected features is a towering series of peaks. 502 00:36:19,760 --> 00:36:21,600 Much, I think, to everyone's surprise, 503 00:36:21,600 --> 00:36:24,160 there were huge mountains that were found on Pluto. 504 00:36:24,160 --> 00:36:26,960 These things are as high as 11,000 feet. 505 00:36:26,960 --> 00:36:30,240 Sort of like the mountains behind me, here in the Rockies in Montana. 506 00:36:32,840 --> 00:36:36,720 Here on Earth, the chemical bonds that bind rock 507 00:36:36,720 --> 00:36:41,040 are strong enough to defy gravity by holding up mountains. 508 00:36:42,640 --> 00:36:46,160 Yet Pluto's crust is not made of rock, but ice. 509 00:36:58,080 --> 00:37:01,880 The -220 degree temperatures there 510 00:37:01,880 --> 00:37:04,560 alter the chemical bonds in Pluto's ice. 511 00:37:06,640 --> 00:37:09,160 And make it as strong as rock is here. 512 00:37:11,560 --> 00:37:16,480 Strong enough to hold up ice mountains as high as the Rockies. 513 00:37:22,000 --> 00:37:25,320 But the arrival of the habitable zone would change this. 514 00:37:27,000 --> 00:37:29,680 Pluto's frosty peaks could be destroyed. 515 00:37:32,400 --> 00:37:34,480 As the sun becomes hotter and hotter, 516 00:37:34,480 --> 00:37:37,400 the ice mountains will start to collapse, I would imagine, 517 00:37:37,400 --> 00:37:40,120 under their own weight because at that point, 518 00:37:40,120 --> 00:37:43,280 the ice won't be as hard as it is today. 519 00:37:43,280 --> 00:37:46,000 And at some point, it may in fact be warm enough 520 00:37:46,000 --> 00:37:48,760 for all this ice on Pluto to melt. 521 00:37:50,880 --> 00:37:55,000 Amid the destruction, something remarkable could emerge. 522 00:37:56,480 --> 00:37:59,440 A water world at the edge of our solar system. 523 00:38:01,440 --> 00:38:04,960 Once you have liquid water and little energy, 524 00:38:04,960 --> 00:38:07,480 that's very good news for life. 525 00:38:07,480 --> 00:38:10,200 At that point, it'll be warm enough 526 00:38:10,200 --> 00:38:13,720 that even, even Pluto will be in a habitable zone. 527 00:38:13,720 --> 00:38:16,120 It will finally have its moment in the sun. 528 00:38:18,120 --> 00:38:21,360 After a 12 billion year long winter, 529 00:38:21,360 --> 00:38:24,560 the expanding sun may bring spring to Pluto. 530 00:38:30,440 --> 00:38:33,520 But while the red giant nurtures Pluto... 531 00:38:34,760 --> 00:38:38,720 ..it poses a grave threat to the planets of the inner solar system. 532 00:38:39,800 --> 00:38:43,040 They face total annihilation. 533 00:38:48,160 --> 00:38:52,880 In 2012, Dr Eva Villaver stumbled across grisly evidence 534 00:38:52,880 --> 00:38:56,320 of what red giants can do to their inner planets. 535 00:38:58,040 --> 00:39:02,520 A search for distant worlds had led to the constellation of Perseus, 536 00:39:02,520 --> 00:39:07,800 where a star called BD+48740 537 00:39:07,800 --> 00:39:10,560 caught her attention for two reasons. 538 00:39:12,680 --> 00:39:16,000 There we have a star, a red giant, 539 00:39:16,000 --> 00:39:17,560 that was very peculiar 540 00:39:17,560 --> 00:39:21,320 because the star itself has a very high content of lithium. 541 00:39:21,320 --> 00:39:24,640 And that's very unusual for this type of star. 542 00:39:24,640 --> 00:39:29,200 So that was one of the pieces of the puzzle and the other one was 543 00:39:29,200 --> 00:39:32,760 that it has a Jupiter-like planet orbiting the star 544 00:39:32,760 --> 00:39:35,200 that has an orbit that is very unusual. 