1 00:00:04,000 --> 00:00:07,080 The planets have fascinated us for millennia. 2 00:00:09,800 --> 00:00:12,600 We've sent dozens of spacecraft to explore them. 3 00:00:15,760 --> 00:00:17,600 Touchdown confirmed... 4 00:00:17,600 --> 00:00:19,280 CHEERING 5 00:00:19,280 --> 00:00:23,160 And now, our exploration of the planets is even taking us out, 6 00:00:23,160 --> 00:00:25,200 beyond our own solar system. 7 00:00:25,200 --> 00:00:27,920 MALE REPORTER: Lift-off for the Delta II rocket with Kepler, 8 00:00:27,920 --> 00:00:32,000 on a search for planets in some way like our own. 9 00:00:32,000 --> 00:00:34,880 We've made discoveries we never imagined. 10 00:00:34,880 --> 00:00:39,160 I have no idea how those mountains could be formed. 11 00:00:39,160 --> 00:00:43,840 Fantastic, to really capture how close Mars was to Earth 12 00:00:43,840 --> 00:00:46,880 in many of its features. I think my jaw hit the floor. 13 00:00:46,880 --> 00:00:49,200 I'm pretty convinced it's really out there. 14 00:00:50,720 --> 00:00:54,080 The Sky At Night has covered every major space science discovery 15 00:00:54,080 --> 00:00:58,760 in more than 60 years of occasionally quirky broadcasting. 16 00:00:58,760 --> 00:01:01,040 Line your finger up with my nose. 17 00:01:01,040 --> 00:01:02,720 It's absolutely tremendous! 18 00:01:02,720 --> 00:01:04,440 I'm going to do just a little mathematics. 19 00:01:04,440 --> 00:01:05,920 Please don't be frightened. 20 00:01:07,360 --> 00:01:09,440 We're going to use this archive 21 00:01:09,440 --> 00:01:12,320 to travel on an epic voyage of discovery. 22 00:01:13,760 --> 00:01:15,640 Good evening. Good evening. Welcome. 23 00:01:15,640 --> 00:01:19,680 Welcome to The Sky At Night's guide to the planets. 24 00:01:26,120 --> 00:01:27,880 For thousands of years, 25 00:01:27,880 --> 00:01:31,920 our only view of the planets was from here on Earth. 26 00:01:33,840 --> 00:01:36,280 But, since the start of the Space Age, 27 00:01:36,280 --> 00:01:39,280 we've sent over 60 probes to visit them. 28 00:01:40,960 --> 00:01:45,040 These probes have led to a transformation in our understanding. 29 00:01:46,320 --> 00:01:48,960 When I was growing up, there were nine of them, 30 00:01:48,960 --> 00:01:51,200 each with its own unique orbit around the sun, 31 00:01:51,200 --> 00:01:53,680 and its own unique characteristics, 32 00:01:53,680 --> 00:01:56,200 unchanged since their formation. 33 00:01:58,760 --> 00:02:00,360 It was even thought possible 34 00:02:00,360 --> 00:02:04,520 that our solar system might be the only one in the galaxy. 35 00:02:06,240 --> 00:02:09,520 The only star with planets in orbit around it. 36 00:02:11,440 --> 00:02:14,720 How wrong we were, about almost everything. 37 00:02:21,520 --> 00:02:24,560 You get some idea of just how far we've come 38 00:02:24,560 --> 00:02:27,080 by looking at the two planets closest to Earth. 39 00:02:27,080 --> 00:02:30,640 The rocky worlds of Venus and Mars. 40 00:02:33,920 --> 00:02:37,280 If there's one planet in the solar system that has fascinated us 41 00:02:37,280 --> 00:02:40,200 more than any other, it's Mars. 42 00:02:40,200 --> 00:02:43,240 Through the years, The Sky At Night has returned to look at Mars 43 00:02:43,240 --> 00:02:45,680 over and over again. 44 00:02:45,680 --> 00:02:48,120 No other planet has been studied more, 45 00:02:48,120 --> 00:02:50,440 and no other planet has revealed 46 00:02:50,440 --> 00:02:53,840 such an extraordinary story of transformation. 47 00:02:53,840 --> 00:02:56,800 PATRICK MOORE: Altair, in the Eagle, you can find Mars quite easily... 48 00:02:56,800 --> 00:02:59,280 As recently as the 1960s, 49 00:02:59,280 --> 00:03:04,320 we thought it might be a living world, with plants on its surface. 50 00:03:07,640 --> 00:03:12,680 But then, in 1969, came Mariner 6, and that changed everything. 51 00:03:15,240 --> 00:03:18,240 And Mariner 6 came down over this direction, 52 00:03:18,240 --> 00:03:20,080 over these so-called deserts. 53 00:03:20,080 --> 00:03:21,760 Well, the question is, of course, 54 00:03:21,760 --> 00:03:25,080 are these dark areas due to organic matter, vegetation if you like, 55 00:03:25,080 --> 00:03:26,440 or are they not? 56 00:03:33,480 --> 00:03:38,480 The answer became clear as soon as Mariner 6 sent back close-up photos. 57 00:03:41,200 --> 00:03:44,480 Nothing but bone-dry desert and craters. 58 00:03:46,000 --> 00:03:48,760 Now, what about this all-important question of life on Mars? 59 00:03:48,760 --> 00:03:51,760 Do you think these dark areas are due to anything organic? 60 00:03:51,760 --> 00:03:53,880 Primitive life, there may be. I don't even think so. 61 00:03:53,880 --> 00:03:55,360 Intelligent life, certainly not. 62 00:03:55,360 --> 00:03:57,760 So, in other words, you think that Mars is a dead planet? 63 00:03:57,760 --> 00:03:59,240 Absolutely, dead as a dodo. 64 00:04:00,400 --> 00:04:02,360 Mars was brutally cold... 65 00:04:03,520 --> 00:04:05,080 ..with virtually no atmosphere. 66 00:04:06,400 --> 00:04:10,360 Vegetation didn't stand a chance, as The Sky At Night demonstrated. 67 00:04:11,640 --> 00:04:14,040 Just to show you the fate of higher plants, 68 00:04:14,040 --> 00:04:16,760 I've brought along tonight two cacti. 69 00:04:16,760 --> 00:04:21,440 This cactus has been quite healthily growing under Earth conditions. 70 00:04:21,440 --> 00:04:25,200 And, you see, it's quite a nice, firm-looking sort of cactus. 71 00:04:25,200 --> 00:04:29,120 This one here has spent one night under Martian conditions, 72 00:04:29,120 --> 00:04:31,680 and I think you can see without any doubt 73 00:04:31,680 --> 00:04:35,400 that it's got a distinctly morning after appearance. 74 00:04:38,120 --> 00:04:41,680 Mariner 6 changed our approach to the Red Planet. 75 00:04:41,680 --> 00:04:45,760 There was no complex life now, that was certain. 76 00:04:45,760 --> 00:04:49,120 But had it always been such a harsh environment? 77 00:04:49,120 --> 00:04:53,640 Our focus shifted to the past, and one big question. 78 00:04:53,640 --> 00:04:56,960 Had there ever been liquid water on Mars? 79 00:05:02,240 --> 00:05:04,000 In 1976, 80 00:05:04,000 --> 00:05:07,440 the Viking orbiter took a series of stunning images 81 00:05:07,440 --> 00:05:09,760 that hinted that the surface of Mars 82 00:05:09,760 --> 00:05:12,160 may have once been flowing with water. 83 00:05:14,000 --> 00:05:18,680 We looked back at Viking's striking achievements in 2017. 84 00:05:20,120 --> 00:05:22,520 So, this is from one of the Viking orbiters, 85 00:05:22,520 --> 00:05:25,040 and it's not a single picture. It is lots and lots of pictures. 86 00:05:25,040 --> 00:05:27,000 It's a mosaic that has been put together. 87 00:05:27,000 --> 00:05:29,160 It's glorious. It's absolutely fantastic. 88 00:05:29,160 --> 00:05:33,000 Just looking at these pictures, started to really capture 89 00:05:33,000 --> 00:05:36,320 how close Mars was to Earth in many of its features. 90 00:05:36,320 --> 00:05:39,960 It's got these volcanoes, which are like the volcanoes on Hawaii. 91 00:05:39,960 --> 00:05:42,760 It's got this rift valley, which is like East Africa. 92 00:05:42,760 --> 00:05:44,360 And, it's got its ice caps. 93 00:05:44,360 --> 00:05:48,440 But you've also got the features that, on Earth, we would interpret 94 00:05:48,440 --> 00:05:49,840 as being from rivers. 95 00:05:49,840 --> 00:05:51,720 So you've got something that looks as if 96 00:05:51,720 --> 00:05:54,000 it's going out into a huge delta. 97 00:05:54,000 --> 00:05:56,400 You've got other channels, meandering channels. 98 00:05:59,080 --> 00:06:03,400 The photographs taken by Viking indicated that, in the past, 99 00:06:03,400 --> 00:06:07,160 huge bodies of liquid water may have shaped the landscape. 100 00:06:07,160 --> 00:06:11,760 But, with only aerial images to go on, this could only be a theory. 101 00:06:11,760 --> 00:06:15,820 To get proof, we needed to land on the surface of Mars. 102 00:06:16,820 --> 00:06:18,360 Easier said than done. 103 00:06:21,660 --> 00:06:24,980 The problem is that Mars has an atmospheric pressure 104 00:06:24,980 --> 00:06:26,780 just 1% of that on Earth. 105 00:06:28,980 --> 00:06:32,040 That makes it hard to use the atmosphere to slow a spacecraft down 106 00:06:32,040 --> 00:06:33,960 enough to make a soft landing. 107 00:06:36,560 --> 00:06:39,920 19 spacecraft have been sent to land on Mars. 108 00:06:40,920 --> 00:06:43,440 Only seven have successfully managed it. 109 00:06:45,880 --> 00:06:50,880 The latest, Nasa's Curiosity Rover, was the biggest and most ambitious. 110 00:06:52,320 --> 00:06:56,200 We have ignition, and lift-off of a Delta II rocket 111 00:06:56,200 --> 00:06:58,920 carrying NASA on an odyssey back to Mars. 112 00:07:00,240 --> 00:07:05,320 It arrived in 2012, and The Sky At Night reported on the complicated 113 00:07:05,840 --> 00:07:08,960 procedure used to deliver the rover to the surface. 114 00:07:10,920 --> 00:07:13,400 Can you start talking us through what had to happen 115 00:07:13,400 --> 00:07:15,200 to get it safely onto the ground? 116 00:07:15,200 --> 00:07:17,360 Yes, there were a number of things that had to happen. 117 00:07:17,360 --> 00:07:20,160 The first is to enter the atmosphere at the right point, 118 00:07:20,160 --> 00:07:23,200 so that the heat shield could slow the craft down enough, 119 00:07:23,200 --> 00:07:27,760 to slow it down from 13,000 mph, down to about 1000 mph, 120 00:07:27,760 --> 00:07:30,440 by the time the atmosphere had done its job, 121 00:07:30,440 --> 00:07:31,920 and, at which point, 122 00:07:31,920 --> 00:07:34,960 the parachute could then come out for the first time. 123 00:07:38,040 --> 00:07:40,880 Parachute deployed. We are decelerating. 