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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 Kvan needs to size the star. Underneath it at a very low level is a little bit of
06:23hiss. That little bit of sssssssss is actually the light flickering that we're interested
06:30bit of ssss is actually the light flickering that we're interested in.
06:38By accurately measuring the level of this background hiss, Kvan can calculate the size
06:43of the star. In this case, it's around the same size as our star, the Sun. Kvan's work
06:52could be the breakthrough exoplanet hunters have been hoping for. It's cheap, the results
06:57are almost instantaneous, and once the size of the star is known, it's quite straightforward
07:02to calculate the size of the planet casting shadows over it.
07:09It feels like a very privileged time to be a scientist, to be an astronomer, working
07:15in this area and contributing to the hunt for the next Earth. Here we are, actually
07:21discovering these worlds by the hundreds, and now on the cusp of being able to identify
07:29the next Earth.
07:35Astronomers suspect there could be tens of billions of rocky, Earth-like planets in the
07:39Milky Way, places where perhaps even life has developed. But life as we know it requires
07:49water. So how can scientists possibly find this miracle substance on planets light-years
07:56away?
08:19Water is crucial to our world. Those with it prosper, those without it suffer.
08:26Remarkably, the water we drink today contains the same atoms as the water dinosaurs drank
08:35100 million years ago. It's the same water that formed clouds over the early Earth 4
08:44million years ago. And every organism that has ever existed on Earth has used this same
08:52supply of water as the biochemical source to keep it alive.
08:59On Earth, all life requires liquid water to grow and reproduce. It's the common ecological
09:05requirement for life.
09:07Liquid water is just so good for getting evolution going. Molecules can dissolve in the water,
09:13or actually interact with each other, form more complex chains.
09:16Now that's very rare. Hardly any other liquids do that. So liquid water is a natural starting
09:25place when you look out into the universe and say, what planets could possibly have
09:28life?
09:32To gauge how much liquid water is in space, astronomers had to first calculate how common
09:38water is in all its forms. Amazingly, it's found wherever they look.
09:46Water is incredibly common. In its gaseous form, we see water vapor filling the space
09:52between the stars. We see it in clouds of material that are actually forming new stars
09:57and planets right now.
10:02Since water is a fundamental building block of stars and planets, then it seems logical
10:06that exoplanets must have it in abundance.
10:10But 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 and the cup of porridge
10:28where one was too hot, one was too cold, and one was just right.
10:31When it comes to cooking up life like a porridge, you need to have an environment that's not
10:36too hot, not too cold, just right.
10:39And 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, on the Earth's
10:51orbit around the sun.
10:55But as they've found more and more exoplanets, they've had to re-evaluate the limits where
11:00liquid water can exist.
11:03There isn't a single distance. It depends on the brightness of your parent star.
11:07A dim star, you need to be closer. A hot star, very bright, you need to be farther away.
11:14But if the planet's atmospheres are rich with greenhouse gases, then they can still have
11:20liquid water even outside the habitable zone.
11:26You could have an atmosphere rich in carbon dioxide that has a strong natural greenhouse
11:30effect. And that means the planet could be much farther out, out of the Goldilocks zone,
11:35but there could still be liquid water because of the composition of the atmosphere.
11:41And planets orbiting much closer to their stars can avoid overheating if their surface
11:46is pale and highly reflective.
11:49When we search for habitable zones, we have to be careful to understand what we're really
11:54looking for.
11:58Astronomers have calculated how many rocky planets may lie within the Goldilocks zone
12:02of their stars. And it's over 30 billion potentially watery worlds.
12:11Even more remarkably, recent discoveries have shown that it's not just planets that can
12:15exist in the Goldilocks zone. There may be moons too, awash with oceans.
12:28Most of the planets we're finding are big Jupiter-sized planets. However, a lot of them
12:33are orbiting roughly where the Earth is orbiting the Sun. So even if the planet that we're
12:37finding can't support life, it could have a moon, a moon with an atmosphere that could
12:44support life.
