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00:00For 25 years, astronomers have been scouring the night sky, looking for a holy grail, the
00:12ultimate hope, a planet like our own.
00:19They found thousands of other worlds, but most are nothing like we expected, and many
00:29are truly bizarre.
00:31The universe has a vivid imagination.
00:36Orphaned planets without stars, worlds made from diamond, and perhaps giant eyeballs circling
00:44their suns.
00:46If humans ever get to visit a planet like this, we're going to lose our darn minds.
00:51Could any of these worlds be an Earth-like twin, with Earth-like life, or is there really
00:58no place like home?
01:16Over 400 years ago, a brave philosopher first posed the question, if the stars in the night
01:23sky are like our sun, could they have planets, too?
01:29The modern hunt for exoplanets, alien worlds orbiting distant stars, began in earnest in
01:35the 1990s.
01:39Pioneering astronomers expected to find planetary systems much like our own.
01:45But from the very first, what they found shocked the world of science.
01:52Of course, we thought, well, if there are other solar systems, they've got to be just
01:55like us, right?
01:57And now we're finding, actually, there's a really wide diversity of planetary systems.
02:02They are nothing like us.
02:06We're finding planets that I would have thought, if I saw in a movie, I'd go, oh, yeah, that's
02:11cute, that makes for a pretty scene, but come on, that's not really going to happen.
02:14And then, boom, we're finding them.
02:18The shocks came early.
02:21The very first confirmed planet found outside our solar system wasn't orbiting anything
02:27like our sun.
02:29It was orbiting one of the most violent and mysterious objects in the universe, a pulsar.
02:39It defied logic.
02:42If you were to wake me up in the middle of the night and say, where's the last place
02:46you would ever, ever, ever find planets, I probably would have said orbiting a pulsar.
02:54A pulsar is honestly one of the most dramatic things the universe offers.
02:59A teaspoonful of the star has about as much mass as Mount Everest, and they can rotate
03:04hundreds of times a second.
03:05Can you imagine a monster like that?
03:10Pulsars are incredibly compact.
03:14They can spin roughly the entire mass of the sun, squeezed down to an object no bigger
03:19than Manhattan.
03:20As they spin, pulsars spit out light and beams of radiation.
03:27But it's the birth of these violent stars that makes them such an unlikely home for
03:32planets, because pulsars are born in the cosmic fires of supernova explosions.
03:45These explosions are so powerful that if one occurs near an ordinary planet like the Earth,
03:51it would be vaporized in the blast.
03:58Finding planets orbiting a pulsar after a supernova explosion is like going to the site
04:03of a devastating earthquake and then finding a China shop with all the plates and everything
04:08is perfectly intact, not a scratch on them.
04:12It just doesn't make any sense.
04:16For a planet to survive a supernova blast, it would have to be made of incredibly tough
04:22material.
04:23But what?
04:25A clue came in 2011, when astronomers found a strange planet closely orbiting a rapidly
04:32spinning pulsar in the Serpens constellation.
04:37The planet is big, 3,000 times the size of the pulsar it orbits.
04:43But the real surprise is its composition.
04:47The planet appears to be very rich in carbon, but so dense it could only be in diamond form.
04:58If you told me 20 years ago we'd be talking about a giant diamond orbiting a pulsar, I
05:04would ask, what were we drinking?
05:07If humans ever get to visit a planet like this, we're going to lose our darn minds.
05:12People love diamonds.
05:13They pay huge amounts of money for it.
05:16And now, we're finding that there's an entire planet that's a diamond.
05:22The diamond planet could be one of the most spectacular places in the universe, shining
05:29like a 37,000 mile wide dirty disco ball.
05:33For planetary geologist Janie Radabaugh, the closest thing we have to the surface of
05:38the diamond planet here on Earth is the shimmering black volcanic floodplain in Iceland.
05:47Imagine we're standing on the surface of a diamond planet.
05:50You can see that it's really flat, really uniform, and yet it's a little bit shiny.
05:55I think it would be really beautiful.
06:02However beautiful, a diamond planet is not a place you want to visit.