124 00:07:40,880 --> 00:07:44,520 APPLAUSE AND CHEERING 125 00:07:46,760 --> 00:07:48,720 We're going to 90 metres per second, 126 00:07:48,720 --> 00:07:51,080 at an altitude of 6.5km per second. 127 00:07:51,080 --> 00:07:53,240 We even got a photo of Curiosity on its parachute. 128 00:07:53,240 --> 00:07:55,360 From the Mars reconnaissance orbiter. 129 00:07:55,360 --> 00:07:56,840 It's great to be able to see this, 130 00:07:56,840 --> 00:07:59,360 to have a spacecraft take a picture of another spacecraft. 131 00:07:59,360 --> 00:08:01,840 But that wasn't the hard bit. What happened next? 132 00:08:01,840 --> 00:08:05,760 Curiosity dropped down, away from its back shell and parachute... 133 00:08:07,040 --> 00:08:08,720 We are in powered flight. 134 00:08:12,760 --> 00:08:15,720 ..and then moved away from the parachute and left it behind 135 00:08:15,720 --> 00:08:18,480 and then started to descend on its own powered rockets then, 136 00:08:18,480 --> 00:08:21,760 so coming down, kind of, eight thrusters slowed it down, 137 00:08:21,760 --> 00:08:23,320 made sure it was all smooth. 138 00:08:23,320 --> 00:08:26,680 This slows down to an almost kind of stop, hovering above the surface. 139 00:08:26,680 --> 00:08:28,000 Sky crane started. 140 00:08:28,000 --> 00:08:29,720 CHEERING 141 00:08:29,720 --> 00:08:32,640 So, at about 20 metres above the surface, then Curiosity, 142 00:08:32,640 --> 00:08:36,520 the rover part, then descended on its own kind of cables 143 00:08:36,520 --> 00:08:38,160 on this sky crane technology. 144 00:08:38,160 --> 00:08:41,000 This was the part that we'd never seen anything like this before. 145 00:08:41,000 --> 00:08:42,480 Ready and stable. 146 00:08:45,920 --> 00:08:47,400 Looks good. 147 00:08:49,120 --> 00:08:50,640 Touchdown confirmed... 148 00:08:50,640 --> 00:08:53,080 CHEERING 149 00:08:57,240 --> 00:08:59,520 Like other Rovers before it, 150 00:08:59,520 --> 00:09:03,120 Curiosity was looking for clues into the past. 151 00:09:05,080 --> 00:09:08,400 But how could we piece together the story of what happened 152 00:09:08,400 --> 00:09:10,840 with photographs of Martian rocks? 153 00:09:13,360 --> 00:09:17,280 The answer was to apply skills we have perfected here on Earth, 154 00:09:17,280 --> 00:09:19,480 as geologist Iain Stewart explained. 155 00:09:21,520 --> 00:09:25,680 This rock face is part of the Jurassic coastline in Devon. 156 00:09:25,680 --> 00:09:28,840 It was formed around 240 million years ago, 157 00:09:28,840 --> 00:09:33,400 and it provides a detailed record of different environmental conditions. 158 00:09:35,160 --> 00:09:37,480 There's a whole set of layers in this cliff, 159 00:09:37,480 --> 00:09:39,360 and, if you know how to read them properly, 160 00:09:39,360 --> 00:09:42,000 then you can create these different environments. 161 00:09:42,000 --> 00:09:44,880 Now, these... These are really distinctive. 162 00:09:44,880 --> 00:09:48,680 You can see how beautifully rounded all those pebbles are. 163 00:09:48,680 --> 00:09:50,200 Look at them, incredible. 164 00:09:50,200 --> 00:09:53,200 So they would have started off as just general angular rocks, 165 00:09:53,200 --> 00:09:56,760 but what's made them all smooth is water. 166 00:09:59,320 --> 00:10:02,000 So this really is evidence that through here, in the past, 167 00:10:02,000 --> 00:10:04,360 once flowed a mighty river. 168 00:10:05,960 --> 00:10:07,640 But just along the coast, 169 00:10:07,640 --> 00:10:09,600 it's a different kind of rock formation 170 00:10:09,600 --> 00:10:13,320 that shows a very different kind of evidence of contact with water. 171 00:10:17,520 --> 00:10:20,960 What we've got in front of us here is just a wall of sand, 172 00:10:20,960 --> 00:10:24,880 layer upon layer of sand dunes, piled on top of each other, 173 00:10:24,880 --> 00:10:28,320 but in amongst it, are these little bands here. 174 00:10:28,320 --> 00:10:30,800 There's another one, and they get a little bit thicker, 175 00:10:30,800 --> 00:10:33,120 and a bit different to the sand above. 176 00:10:33,120 --> 00:10:36,680 In fact, if you look in here, how crumbly this is. 177 00:10:39,320 --> 00:10:41,800 Because what we have got here is a deposit 178 00:10:41,800 --> 00:10:45,440 that's been laid down by a really fine sediment, 179 00:10:45,440 --> 00:10:47,640 settling out of standing water. 180 00:10:47,640 --> 00:10:51,440 Essentially, this is a temporary lake among the sand. 181 00:10:52,840 --> 00:10:56,640 You only get rocks with this incredibly fine-grained structure 182 00:10:56,640 --> 00:10:59,840 if they were formed when water becomes really still, 183 00:10:59,840 --> 00:11:01,960 as it is in lakes and ponds. 184 00:11:01,960 --> 00:11:05,200 So that means that, whenever we see a kind of thin band 185 00:11:05,200 --> 00:11:09,000 of really fine sediment, we know that it must have fallen out 186 00:11:09,000 --> 00:11:10,680 from still water. 187 00:11:16,560 --> 00:11:18,600 Using these geological tricks, 188 00:11:18,600 --> 00:11:21,520 we build up this detailed and compelling picture 189 00:11:21,520 --> 00:11:23,720 of the history of water on Earth. 190 00:11:27,160 --> 00:11:30,840 Using this knowledge, we can look at the rocks on Mars 191 00:11:30,840 --> 00:11:33,680 and reconstruct the planet's watery past. 192 00:11:37,720 --> 00:11:42,000 In early 2018, Sanjeev Gupta from Curiosity's imaging team, 193 00:11:42,000 --> 00:11:45,440 gave the latest update on the search for water. 194 00:11:47,720 --> 00:11:50,520 So where on Mars are we, and what are we looking at here? 195 00:11:50,520 --> 00:11:54,160 So, we are sitting in Gale Crater, this is this crater that's about 196 00:11:54,160 --> 00:11:57,640 150km in diameter that sits on the Martian equator. 197 00:11:57,640 --> 00:12:00,240 What we have got here is this incredible mountain. 198 00:12:00,240 --> 00:12:01,640 This is Mount Sharp. 199 00:12:01,640 --> 00:12:06,160 It's 5km high, and it's made up of beautifully layered, 200 00:12:06,160 --> 00:12:08,560 stratified sedimentary rocks. 201 00:12:08,560 --> 00:12:10,080 You can see that on this image here, 202 00:12:10,080 --> 00:12:12,480 you can see those layers working their way up the mountain. 203 00:12:12,480 --> 00:12:14,560 That's right, these are sedimentary layers. 204 00:12:14,560 --> 00:12:16,560 What these are recording is basically 205 00:12:16,560 --> 00:12:21,280 progressive build-up of sediment on the Martian surface. 206 00:12:21,280 --> 00:12:25,040 OK, so something is depositing regular layers of sediment. 207 00:12:26,560 --> 00:12:28,680 OK, let's have a look, if we can have the next image... 208 00:12:28,680 --> 00:12:30,120 There we go. It's coming up. 209 00:12:30,120 --> 00:12:32,640 It's a big image. OK. 210 00:12:32,640 --> 00:12:36,160 The rock is actually a mudstone, it's a very, very fine-grained rock. 211 00:12:41,000 --> 00:12:43,480 We've got hundreds of thousands, if not millions, 212 00:12:43,480 --> 00:12:47,800 of years of sedimentation in a long-lived lake. 213 00:12:56,640 --> 00:12:59,320 it seems it didn't vanish altogether. 214 00:13:00,400 --> 00:13:02,280 It just went underground. 215 00:13:05,280 --> 00:13:08,280 OK, let's have a look at one of the more recent images. 216 00:13:08,280 --> 00:13:10,840 There we go. And here we are. 217 00:13:10,840 --> 00:13:15,400 This ridge is probably three or four metres high here, in this face, 218 00:13:15,400 --> 00:13:18,120 and just gorgeous detail, in terms of the geology. 219 00:13:24,160 --> 00:13:26,800 We've been able to use the rover's instruments 220 00:13:26,800 --> 00:13:30,760 to actually look at the chemistry of the crack fills, 221 00:13:30,760 --> 00:13:34,720 and actually, they show evidence for calcium sulphate, 222 00:13:34,720 --> 00:13:37,520 so this is actually gypsum that's filling these cracks. 223 00:13:37,520 --> 00:13:40,360 And this tells us an interesting story about Mars. 224 00:13:40,360 --> 00:13:43,800 After these rocks were lithified, they were turned to stone, 225 00:13:43,800 --> 00:13:47,120 they were buried, and then there was ground water flow 226 00:13:47,120 --> 00:13:52,160 through these rocks, and that ground water was full of calcium ions 227 00:13:52,160 --> 00:13:56,520 and sulfate ions, deposited within those crack fills. 228 00:13:56,520 --> 00:13:59,680 The interesting thing about this is that the chemistry, 229 00:13:59,680 --> 00:14:02,920 the chemical changes, are important to habitability, 230 00:14:02,920 --> 00:14:05,920 and that's crucial, and that's why there's so much interest in this. 231 00:14:12,640 --> 00:14:16,600 We now know that Mars once had rivers and lakes, 232 00:14:16,600 --> 00:14:20,400 and this surface water persisted for millions of years. 233 00:14:21,800 --> 00:14:24,560 What's more, there could still be liquid water 234 00:14:24,560 --> 00:14:26,400 deep under the surface today. 235 00:14:30,520 --> 00:14:34,440 As we've studied Mars, an extraordinary story has emerged. 236 00:14:34,440 --> 00:14:37,560 Mars used to be habitable, like the Earth. 237 00:14:37,560 --> 00:14:40,360 But over time, Mars lost its atmosphere, 238 00:14:40,360 --> 00:14:42,360 stripped away by the solar wind. 239 00:14:42,360 --> 00:14:46,080 And, with no atmosphere, Mars also lost its water to space, 240 00:14:46,080 --> 00:14:49,720 gradually becoming the dry, dusty place we know today. 241 00:14:52,920 --> 00:14:56,080 Comparing Earth and Mars reveals that two planets 242 00:14:56,080 --> 00:14:58,840 can start off with similar conditions, 243 00:14:58,840 --> 00:15:02,440 but end up following very different paths of development. 