545 00:39:36,640 --> 00:39:40,360 Eva thought the two strange features must be somehow connected. 546 00:39:41,560 --> 00:39:43,800 Something had happened that had affected 547 00:39:43,800 --> 00:39:46,440 both the planet and the star itself. 548 00:39:49,200 --> 00:39:51,560 The team analysed the possible causes... 549 00:39:52,680 --> 00:39:56,440 ..and concluded there was only one event that could explain both. 550 00:39:58,720 --> 00:40:02,760 The most simple explanation is that something very violent happened. 551 00:40:04,280 --> 00:40:07,560 We think that the star had a multiple planetary system 552 00:40:07,560 --> 00:40:10,200 and what we see is just the leftover planet, 553 00:40:10,200 --> 00:40:13,360 but there was another planet that was eaten by the star. 554 00:40:16,560 --> 00:40:21,520 As one planet was engulfed by the star, it destabilised the other. 555 00:40:23,160 --> 00:40:26,880 Then it triggered lithium production by stirring the hot gases. 556 00:40:30,240 --> 00:40:33,800 So this star has eaten one of its planets 557 00:40:33,800 --> 00:40:36,360 as the star became a red giant. 558 00:40:39,040 --> 00:40:41,000 Eva had found compelling evidence 559 00:40:41,000 --> 00:40:44,520 that ageing stars can grow so large, 560 00:40:44,520 --> 00:40:46,680 they devour their inner planets. 561 00:40:51,000 --> 00:40:53,920 In around 5.5 billion years, 562 00:40:53,920 --> 00:40:57,600 our own sun will enter this extraordinary phase of its life. 563 00:40:58,760 --> 00:41:02,840 Evidence suggests its surface will reach out towards Mercury, 564 00:41:02,840 --> 00:41:05,640 Venus and Earth, 565 00:41:05,640 --> 00:41:08,680 threatening their very existence. 566 00:41:11,040 --> 00:41:13,160 Local astronomer Sheldon Schafer 567 00:41:13,160 --> 00:41:17,040 is leading his weekly inner-planetary bicycle tour. 568 00:41:18,400 --> 00:41:21,640 With the sun's surface hot on his heels. 569 00:41:21,640 --> 00:41:24,160 So right now, we're going at about four miles an hour. 570 00:41:24,160 --> 00:41:26,240 That's about half the speed of light. 571 00:41:26,240 --> 00:41:33,160 This peaceful Midwestern town is about to go on the ride of its life. 572 00:41:33,160 --> 00:41:35,280 And here we are, approaching Mercury. 573 00:41:39,240 --> 00:41:42,600 You can see it's easily a stunt double for the Earth's moon. 574 00:41:42,600 --> 00:41:44,240 It's a heavily-cratered world 575 00:41:44,240 --> 00:41:46,400 without an atmosphere, hot in the sun 576 00:41:46,400 --> 00:41:48,560 and cold in the darkness. 577 00:41:48,560 --> 00:41:53,360 But the solar system's smallest planet will get hotter still. 578 00:41:53,360 --> 00:41:55,040 Off to Venus! 579 00:41:55,040 --> 00:41:56,600 Because astronomers predict 580 00:41:56,600 --> 00:42:01,320 that less than a billion years into the red giant phase, 581 00:42:01,320 --> 00:42:03,960 the sun's surface will reach Mercury. 582 00:42:06,480 --> 00:42:10,200 After more than ten billion years of relative calm, 583 00:42:10,200 --> 00:42:14,320 the solar system will lose a planet. 584 00:42:20,200 --> 00:42:23,440 And the sun will continue to expand. 585 00:42:25,680 --> 00:42:28,720 Growing ever closer to Venus. 