1 00:00:18,639 --> 00:00:23,245 Why do we humans have such a connection to the night sky? 2 00:00:23,280 --> 00:00:29,960 The twinkling lights that seem like oases out there and yet we're not sure. Are there habitable worlds? 3 00:00:31,599 --> 00:00:36,199 Around the world there are a group of highly intelligent, 4 00:00:36,234 --> 00:00:40,764 highly trained scientists that share a surprising belief. 5 00:00:40,799 --> 00:00:45,724 There are a couple of hundred billion stars just in our galaxy and 6 00:00:45,759 --> 00:00:50,039 at least half of them probably have planets. That's 100 billion planetary systems. 7 00:00:50,074 --> 00:00:52,965 How many planets in each system? let's say five. 8 00:00:53,000 --> 00:00:57,500 That's a half trillion, 500 billion planets out there. 9 00:00:57,535 --> 00:01:01,965 Keep in mind, there are 100 billion other galaxies! 10 00:01:02,000 --> 00:01:07,599 For these scientists the vastness of our universe can mean just one thing - 11 00:01:07,634 --> 00:01:10,164 the existence of life. 12 00:01:10,199 --> 00:01:15,840 So to think, "Look, man. This is the only place where anything interesting's happening", 13 00:01:15,875 --> 00:01:19,519 you've gotta be really audacious to take that point of view. 14 00:01:19,554 --> 00:01:22,320 But proving it has not been quite so simple. 15 00:01:24,080 --> 00:01:28,600 I have conducted many, many searches, none of which have produced a discovery. 16 00:01:30,399 --> 00:01:32,319 Until now. 17 00:01:33,920 --> 00:01:38,760 In our local neighbourhood, just 200 trillion kilometres from Earth, 18 00:01:38,795 --> 00:01:42,165 is a planet that we might find rather familiar. 19 00:01:42,200 --> 00:01:47,885 The discovery of Gliese 581c is a marvellous discovery. 20 00:01:47,920 --> 00:01:52,439 It shows how close we're getting to planets that remind us of the Earth. 21 00:02:22,760 --> 00:02:27,159 Occasionally you're sitting on a plane and they guy next to you says, "What do you do for a living?". 22 00:02:27,194 --> 00:02:33,320 "I look for aliens". I explain a little bit, and almost every one everyone is interested. 23 00:02:33,355 --> 00:02:36,919 Nobody says, "That's nice. I'll go back to my magazine now". 24 00:02:40,160 --> 00:02:48,119 In the desert, 300 miles north of San Francisco, Dr Seth Shostak is waiting for a message from an alien. 25 00:02:50,799 --> 00:02:56,520 This is SETI, the front-line in the search for extra-terrestrial intelligence. 26 00:02:56,555 --> 00:03:01,520 If a message ever comes our way, this is where it will be received. 27 00:03:03,400 --> 00:03:08,519 Anybody who can build a transmitter can send messages between the stars. 28 00:03:08,554 --> 00:03:11,564 If we can do that, maybe they can do it. 29 00:03:11,599 --> 00:03:14,645 Here we are, the Allen Telescope Array, - designed to do one thing - 30 00:03:14,680 --> 00:03:21,040 eavesdrop on any signals that might be being broadcast our way by some alien civilisation. 31 00:03:22,640 --> 00:03:26,085 This vast array of telescopes is the latest in a long line 32 00:03:26,120 --> 00:03:31,085 of experiments designed to eavesdrop on our nearest neighbours. 33 00:03:31,120 --> 00:03:36,200 As chief astronomer of the project, Shostak is more confident than most 34 00:03:36,235 --> 00:03:39,160 that he'll be on the receiving end of a close encounter. 35 00:03:41,560 --> 00:03:45,959 There are 42 antennae here now, you can count them up. 36 00:03:45,994 --> 00:03:50,359 But eventually the idea is to have 350 and then this thing 37 00:03:50,394 --> 00:03:53,764 will be able to scan big chunks of the sky, 38 00:03:53,799 --> 00:03:57,839 simultaneously observing five, six, maybe more stars at a time, looking 39 00:03:57,874 --> 00:04:01,879 for the signal that somebody's out there, trying to get our attention. 40 00:04:03,560 --> 00:04:07,564 It's the most powerful experiment humans have ever attempted - 41 00:04:07,599 --> 00:04:12,959 to discover if intelligent life is the exception or the rule in the cosmos. 42 00:04:17,560 --> 00:04:22,719 But although it's early days for Shostak and his team, the omens are not good. 43 00:04:29,319 --> 00:04:34,360 The SETI project has been casting its ear out to the universe for over 50 years. 44 00:04:38,519 --> 00:04:43,320 For the founding father of the search, Dr Frank Drake, the dream has never changed. 45 00:04:44,920 --> 00:04:50,159 Back in the 1950s, there were many scientists interested in ET life, 46 00:04:50,194 --> 00:04:55,359 but we were well aware that there were no means even to detect 47 00:04:55,394 --> 00:04:58,325 planets, let alone microbes, 48 00:04:58,360 --> 00:05:02,204 or any sign of non-intelligent life. 49 00:05:02,239 --> 00:05:07,885 In fact, the only thing open to us was radio transmissions from intelligent civilisations. 50 00:05:07,920 --> 00:05:13,720 Drake was the first scientist to believe that technology could answer the biggest question of all. 51 00:05:22,680 --> 00:05:24,805 Despite widespread scepticism, 52 00:05:24,840 --> 00:05:29,044 he believed that if there were intelligent life forms out there 53 00:05:29,079 --> 00:05:33,760 then the least we could do is to try and listen to any radio signals they may be sending out. 54 00:05:40,919 --> 00:05:46,059 In 1960, we conducted a search for radio signals from the two nearest 55 00:05:46,094 --> 00:05:51,200 stars to the Earth that are like the Sun - Tau Ceti and Epsilon Eridani. 56 00:05:51,235 --> 00:05:55,324 But to no-one's surprise, the search failed. 57 00:05:55,359 --> 00:05:58,965 We searched for two months. We didn't find anything. 58 00:05:59,000 --> 00:06:04,239 And that's actually an important result, because it showed not every star in the sky was radiating. 59 00:06:04,274 --> 00:06:09,800 It also demonstrated that a search was likely to be a very long and difficult one. 60 00:06:11,400 --> 00:06:17,679 Despite putting on a brave face, Drake and his ideas remained on the very fringes of astronomy. 