244 00:15:04,840 --> 00:15:07,280 Neither was this an isolated case. 245 00:15:11,800 --> 00:15:15,880 When The Sky At Night began broadcasting in 1957, 246 00:15:15,880 --> 00:15:20,120 Venus, our closest neighbour, was a planet of mystery... 247 00:15:21,600 --> 00:15:24,120 ..still thought to be potentially habitable... 248 00:15:26,560 --> 00:15:28,920 ..as Patrick Moore remembered in 1982. 249 00:15:31,120 --> 00:15:34,840 Venus, so like the Earth in size and mass, is a curious world. 250 00:15:34,840 --> 00:15:37,800 All we can see from above is the top of a cloud layer. 251 00:15:39,360 --> 00:15:42,240 According to two very famous American astronomers, 252 00:15:42,240 --> 00:15:43,680 Whipple and Menzel, 253 00:15:43,680 --> 00:15:47,440 it was liable to be covered almost, or completely, with ocean, 254 00:15:47,440 --> 00:15:50,000 with only a few islands poking out here and there. 255 00:15:51,480 --> 00:15:55,920 Life on Earth began in our oceans a long, long time ago, 256 00:15:55,920 --> 00:15:59,120 and Venus, therefore, might have been a world 257 00:15:59,120 --> 00:16:01,360 where life was just beginning, and might evolve. 258 00:16:03,480 --> 00:16:05,520 Such speculation was possible 259 00:16:05,520 --> 00:16:08,720 because Venus was completely covered in cloud. 260 00:16:08,720 --> 00:16:11,160 Anything could be hidden underneath. 261 00:16:12,480 --> 00:16:15,680 But when the first successful lander, Russia's Venera 7, 262 00:16:15,680 --> 00:16:18,400 reached its surface in 1970, 263 00:16:18,400 --> 00:16:21,000 all hopes of habitability were dashed. 264 00:16:23,320 --> 00:16:26,920 It revealed an extreme and harsh world. 265 00:16:28,160 --> 00:16:29,760 The pressure at the surface 266 00:16:29,760 --> 00:16:32,560 was greater than being at the bottom of the ocean. 267 00:16:32,560 --> 00:16:36,680 Temperatures were 462 Celsius, 268 00:16:36,680 --> 00:16:40,760 and the atmosphere was almost entirely carbon dioxide. 269 00:16:43,000 --> 00:16:45,280 The conditions were so brutal 270 00:16:45,280 --> 00:16:49,960 that Venera 7 lasted less than an hour, before falling silent. 271 00:16:49,960 --> 00:16:52,240 Conditions on Venus were obviously hellish, 272 00:16:52,240 --> 00:16:55,600 and that provoked a lot of questions. 273 00:16:55,600 --> 00:16:57,720 Just why do you imagine Venus, 274 00:16:57,720 --> 00:17:00,560 which is so like the Earth in size and mass, 275 00:17:00,560 --> 00:17:03,560 has developed an atmosphere which is so utterly unlike ours? 276 00:17:03,560 --> 00:17:06,360 It's closer to the sun, it receives more heat from the sun, 277 00:17:06,360 --> 00:17:08,520 the surface gets hotter therefore, 278 00:17:08,520 --> 00:17:12,080 and the gases rise from the surface, building up the atmosphere, 279 00:17:12,080 --> 00:17:14,800 and, as the atmosphere builds up in capacity, 280 00:17:14,800 --> 00:17:17,200 so the surface temperature will rise too. 281 00:17:17,200 --> 00:17:20,280 It's a kind of runaway effect. It's the runaway greenhouse effect, 282 00:17:20,280 --> 00:17:24,080 in the same way as heat is trapped in your greenhouse, when you're growing your vegetables, 283 00:17:24,080 --> 00:17:26,560 the same heat is getting trapped up on the Venus atmosphere, 284 00:17:26,560 --> 00:17:28,400 building up the surface temperature. 285 00:17:28,400 --> 00:17:32,760 Do you know, Garry, I remember way back in 1957, 1958, 286 00:17:32,760 --> 00:17:34,880 on one of these Sky At Night programmes, 287 00:17:34,880 --> 00:17:38,200 I remember saying that, on the basis of the information we had then, 288 00:17:38,200 --> 00:17:41,200 Venus appeared a rather more promising site for a colony 289 00:17:41,200 --> 00:17:44,880 than Mars, and how wrong I was. How wrong we all were! 290 00:17:48,760 --> 00:17:51,400 Since the Russian Venera triumph, 291 00:17:51,400 --> 00:17:55,120 we've set a total of 21 successful missions to Venus. 292 00:17:55,120 --> 00:17:58,520 They have shown us a planet covered in volcanoes, 293 00:17:58,520 --> 00:18:01,160 and sulphuric acid clouds. 294 00:18:01,160 --> 00:18:04,880 The latest probe to conclude its mission, Venus Express, 295 00:18:04,880 --> 00:18:09,920 revealed just how dynamic the planet is, as I discovered in 2015. 296 00:18:12,200 --> 00:18:14,400 One of the first things these images revealed 297 00:18:14,400 --> 00:18:17,480 was what a constantly changing world Venus is. 298 00:18:17,480 --> 00:18:21,280 That is dramatically illustrated by what happens in the polar regions. 299 00:18:21,280 --> 00:18:25,160 The poles of Venus have these enormous vortex features, 300 00:18:25,160 --> 00:18:28,720 and it really changes shape quite dramatically from day-to-day. 301 00:18:28,720 --> 00:18:31,280 It's also changing from year-to-year. 302 00:18:31,280 --> 00:18:34,720 After eight years of observations we have seen, for example, 303 00:18:34,720 --> 00:18:37,400 that the mean wind speed at the cloud tops of Venus 304 00:18:37,400 --> 00:18:41,320 have increased by 100kmph over the course of the mission. 305 00:18:42,720 --> 00:18:46,560 But Venus Express also suggested that, just like Mars, 306 00:18:46,560 --> 00:18:49,600 in the past, Venus might have been very different. 307 00:18:52,720 --> 00:18:56,080 I met Andrew Coates, who's been studying the way 308 00:18:56,080 --> 00:18:59,240 the solar wind interacts with the atmosphere of Venus. 309 00:19:00,440 --> 00:19:03,160 Venus lacks a magnetic field. You know, unlike the Earth, 310 00:19:03,160 --> 00:19:05,840 which is protected, to some extent, by a magnetic field, 311 00:19:05,840 --> 00:19:09,280 Venus doesn't have such a thing and so the solar wind, 312 00:19:09,280 --> 00:19:11,800 which is a million tonne per second stream of material 313 00:19:11,800 --> 00:19:13,400 coming out of the sun all the time, 314 00:19:13,400 --> 00:19:16,200 can interact with the atmosphere of Venus, actually pull it away. 315 00:19:16,200 --> 00:19:18,520 So, Venus is losing atmosphere? 316 00:19:18,520 --> 00:19:22,080 300kg of the atmosphere per day is actually escaping. 317 00:19:22,080 --> 00:19:23,880 Taken over billions of years, 318 00:19:23,880 --> 00:19:26,440 this means an awful lot of material has been lost. 319 00:19:26,440 --> 00:19:29,040 So, what has surprised you in terms of the results you have 320 00:19:29,040 --> 00:19:30,200 got from Aspera-4? 321 00:19:30,200 --> 00:19:33,240 Looking at the composition of the material escaping from Venus, 322 00:19:33,240 --> 00:19:35,080 it actually turns out to be water. 323 00:19:35,080 --> 00:19:37,160 The composition is very much like... 324 00:19:37,160 --> 00:19:39,200 So, you've got twice the amount of hydrogen, 325 00:19:39,200 --> 00:19:42,240 compared to the amount of oxygen going off, so that's H2O. 326 00:19:42,240 --> 00:19:45,200 Water! Water, yes, and that was a big surprise 327 00:19:45,200 --> 00:19:49,080 because the atmosphere of Venus is basically carbon dioxide, 328 00:19:49,080 --> 00:19:53,680 but it shows that water is still being lost from the Venus atmosphere 329 00:19:53,680 --> 00:19:57,120 so this dehydration is a process which is still going on, 330 00:19:57,120 --> 00:19:59,480 and has been going on for billions of years. 331 00:20:02,240 --> 00:20:06,200 If Venus has been losing water for billions of years, 332 00:20:06,200 --> 00:20:08,800 it might have once been a wet planet. 333 00:20:12,240 --> 00:20:16,640 But then, it got hotter and hotter, thanks to greenhouse gases. 334 00:20:18,120 --> 00:20:19,920 Water turned to steam, 335 00:20:19,920 --> 00:20:23,800 and the solar wind stripped the water vapour away into space. 336 00:20:27,160 --> 00:20:30,440 Seen in the light of what happened to its neighbours, 337 00:20:30,440 --> 00:20:32,760 Earth is the one that is unusual. 338 00:20:35,680 --> 00:20:39,920 Whilst Mars and Venus have both undergone dramatic changes, 339 00:20:39,920 --> 00:20:44,920 Earth alone has remained stable and habitable for billions of years. 340 00:20:51,040 --> 00:20:54,160 Even closer to the sun, we find Mercury. 341 00:20:56,080 --> 00:20:59,080 A planet with massive temperature extremes... 342 00:21:00,440 --> 00:21:02,200 ..and virtually no atmosphere. 343 00:21:06,080 --> 00:21:09,720 Today, these four worlds are all very different, 344 00:21:09,720 --> 00:21:12,160 but they all belong to the same category. 345 00:21:13,280 --> 00:21:14,600 The rocky planets. 346 00:21:18,080 --> 00:21:20,400 As we move further out into the solar system, 347 00:21:20,400 --> 00:21:23,120 we come to an entirely different class of planet. 348 00:21:24,480 --> 00:21:27,600 The gas giants of Jupiter and Saturn. 349 00:21:29,120 --> 00:21:33,520 These spectacular planets have led to some amazing discoveries. 350 00:21:33,520 --> 00:21:37,360 Perhaps the most surprising is how they have changed our understanding 351 00:21:37,360 --> 00:21:40,960 of how the rest of the solar system developed, including Earth. 352 00:21:44,480 --> 00:21:48,120 As one of the brightest objects in the sky, Jupiter, 353 00:21:48,120 --> 00:21:50,000 named after the king of the Roman gods, 354 00:21:50,000 --> 00:21:53,400 has enthralled humans since ancient times. 355 00:21:55,920 --> 00:21:59,880 It was one of the first objects to be observed through a telescope, 356 00:21:59,880 --> 00:22:03,400 so large that its moons and the striped features 357 00:22:03,400 --> 00:22:05,520 were easily visible from Earth. 358 00:22:05,520 --> 00:22:08,040 Jupiter is making quite a brave show and you can see it 359 00:22:08,040 --> 00:22:10,480 in the southern part of the sky late at night. 