586 00:42:32,640 --> 00:42:36,120 OK, so here we are. We've come about 66 million miles 587 00:42:36,120 --> 00:42:38,600 and, er...Venus, you might notice, 588 00:42:38,600 --> 00:42:41,440 is almost exactly the same size as the Earth. 589 00:42:41,440 --> 00:42:45,920 And for that reason alone, it's been called the Earth's sister planet. 590 00:42:45,920 --> 00:42:48,760 But Earth will probably lose its sibling. 591 00:42:50,560 --> 00:42:53,160 Because most models of the sun's evolution 592 00:42:53,160 --> 00:42:56,080 show it easily enveloping Venus. 593 00:43:11,440 --> 00:43:17,640 The next planet...is Earth itself. 594 00:43:19,160 --> 00:43:21,200 You can see from wherever you're standing 595 00:43:21,200 --> 00:43:24,240 that the Earth is blue, with lots and lots of liquid water. 596 00:43:25,680 --> 00:43:28,520 By the time the sun engulfs Venus, 597 00:43:28,520 --> 00:43:32,320 the Earth's oceans are expected to have boiled away. 598 00:43:34,960 --> 00:43:39,160 The ultimate fate of our world appears to be on a knife edge. 599 00:43:46,240 --> 00:43:48,080 - A fortune cookie. - SHE CHUCKLES 600 00:43:50,200 --> 00:43:53,040 For years, scientists have been unsure 601 00:43:53,040 --> 00:43:54,960 what fortunes await the Earth. 602 00:43:54,960 --> 00:43:56,400 Uh-oh! 603 00:43:56,400 --> 00:44:00,280 Will it be swallowed by the sun? 604 00:44:00,280 --> 00:44:02,000 "The world will end in fire." 605 00:44:03,080 --> 00:44:07,920 Or will it outlive the sun, to face a frozen eternity in space? 606 00:44:07,920 --> 00:44:10,080 "The world will end in ice." 607 00:44:13,440 --> 00:44:15,080 Maybe! 608 00:44:15,080 --> 00:44:19,720 In 2001, astronomer Dr Robert Smith decided to investigate. 609 00:44:21,080 --> 00:44:23,960 His first calculations had ominous results. 610 00:44:25,000 --> 00:44:27,960 What we found, to our disappointment, 611 00:44:27,960 --> 00:44:31,200 was that the sun will expand to something like 612 00:44:31,200 --> 00:44:35,160 250 times its present size 613 00:44:35,160 --> 00:44:40,040 and the Earth's orbit is only about 215 times 614 00:44:40,040 --> 00:44:41,800 the present size of the sun. 615 00:44:41,800 --> 00:44:47,240 So it will certainly go beyond the present orbit of the Earth. 616 00:44:48,520 --> 00:44:51,960 But Robert foresaw that there was still hope for our planet. 617 00:44:54,120 --> 00:44:57,160 Another factor that could potentially save the Earth 618 00:44:57,160 --> 00:44:59,040 from the sun's clutches. 619 00:45:08,000 --> 00:45:10,200 He realised the Earth's destiny 620 00:45:10,200 --> 00:45:13,640 hangs on something called the solar wind. 621 00:45:13,640 --> 00:45:17,520 Highly-charged particles that stream out from the sun 622 00:45:17,520 --> 00:45:20,080 as its hot surface evaporates. 623 00:45:20,080 --> 00:45:25,000 Like the wind on Earth, this stream of particles is invisible. 624 00:45:25,000 --> 00:45:26,880 But you can see its effects. 625 00:45:29,080 --> 00:45:32,800 You can see the tail is always downwind of the kite. 626 00:45:32,800 --> 00:45:37,880 And you get the same kind of phenomenon with comets, for example. 627 00:45:37,880 --> 00:45:40,040 You can see that the tail of a comet, 628 00:45:40,040 --> 00:45:42,480 it's not always behind the direction of the comet, 629 00:45:42,480 --> 00:45:45,480 it's streaming away, always away from the sun. 630 00:45:47,920 --> 00:45:50,800 The solar wind also affects the sun itself. 