12:47And the biggest of these moons may be similar to Earth.
12:56There could be billions upon billions of exo-moons out there, and even perhaps countless
13:01paradises teeming with life.
13:05David Kipping searches for exo-moons by looking for double dips in the brightness of distant
13:11stars.
13:13We look for exo-moons in a very similar way to the way that we look for planets, by looking
13:18for them to transit their host star. Now if that planet had a moon, then we should expect
13:25to have one big dip due to the planet, and then one smaller dip either to the left or
13:30to the right due to the moon.
13:35And some of these habitable exo-moons may have some of the most spectacular sights in
13:40the universe.
13:43Imagine a warm, rocky world similar to Earth, with oceans and mountains. But in the sky,
13:51there's a massive ringed planet, and another moon, that's ejecting hot magma into space.
14:08Exoplanets along with exo-moons hint at a galaxy filled with the possibilities for life.
14:14But for life, a rocky planet and liquid ocean may not be enough. Biology requires something
14:21else. Air.
14:34As the sun's light falls on Earth, a halo appears. A pale blue ring of light. Our atmosphere.
14:44And without it, we wouldn't be here.
14:49The Earth's atmosphere provides the gases that fuel the biochemistry of advanced life.
14:56But it also protects the oceans from the full ferocity of the sun's rays, preventing the
15:01water from boiling away into space. Without an atmosphere, there would be no wind, no
15:08rain, no fresh water, and no life.
15:16Atmospheres are absolutely essential for life. Take a look at the planet Earth, and you realize
15:23that just like the skin of the apple, the skin of the apple preserves the apple, well,
15:27the atmosphere of our planet preserves the oceans and makes possible the presence of
15:33life as we know it.
15:39Scientists in search of living exoplanets hope to detect the thin atmosphere that should
15:43surround these worlds. And to do so, they've turned to rainbows for inspiration.
15:54In the same way that water splits sunlight into a rainbow, astronomers use instruments
15:59to split starlight into a band of colors or spectrum. It's one of the oldest techniques
16:05in science, and one of the most revealing.
16:13Several hundred years ago, scientists first began to take something like a prism and put
16:17it in front of their telescope. So they started taking the light from stars like the sun and
16:21actually spreading it out into a spectrum. And what they saw was kind of surprising.
16:26So instead of seeing a continuous rainbow of light, they saw that rainbow, but they
16:30saw these dark lines superimposed on top.
16:36Each chemical element in the star's atmosphere absorbs different parts of the spectrum, creating
16:41signature dark bands.
16:45For instance, up at the top, there's a pair of lines in the yellow part of the spectrum
16:49which are due to sodium.
16:53This spectral analysis has taught us almost all that is known about stars today. But these
16:59same lines may hide a spectacular secret. The faint signal of an atmosphere, and perhaps
17:07life.
17:12So the challenge is that these planets are very small and very faint. So we can't actually
17:17go and directly measure the light emitted from the planet the same way that we go and
17:20measure this lovely spectrum for the sun. Instead we have to rely on more indirect methods.
17:26So one indirect way of doing that is to wait until the planet passes in front of the star.
17:33When the light of a star passes through an exoplanet's atmosphere, the gases that surround
17:37the planet should cast their own faint lines on the star's spectrum.
17:42So as we watch the light from the star transmitted through that atmosphere, its atmosphere is
17:47going to act like a little filter. So part of the starlight is going to pass through
17:51that atmosphere and we're going to see that imprinting extra lines on it which are due
17:56to the planet's atmosphere. So that change in the spectrum tells us something about the
18:00properties of the planet's atmosphere.
18:04The chemical astronomers want to find most is oxygen. This is because only life can produce
18:10enough oxygen to be easily detected. It's known as a biosignature. Astronomers are now
18:20looking to find biosignatures in the atmospheres of rocky exoplanets. But there's more than
18:25one way to search for these chemical signatures.
