WEBVTT 00:00:14.681 --> 00:00:17.244 At this precise moment 00:00:17.268 --> 00:00:21.801 on a planet far, far away... 00:00:42.761 --> 00:00:46.521 ..an alien sunrise ushers in a new day. 00:00:59.481 --> 00:01:02.401 But will alien eyes gaze upon it? 00:01:16.481 --> 00:01:18.081 Or will it go unseen? 00:01:23.121 --> 00:01:27.361 Just another moment in a vast, sterile universe? 00:01:36.681 --> 00:01:39.401 The hunt is on for the answer. 00:01:48.441 --> 00:01:50.401 Magnificent desolation. 00:01:51.641 --> 00:01:54.001 Beautiful, beautiful! Ain't that something? 00:02:18.521 --> 00:02:19.681 The Milky Way... 00:02:21.001 --> 00:02:24.041 ..hundreds of billions of stars... 00:02:31.521 --> 00:02:35.361 ..spread across 100,000 light-years of space. 00:02:41.521 --> 00:02:43.761 Among them, the sun... 00:02:48.161 --> 00:02:53.001 ..with eight planets orbiting around it, including our home. 00:02:55.761 --> 00:03:00.361 Until very recently, these were the only worlds we knew of, 00:03:00.361 --> 00:03:03.401 the only planets we could hope to explore 00:03:03.401 --> 00:03:05.561 for signs of life beyond Earth. 00:03:16.161 --> 00:03:19.121 When I first got into astronomy back in the 1970s, 00:03:19.121 --> 00:03:23.001 we knew of no planets beyond our Solar System. 00:03:23.001 --> 00:03:25.281 We didn't have the technology to detect them 00:03:25.281 --> 00:03:27.921 even if they were there. 00:03:27.921 --> 00:03:32.161 Our neighbourhood was the only place we could look for life. 00:03:35.401 --> 00:03:39.201 And so the hunt for life began in our own backyard. 00:03:46.161 --> 00:03:47.761 Over the last few decades, 00:03:47.761 --> 00:03:51.601 multiple missions have explored our Solar System's planets... 00:03:58.321 --> 00:04:00.281 ..and even some of their moons. 00:04:07.561 --> 00:04:13.881 But to date, even as we continue to look, no convincing evidence of life 00:04:13.881 --> 00:04:16.441 has been found on any of these worlds. 00:04:24.161 --> 00:04:26.641 Earth remains one of a kind... 00:04:32.081 --> 00:04:35.201 ..the only living world around the sun. 00:04:44.881 --> 00:04:48.961 But as the exploration of the Solar System continued, 00:04:48.961 --> 00:04:50.761 another search had begun... 00:04:52.761 --> 00:04:55.921 ...for worlds that lie far beyond these shores. 00:04:57.881 --> 00:05:00.161 You know, the wonderful thing about astronomy is that 00:05:00.161 --> 00:05:02.761 as we develop better and better technology 00:05:02.761 --> 00:05:06.681 and accumulate more and more knowledge about our universe, 00:05:06.681 --> 00:05:10.881 we turn more and more of these points of light in the sky 00:05:10.881 --> 00:05:12.801 into worlds. 00:05:12.801 --> 00:05:17.481 I mean, that, we've known is a world for a long time 00:05:17.481 --> 00:05:19.641 because that is the planet Mars. 00:05:19.641 --> 00:05:23.841 But just above Mars tonight is a constellation called Pegasus. 00:05:23.841 --> 00:05:27.081 This is the Square of Pegasus. 00:05:27.081 --> 00:05:34.681 And we now know that around there is a star called 51 Pegasi, 00:05:34.681 --> 00:05:37.921 which has a planet orbiting around it, 00:05:37.921 --> 00:05:40.801 a gas giant about the size of Jupiter 00:05:40.801 --> 00:05:46.401 that goes round that faint point of light every four days. 00:05:46.401 --> 00:05:49.961 It is wonderful to think that in my lifetime - 00:05:49.961 --> 00:05:53.081 in fact, in my adult lifetime, in the last 25 years - 00:05:53.081 --> 00:05:55.441 we've gone from a universe 00:05:55.441 --> 00:05:59.361 that could have been devoid of planets beyond our Solar System 00:05:59.361 --> 00:06:04.281 to a universe that we know is teeming with places 00:06:04.281 --> 00:06:06.321 that we can search for life. 00:06:16.081 --> 00:06:19.801 Over the last three decades, some of the most powerful telescopes 00:06:19.801 --> 00:06:22.001 on Earth have joined the hunt... 00:06:29.081 --> 00:06:32.441 ..searching for planets unimaginably far away... 00:06:36.121 --> 00:06:38.201 ..hiding in the dark. 00:06:44.681 --> 00:06:47.521 Planets like 51 Pegasi b... 00:06:53.001 --> 00:06:56.641 ..the first world outside our Solar System to be detected 00:06:56.641 --> 00:06:58.521 around a sunlike star. 00:07:08.961 --> 00:07:15.161 51 Pegasi b is a gas giant, around half the mass of Jupiter... 00:07:18.081 --> 00:07:20.441 ..but far closer to its star. 00:07:25.281 --> 00:07:29.081 Just imagine what that world might be like... 00:07:36.001 --> 00:07:40.041 ..a world with skies torn by titanic winds... 00:07:46.201 --> 00:07:51.641 ..where its hot interior is bathed in rain of sapphires. 00:07:57.321 --> 00:08:02.281 In every sense, 51 Pegasi b is an alien world. 00:08:04.881 --> 00:08:08.401 And we soon discovered that the galaxy is full of planets 00:08:08.401 --> 00:08:11.401 unlike anything seen in our Solar System... 00:08:21.761 --> 00:08:25.161 ..planets enveloped by fierce radiation... 00:08:30.641 --> 00:08:33.081 ..their surfaces battered and stripped 00:08:33.081 --> 00:08:37.001 by the high-energy strobing light of their star... 00:08:43.921 --> 00:08:48.521 ..worlds so cold, their atmospheres have frozen solid... 