631 00:45:52,800 --> 00:45:56,560 Solar wind is carrying away particles, so it does reduce the mass. 632 00:45:57,640 --> 00:45:59,400 As the sun loses mass, 633 00:45:59,400 --> 00:46:03,760 so the gravitational field of the sun gets weaker. 634 00:46:03,760 --> 00:46:06,120 It pulls less strongly on the planets 635 00:46:06,120 --> 00:46:10,160 and so the planets tend to move out, the orbits get bigger. 636 00:46:12,640 --> 00:46:17,280 As a red giant, the sun will lose a lot of mass through the solar wind. 637 00:46:18,440 --> 00:46:21,240 Robert wanted to know if it would be enough 638 00:46:21,240 --> 00:46:23,600 for the Earth to escape the advancing sun. 639 00:46:23,600 --> 00:46:27,560 So initially, it was just sheer curiosity. 640 00:46:27,560 --> 00:46:31,280 What happens to the Earth when the sun becomes a red giant? 641 00:46:34,640 --> 00:46:38,160 Robert and his colleague calculated how the sun would evolve. 642 00:46:38,160 --> 00:46:41,440 And, in particular, how much mass it would lose 643 00:46:41,440 --> 00:46:43,680 after it becomes a red giant. 644 00:46:48,240 --> 00:46:50,960 There was an amazing amount of interest in this. 645 00:46:50,960 --> 00:46:53,560 We found that the mass of the sun itself 646 00:46:53,560 --> 00:46:56,120 would go down by something like 20% 647 00:46:56,120 --> 00:46:58,440 at the end of the red giant stage. 648 00:47:02,640 --> 00:47:07,320 As the sun loses mass, the planets will shift further from its centre. 649 00:47:10,040 --> 00:47:13,960 Robert predicts that the Earth will move out millions of miles... 650 00:47:16,560 --> 00:47:18,560 ..as the sun expands. 651 00:47:22,840 --> 00:47:27,000 So here we are at Mars, the last of the terrestrial planets. 652 00:47:27,000 --> 00:47:29,400 But in seven billion years' time, 653 00:47:29,400 --> 00:47:32,960 calculations show that Mars will no longer be here. 654 00:47:34,560 --> 00:47:38,160 Instead, the red planet is forecast to have moved all the way out 655 00:47:38,160 --> 00:47:40,440 to where the asteroid belt is today. 656 00:47:44,240 --> 00:47:46,960 With the Earth in its place. 657 00:47:49,040 --> 00:47:52,360 According to Robert Smith's calculations in 2001, 658 00:47:52,360 --> 00:47:54,720 the sun would then stop growing 659 00:47:54,720 --> 00:47:58,000 when it's still ten million miles away from the Earth. 660 00:47:59,320 --> 00:48:02,720 And our world would survive. 661 00:48:02,720 --> 00:48:06,360 We were quite pleased when we found that the Earth would escape. 662 00:48:06,360 --> 00:48:11,440 Unfortunately, nobody will be around to see it, which is a pity. 663 00:48:16,880 --> 00:48:20,040 But within a few years, scientists began to realise 664 00:48:20,040 --> 00:48:23,120 that there was another effect they hadn't considered... 665 00:48:24,360 --> 00:48:28,760 ..which could potentially draw the Earth back towards the sun. 666 00:48:34,000 --> 00:48:39,480 Dr Eva Villaver has analysed these so-called tidal interactions 667 00:48:39,480 --> 00:48:42,520 that exist between all red giants and their planets. 668 00:48:44,200 --> 00:48:46,280 She foresees that the same forces 669 00:48:46,280 --> 00:48:51,360 will one day act between the sun...and the Earth. 670 00:48:53,160 --> 00:48:57,400 I have a third experiment that maybe can help understanding 671 00:48:57,400 --> 00:48:59,640 how the Earth and the sun will interact 672 00:48:59,640 --> 00:49:02,000 as the sun becomes a red giant. 