1 00:00:12,930 --> 00:00:20,018 At this precise moment on a planet far, far away... 2 00:00:41,020 --> 00:00:44,729 an alien sunrise ushers in a new day. 3 00:00:57,710 --> 00:01:00,634 But will alien eyes gaze upon it? 4 00:01:14,700 --> 00:01:16,293 Or will it go unseen? 5 00:01:21,380 --> 00:01:25,578 Just another moment in a vast, sterile universe? 6 00:01:34,900 --> 00:01:37,608 The hunt is on for the answer. 7 00:01:46,660 --> 00:01:48,617 Magnificent desolation. 8 00:01:49,890 --> 00:01:52,211 Beautiful, beautiful! Ain't that something? 9 00:02:16,741 --> 00:02:17,890 The Milky Way... 10 00:02:19,230 --> 00:02:22,256 hundreds of billions of stars... 11 00:02:29,790 --> 00:02:33,579 spread across 100,000 light-years of space. 12 00:02:39,741 --> 00:02:41,960 Among them, the sun... 13 00:02:46,420 --> 00:02:51,221 with eight planets orbiting around it, including our home. 14 00:02:54,030 --> 00:02:58,591 Until very recently, these were the only worlds we knew of, 15 00:02:58,631 --> 00:03:01,581 the only planets we could hope to explore 16 00:03:01,621 --> 00:03:03,771 for signs of life beyond Earth. 17 00:03:14,430 --> 00:03:17,350 When I first got into astronomy back in the 1970s, 18 00:03:17,390 --> 00:03:21,220 we knew of no planets beyond our Solar System. 19 00:03:21,260 --> 00:03:23,470 We didn't have the technology to detect them 20 00:03:23,510 --> 00:03:26,140 even if they were there. 21 00:03:26,180 --> 00:03:30,378 Our neighbourhood was the only place we could look for life. 22 00:03:33,670 --> 00:03:37,436 And so the hunt for life began in our own backyard. 23 00:03:44,390 --> 00:03:45,980 Over the last few decades, 24 00:03:46,020 --> 00:03:49,820 multiple missions have explored our Solar System's planets... 25 00:03:56,540 --> 00:03:58,508 and even some of their moons. 26 00:04:05,820 --> 00:04:12,110 But to date, even as we continue to look, no convincing evidence of life 27 00:04:12,150 --> 00:04:14,664 has been found on any of these worlds. 28 00:04:22,390 --> 00:04:24,870 Earth remains one of a kind... 29 00:04:30,340 --> 00:04:33,412 the only living world around the sun. 30 00:04:43,160 --> 00:04:47,140 But as the exploration of the Solar System continued, 31 00:04:47,180 --> 00:04:48,978 another search had begun... 32 00:04:51,040 --> 00:04:54,158 for worlds that lie far beyond these shores. 33 00:04:56,150 --> 00:04:58,380 You know, the wonderful thing about astronomy is that 34 00:04:58,420 --> 00:05:00,990 as we develop better and better technology 35 00:05:01,030 --> 00:05:04,920 and accumulate more and more knowledge about our universe, 36 00:05:04,960 --> 00:05:09,070 we turn more and more of these points of light in the sky 37 00:05:09,110 --> 00:05:11,020 into worlds. 38 00:05:11,060 --> 00:05:15,660 I mean, that, we've known is a world for a long time 39 00:05:15,700 --> 00:05:17,860 because that is the planet Mars. 40 00:05:17,900 --> 00:05:22,020 But just above Mars tonight is a constellation called Pegasus. 41 00:05:22,060 --> 00:05:25,310 This is the Square of Pegasus. 42 00:05:25,350 --> 00:05:32,910 And we now know that around there is a star called 51 Pegasi, 43 00:05:32,950 --> 00:05:36,150 which has a planet orbiting around it, 44 00:05:36,190 --> 00:05:39,020 a gas giant about the size of Jupiter 45 00:05:39,060 --> 00:05:44,640 that goes round that faint point of light every four days. 46 00:05:44,680 --> 00:05:48,190 It is wonderful to think that in my lifetime — 47 00:05:48,230 --> 00:05:51,270 in fact, in my adult lifetime, in the last 25 years — 48 00:05:51,310 --> 00:05:53,670 we've gone from a universe 49 00:05:53,710 --> 00:05:57,581 that could have been devoid of planets beyond our Solar System 50 00:05:57,621 --> 00:06:02,471 to a universe that we know is teeming with places 51 00:06:02,511 --> 00:06:04,548 that we can search for life. 52 00:06:14,350 --> 00:06:17,990 Over the last three decades, some of the most powerful telescopes 53 00:06:18,030 --> 00:06:20,237 on Earth have joined the hunt... 54 00:06:27,360 --> 00:06:30,671 searching for planets unimaginably far away... 55 00:06:34,350 --> 00:06:36,409 hiding in the dark. 56 00:06:42,910 --> 00:06:45,754 Planets like 51 Pegasi b... 57 00:06:51,280 --> 00:06:54,820 the first world outside our Solar System to be detected 58 00:06:54,860 --> 00:06:56,749 around a sun-like star. 59 00:07:07,220 --> 00:07:13,398 51 Pegasi b is a gas giant, around half the mass of Jupiter... 60 00:07:16,320 --> 00:07:18,664 but far closer to its star. 61 00:07:23,550 --> 00:07:27,316 Just imagine what that world might be like... 62 00:07:34,270 --> 00:07:38,275 a world with skies torn by titanic winds... 63 00:07:44,480 --> 00:07:49,862 where its hot interior is bathed in rain of sapphires. 64 00:07:55,590 --> 00:08:00,505 In every sense, 51 Pegasi b is an alien world. 65 00:08:03,150 --> 00:08:06,640 And we soon discovered that the galaxy is full of planets 66 00:08:06,680 --> 00:08:09,638 unlike anything seen in our Solar System... 67 00:08:20,040 --> 00:08:23,396 planets enveloped by fierce radiation... 68 00:08:28,920 --> 00:08:31,320 their surfaces battered and stripped 69 00:08:31,360 --> 00:08:35,228 by the high-energy strobing light of their star... 70 00:08:42,200 --> 00:08:46,740 worlds so cold, their atmospheres have frozen solid... 71 00:09:06,790 --> 00:09:08,792 or great swollen planets... 72 00:09:13,631 --> 00:09:16,578 with the density of Styrofoam... 73 00:09:21,280 --> 00:09:23,260 and fathomless atmospheres. 