61 00:06:17,714 --> 00:06:23,164 Listening for aliens just wasn't science. 62 00:06:23,199 --> 00:06:26,479 Congress people would see that they could get publicity by 63 00:06:26,514 --> 00:06:29,725 attacking this project as a waste of tax-payers' money. 64 00:06:29,760 --> 00:06:34,039 Using tax-payers' money to search for little green men was a common theme. 65 00:06:36,120 --> 00:06:40,800 So Drake would have to wait for another generation of scientists 66 00:06:40,835 --> 00:06:43,799 to bring alien-hunting in from the cold. 67 00:06:59,839 --> 00:07:03,120 Being an astronomer is a bit sacrificial. 68 00:07:03,155 --> 00:07:06,244 My wife is at home and she misses me. 69 00:07:06,279 --> 00:07:08,829 I call her up and she says, "When are you coming home?" 70 00:07:08,864 --> 00:07:11,379 It's another four, five nights, I have to tell her. 71 00:07:12,479 --> 00:07:18,919 Professor Geoff Marcy is a planet hunter, an explorer of alien worlds. 72 00:07:22,239 --> 00:07:25,564 Staying up all night means you don't get much sleep. 73 00:07:25,599 --> 00:07:30,039 So it's a bit of a sacrifice, but I wouldn't give it up for anything. It's such a treasure. 74 00:07:38,880 --> 00:07:42,485 For the last ten years, Marcy has come here to 75 00:07:42,520 --> 00:07:46,920 use the planet's largest telescope in the hope of finding other worlds. 76 00:07:51,079 --> 00:07:58,520 The Keck Telescope high up on Mauna Kea, Hawaii is about as close as you can get to the stars. 77 00:08:01,160 --> 00:08:06,759 It's the world's largest, because the collecting area of the mirror is the largest in the world. 78 00:08:06,794 --> 00:08:09,044 The mirror is ten metres across, 79 00:08:09,079 --> 00:08:13,759 1/10 the length of a football field, all to collect the starlight 80 00:08:13,794 --> 00:08:17,239 coming from hundreds or thousands of light years away. 81 00:08:19,880 --> 00:08:26,639 But even using the mighty Keck telescope, 82 00:08:28,679 --> 00:08:36,159 Pluto, at the edge of our solar system, is a colossal 4½ billion kilometres away. 83 00:08:37,760 --> 00:08:41,079 And this is the best image astronomers have achieved. 84 00:08:44,480 --> 00:08:48,645 And yet Marcy wanted to look beyond our solar system, 85 00:08:48,680 --> 00:08:55,799 to find the hypothetical worlds that astronomers call exoplanets, which lie around other stars. 86 00:08:59,000 --> 00:09:01,765 Like our nearest star, Proxima Centauri, 87 00:09:01,800 --> 00:09:07,200 a staggering 40 trillion kilometres away, or four light years. 88 00:09:09,680 --> 00:09:17,079 Beyond lies the rest of our galaxy - an unimaginable 100,000 light years across. 89 00:09:17,114 --> 00:09:21,239 It's our local neighbourhood of 200 billion stars. 90 00:09:23,359 --> 00:09:27,004 Astronomers knew there had to be planets out there. 91 00:09:27,039 --> 00:09:31,400 We saw young stars with proto-planetary disks of gas and dust 92 00:09:31,435 --> 00:09:34,417 around them, surely making planets. 93 00:09:34,452 --> 00:09:37,364 But we couldn't detect the planets. 94 00:09:37,399 --> 00:09:42,139 And the reason is that even with the largest telescope, like this one, 95 00:09:42,174 --> 00:09:46,880 the mighty Keck, the planets were lost in the glare of the host stars. 96 00:09:49,359 --> 00:09:53,125 The problem is that compared to the light of a star, 97 00:09:53,160 --> 00:09:57,560 the reflected light from an exoplanet is all but invisible. 98 00:09:57,595 --> 00:10:00,879 The star burns a billion times more brightly. 99 00:10:05,720 --> 00:10:13,040 Indeed, even with the Hubble space telescope, we can't detect planets directly around nearby stars. 100 00:10:14,640 --> 00:10:21,320 Using direct observational methods, astronomers were confined within our own solar system, 101 00:10:21,355 --> 00:10:26,039 unable to prove even the existence of exoplanets, let alone life. 102 00:10:27,640 --> 00:10:31,519 So how do you look for something you can't see? 103 00:10:33,119 --> 00:10:38,319 The answer was first proposed in an obscure paper published in 1952, 104 00:10:38,354 --> 00:10:42,445 by a Russia astronomer called Otto Struve. 105 00:10:42,480 --> 00:10:45,885 Struve theorised that even though the planets themselves 106 00:10:45,920 --> 00:10:50,160 were invisible, there was still a way of unlocking their secrets. 107 00:10:54,200 --> 00:11:01,199 He knew that each planet was held in orbit around its star by an immense gravitational force. 108 00:11:01,234 --> 00:11:04,365 This force works in two directions. 109 00:11:04,400 --> 00:11:09,639 The star pulls on the planet but the planet also pulls back on the star, 110 00:11:09,674 --> 00:11:12,479 making the star move with the minutest wobble. 111 00:11:16,359 --> 00:11:19,365 It's not much, but this wobble is just big enough 112 00:11:19,400 --> 00:11:23,120 to make it theoretically detectable back here on Earth. 113 00:11:24,680 --> 00:11:27,924 If you have the right technology. 114 00:11:27,959 --> 00:11:34,199 The real tipping point was not in the telescopes, we've had big telescopes for several decades. 115 00:11:34,234 --> 00:11:37,845 The tipping point was having digital detectors, like the digital cameras 116 00:11:37,880 --> 00:11:43,560 that most of us enjoy, with the CCD light detectors at their backs - and computers. 117 00:11:45,239 --> 00:11:47,600 Let's see what kind of a night we're gonna have here. 118 00:11:47,635 --> 00:11:52,359 Marcy script underscore ETA underscore Earth. 119 00:11:52,394 --> 00:11:55,324 Great. First object on the list... 120 00:11:55,359 --> 00:11:57,840 Why don't you go ahead and open the dome slit? 121 00:12:01,400 --> 00:12:09,299 We're observing 80 stars, night after night, with one key goal, and that's to detect Earth-like planets. 122 00:12:09,334 --> 00:12:17,199 Since the early 1990s, planet hunters like Marcy have been gazing to the stars for the tiniest wobble 123 00:12:17,234 --> 00:12:19,845 that could signal the 124 00:12:19,880 --> 00:12:22,644 The theory seemed correct. 