673 00:49:02,000 --> 00:49:05,120 Imagine that you have a carousel, which is the sun, 674 00:49:05,120 --> 00:49:07,160 and you have a bicycle orbiting it, 675 00:49:07,160 --> 00:49:09,600 going around it, and the bike is the Earth. 676 00:49:11,280 --> 00:49:15,640 As the sun expands, the rate it spins will slow down. 677 00:49:15,640 --> 00:49:18,400 So by the time it reaches its maximum size, 678 00:49:18,400 --> 00:49:21,240 the Earth will be going around the sun 679 00:49:21,240 --> 00:49:24,200 much faster than the sun itself is turning, 680 00:49:24,200 --> 00:49:26,240 which has a critical effect. 681 00:49:28,360 --> 00:49:30,560 The rotation of both is going to be connected. 682 00:49:30,560 --> 00:49:34,120 So imagine that you have a rope tied on the bike. 683 00:49:34,120 --> 00:49:37,360 If the carousel rotates more slowly than the bike, 684 00:49:37,360 --> 00:49:42,240 it will pull whatever is rotating around it. 685 00:49:42,240 --> 00:49:47,000 And as a consequence of that, the planet will be forced to slow down. 686 00:49:48,400 --> 00:49:50,480 So that's basically the tidal force, 687 00:49:50,480 --> 00:49:53,440 this connection between the carousel and the bike. 688 00:49:54,560 --> 00:49:58,440 As a result, the Earth would lose speed. 689 00:49:58,440 --> 00:50:01,920 The planet will be moving more slowly 690 00:50:01,920 --> 00:50:03,240 and as a consequence of that, 691 00:50:03,240 --> 00:50:06,200 the Earth would get closer to the surface of the sun. 692 00:50:08,560 --> 00:50:11,920 Our world would be drawn towards the sun. 693 00:50:21,240 --> 00:50:23,800 Could the Earth be doomed after all? 694 00:50:25,800 --> 00:50:27,800 Dr Robert Smith went back to work 695 00:50:27,800 --> 00:50:31,800 to calculate whether the tidal force pulling the Earth in 696 00:50:31,800 --> 00:50:33,720 could counteract the solar wind 697 00:50:33,720 --> 00:50:35,960 reducing the sun's grip on our planet. 698 00:50:37,440 --> 00:50:39,160 Unfortunately, we found that 699 00:50:39,160 --> 00:50:42,720 the tidal effect was really quite important. 700 00:50:42,720 --> 00:50:47,640 And it caused the Earth to spiral in towards the sun. 701 00:50:47,640 --> 00:50:49,480 And the overall effect was that 702 00:50:49,480 --> 00:50:52,880 the Earth actually was swallowed by the sun. 703 00:51:01,840 --> 00:51:04,560 Well, that was a very disappointing result 704 00:51:04,560 --> 00:51:08,720 because we had hoped that the Earth would still nonetheless escape, 705 00:51:08,720 --> 00:51:11,440 but unfortunately, that's the way things are. 706 00:51:11,440 --> 00:51:15,640 And the Earth, by that stage, wouldn't have been liveable on, 707 00:51:15,640 --> 00:51:18,360 so perhaps it doesn't matter too much. 708 00:51:24,360 --> 00:51:27,280 For a vision of those final days on Earth, 709 00:51:27,280 --> 00:51:33,360 Dr Eva Villaver has come to a unique facility in Odeillo, France. 710 00:51:33,360 --> 00:51:36,280 The world's largest solar furnace. 711 00:51:39,920 --> 00:51:41,960 As the sun becomes a red giant, 712 00:51:41,960 --> 00:51:46,040 we will have a red star occupying most of the sky. 713 00:51:46,040 --> 00:51:49,480 And the energy that every single inch of the Earth 714 00:51:49,480 --> 00:51:51,520 will receive will increase. 715 00:51:51,520 --> 00:51:54,720 And here, this is exactly what these mirrors are doing. 