06:09The planet's immense density exerts a gravitational pull over 15 times stronger than on the Earth,
06:16making life extremely uncomfortable for any visitors.
06:21So we're standing on a very high mass body.
06:24This means that the gravity is extremely large.
06:27It's so big that we would probably just be squished flat to a paste.
06:33So how did the diamond planet form?
06:37Scientists believe there can be only one explanation.
06:41It must have started life as a star, but a star in binary partnership with a much larger
06:49companion.
06:50We're actually sort of rare.
06:53Our solar system only has one star.
06:56Other solar systems have more than one, and the stars orbit around each other.
07:00Billions of years ago, the diamond planet is a yellow star, much like our sun.
07:08Its much larger binary partner quickly burns through its fuel supply and then explodes
07:15in a violent supernova, leaving behind a remnant, a pulsar.
07:26The sunlight star is big enough to survive the blast.
07:32But billions of years later, it too runs out of fuel and dies, leaving behind a burnt-out
07:39core known as a white dwarf.
07:43The gravity of the pulsar starts to steal the lighter elements, hydrogen and helium,
07:48from the outer layers of the white dwarf, until pretty much all that's left are the
07:55molecules of carbon the dead star produced during its lifetime.
08:02The pressures in the leftover ball are so intense that the carbon crystallizes, turning
08:07it into a compacted diamond.
08:11But it turns out these cosmic diamonds aren't forever.
08:21The gravity of the pulsar pulls on the diamond planet, hauling inwards on a slow and ever-decreasing
08:28death spiral.
08:34At some point, as it gets closer and closer to that pulsar, it's feeling a harder and
08:38harder gravitational stretching.
08:42When it gets very close, that force will get so strong, it will rip that planet apart.
08:48Imagine the view as a 10,000 billion, billion, billion-carat diamond is ripped to pieces.
08:58Gravity cannot hold the body together anymore, and it basically gets torn apart.
09:04And now what you have is a shower of diamonds.
09:07I mean, that sounds amazing.
09:15Extreme diamond planets, forged from the hearts of dead stars, fire the imagination.
09:23But what exoplanet hunters really want to find are worlds like our own.
09:29Astronomers in general are looking for exoplanets because they're cool.
09:33But come on.
09:34We're human beings, and we want to find another planet like Earth.
09:40And in 2016, astronomers shock the world when they discover a potentially Earth-like planet
09:47orbiting our closest neighboring star.
09:50Stunned scientists wonder, will this nearest of exoplanets have what it takes to give birth
09:57to life, intelligent life, even civilizations?
10:19Take a look at the night sky.
10:22We now know that planets probably surround almost every point of light you see.
10:31But there's more.
10:33For every star we can see, there are countless others too dim for our eyes to pick out.
10:41These are the red dwarfs, far smaller than the sun, but home to planets by the truckload.
10:48And many of these worlds are made from rock, like the Earth.
10:55There are more red dwarfs than any other kind of star in the galaxy.
10:59And these stars have planets too.
11:01So the most common type of planet in our galaxy is one that lives around a red dwarf.
11:09Of the billions of red dwarfs in the Milky Way, more than one in 20 have planets that
11:14orbit at just the right distance for liquid water to collect on the surface.
11:22Any closer, the temperature would be too hot and the surface water would boil.
11:28Any farther away, the water would freeze.
11:34It's often called the Goldilocks Zone because it's just right for oceans to form, just as
11:40they did on the Earth.
11:43Could one of these warm, wet worlds provide a habitat for life like us?
11:51That's kind of the holy grail of this whole thing, to find something that looks like home.
11:56For one thing, that means that it could have life on it.
11:59It could be habitable.
12:01And that's something that we've always wanted to know the answer to.
12:04Are we alone?
12:08These rocky, wet planets may seem like dead ringers for the Earth, but there's a catch.
12:18Many red dwarf worlds orbit so closely to their dim host stars that the same side of
12:24the planet would always face inwards, held in place forever by the star's immense gravity.
12:33Scientists call this phenomenon tidal locking.
12:37We're all familiar with the fact that the Earth spins on its axis and gives us night
12:42and day, but if you move in closer to a star, eventually the gravitational interaction will
12:47stop that rotation.