125 00:12:22,679 --> 00:12:28,759 They had the right technology, and yet after years of searching, the exoplanets were still missing. 126 00:12:31,400 --> 00:12:33,685 We were confronted with a contradiction. 127 00:12:33,720 --> 00:12:39,679 On the one hand, it appeared that young stars had the right kind of planet building material, 128 00:12:39,714 --> 00:12:44,159 but on the other hand, humanity had failed to find any. 129 00:12:46,199 --> 00:12:51,279 All the planet hunters had to keep them going was their faith. 130 00:12:51,314 --> 00:12:56,360 And the belief of the one man who has never given up hope. 131 00:13:02,400 --> 00:13:07,679 It occurred to me, we need to know how often does life arise, how often does intelligence arise. 132 00:13:07,714 --> 00:13:12,440 I recognised that all you had to do was multiply these factors together 133 00:13:12,475 --> 00:13:15,957 and you have a very prime important equation, of basic interest, 134 00:13:15,992 --> 00:13:19,440 because it tells us how many civilisations there are out there. 135 00:13:23,439 --> 00:13:27,285 Without any hard evidence, back in 1960, Frank Drake went 136 00:13:27,320 --> 00:13:32,444 about creating an equation that would answer the big question once and for all. 137 00:13:32,479 --> 00:13:39,639 We have an equation which gives us N, the number of detectable civilisations in our galaxy. 138 00:13:39,674 --> 00:13:43,045 It's based on what we know of the history of our galaxy 139 00:13:43,080 --> 00:13:47,159 and particularly the history of our solar system and of life on Earth. 140 00:13:48,799 --> 00:13:54,360 The equation defines all the necessary ingredients for intelligent life to arise. 141 00:13:54,395 --> 00:13:57,964 There are seven factors in the equation. 142 00:13:57,999 --> 00:14:02,559 Since life needs a home, it begins with a known observation. 143 00:14:02,594 --> 00:14:04,799 For the rate of star formation, we know that very well. 144 00:14:04,834 --> 00:14:07,605 It's about 20 stars per year. 145 00:14:07,640 --> 00:14:11,724 For the fraction of planets, we didn't used to know that at all. 146 00:14:11,759 --> 00:14:16,405 Everything else in the equation, from the number of stars with planets 147 00:14:16,440 --> 00:14:22,119 to the number of planets per star capable of supporting life, was a total mystery. 148 00:14:22,154 --> 00:14:26,760 But over the years, it hasn't stopped people from guessing. 149 00:14:26,795 --> 00:14:30,004 This is our number N. 150 00:14:30,039 --> 00:14:34,324 The number of technical civilisations in the galaxy. 151 00:14:34,359 --> 00:14:40,179 Into the now famous, or infamous, Drake equation goes everything, from astrophysics, through 152 00:14:40,214 --> 00:14:45,999 evolutionary biology to whatever it is that governs the lifetime of a detectable civilisation. 153 00:14:46,034 --> 00:14:50,439 Not surprisingly, no-one's solved it yet, but anyone can have a go. 154 00:14:50,474 --> 00:14:52,719 It's almost a game the whole family can play. 155 00:14:55,760 --> 00:14:59,579 If you sort of take the average of people's guesses, 156 00:14:59,614 --> 00:15:03,399 it gives you a total number of detectable civilisations, 157 00:15:03,434 --> 00:15:05,200 which is about 10,000. 158 00:15:07,240 --> 00:15:08,720 A big number. 159 00:15:10,919 --> 00:15:17,760 And yet with no call from ET, and no sign of another Earth, Drake's guess seemed wildly optimistic. 160 00:15:36,119 --> 00:15:40,404 When I was a child, I was living in 161 00:15:40,439 --> 00:15:46,360 We spent evenings with my sister laying down on the grass and looking at the sky. 162 00:15:46,395 --> 00:15:49,839 And that's really good for the imagination. 163 00:15:51,640 --> 00:15:56,439 I'm sure there are other Earths similar to our own Earth. 164 00:15:56,474 --> 00:16:01,239 And on some of them, you even may have life developing. 165 00:16:02,999 --> 00:16:07,019 Professor Stephane Udry is part of a Swiss team of planet hunters 166 00:16:07,054 --> 00:16:11,039 who began searching for life-bearing planets in the mid-'90s. 167 00:16:14,559 --> 00:16:16,965 They had developed a new planet detector, 168 00:16:17,000 --> 00:16:21,080 which had just been installed at their observatory in central France. 169 00:16:21,115 --> 00:16:23,124 When you have a new instrument, 170 00:16:23,159 --> 00:16:27,240 you want to check the short-term precision of the instrument. 171 00:16:29,559 --> 00:16:34,245 Tests of their new detector where scheduled to last a few weeks. 172 00:16:34,280 --> 00:16:39,159 Among their target stars was one similar to our own sun, called Pegasi 51. 173 00:16:40,760 --> 00:16:46,079 The light from Peg 51 should remain constant, but there seemed to be a problem. 174 00:16:48,360 --> 00:16:49,884 The star... 175 00:16:49,919 --> 00:16:52,040 appeared to be wobbling. 176 00:16:54,200 --> 00:17:01,520 The thinking was, "Oh, that should be some crazy effect of the star that could explain the observations". 177 00:17:03,399 --> 00:17:10,239 They tried to reject, one after the other, all the possible explanations. 178 00:17:10,274 --> 00:17:14,719 And at the end, the best explanation was the presence of a planet. 179 00:17:18,519 --> 00:17:21,525 With ingenuity and a little bit of luck, 180 00:17:21,560 --> 00:17:26,079 the Swiss had discovered the first planet outside our solar system. 181 00:17:28,919 --> 00:17:35,559 It was massive, half the mass of Jupiter, but in a rapid orbit lasting only four days. 182 00:17:37,160 --> 00:17:43,200 The planet was so close to its star, that surface temperatures exceed 1,000 degrees Centigrade. 183 00:17:50,759 --> 00:17:55,119 Being part of these teams finding planets around other stars 184 00:17:55,154 --> 00:17:57,445 is very exciting in that sense. 185 00:17:57,480 --> 00:18:02,360 That's really a new domain that is opening in science. 186 00:18:04,239 --> 00:18:08,319 The discovery of this planet opened the flood gates. 