18:2927 to 29. All right, we're off.
18:37Ben Oppenheimer is part of a team trying to take direct photographs of exoplanets using
18:42massive ground-based telescopes.
18:44Awaiting instructions.
18:47We're within minutes of taking our first long exposure and I hope it's good.
18:55The toughest aspect of photographing exoplanets is the blinding light of the parent star which
19:01shines tens of millions of times brighter than the planet itself. The key is to stop
19:08the light of the star entering the telescope's sensors by blocking it, achieved by using
19:13a series of masks and lenses called a coronagraph.
19:18Right now we're standing right underneath the telescope's primary mirror and the light
19:21comes through a hole in the middle of the mirror and goes into this crazy box here which
19:27is full of optics, motors, sensors and electronics that all allow us to precisely control the
19:33starlight that's coming through the system.
19:38Software is then used to manipulate the coronagraph to block out the unwanted sunlight.
19:44Under good conditions we can actually carve dark holes into this image of the star so
19:51that we can see really faint things in those regions.
19:54But coronagraphs produce an intriguing problem. Errors within the optics produce tiny flares
20:00of starlight called speckles which appear just like exoplanets. However, Ben has developed
20:07an ingenious way to distinguish the planets from the speckles.
20:11So we've developed a technique where we exploit an aspect of speckles which is that they change
20:17position in the image depending on what colour you take your image at.
20:25Ben takes the same image of the star through different colour filters and runs them like
20:30a movie. The speckles appear to move across the screen, however the planets don't, allowing
20:38Ben to easily identify them.
20:41And so I'd like to point out that there is a little thing right here that if you watch
20:47for, you're careful, you'll notice that it doesn't move and the speckles are washing
20:52over it.
21:00This stationary blob is a candidate exoplanet. And below it to the left is a second. They
21:07both appear to orbit a star that's about 200 light years from the Earth. Just a decade
21:13ago capturing an image such as this through a telescope was unthinkable.
21:28But today there are hundreds of such pictures. And by analysing the light from these distant
21:34worlds, scientists can decipher their chemical composition and potentially the fingerprints
21:40of life.
21:45At this point we're studying much larger planets, gaseous things like Jupiter that
21:49most likely don't have any kind of life like we know it. But that's a first step and we're
21:55going to fainter and smaller and smaller planets as time goes on as we develop this technology.
22:02In the not too distant future, scientists may be able to simply scan a star for Earth-like
22:08planets and find it has the biosignature of life.
22:14We can look right at the light from a little planet around its distant star and that opens
22:18up a whole range of possibilities for us to not just detect the planet but to study the
22:23planet.
22:24I mean this all sounds like science fiction but there is a reality to this. We have a
22:28scientific method to actually determine whether there is life on another planet.
22:36Life is one thing, but life that's intelligent is another altogether. It requires billions
22:43of years of evolution and a powerful force field, similar to that which we owe our lives
22:50to every day.
22:58If somebody from outside our solar system were to study it, they may well make a surprising
23:20observation. Of the 8 planets that orbit our sun, they might calculate that 2, not 1, are
23:28suitable for life. And this would be because the sun has 2 planets orbiting within its
23:34Goldilocks zone, the Earth and Mars.
23:40Both planets have surfaces warm enough for liquid water to pool on. Yet while the Earth
23:45is blessed with warm oceans, Mars is dry and dead.
23:51There is one crucial difference between the two planets and it could be the key to finding
23:56habitable exoplanets. A magnetic field.
24:08Our sun is constantly emitting deadly radiation out towards Earth. And we're only protected
24:14because of our magnetic field, or magnetosphere. Without it, the solar wind would blow our
24:22atmosphere away. And without an atmosphere, liquid water couldn't exist on the surface.