716 00:51:56,080 --> 00:51:59,720 Around 10,000 mirrors focus the sun's rays, 717 00:51:59,720 --> 00:52:02,120 like a giant magnifying glass. 718 00:52:03,880 --> 00:52:07,280 Which allows them to replicate the conditions the Earth will face 719 00:52:07,280 --> 00:52:10,040 when the sun becomes a red giant. 720 00:52:15,200 --> 00:52:18,600 Eva calculates that the radiation shining on the Earth's surface 721 00:52:18,600 --> 00:52:22,840 will be nearly 3,000 times more intense than today. 722 00:52:29,680 --> 00:52:31,480 So, to simulate our future, 723 00:52:31,480 --> 00:52:36,040 the solar furnace has magnified the sun's power by 3,000 times. 724 00:52:40,000 --> 00:52:42,640 We are focusing the light of the sun in a bin 725 00:52:42,640 --> 00:52:45,480 and trying to see what will be the effect on a rock. 726 00:52:45,480 --> 00:52:48,040 Because the Earth is a rock floating around the sun. 727 00:52:56,760 --> 00:52:58,360 Wow, look at this! 728 00:53:04,120 --> 00:53:05,640 There it goes. 729 00:53:11,720 --> 00:53:14,560 The temperature at the surface of the Earth at that point 730 00:53:14,560 --> 00:53:17,920 will be of the order of 1,400 degrees. 731 00:53:17,920 --> 00:53:20,000 Enough to melt rock. 732 00:53:20,000 --> 00:53:22,440 Enough to melt the whole surface of the Earth. 733 00:53:27,760 --> 00:53:31,720 It's thought the planet will be covered in a vast ocean of molten lava. 734 00:53:34,120 --> 00:53:36,400 But even after the Earth's surface has melted, 735 00:53:36,400 --> 00:53:41,120 the heat is expected to increase further as the planet is engulfed. 736 00:53:42,480 --> 00:53:44,960 The maximum intensity of the solar furnace 737 00:53:44,960 --> 00:53:49,080 is 16,000 times the sun's power today. 738 00:53:51,320 --> 00:53:54,440 Still only a fraction of what the Earth would encounter 739 00:53:54,440 --> 00:53:56,480 inside the red giant. 740 00:53:59,120 --> 00:54:00,960 The rock would be stripped away, 741 00:54:00,960 --> 00:54:03,640 leaving just the planet's iron core. 742 00:54:12,600 --> 00:54:14,520 Wow, look at this! 743 00:54:14,520 --> 00:54:16,960 Just the sun's radiation. 744 00:54:18,520 --> 00:54:21,560 That's iron being melted by the radiation of the sun. 745 00:54:23,720 --> 00:54:26,960 This is how the last moments of our world would be. 746 00:54:28,960 --> 00:54:31,680 So everything, the whole material of the Earth, 747 00:54:31,680 --> 00:54:33,640 will melt all the way down through the core. 748 00:54:33,640 --> 00:54:35,520 Even the iron core will melt. 749 00:54:35,520 --> 00:54:39,120 The whole material of the Earth will be part of the material of the sun. 750 00:54:39,120 --> 00:54:41,040 Everything will be mixed together. 751 00:54:42,240 --> 00:54:44,880 According to the latest calculations, 752 00:54:44,880 --> 00:54:47,720 the world will end in fire. 753 00:54:51,520 --> 00:54:54,520 But our solar system's story is not quite over yet. 754 00:54:56,000 --> 00:54:58,920 Because the final phase of the sun's life 755 00:54:58,920 --> 00:55:01,200 will be the most spectacular of all. 756 00:55:04,040 --> 00:55:06,680 - There's Seven Sisters... - It's like an upside-down owl. 757 00:55:06,680 --> 00:55:08,560 Nick, you wanted to see the Andromeda? 