12:52One side of the planet will always face the star, one side will always face away, and
12:56you're tidally locked.
13:00If you land on this planet and look around, you're going to see stuff that's very different
13:03depending on where you are.
13:08If you're on the point on the planet that is underneath the star, so that when you look
13:11up, that star is straight up, that's where it's always going to be.
13:15It never rises, it never sets, it never moves.
13:18That's where the star is.
13:20Now, if you're on the other side of the planet, it's always nighttime.
13:26You look up, you never see your host star in the sky.
13:32In 2016, astronomers discovered a potentially Earth-like planet orbiting the sun's closest
13:40neighboring star, Proxima Centauri.
13:45The planet, dubbed Proxima b, sits close enough to its host to have oceans like the Earth,
13:52but scientists suspect it's tidally locked.
13:56As a result, Proxima b may be staring at its parent star like a giant eyeball.
14:04The night side is a dark and frozen wilderness.
14:08The day side, a sun-blasted desert.
14:12But between the two lies hope for life.
14:16A long, thin strip of land in permanent dusk wraps around the planet like a ribbon.
14:27Jenny Radebaugh pictures an eerie twilight zone, where dark glaciers melt to fill vast
14:34lagoons just as Earth's glaciers do during the summer.
14:40The boundary between these two completely different landscapes is something like what
14:45we're sitting on right now.
14:47Basically we would have liquid water flowing off of these glaciers, pouring off the glaciers
14:52and heading out into the wasteland.
14:56And icebergs calving off into this beautiful bay.
15:01We've got liquid water, we've got energy from this burning sun, and so this is exactly the
15:05kind of place where we would look for life.
15:09A ring of hope for life on a planet orbiting the sun's nearest neighbor.
15:15Could Proxima b really be home to alien civilizations?
15:21Until we go there, the jury's out.
15:27But red dwarfs burn for trillions of years, and they outnumber the other stars in the
15:33sky three to one.
15:36If advanced alien civilizations really are out there, the planets of red dwarf stars
15:42could be ideal places to find them.
15:46Let's say that we were sitting on a planet around one of these red dwarfs, maybe a trillion
15:50years from now.
15:51We could still be sitting here.
15:53We've never seen a red dwarf die.
15:55And so if that's true, then there's plenty of time for life to have gotten started.
15:59So not only do we have microbial life, but maybe even more complex life.
16:04And then taken a step further, we might even have intelligent life in these locations.
16:11Astronomers have been amazed by the potential for tiny stars to host habitable rocky planets,
16:18no matter how weird those worlds may turn out to be.
16:24But when astronomers turn their telescopes to bigger stars, the surprises keep coming.
16:32These sun-sized stars have planets too, and some of them could be the weirdest yet.
16:40Imagine a giant, gassy planet so black it appears to be eating the light from its own
16:46star.
17:02For over two decades, astronomers have scanned the heavens for alien worlds, and they've
17:08found them pretty much everywhere they've looked.
17:12They hope to discover planets like Earth.
17:15Instead, they've uncovered a galaxy filled with the bizarre.
17:22When you search for all of these exoplanets, you can see that the universe has a vivid
17:27imagination.
17:31Some of them have supergravity, others have two stars and not one.
17:39There are all kinds of interesting configurations.
17:43In 2011, astronomer David Kipping was reviewing data on a gas giant that was closely orbiting
17:51a distant star.
17:54As he studied the brightness of the parent star, he found something really weird.
18:00It looked like the orbiting planet was eating light.
18:05When I first saw this data, I actually thought we'd made a mistake.
18:09I thought we must have screwed up in our analysis.
18:14David was measuring the brightness of the star as it went from its day side to its night
18:19side.
18:20It had been predicted that the brightness would drop significantly.
18:25But instead, there was only a tiny drop in light levels.
18:28The only explanation was that the planet was barely reflecting any light at all.
18:34This planet, TrES-2b, reflects less than 1% of the light which hits it.
18:41So that's darker than black acrylic paint.
18:44That's darker than a piece of coal that you might hold in your hand.
18:47And it's darker than virtually any material you will ever encounter in your life.
18:52And for that reason, we started to nickname this planet the Dark Knight.
18:57Dark objects appear black because they absorb most of the light that hits them.
19:02The more light an object absorbs, the blacker it appears.
19:07But the Dark Knight is so intensely black, it almost appears to be eating the light around
19:14it.
19:16And this weird planet has another secret.
19:21It sits so close to its star that its atmosphere reaches incredible temperatures, 2,000 degrees
19:28Fahrenheit, hot enough to melt gold.
19:33Could the Dark Knight's scorching atmosphere help explain its weird, ultra-dark appearance?
19:41There is some very alien chemistry happening in the atmosphere of this exoplanet.
19:46You're going to have vaporous sodium, vaporous potassium, maybe even titanium oxide in the
19:51atmosphere.
19:54On Earth, these substances are solid.
19:57But here, it's so hot, they're gas.
20:01And these gases, like sodium, are excellence absorbers of light, creating the Dark Knight's
20:08weird, shadowy appearance.
20:12Usually this absorption is limited to a very narrow color, such as yellow light.
20:18But this planet is so hot that that absorption feature gets broadened out.
20:22And this absorption feature then spans the entire visible wavelength of light.
20:27And the planet basically looks extremely dark.
20:32Planets like the Dark Knight fascinate astronomers because these star-skimming giant planets
20:38aren't expected to form so close to their star.
20:43When young stars burst into life, they blow away the gas and dust that surrounds them,
20:49leaving little material behind to build giant planets the size of Jupiter.
20:55Instead, the Dark Knight must have started life far away from the heat of its star, where
21:02ice and gas were abundant, and then gradually spiraled inwards to its current position.
21:12We think these hot Jupiters form much like the planets in our solar system.
21:15You have a massive planet that's orbiting very far from the star, building up material
21:20from this disk of gas and dust swirling around the star.
21:25But as it's moving through this disk of material, it's slamming into it.
21:29And that's acting like a drag.
21:30So very slowly, over millions of years, it's going to spiral into the star, get closer
21:36and closer and closer.
21:37And what you're left with is a massive planet orbiting very close to its star, a hot Jupiter.
21:45TrES-2b most likely started life looking much like Jupiter does today, bright in color and
21:53with a raging, stormy atmosphere.
21:58But as it moved closer to its hot star, the planet's atmosphere began to cook.
22:05It blackened and became the Dark Knight.
22:11So this shows us how dynamic planetary systems must be.
22:15Planets migrate.
22:18Where we see planets today may not be where they formed.
22:22In fact, we're pretty sure it's not where they form.
22:29Thanks to strange worlds like TrES-2b, we now know for sure that gas giant planets can
22:35drift in towards their star.
22:39But could they sometimes drift out?
22:44Astronomers turn their telescopes to the spaces between stars, and they're amazed to find
22:50scores of exoplanets set adrift in open space.
22:55And one of these lonely, orphan worlds could be the weirdest exoplanet we have ever seen.
23:102013, a group of astronomers scan the night sky for a mysterious class of object called
23:25a brown dwarf.
23:30Brown dwarfs are failed stars, too small for fusion to fully ignite in their cores.
23:38But these dim, gassy balls do radiate some heat, allowing the astronomers to pick them
23:45out with infrared telescopes.
23:49A star is just a big ball of hydrogen that happens to have enough mass that the interior
23:54gets hot enough inside to start a nuclear reaction.
23:57Now the universe is perfectly capable of producing things that just didn't quite have enough
24:01mass to get those nuclear reactions going.
24:04And we call that a brown dwarf.
24:09The astronomers get a hit on their infrared detectors, but it's not what they were expecting.
24:15The object they see is too low mass to be a brown dwarf.
24:20It had to be a gas planet.
24:24But its host star was nowhere to be seen.
24:29The planet appeared to be set adrift in open space.
24:34The astronomers had stumbled across one of the most mysterious objects in the galaxy,
24:40a lonely, orphaned, rogue planet.
24:44A planet, by definition, in some sense, you might have thought, is something that orbits a star.
24:50These are planets that don't have a star.
24:53The idea of a rogue planet is just crazy.
24:55That's bonkers.
24:57But yet we've seen them, so these things exist.
24:59I mean, that already is crazy.
25:02Our galaxy could be home to billions of rogue planets wandering across interstellar space.
25:10Without a star to keep them warm, you'd expect rogue planets to be cold.
25:15But the planet discovered in 2013 is seriously weird, because it's hot.
25:23The clouds in the planet's deep, gassy atmosphere burn at over 1,400 degrees Fahrenheit.
25:30It's hard to imagine a more hostile place to visit.
25:34Now, if you were to try to get into this planet, if you could protect yourself from the massive
25:42amounts of pressure, the atmosphere squeezing on you, and the huge amount of heat trying
25:46to basically vaporize you, it still would be a pretty rough neighborhood.
25:51This thing almost certainly has incredible weather, tremendous storms, huge parcels of
25:56hot air rising and cold air falling.
26:03Not only that, we found that in a lot of these objects, if they're hot enough, iron and other
26:07metals can be vaporized.
26:10As they cool, though, this stuff can rain out, and you can imagine droplets of molten
26:16iron falling out of the sky.
26:18This is not a great place to, you know, buy a condo.
26:24Diving deep below the searing cloud tops, pressure builds until the crush of gravity
26:29is so great, hydrogen gases turn into a weird liquid metal.
26:39But where did this planet come from, and how did it get so hot?
26:46The planet's scorching heat reminds scientists of infant gas giants in the hot, violent crossfire
26:53of a newly forming planetary system.
26:58Is it possible this rogue planet was actually a baby, thrown from its nursery by its jealous
27:05planetary siblings?
27:08Planets obviously form in the vicinity of a star, but, you know, they're in a nest with
27:12other planets, and when they get too close in the early days, their gravitational tugs
27:17will usually slow one of the planets down and speed the other planet up, and if it speeds
27:21up enough, that planet can escape the stellar system altogether and go out into the depths
27:26of space.
27:31An orphaned baby gas giant cast out into the cosmos.
27:38But this world may not be truly alone.
27:42It could have companion moons.
27:47In our solar system, families of icy moons swing around Jupiter and Saturn on elliptical
27:53orbits, and scientists have discovered icy exteriors that cover vast liquid water oceans.
28:06As we've seen in the moons in our solar system, they can be very active.
28:10If they're icy, the gravitational interaction with their host planet can stretch them and
28:15squeeze them and heat them up.
28:19These so-called tidal forces could power hot volcanic vents, just like the vents found
28:25deep in the Earth's oceans.
28:30On our planet, ocean vents support all kinds of strange life without the benefit of sunlight.
28:38Could something similar be happening on the tidally heated moon of a rogue planet?
28:45It's entirely possible, hugely speculative, and let me be clear about that, but it's possible
28:51that you could have life arising on the moon of a sunless planet.
28:56You could have these moons kind of snuggled up around this exoplanet, keeping warm through
29:00its gravity.
29:01So even with no star around, there's still possibility for life.
29:07Most rogue planets allow us to dream of interstellar alien life, but it's hard to imagine these
29:16deep sea creatures developing into advanced, complex, intelligent life on the meager energy
29:23of volcanic vents.
29:28To find evidence of civilizations, we need to look elsewhere, and it's just possible
29:34The weird behavior of a distant star has provided our first clue that these advanced civilizations
29:42really could exist.
29:56We used to think our galaxy was only filled with stars.
30:00We now know those stars have planets too, hundreds of billions of them.
30:07Astronomers call these alien worlds exoplanets.
30:11We didn't even know that there were exoplanets, and now it's just mind-blowing to think about
30:16how much we've learned about our universe just in the last, you know, 10 to 15 years.
30:24This is a really fundamental shift in our understanding of what is out there.
30:32Since 2009, a planet-hunting space observatory called Kepler has revolutionized the search
30:38for exoplanets, locating more than all the telescopes on Earth combined.
30:45Kepler's success is due to its ability to keep a watch on 150,000 stars simultaneously.