360 00:22:10,480 --> 00:22:13,480 It looks like a very brilliant star, and in fact, it's so bright 361 00:22:13,480 --> 00:22:15,000 that you can't really mistake it. 362 00:22:15,000 --> 00:22:16,640 If you've got a telescope, 363 00:22:16,640 --> 00:22:18,960 you'll be able to see its belts and its moons. 364 00:22:18,960 --> 00:22:21,200 This is a picture of Jupiter, which also shows 365 00:22:21,200 --> 00:22:23,840 that extraordinary feature, the great red spot. 366 00:22:26,920 --> 00:22:30,640 Our exploration of Jupiter began in earnest in 1979 367 00:22:30,640 --> 00:22:33,160 with the two Voyager spacecraft. 368 00:22:33,160 --> 00:22:36,560 571, item one, 369 00:22:36,560 --> 00:22:39,240 NA, item two command... 370 00:22:39,240 --> 00:22:43,000 Good evening. As I think most people will know, 371 00:22:43,000 --> 00:22:46,640 the American probe Voyager 1 has flown past the giant planet Jupiter, 372 00:22:46,640 --> 00:22:49,520 and sent back incredible results. 373 00:22:49,520 --> 00:22:52,600 And this, in fact, is a film made up of the various stills taken 374 00:22:52,600 --> 00:22:53,960 as the probe went in, 375 00:22:53,960 --> 00:22:56,720 and you can actually see Jupiter spinning around. 376 00:22:56,720 --> 00:23:00,000 Don't forget, Jupiter has the shortest day in the solar system, 377 00:23:00,000 --> 00:23:01,440 less than ten hours long. 378 00:23:01,440 --> 00:23:04,240 There it is, there is the great red spot below the centre. 379 00:23:09,800 --> 00:23:13,240 What we see as Jupiter's coloured bands are, in fact, 380 00:23:13,240 --> 00:23:15,880 the tops of giant weather systems. 381 00:23:17,280 --> 00:23:22,160 A layer of swirling cloud tops that go down 3,000 kilometres. 382 00:23:27,240 --> 00:23:30,880 The Voyagers confirmed theories that the great red spot, 383 00:23:30,880 --> 00:23:33,160 observed for hundreds of years, 384 00:23:33,160 --> 00:23:36,920 was a gigantic storm, twice the size of Earth. 385 00:23:36,920 --> 00:23:39,640 But how had it persisted for so long? 386 00:23:40,960 --> 00:23:44,520 Helen Czerski met Professor Peter Read to find out 387 00:23:44,520 --> 00:23:47,800 how temperature differences between the different bands of weather 388 00:23:47,800 --> 00:23:50,800 can create very stable features in the atmosphere. 389 00:23:55,000 --> 00:23:57,680 What we tend to see is that the main temperature differences 390 00:23:57,680 --> 00:23:59,360 that we can measure in the atmosphere 391 00:23:59,360 --> 00:24:01,240 are actually between the bright bands 392 00:24:01,240 --> 00:24:04,400 and the dark bands, so that you'll typically have a bright band that's 393 00:24:04,400 --> 00:24:07,840 relatively warm and then, to the north and to the south of that, 394 00:24:07,840 --> 00:24:11,320 there'll be a dark band that is relatively cold. 395 00:24:11,320 --> 00:24:12,840 With this experiment, 396 00:24:12,840 --> 00:24:15,560 we are trying to create a simulation of what happens 397 00:24:15,560 --> 00:24:17,480 within one of Jupiter's bands. 398 00:24:18,480 --> 00:24:19,840 The water is heated, 399 00:24:19,840 --> 00:24:22,720 but the edges and the centre of the vessel are cooled. 400 00:24:22,720 --> 00:24:27,360 This represents a warm band on Jupiter, surrounded by colder air. 401 00:24:27,360 --> 00:24:30,080 And then the whole experiment is rotated. 402 00:24:31,320 --> 00:24:35,600 After a few minutes, some vortices are created that are so stable 403 00:24:35,600 --> 00:24:37,280 they barely appear to move. 404 00:24:39,160 --> 00:24:43,440 So, what you can see now is a whole chain of these eddies 405 00:24:43,440 --> 00:24:45,840 that are all circulating in the same sense. 406 00:24:45,840 --> 00:24:48,000 So if this was a band on Jupiter, 407 00:24:48,000 --> 00:24:51,120 at the bottom of the band the winds are going this way round, 408 00:24:51,120 --> 00:24:53,760 and then you've got these eddies spinning, like this. 409 00:24:53,760 --> 00:24:56,880 But, at the top, the winds are going the other way. 410 00:24:56,880 --> 00:25:00,680 That's right, so this is just like the bright bands on Jupiter. 411 00:25:00,680 --> 00:25:03,400 So, the south of the jets will be going in one direction, 412 00:25:03,400 --> 00:25:05,960 and to the north they'll be going in the opposite direction, 413 00:25:05,960 --> 00:25:07,960 with these vortices rolling in between them. 414 00:25:07,960 --> 00:25:11,640 These rotating storms are trapped within the bands 415 00:25:11,640 --> 00:25:14,680 that have been created by Jupiter's fast rotation. 416 00:25:28,920 --> 00:25:32,880 Since the Voyager probes, we've sent another five probes to Jupiter... 417 00:25:35,840 --> 00:25:38,480 ..and discovered it has northern lights 418 00:25:38,480 --> 00:25:41,720 up to 1000 times brighter than on Earth, 419 00:25:41,720 --> 00:25:44,920 an intense magnetic field, 420 00:25:44,920 --> 00:25:47,320 and deadly radiation zones. 421 00:25:49,320 --> 00:25:52,880 But perhaps the most surprising discovery we've made about Jupiter 422 00:25:52,880 --> 00:25:56,560 is that it plays a key role in keeping Earth habitable. 423 00:25:58,920 --> 00:26:00,400 In 1992, 424 00:26:00,400 --> 00:26:04,040 scientists spotted a comet they called Shoemaker-Levy 9, 425 00:26:04,040 --> 00:26:06,200 which had broken up into pieces, 426 00:26:06,200 --> 00:26:08,520 and was on a collision course with Jupiter. 427 00:26:10,640 --> 00:26:13,200 Telescopes, including the Hubble space telescope, 428 00:26:13,200 --> 00:26:16,400 were trained on Jupiter to record the impact. 429 00:26:18,680 --> 00:26:22,240 The Sky At Night looked back at the event two decades on. 430 00:26:26,120 --> 00:26:27,920 In July 1994, 431 00:26:27,920 --> 00:26:32,080 21 fragments of what was the comet Shoemaker-Levy 9 432 00:26:32,080 --> 00:26:34,720 crashed into the atmosphere of Jupiter, 433 00:26:34,720 --> 00:26:37,720 and Hubble was on hand to witness the event. 434 00:26:40,600 --> 00:26:44,480 The impacts happened just over the limb, on the planet's far side, 435 00:26:44,480 --> 00:26:46,920 though the plumes were detected from Earth. 436 00:26:48,240 --> 00:26:51,400 But the true enormity of what had happened became apparent 437 00:26:51,400 --> 00:26:54,480 slightly afterwards, as Jupiter rotated, 438 00:26:54,480 --> 00:26:56,960 bringing the impact sites into Hubble's view. 439 00:26:58,440 --> 00:27:00,320 The wonderful thing about this event 440 00:27:00,320 --> 00:27:03,200 was that no-one had any real idea what to expect. 441 00:27:03,200 --> 00:27:07,200 We hoped we might see the impact with Hubble, but that was about it. 442 00:27:07,200 --> 00:27:10,080 I remember going outside with my own small telescope, 443 00:27:10,080 --> 00:27:13,520 and being able to see these bruises on Jupiter's surface, 444 00:27:13,520 --> 00:27:16,120 and then to see the Hubble images afterwards 445 00:27:16,120 --> 00:27:18,040 made for a really thrilling event. 446 00:27:19,760 --> 00:27:23,640 The resulting impacts were more profound than anyone had predicted. 447 00:27:24,920 --> 00:27:26,920 Using different wavelengths of light, 448 00:27:26,920 --> 00:27:30,040 Hubble captured striking images of the impact scars, 449 00:27:30,040 --> 00:27:33,600 revealing some to be twice the size of Earth. 450 00:27:37,200 --> 00:27:40,320 At the time, we thought it was a once-in-a-lifetime event. 451 00:27:41,560 --> 00:27:43,680 But we now know differently. 452 00:27:43,680 --> 00:27:46,600 Thanks to the work of amateur astronomers, 453 00:27:46,600 --> 00:27:48,720 who monitor Jupiter constantly. 454 00:27:50,400 --> 00:27:53,840 They've spotted several unusual spots over the years. 455 00:27:55,360 --> 00:27:58,000 In 2009, quite unexpectedly, an amateur, Anthony Wesley, 456 00:27:58,000 --> 00:28:00,680 discovered such a spot on the planet. 457 00:28:00,680 --> 00:28:02,880 This was an image he took two nights earlier, 458 00:28:02,880 --> 00:28:06,200 and then he saw this remarkably black spot appearing, 459 00:28:06,200 --> 00:28:08,800 and realised this might well be an impact. 460 00:28:08,800 --> 00:28:12,240 So, other amateurs immediately started taking images to confirm, 461 00:28:12,240 --> 00:28:15,280 and professional scientists took this image 462 00:28:15,280 --> 00:28:17,000 in a far infrared wavelength. 463 00:28:17,000 --> 00:28:19,680 The Hubble space telescope took this image a few days later, 464 00:28:19,680 --> 00:28:23,080 and so the professional astronomers managed to follow this event 465 00:28:23,080 --> 00:28:24,600 over several months, 466 00:28:24,600 --> 00:28:27,640 while we amateurs were also tracking it over several months. 467 00:28:27,640 --> 00:28:30,240 So, the amateurs keep watch, they keep an eye on Jupiter, 468 00:28:30,240 --> 00:28:32,880 and alert the professionals when something exciting happens? 469 00:28:32,880 --> 00:28:35,400 Yes, indeed. And, more recently, 470 00:28:35,400 --> 00:28:38,320 amateurs have been noticing impacts while they happen. 471 00:28:38,320 --> 00:28:41,680 They're much smaller impacts, they don't leave visible scars, 472 00:28:41,680 --> 00:28:45,680 but they're more frequent, and so now three times since 2009, 473 00:28:45,680 --> 00:28:49,600 amateurs have actually seen fireballs in Jupiter's atmosphere, 474 00:28:49,600 --> 00:28:52,840 which previously, if anyone had seen them, they didn't notice them, 475 00:28:52,840 --> 00:28:55,320 or didn't believe they were seeing them. 476 00:28:55,320 --> 00:28:58,000 But now, we have real webcam videos and it is clear 477 00:28:58,000 --> 00:29:01,720 that amateurs are actually detecting flashes as they occur. 478 00:29:04,440 --> 00:29:08,480 Jupiter's enormous gravity acts like a giant magnet, 479 00:29:08,480 --> 00:29:12,920 sweeping up comets and meteors that might otherwise collide with Earth. 480 00:29:15,120 --> 00:29:19,280 Without Jupiter, Earth might have faced many more meteorite impacts, 481 00:29:19,280 --> 00:29:22,040 like the one that wiped out the dinosaurs. 482 00:29:23,480 --> 00:29:25,640 That would have made it much more difficult 483 00:29:25,640 --> 00:29:27,440 for complex life to develop. 484 00:29:31,280 --> 00:29:34,320 Jupiter may be the biggest planet in the solar system, 485 00:29:34,320 --> 00:29:36,520 but it isn't the most spectacular. 486 00:29:38,160 --> 00:29:40,880 Saturn, with its incredible ring system, 487 00:29:40,880 --> 00:29:44,840 is perhaps one of the solar system's greatest planetary wonders. 488 00:29:44,840 --> 00:29:47,520 It's also the planet that sits furthest away from the sun 489 00:29:47,520 --> 00:29:50,880 that can be seen from Earth with just the naked eye. 490 00:29:50,880 --> 00:29:54,920 And its iconic rings have puzzled astronomers for centuries. 491 00:29:54,920 --> 00:29:56,480 What are they made of? 492 00:29:56,480 --> 00:29:57,960 How are they formed? 493 00:29:57,960 --> 00:30:00,000 And, how many rings are there? 494 00:30:02,400 --> 00:30:03,560 PATRICK MOORE: Saturn. 495 00:30:03,560 --> 00:30:06,360 This is the planet with the rings, a magnificent sight, 496 00:30:06,360 --> 00:30:08,920 there's nothing else like it in the sky, so far as we know. 497 00:30:10,720 --> 00:30:14,720 Saturn sits 1.4 billion kilometres from the sun. 498 00:30:16,120 --> 00:30:19,200 And, so far, we've only sent four missions there. 499 00:30:21,040 --> 00:30:25,240 The first was Pioneer 11 in 1979. 500 00:30:27,240 --> 00:30:30,360 This is Saturn, photographed from close range. 501 00:30:30,360 --> 00:30:32,440 Since our last Sky At Night programme, 502 00:30:32,440 --> 00:30:35,840 the space probe Pioneer 11 has flown past the ringed planet, 503 00:30:35,840 --> 00:30:38,320 and has sent back pictures which are surprisingly good, 504 00:30:38,320 --> 00:30:41,160 as well as a great deal of fascinating general information. 505 00:30:41,160 --> 00:30:45,800 But we had to wait 25 years for the definitive mission to Saturn. 506 00:30:47,560 --> 00:30:50,040 And, lift-off of the Cassini spacecraft 507 00:30:50,040 --> 00:30:52,200 on a billion-mile trek to Saturn. 508 00:30:53,920 --> 00:30:57,160 Cassini, which arrived in orbit in 2004, 509 00:30:57,160 --> 00:31:00,200 spent 13 years orbiting the planet. 510 00:31:04,640 --> 00:31:07,320 It revealed enormous hurricanes, 511 00:31:07,320 --> 00:31:09,960 8,000 kilometres across. 512 00:31:09,960 --> 00:31:15,000 And we finally saw Saturn's rings in all their complex glory. 513 00:31:19,720 --> 00:31:22,960 They're made almost entirely of water ice, 514 00:31:22,960 --> 00:31:25,960 Some particles the size of grains of sand, 515 00:31:25,960 --> 00:31:28,200 and others as big as mountains. 516 00:31:29,160 --> 00:31:32,840 They're 280,000 kilometres across, 517 00:31:32,840 --> 00:31:36,120 but, astonishingly, only ten metres deep. 518 00:31:38,600 --> 00:31:41,120 And they show incredible complexity. 519 00:31:43,760 --> 00:31:45,880 There are seven groups of rings, 520 00:31:45,880 --> 00:31:49,920 and each one has its own unique, intricate structure. 521 00:31:51,320 --> 00:31:54,920 What's more, Cassini discovered remarkable and unexpected 522 00:31:54,920 --> 00:31:58,320 interactions between Saturn's rings and its moons. 523 00:31:59,520 --> 00:32:03,840 Gravity from the moons could clearly be seen to be dragging the rings out 524 00:32:03,840 --> 00:32:06,600 of shape, but the relationship between the moons and the rings 525 00:32:06,600 --> 00:32:08,520 went further than that. 526 00:32:12,880 --> 00:32:15,000 This is the moon Enceladus. 527 00:32:15,000 --> 00:32:16,600 So, this is active. 528 00:32:16,600 --> 00:32:18,240 We think there is liquid water, 529 00:32:18,240 --> 00:32:21,040 possibly even a global ocean of liquid water underneath, 530 00:32:21,040 --> 00:32:23,720 and this material is coming out at pressure, 531 00:32:23,720 --> 00:32:25,800 and going out several hundred kilometres, 532 00:32:25,800 --> 00:32:28,200 and then just sort of wandering around. 533 00:32:28,200 --> 00:32:31,640 If you look at it really closely, you can actually see the plumes 534 00:32:31,640 --> 00:32:34,080 of material coming out of the south polar, 535 00:32:34,080 --> 00:32:36,480 and that has created the entire ring, 536 00:32:36,480 --> 00:32:39,600 so it shows this connection between rings and moons. 537 00:32:39,600 --> 00:32:41,760 Yeah, and it seems to be quite a strong one. 538 00:32:43,680 --> 00:32:46,680 It seems that at least some of Saturn's rings, 539 00:32:46,680 --> 00:32:50,120 are formed from material ejected from Saturn's moons. 540 00:32:51,160 --> 00:32:53,600 But there's a twist to this discovery. 541 00:32:55,000 --> 00:32:59,680 So, moons forming rings, and then sometimes rings forming moons? 542 00:32:59,680 --> 00:33:01,920 Yes. I find that surprising! 543 00:33:01,920 --> 00:33:04,280 I did, too, when it was first proposed, 544 00:33:04,280 --> 00:33:07,360 but we know that material kind of naturally can accrete in the rings. 545 00:33:07,360 --> 00:33:09,760 Supposing you get a large enough mass, 546 00:33:09,760 --> 00:33:13,280 then it could start evolving as it interacts with the ring material 547 00:33:13,280 --> 00:33:16,520 around it, and maybe even escape from the rings, 548 00:33:16,520 --> 00:33:19,640 and then move outwards, and maybe catch up with another one 549 00:33:19,640 --> 00:33:21,840 that had already escaped and get larger. 550 00:33:21,840 --> 00:33:24,280 And they originally proposed it for small moons, 551 00:33:24,280 --> 00:33:27,720 and they think now, maybe even the larger moons of Saturn, 552 00:33:27,720 --> 00:33:30,720 and indeed other giant planets, could have formed in this way. 553 00:33:32,160 --> 00:33:36,680 This process helps us understand the early days of the solar system. 554 00:33:38,240 --> 00:33:41,040 You have this sort of, what I might call an accretion disc, 555 00:33:41,040 --> 00:33:42,960 with a large mass in the centre. 556 00:33:42,960 --> 00:33:46,320 It's very similar to what we think the early solar system was like. 557 00:33:46,320 --> 00:33:49,160 That there was a disc of material out of which the planets formed, 558 00:33:49,160 --> 00:33:53,680 in that case, and this is a disc that we can study in detail now, 559 00:33:53,680 --> 00:33:56,880 and we can see how things evolve now, 560 00:33:56,880 --> 00:34:00,120 and then try and extrapolate to what went on in the early solar system. 561 00:34:00,120 --> 00:34:02,040 So, by studying the rings of Saturn, 562 00:34:02,040 --> 00:34:05,160 we can actually look back to the formation of our whole solar system. 563 00:34:09,880 --> 00:34:13,840 When we leave the gas giants behind, everything changes again. 564 00:34:13,840 --> 00:34:16,560 We meet a whole new class of planet, 565 00:34:16,560 --> 00:34:19,240 the ice giants of Uranus and Neptune. 566 00:34:35,160 --> 00:34:40,200 Even as recently as 1975, we had no clear photos of Uranus and Neptune. 567 00:34:44,640 --> 00:34:47,520 Well, I'm afraid you're not going to see very much on Uranus 568 00:34:47,520 --> 00:34:50,320 with any telescope. I can show you a picture of it, 569 00:34:50,320 --> 00:34:52,840 and here is one, taken with a major telescope. 570 00:34:54,360 --> 00:34:57,320 You can see there some of Uranus's satellites as well, 571 00:34:57,320 --> 00:35:00,520 but not even a giant telescope is going to show you much on the disc. 572 00:35:01,640 --> 00:35:03,160 And here is Neptune. 573 00:35:04,160 --> 00:35:05,560 But, as I've said, 574 00:35:05,560 --> 00:35:08,520 I'm afraid no telescope is going to show you very much on it. 575 00:35:08,520 --> 00:35:11,680 We've got, I think, to wait for the era of space probes. 576 00:35:13,560 --> 00:35:16,840 Only one probe has ever visited these outer planets. 577 00:35:19,200 --> 00:35:20,680 Voyager II. 578 00:35:20,680 --> 00:35:24,280 And only by flying past, not going into orbit. 579 00:35:28,440 --> 00:35:32,040 When Voyager arrived at Uranus in 1986, 580 00:35:32,040 --> 00:35:35,040 what it found was far from what we had imagined. 581 00:35:36,520 --> 00:35:40,120 Voyager flew past Uranus and saw a greenish disc 582 00:35:40,120 --> 00:35:43,400 with very little cloud activity taking place over the disc itself. 583 00:35:43,400 --> 00:35:46,480 It was a really boring planet, let's be honest about this. 584 00:35:46,480 --> 00:35:48,960 Lots of scientists looked at it, and were quite disappointed. 585 00:35:51,480 --> 00:35:55,480 But observations over the next quarter century showed us 586 00:35:55,480 --> 00:35:58,440 that first impressions aren't always accurate. 587 00:36:00,480 --> 00:36:03,320 We've got a large dataset of Hubble space telescope, 588 00:36:03,320 --> 00:36:05,720 and ground-based observations of Uranus, 589 00:36:05,720 --> 00:36:08,960 which show that we simply flew past at a rather boring time, 590 00:36:08,960 --> 00:36:12,480 and, in fact, it's become a lot more active now, with white spots, 591 00:36:12,480 --> 00:36:15,520 incredible colours around the north pole and the south pole. 592 00:36:17,080 --> 00:36:19,840 Uranus also has its own ring system, 593 00:36:19,840 --> 00:36:23,960 and a bizarre feature that makes it unique amongst the planets. 594 00:36:25,200 --> 00:36:29,240 It's tilted so far over that it's completely on its side, 595 00:36:29,240 --> 00:36:31,720 giving it extreme seasons. 596 00:36:33,080 --> 00:36:34,840 In contrast to Uranus, 597 00:36:34,840 --> 00:36:38,720 Voyager found Neptune's surface to be turbulent and active, 598 00:36:38,720 --> 00:36:41,520 with the strongest winds in the solar system. 599 00:36:41,520 --> 00:36:45,600 Neptune has its own source of internal heat. 600 00:36:45,600 --> 00:36:49,640 Basically, it emits out more energy than it's receiving from the sun. 601 00:36:49,640 --> 00:36:53,800 That emission of heat is driving a really complicated weather pattern. 602 00:36:53,800 --> 00:36:57,600 We see incredible storms, such as the great dark spot that was present 603 00:36:57,600 --> 00:37:01,360 in Neptune when Voyager II flew past in 1989. 604 00:37:05,200 --> 00:37:08,520 Uranus and Neptune have different characteristics, 605 00:37:08,520 --> 00:37:11,480 but also, they share significant similarities. 606 00:37:12,560 --> 00:37:15,400 They both have a blue-green hue. 607 00:37:15,400 --> 00:37:18,000 And they're both made of similar materials, 608 00:37:18,000 --> 00:37:20,720 including methane, ammonia, and water. 609 00:37:22,560 --> 00:37:25,640 Looking back at all the planets in the solar system, 610 00:37:25,640 --> 00:37:30,000 it's striking to consider how they fall into three distinct groups. 611 00:37:30,000 --> 00:37:33,080 The rocky planets closest to the sun. 612 00:37:34,120 --> 00:37:35,920 Then the gas giants. 613 00:37:35,920 --> 00:37:38,240 And, finally, the ice giants. 614 00:37:39,760 --> 00:37:42,880 The question is, what can explain this pattern? 615 00:37:45,640 --> 00:37:50,400 In 2014, physicist Chamkaur Ghag took on the challenge. 616 00:37:53,480 --> 00:37:56,200 It meant going back to the formation of the solar system... 617 00:37:58,680 --> 00:38:01,320 ..when there was a huge cloud of gas and dust, 618 00:38:01,320 --> 00:38:03,760 and a newly formed star in the middle. 619 00:38:06,600 --> 00:38:09,440 Imagine this bench is our disc of stellar dust. 620 00:38:09,440 --> 00:38:10,640 All the bits and pieces 621 00:38:10,640 --> 00:38:12,960 are going to go into making our brand-new solar system. 622 00:38:12,960 --> 00:38:14,320 Now, in the centre, 623 00:38:14,320 --> 00:38:18,320 the sun pushes the temperature up by over 1,700 Celsius. 624 00:38:18,320 --> 00:38:21,880 But as we move away, the temperature drops, 625 00:38:21,880 --> 00:38:24,760 and down towards the edge of this disc, 626 00:38:24,760 --> 00:38:27,240 an incredible 9 billion miles away, 627 00:38:27,240 --> 00:38:31,480 the temperature has dropped to a chilly minus 250 Celsius. 628 00:38:33,160 --> 00:38:36,160 This temperatures gradient, from inferno to freezer, 629 00:38:36,160 --> 00:38:39,040 is critical to the way each planet is formed. 630 00:38:40,080 --> 00:38:43,120 Here, close to the centre of our primordial solar system, 631 00:38:43,120 --> 00:38:46,240 the temperature's upwards of 1,000 Celsius. 632 00:38:46,240 --> 00:38:50,600 In this searing heat, only certain rocks and metals can remain solid, 633 00:38:50,600 --> 00:38:53,400 and that's why Mercury, Venus, 634 00:38:53,400 --> 00:38:55,720 Earth and Mars, 635 00:38:55,720 --> 00:38:59,640 are made up mostly of rocks and silicates. 636 00:39:01,520 --> 00:39:05,000 As we move further out into the solar system beyond Mars, 637 00:39:05,000 --> 00:39:08,400 we reach the snow line, an imaginary line, 638 00:39:08,400 --> 00:39:12,600 where the temperatures drop to zero degrees, and water ice can form. 639 00:39:12,600 --> 00:39:16,680 Now, what's really interesting about this 640 00:39:16,680 --> 00:39:21,160 is that ice sticks together much more easily than rocks do. 641 00:39:22,840 --> 00:39:25,760 And this explains why gas giants Jupiter and Saturn 642 00:39:25,760 --> 00:39:29,640 are a bit like these balloons, filled with hydrogen and helium. 643 00:39:31,320 --> 00:39:34,760 The ice helped the cores of these giant planets to form very early, 644 00:39:34,760 --> 00:39:38,840 and suck up vast amounts of gas from the protoplanetary disc. 645 00:39:40,480 --> 00:39:43,480 It's only when we get further out, much further out, 646 00:39:43,480 --> 00:39:45,960 double the distance from Earth to Saturn, 647 00:39:45,960 --> 00:39:48,560 that we start to see the ingredients of the ice giants appear. 648 00:39:49,880 --> 00:39:52,320 By the time we're at the orbit of Uranus, 649 00:39:52,320 --> 00:39:55,480 the temperature is so low that it's not just water turning into ice, 650 00:39:55,480 --> 00:39:59,120 complex gases like methane and ammonia are forming ice as well. 651 00:40:00,240 --> 00:40:04,560 Suddenly, now that these volatile gases are locked into solid form, 652 00:40:04,560 --> 00:40:06,480 they can start to coalesce, 653 00:40:06,480 --> 00:40:09,920 and they seed what will become Uranus and Neptune. 654 00:40:11,680 --> 00:40:15,160 Although today, the planets are considered to end at Neptune... 655 00:40:16,360 --> 00:40:18,200 ..that hasn't always been the case. 656 00:40:21,480 --> 00:40:23,080 When I was growing up, 657 00:40:23,080 --> 00:40:26,720 Pluto was very definitely considered to be 658 00:40:26,720 --> 00:40:28,200 the solar system's ninth planet. 659 00:40:28,200 --> 00:40:30,520 But not any more. 660 00:40:30,520 --> 00:40:35,280 In 2006, it was demoted, and given dwarf planet status. 661 00:40:35,280 --> 00:40:38,320 But this raised a rather interesting question. 662 00:40:38,320 --> 00:40:40,400 Just what is a planet? 663 00:40:42,840 --> 00:40:44,560 Pluto was demoted, 664 00:40:44,560 --> 00:40:48,560 partly because it's small, but also because it's not something known as 665 00:40:48,560 --> 00:40:50,400 "gravitationally dominant". 666 00:40:52,360 --> 00:40:56,000 Pluto sits in a region of the solar system called the Kuiper belt, 667 00:40:56,000 --> 00:40:59,920 a vast realm of icy bodies of many different sizes. 668 00:41:01,520 --> 00:41:04,960 Its weak gravity means it has not cleared the area around it 669 00:41:04,960 --> 00:41:06,480 of other objects. 670 00:41:08,560 --> 00:41:12,840 Nevertheless, the debate over Pluto's status hasn't gone away. 671 00:41:17,000 --> 00:41:21,480 When Nasa's New Horizons mission arrived at Pluto in 2015, 672 00:41:21,480 --> 00:41:24,080 it turned out to be one of the most exciting, 673 00:41:24,080 --> 00:41:27,080 and surprising space missions of all time. 674 00:41:33,360 --> 00:41:35,160 Stand by for telemetry. 675 00:41:35,160 --> 00:41:36,640 This is good. 676 00:41:36,640 --> 00:41:39,840 For Alice Bowman, it had been a ten-year wait. 677 00:41:40,920 --> 00:41:44,600 OK. Copy that. We're in luck with telemetry with the spacecraft. 678 00:41:44,600 --> 00:41:47,040 CHEERING 679 00:41:49,800 --> 00:41:53,480 I told you it would be fine! I wasn't worried! 680 00:41:53,480 --> 00:41:55,520 These people weren't worried! 681 00:41:55,520 --> 00:41:57,680 I was worried. I was worried! 682 00:41:57,680 --> 00:42:00,880 This is such a culmination of so much work! 683 00:42:00,880 --> 00:42:04,560 They'd travelled three billion miles, 684 00:42:04,560 --> 00:42:08,240 and hit a target that was only 60 by 90 miles, 685 00:42:08,240 --> 00:42:11,480 the equivalent of throwing something across the Atlantic Ocean, 686 00:42:11,480 --> 00:42:13,600 and it landing in my hand. 687 00:42:18,240 --> 00:42:20,520 Pluto astonished everyone. 688 00:42:21,840 --> 00:42:24,240 It had huge nitrogen glaciers... 689 00:42:25,960 --> 00:42:28,560 ..and mountains made out of water ice, 690 00:42:28,560 --> 00:42:30,960 completely impossible here on Earth. 691 00:42:33,320 --> 00:42:37,160 Carly Howett explained why the team was so excited. 692 00:42:38,480 --> 00:42:40,560 You have to remember that the surface isn't rock, 693 00:42:40,560 --> 00:42:42,000 this is water ice mountains. 694 00:42:42,000 --> 00:42:44,720 Which sounds crazy. That's crazy, right? We don't... 695 00:42:44,720 --> 00:42:47,800 Water ice in the earth just wouldn't be strong enough 696 00:42:47,800 --> 00:42:49,400 to sustain that sort of structure. 697 00:42:49,400 --> 00:42:52,200 But what you have on Pluto are surface temperatures that are 698 00:42:52,200 --> 00:42:56,560 so, so cold that water ice almost has a sort of rocklike quality, 699 00:42:56,560 --> 00:42:59,080 it's just absolutely solid. 700 00:42:59,080 --> 00:43:02,560 And so it's able to support these massive structures. 701 00:43:02,560 --> 00:43:05,520 I don't remember anyone predicting mountains. 702 00:43:05,520 --> 00:43:09,240 Do we understand how these things could have grown? 703 00:43:09,240 --> 00:43:12,560 I have no idea how those mountains could be formed. 704 00:43:12,560 --> 00:43:15,360 Pluto has quite low gravity, it's a small body. 705 00:43:15,360 --> 00:43:17,760 We don't get winds and water, like you do on the Earth, 706 00:43:17,760 --> 00:43:21,240 so how you get uplift, how does that even happen? 707 00:43:21,240 --> 00:43:22,480 I don't know. 708 00:43:24,680 --> 00:43:28,600 The discoveries reignited the debate about Pluto's status. 709 00:43:33,440 --> 00:43:37,120 In 2017, a team of scientists submitted a proposal 710 00:43:37,120 --> 00:43:39,600 to reclassify Pluto as a planet. 711 00:43:45,880 --> 00:43:50,280 For now, though, Pluto is just one of a vast number of bodies 712 00:43:50,280 --> 00:43:53,400 going round the sun in distant orbits. 713 00:43:53,400 --> 00:43:55,880 All part of the Kuiper belt. 714 00:43:55,880 --> 00:43:58,280 And it's in this distant realm 715 00:43:58,280 --> 00:44:02,400 that our solar system has given us one last planetary surprise. 716 00:44:08,080 --> 00:44:10,840 In 2016 came a dramatic announcement. 