74 00:09:28,280 --> 00:09:33,798 These discoveries proved that, in one sense, we really are not alone. 75 00:09:34,910 --> 00:09:39,916 There are other worlds out there waiting to be explored. 76 00:09:49,720 --> 00:09:52,630 We estimate that in the Milky Way galaxy, 77 00:09:52,670 --> 00:09:57,676 there are more planets than stars — hundreds of billions of them. 78 00:10:00,000 --> 00:10:04,278 That's hundreds of billions of places to look for life. 79 00:10:07,200 --> 00:10:12,360 But there's a catch, because not all worlds — by a long stretch — 80 00:10:12,400 --> 00:10:13,765 are like this one. 81 00:10:26,230 --> 00:10:30,840 The first planets we found appeared too bizarre, too large 82 00:10:30,880 --> 00:10:35,499 and often too close to their stars for living things to survive. 83 00:10:45,310 --> 00:10:49,120 To find worlds where life could exist, we needed to look for 84 00:10:49,160 --> 00:10:53,836 smaller, rocky planets in orbits further from their stars. 85 00:10:56,720 --> 00:11:00,351 T-minus 10, 9, 8, 7... 86 00:11:00,391 --> 00:11:03,840 We needed to look for another Earth... 87 00:11:03,880 --> 00:11:06,481 ...3, 2... 88 00:11:06,521 --> 00:11:11,990 Engine start. 1, 0, and liftoff of the Delta II rocket with Kepler, 89 00:11:12,030 --> 00:11:15,273 on a search for planets in some way like our own. 90 00:11:18,070 --> 00:11:20,520 ...so the hunt moved to space 91 00:11:20,560 --> 00:11:23,666 with the launch of NASA's Kepler Space Telescope... 92 00:11:26,130 --> 00:11:27,791 And we have separation. 93 00:11:31,641 --> 00:11:36,920 ...searching for Earth-like worlds in the galaxy beyond. 94 00:11:46,110 --> 00:11:49,671 Kepler crossed 94 million miles of space... 95 00:11:54,290 --> 00:11:57,749 until it arrived in a steady orbit around the sun... 96 00:12:07,450 --> 00:12:11,512 from where it looked out with a fixed and clear gaze... 97 00:12:15,570 --> 00:12:20,030 to a single patch of sky in the constellation of Cygnus. 98 00:12:32,740 --> 00:12:35,300 Exposing its sensitive light meter... 99 00:12:41,210 --> 00:12:44,635 to the light of 150,000 stars... 100 00:12:51,930 --> 00:12:56,447 it began to look for Earth-like alien worlds. 101 00:13:17,010 --> 00:13:21,351 Kepler doesn't detect planets directly. They are far too small. 102 00:13:21,391 --> 00:13:24,747 They're just specks of dust relative to their parent star. 103 00:13:26,730 --> 00:13:28,200 They're also very faint. 104 00:13:28,240 --> 00:13:32,770 They don't emit light of their own, so they just glow very dimly 105 00:13:32,810 --> 00:13:35,970 in the reflected ambient light of their stars. 106 00:13:36,010 --> 00:13:39,601 So, Kepler has to detect planets indirectly. 107 00:13:39,641 --> 00:13:44,880 Imagine that a moth just flew across the beam of light 108 00:13:44,920 --> 00:13:47,970 from the lighthouse. Now, I wouldn't see the moth, 109 00:13:48,010 --> 00:13:50,200 but if I had a sensitive enough detector 110 00:13:50,240 --> 00:13:52,000 and everything was lined up properly, 111 00:13:52,040 --> 00:13:56,920 I might just see the brightness of the light dim. 112 00:13:56,960 --> 00:13:59,770 And that is how Kepler detects planets. 113 00:13:59,810 --> 00:14:01,550 We imagine there's an alien astronomer 114 00:14:01,590 --> 00:14:04,390 in some distant solar system looking back at the sun, 115 00:14:04,430 --> 00:14:07,361 and everything's lined up so they see the Earth 116 00:14:07,401 --> 00:14:10,150 trace across the face of our star. 117 00:14:10,190 --> 00:14:15,461 They would see the light from the sun dim by one-hundredth of 1%. 118 00:14:15,501 --> 00:14:18,800 It's a tiny amount, but it's enough. 119 00:14:18,840 --> 00:14:21,970 And if they saw that dimming was regular, 120 00:14:22,010 --> 00:14:25,260 if they saw the star dim once every year, in this case, 121 00:14:25,300 --> 00:14:30,909 then they would infer that there's a planet orbiting around a star. 122 00:14:49,531 --> 00:14:52,546 With its exquisitely sensitive light meter... 123 00:14:56,970 --> 00:15:01,089 Kepler sees only the regular dimming of pixels... 124 00:15:05,411 --> 00:15:07,641 just a few bits of information. 125 00:15:12,010 --> 00:15:16,160 But from those bits, astronomers can begin to build a picture 126 00:15:16,200 --> 00:15:18,828 of the worlds that dim the starlight... 127 00:15:26,940 --> 00:15:31,468 worlds that might, in some way, resemble our own... 128 00:15:40,370 --> 00:15:43,681 worlds like Kepler-36b. 129 00:15:53,100 --> 00:15:56,866 The planet was one of Kepler's earliest discoveries. 130 00:16:04,411 --> 00:16:07,780 Orbiting a star similar to our own, 131 00:16:07,820 --> 00:16:12,735 we'd found a world that, at first glance, might seem familiar. 132 00:16:18,380 --> 00:16:22,580 Weighing in at around four times the mass of our own planet, 133 00:16:22,620 --> 00:16:27,770 Kepler-36b was one of the first of a new class of planet — 134 00:16:27,810 --> 00:16:29,403 a super-earth. 135 00:16:39,460 --> 00:16:42,491 The Kepler data doesn't just allow us to say there's a planet 136 00:16:42,531 --> 00:16:47,500 around that star. It allows us to characterize those planets. 137 00:16:47,540 --> 00:16:51,270 So by looking at the precise way that the light fades 138 00:16:51,310 --> 00:16:55,270 and then rises again, and the timing between the dips, 139 00:16:55,310 --> 00:16:58,430 we can measure the orbits of the planets. 140 00:16:58,470 --> 00:17:00,580 And if there are multiple planets in the system, 141 00:17:00,620 --> 00:17:03,060 we can even estimate their masses, 142 00:17:03,100 --> 00:17:07,231 so the Kepler data allows astronomers to paint a picture 143 00:17:07,271 --> 00:17:09,239 of the worlds it discovers. 144 00:17:22,820 --> 00:17:26,779 But the more detailed our picture of Kepler-36b became... 145 00:17:28,521 --> 00:17:32,788 the less Earth-like this super-earth appeared to be. 146 00:17:40,853 --> 00:17:46,895 It orbits very close to its star, circling once every 14 days. 147 00:17:54,290 --> 00:17:56,486 And it has company... 148 00:17:59,698 --> 00:18:02,042 a gigantic gaseous companion... 149 00:18:04,050 --> 00:18:08,044 with an albeit unusually close to its smaller sibling. 150 00:18:09,520 --> 00:18:13,460 The proximity of both its star and sister planet 151 00:18:13,500 --> 00:18:16,770 allows us to imagine the bizarre conditions 152 00:18:16,810 --> 00:18:20,610 that may exist on the surface of Kepler-36b. 153 00:18:31,720 --> 00:18:34,400 The planet may be tidally locked, 154 00:18:34,440 --> 00:18:38,604 which would mean that one hemisphere always faces the star. 155 00:18:46,250 --> 00:18:51,848 On this side, the punishing heat could turn the ground molten... 156 00:18:56,090 --> 00:19:00,209 creating rivers of lava that would crisscross the surface. 157 00:19:10,390 --> 00:19:13,712 The planet could experience violent eruptions... 158 00:19:14,870 --> 00:19:17,630 as the gravitational pull of the gas giant 159 00:19:17,670 --> 00:19:19,946 triggers intense volcanism... 160 00:19:27,690 --> 00:19:30,682 each time it passes by. 161 00:19:54,980 --> 00:19:58,154 But Kepler-36b could also be... 162 00:19:58,920 --> 00:20:00,638 a planet of ice. 163 00:20:09,140 --> 00:20:11,410 Because if it's tidally locked, 164 00:20:11,450 --> 00:20:15,933 the far side would face permanently away from the star... 165 00:20:19,291 --> 00:20:22,480 and we could imagine a freezing cold hemisphere 166 00:20:22,520 --> 00:20:25,205 shrouded in eternal darkness. 167 00:20:39,700 --> 00:20:43,568 For now, this is all just informed speculation. 168 00:20:46,301 --> 00:20:50,740 But we are beginning to build a picture of these worlds. 169 00:20:50,780 --> 00:20:54,840 I mean, imagine a world where the sun stays at the same point 170 00:20:54,880 --> 00:20:57,410 in the sky forever, 171 00:20:57,450 --> 00:21:00,490 so one side of the planet is in eternal night 172 00:21:00,530 --> 00:21:02,960 and the other side in eternal day. 173 00:21:03,000 --> 00:21:07,690 And even the twilight strip between day and night, 174 00:21:07,730 --> 00:21:10,768 we think, would suffer from extreme conditions. 175 00:21:15,790 --> 00:21:20,010 So Kepler-36b just goes to show there's so much more 176 00:21:20,050 --> 00:21:25,670 to having a habitable world than just the composition of the planets. 177 00:21:25,710 --> 00:21:27,510 There's the details of its orbits 178 00:21:27,550 --> 00:21:32,600 and also the nature of the other objects in the solar system 179 00:21:32,640 --> 00:21:35,337 that are orbiting around the star with it. 180 00:21:50,370 --> 00:21:53,980 Kepler-36b is just one of thousands of planets 181 00:21:54,020 --> 00:21:56,296 that Kepler has discovered. 182 00:22:03,230 --> 00:22:07,100 We now know beyond doubt that our galaxy is home 183 00:22:07,140 --> 00:22:11,088 to a diverse collection of alien worlds. 184 00:22:18,610 --> 00:22:22,740 Each one of the over 4,000 planets that we've discovered to date 185 00:22:22,780 --> 00:22:24,740 is different from all the others. 186 00:22:24,780 --> 00:22:27,630 They really are an alien and exotic bunch, 187 00:22:27,670 --> 00:22:30,620 and there's certainly no planet that's identical to the planets 188 00:22:30,660 --> 00:22:32,446 that we know of in our Solar System. 189 00:22:34,790 --> 00:22:38,381 And I think that reveals a deep truth about the universe, 190 00:22:38,421 --> 00:22:42,930 because although the laws of nature that form the planets are simple 191 00:22:42,970 --> 00:22:46,381 and the same everywhere and the fundamental ingredients 192 00:22:46,421 --> 00:22:50,820 out of which the planets are made are simple and the same everywhere, 193 00:22:50,860 --> 00:22:55,420 the nature of a planet also depends on the history of its formation 194 00:22:55,460 --> 00:22:58,251 and the environment around its parent star 195 00:22:58,291 --> 00:23:00,210 out of which the planet formed. 196 00:23:00,250 --> 00:23:03,447 And those are all radically different. 197 00:23:05,590 --> 00:23:08,141 So, each planet has a different story to tell. 198 00:23:08,181 --> 00:23:11,340 I suppose, in that sense, planets are like human beings. 199 00:23:11,380 --> 00:23:15,540 And this wholly unexpected but exciting discovery 200 00:23:15,580 --> 00:23:18,447 certainly complicates the search for life. 201 00:23:25,890 --> 00:23:28,211 We needed to narrow the search... 202 00:23:29,540 --> 00:23:34,728 for planets further but not too far away from their parent stars... 203 00:23:39,150 --> 00:23:43,391 planets at just the right distance for their surfaces 204 00:23:43,431 --> 00:23:45,945 potentially to be habitable... 205 00:23:48,890 --> 00:23:52,420 alien worlds with one precious ingredient 206 00:23:52,460 --> 00:23:55,373 that makes Earth a living planet. 207 00:24:16,220 --> 00:24:17,980 Now, you might legitimately ask, 208 00:24:18,020 --> 00:24:22,470 "Can we transfer all the knowledge we have of life here on Earth 209 00:24:22,510 --> 00:24:25,070 "to planets elsewhere in the universe?" 210 00:24:25,110 --> 00:24:28,580 Well, I would answer emphatically, yes, we can, 211 00:24:28,620 --> 00:24:30,750 because the laws of nature are universal. 212 00:24:30,790 --> 00:24:34,910 So, the laws of physics and chemistry that underpin biology here 213 00:24:34,950 --> 00:24:39,980 on this planet will apply to every planet out there in the universe, 214 00:24:40,020 --> 00:24:42,728 whether we've discovered it or not. 215 00:24:49,470 --> 00:24:52,750 The chemistry of life requires a few basic ingredients — 216 00:24:52,790 --> 00:24:56,249 carbon, nitrogen, oxygen, iron. 217 00:24:57,690 --> 00:25:01,170 And it also requires a ready supply of high-quality energy 218 00:25:01,210 --> 00:25:05,215 from heats within the planets or perhaps from starlight. 219 00:25:09,790 --> 00:25:15,250 But life here on Earth also requires one very important, fundamental 220 00:25:15,290 --> 00:25:18,260 extra ingredient, which is liquid water. 221 00:25:18,300 --> 00:25:21,590 Liquid water is a deceptively complicated substance. 222 00:25:21,630 --> 00:25:25,111 It's a very powerful solvent, but it also has structures 223 00:25:25,151 --> 00:25:28,370 which are constantly forming and disappearing within it, 224 00:25:28,410 --> 00:25:33,064 which act as a kind of scaffolding around which biology happens. 225 00:25:37,190 --> 00:25:40,700 Organic molecules are orientated by that scaffolding 226 00:25:40,740 --> 00:25:42,697 so they can react together. 227 00:25:44,420 --> 00:25:49,111 Now, it is certain that every living thing here on Earth 228 00:25:49,151 --> 00:25:52,310 requires liquid water to survive, 229 00:25:52,350 --> 00:25:56,900 and I would say it is a very good assumption that every living thing 230 00:25:56,940 --> 00:26:01,184 anywhere out there in the universe will require it too. 231 00:26:23,100 --> 00:26:25,558 The universe is filled with water. 232 00:26:26,940 --> 00:26:30,660 Great reservoirs have been detected throughout the galaxy 233 00:26:30,700 --> 00:26:33,704 amongst the gas clouds of giant nebulae. 234 00:26:38,261 --> 00:26:42,900 But just because water is plentiful, that doesn't mean that 235 00:26:42,940 --> 00:26:47,013 it necessarily ends up in oceans on planetary surfaces. 236 00:26:54,450 --> 00:26:57,150 Of the eight planets in our Solar System, 237 00:26:57,190 --> 00:27:02,629 only one has liquid water flowing permanently on its surface today... 238 00:27:08,021 --> 00:27:12,731 an ocean world where, long ago, life began. 239 00:27:32,220 --> 00:27:35,900 Around 4 billion years ago, life on Earth would have begun, 240 00:27:35,940 --> 00:27:39,540 probably in places not dissimilar to this, 241 00:27:39,580 --> 00:27:44,460 where there's geothermal activity, a source of energy in contact 242 00:27:44,500 --> 00:27:49,270 with rich concentrations of reactive chemical elements and minerals, 243 00:27:49,310 --> 00:27:56,340 but also, crucially, that — the magical solvent, liquid water. 244 00:27:56,380 --> 00:28:02,060 Now, many rocky planets out there in the galaxy will probably have this, 245 00:28:02,100 --> 00:28:06,480 but far fewer, we think, will have that — 246 00:28:06,520 --> 00:28:10,190 large bodies of liquid water on the surface. 247 00:28:10,230 --> 00:28:12,600 So that's why there's a kind of a catchphrase 248 00:28:12,640 --> 00:28:14,880 in the astrobiology community, which is, 249 00:28:14,920 --> 00:28:19,699 "If you want to search for life, follow the water." 250 00:28:33,790 --> 00:28:35,747 Whilst life on Earth was evolving... 251 00:28:43,220 --> 00:28:45,370 124 light-years away... 252 00:28:46,710 --> 00:28:50,635 amidst a collapsing cloud of gas, dust and ice... 253 00:28:55,750 --> 00:28:57,502 a small star was born... 254 00:29:04,720 --> 00:29:10,420 and the cloud-swirling leftovers condensed to form a brand-new world. 255 00:29:17,750 --> 00:29:20,430 In 2015, Kepler found a planet 256 00:29:20,470 --> 00:29:24,509 orbiting comfortably within its star's habitable zone. 257 00:29:39,790 --> 00:29:45,092 More than eight times the mass of the Earth, K2-18b is a giant... 258 00:30:08,070 --> 00:30:11,480 If the planet is rocky, this may have allowed it to hang on 259 00:30:11,520 --> 00:30:13,147 to a thick atmosphere. 260 00:30:16,360 --> 00:30:21,139 K2-18b might have all the makings of a water world. 261 00:30:26,990 --> 00:30:31,040 And a legendary space telescope had Kepler's new discovery 262 00:30:31,080 --> 00:30:32,423 in its sights. 263 00:30:38,870 --> 00:30:43,751 The most powerful space telescope of them all had joined the hunt. 264 00:30:49,720 --> 00:30:54,400 Hubble examined the light from K2-18b's parent star 265 00:30:54,440 --> 00:30:56,704 as the planet passed in front of it... 266 00:30:58,870 --> 00:31:03,068 and detected what may be a faint signature... 267 00:31:04,810 --> 00:31:06,244 of water vapor. 268 00:31:11,680 --> 00:31:14,138 124 light-years from Earth... 269 00:31:15,700 --> 00:31:19,852 we may have at last found the evidence of water 270 00:31:19,892 --> 00:31:21,280 on an alien world. 271 00:31:23,112 --> 00:31:27,472 This was the first observation of water vapor in the atmosphere 272 00:31:27,512 --> 00:31:31,903 of a planet orbiting in the habitable zone around its star. 273 00:31:31,943 --> 00:31:34,392 Now, admittedly, measurements of the amount of water vapor 274 00:31:34,432 --> 00:31:35,882 in the atmosphere is pretty wide. 275 00:31:35,922 --> 00:31:40,492 It's somewhere between 0.01% and 50%. 276 00:31:40,532 --> 00:31:42,592 I mean, this is a planet that's a long way away, 277 00:31:42,632 --> 00:31:47,962 but for comparison, our planet has a few percent water vapor 278 00:31:48,002 --> 00:31:49,602 in its atmosphere. 279 00:31:49,642 --> 00:31:52,232 So, that observation is important for two reasons. 280 00:31:52,272 --> 00:31:54,552 One is, it is not zero. 281 00:31:54,592 --> 00:31:56,903 There is water vapor in the atmosphere. 282 00:31:56,943 --> 00:32:00,672 But secondly, if the measurement is at the lower end, 283 00:32:00,712 --> 00:32:03,542 a few percent of the water vapor in the atmosphere, 284 00:32:03,582 --> 00:32:08,398 then that is consistent with this world being a planet 285 00:32:08,438 --> 00:32:12,227 with oceans on its surface. 286 00:32:18,348 --> 00:32:21,028 The nature of this planet is currently the subject 287 00:32:21,068 --> 00:32:23,184 of intense scientific debate. 288 00:32:25,039 --> 00:32:28,191 The planet may be more like a mini-Neptune... 289 00:32:29,598 --> 00:32:30,793 a gas planet. 290 00:32:34,968 --> 00:32:38,918 But it is possible to dream of a rocky alien world 291 00:32:38,958 --> 00:32:41,063 with skies full of clouds... 292 00:32:45,648 --> 00:32:47,764 where water droplets collect... 293 00:32:50,068 --> 00:32:51,513 and eventually fall... 294 00:32:56,798 --> 00:32:59,221 feeding vast oceans... 295 00:33:01,658 --> 00:33:05,447 that cover the surface of a massive planet... 296 00:33:08,578 --> 00:33:10,012 a water world... 297 00:33:19,019 --> 00:33:21,238 where the elixir of life... 298 00:33:23,048 --> 00:33:24,880 is in plentiful supply. 299 00:33:32,118 --> 00:33:36,438 K2-18b is exciting because it's the smallest world 300 00:33:36,478 --> 00:33:39,148 with an atmosphere that we've been able to analyze, 301 00:33:39,188 --> 00:33:42,018 and we've found that its mass and density, 302 00:33:42,058 --> 00:33:45,158 composition of its atmosphere and its orbits 303 00:33:45,198 --> 00:33:48,475 are consistent with it being a world with water. 304 00:33:50,108 --> 00:33:54,278 And it might be a world with oceans on its surface. 305 00:33:54,318 --> 00:33:56,198 We don't know for sure. 306 00:33:56,238 --> 00:34:00,488 But just imagine what that small, faraway world 307 00:34:00,528 --> 00:34:03,987 around a faint red star might be like. 308 00:34:11,608 --> 00:34:14,782 Kepler went on to make many more discoveries... 309 00:34:26,239 --> 00:34:31,928 until, in October 2018, it finally ran out of fuel. 310 00:34:38,755 --> 00:34:44,649 After nine years, it had found over 2,500 alien worlds... 311 00:34:51,796 --> 00:34:53,996 showing us just how common 312 00:34:54,036 --> 00:34:56,858 potentially Earth-like planets might be. 313 00:35:10,633 --> 00:35:14,563 We estimate that there may be around 20 billion 314 00:35:14,603 --> 00:35:16,653 potentially Earth-like worlds — 315 00:35:16,693 --> 00:35:20,283 that's rocky planets in the habitable zone around a star 316 00:35:20,323 --> 00:35:25,153 that may support liquid water on the surface — in our galaxy. 317 00:35:25,193 --> 00:35:29,323 That is 20 billion potential homes for life. 318 00:35:40,344 --> 00:35:44,174 Now, we don't know the probability that, given the right conditions, 319 00:35:44,214 --> 00:35:50,015 life will begin on a planet, but we do have evidence from our world. 320 00:35:50,055 --> 00:35:53,654 What we know is that here on Earth, life began pretty much 321 00:35:53,694 --> 00:35:57,494 as soon as it could after the Earth had formed and cooled down 322 00:35:57,534 --> 00:36:00,015 and the oceans formed on its surface. 323 00:36:00,055 --> 00:36:03,714 So that might suggest that whilst there isn't a sense of inevitability 324 00:36:03,754 --> 00:36:06,544 about the origin of life given the right conditions, 325 00:36:06,584 --> 00:36:10,124 it might at least be reasonably probable, 326 00:36:10,164 --> 00:36:15,194 so I think that there is at least a chance that life may have begun 327 00:36:15,234 --> 00:36:19,334 on some, perhaps many, of those 20 billion Earth-like worlds 328 00:36:19,374 --> 00:36:21,240 out there in our galaxy. 329 00:36:23,394 --> 00:36:27,684 But I think there are two questions about life. 330 00:36:27,724 --> 00:36:31,191 One question is about the origin and the existence of microbes. 331 00:36:31,231 --> 00:36:34,911 But often, when we speak about aliens, what we really mean 332 00:36:34,951 --> 00:36:38,461 is not microbes but complex creatures. 333 00:36:38,501 --> 00:36:42,381 We need things that we can speak to, civilizations. 334 00:36:42,421 --> 00:36:45,752 What is the probability there will be other civilizations out there 335 00:36:45,792 --> 00:36:47,391 in the Milky Way? 336 00:36:47,431 --> 00:36:50,831 Well, again, the answer is we don't know. 337 00:36:50,871 --> 00:36:55,231 But there are observations we can make, patterns we can see 338 00:36:55,271 --> 00:37:00,209 in the Milky Way, that might allow us to make an educated guess. 339 00:37:38,333 --> 00:37:40,343 We don't know precisely 340 00:37:40,383 --> 00:37:44,593 how we highly intelligent, complex creatures came to be here. 341 00:37:54,583 --> 00:37:59,441 But we do know for certain that life on Earth didn't begin this way. 342 00:38:03,143 --> 00:38:06,173 We are the product of a story that has been playing out 343 00:38:06,213 --> 00:38:09,524 for over a quarter of the age of the universe... 344 00:38:12,694 --> 00:38:13,729 from microbes... 