How the Universe Works - S03E09 - The Search for a Second Earth
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00:00The Earth is not unique. In the last few years, scientists have discovered that our planet
00:09is just one of billions in the Milky Way galaxy.
00:13There's a really decent chance that there are more planets in the galaxy than there
00:17actually are stars.
00:20These planets are now being scrutinized for evidence of atmospheres, liquid water, and
00:26perhaps even life itself.
00:31We have a scientific method to actually determine whether there is life on another planet.
00:37Another Earth. Alien life. The truth lies beyond our skies. But are we prepared for
00:45it?
00:56Our Earth informs us about life as we know it. The sun warms our oceans, creating the
01:09perfect environment for all scales of life, from the very smallest creatures to the giants
01:16that rule the seas.
01:20Mountains, plains, and forests team with plant and animal species, all within a thick
01:29atmosphere that nurtures and protects them.
01:33For us, it's a paradise.
01:41Twenty years ago, a group of scientists decided to find out if there were other such paradises
01:46out in space. Orbiting stars that light up our night sky, they began to hunt for exoplanets.
01:56Just in the last decade, we've had this explosion in the discovery of these exoplanets,
02:01which has revolutionized the whole field of astronomy.
02:08During the first few years of searching for exoplanets, scientists discovered lots of
02:12Jupiter-sized planets.
02:19These hot gas giants proved easy to find, yet are not suitable for life as we know it.
02:26But with the advancement of technology and telescopes, astronomers were able to look
02:30for smaller planets, Earth-sized worlds. The results were so stunning that it transformed
02:38how we see our place in the universe.
02:48The increase in exoplanet discoveries was because of the Kepler Space Telescope.
02:55The Kepler Space Telescope is an observatory in space that is staring at one spot in the
03:00sky. It's looking at roughly 150,000 stars, and it's looking for the telltale sign of
03:07planets orbiting those stars. Then every time the planet passes in front of the star, it'll
03:12block a little bit of that starlight. And if you plot the amount of light you get from
03:16the star, it drops and then goes back up as the planet passes.
03:23In just four years, scientists have detected over a thousand exoplanets just from their
03:28shadows. Yet Kepler is unable to determine if the shadow was made by a giant gas planet
03:37unsuitable for life, or by a potentially habitable Earth-like planet.
03:45What we're measuring when a planet passes in front of its host star is, what is the
03:51area of the planet relative to the area of the star that it's passing in front of? It's
03:59a ratio, basically.
04:03Jupiter-sized planets passing giant stars fool Kepler, because they block the same fraction
04:08of light as Earth-sized planets crossing smaller stars.
04:15To prove a planet is Earth-sized, the size of its star needs to be measured first using
04:19the world's largest telescopes. But it's time-consuming and expensive, so creates a huge list of exoplanets
04:26waiting to be scrutinized. However, astronomer Kevan Stassen has developed an ingenious shortcut
04:35by turning the raw Kepler data into sound.
04:41What the Kepler telescope directly measures in the data that we use is small changes in
04:47brightness that a star produces due to the flickering arising from the boiling and roiling
04:54motions of gas at its surface. What we can do then is take that light flickering data
05:01and transform it in a sound studio, for example, into audio frequencies, and so then we can
05:09represent with sound what we're actually detecting with light.
05:15The bigger the star is, the more its surface boils with activity, making big stars flicker
05:22more powerfully. Converted to sound, this boiling becomes a deafening hiss.
05:31Well let's listen to some stars. Can we hear the red giant star, please? I'm going to bring
05:37up the volume here. This is a very large star, very low density, and so that large amount
05:45of hiss is the result of vigorous boiling and churning at the surface of this large
05:51red giant star. Can we get the dwarf star, please?
06:00On smaller stars, sunspots dominate the sound profile, and these create a low frequency
06:08drone. It actually sounds like a series of clicks. However, among these clicks lies the
06:16faint hiss Cave-Anne needs to size the star. Underneath it at a very low level is a little
06:23bit of hiss. That little bit of sssssssss is actually the light flickering that we're interested
06:30That little bit of ssss is actually the light flickering that we're interested in.
06:38By accurately measuring the level of this background hiss,
06:41Kvan can calculate the size of the star.
06:45In this case, it's around the same size as our star, the Sun.
06:51Kvan's work could be the breakthrough exoplanet hunters have been hoping for.
06:55It's cheap, the results are almost instantaneous,
06:58and once the size of the star is known,
07:01it's quite straightforward to calculate the size of the planet casting shadows over it.
07:08It feels like a very privileged time to be a scientist,
07:12to be an astronomer working in this area and contributing to the hunt for the next Earth.
07:19Here we are, actually discovering these worlds by the hundreds,
07:24and now on the cusp of being able to identify the next Earth.
07:34Astronomers suspect there could be tens of billions of rocky, Earth-like planets in the Milky Way,
07:40places where perhaps even life has developed.
07:47But life as we know it requires water.
07:51So how can scientists possibly find this miracle substance on planets light-years away?
07:57Transcribed by ESO, translated by —
08:28Remarkably, the water we drink today contains the same atoms as the water dinosaurs drank 100 million years ago.
08:40It's the same water that formed clouds over the early Earth 4 billion years ago.
08:47And every organism that has ever existed on Earth has used this same supply of water
08:53as the biochemical source to keep it alive.
08:58On Earth, all life requires liquid water to grow and reproduce.
09:03It's the common ecological requirement for life.
09:06Liquid water is just so good for getting evolution going.
09:10Molecules can dissolve in the water, actually interact with each other, form more complex chains.
09:18Now that's very rare. Hardly any other liquids do that.
09:22So liquid water is a natural starting place when you look out into the universe and say,
09:27what planets could possibly have life?
09:32To gauge how much liquid water is in space,
09:35astronomers had to first calculate how common water is in all its forms.
09:41Amazingly, it's found wherever they look.
09:46Water is incredibly common.
09:49In its gaseous form, we see water vapor filling the space between the stars.
09:53We see it in clouds of material that are actually forming new stars and planets right now.
10:01Since water is a fundamental building block of stars and planets,
10:05then it seems logical that exoplanets must have it in abundance.
10:11But if searching for life, liquid water needs to be found, and plenty of it.
10:18To find it, astronomers took inspiration from a fairy tale.
10:23Everybody knows the famous story of Goldilocks and the Three Bears
10:26and the cup of porridge where one was too hot, one was too cold, and one was just right.
10:31When it comes to cooking up life like a porridge,
10:34you need to have an environment that's not too hot, not too cold, just right.
10:38And traditionally, we look for that at a certain distance around a star.
10:44At first, astronomers based this magical distance, known as the Goldilocks zone,
10:49on the Earth's orbit around the Sun.
10:53But as they've found more and more exoplanets,
10:56they've had to re-evaluate the limits where liquid water can exist.
11:01There isn't a single distance that depends on the brightness of your parent star.
11:05A dim star, you need to be closer.
11:08A hot star, very bright, you need to be farther away.
11:11But if the planet's atmospheres are rich with greenhouse gases,
11:15then they can still have liquid water even outside the habitable zone.
11:22You could have an atmosphere rich in carbon dioxide
11:25that has a strong natural greenhouse effect,
11:28and that means the planet could be much farther out, out of the Goldilocks zone,
11:32but there could still be liquid water because of the composition of the atmosphere.
11:36And planets orbiting much closer to their stars
11:39can avoid overheating if their surface is pale and highly reflective.
11:45When we search for habitable zones,
11:47we have to be careful to understand what we're really looking for.
11:52Scientists have calculated how many rocky planets
11:55may lie within the Goldilocks zone of their stars,
11:58and it's over 30 billion potentially habitable stars.
12:02Even more remarkably, recent discoveries have shown
12:05that it's not just planets that can exist in the Goldilocks zone.
12:09There may be moons, too.
12:11Awash with oceans.
12:18Most of the planets we're finding are big Jupiter-sized planets.
12:22How do we know they're habitable?
12:24Well, we know that most of the planets in the Goldilocks zone
12:28Most of the planets we're finding are big Jupiter-sized planets.
12:31However, a lot of them are orbiting roughly where the Earth is orbiting the Sun.
12:36So even if the planet that we're finding can't support life,
12:40it could have a moon, a moon with an atmosphere, that could support life.
12:47And the biggest of these moons may be similar to Earth.
12:52There could be billions upon billions of exomoons out there,
12:56and even, perhaps, countless paradises teeming with life.
13:02David Kipping searches for exomoons
13:05by looking for double dips in the brightness of distant stars.
13:09We look for exomoons in a very similar way to the way that we look for planets,
13:14by looking for them transit their host star.
13:18Now, if that planet had a moon,
13:20then we should expect to have one big dip due to the planet,
13:23and then one smaller dip either to the left or to the right due to the moon.
13:31And some of these habitable exomoons
13:33may have some of the most spectacular sights in the universe.
13:38Imagine a warm, rocky world similar to Earth,
13:42with oceans and mountains.
13:45But in the sky, there's a massive ringed planet,
13:49and another moon that's ejecting hot magma into space.
14:04Exoplanets, along with exomoons,
14:06hint at a galaxy filled with the possibilities for life.
14:10But for life, a rocky planet and liquid ocean may not be enough.
14:14Biology requires something else.
14:17Air.
14:28As the sun's light falls on Earth, a halo appears.
14:32A pale blue ring of light.
14:35Our atmosphere.
14:37And without it, we wouldn't be here.
14:42The Earth's atmosphere provides the gases
14:44that fuel the biochemistry of advanced life.
14:49But it also protects the oceans from the full ferocity of the sun's rays,
14:53preventing the water from boiling away into space.
14:58Without it, we wouldn't be here.
15:05Without an atmosphere, there would be no wind,
15:08no rain,
15:10no fresh water,
15:12and no life.
15:17Atmospheres are absolutely essential for life.
15:20Take a look at the planet Earth,
15:22and you realize that just like the skin of the apple,
15:25the skin of the apple preserves the apple,
15:27well, the atmosphere of our planet preserves the oceans
15:31and makes possible the presence of life as we know it.
15:38Scientists in search of living exoplanets
15:41hope to detect the thin atmosphere that should surround these worlds.
15:45And to do so, they've turned to rainbows for inspiration.
15:54In the same way that water splits sunlight into a rainbow,
15:58astronomers use instruments to split starlight
16:01into a band of colours or spectrum.
16:04It's one of the oldest techniques in science
16:07and one of the most revealing.
16:13Several hundred years ago, scientists first began to
16:16take something like a prism and put it in front of their telescope.
16:19So they started taking the light from stars like the sun
16:22and actually spreading it out into a spectrum.
16:24And what they saw was kind of surprising.
16:26So instead of seeing a continuous rainbow of light,
16:29they saw that rainbow, but they saw these dark lines superimposed on top.
16:36Each chemical element in the star's atmosphere
16:38absorbs different parts of the spectrum,
16:41creating signature dark bands.
16:45For instance, up at the top, there's a pair of lines
16:48in the yellow part of the spectrum which are due to sodium.
16:52This spectral analysis has taught us almost all
16:55that is known about stars today.
16:58But these same lines may hide a spectacular secret.
17:03The faint signal of an atmosphere, and perhaps life.
17:12So the challenge is that these planets are very small and very faint.
17:15So we can't actually go and directly measure the light emitted from the planet
17:19the same way that we go and measure this lovely spectrum for the sun.
17:22Instead, we have to rely on more indirect methods.
17:25So one indirect way of doing that
17:27is to wait until the planet passes in front of the star.
17:32When the light of a star passes through an exoplanet's atmosphere,
17:36the gases that surround the planet
17:38should cast their own faint lines on the star's spectrum.
17:42So as we watch the light from the star transmitted through that atmosphere,
17:46its atmosphere is going to act like a little filter.
17:49So part of the starlight is going to pass through that atmosphere,
17:52and we're going to see that imprinting extra lines on it
17:55which are due to the planet's atmosphere.
17:57So that change in the spectrum tells us something
18:00about the properties of the planet's atmosphere.
18:04The chemical astronomers want to find most is oxygen.
18:08This is because only life can produce enough oxygen to be easily detected.
18:13It's known as a biosignature.
18:17Astronomers are now looking to find biosignatures
18:21in the atmospheres of rocky exoplanets.
18:25But there's more than one way to search for these chemical signatures.
18:3227 to 29. All right, we're off.
18:36Ben Oppenheimer is part of a team
18:38trying to take direct photographs of exoplanets
18:41using massive, ground-based telescopes.
18:45Awaiting instructions.
18:47We're within minutes of taking our first long exposure,
18:51and I hope it's good.
18:55The toughest aspect of photographing exoplanets
18:58is the blinding light of the parent star,
19:01which shines tens of millions of times brighter than the planet itself.
19:06The key is to stop the light of the star
19:08entering the telescope's sensors by blocking it,
19:11achieved by using a series of masks and lenses called a coronagraph.
19:17Right now we're standing right underneath the telescope's primary mirror,
19:21and the light comes through a hole in the middle of the mirror
19:24and goes into this crazy box here,
19:27which is full of optics, motors, sensors and electronics
19:31that all allow us to precisely control the starlight
19:34that's coming through the system.
19:37Software is then used to manipulate the coronagraph
19:41to block out the unwanted sunlight.
19:44Under good conditions, we can actually carve dark holes
19:48into this image of the star
19:50so that we can see really faint things in those regions.
19:54But coronagraphs produce an intriguing problem.
19:57Errors within the optics produce tiny flares of starlight called speckles,
20:02which appear just like exoplanets.
20:05However, Ben has developed an ingenious way
20:08to distinguish the planets from the speckles.
20:11So we've developed a technique where we exploit an aspect of speckles,
20:16which is that they change position in the image
20:19depending on what colour you take your image at.
20:25Ben takes the same image of the star through different colour filters
20:29and runs them like a movie.
20:32The speckles appear to move across the screen.
20:35However, the planets don't,
20:37allowing Ben to easily identify them.
20:41And so I'd like to point out that there is a little thing right here
20:46that if you watch for... you're careful,
20:49you'll notice that it doesn't move and the speckles are washing over it.
20:54This stationary blob is a candidate exoplanet,
20:58and below it to the left is a second.
21:01They both appear to orbit a star that's about 200 light-years from the Earth.
21:06Just a decade ago,
21:08capturing an image such as this through a telescope was unthinkable.
21:13But today, there are hundreds of such pictures,
21:16and by analysing the light from these distant worlds,
21:19scientists can decipher their chemical composition
21:23and, potentially, the fingerprints of life.
21:28This is the first time in the history of science
21:31that we've seen a speckle in a star.
21:34It's the first time that we've seen a speckle in a star.
21:38And, potentially, the fingerprints of life.
21:45At this point, we're studying much larger planets,
21:48gaseous things like Jupiter,
21:50that most likely don't have any kind of life like we know it.
21:53But that's a first step,
21:55and we're going to fainter and smaller and smaller planets
21:58as time goes on, as we develop this technology.
22:03In the not-too-distant future,
22:05scientists may be able to simply scan a star for Earth-like planets
22:09and find it has the biosignature of life.
22:13We can look right at the light
22:15from a little planet around its distant star,
22:18and that opens up a whole range of possibilities for us
22:21to not just detect the planet, but to study the planet.
22:24I mean, this all sounds like science fiction,
22:26but there is a reality to this.
22:28We have a scientific method
22:30to actually determine whether there is life on another planet.
22:35Life is one thing,
22:37but life that's intelligent is another altogether.
22:42It requires billions of years of evolution,
22:45and a powerful force field,
22:48similar to that which we owe our lives to every day.
23:06If somebody from outside our solar system were to study it,
23:11they may well make a surprising observation.
23:14Of the eight planets that orbit our Sun,
23:17they might calculate that two, not one,
23:20is suitable for life.
23:22And this would be because the Sun has two planets
23:25orbiting within its Goldilocks zone.
23:28The first planet is the Sun,
23:30and the second planet is the Earth.
23:33Both planets orbiting within its Goldilocks zone,
23:36the Earth and Mars.
23:40Both planets have surfaces warm enough
23:42for liquid water to pool on.
23:44Yet while the Earth is blessed with warm oceans,
23:47Mars is dry and dead.
23:51There is one crucial difference between the two planets,
23:54and it could be the key to finding habitable exoplanets.
23:59A magnetic field.
24:04Our Sun is constantly emitting deadly radiation out towards Earth.
24:09And we're only protected because of our magnetic field,
24:12or magnetosphere.
24:16Without it, the solar wind would blow our atmosphere away.
24:20And without an atmosphere,
24:22liquid water couldn't exist on the surface.
24:27In order to have liquid water,
24:29not only do you need the right temperature,
24:31but you need the right pressure.
24:33If there were no atmosphere here right now,
24:35even at the same temperature we are today,
24:37all of the water would boil off into vapor immediately.
24:41So where does Earth's magnetosphere come from?
24:44And why doesn't Mars have one?
24:49Actually, in the past,
24:51both Earth and Mars had magnetospheres.
24:54However, Mars lost it around 4 billion years ago.
24:58And along with it, the potential for life to develop.
25:04Earth and Mars were created during a period of violence.
25:09Asteroids smashed into their surfaces,
25:12turning rock and metal into a molten mass.
25:17As they started to cool, a solid crust formed on the surface.
25:21But the molten metal below churned as the planets turned.
25:25Inducing a magnetic field,
25:27which rose high up above the surface of both planets.
25:34Active volcanoes expelled gas into the space around each planet.
25:39Protected by the newly formed magnetic field,
25:42these gases built up into thick atmospheres,
25:45creating the air pressure for liquid water to run on the surface.
25:50For over 100 million years,
25:52both Mars and Earth were warm, wet paradises,
25:56ready for life to evolve.
25:59Then Mars' magnetic protection suddenly disappeared.
26:06The solar wind blew its atmosphere into space.
26:10Then its oceans boiled away,
26:12leaving the dry, sterile red rock we know today.
26:17Mars is fine.
26:19Mars' fundamental problem is that it's smaller than Earth.
26:22And because it's smaller,
26:24the internal core of Mars cooled down and solidified.
26:29And once it becomes a solid metal,
26:31there's no more magnetic field.
26:33The magnetic field shuts off, essentially,
26:35and the atmosphere, therefore, is vulnerable
26:38to both energy and radiation from the Sun
26:41and the rest of the galaxy.
26:44And probably just blew off.
26:46Whatever life was on there, at least on the surface,
26:49is now completely exposed.
26:52All rocky planets will eventually lose their magnetospheres
26:55as their cores cool and turn solid.
26:59So one way to find out if an exoplanet is alive
27:03is to discover if its magnetosphere is still active.
27:07Yet magnetospheres are difficult to measure
27:10because they're incredibly weak.
27:12The Earth has a magnetic field of approximately half a gauss,
27:16which, when you think about it, is actually really weak.
27:19Our fridge magnets are about 100 gausses.
27:21They're much stronger.
27:26Exoplanets are far too far away
27:28for these weak magnetic fields to be measured directly.
27:32But there is an indirect method.
27:34When electrons in the solar wind interact with the planet's magnetosphere,
27:38they emit radio waves that beam out into space.
27:42It turns the planet into a giant radio beacon.
27:49Astronomers such as Evgenia hope to use these signals
27:52to detect habitable exoplanets.
27:55Not only that, but they also hope to use them
27:58as a means of communicating with other planets.
28:02If we're looking for the magnetic signature in the radio waves
28:06of a giant planet, say a hot Jupiter,
28:09we expect it to have a strong magnetic field
28:12and therefore it would have a high frequency,
28:15at around 100 megahertz, kind of where the limit of this radio is.
28:20However, we don't know for sure
28:22because we haven't been able to measure it yet.
28:25But looking for exoplanets at 10 megahertz presents a unique challenge
28:29because the Earth's own magnetosphere generates a signal
28:32that interferes with this search.
28:37The radio frequency is a very high frequency,
28:39and around 100 megahertz is kind of where the limit of this radio is.
28:43However, a weaker field like Earth's
28:45requires us to go down to lower and lower frequencies.
28:48So instead of 100 megahertz, we go down to 10 megahertz.
28:56So to find other Earths using radio requires a dish in space.
29:05When we want to look for magnetospheres of extrasolar planets,
29:10we really need to get outside of the Earth-Moon system
29:14in order to get away from all the radio frequencies
29:17that are bouncing around the Earth.
29:21With the development of new techniques
29:23and technologies,
29:25scientists are edging closer to finding a second Earth.
29:33I wouldn't be surprised if we have that data about an Earth
29:37and about life on it around another star in 10 or 15 years.
29:41I'm hoping to see that soon.
29:45Using shadows, light spectrums and now radio,
29:51scientists have the tools to detect a planet similar to our own.
29:56But in the rush to find a planet like Earth,
29:59are we missing something?
30:03What if Earth is an outlier,
30:05an inexplicably lucky place on the very fringes of habitability?
30:12Is there another sort of planet that's even better for life to evolve?
30:21EARTH
30:27For years, astronomers have scanned the skies for planets that could sustain life.
30:33They've based their search on the Earth,
30:35seeking the exact same conditions and the exact same size.
30:42I think right now there's a huge focus to finding Earth-like planets.
30:47Now whether or not there actually is life there,
30:50that is another question altogether.
30:54But after 20 years of searching for another Earth,
30:57the scientists may be about to change their focus.
31:01Recent observations have revealed a brand new class of planet,
31:05one that may eclipse our own.
31:09SUPER-EARTHS
31:16Astronomers call this mysterious new class of planets super-Earths.
31:21Super-Earths are about three to five times the mass of the Earth,
31:24and there's nothing like that here.
31:26We don't know what they're like. It's an entirely alien sort of planet.
31:31In recent years, astronomers have begun to imagine the conditions on this new class of planet,
31:36and they've come to a startling conclusion.
31:40Super-Earths could be even more habitable.
31:46There are probably planets out there that are even more hospitable for life,
31:50planets that have even more chemicals necessary to create the organic materials that create life,
31:57conditions that make it more likely to get life off the ground.
32:02Imagine a rocky planet twice the size of the Earth,
32:06with volcanism on the surface that indicates a vast source of heat and magma within its core.
32:16We expect that a heavier Earth will be more geologically active,
32:23that the increased amount of geothermal heat within the super-Earth
32:29will lead to stronger motions of the magma underneath the crust.
32:38Erupting volcanoes dot the surface of this super-Earth.
32:43Their gases feed a super-thick atmosphere and help to regulate a super-stable climate.
32:49Many times life on Earth was nearly extinguished.
32:53For example, once upon a time the Earth was snowball Earth,
32:57completely covered in ice.
33:00Maybe in these other planets there are Earths in which snowball Earth never happened,
33:06that the climate was always stable and temperate.
33:12Gravity is three times stronger here than on our planet.
33:16It pulls mountain ranges down to a third of the height they'd be on Earth.
33:21Gravity also flattens the ocean bed, creating shallower seas,
33:25filled with volcanic island chains.
33:29And the nutrient-rich waters that surround these archipelagos provide the perfect conditions for life.
33:39In these other planets perhaps they have conditions which would make DNA get off the ground much earlier
33:45and flourish much more quickly.
33:49Finally, the super-Earth may be protected by a super-magnetosphere.
33:56The magnetic field strength is a condition both of the mass of the planet as well as its rotation speed.
34:04And so it is quite likely that a planet that is a couple of times bigger than the Earth's
34:10would be able to develop a stronger magnetic field,
34:13may shield the planet even better than our magnetic field shields us.
34:18Having a stronger magnetosphere would be advantageous for life on a super-Earth.
34:23If it were orbiting the Milky Way's most abundant type of star,
34:27the M-dwarf or red dwarf star.
34:34Red dwarf habitable zones are much closer in because their host star is so dim.
34:41It's as if you took the terrestrial planets in our own solar system and zapped it with a shrink ray gun
34:46and shrunk them down to orbital periods that are less than about 30 days,
34:50meaning that they are very close to their stars.
34:54Some astronomers believe this puts these planets at risk from solar activity, such as deadly flares.
35:04But a super-Earth with a super-protective magnetosphere may well deflect these deadly rays,
35:11allowing life to flourish under a sky full of stunning auroras.
35:15If one was standing on a super-Earth, we would see the aurorae come down to lower latitudes.
35:23You might get different colours.
35:27If I had the opportunity to travel to one of these exoplanets, I would snap that up pretty quickly.
35:36Most intriguing of all, if life does exist on a red dwarf super-Earth,
35:41it could be home to the oldest civilizations in the entire universe.
35:47The advantage of the M-dwarfs is that they last for much longer.
35:50And if you had a super-Earth then, keeping a strong magnetic field,
35:54going for billions and billions of years, especially now around a red dwarf
35:59that is going to exist for billions and billions of years,
36:02you might be in that perfect system where life can exist and evolve for billions and billions of years.
36:08A perfect system where life can exist and evolve into even more complex beings than us.
36:17And in among the stars near to us, red dwarfs are abundant,
36:22bursting with the potential for advanced life.
36:27But in our galaxy there are also cosmic events that can destroy life on a regular basis.
36:34So is there anywhere that is safe?
36:38Transcribed by ESO, translated by —
37:08And has found them by the thousands.
37:14For a long time, we didn't know if the other stars in our galaxy had planets.
37:19And for thousands of years there was no way to answer that question.
37:23Finally, now with modern technology, we can do that.
37:27And to our surprise, we found they are extremely common.
37:32From Kepler's small sample, astronomers believe there could be tens of billions
37:37of rocky Earth-like planets throughout the Milky Way,
37:40where life may already be thriving.
37:45But how many of these countless worlds have held on to this life long enough for intelligence to evolve?
37:55The answer may depend on a planet's position,
37:58The answer may depend on a planet's position in space.
38:05The universe is not a happy, safe place. The universe wants to kill us.
38:09It's incredibly violent out there.
38:12There are solar flares and supernovae and black holes and colliding galaxies
38:17and all these really amazingly dangerous and violent events.
38:20It's actually kind of amazing that we're here at all.
38:24In order to develop advanced intelligent life,
38:28an exoplanet may have to avoid these cosmic violent events for over 3 billion years.
38:34If we look at the history of the Earth, the first thing that happens that's important
38:39is the origin of life, right away, very quickly.
38:42But then, nothing for a long time.
38:44You have nothing but microbes stomping on the Earth.
38:47Single-celled microbes ruled the Earth for 2.5 billion years.
38:52Multicellular life has only been around for a billion years.
38:58Fish for 500 million.
39:00Mammals for 200 million.
39:03And modern humans have only walked the Earth for the last 200,000 years.
39:11It's clear that life is not a happy, safe place.
39:15It's clear that it takes a long time for intelligent life to develop.
39:23But most planets in the Milky Way aren't afforded this amount of time.
39:31Astronomers believe that a planet's position within a galaxy
39:35may determine whether it'll experience mass global extinction events.
39:40There's an idea of a habitable zone for a galaxy.
39:44And it's in analogy to the habitable zone around stars.
39:48Stars too close to the galactic centre are in the direct line of their violent neighbours,
39:54which frequently blast them with deadly high-energy radiation.
40:02In the middle of a galaxy, we have a lot of bright stars
40:06and young stars and maybe even supernovae going off.
40:09And so there's a very harsh radiation field. That's not good for life.
40:16Generated by the supermassive black hole that sits at the centre of the Milky Way,
40:20this cosmic destruction area stretches out around 8,000 light-years from the galactic centre
40:27and extends out along the densely packed spiral arms.
40:32Any planets that exist within this zone
40:35are likely to have life regularly obliterated from their surface.
40:41Fortunately for us, our star, the Sun, sits in a relatively empty, quiet zone
40:47between two of the galaxy's spiral arms.
40:54So there's this idea that there's a band in the middle of the galaxy
40:58that's the galactic habitable zone, where you don't have too many stars going off,
41:02you don't have too many supernovae, so it's quiet in that way.
41:05Those might be great places for complex life.
41:09These green zones are the quiet areas of our Milky Way galaxy.
41:14They're sheltered from the worst of the galaxy's radiation.
41:18It's these zones where Earth-like planets would experience long, uninterrupted periods of time,
41:24where life could take hold, to develop into more complex forms,
41:29and eventually, possibly, into intelligent life like us, or even superior to us.
41:38The galactic habitable zone is no more than a fledgling theory.
41:43If it's true, it reduces the number of places where advanced life could flourish in the Milky Way.
41:49However, it does mean that the places where advanced life could be should be near to us,
41:55which means that aliens are likely to be close by, too.
42:00And with our technology getting better every day,
42:03it surely won't be long before we find them.
42:07I think in 20 years' time, I'm going to be able to look up into the night sky
42:11and say there really is another galaxy out there,
42:15And in 20 years' time, I'm going to be able to look up into the night sky
42:17and say there really is another place I could stand like this and feel at home.
42:26To have cousins that we one day may communicate with
42:30seems to me to be potentially one of the greatest developments that humanity will ever experience.
42:36And if that isn't worth doing, I don't know what is.