00:09:08.561 --> 00:09:10.561 ..or great swollen planets... 00:09:15.401 --> 00:09:18.361 ..with the density of Styrofoam... 00:09:23.041 --> 00:09:25.041 ..and fathomless atmospheres. 00:09:30.001 --> 00:09:35.561 These discoveries proved that, in one sense, we really are not alone. 00:09:36.641 --> 00:09:41.681 There are other worlds out there waiting to be explored. 00:09:51.441 --> 00:09:54.401 We estimate that in the Milky Way galaxy, 00:09:54.401 --> 00:09:59.441 there are more planets than stars - hundreds of billions of them. 00:10:01.721 --> 00:10:06.041 That's hundreds of billions of places to look for life. 00:10:08.921 --> 00:10:14.161 But there's a catch, because not all worlds - by a long stretch - 00:10:14.161 --> 00:10:15.521 are like this one. 00:10:27.961 --> 00:10:32.601 The first planets we found appeared too bizarre, too large 00:10:32.601 --> 00:10:37.281 and often too close to their stars for living things to survive. 00:10:47.041 --> 00:10:50.881 To find worlds where life could exist, we needed to look for 00:10:50.881 --> 00:10:55.601 smaller, rocky planets in orbits further from their stars. 00:10:58.441 --> 00:11:02.121 T-minus 10, 9, 8, 7... 00:11:02.121 --> 00:11:05.641 We needed to look for another Earth... 00:11:05.641 --> 00:11:08.241 ..3, 2... 00:11:08.241 --> 00:11:13.761 Engine start. 1, 0, and liftoff of the Delta II rocket with Kepler, 00:11:13.761 --> 00:11:17.041 on a search for planets in some way like our own. 00:11:19.841 --> 00:11:22.281 ..so the hunt moved to space 00:11:22.281 --> 00:11:25.441 with the launch of Nasa's Kepler Space Telescope... 00:11:27.881 --> 00:11:29.561 And we have separation. 00:11:33.361 --> 00:11:38.681 ..searching for Earth-like worlds in the galaxy beyond. 00:11:47.841 --> 00:11:51.441 Kepler crossed 94 million miles of space... 00:11:56.001 --> 00:11:59.521 ..until it arrived in a steady orbit around the sun... 00:12:09.161 --> 00:12:13.281 ..from where it looked out with a fixed and clear gaze... 00:12:17.281 --> 00:12:21.801 ..to a single patch of sky in the constellation of Cygnus. 00:12:34.481 --> 00:12:37.081 Exposing its sensitive light meter... 00:12:42.961 --> 00:12:46.401 ..to the light of 150,000 stars... 00:12:53.641 --> 00:12:58.201 ..it began to look for Earth-like alien worlds. 00:13:18.721 --> 00:13:23.121 Kepler doesn't detect planets directly. They are far too small. 00:13:23.121 --> 00:13:26.521 They're just specks of dust relative to their parent star. 00:13:28.481 --> 00:13:29.961 They're also very faint. 00:13:29.961 --> 00:13:34.561 They don't emit light of their own, so they just glow very dimly 00:13:34.561 --> 00:13:37.721 in the reflected ambient light of their stars. 00:13:37.721 --> 00:13:41.361 So, Kepler has to detect planets indirectly. 00:13:41.361 --> 00:13:46.641 Imagine that a moth just flew across the beam of light 00:13:46.641 --> 00:13:49.721 from the lighthouse. Now, I wouldn't see the moth, 00:13:49.721 --> 00:13:51.961 but if I had a sensitive enough detector 00:13:51.961 --> 00:13:53.761 and everything was lined up properly, 00:13:53.761 --> 00:13:58.681 I might just see the brightness of the light dim. 00:13:58.681 --> 00:14:01.521 And that is how Kepler detects planets. 00:14:01.521 --> 00:14:03.321 We imagine there's an alien astronomer 00:14:03.321 --> 00:14:06.161 in some distant solar system looking back at the sun, 00:14:06.161 --> 00:14:09.161 and everything's lined up so they see the Earth 00:14:09.161 --> 00:14:11.921 trace across the face of our star. 00:14:11.921 --> 00:14:17.281 They would see the light from the sun dim by one-hundredth of 1%. 00:14:17.281 --> 00:14:20.561 It's a tiny amount, but it's enough. 00:14:20.561 --> 00:14:23.721 And if they saw that dimming was regular, 00:14:23.721 --> 00:14:27.041 if they saw the star dim once every year, in this case, 00:14:27.041 --> 00:14:32.681 then they would infer that there's a planet orbiting around a star. 00:14:51.241 --> 00:14:54.321 With its exquisitely sensitive light meter... 00:14:58.681 --> 00:15:02.881 ..Kepler sees only the regular dimming of pixels... 00:15:07.161 --> 00:15:09.401 ..just a few bits of information. 00:15:13.721 --> 00:15:17.921 But from those bits, astronomers can begin to build a picture 00:15:17.921 --> 00:15:20.601 of the worlds that dim the starlight... 00:15:28.681 --> 00:15:33.241 ..worlds that might, in some way, resemble our own... 00:15:42.121 --> 00:15:45.441 ..worlds like Kepler-36b. 00:15:54.841 --> 00:15:58.641 The planet was one of Kepler's earliest discoveries. 00:16:06.161 --> 00:16:09.561 Orbiting a star similar to our own, 00:16:09.561 --> 00:16:14.481 we'd found a world that, at first glance, might seem familiar. 00:16:20.081 --> 00:16:24.361 Weighing in at around four times the mass of our own planet, 00:16:24.361 --> 00:16:29.561 Kepler-36b was one of the first of a new class of planet - 00:16:29.561 --> 00:16:31.161 a super-earth. 00:16:41.201 --> 00:16:44.241 The Kepler data doesn't just allow us to say there's a planet 00:16:44.241 --> 00:16:49.281 around that star. It allows us to characterise those planets. 00:16:49.281 --> 00:16:53.041 So by looking at the precise way that the light fades 00:16:53.041 --> 00:16:57.041 and then rises again, and the timing between the dips, 00:16:57.041 --> 00:17:00.201 we can measure the orbits of the planets. 00:17:00.201 --> 00:17:02.401 And if there are multiple planets in the system, 00:17:02.401 --> 00:17:04.801 we can even estimate their masses, 00:17:04.801 --> 00:17:09.041 so the Kepler data allows astronomers to paint a picture 00:17:09.041 --> 00:17:11.001 of the worlds it discovers. 00:17:24.521 --> 00:17:28.561 But the more detailed our picture of Kepler-36b became... 00:17:30.241 --> 00:17:34.561 ..the less Earth-like this super-earth appeared to be. 00:17:42.521 --> 00:17:48.641 It orbits very close to its star, circling once every 14 days. 00:17:56.041 --> 00:17:58.241 And it has company... 00:18:01.481 --> 00:18:03.841 ..a gigantic gaseous companion... 00:18:05.761 --> 00:18:09.801 ..with an albeit unusually close to its smaller sibling. 00:18:11.201 --> 00:18:15.201 The proximity of both its star and sister planet 00:18:15.201 --> 00:18:18.521 allows us to imagine the bizarre conditions 00:18:18.521 --> 00:18:22.361 that may exist on the surface of Kepler-36b. 00:18:33.481 --> 00:18:36.161 The planet may be tidally locked, 00:18:36.161 --> 00:18:40.361 which would mean that one hemisphere always faces the star. 00:18:47.961 --> 00:18:53.601 On this side, the punishing heat could turn the ground molten... 00:18:57.801 --> 00:19:01.961 ..creating rivers of lava that would crisscross the surface. 00:19:12.081 --> 00:19:15.441 The planet could experience violent eruptions... 00:19:16.561 --> 00:19:19.401 ..as the gravitational pull of the gas giant 00:19:19.401 --> 00:19:21.681 triggers intense volcanism... 00:19:29.441 --> 00:19:32.441 ..each time it passes by. 00:19:56.721 --> 00:19:59.401 But Kepler-36b could also be... 00:20:00.641 --> 00:20:02.361 ..a planet of ice. 00:20:10.881 --> 00:20:13.161 Because if it's tidally locked, 00:20:13.161 --> 00:20:17.681 the far side would face permanently away from the star... 00:20:21.001 --> 00:20:24.281 ..and we could imagine a freezing cold hemisphere 00:20:24.281 --> 00:20:26.961 shrouded in eternal darkness. 00:20:41.401 --> 00:20:45.321 For now, this is all just informed speculation. 00:20:48.001 --> 00:20:52.481 But we are beginning to build a picture of these worlds. 00:20:52.481 --> 00:20:56.641 I mean, imagine a world where the sun stays at the same point 00:20:56.641 --> 00:20:59.161 in the sky forever, 00:20:59.161 --> 00:21:02.281 so one side of the planet is in eternal night 00:21:02.281 --> 00:21:04.721 and the other side in eternal day. 00:21:04.721 --> 00:21:09.441 And even the twilight strip between day and night, 00:21:09.441 --> 00:21:12.521 we think, would suffer from extreme conditions. 00:21:17.481 --> 00:21:21.761 So Kepler-36b just goes to show there's so much more 00:21:21.761 --> 00:21:27.441 to having a habitable world than just the composition of the planets. 00:21:27.441 --> 00:21:29.241 There's the details of its orbits 00:21:29.241 --> 00:21:34.321 and also the nature of the other objects in the solar system 00:21:34.321 --> 00:21:37.081 that are orbiting around the star with it. 00:21:52.081 --> 00:21:55.761 Kepler-36b is just one of thousands of planets 00:21:55.761 --> 00:21:58.041 that Kepler has discovered. 00:22:04.921 --> 00:22:08.841 We now know beyond doubt that our galaxy is home 00:22:08.841 --> 00:22:12.841 to a diverse collection of alien worlds. 00:22:20.321 --> 00:22:24.521 Each one of the over 4,000 planets that we've discovered to date 00:22:24.521 --> 00:22:26.481 is different from all the others. 00:22:26.481 --> 00:22:29.361 They really are an alien and exotic bunch, 00:22:29.361 --> 00:22:32.401 and there's certainly no planet that's identical to the planets 00:22:32.401 --> 00:22:34.201 that we know of in our Solar System. 00:22:36.481 --> 00:22:40.121 And I think that reveals a deep truth about the universe, 00:22:40.121 --> 00:22:44.681 because although the laws of nature that form the planets are simple 00:22:44.681 --> 00:22:48.121 and the same everywhere and the fundamental ingredients 00:22:48.121 --> 00:22:52.561 out of which the planets are made are simple and the same everywhere, 00:22:52.561 --> 00:22:57.161 the nature of a planet also depends on the history of its formation 00:22:57.161 --> 00:23:00.041 and the environment around its parent star 00:23:00.041 --> 00:23:01.961 out of which the planet formed. 00:23:01.961 --> 00:23:05.201 And those are all radically different. 00:23:07.281 --> 00:23:09.881 So, each planet has a different story to tell. 00:23:09.881 --> 00:23:13.081 I suppose, in that sense, planets are like human beings. 00:23:13.081 --> 00:23:17.281 And this wholly unexpected but exciting discovery 00:23:17.281 --> 00:23:20.201 certainly complicates the search for life. 00:23:27.641 --> 00:23:29.961 We needed to narrow the search... 00:23:31.241 --> 00:23:36.481 ..for planets further but not too far away from their parent stars... 00:23:40.841 --> 00:23:45.161 ..planets at just the right distance for their surfaces 00:23:45.161 --> 00:23:47.681 potentially to be habitable... 00:23:50.601 --> 00:23:54.201 ..alien worlds with one precious ingredient 00:23:54.201 --> 00:23:57.121 that makes Earth a living planet. 00:24:17.961 --> 00:24:19.721 Now, you might legitimately ask, 00:24:19.721 --> 00:24:24.201 "Can we transfer all the knowledge we have of life here on Earth 00:24:24.201 --> 00:24:26.801 "to planets elsewhere in the universe?" 00:24:26.801 --> 00:24:30.321 Well, I would answer emphatically, yes, we can, 00:24:30.321 --> 00:24:32.481 because the laws of nature are universal. 00:24:32.481 --> 00:24:36.641 So, the laws of physics and chemistry that underpin biology here 00:24:36.641 --> 00:24:41.761 on this planet will apply to every planet out there in the universe, 00:24:41.761 --> 00:24:44.481 whether we've discovered it or not. 00:24:51.161 --> 00:24:54.521 The chemistry of life requires a few basic ingredients - 00:24:54.521 --> 00:24:57.961 carbon, nitrogen, oxygen, iron. 00:24:59.361 --> 00:25:02.961 And it also requires a ready supply of high-quality energy 00:25:02.961 --> 00:25:07.001 from heats within the planets or perhaps from starlight. 00:25:11.521 --> 00:25:17.041 But life here on Earth also requires one very important, fundamental 00:25:17.041 --> 00:25:20.041 extra ingredient, which is liquid water. 00:25:20.041 --> 00:25:23.361 Liquid water is a deceptively complicated substance. 00:25:23.361 --> 00:25:26.881 It's a very powerful solvent, but it also has structures 00:25:26.881 --> 00:25:30.161 which are constantly forming and disappearing within it, 00:25:30.161 --> 00:25:34.841 which act as a kind of scaffolding around which biology happens. 00:25:38.921 --> 00:25:42.521 Organic molecules are orientated by that scaffolding 00:25:42.521 --> 00:25:44.481 so they can react together. 00:25:46.161 --> 00:25:50.921 Now, it is certain that every living thing here on Earth 00:25:50.921 --> 00:25:54.081 requires liquid water to survive, 00:25:54.081 --> 00:25:58.681 and I would say it is a very good assumption that every living thing 00:25:58.681 --> 00:26:02.961 anywhere out there in the universe will require it too. 00:26:24.881 --> 00:26:27.321 The universe is filled with water. 00:26:28.681 --> 00:26:32.441 Great reservoirs have been detected throughout the galaxy 00:26:32.441 --> 00:26:35.481 amongst the gas clouds of giant nebulae. 00:26:40.001 --> 00:26:44.681 But just because water is plentiful, that doesn't mean that 00:26:44.681 --> 00:26:48.801 it necessarily ends up in oceans on planetary surfaces. 00:26:56.201 --> 00:26:58.921 Of the eight planets in our Solar System, 00:26:58.921 --> 00:27:04.401 only one has liquid water flowing permanently on its surface today... 00:27:09.761 --> 00:27:14.521 ..an ocean world where, long ago, life began. 00:27:33.961 --> 00:27:37.681 Around 4 billion years ago, life on Earth would have begun, 00:27:37.681 --> 00:27:41.321 probably in places not dissimilar to this, 00:27:41.321 --> 00:27:46.241 where there's geothermal activity, a source of energy in contact 00:27:46.241 --> 00:27:51.041 with rich concentrations of reactive chemical elements and minerals, 00:27:51.041 --> 00:27:58.121 but also, crucially, that - the magical solvent, liquid water. 00:27:58.121 --> 00:28:03.841 Now, many rocky planets out there in the galaxy will probably have this, 00:28:03.841 --> 00:28:08.241 but far fewer, we think, will have that - 00:28:08.241 --> 00:28:11.961 large bodies of liquid water on the surface. 00:28:11.961 --> 00:28:14.361 So that's why there's a kind of a catchphrase 00:28:14.361 --> 00:28:16.641 in the astrobiology community, which is, 00:28:16.641 --> 00:28:21.481 "If you want to search for life, follow the water." 00:28:35.561 --> 00:28:37.521 Whilst life on Earth was evolving... 00:28:45.001 --> 00:28:47.161 ..124 light-years away... 00:28:48.441 --> 00:28:52.401 ..amidst a collapsing cloud of gas, dust and ice... 00:28:57.521 --> 00:28:59.281 ..a small star was born... 00:29:06.441 --> 00:29:12.201 ..and the cloud-swirling leftovers condensed to form a brand-new world. 00:29:19.521 --> 00:29:22.201 In 2015, Kepler found a planet 00:29:22.201 --> 00:29:26.281 orbiting comfortably within its star's habitable zone. 00:29:41.521 --> 00:29:46.881 More than eight times the mass of the Earth, K2-18b is a giant... 00:29:56.961 --> 00:29:59.601 ..with a powerful gravitational pull. 00:30:09.801 --> 00:30:13.281 If the planet is rocky, this may have allowed it to hang on 00:30:13.281 --> 00:30:14.921 to a thick atmosphere. 00:30:18.081 --> 00:30:22.921 K2-18b might have all the makings of a water world. 00:30:28.721 --> 00:30:32.841 And a legendary space telescope had Kepler's new discovery 00:30:32.841 --> 00:30:34.201 in its sights. 00:30:40.601 --> 00:30:45.521 The most powerful space telescope of them all had joined the hunt. 00:30:51.441 --> 00:30:56.161 Hubble examined the light from K2-18b's parent star 00:30:56.161 --> 00:30:58.481 as the planet passed in front of it... 00:31:00.601 --> 00:31:04.881 ..and detected what may be a faint signature... 00:31:06.601 --> 00:31:08.041 ..of water vapour. 00:31:13.441 --> 00:31:15.921 124 light-years from Earth... 00:31:17.441 --> 00:31:21.641 ..we may have at last found the evidence of water 00:31:21.641 --> 00:31:23.001 on an alien world. 00:31:24.801 --> 00:31:29.201 This was the first observation of water vapour in the atmosphere 00:31:29.201 --> 00:31:33.641 of a planet orbiting in the habitable zone around its star. 00:31:33.641 --> 00:31:36.161 Now, admittedly, measurements of the amount of water vapour 00:31:36.161 --> 00:31:37.601 in the atmosphere is pretty wide. 00:31:37.601 --> 00:31:42.241 It's somewhere between 0.01% and 50%. 00:31:42.241 --> 00:31:44.321 I mean, this is a planet that's a long way away, 00:31:44.321 --> 00:31:51.321 but for comparison, our planet has a few percent water vapour in its atmosphere. 00:31:51.321 --> 00:31:54.001 So, that observation is important for two reasons. 00:31:54.001 --> 00:31:56.281 One is, it is not zero. 00:31:56.281 --> 00:31:58.641 There is water vapour in the atmosphere. 00:31:58.641 --> 00:32:02.401 But secondly, if the measurement is at the lower end, 00:32:02.401 --> 00:32:05.281 a few percent of the water vapour in the atmosphere, 00:32:05.281 --> 00:32:10.201 then that is consistent with this world being a planet 00:32:10.201 --> 00:32:14.001 with oceans on its surface. 00:32:20.081 --> 00:32:22.841 The nature of this planet is currently the subject 00:32:22.841 --> 00:32:24.961 of intense scientific debate. 00:32:26.761 --> 00:32:29.961 The planet may be more like a mini-Neptune... 00:32:31.361 --> 00:32:32.561 ..a gas planet. 00:32:36.721 --> 00:32:40.721 But it is possible to dream of a rocky alien world 00:32:40.721 --> 00:32:42.841 with skies full of clouds... 00:32:47.441 --> 00:32:49.561 ..where water droplets collect... 00:32:51.841 --> 00:32:53.281 ..and eventually fall... 00:32:58.561 --> 00:33:01.001 ..feeding vast oceans... 00:33:03.401 --> 00:33:07.241 ..that cover the surface of a massive planet... 00:33:10.361 --> 00:33:11.801 ..a water world... 00:33:20.801 --> 00:33:23.041 ..where the elixir of life... 00:33:24.841 --> 00:33:26.681 ..is in plentiful supply. 00:33:33.841 --> 00:33:38.201 K2-18b is exciting because it's the smallest world 00:33:38.201 --> 00:33:40.921 with an atmosphere that we've been able to analyse, 00:33:40.921 --> 00:33:43.801 and we've found that its mass and density, 00:33:43.801 --> 00:33:46.921 composition of its atmosphere and its orbits 00:33:46.921 --> 00:33:50.241 are consistent with it being a world with water. 00:33:51.841 --> 00:33:56.081 And it might be a world with oceans on its surface. 00:33:56.081 --> 00:33:57.961 We don't know for sure. 00:33:57.961 --> 00:34:02.241 But just imagine what that small, faraway world 00:34:02.241 --> 00:34:05.761 around a faint red star might be like. 00:34:13.321 --> 00:34:16.561 Kepler went on to make many more discoveries... 00:34:27.921 --> 00:34:33.641 ..until, in October 2018, it finally ran out of fuel. 00:34:40.481 --> 00:34:46.441 After nine years, it had found over 2,500 alien worlds... 00:34:53.521 --> 00:34:55.721 ..showing us just how common 00:34:55.721 --> 00:34:58.601 potentially Earth-like planets might be. 00:35:12.361 --> 00:35:18.441 We estimate that there may be around 20 billion potentially Earth-like worlds - 00:35:18.441 --> 00:35:22.041 that's rocky planets in the habitable zone around a star 00:35:22.041 --> 00:35:26.921 that may support liquid water on the surface - in our galaxy. 00:35:26.921 --> 00:35:31.121 That is 20 billion potential homes for life. 00:35:42.041 --> 00:35:45.921 Now, we don't know the probability that, given the right conditions, 00:35:45.921 --> 00:35:51.761 life will begin on a planet, but we do have evidence from our world. 00:35:51.761 --> 00:35:55.401 What we know is that here on Earth, life began pretty much 00:35:55.401 --> 00:35:59.241 as soon as it could after the Earth had formed and cooled down 00:35:59.241 --> 00:36:01.761 and the oceans formed on its surface. 00:36:01.761 --> 00:36:05.441 So that might suggest that whilst there isn't a sense of inevitability 00:36:05.441 --> 00:36:08.281 about the origin of life given the right conditions, 00:36:08.281 --> 00:36:11.841 it might at least be reasonably probable, 00:36:11.841 --> 00:36:16.921 so I think that there is at least a chance that life may have begun 00:36:16.921 --> 00:36:21.081 on some, perhaps many, of those 20 billion Earth-like worlds 00:36:21.081 --> 00:36:22.961 out there in our galaxy. 00:36:25.081 --> 00:36:29.401 But I think there are two questions about life. 00:36:29.401 --> 00:36:32.961 One question is about the origin and the existence of microbes. 00:36:32.961 --> 00:36:36.681 But often, when we speak about aliens, what we really mean 00:36:36.681 --> 00:36:40.241 is not microbes but complex creatures. 00:36:40.241 --> 00:36:44.161 We need things that we can speak to, civilisations. 00:36:44.161 --> 00:36:47.561 What is the probability there will be other civilisations out there 00:36:47.561 --> 00:36:49.161 in the Milky Way? 00:36:49.161 --> 00:36:52.601 Well, again, the answer is we don't know. 00:36:52.601 --> 00:36:57.001 But there are observations we can make, patterns we can see 00:36:57.001 --> 00:37:02.001 in the Milky Way, that might allow us to make an educated guess. 00:37:40.081 --> 00:37:42.081 We don't know precisely 00:37:42.081 --> 00:37:46.361 how we highly intelligent, complex creatures came to be here. 00:37:56.281 --> 00:38:01.201 But we do know for certain that life on Earth didn't begin this way. 00:38:04.841 --> 00:38:07.921 We are the product of a story that has been playing out 00:38:07.921 --> 00:38:11.281 for over a quarter of the age of the universe... 00:38:14.441 --> 00:38:15.481 ..from microbes... 00:38:17.161 --> 00:38:19.984 ..to a global technological civilisation reaching out 00:38:19.984 --> 00:38:22.761 for others. 00:38:24.761 --> 00:38:28.601 For now, at least, we remain surrounded by silence. 00:38:28.601 --> 00:38:32.401 The messages we've sent out into the cosmos remain unanswered, 00:38:32.401 --> 00:38:36.681 and the telescopes we use to scan the skies for alien signals 00:38:36.681 --> 00:38:38.361 remain quiet. 00:38:38.361 --> 00:38:39.881 Now that's not to say, of course, 00:38:39.881 --> 00:38:42.521 that there aren't other civilisations out there. 00:38:42.521 --> 00:38:46.441 We may have been looking for the wrong thing in the wrong place. 00:38:46.441 --> 00:38:49.921 But I think the answer to the question of the great silence 00:38:49.921 --> 00:38:53.601 can be found here on Earth, because, here, 00:38:53.601 --> 00:38:58.481 it took 4 billion years of stability for a civilisation to emerge. 00:38:58.481 --> 00:39:01.161 That is a vast amount of time. 00:39:01.161 --> 00:39:04.321 And when we look to the other worlds out there in the Milky Way, 00:39:04.321 --> 00:39:08.281 it's those two things - stability and time - 00:39:08.281 --> 00:39:11.681 that appear to be very rare commodities indeed. 00:39:21.641 --> 00:39:26.601 In 2013, the European Space Agency launched the Gaia Space Telescope. 00:39:30.041 --> 00:39:35.241 Its mission? To survey the stars of our galaxy, the Milky Way. 00:39:37.721 --> 00:39:40.281 Billions of stars have been mapped... 00:39:44.601 --> 00:39:49.361 ..each star a potential host for alien worlds. 00:39:53.961 --> 00:39:57.441 And patterns are already beginning to emerge. 00:40:11.721 --> 00:40:14.481 Not all stars exist alone. 00:40:19.921 --> 00:40:22.521 Some have company. 00:40:30.681 --> 00:40:32.641 And bizarre as they seem, 00:40:32.641 --> 00:40:36.521 Gaia has discovered around a million of these binary 00:40:36.521 --> 00:40:38.361 or multiple star systems. 00:40:43.361 --> 00:40:45.841 We've known for a long time that binary star systems 00:40:45.841 --> 00:40:48.961 and, indeed, multiple star systems exist, 00:40:48.961 --> 00:40:52.241 but we didn't know precisely how common they are. 00:40:54.561 --> 00:40:57.881 But now we have a huge amount of high-precision data, 00:40:57.881 --> 00:41:02.881 including the Gaia data, which tells us that around 50% 00:41:02.881 --> 00:41:06.441 of all sun-like stars are in multiple star systems. 00:41:06.441 --> 00:41:10.761 And for more massive stars, that number is 80%. 00:41:15.561 --> 00:41:20.041 So how does the prevalence of multiple star systems in the galaxy 00:41:20.041 --> 00:41:23.201 shift the odds in the hunt for another Earth? 00:41:25.641 --> 00:41:30.841 Could Earth-like planets exist in multiple star systems? 00:41:30.841 --> 00:41:34.321 And if so, what might their fate be? 00:41:40.201 --> 00:41:44.681 In 2020, we may have found a clue, 00:41:44.681 --> 00:41:51.681 a planet the size of Mars floating freely through the galaxy, 00:41:51.681 --> 00:41:54.001 a so-called "rogue world". 00:41:56.041 --> 00:42:00.641 But planets can't form alone in interstellar space, 00:42:00.641 --> 00:42:02.601 so where did it come from? 00:42:19.081 --> 00:42:20.241 Dawn... 00:42:28.041 --> 00:42:30.361 ..ushered in not by one star... 00:42:35.361 --> 00:42:36.521 ..but two. 00:42:46.121 --> 00:42:49.841 Perhaps the rogue world grew up in a close binary system... 00:43:04.161 --> 00:43:07.681 ..subject to the gravitational pull of two stars. 00:43:15.361 --> 00:43:18.201 Its orbit may have been unstable... 00:43:26.161 --> 00:43:30.361 ..as its parent stars fought to control its destiny. 00:43:44.281 --> 00:43:48.761 Even in single star systems, the weak gravitational interactions 00:43:48.761 --> 00:43:51.761 between the planets can change their orbits. 00:43:51.761 --> 00:43:53.561 Now, in a double star system, 00:43:53.561 --> 00:43:56.641 the planets are not only subjected to the gravitational pulls 00:43:56.641 --> 00:44:02.521 of each other, they're subjected to the stronger gravitational pull of another star. 00:44:02.521 --> 00:44:05.841 So, even if a planet gets into a stable orbit, 00:44:05.841 --> 00:44:10.241 it's very likely that it won't stay in that orbit for long. 00:44:10.241 --> 00:44:14.281 So, in double star systems, the line between order and chaos 00:44:14.281 --> 00:44:16.161 is very thin indeed. 00:44:21.641 --> 00:44:24.441 Even subtle changes in a planet's orbit 00:44:24.441 --> 00:44:27.201 can lead to dramatic changes in climate. 00:44:28.361 --> 00:44:30.721 And that's why the surface conditions on planets 00:44:30.721 --> 00:44:36.401 in double star systems may be unlikely to remain stable enough 00:44:36.401 --> 00:44:39.921 for long enough for intelligent life to evolve. 00:44:48.601 --> 00:44:50.641 And the changes in the orbits of planets 00:44:50.641 --> 00:44:55.601 can sometimes be anything but subtle. 00:45:14.561 --> 00:45:20.481 A close encounter may have given the rogue world 00:45:20.481 --> 00:45:22.841 a final gravitational kick. 00:45:43.641 --> 00:45:45.201 Flinging it outwards. 00:45:49.281 --> 00:45:52.961 And releasing it from the grip of its parent stars. 00:45:59.081 --> 00:46:00.481 Setting it loose... 00:46:06.361 --> 00:46:08.641 ..on a journey through the galaxy. 00:46:23.881 --> 00:46:28.081 Far from the warmth of its stars, 00:46:28.081 --> 00:46:31.881 any liquid water the rogue world might once have had... 00:46:36.721 --> 00:46:38.441 ..would have frozen solid. 00:46:43.881 --> 00:46:46.921 Any atmosphere that once protected it... 00:46:54.761 --> 00:46:57.801 ..would have frozen out on to the surface. 00:47:02.241 --> 00:47:04.321 The rogue would have become a world... 00:47:07.121 --> 00:47:11.321 ..with conditions that no living thing could endure. 00:47:14.961 --> 00:47:19.001 An entire planet alone and adrift. 00:47:26.041 --> 00:47:30.761 Only to be detected by us millions of years later. 00:47:32.761 --> 00:47:36.201 A small Earth-like rogue planet... 00:47:38.201 --> 00:47:42.401 ..roaming the darkness of space for eternity. 00:47:53.761 --> 00:47:57.761 This lonely wandering planet is not a unique world. 00:47:57.761 --> 00:48:00.361 Although rogue planets are very difficult to detect, 00:48:00.361 --> 00:48:02.241 it's estimated that there may be 00:48:02.241 --> 00:48:05.241 over 100 billion of them in our galaxy. 00:48:05.241 --> 00:48:07.321 Rogue planets might be the most common 00:48:07.321 --> 00:48:09.481 type of planet in the Milky Way. 00:48:10.921 --> 00:48:12.521 And although we think most of them 00:48:12.521 --> 00:48:16.201 were torn away from their star soon after formation, 00:48:16.201 --> 00:48:21.361 this does suggest that star systems are not always stable places 00:48:21.361 --> 00:48:24.721 where complex life could evolve over billions of years. 00:48:37.601 --> 00:48:41.161 Our hunt for another living planet has only just begun. 00:48:45.201 --> 00:48:47.201 Yet we've already learned so much. 00:48:54.721 --> 00:48:57.561 We found our first rocky worlds. 00:48:59.841 --> 00:49:03.601 Some in the habitable zone around their stars. 00:49:12.081 --> 00:49:16.881 Some, potentially, with liquid water on the surface. 00:49:18.481 --> 00:49:23.401 Candidate worlds for future missions to search for evidence of life. 00:49:28.841 --> 00:49:32.881 But we've also found hordes of bizarre, tortured worlds... 00:49:35.481 --> 00:49:37.801 ..orbiting around violent stars. 00:49:46.441 --> 00:49:49.201 And a multitude of rogue planets... 00:49:51.401 --> 00:49:55.961 ..where complex life as we understand it seems impossible. 00:50:04.441 --> 00:50:08.641 Perhaps it's these worlds that hint at the reason why... 00:50:10.361 --> 00:50:14.081 ..for now, one planet stands apart. 00:50:19.881 --> 00:50:21.321 Alone. 00:50:34.001 --> 00:50:37.641 Our planet seems to have largely escaped the violence, 00:50:37.641 --> 00:50:39.921 the chaos and the constant change 00:50:39.921 --> 00:50:43.281 that seems to characterise a galaxy like the Milky Way. 00:50:43.281 --> 00:50:46.121 Yes, there's been the odd mass extinction, 00:50:46.121 --> 00:50:49.441 but there's been an unbroken chain of life here on Earth 00:50:49.441 --> 00:50:52.241 stretching back four billion years. 00:50:52.241 --> 00:50:55.361 And if that's what you need to go from the origin of life 00:50:55.361 --> 00:50:59.361 to a civilisation, then although there may be billions of worlds 00:50:59.361 --> 00:51:03.961 out there where life began, there may be very few civilisations. 00:51:05.041 --> 00:51:07.561 But that's just an opinion. 00:51:07.561 --> 00:51:09.361 It's an educated guess. 00:51:09.361 --> 00:51:12.241 And given the profound nature of the question, 00:51:12.241 --> 00:51:17.041 no matter how educated the guess, I think it would be ridiculous 00:51:17.041 --> 00:51:21.801 for us to stop looking, both inside our galaxy and beyond. 00:51:25.721 --> 00:51:30.161 For we may have just received the first glimpse 00:51:30.161 --> 00:51:32.881 of a world beyond the Milky Way... 00:51:41.361 --> 00:51:44.721 ..around 30 million light years away, 00:51:44.721 --> 00:51:48.681 nestled in the spiral arms of the Whirlpool Galaxy. 00:51:53.361 --> 00:51:55.801 A world the size of Saturn. 00:52:04.481 --> 00:52:07.921 A find that marks an expansion of our horizons. 00:52:13.001 --> 00:52:17.361 The beginning of the hunt for extragalactic planets. 00:52:20.961 --> 00:52:24.481 The potential discovery of a planet orbiting around a star 00:52:24.481 --> 00:52:28.401 in another galaxy is something that I never thought I'd see. 00:52:28.401 --> 00:52:32.361 And it opens up the intriguing possibility that we might be able 00:52:32.361 --> 00:52:36.001 to explore not only the question, "Are we alone in our galaxy?" 00:52:36.001 --> 00:52:38.401 but "Are we alone in the universe?" 00:52:42.721 --> 00:52:46.801 The answer to that question may lie far in the future. 00:52:46.801 --> 00:52:49.641 We might never answer that question, 00:52:49.641 --> 00:52:53.641 but I said the question "Are we alone?" is profound 00:52:53.641 --> 00:52:56.481 because answering it would teach us much more 00:52:56.481 --> 00:52:58.281 about what it means to be human. 00:53:02.721 --> 00:53:07.401 Well, I think we become a little bit more human with every world 00:53:07.401 --> 00:53:12.801 that we explore because that ability to lay the foundations, 00:53:12.801 --> 00:53:16.521 to explore questions to which we may never receive answers 00:53:16.521 --> 00:53:19.921 in our lifetime, questions for our children 00:53:19.921 --> 00:53:21.841 or our grandchildren to answer 00:53:21.841 --> 00:53:25.161 is a fundamental part of what it means to be human. 00:53:25.161 --> 00:53:30.041 It's a fundamental part of what makes us so special 00:53:30.041 --> 00:53:34.881 here on this little world, looking up at the stars, 00:53:34.881 --> 00:53:37.321 whether we're alone or not. 00:53:53.801 --> 00:53:56.081 Five, four, 00:53:56.081 --> 00:53:58.521 three, two... 00:53:58.521 --> 00:54:00.601 Engine start. One, zero, 00:54:00.601 --> 00:54:03.881 and liftoff of the Delta II rocket with Kepler 00:54:03.881 --> 00:54:07.201 on a search for planets in some way like our own. 00:54:09.241 --> 00:54:12.081 We had worked to get thousands of people to work together 00:54:12.081 --> 00:54:14.361 and it's all coming together. 00:54:15.801 --> 00:54:17.201 And we have separation. 00:54:19.001 --> 00:54:21.921 It was so emotional to see the project they had worked on 00:54:21.921 --> 00:54:25.401 for so many years or decades finally go to space 00:54:25.401 --> 00:54:28.681 and all that hope and promise all bundled up in the machinery. 00:54:36.881 --> 00:54:39.601 Kepler was an immediate success, 00:54:39.601 --> 00:54:42.041 discovering over 2,000 new planets 00:54:42.041 --> 00:54:44.761 in its first four years of operation. 00:54:50.001 --> 00:54:52.201 But in the summer of 2012, 00:54:52.201 --> 00:54:55.561 the team faced a challenge that threatened the entire mission. 00:54:59.881 --> 00:55:02.521 One of the things that the Kepler mission needs to operate 00:55:02.521 --> 00:55:05.121 are reaction wheels that spin and hold it on target. 00:55:07.521 --> 00:55:10.961 So it always points at the same stars and doesn't jiggle. 00:55:10.961 --> 00:55:13.201 Well, we had four wheels that did that. 00:55:15.681 --> 00:55:18.201 And we knew that we only had a couple of spare gyroscopes, 00:55:18.201 --> 00:55:22.161 and we knew that spacecraft tend to have gyros fail. 00:55:27.001 --> 00:55:28.881 And after a while, it failed. 00:55:28.881 --> 00:55:31.241 Three months later, the second one failed. 00:55:31.241 --> 00:55:32.841 And since we needed three, 00:55:32.841 --> 00:55:36.081 we could no longer look at the Kepler field of view. 00:55:38.721 --> 00:55:41.801 I had hoped that they'll figure out a way to work with two gyros, 00:55:41.801 --> 00:55:43.441 and indeed they did. 00:55:47.361 --> 00:55:50.321 So the very clever people, the engineers and scientists, 00:55:50.321 --> 00:55:55.321 said, "What we can use is we'll use the sunshine for the third wheel. 00:55:55.321 --> 00:55:57.761 "We'll make this thing reflect sunlight off it, 00:55:57.761 --> 00:56:01.161 "we use the other two wheels and now we can point in the sky." 00:56:03.801 --> 00:56:08.121 The faint pressure of sunlight helped stabilise the telescope. 00:56:09.201 --> 00:56:12.321 That was kind of good news, actually, because it meant Kepler 00:56:12.321 --> 00:56:14.841 was going to have to go off the Kepler field now 00:56:14.841 --> 00:56:18.361 and we could get all kinds of other stars and observe them. 00:56:18.361 --> 00:56:21.441 And so, it actually was a boon for stellar astronomy. 00:56:24.281 --> 00:56:27.081 After another four years of discoveries, 00:56:27.081 --> 00:56:31.921 in total it had found over 2,600 planets, 00:56:31.921 --> 00:56:36.641 making it, by far, our most successful planet hunter to date. 00:56:40.041 --> 00:56:44.041 It was sad when they said to command to shut everything down. 00:56:44.041 --> 00:56:45.801 You know, it's asleep now, 00:56:45.801 --> 00:56:49.601 it's in orbit around the Sun and it will continue that orbit, 00:56:49.601 --> 00:56:52.761 but since it launched from Earth, it will come back to Earth. 00:56:52.761 --> 00:56:56.761 It'll come and visit us again in about 40 years. 00:56:56.761 --> 00:56:58.921 And my hope is people will say, 00:56:58.921 --> 00:57:02.441 "This is a historic telescope. It told us about all these planets." 00:57:02.441 --> 00:57:06.361 And they will go up and pick up this telescope and bring it back to Earth 00:57:06.361 --> 00:57:10.241 and put it in the Air and Space Museum for us all to admire.