187 00:18:08,354 --> 00:18:12,176 Hundreds of exoplanets have since been discovered, 188 00:18:12,211 --> 00:18:15,999 but none of them have proved suitable for life. 189 00:18:25,480 --> 00:18:31,079 My favourite planet is a little planet that orbits the star Gliese 756. 190 00:18:31,114 --> 00:18:33,880 My favourite planetary system is called 55 Cancri. 191 00:18:34,000 --> 00:18:34,045 My favourite planetary system is called 55 Cancri. 192 00:18:34,080 --> 00:18:37,599 There are two planets which I have an emotional attachment to. 193 00:18:37,634 --> 00:18:39,684 It has an orbit of two days. 194 00:18:39,719 --> 00:18:41,884 The planets of the star Tau Ceti. 195 00:18:41,919 --> 00:18:44,319 That means its seasons occur in two days. 196 00:18:44,354 --> 00:18:46,365 And the star Epsilon Eridani. 197 00:18:46,400 --> 00:18:49,284 So summer-winter would alternate in two days. 198 00:18:49,319 --> 00:18:53,440 It's like the solar system because it's a planet like Jupiter on a Jupiter-like orbit. 199 00:18:53,475 --> 00:18:55,845 Those stars were my targets 200 00:18:55,880 --> 00:19:01,080 when I first searched for the first evidence of extra terrestrial life in 1960. 201 00:19:01,115 --> 00:19:04,879 It's unlike the solar system because it has three other planets that are 202 00:19:04,914 --> 00:19:07,720 very close into the star, hot Jupiter-type planets. 203 00:19:09,800 --> 00:19:15,080 In the last decade, astronomers have found over 260 exoplanets, 204 00:19:15,115 --> 00:19:18,279 most of them searingly hot gas-giants. 205 00:19:19,920 --> 00:19:23,725 As a biologist, I don't really have a favourite exoplanet at the moment, 206 00:19:23,760 --> 00:19:27,999 because the astronomers keep finding me hot Jupiters and they don't do much good for biology. 207 00:19:28,034 --> 00:19:30,836 What I'm looking for is something really Earth-like. 208 00:19:30,871 --> 00:19:33,639 Something that's got a good chance of liquid water. 209 00:19:33,674 --> 00:19:35,320 And then I'll have a favourite. 210 00:19:40,719 --> 00:19:43,960 So just how rare is our blue planet? 211 00:19:47,879 --> 00:19:52,599 Dr Lynn Rothschild is an astrobiologist who has studied our own solar system 212 00:19:52,634 --> 00:19:55,999 in an effort to understand what makes Earth so special. 213 00:19:58,879 --> 00:20:03,519 Let's pretend that this fire here is our sun and that this rock is Venus. 214 00:20:03,554 --> 00:20:08,160 It's about as close to the sun you can get and still have liquid water. 215 00:20:08,195 --> 00:20:11,760 So the orbit of Venus would be, say, like this. 216 00:20:13,879 --> 00:20:18,320 This is getting pretty hot, cos I'm awfully close to the sun here. 217 00:20:20,679 --> 00:20:24,840 Now on the other extreme, this is Mars, which is the farthest planet 218 00:20:24,875 --> 00:20:27,877 from the sun that has any chance of liquid water. 219 00:20:27,912 --> 00:20:30,844 There's no liquid surface water today, but we know 220 00:20:30,879 --> 00:20:34,320 that there used to be in the past. So let's trace the orbit of Mars. 221 00:20:37,440 --> 00:20:42,520 Now right between these two circles, where the orbit of Mars would be 222 00:20:42,555 --> 00:20:47,005 and the orbit of Venus, this is where liquid water is stable. 223 00:20:47,040 --> 00:20:50,959 And right in this habitable zone in our solar system is planet Earth. 224 00:20:50,994 --> 00:20:53,960 Our beautiful watery world that's just covered with life. 225 00:21:00,220 --> 00:21:02,924 Just 10% closer in, 226 00:21:02,959 --> 00:21:04,720 and Earth would no longer be capable 227 00:21:04,755 --> 00:21:07,519 of supporting liquid water. 228 00:21:15,120 --> 00:21:20,040 Almost miraculously, Earth slots right into the heart of the habitable zone. 229 00:21:25,280 --> 00:21:30,639 Giving this once lifeless rock just the right elements for life to take hold and flourish. 230 00:21:41,119 --> 00:21:45,079 Over billions of years, microbes, plants and animals 231 00:21:45,114 --> 00:21:49,040 have transformed Earth into a living, breathing world. 232 00:21:56,199 --> 00:21:59,085 A world where one evolutionary line 233 00:21:59,120 --> 00:22:02,959 has led to modern humankind and civilisation. 234 00:22:11,720 --> 00:22:15,659 But even with our civilisation's most advanced technology, 235 00:22:15,694 --> 00:22:19,599 finding other planets like Earth has proved impossible. 236 00:22:21,759 --> 00:22:26,044 Rotator is vertical, angle mode zero. 237 00:22:26,079 --> 00:22:31,519 The very factors that enable life, a small planet at a safe distance from 238 00:22:31,554 --> 00:22:37,417 the sun, means the telltale wobbles that these planets produce are tiny. 239 00:22:37,452 --> 00:22:42,985 Our Earth, when it orbits the sun, causes our sun to wobble with a speed 240 00:22:43,020 --> 00:22:48,519 of 1/10th of one metre per second, a smaller motion than we can detect. 241 00:22:50,879 --> 00:22:56,919 Or there could be another, more profound explanation for the missing Earth-like planets. 242 00:22:58,479 --> 00:23:03,045 It's possible that other stars didn't have planets around them. 243 00:23:03,080 --> 00:23:07,719 That we're just one of the freaks of nature that grew up on a rocky planet. 244 00:23:09,479 --> 00:23:14,399 Either way, despite decades of searching, until 2007, 245 00:23:14,434 --> 00:23:17,319 Earth remained entirely alone. 246 00:23:31,040 --> 00:23:38,720 Between the Andes mountains and the Pacific Ocean, on the remote southern edge of the Atacama desert 247 00:23:38,755 --> 00:23:43,400 lies one of the most extraordinary observatories on Earth. 248 00:23:47,600 --> 00:23:50,484 The high elevation and the low rainfall, 249 00:23:50,519 --> 00:23:57,000 just one millimetre a year, makes it the perfect place for uninterrupted views of the southern night sky. 250 00:23:57,035 --> 00:24:00,159 Please come in, I have something to show you in here. 251 00:24:03,920 --> 00:24:11,199 Professor Stephane Udry is the proud owner of a machine which could change the course of human history. 