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00:00Earth, the cradle of humanity throughout our existence.
00:17But it won't be forever.
00:22All it would take would be one giant meteorite to wipe us off the face of the Earth.
00:33It's not just meteors.
00:36Our planet will change.
00:39Our planet could freeze over or it could heat up.
00:43And our sun will eventually die.
00:47We are actually near the end of habitability of Earth.
00:52To survive in this universe, we need an insurance policy to colonize other worlds.
01:04Having multiple planets which are colonized is really in our interest for our own survival.
01:11So can we find a new home in the galaxy?
01:17Can we find Earth 2.0?
01:39For centuries, we only knew of the handful of planets in our own solar system.
01:49Now astronomers are finding thousands of new worlds around alien stars, exoplanets.
01:59We are discovering exoplanets by the bucketful.
02:05There are as many planets out there as there are stars.
02:11And there are hundreds of billions of stars in the galaxy.
02:17But out of billions of exoplanets, are there any that could offer new opportunities for
02:23humanity to thrive and provide sanctuary in a dangerous universe?
02:31Is there an Earth 2.0?
02:37Is Earth 2.0 out there?
02:40That would be truly amazing.
02:49August 2016.
02:53Astronomers announce that Earth 2.0 could be closer than anyone ever expected.
03:02A planet orbiting the sun's nearest stellar neighbor, the red dwarf, Proxima Centauri.
03:14So it turns out that our nearest star neighbor has an exoplanet.
03:19It's only about four light years away.
03:21That means that it's actually potentially possible for us to get there and to explore
03:26it.
03:29Scientists name the planet after the star, Proxima Centauri b, or Proxima b for short.
03:37A world that appears to be a lot like Earth.
03:45From the way it's tugging on the star, Proxima Centauri, we know that it has 1.3 times the
03:51Earth's mass, but it's roughly the same size as the Earth.
03:58Of the exoplanets we know about, most are uninhabitable gas giants, like Jupiter.
04:05Proxima b is a rare find, an Earth-sized planet.
04:11But an Earth-sized planet might not be Earth-like.
04:16A true second Earth must also be the right distance from its star.
04:24The habitable zone, or some people call it the Goldilocks region, is a distance away
04:29from the star where you're not so close where you're going to burn up and evaporate all
04:34of your liquids, and you're not so far away where you're frigid and cold.
04:39So it's that special region where it's just right.
04:46Does Proxima b lie in this region?
04:49Could it have liquid water?
04:53Proxima Centauri b orbits its star once every 11.2 days.
04:57So compare that to the Earth, which goes around the sun once every 365 days.
05:02That's because the planet is much, much closer to the star than the Earth is to the sun.
05:12Earth orbits 93 million miles from the sun.
05:18Proxima b orbits 20 times closer, under 5 million miles from its star.
05:26You might think that Proxima b should be a fried world, a burnt-out husk, if you will.
05:36But Proxima b's sun is very different than ours.
05:44At just over 5,000 degrees Fahrenheit, it's half as hot and roughly eight times smaller.
05:52An M-class red dwarf star.
05:56An M-dwarf, the Proxima b is around, is much less bright, much less hot.
06:01So you can orbit much closer to that star and be at the same temperature that we are
06:05here on Earth.
06:12Proxima b's tight orbit around the red dwarf could make the planet habitable.
06:18But it would be very different from Earth.
06:24The star dominates the sky, lighting any oceans and mountains with an alien red glow.
06:35So Proxima b may be the Earth 2.0 that we've been looking for.
06:42But in 2017, the dim red dwarf star erupts in a way that's unlike anything we've seen
06:50before, blasting the planet with radiation, a megaflare.
07:03Megaflares are like solar flares, but they can be much more powerful.
07:06In fact, they can outshine the star itself.
07:10Our sun releases powerful solar flares when its magnetic field becomes tangled.
07:20But the megaflare is 10 times stronger than our sun's strongest flares.
07:29On an M-dwarf star, that magnetic field can get a lot more tangled than on our own sun.
07:35That means that when a flare happens, it can release a lot more energy.
07:43Scientists believe that megaflares like this are planet killers.
07:51Radiation tears the atmosphere from the planet, and these megaflares hit Proxima b roughly
07:59once every year.
08:03Red dwarf stars are incredibly temperamental.
08:07They are not good parents to their planets.
08:11So if Proxima b did have an atmosphere at one point, it would have been stripped away
08:17by one of these violent outbursts.
08:22Leaving Proxima b dangerously exposed to space.
08:29An atmosphere dampens the temperature gradients between light and shadow.
08:34So in sunlight, it is just burning hot.
08:38But right next door in a shadow, it is freezing cold.
08:44Without an atmosphere, Proxima b would be a barren wasteland, blasted by intense radiation
08:51from its star, completely uninhabitable.
08:59Proxima b, perhaps best shot at finding Earth 2.0 so far, is actually a dried out husk of
09:06a world that has lost its atmosphere, maybe lost any water that it also harbored, simply
09:11by being that close to its parent star.
09:20Proxima b may be the nearest exoplanet, but it's not the only option.
09:25The future of humanity may lie in an incredible star system just 40 light years away.
09:37We've just found a really exciting system where there's not just one chance to have
09:40a new Earth, but seven.
09:51In an unforgiving galaxy, finding Earth 2.0 could be the difference between extinction
10:02and survival.
10:07It's a pretty wild place out there.
10:11Our planet's not going to be here forever, and it would be wonderful if we could find
10:14a place like it that we could live.
10:21The future of humanity lies on an alien exoplanet.
10:26The question is, where?
10:312016, astronomers scan the skies with the new Transiting Planets and Planetesimals Small
10:40Telescope, or TRAPPIST.
10:46They look for the flickering of a star caused by the silhouette of a planet.
10:54The planet can pass in front of the disk of its star once per orbit, causing a little
10:59mini-eclipse, a little dimming temporarily in the light of the star.
11:05They spot the dimming of a nearby red dwarf star just 40 light years from Earth.
11:12The first alien system detected by the telescope, the incredible TRAPPIST-1 system.
11:23The TRAPPIST-1 discovery was a really great bang for our buck, in a sense, because we
11:30found seven exoplanets all at once.
11:35But are any of these seven planets actually habitable?
11:39With the worlds of the TRAPPIST system, there's probably a range of climates. The innermost
11:43ones are probably very hot. You might even be looking at lava worlds. Farther away, they're
11:50probably worlds of ice.
11:55But the middle planets, D, E, and F, are all prime candidates.
12:02It's exciting to think that three of the planets orbiting TRAPPIST-1 are in the habitable zone,
12:08are at the right distance from that star to have liquid water on their surface.
12:17And one planet stands out as a new Earth, orbiting just 2.7 million miles from the star,
12:25TRAPPIST-1e.
12:29The composition of TRAPPIST-1e suggests that it could have a pretty significant iron core,
12:35kind of like the Earth does. There's a potential there for a very powerful magnetic field.
12:42Like Earth, TRAPPIST-1e could host a protective magnetic field, deflecting the harsh solar
12:48winds and powerful outbursts that strip away atmospheres.
12:55So a magnetic field is a good thing. It's a kind of a protection from the evil forces
13:00of the star that you're orbiting around.
13:03And unlike Proxima Centauri, the TRAPPIST-1 star appears unusually quiet.
13:11TRAPPIST-1 is actually a very old, much calmer star and doesn't undergo a lot of these huge
13:16flares like Proxima Centauri does.
13:18And so it's a somewhat, perhaps, better system to look for an Earth-like planet, an Earth 2.0.
13:28The data suggests that TRAPPIST-1e could have vast oceans, a protective atmosphere, and
13:35habitable temperatures.
13:43But living here would be nothing like living on Earth.
13:52The thing to keep in mind about the TRAPPIST system is that it's very unlike our own. The
13:57planets are much closer in. So because they're closer in, their orbits are faster and smaller.
14:04On TRAPPIST-1e, an entire year takes just six Earth days.
14:11Can you imagine you're just basically tearing your calendar days off, day after day after
14:14day, really quickly? Your birthday would be today, and then tomorrow, happy birthday again!
14:19Wedding anniversaries, you're constantly forgetting your wedding anniversary. It would be hard.
14:26And on this strange and alien world, explorers would witness sights unlike anything seen
14:33before.
14:33In a lot of ways, it really is sort of a science fiction sky, the kind of things that are envisioned
14:40in movies.
14:41You could look up and see the other planets in your sky, much like how we can see our
14:45own moon. You could physically resolve features on the surface, such as continents, with your
14:51own eyes.
14:54But could this planet be too good to be true?
14:59So we could have a potentially habitable planet that's really close to its star, but other
15:06issues arise when you have a solar system that's that compressed. And one of those is
15:10the potential for tidal locking.
15:16Orbiting just a few million miles from the star, TRAPPIST-1e is likely tidally locked,
15:25with one side facing the star forever.
15:30So you could imagine a situation where, gosh, it's constant day, and it might just produce
15:35something that's like a scorched earth, kind of like what we see behind me. But on the
15:40other side, it is constant night. And so in that case, it might just be like a frozen
15:44wasteland.
15:47And TRAPPIST-1e's problems get even more extreme.
15:51If you have a permanent day side and a permanent night side, the night side of the planet is
15:57going to get so cold that everything just freezes out, including the atmosphere.
16:06The gases of TRAPPIST-1e's atmosphere could freeze into solid ice on the frigid night
16:12side of the planet. And the gases on the day side burn away. The atmosphere thins, and
16:22eventually disappears. And TRAPPIST-1e ends up completely inhospitable.
16:31So even though we found maybe a perfect planet around a star, the type of star and where
16:38it's orbiting can have a really important effect as to whether or not that planet might
16:42be habitable.
16:46Despite its apparent potential, our future is not in the TRAPPIST-1e system. The search
16:54for truly Earth-like planets continues.
16:59In the TRAPPIST-1 system, we find a very Earth-like world, but the star it's orbiting
17:05is not very Sun-like. So what we should be looking for, perhaps, is an Earth-like planet
17:11around a Sun-like star.
17:15To find Earth 2.0, we need a Sun 2.0.
17:29The Milky Way. Home to hundreds of billions of stars. Ranging from dim, explosive red
17:46dwarves to short-lived blazing giants.
17:54But in the middle are stars like our Sun.
18:03Strictly speaking, if we really want Earth 2.0, we need to look for planets around stars
18:08like our Sun.
18:11Stars like our Sun are calm and stable, with long lives. And the habitable zone lies far
18:19enough away that planets avoid tidal locking.
18:23We stand a much better chance of colonizing a planet around a Sun-like star.
18:30We're hunting for planets in the habitable zone of stars like our own Sun, and we have
18:37found worlds there.
18:45Worlds like Kepler-452b, an exoplanet 1,800 light-years away, orbiting the same type of
18:56star as our Sun.
18:59You really couldn't ask for a more Earth-like orbit around the star. The year is about 385
19:06days. We're 365 days. This really is very much like the Earth.
19:12The planet orbits its star at roughly the same distance as the Earth orbits the Sun,
19:19and could be very much like home.
19:24Kepler-452b is in the habitable zone of its star, so if there is liquid water there, there
19:31could be oceans and lakes and rivers and streams and blue skies and cloudy days.
19:41Sounds nice, but Kepler-452b is a lot larger than Earth.
19:50Kepler-452b is a great Earth 2.0 candidate, but it's sort of like Earth on steroids.
19:59This world is about five times more massive than our own planet, and about 60 percent
20:06wider.
20:08Scientists call large worlds like Kepler-452b super-Earths.
20:18These worlds are maybe one and a half or two times the size of the Earth, with maybe as
20:23much as ten times the mass.
20:26Could this super-sized, Earth-like planet be our second home?
20:31A planet like this seems to meet a lot of our standards for an Earth 2.0. It's around
20:36a star like our Sun, it's smack dab in the middle of the habitable zone.
20:41The problem is there are other factors at play. One of those is simply the mass of the
20:45planet.
20:53Kepler-452b's size has an extreme effect on its gravity.
20:59Because of its incredible mass, the gravity on the surface is about twice what we feel
21:05here on the Earth.
21:09That extra gravity would make colonizing the planet difficult.
21:14Just about any chore you can imagine doing that you don't like doing on the Earth, you're
21:18going to like it even less on a planet like that. When the garbage can weighs twice as
21:22much as it does here on the Earth, that's not going to be very much fun.
21:26Maybe LeBron James and I will be okay, but normal humans, I'm not so sure.
21:33And we could be stuck on the planet's surface.
21:38If you landed on the surface of one of these super-Earths, it would be pretty easy to get
21:42down onto the surface, but it would be very difficult to get back up.
21:47It's already incredibly difficult for us to leave the Earth. Think of our giant engines
21:52and rockets, these incredible miracles of engineering that we need to blast off. You
21:58need twice that to get off of Kepler-452b.
22:05And to make matters worse, Kepler-452b's atmosphere is thought to be radically different
22:12from Earth's.
22:14In some sense, how big the planet is, how massive it is, will determine what its atmosphere
22:19is like. If you have a lot of mass and a lot of gravity, you can hold on to a lot of air.
22:25You can have a much larger atmosphere, much thicker, much denser, and higher pressure
22:29at the surface.
22:32A thick atmosphere could trap heat from the star. Surface temperatures become ferociously
22:40hot, and crushing pressures make the surface completely uninhabitable.
22:47So it's possible this planet has a very thick atmosphere that's become more of a runaway
22:51greenhouse effect. The planet's gotten hotter and hotter over time. Maybe instead of finding
22:56an Earth 2.0, what we've found is a Venus 2.0.
23:05Super-Earths may have an appealing name, but their intense gravity would make them difficult
23:11to live on, and we could not survive in their thick atmospheres.
23:22So far, all the worlds we've found have turned out to be uninhabitable.
23:28But what if our new home is not a planet?
23:34Earth 2.0 may not be an exoplanet at all. It might be an exomoon.
23:45We live in a cosmic shooting range, where planets die every day.
24:05But backup planets, like our own, seem almost impossible to find.
24:11Have we been looking for the wrong thing?
24:15I think there's a pretty good chance that Earth 2.0 might not be a planet per se,
24:20but actually a moon of a giant planet.
24:24The exciting thing about an exomoon is that they could potentially be habitable.
24:28So is it possible that as we look at different solar systems, the real analog for Earth 2.0
24:33will turn out to be an exomoon?
24:422017. The Kepler telescope scanned a sun-like star 8,000 light-years away,
24:50and Professor David Kipping and his team watched the transiting exoplanet,
24:57Kepler-1625b.
25:02Kepler-1625 was one of the many thousands of planets discovered by Kepler,
25:08but what made it different from our perspective as a moon hunter was that this is a planet
25:14which was Jupiter-sized, far away from its star, and apparently on a near-circular orbit.
25:20So everything that we want for finding exomoons.
25:29The exoplanet Kepler-1625b is an uninhabitable gas giant, like Jupiter.
25:38But it is in the habitable zone, and that means its moons would be, too.
25:47Unfortunately, these exomoons are incredibly hard to see.
25:55The way that Kepler finds exoplanets out there really does relate to the size of the planet,
26:01and for moons it's much, much more difficult because it's smaller, so it's harder to detect.
26:06The largest moon in the solar system is Ganymede around Jupiter.
26:10It's about 40% the size of the Earth, and we really very rarely detect planets that small.
26:16So, of course, looking for exomoons is going to be very, very challenging.
26:26In 2018, the team recruited the powerful Hubble Space Telescope
26:32and used the data to hunt for the tiny silhouette of any moons.
26:39If you have an exomoon orbiting a planet, sometimes it's going to lead the planet when it transits the star,
26:44and sometimes it's going to trail behind as it transits the star,
26:48and you see a little bump in the transit dip itself at different places.
26:54And the team detected the signal.
26:58Not one, but two objects orbiting together.
27:04Confirmation of the first exomoon ever discovered.
27:18It was an amazing discovery.
27:21I've been looking for exomoons my entire career.
27:24For 10 years we have been in this quest to try and find these things.
27:28This discovery, this announcement, was absolutely remarkable.
27:33Not only does it mean that we might find Earth twins everywhere in the Milky Way,
27:37but it gives us something to strive for for human exploration.
27:45On this alien exomoon, the skies would be nothing like Earth's.
27:51Visually, I think it would be an absolutely stunning place to be.
27:55You look up in the sky and you see this ringed planet looming huge in the sky.
28:03A world that could be like Earth, only orbiting another planet.
28:15But don't pack your space suit just yet.
28:19Even though the planet and the moon are potentially the right distance away from the star
28:24that we might imagine to be in liquid water on the surface,
28:27both the moon and the planet are likely gaseous objects with no solid surface to speak of.
28:35Although the moon probably isn't habitable,
28:38it is an important step for finding worlds like our own in the galaxy.
28:44If we find exomoons around exoplanets,
28:47that potentially hugely increases the number of habitable worlds that are out there.
28:52We just need more accurate measurements,
28:54and then all of a sudden the universe is going to be full of exomoons.
29:03But these worlds need to be more than just Earth lookalikes.
29:09Everyone gets very excited when we find Earth-like planets around other stars.
29:13But Earth-like kind of just means how big it is and whether it can support liquid water,
29:18where it is in relation to its star.
29:20All of that is great, but it's just not enough.
29:28A planet's composition could be make or break for our new home.
29:34The difference between the perfect world and a ticking time bomb.
29:55The hunt for Earth 2.0 is still on.
30:00We've examined intense red dwarf systems,
30:05massive super-Earths,
30:08and alien exomoons.
30:11But so far, there's no place like home.
30:16There are all these criteria we have to tick off.
30:19A sun-like star,
30:21an orbit that puts it at about the right temperature,
30:24a solid surface, something that could retain an atmosphere.
30:29But a planet that appears Earth-like on the outside
30:32may not be Earth-like on the inside.
30:39One of the things that makes our world so unique is its plate tectonics,
30:43and that actually regulates our climate.
30:48The Earth's climate depends on cycles of materials,
30:51like carbon dioxide and water.
30:54Molecules move between the Earth's molten interior and the surface
30:59through active plate tectonics and volcanic eruptions.
31:05These cycles help to regulate the temperature
31:08and composition of the Earth's atmosphere.
31:14If we were to find another Earth-like planet out there
31:17and it had geologic activity,
31:19that means that at least it has the means to sustain
31:22the carbon cycle and all of these natural phenomenon
31:25that makes this planet habitable and sustainable.
31:34How can we know what's happening inside a planet?
31:40A clue can be found in vast ranges across our world.
31:45Mountains.
31:53These topographical features are an indicator that the planet is alive
31:58and there is still processes happening underneath its surface.
32:06Mountain ranges are created when a planet's tectonic plates collide.
32:13And even though exoplanets are light-years away,
32:16astronomers could work out whether their surfaces are smooth
32:21or covered in peaks.
32:27Those mountain ranges are poking out,
32:29and depending on which rotation the planet is in,
32:32the planet will appear very slightly bigger or very slightly smaller
32:36depending on the silhouette which is being cast.
32:43These tiny changes in light could be the sign
32:47that an exoplanet is healthy and active.
32:52But we can only use this method when a planet is in front of its star.
32:59What if astronomers could use starlight itself
33:02to determine the geology of a planet?
33:07We think that planets form at roughly the same sort of time that stars form,
33:12and they all form from this same giant cloud of material.
33:17And so if you measure the composition of a star,
33:20then it seems reasonable to take those values
33:23and assume they're somewhat similar for the planets as well.
33:29Astronomers can work out what chemical elements are present in the star
33:34by splitting its light into different wavelengths.
33:39And any planets around that star will have a similar chemical composition.
33:45Composition is actually a really important part
33:48of whether or not it's actually going to be habitable.
33:51The composition really is its geology.
33:57Rocky exoplanets are all made from the same basic ingredients,
34:03chemical elements like oxygen, silicon, and aluminum.
34:08Change the balance of ingredients, and you get very different planets.
34:14If we have some idea of the composition of a rocky planet,
34:18we can actually use that to give us clues
34:21as to whether a world has or doesn't have plate tectonics.
34:29New research indicates that exoplanets with too much silicon and sodium
34:33form different types of rock than those on Earth,
34:38creating rigid planets where plate tectonics stall
34:42and carbon dioxide builds up with devastating consequences.
34:50Without active geology, we end up with maybe a Venetian atmosphere.
34:54That means there's a runaway greenhouse effect.
34:57It's gotten hotter and hotter. Gases are baked out of the rocks.
35:00There's no way to actually rain them back out.
35:03Not a good place for life at all.
35:06And worse, the planet becomes a pressure cooker waiting to explode.
35:13If we change the composition of a planet, it affects its tectonic system.
35:17That entirely changes how a planet loses heat,
35:20and the heat builds up and builds up and builds up,
35:23and then maybe there's a catastrophic overturn of the crust.
35:36The solid crust of the planet collapses.
35:43Oceans of lava bubble up,
35:46and a greenhouse atmosphere boils the surface.
35:52A violent end to a potential new home.
35:58Clearly, you need to know about the composition of those planets
36:02before you can start making statements about how habitable those worlds truly are.
36:10But there's something else that a planet needs to be Earth-like.
36:15An invisible shield that protects it from the dangers of space,
36:21providing warmth and life-giving water, an atmosphere.
36:33Explosions
36:40The hunt for Earth 2.0 has turned up plenty of planets.
36:45But for a planet to be like Earth, it has to check a lot of boxes.
36:52If you're really looking for Earth 2.0,
36:55then you're going to have to find a planet that's the same mass and size as Earth,
36:59orbiting a sun-like star at about the same distance,
37:03with a similar atmosphere and a lot of surface water that's in liquid form.
37:09Good luck.
37:15And on the list of requirements, an exoplanet's atmosphere is critical.
37:22It protects the planet from huge temperature swings.
37:25It protects the planet from small asteroid impacts.
37:28It protects the planet from dangerous radiation from space and from the star.
37:32It is almost literally a shield around the planet, protecting us from outer space.
37:38But it also has to be the right kind of atmosphere.
37:46Get it wrong, and the planet can have crushing, boiling conditions on the surface.
37:52Look at our own solar system.
37:54The sun's habitable zone includes three different planets, Venus, Earth and Mars.
37:59But Mars has a thin atmosphere and is too cold.
38:02Venus has too thick of an atmosphere and is too hot.
38:05We're the only planet that happens to be just right.
38:13So far, astronomers have mostly had to guess if these exoplanets have atmospheres.
38:19But now, we're looking for them directly.
38:24Searching for Earth-like atmospheres around Earth-like planets.
38:31This is incredibly hard to do.
38:34So in order to look at the details of these atmospheres and the glare of the star
38:39requires incredibly precise technology and precise measurements.
38:46Astronomers detect atmospheres by watching a planet pass in front of the star.
38:54A small fraction of light shines around the edge of the planet and through the atmosphere,
39:01where molecules like water, hydrogen and carbon dioxide
39:05absorb particular wavelengths of light from the star.
39:09If we can see the light of the star shining through around the planet,
39:14we can maybe deduce some information about, does it have an atmosphere?
39:18What are the properties of that atmosphere? How hot is it? What's it made out of?
39:23That's how we'll be able to determine if things in the atmosphere
39:27might indicate that the surface is hospitable to life.
39:30So far, we haven't seen any exoplanets with atmospheres that we could live in.
39:36But that's about to change.
39:44Scientists around the world are working on the next generation of telescopes
39:49to represent the atmosphere around the planet.
39:53Scientists around the world are working on the next generation of telescopes
39:58to revolutionize exoplanet astronomy.
40:02We've got some ideas and some telescopes that are going to be built
40:05probably in the next couple of decades will be big enough,
40:09will be sophisticated enough to be able to see this sort of thing.
40:14Missions like the James Webb Space Telescope,
40:18seven times more powerful than Hubble,
40:21it should allow us to see the atmospheres of planets across the galaxy
40:27and be a tool that finally finds a second Earth.
40:34The key things we'd be looking for in these atmospheres
40:37are in the infrared part of the electromagnetic spectrum,
40:40which is where Webb is designed to work.
40:43The James Webb Space Telescope is, I believe,
40:45going to be the next really critical mission
40:49to help us in our search for a potentially Earth-like planet.
40:58We're still searching for that perfect Earth twin.
41:03And every day, we get closer to finding it.
41:09We get closer to finding it.
41:13Thirty years ago, we had zero exoplanets.
41:17Today, we know of thousands.
41:20With the next generation of instruments,
41:23we're going to uncover tens of thousands, hundreds of thousands,
41:27even millions of exoplanets.
41:32All with the ultimate aim of leaving Earth,
41:35a civilization spread across the stars.
41:40One of the things I love about being a human
41:42is the fact that I'm born with this curiosity.
41:44This curiosity drives us to explore,
41:47explore Earth, explore our solar system, and beyond, into the galaxy.
41:52We'll be learning about these planets for a long time.
41:55We have just started this journey.