717 00:44:12,200 --> 00:44:15,960 The claim that scientists had discovered a new ninth planet. 718 00:44:19,200 --> 00:44:23,520 If true, it will transform our ideas about the solar system. 719 00:44:26,760 --> 00:44:29,760 The strangeness of this proposed Planet Nine 720 00:44:29,760 --> 00:44:32,840 began with its extraordinary orbit about the sun, 721 00:44:32,840 --> 00:44:35,160 as I try to demonstrate. 722 00:44:38,240 --> 00:44:43,080 On this scale, every centimetre is about 35 million kilometres. 723 00:44:44,960 --> 00:44:47,720 The Earth is 150 million kilometres from the sun. 724 00:44:49,160 --> 00:44:51,960 Pluto is 40 times further away, 725 00:44:51,960 --> 00:44:54,360 and the outer edge of the Kuiper belt is... 726 00:44:54,360 --> 00:44:55,560 ..still further. 727 00:44:58,000 --> 00:45:02,200 But if it exists, the orbit of Planet Nine is far beyond that. 728 00:45:04,360 --> 00:45:06,440 At the point of closest approach in its orbit, 729 00:45:06,440 --> 00:45:10,360 it sits 200 times further from the sun than the Earth. 730 00:45:10,360 --> 00:45:13,960 That's a whopping 30 billion kilometres away from the sun. 731 00:45:15,600 --> 00:45:17,400 But unlike the other planets, 732 00:45:17,400 --> 00:45:20,160 the proposed orbit of Planet Nine is not circular. 733 00:45:22,400 --> 00:45:26,640 Its highly eccentric path takes it much, much deeper into space. 734 00:45:28,720 --> 00:45:32,160 At its furthest possible point, its aphelion... 735 00:45:33,360 --> 00:45:37,400 ..in reality, it would be 180 billion kilometres from the sun. 736 00:45:39,160 --> 00:45:41,760 This discovery was enormous news. 737 00:45:41,760 --> 00:45:45,320 But there was one crucial stumbling block. 738 00:45:45,320 --> 00:45:47,600 No-one had actually seen Planet Nine. 739 00:45:48,880 --> 00:45:51,960 Its existence had only been predicted by computer models 740 00:45:51,960 --> 00:45:54,560 that have been built by Caltech astronomers 741 00:45:54,560 --> 00:45:56,640 Mike Brown and Konstantin Batygin. 742 00:45:57,880 --> 00:45:59,360 In 2003, 743 00:45:59,360 --> 00:46:03,080 Mike discovered the first of six objects with bizarre orbits 744 00:46:03,080 --> 00:46:04,640 in the Kuiper belt. 745 00:46:04,640 --> 00:46:08,120 All of the orbits pointed off in the same direction. 746 00:46:08,120 --> 00:46:11,560 They were all tilted downwards, and off to the side. 747 00:46:14,440 --> 00:46:18,880 And the culmination of those two is what really shows you 748 00:46:18,880 --> 00:46:20,800 that there's something strange going on. 749 00:46:20,800 --> 00:46:25,640 So, it's about a 0.01% chance that this is all just randomly aligned. 750 00:46:25,640 --> 00:46:29,240 So, is that what got your attention, got you working on the problem? 751 00:46:29,240 --> 00:46:31,760 That was the big clue that something's going on 752 00:46:31,760 --> 00:46:33,040 in the outer solar system. 753 00:46:34,400 --> 00:46:36,680 This was an extraordinary finding. 754 00:46:36,680 --> 00:46:40,320 The tight clustering of the six orbits might have been caused by the 755 00:46:40,320 --> 00:46:43,840 gravitational influence of something massive in the outer solar system. 756 00:46:47,560 --> 00:46:50,400 For Mike Brown, the question now became, 757 00:46:50,400 --> 00:46:52,920 what was out there, and how could he find it? 758 00:46:54,400 --> 00:46:56,680 Working down the corridor from Mike 759 00:46:56,680 --> 00:46:59,200 is theoretical astronomer Konstantin Batygin. 760 00:47:01,360 --> 00:47:03,960 Hey, how are you doing? Nice to see you. 761 00:47:03,960 --> 00:47:07,480 So, Mike challenged him to build a computer model of the solar system 762 00:47:07,480 --> 00:47:11,360 and replicate the strange orbits of the six Kuiper belt objects. 763 00:47:14,600 --> 00:47:19,080 After many calculations, Batygin came up with a model that worked. 764 00:47:21,120 --> 00:47:23,960 But there was another, unexpected effect. 765 00:47:23,960 --> 00:47:27,640 The model also predicted a second set of Kuiper belt objects, 766 00:47:27,640 --> 00:47:29,240 with even stranger orbits. 767 00:47:30,840 --> 00:47:33,680 Trouble was, they didn't seem to exist. 768 00:47:33,680 --> 00:47:38,640 The biggest twist to the whole story came when our numerical models 769 00:47:39,200 --> 00:47:44,200 consistently would generate orbits that looked more or less like this. 770 00:47:45,120 --> 00:47:47,520 Sort of coming in and out of the blackboard? That's right. 771 00:47:47,520 --> 00:47:48,960 They're on their sides, 772 00:47:48,960 --> 00:47:53,000 they're perpendicular to the plane of the solar system. 773 00:47:53,000 --> 00:47:54,840 And this is just so weird, 774 00:47:54,840 --> 00:47:58,520 the fact that the simulations producing these orbits 775 00:47:58,520 --> 00:48:01,400 that should be perpendicular, they should be readily observable, 776 00:48:01,400 --> 00:48:05,280 and the fact that we don't know of them, is big trouble. 777 00:48:05,280 --> 00:48:08,880 So, it's not just that they might be there, they HAVE to be there? 778 00:48:08,880 --> 00:48:11,080 We initially thought of this as counter-evidence 779 00:48:11,080 --> 00:48:12,640 to the existence of the planet. 780 00:48:12,640 --> 00:48:14,360 Fine, it explains alignment, 781 00:48:14,360 --> 00:48:17,400 but it also predicts stuff that's clearly not there. 782 00:48:18,720 --> 00:48:21,360 So, there was the key moment where we looked at it and said, 783 00:48:21,360 --> 00:48:23,560 "This makes no sense." 784 00:48:23,560 --> 00:48:25,680 Then I looked more carefully at the other objects 785 00:48:25,680 --> 00:48:28,760 in the dataset that I had not been paying much attention to 786 00:48:28,760 --> 00:48:30,040 in the last couple of years. 787 00:48:30,040 --> 00:48:33,240 There are five of these objects that are on perpendicular orbits 788 00:48:33,240 --> 00:48:36,880 that I didn't... I remember when one of them was discovered a few years 789 00:48:36,880 --> 00:48:40,160 ago and thinking it was just weird, nobody had any explanation for them. 790 00:48:40,160 --> 00:48:43,280 And I said, "Konstantin, I'm going to go plot these right now, 791 00:48:43,280 --> 00:48:44,840 "and we'll see where they are, 792 00:48:44,840 --> 00:48:47,440 "and if they're sitting right at these two spots right here, 793 00:48:47,440 --> 00:48:48,880 "my head's going to explode." 794 00:48:48,880 --> 00:48:51,120 And we plotted them, one of them is right here, 795 00:48:51,120 --> 00:48:54,040 four of them are exactly right here, where we predicted. 796 00:48:54,040 --> 00:48:58,400 We just both sat there and stared at that, and my jaw hit the floor. 797 00:48:58,400 --> 00:49:01,360 It really was an honest blind prediction of something 798 00:49:01,360 --> 00:49:02,960 we didn't know was there. 799 00:49:02,960 --> 00:49:07,040 And that... When I look at those objects, and those rings, 800 00:49:07,040 --> 00:49:09,720 and then I see those aligned objects, too... 801 00:49:10,960 --> 00:49:15,960 ..I think I really am pretty convinced it's really out there. 802 00:49:20,080 --> 00:49:23,000 So far, Planet Nine is just a theory. 803 00:49:24,480 --> 00:49:26,480 It has yet to be discovered. 804 00:49:28,520 --> 00:49:32,400 The planets of our solar system are amazingly varied, 805 00:49:32,400 --> 00:49:37,000 but they are just one case study orbiting our star, the sun. 806 00:49:37,000 --> 00:49:40,680 But the sun is amongst billions that lie in our galaxy. 807 00:49:42,040 --> 00:49:45,200 For many years, we've thought that there must be other planets 808 00:49:45,200 --> 00:49:47,960 out there in the galaxy beyond our solar system, 809 00:49:47,960 --> 00:49:49,720 the so-called exoplanets. 810 00:49:49,720 --> 00:49:52,800 And, some might even be habitable, like our Earth. 811 00:49:52,800 --> 00:49:55,080 But we've had no way of finding them. 812 00:49:57,880 --> 00:50:02,200 Planets with no light of their own were rendered invisible in the glare 813 00:50:02,200 --> 00:50:06,680 from their parent star, as Patrick explained in 1975. 814 00:50:06,680 --> 00:50:09,640 Now, that's the view from close range. 815 00:50:09,640 --> 00:50:13,520 Now we're going to pull the cameras back, and you will see what happens. 816 00:50:13,520 --> 00:50:17,040 As the sun shrinks from a large disc into a smaller one, 817 00:50:17,040 --> 00:50:19,320 so the little Earth gets closer and closer to it, 818 00:50:19,320 --> 00:50:21,240 and finally disappears. 819 00:50:21,240 --> 00:50:24,920 Now, just imagine what would happen if we could go so far away 820 00:50:24,920 --> 00:50:28,720 from the sun that it looked merely like a point of light. 821 00:50:28,720 --> 00:50:31,520 What would be the chances of our being able to see 822 00:50:31,520 --> 00:50:33,880 a world such as the Earth going round it? 823 00:50:33,880 --> 00:50:35,240 Frankly, none. 824 00:50:38,320 --> 00:50:42,760 But then, scientists came up with a new way to search for exoplanets 825 00:50:42,760 --> 00:50:46,280 that meant they didn't have to see the planet to know it was there. 826 00:50:50,680 --> 00:50:53,880 It involved looking for tiny movements in the stars, 827 00:50:53,880 --> 00:50:56,840 wobbles, caused by the planets pulling them off-centre 828 00:50:56,840 --> 00:50:58,520 due to gravity. 829 00:51:00,040 --> 00:51:03,240 In 1995, two young scientists from Geneva 830 00:51:03,240 --> 00:51:06,080 studying a star called 51 Pegasi 831 00:51:06,080 --> 00:51:08,000 made a breakthrough. 832 00:51:08,000 --> 00:51:12,760 Didier Queloz, who made the discovery, told me what happened. 833 00:51:12,760 --> 00:51:14,920 I was a PhD student at the time, 834 00:51:14,920 --> 00:51:18,920 so the first time I saw something a bit strange going on 835 00:51:18,920 --> 00:51:22,680 on the series of measurements I made on 51 Peg, 836 00:51:22,680 --> 00:51:24,880 I never thought it would be a planet, 837 00:51:24,880 --> 00:51:26,640 because it was just impossible. 838 00:51:26,640 --> 00:51:28,720 I thought it was a bug in the instrument, 839 00:51:28,720 --> 00:51:31,520 something was wrong with the instrument, with the machinery, 840 00:51:31,520 --> 00:51:34,840 and it took me a long time to figure out exactly what it was. 841 00:51:37,760 --> 00:51:40,040 This was no data error. 842 00:51:40,040 --> 00:51:45,080 Didier had found the first exoplanet orbiting a stable main sequence star 843 00:51:45,560 --> 00:51:48,040 50 light years away from Earth. 844 00:51:49,880 --> 00:51:51,880 Did you hope that the planet you discovered 845 00:51:51,880 --> 00:51:53,760 was going to be Earth-like? 846 00:51:53,760 --> 00:51:58,680 Well, it was impossible, because the instrument we were using, 847 00:51:58,680 --> 00:52:01,880 the best it could do is detecting something like Jupiter. 848 00:52:01,880 --> 00:52:04,080 It was designed to do that, 849 00:52:04,080 --> 00:52:06,360 that was already a tremendous achievement at that time. 850 00:52:07,720 --> 00:52:11,520 The technology then could only detect the most massive planets, 851 00:52:11,520 --> 00:52:14,560 because they exerted the biggest gravitational pull 852 00:52:14,560 --> 00:52:17,920 on their host star, resulting in a bigger wobble. 853 00:52:20,760 --> 00:52:24,200 51 Pegasi b was spectacularly weird. 854 00:52:27,400 --> 00:52:29,080 It was the size of Jupiter, 855 00:52:29,080 --> 00:52:32,280 yet it took just four days to orbit its star. 856 00:52:33,520 --> 00:52:37,600 And its surface temperature exceeded 1,000 degrees Celsius. 857 00:52:39,640 --> 00:52:44,040 It was a type of planet that has come to be known as a hot Jupiter. 858 00:52:45,360 --> 00:52:48,560 And it rewrote our ideas of what a planet could be like. 859 00:52:49,960 --> 00:52:52,640 But there was no chance it might be habitable. 860 00:52:54,440 --> 00:52:57,480 Now, the hunt was on for more exoplanets. 861 00:52:58,680 --> 00:53:00,720 Engines start, one-zero... 862 00:53:00,720 --> 00:53:03,800 And lift-off of the Delta II rocket with Kepler, 863 00:53:03,800 --> 00:53:07,400 on a search for planets in some way like our own. 864 00:53:10,680 --> 00:53:14,400 When Nasa's Kepler telescope was launched in 2009, 865 00:53:14,400 --> 00:53:17,640 it unleashed a planet-hunting gold rush. 866 00:53:19,520 --> 00:53:22,600 Designed to stare at a patch of sky continuously, 867 00:53:22,600 --> 00:53:26,520 Kepler measures any subtle dimming in a star's brightness, 868 00:53:26,520 --> 00:53:30,400 a signal that a planet could have passed in front of a star. 869 00:53:31,760 --> 00:53:33,040 Using this technique, 870 00:53:33,040 --> 00:53:35,920 Kepler has found over 2,600 exoplanets 871 00:53:35,920 --> 00:53:38,240 in many different flavours. 872 00:53:39,600 --> 00:53:44,480 Hot Jupiters, mini Neptunes, and even so-called super-Earths, 873 00:53:44,480 --> 00:53:47,400 just a few times bigger than our own planet. 874 00:53:51,600 --> 00:53:55,080 To explore just how strange some of these planets are, 875 00:53:55,080 --> 00:53:58,640 The Sky At Night decided to compare the planets that science fiction had 876 00:53:58,640 --> 00:54:02,400 imagined in movies with the real planets that had been discovered. 877 00:54:06,440 --> 00:54:09,640 So, how realistic are some of these exoplanets 878 00:54:09,640 --> 00:54:11,000 that we see in the movies? 879 00:54:11,000 --> 00:54:14,000 The planet Tatooine from Star Wars, for example, 880 00:54:14,000 --> 00:54:18,360 you've got this wonderful, evocative desert landscape, 881 00:54:18,360 --> 00:54:22,400 and Luke Skywalker stands there, contemplating his destiny, 882 00:54:22,400 --> 00:54:27,240 bathed in the afternoon light from not one, but two, suns. 883 00:54:27,240 --> 00:54:29,560 Could something like that happen? 884 00:54:29,560 --> 00:54:31,760 If so, have we found anything like that? 885 00:54:31,760 --> 00:54:34,240 Yes, absolutely, we've found more than one, actually. 886 00:54:34,240 --> 00:54:36,760 The first one we found was Kepler 16 b. 887 00:54:36,760 --> 00:54:38,920 We found that in 2011. 888 00:54:38,920 --> 00:54:41,920 And it's the first circumbinary planet. 889 00:54:41,920 --> 00:54:45,680 That means it's a binary star, two stars that orbit each other, 890 00:54:45,680 --> 00:54:48,840 and a planet on the outside that orbits both of them. 891 00:54:48,840 --> 00:54:51,680 So, the planet has... So the stars are orbiting each other in a little 892 00:54:51,680 --> 00:54:53,760 merry dance, and the planet is going round all... 893 00:54:53,760 --> 00:54:57,880 Exactly, yes. Wow, OK. So, here is video of that system. 894 00:54:57,880 --> 00:55:00,640 And you can see, there's the binary star in the centre, 895 00:55:00,640 --> 00:55:03,040 one is a little bit more massive than the other, 896 00:55:03,040 --> 00:55:05,600 and it's slightly whiter, the other one is a little red. 897 00:55:05,600 --> 00:55:07,280 Almost exactly like in Star Wars. 898 00:55:07,280 --> 00:55:10,000 It is! Almost exactly like in Star Wars. 899 00:55:10,000 --> 00:55:12,880 One of the things about Planet Miller in Interstellar 900 00:55:12,880 --> 00:55:15,040 is that it's orbiting a black hole 901 00:55:15,040 --> 00:55:17,680 and you get into all kinds of exotic physics, 902 00:55:17,680 --> 00:55:19,920 time dilation, all this sort of stuff. 903 00:55:19,920 --> 00:55:22,640 But the fact that it's orbiting a black hole 904 00:55:22,640 --> 00:55:26,080 is a really interesting idea. Is that at all feasible? 905 00:55:26,080 --> 00:55:30,240 Yes, there's no reason why you can't have a planet orbiting a black hole. 906 00:55:30,240 --> 00:55:35,240 In fact, there is some evidence to suggest that we've even seen 907 00:55:35,640 --> 00:55:39,040 a super Jupiter being eaten by a black hole. 908 00:55:40,920 --> 00:55:45,080 We've even found a planet that might be made of diamonds. 909 00:55:46,800 --> 00:55:50,320 There are lots of things we've found that are stranger than things we can imagine. 910 00:55:50,320 --> 00:55:52,760 That's the beauty of the exoplanet world at the moment. 911 00:55:52,760 --> 00:55:55,320 We're discovering things we didn't know were possible, 912 00:55:55,320 --> 00:55:57,280 new physics is being discovered all the time. 913 00:55:59,080 --> 00:56:02,560 Although we were constantly being surprised by new exoplanets, 914 00:56:02,560 --> 00:56:06,440 we still hadn't found anything that seemed potentially habitable. 915 00:56:07,880 --> 00:56:11,760 What we needed was a rocky planet in the Goldilocks zone. 916 00:56:13,040 --> 00:56:16,720 Not too hot, not too cold, but just right. 917 00:56:16,720 --> 00:56:19,840 Hopefully, with liquid water on its surface, 918 00:56:19,840 --> 00:56:24,480 and therefore the potential to support life as we know it. 919 00:56:26,320 --> 00:56:28,680 In 2016, came a breakthrough. 920 00:56:32,640 --> 00:56:35,480 Now, scientists are hailing a major discovery, 921 00:56:35,480 --> 00:56:38,080 a new planet which they've called Proxima b. 922 00:56:40,160 --> 00:56:42,760 The planet was found orbiting Proxima Centauri, 923 00:56:42,760 --> 00:56:45,000 our closest neighbouring star. 924 00:56:45,000 --> 00:56:47,280 At just over four light years away, 925 00:56:47,280 --> 00:56:50,280 it's close enough to imagine that we might, one day, 926 00:56:50,280 --> 00:56:52,520 be able to send a spacecraft there. 927 00:56:53,760 --> 00:56:56,320 And so a French team have tried to work out 928 00:56:56,320 --> 00:56:59,280 the planet's characteristics by using comparisons 929 00:56:59,280 --> 00:57:02,080 with the planets in our own solar system. 930 00:57:04,320 --> 00:57:08,840 If it has a large metallic core and a rocky mantle, like Mercury does, 931 00:57:08,840 --> 00:57:11,840 then it's probably a little bit smaller than the Earth. 932 00:57:11,840 --> 00:57:14,960 But there are other, more exotic possibilities. 933 00:57:14,960 --> 00:57:19,440 It could be much bigger than the Earth, 1.4 times the size, 934 00:57:19,440 --> 00:57:22,840 in which case it would have to have a rocky core, 935 00:57:22,840 --> 00:57:24,720 surrounded by a mantle of ice. 936 00:57:24,720 --> 00:57:28,360 And, excitingly, the model suggests that the entire planet 937 00:57:28,360 --> 00:57:32,600 would then be covered in a vast ocean, 200km deep. 938 00:57:39,160 --> 00:57:41,640 Proxima b was tantalising enough. 939 00:57:41,640 --> 00:57:46,480 But then, in 2017, came the most exciting discovery yet. 940 00:57:47,800 --> 00:57:52,120 A whole system of Earth-like, potentially habitable, planets, 941 00:57:52,120 --> 00:57:54,200 known as TRAPPIST-1. 942 00:57:58,960 --> 00:58:00,720 In just 60 years, 943 00:58:00,720 --> 00:58:04,000 we've gone from one solar system of nine planets, 944 00:58:04,000 --> 00:58:06,480 to a galaxy teeming with them. 945 00:58:10,160 --> 00:58:13,960 So, what's next for planetary exploration? 946 00:58:13,960 --> 00:58:17,800 Well, if there's one lesson that we've learned through over 60 years 947 00:58:17,800 --> 00:58:20,560 of The Sky At Night, it's this. 948 00:58:20,560 --> 00:58:23,640 Every mission that we send out to another world 949 00:58:23,640 --> 00:58:26,360 throws up incredible discoveries, 950 00:58:26,360 --> 00:58:29,320 and provokes many new questions. 951 00:58:29,320 --> 00:58:32,240 Really, we've only just begun.