24:30In order to have liquid water, not only do you need the right temperature, but you need
24:33the right pressure. You know, if there were no atmosphere here right now, even at the
24:37same temperature we are today, all of the water would boil off into vapor immediately.
24:43So where does Earth's magnetosphere come from? And why doesn't Mars have one?
24:52Actually in the past, both Earth and Mars had magnetospheres. However Mars lost it around
24:584 billion years ago. And along with it, the potential for life to develop.
25:07Earth and Mars were created during a period of violence. Asteroids smashed into their
25:13surfaces, turning rock and metal into a molten mass. As they started to cool, Earth's magnetosphere
25:21became a solid crust formed on the surface. But the molten metal below churned as the
25:26planets turned, inducing a magnetic field, which rose high up above the surface of both
25:32planets. Active volcanoes expelled gas into the space around each planet. Protected by
25:42the newly formed magnetic field, these gases built up into thick atmospheres, creating
25:49the air pressure for liquid water to run on the surface. For over 100 million years,
25:55both Mars and Earth were warm, wet paradises, ready for life to evolve. Then Mars' magnetic
26:04protection suddenly disappeared. The solar wind blew its atmosphere into space. Then
26:12its oceans boiled away, leaving the dry, sterile red rock we know today.
26:21Mars' fundamental problem is that it's smaller than Earth. And because it's smaller, the
26:26internal core of Mars cooled down and solidified. And once it becomes a solid metal, there's
26:34no more magnetic field. The magnetic field shuts off, essentially, and the atmosphere
26:39therefore is vulnerable to both energy and radiation from the sun and the rest of the
26:45galaxy, and probably just blew off. Whatever life was on there, at least on the surface,
26:52is now completely exposed. All rocky planets will eventually lose their magnetospheres
26:58as their cores cool and turn solid. So one way to find out if an exoplanet is alive is
27:06to discover if its magnetosphere is still active. Yet magnetospheres are difficult to
27:12measure, because they're incredibly weak. The Earth has a magnetic field of approximately
27:20half a gauss, which, when you think about it, is actually really weak. Our fridge magnets
27:25are about 100 gausses. They're much stronger. Exoplanets are far too far away for these
27:33weak magnetic fields to be measured directly. But there is an indirect method. When electrons
27:42in the solar wind interact with the planet's magnetosphere, they emit radio waves that
27:47beam out into space. It turns the planet into a giant radio beacon. Astronomers such as
27:55Evgenia hope to use these signals to detect habitable exoplanets. Not only that, the frequency
28:05of the signal should also indicate how big the planet is. If we're looking for the magnetic
28:15signature in the radio waves of a giant planet, say a hot Jupiter, we expect it to have a
28:21strong magnetic field, and therefore it would have a high frequency, at around 100 megahertz,
28:27kind of where the limit of this radio is. However, a weaker field like Earth's requires
28:33us to go down to lower and lower frequencies. So instead of 100 megahertz, we go down to
28:3810 megahertz. But looking for exoplanets at 10 megahertz presents a unique challenge,
28:46because the Earth's own magnetosphere generates a signal that interferes with this search.
28:58So to find other Earths using radio requires a dish in space.
29:06When we want to look for magnetospheres of extrasolar planets, we really need to get
29:12outside of the Earth-Moon system in order to get away from all the radio frequencies
29:17that are bouncing around the Earth. With the development of new techniques and
29:25technologies, scientists are edging closer to finding a second Earth.
29:34I wouldn't be surprised if we have that data about an Earth and about life on it around
29:39another star in 10 or 15 years. I'm hoping to see that soon.
29:46Using shadows, light spectrums, and now radio, scientists have the tools to detect a planet
29:53similar to our own. But in the rush to find a planet like Earth,
30:00are we missing something? What if Earth is an outlier, an inexplicably
30:07lucky place on the very fringes of habitability? Is there another sort of planet that's even
30:14better for life to evolve? For years, astronomers have scanned the skies
30:29for planets that could sustain life. They've based their search on the Earth, seeking the
30:36exact same conditions and the exact same size. I think right now there's a huge focus to
30:45finding Earth-like planets. Now, whether or not there actually is life there, that is
30:52another question altogether. But after 20 years of searching for another
30:57Earth, the scientists may be about to change their focus.
31:02Recent observations have revealed a brand new class of planet, one that may eclipse
31:07our own. Astronomers call this mysterious new class
31:18of planets super-Earths. Super-Earths are about three to five times
31:23the mass of the Earth, and there's nothing like that here. We don't know what they're
31:27like. It's an entirely alien sort of planet. In recent years, astronomers have begun to
31:34imagine the conditions on this new class of planet, and they've come to a startling conclusion.
31:41Super-Earths could be even more habitable. There are probably planets out there that
31:48are even more hospitable for life, planets that have even more chemicals necessary to
31:54create the organic materials that create life, conditions that make it more likely
31:59to get life off the ground. Imagine a rocky planet twice the size of
32:05the Earth, with volcanism on the surface that indicates a vast source of heat and magma
32:11within its core. We expect that a heavier Earth will be more
32:19geologically active, that the increased amount of geothermal heat within the super-Earth
32:27will lead to stronger motions of the magma underneath the crust.
32:36Erupting volcanoes dot the surface of this super-Earth. Their gases feed a super-thick
32:44atmosphere, and help to regulate a super-stable climate.
32:50Many times, life on Earth was nearly extinguished. For example, once upon a time the Earth was
32:56snowball Earth, completely covered in ice. Maybe in these other planets, there are Earths
33:03in which snowball Earth never happened, that the climate was always stable and temperate.
33:11Gravity is three times stronger here than on our planet. It pulls mountain ranges down
33:18to a third of the height they'd be on Earth. Gravity also flattens the ocean bed, creating
33:24shallower seas, filled with volcanic island chains. And the nutrient-rich waters that
33:31surround these archipelagos provide the perfect conditions for life.
33:39In these other planets, perhaps they have conditions which would make DNA get off the
33:43ground much earlier, and flourish much more quickly.
33:48Finally, the super-Earth may be protected by a super-magnetosphere.
33:58The magnetic field strength is a condition both of the mass of the planet, as well as
34:03its rotation speed. And so it is quite likely that a planet that is a couple of times bigger
34:10than the Earth's would be able to develop a stronger magnetic field, may shield the
34:15planet even better than our magnetic field shields us.
34:20Having a stronger magnetosphere would be advantageous for life on a super-Earth, if it were orbiting
34:25the Milky Way's most abundant type of star, the M-dwarf or red dwarf star.
34:34Red dwarf habitable zones are much closer in, because their host star is so dim.
34:42It's as if you took the terrestrial planets in our own solar system, and zapped it with
34:46a shrink ray gun, and shrunk them down to orbital periods that are less than about 30
34:51days, meaning that they're very close to their stars.
34:56Some astronomers believe this puts these planets at risk from solar activity, such as deadly
35:01flares. But a super-Earth with a super-protective magnetosphere may well deflect these deadly
35:10rays, allowing life to flourish under a sky full of stunning auroras.
35:20If one was standing on a super-Earth, we would see the aurorae come down to lower latitudes.
35:26You might get different colors.
35:31If I had the opportunity to travel to one of these exoplanets, I would snap that up
35:37pretty quickly.
35:40Most intriguing of all, if life does exist on a red dwarf super-Earth, it could be home
35:46to the oldest civilizations in the entire universe.
35:52The advantage of the M dwarfs is that they last for much longer. And if you had a super-Earth
35:56then, keeping a strong magnetic field, going for billions and billions of years, especially
36:02now around a red dwarf that is going to exist for billions and billions of years, you might
36:07be in that perfect system where life can exist and evolve into even more complex beings than
36:13us.
36:16Located in among the stars near to us, red dwarfs are abundant, bursting with the potential
36:22for advanced life.
36:27But in our galaxy, there are also cosmic events that can destroy life on a regular basis.
36:33So is there anywhere that is safe?
36:47The hunt for exoplanets is well underway. The Kepler space telescope has scanned the
37:05stars near to us for planets, and has found them by the thousands.
37:13For a long time, we didn't know if the other stars in our galaxy had planets. And for thousands
37:20of years, there was no way to answer that question. Finally, now, with modern technology,
37:26we can do that. And to our surprise, we found they are extremely common.
37:35From Kepler's small sample, astronomers believe there could be tens of billions of rocky Earth-like
37:41planets throughout the Milky Way, where life may already be thriving.
37:47But how many of these countless worlds have held on to this life long enough for intelligence
37:53to evolve?
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. It's incredibly
38:11violent out there. There are solar flares and supernovae and black holes and colliding
38:16galaxies and all these really amazingly dangerous and violent events. It's actually kind of
38:21amazing that we're here at all.
38:28In order to develop advanced, intelligent life, an exoplanet may have to avoid these
38:33cosmic violent events for over three billion years.
38:38If we look at the history of the Earth, the first thing that happens that's important
38:42is the origin of life right away, very quickly. But then, nothing for a long time. You have
38:47nothing but microbes stomping on the Earth.
38:52Single-celled microbes ruled the Earth for two and a half billion years. Multicellular
38:58life has only been around for a billion years. Fish for 500 million. Mammals for 200 million.
39:08And modern humans have only walked the Earth for the last 200,000 years.
39:16It's clear that it takes a long time for intelligent life to develop. But most planets in the Milky
39:24Way aren't afforded this amount of time.
39:31Astronomers believe that a planet's position within a galaxy may determine whether it'll
39:36experience mass global extinction events.
39:40There's an idea of a habitable zone for a galaxy. And it's in analogy to the habitable
39:46zone around stars.
39:49Stars too close to the galactic center are in the direct line of their violent neighbors,
39:55which frequently blast them with deadly high-energy radiation.
40:02In the middle of a galaxy, we have a lot of bright stars and young stars and maybe even
40:07supernovae going off. And so there's a very harsh radiation field. That's not good for
40:12life.
40:17Surrounded by the supermassive black hole that sits at the center of the Milky Way,
40:21this cosmic destruction area stretches out around 8,000 light-years from the galactic
40:25center and extends out along the densely packed spiral arms.
40:35Any planets that exist within this zone are likely to have life regularly obliterated
40:40from their surface.
40:44Fortunately for us, our star, the Sun, sits in a relatively empty, quiet zone between
40:50two of the galaxy's spiral arms.
40:57So there's this idea that there's a band in the middle of the galaxy that's the galactic
41:01habitable zone, where you don't have too many stars going off, you don't have too many supernovae,
41:06so it's quiet in that way. Those might be great places for complex life.
41:11These green zones are the quiet areas of our Milky Way galaxy. They're sheltered from the
41:17worst of the galaxy's radiation. It's these zones where Earth-like planets would experience
41:23long uninterrupted periods of time, where life could take hold, to develop into more
41:30complex forms, and eventually, possibly, into intelligent life like us, or even superior
41:38to us.
41:41The galactic habitable zone is no more than a fledgling theory. If it's true, it reduces
41:48the number of places where advanced life could flourish in the Milky Way.
41:55However, it does mean that the places where advanced life could be should be near to us,
42:01which means that aliens are likely to be close by, too. And with our technology getting better
42:08every day, it surely won't be long before we find them.
42:13I think in 20 years' time, I'm going to be able to look up into the night sky and say
42:17there really is another place I could stand like this and feel at home.
42:25To have cousins that we one day may communicate with seems to me to be potentially one of
42:32the greatest developments that humanity will ever, ever experience. And if that isn't worth
42:38doing, I don't know what is.

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