30:54If Kepler spots the light from one of those stars dimming, just for a moment, an exoplanet
30:59could be crossing its path.
31:02And the greater the drop detected, the bigger the exoplanet could be.
31:11But in 2011 and 2013, Kepler spots dips in brightness that baffle scientists.
31:18They're off the scale.
31:20The mystery star is dubbed Tabby's Star, named for the astronomer who investigated
31:26its strange behavior.
31:27And that behavior has forced scientists to consider a completely universe-changing possibility.
31:35The Tabby's Star is really interesting because we know there's an object that's orbiting
31:40it, and it's blocking a really large amount of the light.
31:43That's how we know it's there, but it's much more than a planet typically would block.
31:46If you have a Jupiter-sized planet orbiting a normal star, you block a percent of the
31:51light.
31:52It's a very, very small dip.
31:53But with Tabby's Star, what we're seeing are huge dips where 20% of the light is being
31:59blocked.
32:03If Kepler's readings are right, something monstrous and bizarre is moving around Tabby's
32:09Star.
32:11And there's an outside chance that something is the remnants of a planet taken apart by
32:18aliens and repurposed as a giant solar power plant.
32:24What really advanced societies would do is just take apart some worthless planet, like
32:28Neptune in our own solar system, and rebuild it as a giant sphere outside the orbit of
32:32their planet, collect all that sunlight, and then use that.
32:37The best energy source anywhere is your star.
32:40So you might build giant structures to actually harvest the energy.
32:45Freeman Dyson called this a Dyson sphere.
32:47Could we have found a Dyson sphere?
32:54If the remains of an exoplanet are blocking out the light from Tabby's Star, the aliens
33:00have been busy.
33:02To account for the dimming seen by scientists, their light-gathering megastructure would
33:07need to be vast.
33:10A Dyson sphere is the stuff of sci-fi dreams.
33:15But how can we prove it's real?
33:20Astronomers at SETI turned their telescopes to Tabby's Star in the hope of eavesdropping
33:26on the radio chatter of its alien builders.
33:29As soon as we heard about Tabby's Star, we swung the antennas of our Allen telescope
33:34array in the direction of that star, and we spent about a week or two actually looking
33:38at it over a wide range of the radio dial, looking for any signals.
33:42We didn't find any.
33:43We also used a telescope down in Panama to look for flashing laser lights.
33:46We didn't find those either.
33:47If a type of Dyson sphere is being constructed around Tabby's Star, its workforce is suspiciously
33:56quiet.
33:57Every big claim requires big proof.
34:01Just seeing a change in a light profile is interesting and is intriguing, but is not
34:07compelling and is not evidence to me yet.
34:10I think that just tells us we just have to keep looking and keep learning.
34:17I would love for it to be aliens.
34:18That would be job security for me.
34:20But honestly, I'm trying to be realistic about this.
34:24But if it's not aliens repurposing a planet, what else could be causing the dramatic dimming
34:29in light?
34:31Could it be stellar activity?
34:33Could it be there's a disk around the star?
34:35Could it be disintegrating planets?
34:37But probably right now the leading contender is a family of comets which are orbiting this
34:42star and causing these dips.
34:45Comets are vast, dirty snowballs left over from the formation of planetary systems.
34:51But if a giant cluster of comets exists around Tabby's Star, they could appear a whole lot
34:56bigger.
34:57If they get close to their star, that ice can heat up, turn into a gas, and expand.
35:03And that expanding cloud can then be much larger than a planet.
35:07We see that in our own solar system.
35:09And it could be something like that that's blocking the star.
35:14It makes sense and it explains a lot of what we're seeing, but it doesn't explain everything
35:19that we're seeing.
35:22A vast cloud of comets could be blocking the light from Tabby's Star.
35:27But there's a problem.
35:28It's unlikely there would be enough comets to account for these dips.
35:33And the infrared signature hasn't been anything out of the ordinary.
35:38So either we've missed it, that dust disperses relatively rapidly, which is possible.
35:42Or there's something more going on here than just comets breaking up.
35:48None of the current theories offered by scientists can fully solve the mystery of Tabby's Star.
35:55So for now, an alien megastructure built from a planet remains an intriguing, if distant,
36:03possibility.
36:05We've seen worlds made of diamond, planets where molten rock rains from the sky, and
36:14perhaps now, the first signs of alien intelligence.
36:19But there's another planet in the galaxy that could be the weirdest of them all.
36:24Because we simply can't find another to match it.
36:42Our hunts for exoplanets has revealed a galaxy filled with bizarre planetary systems and
36:48strange alien worlds.
36:52But the biggest surprise is how few of these worlds look like our planet, the Earth.
36:59So far, we haven't found many, if any, solar systems that are kind of identical to ours.
37:03We always assume there'd be small planets on the inside, big planets on the outside,
37:07like our own solar system.
37:09We don't find too many of those.
37:12What we have found is that the most common type of planet around other stars is what's
37:16called a super-Earth.
37:20These super-Earths have atmospheres that are too thick, creating pressures too high to
37:25support life like us.
37:30So why does our small planet, with its exposed continents and relatively thin atmosphere,
37:36appear to be so rare?
37:40A new and radical idea suggests the Earth could be a second-generation planet, built
37:47from the remains of a demolition derby between giant, rocky worlds that once dominated the
37:54early solar system.
37:58There may have been planets before the ones we know that were destroyed, that were kicked
38:02out or sent careening into the sun.
38:05What makes the Earth a second-generation planet?
38:08We may have had brothers and sisters that we never knew about.
38:13Imagine the scene 4.6 billion years ago.
38:18The planets in our solar system begin to form from a vast disk of dust and debris.
38:26Giant Jupiter is the first to form and settles into a distant orbit.
38:33Where the Earth sits today, a band of super-Earths are born, vast worlds up to ten times the
38:40mass of the Earth and with thick, crushing atmospheres.
38:46The gravity of these giant, inhospitable worlds dominates the inner solar system, preventing
38:53smaller Earth-sized planets from forming.
38:58That is, until Jupiter crashes the party.
39:04We believe that this first generation of planets was driven onto the surface of the young sun
39:12by the migration of our big bully in the solar system, Jupiter.
39:19A few million years after the birth of the solar system, the gravity of the protoplanetary
39:24disk slows Jupiter down and it starts to spiral in towards the sun.
39:31As it moves inwards, it pushes waves of asteroids and debris towards the super-Earths.
39:39The incoming debris disturbs their orbits.
39:44Some may smash each other to pieces.
39:49Others are sent hurtling into the sun.
39:57Once the planetary carnage is over, Jupiter leaves the scene of the crime, tugged back
40:04out by the gravity of the newly formed Saturn.
40:08The solar system that is left behind is a place that's depleted in solid material and
40:13gaseous material.
40:15It's a place where debris will coalesce over hundreds of millions of years into the terrestrial
40:21planets that we have today.
40:25In the inner solar system, planet building starts over.
40:29But the new planets feed on scraps, growing much smaller than their predecessors, with
40:36thinner atmospheres and lower pressures.
40:41Among them is our own planet, the Earth, a second-generation planet primed for life.
40:49For so long, we've been trying to identify all the different factors that make life likely
40:53on a planet.
40:54We know that it has to be warm, it has to be stable.
40:56Well, maybe we've got a new one now.
40:58Maybe being a second-generation planet is one of the advantages we need to look for.
41:05If this is true, the incredible birth story of our planet could offer a reason why we
41:11don't see other worlds out there teeming with intelligent life.
41:16Perhaps the Earth is the planet that broke the rules.
41:21What we're seeing is that our own solar system may be the exception.
41:26And when you look at our solar system, what's the weirdest planet?
41:29Well, the weirdest planet would have to be Earth.
41:36Right now, Earth seems to be the oddball in the universe because we haven't found life
41:40in the universe.
41:41But we continue to search, and hopefully we'll prove ourselves wrong.
41:46Eventually, among the billions of worlds out there, we might find another planet that beat
41:52the odds, gave birth to life, nurtured intelligence, and created a civilization smart enough not
42:01to extinguish itself.
42:04Only then will we know whether the Earth is truly unique.