252 00:24:11,234 --> 00:24:16,485 Inside this big box is an enclosure and inside there is a vacuum tank 253 00:24:16,520 --> 00:24:21,765 with the instrument, that is the most sensitive in the world now for planet detection. 254 00:24:21,800 --> 00:24:29,879 With this instrument we can detect low mass planet five, ten times the mass of the Earth. Can we go in? 255 00:24:29,914 --> 00:24:36,405 No. Of course not, because just opening the door will destroy the measurement for a few days. 256 00:24:36,440 --> 00:24:44,440 Because we need to have a very stable instrument to be able to repeat the measurement with the same precision, 257 00:24:44,475 --> 00:24:47,559 day after day, month after month, years after years. 258 00:24:53,640 --> 00:24:56,799 And that's exactly what they've been doing. 259 00:24:59,240 --> 00:25:04,680 They drew up a list of a thousand targets taken from the Gliese Catalogue of Nearby Stars 260 00:25:04,715 --> 00:25:07,204 and began measuring and re-measuring each candidate, 261 00:25:07,239 --> 00:25:12,479 hunting for wobbles that had previously been too small to detect. 262 00:25:23,559 --> 00:25:27,804 But one star caught Stephane's attention. 263 00:25:27,839 --> 00:25:33,964 Gliese 581 was in our target list since the beginning. 264 00:25:33,999 --> 00:25:41,200 Categorised as Gliese 581a, it's a red dwarf star, a third of the mass of our own sun. 265 00:25:45,200 --> 00:25:52,520 When the wobble was plotted it revealed 581b, a massive planet the size of Neptune, 266 00:25:52,555 --> 00:25:54,684 close into the star 267 00:25:54,719 --> 00:25:58,559 and orbiting once every 5½ days. 268 00:25:58,594 --> 00:26:00,525 It was no Earth, 269 00:26:00,560 --> 00:26:05,240 but the star's wobble held some fine detail that intrigued Stephane. 270 00:26:06,960 --> 00:26:10,765 We noticed that there was something else in the system. 271 00:26:10,800 --> 00:26:16,165 There seemed to another, smaller planet lurking in the detail. 272 00:26:16,200 --> 00:26:23,685 That something else could be a five Earth mass planet very close to the star. 273 00:26:23,720 --> 00:26:31,119 If Stephan's hunch was right, it would be the smallest planet ever detected around a distant sun. 274 00:26:31,154 --> 00:26:33,559 And this planet seemed to be habitable. 275 00:26:35,199 --> 00:26:41,000 We got excited because the distance was just right for the planet to possibly be in the habitable zone. 276 00:26:46,520 --> 00:26:49,605 After years of hunting, the search 277 00:26:49,640 --> 00:26:52,924 for the first "Second Earth" was over. 278 00:26:52,959 --> 00:26:56,559 European astronomers have spotted a new planet outside our solar system 279 00:26:56,594 --> 00:26:58,725 which closely resembles the planet Earth. 280 00:26:58,760 --> 00:27:02,280 The probability that there is life somewhere else in the Universe goes up a bit. 281 00:27:02,315 --> 00:27:06,120 This latest find has set the world of astronomy alight. 282 00:27:14,239 --> 00:27:17,319 For the Swiss team, the breakthrough was a triumph. 283 00:27:22,199 --> 00:27:26,919 It is always very exciting to be the first one to know. 284 00:27:26,954 --> 00:27:31,605 The discovery of Gliese 581c is a marvellous discovery. 285 00:27:31,640 --> 00:27:36,200 It shows how close we are were getting to planets that remind us of the Earth. 286 00:27:36,235 --> 00:27:39,804 It shows that potential life-bearing planets exist. 287 00:27:39,839 --> 00:27:44,600 When you know, when you realise it, and you are the only one, it's like being in the spaceship 288 00:27:44,635 --> 00:27:48,519 coming to a planet and being the first one to see the landscape. 289 00:27:50,559 --> 00:27:54,759 For those tempted to make the journey, pick a clear night 290 00:27:54,794 --> 00:27:57,719 and look for the constellation Libra. 291 00:27:57,754 --> 00:27:59,644 Invisible to the naked eye, 292 00:27:59,679 --> 00:28:04,760 Gliese 581 lies just north of the brightest star in the constellation. 293 00:28:08,640 --> 00:28:14,360 Remarkably, it's one of our closest neighbours, a shade over 20 light years distant. 294 00:28:17,360 --> 00:28:20,565 At the heart of the system is the parent star. 295 00:28:20,600 --> 00:28:28,239 Close by is 581b, 16 times more massive than Earth and too hot for life to survive. 296 00:28:30,080 --> 00:28:34,760 Beyond, just on the inner warm edge of the habitable zone, 297 00:28:34,795 --> 00:28:38,404 lies Gliese 581c - 298 00:28:38,439 --> 00:28:43,000 the smallest and most Earth-like exoplanet yet detected. 299 00:28:54,800 --> 00:29:02,079 At last, scientists have found another planet that may just be capable of supporting life. 300 00:29:05,559 --> 00:29:07,444 Not much out here. 301 00:29:07,479 --> 00:29:10,279 See if there's any under the rock. 302 00:29:10,314 --> 00:29:13,879 Nope. A lot of UV radiation. 303 00:29:22,119 --> 00:29:25,759 Nothing green, nothing coloured I can see. Very dry. 304 00:29:25,794 --> 00:29:31,404 For astrobiologists like Dr Lynn Rothschild, 305 00:29:31,439 --> 00:29:37,199 its discovery means they can begin to imagine what it would be like to spend a day on a Super-Earth. 306 00:29:43,080 --> 00:29:48,280 We're up here on the edge of the Atacama desert in Chile right near the Bolivian border. 307 00:29:50,319 --> 00:29:55,080 You can see it's very dry, in fact, one of the driest places on Earth. 308 00:29:58,920 --> 00:30:03,999 This is a great place to get an idea of what an extra-solar planet, 309 00:30:04,034 --> 00:30:06,679 for example Gliese 581c, might be like. 310 00:30:31,160 --> 00:30:36,604 Let's imagine that we're on Gliese 581c. 311 00:30:36,639 --> 00:30:40,319 There's an awful lot of rocks around. It's dry. 312 00:30:41,919 --> 00:30:45,619 The planet's mass is five times that of Earth. 313 00:30:45,654 --> 00:30:49,284 This means that gravity will pull twice as hard. 314 00:30:49,319 --> 00:30:52,160 Whereas on the moon, the astronauts could jump with no effort, 315 00:30:52,195 --> 00:30:56,405 on this planet you would be suffering from extra gravity. 