345 00:38:15,453 --> 00:38:19,923 to a global technological civilization reaching out 346 00:38:19,963 --> 00:38:21,010 for others. 347 00:38:23,053 --> 00:38:26,853 For now, at least, we remain surrounded by silence. 348 00:38:26,893 --> 00:38:30,654 The messages we've sent out into the cosmos remain unanswered, 349 00:38:30,694 --> 00:38:34,904 and the telescopes we use to scan the skies for alien signals 350 00:38:34,944 --> 00:38:36,583 remain quiet. 351 00:38:36,623 --> 00:38:38,133 Now that's not to say, of course, 352 00:38:38,173 --> 00:38:40,784 that there aren't other civilizations out there. 353 00:38:40,824 --> 00:38:44,703 We may have been looking for the wrong thing in the wrong place. 354 00:38:44,743 --> 00:38:48,183 But I think the answer to the question of the great silence 355 00:38:48,223 --> 00:38:51,853 can be found here on Earth, because, here, 356 00:38:51,893 --> 00:38:56,703 it took 4 billion years of stability for a civilization to emerge. 357 00:38:56,743 --> 00:38:59,413 That is a vast amount of time. 358 00:38:59,453 --> 00:39:02,593 And when we look to the other worlds out there in the Milky Way, 359 00:39:02,633 --> 00:39:06,563 it's those two things — stability and time — 360 00:39:06,603 --> 00:39:09,891 that appear to be very rare commodities indeed. 361 00:39:19,913 --> 00:39:24,805 In 2013, the European Space Agency launched the Gaia Space Telescope. 362 00:39:28,351 --> 00:39:33,448 Its mission? To survey the stars of our galaxy, the Milky Way. 363 00:39:35,980 --> 00:39:38,517 Billions of stars have been mapped... 364 00:39:42,870 --> 00:39:47,580 each star a potential host for alien worlds. 365 00:39:52,240 --> 00:39:55,665 And patterns are already beginning to emerge. 366 00:40:09,950 --> 00:40:12,715 Not all stars exist alone. 367 00:40:18,160 --> 00:40:20,754 Some have company. 368 00:40:28,921 --> 00:40:30,881 And bizarre as they seem, 369 00:40:30,921 --> 00:40:34,710 Gaia has discovered around a million of these binary 370 00:40:34,750 --> 00:40:36,593 or multiple star systems. 371 00:40:41,640 --> 00:40:44,050 We've known for a long time that binary star systems 372 00:40:44,090 --> 00:40:47,200 and, indeed, multiple star systems exist, 373 00:40:47,240 --> 00:40:50,460 but we didn't know precisely how common they are. 374 00:40:52,820 --> 00:40:56,110 But now we have a huge amount of high-precision data, 375 00:40:56,150 --> 00:41:01,120 including the Gaia data, which tells us that around 50% 376 00:41:01,160 --> 00:41:04,680 of all sun-like stars are in multiple star systems. 377 00:41:04,720 --> 00:41:08,998 And for more massive stars, that number is 80%. 378 00:41:13,830 --> 00:41:18,280 So how does the prevalence of multiple star systems in the galaxy 379 00:41:18,320 --> 00:41:21,438 shift the odds in the hunt for another Earth? 380 00:41:23,921 --> 00:41:29,020 Could Earth-like planets exist in multiple star systems? 381 00:41:29,060 --> 00:41:32,553 And if so, what might their fate be? 382 00:41:38,440 --> 00:41:42,890 In 2020, we may have found a clue, 383 00:41:42,930 --> 00:41:49,891 a planet the size of Mars floating freely through the galaxy, 384 00:41:49,931 --> 00:41:52,229 a so-called "rogue world". 385 00:41:54,310 --> 00:41:58,840 But planets can't form alone in interstellar space, 386 00:41:58,880 --> 00:42:00,837 so where did it come from? 387 00:42:17,330 --> 00:42:18,468 Dawn... 388 00:42:26,300 --> 00:42:28,598 ushered in not by one star... 389 00:42:33,600 --> 00:42:34,738 but two. 390 00:42:44,400 --> 00:42:48,075 Perhaps the rogue world grew up in a close binary system... 391 00:43:02,430 --> 00:43:05,912 subject to the gravitational pull of two stars. 392 00:43:13,600 --> 00:43:16,433 Its orbit may have been unstable... 393 00:43:24,450 --> 00:43:28,592 as its parent stars fought to control its destiny. 394 00:43:42,561 --> 00:43:47,000 Even in single star systems, the weak gravitational interactions 395 00:43:47,040 --> 00:43:49,960 between the planets can change their orbits. 396 00:43:50,000 --> 00:43:51,800 Now, in a double star system, 397 00:43:51,840 --> 00:43:54,870 the planets are not only subjected to the gravitational pulls 398 00:43:54,910 --> 00:43:58,920 of each other, they're subjected to the stronger gravitational pull 399 00:43:58,960 --> 00:44:00,761 of another star. 400 00:44:00,801 --> 00:44:04,080 So, even if a planet gets into a stable orbit, 401 00:44:04,120 --> 00:44:08,480 it's very likely that it won't stay in that orbit for long. 402 00:44:08,520 --> 00:44:12,490 So, in double star systems, the line between order and chaos 403 00:44:12,530 --> 00:44:14,396 is very thin indeed. 404 00:44:19,920 --> 00:44:22,690 Even subtle changes in a planet's orbit 405 00:44:22,730 --> 00:44:25,438 can lead to dramatic changes in climate. 406 00:44:26,640 --> 00:44:28,960 And that's why the surface conditions on planets 407 00:44:29,000 --> 00:44:34,641 in double star systems may be unlikely to remain stable enough 408 00:44:34,681 --> 00:44:38,163 for long enough for intelligent life to evolve. 409 00:44:46,890 --> 00:44:48,880 And the changes in the orbits of planets 410 00:44:48,920 --> 00:44:53,835 can sometimes be anything but subtle. 411 00:45:12,840 --> 00:45:18,680 A close encounter may have given the rogue world 412 00:45:18,720 --> 00:45:21,087 a final gravitational kick. 413 00:45:41,890 --> 00:45:43,449 Flinging it outwards. 414 00:45:47,571 --> 00:45:51,201 And releasing it from the grip of its parent stars. 415 00:45:57,340 --> 00:45:58,717 Setting it loose... 416 00:46:04,610 --> 00:46:06,897 on a journey through the galaxy. 417 00:46:22,130 --> 00:46:26,320 Far from the warmth of its stars, 418 00:46:26,360 --> 00:46:30,115 any liquid water the rogue world might once have had... 419 00:46:34,970 --> 00:46:36,699 would have frozen solid. 420 00:46:42,180 --> 00:46:45,172 Any atmosphere that once protected it... 421 00:46:53,060 --> 00:46:56,018 would have frozen out on to the surface. 422 00:47:00,530 --> 00:47:02,555 The rogue would have become a world... 423 00:47:05,400 --> 00:47:09,553 with conditions that no living thing could endure. 424 00:47:13,200 --> 00:47:17,228 An entire planet alone and adrift. 425 00:47:24,320 --> 00:47:28,996 Only to be detected by us millions of years later. 426 00:47:30,990 --> 00:47:34,426 A small Earth-like rogue planet... 427 00:47:36,470 --> 00:47:40,634 roaming the darkness of space for eternity. 428 00:47:52,050 --> 00:47:56,000 This lonely wandering planet is not a unique world. 429 00:47:56,040 --> 00:47:58,560 Although rogue planets are very difficult to detect, 430 00:47:58,600 --> 00:48:00,490 it's estimated that there may be 431 00:48:00,530 --> 00:48:03,470 over 100 billion of them in our galaxy. 432 00:48:03,510 --> 00:48:05,521 Rogue planets might be the most common 433 00:48:05,561 --> 00:48:07,711 type of planet in the Milky Way. 434 00:48:09,150 --> 00:48:10,750 And although we think most of them 435 00:48:10,790 --> 00:48:14,440 were torn away from their star soon after formation, 436 00:48:14,480 --> 00:48:19,590 this does suggest that star systems are not always stable places 437 00:48:19,630 --> 00:48:22,952 where complex life could evolve over billions of years. 438 00:48:35,890 --> 00:48:39,394 Our hunt for another living planet has only just begun. 439 00:48:43,431 --> 00:48:45,422 Yet we've already learned so much. 440 00:48:52,970 --> 00:48:55,826 We found our first rocky worlds. 441 00:48:58,175 --> 00:49:01,850 Some in the habitable zone around their stars. 442 00:49:10,362 --> 00:49:15,129 Some, potentially, with liquid water on the surface. 443 00:49:16,762 --> 00:49:21,632 Candidate worlds for future missions to search for evidence of life. 444 00:49:27,122 --> 00:49:31,127 But we've also found hordes of bizarre, tortured worlds... 445 00:49:33,732 --> 00:49:36,042 orbiting around violent stars. 446 00:49:44,722 --> 00:49:47,441 And a multitude of rogue planets... 447 00:49:49,693 --> 00:49:54,199 where complex life as we understand it seems impossible. 448 00:50:02,722 --> 00:50:06,875 Perhaps it's these worlds that hint at the reason why... 449 00:50:08,602 --> 00:50:12,334 for now, one planet stands apart. 450 00:50:18,132 --> 00:50:19,566 Alone. 451 00:50:32,292 --> 00:50:35,832 Our planet seems to have largely escaped the violence, 452 00:50:35,872 --> 00:50:38,162 the chaos and the constant change 453 00:50:38,202 --> 00:50:41,533 that seems to characterize a galaxy like the Milky Way. 454 00:50:41,573 --> 00:50:44,372 Yes, there's been the odd mass extinction, 455 00:50:44,412 --> 00:50:47,653 but there's been an unbroken chain of life here on Earth 456 00:50:47,693 --> 00:50:50,482 stretching back four billion years. 457 00:50:50,522 --> 00:50:53,602 And if that's what you need to go from the origin of life 458 00:50:53,642 --> 00:50:57,602 to a civilization, then although there may be billions of worlds 459 00:50:57,642 --> 00:51:02,204 out there where life began, there may be very few civilizations. 460 00:51:03,292 --> 00:51:05,742 But that's just an opinion. 461 00:51:05,782 --> 00:51:07,602 It's an educated guess. 462 00:51:07,642 --> 00:51:10,482 And given the profound nature of the question, 463 00:51:10,522 --> 00:51:15,283 no matter how educated the guess, I think it would be ridiculous 464 00:51:15,323 --> 00:51:20,045 for us to stop looking, both inside our galaxy and beyond. 465 00:51:24,012 --> 00:51:28,362 For we may have just received the first glimpse 466 00:51:28,402 --> 00:51:31,110 of a world beyond the Milky Way... 467 00:51:39,632 --> 00:51:42,942 around 30 million light years away, 468 00:51:42,982 --> 00:51:46,930 nestled in the spiral arms of the Whirlpool Galaxy. 469 00:51:51,602 --> 00:51:54,037 A world the size of Saturn. 470 00:52:02,772 --> 00:52:06,163 A find that marks an expansion of our horizons. 471 00:52:11,292 --> 00:52:15,593 The beginning of the hunt for extragalactic planets. 472 00:52:19,242 --> 00:52:22,732 The potential discovery of a planet orbiting around a star 473 00:52:22,772 --> 00:52:26,642 in another galaxy is something that I never thought I'd see. 474 00:52:26,682 --> 00:52:30,612 And it opens up the intriguing possibility that we might be able 475 00:52:30,652 --> 00:52:34,202 to explore not only the question, "Are we alone in our galaxy?" 476 00:52:34,242 --> 00:52:36,643 But "Are we alone in the universe?" 477 00:52:41,002 --> 00:52:45,002 The answer to that question may lie far in the future. 478 00:52:45,042 --> 00:52:47,892 We might never answer that question, 479 00:52:47,932 --> 00:52:51,902 but I said the question "Are we alone?" is profound 480 00:52:51,942 --> 00:52:54,682 because answering it would teach us much more 481 00:52:54,722 --> 00:52:56,520 about what it means to be human. 482 00:53:01,012 --> 00:53:05,642 Well, I think we become a little bit more human with every world 483 00:53:05,682 --> 00:53:11,042 that we explore because that ability to lay the foundations, 484 00:53:11,082 --> 00:53:14,762 to explore questions to which we may never receive answers 485 00:53:14,802 --> 00:53:18,122 in our lifetime, questions for our children 486 00:53:18,162 --> 00:53:20,102 or our grandchildren to answer 487 00:53:20,142 --> 00:53:23,413 is a fundamental part of what it means to be human. 488 00:53:23,453 --> 00:53:28,283 It's a fundamental part of what makes us so special 489 00:53:28,323 --> 00:53:33,102 here on this little world, looking up at the stars, 490 00:53:33,142 --> 00:53:35,577 whether we're alone or not. 491 00:53:52,052 --> 00:53:54,283 Five, four, 492 00:53:54,323 --> 00:53:56,722 three, two... 493 00:53:56,762 --> 00:53:58,852 Engine start. One, zero, 494 00:53:58,892 --> 00:54:02,142 and liftoff of the Delta II rocket with Kepler 495 00:54:02,182 --> 00:54:05,447 on a search for planets in some way like our own. 496 00:54:07,532 --> 00:54:10,293 We had worked to get thousands of people to work together 497 00:54:10,333 --> 00:54:12,609 and it's all coming together. 498 00:54:14,052 --> 00:54:15,451 And we have separation. 499 00:54:17,302 --> 00:54:20,172 It was so emotional to see the project they had worked on 500 00:54:20,212 --> 00:54:23,642 for so many years or decades finally go to space 501 00:54:23,682 --> 00:54:26,982 and all that hope and promise all bundled up in the machinery. 502 00:54:35,112 --> 00:54:37,812 Kepler was an immediate success, 503 00:54:37,852 --> 00:54:40,273 discovering over 2,000 new planets 504 00:54:40,313 --> 00:54:43,010 in its first four years of operation. 505 00:54:48,242 --> 00:54:50,442 But in the summer of 2012, 506 00:54:50,482 --> 00:54:53,793 the team faced a challenge that threatened the entire mission. 507 00:54:58,172 --> 00:55:00,772 One of the things that the Kepler mission needs to operate 508 00:55:00,812 --> 00:55:03,361 are reaction wheels that spin and hold it on target. 509 00:55:05,792 --> 00:55:09,202 So it always points at the same stars and doesn't jiggle. 510 00:55:09,242 --> 00:55:11,449 Well, we had four wheels that did that. 511 00:55:13,962 --> 00:55:16,432 And we knew that we only had a couple of spare gyroscopes, 512 00:55:16,472 --> 00:55:20,386 and we knew that spacecraft tend to have gyros fail. 513 00:55:25,252 --> 00:55:27,122 And after a while, it failed. 514 00:55:27,162 --> 00:55:29,412 Three months later, the second one failed. 515 00:55:29,452 --> 00:55:31,062 And since we needed three, 516 00:55:31,102 --> 00:55:34,322 we could no longer look at the Kepler field of view. 517 00:55:37,002 --> 00:55:40,002 I had hoped that they'll figure out a way to work with two gyros, 518 00:55:40,042 --> 00:55:41,680 and indeed they did. 519 00:55:45,652 --> 00:55:48,562 So the very clever people, the engineers and scientists, 520 00:55:48,602 --> 00:55:53,562 said, "What we can use is we'll use the sunshine for the third wheel. 521 00:55:53,602 --> 00:55:56,002 "We'll make this thing reflect sunlight off it, 522 00:55:56,042 --> 00:55:59,398 "we use the other two wheels and now we can point in the sky." 523 00:56:02,092 --> 00:56:06,370 The faint pressure of sunlight helped stabilize the telescope. 524 00:56:07,482 --> 00:56:10,572 That was kind of good news, actually, because it meant Kepler 525 00:56:10,612 --> 00:56:13,082 was going to have to go off the Kepler field now 526 00:56:13,122 --> 00:56:16,602 and we could get all kinds of other stars and observe them. 527 00:56:16,642 --> 00:56:19,680 And so, it actually was a boon for stellar astronomy. 528 00:56:22,552 --> 00:56:25,332 After another four years of discoveries, 529 00:56:25,372 --> 00:56:30,163 in total it had found over 2,600 planets, 530 00:56:30,203 --> 00:56:34,879 making it, by far, our most successful planet hunter to date. 531 00:56:38,323 --> 00:56:42,252 It was sad when they said to command to shut everything down. 532 00:56:42,292 --> 00:56:44,052 You know, it's asleep now, 533 00:56:44,092 --> 00:56:47,822 it's in orbit around the Sun and it will continue that orbit, 534 00:56:47,862 --> 00:56:51,002 but since it launched from Earth, it will come back to Earth. 535 00:56:51,042 --> 00:56:54,972 It'll come and visit us again in about 40 years. 536 00:56:55,012 --> 00:56:57,163 And my hope is people will say, 537 00:56:57,203 --> 00:57:00,692 "This is a historic telescope. It told us about all these planets." 538 00:57:00,732 --> 00:57:04,622 And they will go up and pick up this telescope and bring it back to Earth 539 00:57:04,662 --> 00:57:08,485 and put it in the Air and Space Museum for us all to admire. 540 00:57:16,463 --> 00:57:18,293 Next time... 541 00:57:18,333 --> 00:57:23,002 a powerful new space mission uncovers the history of our galaxy, 542 00:57:23,042 --> 00:57:24,407 the Milky Way. 543 00:57:25,652 --> 00:57:29,452 How it arose from the universe's mysterious dark age... 544 00:57:31,992 --> 00:57:36,163 and overcame a series of enormous collisions 545 00:57:36,203 --> 00:57:39,012 with rival galaxies 546 00:57:39,052 --> 00:57:42,112 that paved the way for our own arrival 547 00:57:42,152 --> 00:57:45,144 inside one of its magnificent spiral arms. 548 00:57:51,022 --> 00:57:53,682 Journey through the universe with the Open University 549 00:57:53,722 --> 00:57:58,080 and learn more about stars, planets and galaxies with this free poster. 550 00:57:59,382 --> 00:58:03,852 Order your poster by calling 03003035746. 551 00:58:03,892 --> 00:58:07,602 Or go to bbc.co.uk/theuniverse 552 00:58:07,642 --> 00:58:10,270 and follow the links to the Open University.