758 00:55:08,560 --> 00:55:11,280 - It's really cool. - Wow! 759 00:55:11,280 --> 00:55:13,840 In Peoria, every Saturday night, 760 00:55:13,840 --> 00:55:16,000 the Astronomy Society meets 761 00:55:16,000 --> 00:55:19,360 by the Northmoor Observatory at the edge of town. 762 00:55:20,600 --> 00:55:23,680 And between these two stars is the remnant 763 00:55:23,680 --> 00:55:26,000 of what's going to happen to our sun. 764 00:55:27,800 --> 00:55:29,520 So we're going to move the telescope 765 00:55:29,520 --> 00:55:31,400 and, Brian, do you want to move the dome? 766 00:55:32,520 --> 00:55:36,200 Tonight, Sheldon is searching for a distant, dying star. 767 00:55:38,400 --> 00:55:40,200 The Ring Nebula. 768 00:55:42,760 --> 00:55:44,120 OK, that's good, Brian. 769 00:55:46,040 --> 00:55:47,560 Ha! I think it's there. 770 00:55:49,120 --> 00:55:52,240 OK. So come on over and take a look. 771 00:55:52,240 --> 00:55:55,240 Look through the eyepiece and you should see a lot of stars 772 00:55:55,240 --> 00:55:57,960 and then right in the middle, do you see that little smoke ring? 773 00:55:57,960 --> 00:56:01,240 - Yes. - It's just barely there, right? - Yeah. 774 00:56:01,240 --> 00:56:06,320 - Wow! - So this is a star that, after the red giant stage, 775 00:56:06,320 --> 00:56:08,920 it puffs off shells of itself. 776 00:56:08,920 --> 00:56:13,520 It expels most of its matter into, like, bubbles of gas. 777 00:56:14,760 --> 00:56:17,720 The planetary nebulae produced by dying stars 778 00:56:17,720 --> 00:56:21,640 are some of the most spectacular celestial objects in the night sky. 779 00:56:23,760 --> 00:56:26,880 When our sun dies, it, too, could make a nebula. 780 00:56:28,920 --> 00:56:32,480 Astronomers have calculated that up to half of the sun's mass 781 00:56:32,480 --> 00:56:36,320 would be thrown off into space as gas and dust. 782 00:56:37,320 --> 00:56:40,560 Including much of the material that came from the Earth. 783 00:56:43,080 --> 00:56:46,240 And then the star itself shrinks from the red giant 784 00:56:46,240 --> 00:56:47,800 down to a white dwarf, 785 00:56:47,800 --> 00:56:50,160 which is a star about the size of the Earth. 786 00:56:50,160 --> 00:56:52,840 Very, very hot, but extremely tiny. 787 00:56:52,840 --> 00:56:55,200 And, er...then the shells of gas 788 00:56:55,200 --> 00:56:58,680 are really the only thing that's left to see. 789 00:57:01,920 --> 00:57:04,480 The vaporised remains of half the solar system 790 00:57:04,480 --> 00:57:08,520 would glow brilliantly for around 10,000 years. 791 00:57:12,920 --> 00:57:15,520 Then, as it spreads into space, 792 00:57:15,520 --> 00:57:17,600 the light would slowly fade. 793 00:57:20,080 --> 00:57:23,120 And our solar system will end. 794 00:57:27,840 --> 00:57:31,960 But in a sense, it's just a new beginning. 795 00:57:35,320 --> 00:57:37,920 The materials that make up our bodies 796 00:57:37,920 --> 00:57:43,440 may well ultimately get spat out into the cosmos 797 00:57:43,440 --> 00:57:47,520 and be the raw materials for another generation of stars, 798 00:57:47,520 --> 00:57:49,920 planets and maybe even life forms. 799 00:57:53,800 --> 00:57:57,040 We're all famously made of star stuff. 800 00:57:59,640 --> 00:58:01,640 One day, we may return to a star. 801 00:58:03,840 --> 00:58:05,960 Our sun. 802 00:58:08,320 --> 00:58:13,360 But then, in an extraordinary process of cosmic rebirth, 803 00:58:13,360 --> 00:58:17,880 the sun would return our atoms to interstellar space. 804 00:58:19,920 --> 00:58:22,560 To form new worlds... 805 00:58:24,320 --> 00:58:27,120 ..and perhaps new life.