316 00:30:56,440 --> 00:31:01,920 If you took a rock and you threw it, it would come crashing down, 317 00:31:01,955 --> 00:31:04,285 much faster than that of the Earth. 318 00:31:04,320 --> 00:31:08,679 High gravity will affect the look of the planet. No mountains. 319 00:31:08,714 --> 00:31:11,280 Just low hills and vast plains. 320 00:31:12,920 --> 00:31:16,485 And the last thing is it's close to the parent star, 321 00:31:16,520 --> 00:31:22,040 and so the radiation from the sun would be much stronger on Earth. 322 00:31:22,075 --> 00:31:25,400 Here we're getting burned, there we would probably be fried. 323 00:31:34,080 --> 00:31:37,244 The planet's red dwarf star will dominate the sky - 324 00:31:37,279 --> 00:31:40,959 a fiery ball five times larger than our own sun back home. 325 00:31:40,994 --> 00:31:43,204 And a few hours into their trip, 326 00:31:43,239 --> 00:31:48,159 interstellar visitors will discover that this sun never moves. 327 00:31:54,879 --> 00:31:57,159 The planet is so close to its star 328 00:31:57,194 --> 00:32:01,320 that immense gravitational forces have united the two. 329 00:32:04,520 --> 00:32:09,639 They're tidally locked, with the planet presenting just one face to the light. 330 00:32:12,399 --> 00:32:15,525 On the Earth, we're used to getting up in the morning, the sun rises. 331 00:32:15,560 --> 00:32:20,920 We have our midday meal, in the evening we have dinner, if we're lucky, we get a nice sunset. 332 00:32:20,955 --> 00:32:25,320 But on something like Gliese 581c it would be totally different. 333 00:32:30,279 --> 00:32:35,760 If I wanted to see the equivalent of a sunset, I'd be the one who'd have to get into the car and move. 334 00:32:39,879 --> 00:32:42,604 Beyond this point is the dark side 335 00:32:42,639 --> 00:32:47,999 of the planet, perpetually turned outwards to the cold of space. 336 00:32:49,800 --> 00:32:53,360 I wouldn't want to live here, I wouldn't want to be a colonist 337 00:32:53,395 --> 00:32:57,204 on another world that was barren like this. 338 00:32:57,239 --> 00:33:03,240 I'd take even a year-long field trip, but I wouldn't sign up for the rest of my life. 339 00:33:07,640 --> 00:33:12,525 Comfortable as Gliese 581c may be for a day trip, 340 00:33:12,560 --> 00:33:20,280 for life to exist there, for it truly to be second Earth, it must have one other vital ingredient. 341 00:33:31,560 --> 00:33:35,120 Water is the one thing life on Earth has in common, so we think 342 00:33:35,155 --> 00:33:37,244 looking for water on other planets 343 00:33:37,279 --> 00:33:39,680 is a way to look for life on those planets. 344 00:33:46,879 --> 00:33:50,600 For astrophysicists like Sean Raymond, finding water 345 00:33:50,635 --> 00:33:53,360 on other worlds is the key to finding life. 346 00:33:56,479 --> 00:34:02,199 Every day in his laboratory, he makes new solar systems from scratch. 347 00:34:02,234 --> 00:34:06,757 So the way we do this is computer simulations of a disc of rocks orbiting 348 00:34:06,792 --> 00:34:11,280 a star, and we let them collide and let their orbits evolve, and such. 349 00:34:11,315 --> 00:34:14,439 And it turns out these take quite a long time to do. 350 00:34:14,474 --> 00:34:18,324 Over the months, Sean's computer calculates 351 00:34:18,359 --> 00:34:22,599 how alien planetary systems evolve over millions of years. 352 00:34:33,399 --> 00:34:36,599 Here's a movie of one of these simulations. You can see everything 353 00:34:36,634 --> 00:34:39,045 on the inner disc starts off red meaning, quite dry. 354 00:34:39,080 --> 00:34:42,325 All these guys start off being the size of the moon, or actually a little smaller. 355 00:34:42,360 --> 00:34:47,245 And then the number of bodies is going down as they collide and grow into larger things. 356 00:34:47,280 --> 00:34:51,160 And by about 10 million years or so, a planet almost the size of the Earth is formed right there. 357 00:34:51,195 --> 00:34:53,600 And you can see it's still red. 358 00:34:55,239 --> 00:34:59,400 These new planets are all dry. Only far out from the star 359 00:34:59,435 --> 00:35:03,240 are temperatures low enough for water to collect. 360 00:35:05,439 --> 00:35:09,640 It's not until a little later... you'll see in a second, it gets collided by something that's 361 00:35:09,675 --> 00:35:15,037 blue and turns - right there, it went from being completely dry to having some water. 362 00:35:15,072 --> 00:35:20,400 And that process of water delivery continues over the next 100 million years or so. 363 00:35:23,120 --> 00:35:26,805 Over this time, icy comets and asteroids from 364 00:35:26,840 --> 00:35:32,679 the outer solar system are drawn inwards towards the young planets. 365 00:35:32,714 --> 00:35:37,400 Shaun's theory is they bring with them vast amounts of water, 366 00:35:40,080 --> 00:35:44,360 transforming dead worlds into blue planets. 367 00:35:44,395 --> 00:35:46,364 That's a pretty good Earth analogue. 368 00:35:46,399 --> 00:35:50,120 And we think this is how the solar system terrestrial planets formed. 369 00:35:51,759 --> 00:35:55,204 Sean has run hundreds of simulations. 370 00:35:55,239 --> 00:35:59,199 And each time, some something happens to the planets in the habitable zone - 371 00:36:01,040 --> 00:36:06,524 they nearly all have water. Water is very abundant. 372 00:36:06,559 --> 00:36:12,240 In the solar system, water is two to four times more abundant than rock and iron. 373 00:36:15,560 --> 00:36:19,680 It looks like Earth might, on average, be a little bit water-poor. 374 00:36:19,715 --> 00:36:23,800 And many planets may end up with a lot more water than the Earth. 375 00:36:23,835 --> 00:36:27,725 Including the newly discovered 581c. 376 00:36:27,760 --> 00:36:32,919 Gliese 581c especially, is very exciting, a very big discovery. 377 00:36:32,954 --> 00:36:37,077 These planets would have acquired some water-rich material, so they 378 00:36:37,112 --> 00:36:41,200 probably have some water contents comparable to Earth at least. 379 00:36:42,839 --> 00:36:45,244 Far from being a barren rock, 380 00:36:45,279 --> 00:36:49,359 this new planet may be awash with liquid water. 381 00:36:54,480 --> 00:36:58,564 But in their rush to tell the world of another world, 382 00:36:58,599 --> 00:37:03,600 the Swiss had overlooked one thing - the planet's atmosphere. 383 00:37:05,199 --> 00:37:08,205 We got very excited about Gliese 581c 384 00:37:08,240 --> 00:37:13,040 when we realised that it was just at the right distance from the star. 385 00:37:13,075 --> 00:37:17,805 But then, talking with specialists of the evolution of atmospheres 386 00:37:17,840 --> 00:37:21,800 on the planet, they told us that maybe the greenhouse effect could be big. 387 00:37:21,835 --> 00:37:25,804 And so the temperature could be too high for the development of life. 388 00:37:25,839 --> 00:37:30,960 If the planet's atmosphere contains too much water vapour or carbon dioxide... 389 00:37:30,995 --> 00:37:36,124 a runaway greenhouse effect could take hold. 390 00:37:36,159 --> 00:37:41,279 Rather than resembling Earth, 581c could be a super-Venus. 391 00:37:46,239 --> 00:37:48,124 Instead of liquid water, 392 00:37:48,159 --> 00:37:53,480 steam would shroud a searingly hot world, incapable of supporting life. 393 00:38:00,800 --> 00:38:03,924 It's probably too hot to be habitable. 394 00:38:03,959 --> 00:38:11,880 If it has water at all, which is doubtful, that water would be boiled off, evaporated and gone. 395 00:38:11,915 --> 00:38:17,364 581c may, after all, lie on the hot side of the habitable zone, 396 00:38:17,399 --> 00:38:22,399 but the light the Swiss team were collecting from the planet's star held another surprise. 397 00:38:24,479 --> 00:38:30,480 After decades of fruitless searching for habitable worlds, out popped another one. 398 00:38:32,079 --> 00:38:34,364 We had to wait for one more year 399 00:38:34,399 --> 00:38:39,879 before being able to actually find another planet a bit further out. 400 00:38:39,914 --> 00:38:42,480 There is a third planet in the system. 401 00:38:48,319 --> 00:38:54,520 They'd discovered a second super-Earth in the same system - 402 00:38:54,555 --> 00:38:56,840 Gliese 581d. 403 00:38:57,960 --> 00:39:02,739 This world lies on the far, cold edge of the habitable zone. 404 00:39:02,774 --> 00:39:07,484 On first calculations, this would make it a giant frozen world. 405 00:39:07,519 --> 00:39:14,399 But if it too enjoys a greenhouse effect, then it could be just warm enough for liquid water. 406 00:39:15,999 --> 00:39:20,400 If there is some atmosphere, and a greenhouse effect, then the temperature could 407 00:39:20,435 --> 00:39:22,959 be even better on that planet for the development of life. 408 00:39:28,119 --> 00:39:31,764 Perched on opposite edges of the habitable zone, 409 00:39:31,799 --> 00:39:35,439 the conditions on the planets in this system will be harsh. 410 00:39:37,040 --> 00:39:39,800 Perhaps too harsh for life to survive. 411 00:39:46,799 --> 00:39:52,959 Here on Earth, Dr Lynn Rothschild is investigating places where conditions mirror the extreme 412 00:39:52,994 --> 00:39:57,200 environments found on both the G581 planets. 413 00:39:59,079 --> 00:40:03,719 We're up here in the altiplano in Bolivia. Up at about... 414 00:40:03,754 --> 00:40:06,644 well over 4,000m, or 15,000ft. 415 00:40:06,679 --> 00:40:11,959 In the winter it's frozen - it's not a whole lot warmer in the summer, 416 00:40:11,994 --> 00:40:14,125 and yet life lives up here. 417 00:40:14,160 --> 00:40:19,799 Every place we've gone that's cold - the Antarctic, the ice caps, we've found life. 418 00:40:19,834 --> 00:40:23,360 And even under here, there's plenty that's growing. It's just amazing. 419 00:40:28,959 --> 00:40:33,840 So life in the freezing conditions of the outer planet is a possibility. 420 00:40:43,599 --> 00:40:49,360 And even the on the inner hotter world, where temperatures could exceed the boiling point of water, 421 00:40:49,395 --> 00:40:52,917 scientists are beginning to understand how life could survive. 422 00:40:52,952 --> 00:40:56,439 We don't actually know how life actually got started on Earth. 423 00:40:56,474 --> 00:40:59,919 But we do know that when we look at modern organisms, 424 00:40:59,954 --> 00:41:02,804 and at their evolution, the most ancient ones 425 00:41:02,839 --> 00:41:06,960 seem to be the ones that live at extremely high temperatures, 426 00:41:06,995 --> 00:41:09,320 just like these areas around here. 427 00:41:15,519 --> 00:41:17,725 Indeed, the more scientists look, 428 00:41:17,760 --> 00:41:22,800 the wider the range of habitats they find in which living organisms can thrive. 429 00:41:22,835 --> 00:41:27,920 So this gives us hope, this gives us optimism that when we go elsewhere 430 00:41:27,955 --> 00:41:31,045 to other worlds, that there might be life. 431 00:41:31,080 --> 00:41:35,840 For now, no-one knows for sure if life could survive in the massive, 432 00:41:35,875 --> 00:41:38,319 strange worlds of the G581 system. 433 00:41:43,520 --> 00:41:47,919 And Earth-bound planet hunting may have reached the end of the line. 434 00:41:47,954 --> 00:41:51,639 Because to find true Earth-sized planets, 435 00:41:51,674 --> 00:41:54,200 the hunt is moving into space. 436 00:42:11,679 --> 00:42:15,200 This is a spaceship factory. 437 00:42:17,960 --> 00:42:22,325 In these category A clean rooms, machines are built 438 00:42:22,360 --> 00:42:26,640 that their designers hope will unlock the secrets of the universe. 439 00:42:26,675 --> 00:42:29,240 That's the interferometer. 440 00:42:33,519 --> 00:42:38,204 There's the focus mechanisms right here. Here's one focus mechanism. 441 00:42:38,239 --> 00:42:42,960 This is the actual focus mechanism. This is the flight hardware. Wonderful. 442 00:42:42,995 --> 00:42:48,320 Today the team are midway through assembling their latest mission - 443 00:42:48,355 --> 00:42:51,519 the giant Kepler space telescope. 444 00:42:57,719 --> 00:43:05,119 But it's not scheduled to fly until 2009, so currently the spaceship is in bits. 445 00:43:05,154 --> 00:43:09,200 This is where the primary mirror is gonna sit, on top of this. 446 00:43:09,235 --> 00:43:13,277 So that measures how well you've got the optics aligned? 447 00:43:13,312 --> 00:43:17,284 That's right, you can measure how well it's working. 448 00:43:17,319 --> 00:43:21,360 Leading the NASA team assembling the space telescope is Bill Borucki. 449 00:43:21,395 --> 00:43:26,000 It's magnificent, it's just wonderful to see it come together. 450 00:43:26,035 --> 00:43:29,799 We've been planning this for years and years. 451 00:43:29,834 --> 00:43:31,324 So to actually see it here... 452 00:43:31,359 --> 00:43:34,160 This is the flight equipment, this will go into space. 453 00:43:34,195 --> 00:43:36,165 It's this that will make our discovery. 454 00:43:36,200 --> 00:43:41,080 I'm delighted to see all the details that seem to be right. 455 00:43:46,399 --> 00:43:52,359 When Kepler flies, it will undertake a four-year mission to seek out new worlds. 456 00:43:54,519 --> 00:43:56,924 But it won't be looking for wobbles. 457 00:43:56,959 --> 00:44:04,640 Instead, Kepler will be hunting for planets that pass in front of their stars, creating a tell-tale wink. 458 00:44:04,675 --> 00:44:08,840 Looking at the star, it seems to wink, it gets dimmer for a while. 459 00:44:08,875 --> 00:44:12,124 Like it closed its eye for a second and then opened it. 460 00:44:12,159 --> 00:44:16,359 This is because the planet moved in front of it and blocked some of its light. 461 00:44:16,394 --> 00:44:18,604 It happens in our solar system too. 462 00:44:18,639 --> 00:44:25,640 We had a transit, Mercury going in front of the sun fairly recently, we could see that with a telescope. 463 00:44:27,600 --> 00:44:30,439 For the wink technique to work, 464 00:44:30,474 --> 00:44:33,244 a space telescope is essential. 465 00:44:33,279 --> 00:44:36,924 Free from the interference of Earth's atmosphere, it gives 466 00:44:36,959 --> 00:44:41,279 Kepler an uninterrupted view of a very special part of the galaxy. 467 00:44:41,314 --> 00:44:43,764 Kepler only looks at one area of the sky. 468 00:44:43,799 --> 00:44:47,960 It's a good area for us, in that it has a huge number of stars. 469 00:44:47,995 --> 00:44:51,279 Kepler will scan the same 100,000 stars 470 00:44:51,314 --> 00:44:54,285 over its entire four-year mission, 471 00:44:54,320 --> 00:44:58,524 constantly measuring the brightness of each one. 472 00:44:58,559 --> 00:45:04,139 And from day one, it will be sensitive enough to detect the wink 473 00:45:04,174 --> 00:45:09,719 of an Earth-sized planet crossing its sun, tens of light years away. 474 00:45:12,440 --> 00:45:18,279 It's always very exciting, because we've always wanted to know - are there lots of Earths out there? 475 00:45:18,314 --> 00:45:22,440 Geoff Marcy and Stephane Udry and all these other people are extremely competitive. 476 00:45:22,475 --> 00:45:25,564 They want to find planets, they want the answers too. 477 00:45:25,599 --> 00:45:29,319 Well, we all do, and the best way to do that is to co-operate. 478 00:45:29,354 --> 00:45:33,004 There's a bit of a race going on, but it's a delightful race. 479 00:45:33,039 --> 00:45:38,519 The competition is lovely, and it makes us get up in the morning, go to work, and work a little harder. 480 00:45:38,554 --> 00:45:41,245 So who's gonna find the first Earth-sized object? 481 00:45:41,280 --> 00:45:46,239 We are. Kepler's going to find the first Earths in the habitable zone. 482 00:45:49,240 --> 00:45:56,639 Between them, the planet hunters are beginning to define the first galactic map of Earth-like worlds. 483 00:46:00,879 --> 00:46:07,920 At last, a phone directory for those listening for a message from ET. 484 00:46:11,359 --> 00:46:14,120 They're gonna allow us to sharpen our gaze of the heavens, 485 00:46:14,155 --> 00:46:16,045 where we're pointing these antennas, 486 00:46:16,080 --> 00:46:20,400 trying to pick up a signal, they're gonna tell us, "You don't have to look at every star, 487 00:46:20,435 --> 00:46:24,537 "these ones have planets", and eventually they'll be able to say, 488 00:46:24,572 --> 00:46:28,639 "These are the ones that have planets the same size as Earth." 489 00:46:28,674 --> 00:46:31,444 And ten years after that, they'll be able to say, 490 00:46:31,479 --> 00:46:36,205 "These are the ones with oxygen or methane in their atmosphere. 491 00:46:36,240 --> 00:46:41,400 "So they have some biology, and it's up to you to find out if any of that biology is smart or not." 492 00:46:43,760 --> 00:46:49,799 Rather than the entire galaxy of 200 billion stars, in the future, SETI 493 00:46:49,834 --> 00:46:55,839 need only tune into the handful of star systems that Kepler discovers. 494 00:47:01,680 --> 00:47:05,404 Everything has caused us to become more optimistic. 495 00:47:05,439 --> 00:47:09,320 We really believe in the next 20 years or so, we're going to learn 496 00:47:09,355 --> 00:47:12,760 a great deal more about life beyond Earth 497 00:47:12,795 --> 00:47:16,404 detected that life 498 00:47:16,439 --> 00:47:21,004 and perhaps even intelligent life elsewhere in our galaxy. 499 00:47:21,039 --> 00:47:26,004 Remember, there's a flip side to this - it could be that advanced 500 00:47:26,039 --> 00:47:32,880 technological civilisations, species, are a rarity, one in a million, maybe one in a billion. 501 00:47:32,915 --> 00:47:38,640 If so, we humans could be quite a precious rarity in the Milky Way galaxy. 502 00:47:38,675 --> 00:47:42,405 Maybe, in fact, they're not out there watching us. 503 00:47:42,440 --> 00:47:46,920 We may be the ones to be the first to go out and explore the galaxy. 504 00:47:55,519 --> 00:47:57,365 If you'd like to explore 505 00:47:57,400 --> 00:47:59,420 Dr Frank Drake's famous equation 506 00:47:59,455 --> 00:48:01,405 and come up with your own estimate 507 00:48:01,440 --> 00:48:05,280 of the number of alien civilisations in the galaxy, 508 00:48:05,315 --> 00:48:08,119 log on to: