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00:00It used to be the only planets we knew about were the ones that orbit our sun, but now
00:10we've discovered rocky worlds and gas giants orbiting other stars.
00:16They tell an amazing story.
00:19The early history of these planets would have been very, very violent.
00:25Planets are made everywhere in the same way.
00:29They form from the dust and debris left over from the birth of stars.
00:35So if they're all made the same way, what makes them all so different?
00:57The universe is full of galaxies, gas clouds, stars, and planets, as it turns out.
01:13Our solar system has eight planets, but we now know they're a tiny group compared to
01:18the huge cosmic family of planets across the galaxy.
01:25It's an extraordinary moment in scientific history.
01:31To know for sure that there are other planetary systems out there, they're very common.
01:36And out of the 200 billion stars in our Milky Way galaxy, there are surely dozens of billions
01:44of planets out there.
01:48In 2009, NASA launched the Kepler Space Telescope on a six-year mission to find new planets
01:55orbiting other stars.
02:00So far, astronomers have found over 400.
02:08Some are colossal balls of churning gas, five times the size of Jupiter.
02:16Others are huge, rocky worlds, many times larger than Earth.
02:23Some follow wild, erratic orbits, so close to a star they're burning up.
02:35One thing is clear.
02:36No two planets are the same.
02:41Each is one of a kind.
02:45But most of these new planets are far away and hard to study.
02:51Most of what we know about how planets work comes from the eight that orbit our own star.
02:58Our own planets come in two main types.
03:01There are four rocky planets in the inner solar system, Mercury, Venus, Earth, and Mars.
03:13And in the outer solar system, there are four giant gas planets, Jupiter, Saturn, Uranus,
03:24and Neptune.
03:28Each of the eight planets is distinct and very different.
03:36Their unique personalities began to form at the birth of our solar system, 4.6 billion
03:42years ago.
03:47When the sun ignited, it left behind a huge cloud of gas and dust.
03:55All eight planets, the inner rocky and the outer gas planets, came from this cloud of
04:01cosmic debris.
04:03The planets in our solar system are all made from the same stuff.
04:07They're made from the same cloud of gas and dust.
04:10But they formed under very different conditions.
04:13Some of them formed in close to the sun, where it was much hotter.
04:15Some much farther away, where it was much colder.
04:18And because the conditions were so different, the end result, the product of that formation
04:23was different as well.
04:28So you start the solar system, in my view, with a pretty homogeneous mix of silicates
04:32and water vapor and hydrogen, lots of hydrogen and methane and other elements.
04:38These elements in the dust cloud are like ingredients in a cake.
04:41They cook differently, depending on the combination of the ingredients and the temperature of
04:46the oven.
04:47And just like with the cake, you'd mix the ingredients and then you'd put it in the oven
04:51and bake it, and it would change.
04:55And so this is kind of what happened in the solar system.
04:58Overall, the planet cooks in a slightly different way, depending on how close it is to the sun.
05:05When the sun goes in, where it's hot, the sun burns off gases and boils away water.
05:13Only materials that stay solid at high temperatures, like metals and rock, can survive.
05:20Which is why only rocky planets form close to the sun.
05:27Move farther away from the heat of the sun, and you get different kinds of planets cooking.
05:33But it's the ingredients in the cloud that determine precisely what kinds of planets
05:38will form.
05:39Well, depending on the type of cloud a solar system forms in, you could have solar systems
05:45that don't have rocky planets because it was just too poor in the materials to build something
05:51like the Earth.
05:52And instead, you could end up with more gas giants and no rocky planets at all.
06:01If you want rocky planets, you need a cloud full of metals and rock.
06:09Next step, turn the heat down.
06:13As it cools down, some of the elements in there that have a high boiling point start
06:19to condense out as solids.
06:22And you can get these very tiny little mineral grains forming.
06:27These tiny mineral grains are the seeds of a new rocky planet.
06:31Over time, they start to stick together.
06:34You'd have one dust molecule and another dust molecule, and they would basically slam
06:38into each other and become one slightly bigger dust molecule.
06:42And they would pick up more and more and more.
06:44This process is called accretion.
06:47As these things got bigger, they became basically rocks.
06:54Then rocks slam into other rocks and form boulders.
07:02Boulders smash together to form bigger boulders.
07:06Eventually, you got something big enough that its gravity was strong enough that it could
07:12start drawing material in.
07:14So instead of just slamming into material and gaining mass that way, it was actually
07:18actively pulling material in.
07:22In our own solar system, there were many growing infant planets at first, maybe a hundred.
07:30Most of them didn't make it.
07:33If you go to the asteroid belt and look at the asteroid 4 Vesta, that is a good indicator
07:42of how big a rocky planet has to be before it can pull itself into a spherical shape.
07:48Vesta is only 329 miles across, not quite big enough to become a sphere.
07:57For a growing planet to become round, it has to reach 500 miles across.
08:03Then it has enough gravity to crush it into a sphere.
08:07Any smaller and it stays in irregular shape.
08:13As round infant planets keep eating up stuff, each collision makes them hotter and hotter
08:20until they start to melt.
08:22Now gravity begins to separate the heavy stuff from the light.
08:28Lighter materials tend to float up into crusty film and heavier materials, many of the metals,
08:35falling down and forming a much denser core at the center of the planet.
08:44The young planets are finally beginning to look like planets, but now they have to survive
08:54a period of violence and destruction, a brutal phase that determines which planets will live
09:05and which planets will die.
09:10After the birth of the sun, our eight planets all evolved from the same cloud of dust and gas.
09:17And yet, they ended up completely different.
09:22There was no real blueprint for each of the newborn planets.
09:26They did obey the laws of physics and chemistry, but the most important things happened by pure chance.
09:354.5 billion years ago, around a hundred baby planets circled our sun.
09:46It turned into a demolition derby.
09:49Planet hit planet.
09:51Most were destroyed.
09:56The early history of these planets would have been very, very violent with lots of these
10:02impacts taking place in the final stages of the growth of each planet.
10:07As these impacts took place, as objects ran into each other, certain objects began to
10:14grow at the expense of all the others in this swarm of planetesimals.
10:18And these planets, these things that would become planets, grew and grew, and as they
10:22got bigger, they swept up all the smaller planetesimals around them, the consequence
10:28on the surface of that proto-planet being an enormous amount of bombardment by debris
10:33from space.
10:40When it was over, all that was left were four very different rocky planets.
10:49Each planet's impact history left its stamp, and that's why they're all so different from
10:54each other.
10:56Mars is a frozen wasteland.
11:01Earth flows with liquid water.
11:04Venus is a volcanic hellhole.
11:10And Mercury is tiny, bleak, and super hot, the result of a monster collision.
11:26Mercury, for example, is extremely dense, and has a very thin crust, so it's possible
11:31it started off as a bigger planet, and then something hit it at an angle, and it sheared
11:37off the lighter weight crust, leaving only the dense core.
11:42The young Earth also took a big hit.
11:48Sometime late in its development, the Earth was impacted by another object that ripped
11:56debris out of the Earth's mantle.
12:00Which then went into orbit around the Earth, and re-accumulated to form what is now the
12:05Moon.
12:07There's also evidence of another object that was impact, and that would've been the Moon.
12:22is now the moon.
12:35There's also evidence that something crashed into Mars.
12:40The northern hemisphere has a thinner crust than the southern.
12:47A theory that has emerged for how this happened is that early in the planet's history, the
12:53northern hemisphere of Mars was whacked by some object that blasted a lot of the crust
12:58off of it, and that crust re-accumulated on the southern half of Mars.
13:15All these collisions did two things.
13:18They cut down the number of surviving infant planets, and they brought more ingredients
13:25to the survivors.
13:28If you had a collision with something that was metal-rich, those chunks would tend to
13:35descend down into what was becoming the core.
13:42Or if you collided with something light, icy, they would tend to just float about and form
13:49part of the crust instead.
13:54The four rocky planets close to the sun were almost complete.
13:59They had a solid, hot iron core, surrounded by a layer of liquid iron, all wrapped in
14:05a jacket of molten rock.
14:11Above that, an outer surface crust.
14:16These rocky planets all formed in the same basic way, from the same basic stuff.
14:25But each of them was very different.
14:30Different sizes, and very different destinies.
14:42Space may look empty, but it's not.
14:46It's full of stuff blown out of the sun.
14:50The sun generates powerful magnetic fields that rise above the surface in giant loops.
14:59When they clash, it triggers a storm of super-hot, highly-charged particles blasting out into
15:05space.
15:11It's called the solar wind.
15:18Astronauts in space can see it, but only when they close their eyes.
15:25Then you see a little flash with your eyes shut, and that is an energetic particle coming
15:31through your head and interacting with the fluid inside your eye, and it makes a little
15:35light flash.
15:36And you see these every couple of minutes or so, that you're awake with your eyes shut.
15:43If the astronauts were exposed to a lot more of the solar wind, it could be a killer.
15:49During the Apollo program, in between two of the moon missions, there was an outburst
15:55on the sun that would have killed the astronauts if they had been there.
16:01So space radiation is a serious business.
16:04But here on Earth, the solar wind isn't much of a threat, because we have an invisible
16:10protective shield, a magnetic field generated by the planet's core.
16:21The very center of the Earth is the solid inner core, this hard iron crystalline ball.
16:33Then there's a thick layer of liquid iron, which is convecting, churning motions, which
16:40give rise to the magnetic field.
16:42Well, that's the theory.
16:49To prove that an iron core can generate a magnetic shield, scientists built their own
16:55planet in a lab.
16:59This 10-foot, 26-ton sphere simulates conditions deep inside the Earth.
17:07A metal ball in the center acts as the planet's inner core.
17:14Liquid sodium spins around it at 90 miles an hour, imitating the effects of molten metal
17:19spinning around the Earth's core.
17:26We built this experiment to try to generate a magnetic field to attempt to understand
17:32why the Earth has a magnetic field and why other planets do not have magnetic fields.
17:38It works like the generator in your car, where rotating coils of wire produce electricity.
17:48In the experiment, liquid sodium churns around the core and generates a magnetic field.
17:56It's very much like an electrical generator.
17:59You have motion that is able to generate magnetic fields by turning the energy of the motion
18:05into magnetic energy.
18:09The same thing happens deep inside the Earth.
18:14As the Earth spins, the hot liquid metal flows around the solid core, transforming
18:19its energy into a magnetic field that emerges from the poles.
18:28It protects the planet's atmosphere from the solar wind.
18:35And if the planet has a magnetic field, that solar wind will be diverted around the planet
18:41by the magnetic field.
18:43The magnetic field deflects the solar wind around the planet, protecting the atmosphere
18:49and everything on Earth's surface.
18:52Sometimes big storms of solar radiation will mix it up with the magnetic field.
18:58Then we get big light shows over the poles, the auroras.
19:10Without a magnetic force field, the solar wind would blast away Earth's atmosphere and
19:15water, leaving a dead, arid planet, a lot like Mars.
19:29Mars formed just like Earth, but today it's cold and dry, with little atmosphere.
19:40So why are the two planets now so different?
19:46In 2004, NASA sent two robot explorers to Mars to find out.
19:54The rovers, named Spirit and Opportunity, explored miles of the Martian surface.
20:00They confirmed that Mars is a dry and hostile desert, with only 1% the atmosphere of Earth.
20:07But they did find evidence of water in the past.
20:13Mars was not always a desert.
20:16We have found compelling evidence that water was once beneath the surface, came to the
20:22surface and evaporated away.
20:27We also see, in a few places, ripples preserved of the sort that are formed when water flows
20:33over sand.
20:34So not only did water exist below the surface, it exploded across the surface.
20:40If Mars had water once, it probably also had a thick atmosphere.
20:45So what happened?
20:48We can see that Mars once had active volcanoes, so it had a hot interior at some point.
20:54And because it was made of the same stuff as Earth, it would have had a hot iron core,
21:00surrounded by liquid metal at its center.
21:03So it should have had a magnetic field, too.
21:06The question is, where did it go?
21:11Early in the planet's history, Mars apparently had a strong magnetic field, and it was probably
21:17caused in the same way as it is on Earth.
21:22But Mars is a smaller planet than Earth.
21:24It's going to lose its heat more rapidly as a consequence, and what that means is that
21:29liquid core can freeze solid.
21:33Freeze the core solid, the convection will stop.
21:37The convection stops, the magnetic field goes away.
21:41As the magnetic shield died, the solar wind blasted away the atmosphere, and the water
21:46evaporated.
21:48Mars became a cold, barren planet.
21:52Mars, Earth, Venus, and Mercury, the rocky planets, all formed within 150 million miles
22:00of the sun.
22:02But four times farther out, the sun baked a very different kind of planet.
22:09They're gigantic, they're made of gas, and these monsters have no solid surfaces at all.
22:22So far, astronomers have discovered over 400 new planets orbiting in far-off solar systems.
22:33Nearly all of them are gigantic and made of gas.
22:40We have four of these so-called gas giants in our own solar system.
22:48Jupiter, Saturn, Uranus, Neptune, which all have these very thick, very soupy atmospheres.
22:59Lots of hydrogen, lots of helium, lots of methane.
23:04Why are these outer four made of gas when the inner ones are rocky?
23:11It all has to do with location.
23:15But here, 500 million miles from the sun, it's very cold.
23:23At the start of the solar system, there was some dust, but mostly gas and water, frozen
23:29in ice grains.
23:33Where the giant planets started to form, it was cold enough to get solid snow.
23:38And we think we were able to make ice snowflakes, and these things were able to clump together
23:45to form the cores of the giant planets.
23:47We think that's maybe why the giant planets got to be so big.
23:51There was so much ice and gas, their cores grew huge, around ten times larger than the
23:59Earth.
24:01These giant cores generated a lot of gravity.
24:05They had so much pulling power, they sucked in all the surrounding gas and built up thick,
24:11soupy atmospheres, tens of thousands of miles deep.
24:17The larger they got, the more gravity they generated.
24:22More and more dust and debris got pulled in towards the planets, and this became the building
24:28blocks of their moons.
24:36Jupiter and Saturn have over 60 moons each.
24:45The gas planets have another special feature, rings.
24:51Saturn is unique among the planets in that it has this gorgeous ring system.
24:55It turns out Jupiter and Uranus and Neptune, they have ring systems as well, but they're
24:59really weak and pathetic and extremely hard to detect.
25:05They are there.
25:06All four of the gas giants have rings, but Saturn's are the most obvious.
25:12From a distance, Saturn's rings look like a single, flat disk.
25:17However, they're actually thousands of separate ringlets, each only a few miles wide.
25:23When the Cassini probe flew past, it detected billions of pieces of ice and cosmic rubble
25:29orbiting inside the rings at speeds of up to 50,000 miles an hour.
25:35These bits of ice and rock constantly crash into each other.
25:39Some grow into tiny moons, others smash apart.
25:45But they never form into larger moons because Saturn's immense gravity tears them apart.
25:53Scientists are only just beginning to figure out how the rings formed in the first place.
26:02The theory goes like this.
26:06A comet smashed into a moon and knocked it out of its orbit and closer to the planet.
26:14Saturn's gravity tore it to pieces.
26:26And all of that debris got trapped in rings around the planet.
26:34But the real mysteries of the gas giants lie deep inside them, tens of thousands of miles
26:41beneath the clouds.
26:44This is where the real action is.
26:50It's a place so extreme, it challenges the laws of nature.
27:00Most of the new planets we're finding around distant stars are gas giants.
27:06They're so huge, they make Jupiter look small.
27:09But what goes on inside all gas giant planets, both in our solar system and way out there,
27:16is a mystery.
27:19We know Jupiter's dense atmosphere is 40,000 miles deep, and we can see high-speed bands
27:26of gas creating violent storms that rage across its surface.
27:31But what we don't know is what's going on deep inside, far beneath the storms.
27:39To find out, NASA launched the spacecraft Galileo on a 14-year mission to Jupiter.
27:463-2-1. We have ignition and liftoff of Atlantis and the Galileo spacecraft bound for Jupiter.
28:02December 7, 1995.
28:06Galileo dropped a probe that dove into Jupiter's atmosphere at 160,000 miles an hour.
28:17Parachutes slowed it down as it dropped through the thick atmosphere.
28:26It detected lightning in the clouds and winds of 450 miles an hour.
28:33The probe transmitted data back to Earth for 58 minutes.
28:39So people have asked me what happened to the Galileo probe that we dropped in.
28:43It didn't hit anything.
28:46It just fell continually into the Jupiter environment and the pressure increased and
28:51increased and increased.
28:54As it descended, it recorded pressures 23 times greater than on Earth and temperatures
29:00of over 300 degrees.
29:05When you're in the gas giant environment and you go deeper and deeper into this hydrogen
29:10soup that has no solid surface, it nevertheless can have a tremendous weight and so eventually
29:17you would be crushed by the overlying weight of the material that's there.
29:23Even though the probe descended for only 124 miles before it was crushed, it gave scientists
29:29a glimpse of Jupiter's interior.
29:34But the dark heart of the planet still remains a mystery.
29:44Like some rocky planets, the gas giants have a magnetic field too.
29:49But these are off the charts.
29:53Jupiter's magnetic field is 20,000 times more powerful than Earth's.
29:59And so huge, it extends all the way to Saturn, more than 400 million miles away.
30:07Like on Earth, the magnetic field deflects the solar wind and protects Jupiter's atmosphere.
30:16When scientists studied Jupiter's magnetic field, they discovered it was affecting Jupiter's moons.
30:22The volcanic moon, Io, orbits only 217,000 miles from the planet.
30:32Io's volcanoes blast a ton of gas and dust into space every second.
30:43And Jupiter's magnetic field supercharges it, creating powerful black holes.
30:52And that makes the vicinity of Jupiter very active in many different ways.
31:00If you point a radio antenna at Jupiter, one can hear all sorts of interactions happening
31:06between the planets and the magnetic field.
31:11This is the sound of Jupiter's magnetic field.
31:22Jupiter and Saturn don't need the solar wind to make auroras.
31:29They have huge magnetic fields that create their own.
31:35The Chandra Space Telescope took these images of Jupiter's auroras.
31:41And NASA's Cassini probe took these beautiful pictures of auroras on Saturn.
31:48These auroras are proof that gas planets have magnetic fields, too.
31:55But how do gas planets generate magnetic fields?
32:00On Earth, a superhot liquid metal spinning around the planet's solid iron core does the job.
32:07Gas planets probably do roughly the same thing.
32:12But gas planets don't have hot iron cores.
32:18They formed around frozen cores of dust and ice.
32:23So exactly what's going on deep inside is a mystery.
32:29At the very deepest interior of Jupiter, we really don't understand
32:35what composes those deep interior states.
32:38It could be that the very center of Jupiter has a solid core.
32:46Or it could actually just be still fluid.
32:55We may never find out.
32:57No probe could ever make the 44,000-mile journey to the planet's center to investigate.
33:05The sail was crushed before it got anywhere near the planet's core.
33:12So now scientists are recreating Jupiter's interior right here in a lab on Earth.
33:20Here at the National Ignition Facility in Livermore, California,
33:24they're simulating Jupiter's core using the world's most powerful laser.
33:31This facility is really designed to compress hydrogen to extreme densities and temperatures.
33:41Inside Jupiter, extreme pressures are created by the weight
33:45of 40,000 miles of hydrogen gas crushing down on the core.
33:53In the lab, it's done by focusing 192 laser beams on a tiny sample of hydrogen.
34:03As the pressure in the sample reaches over a million times the surface pressure on Earth,
34:08the hydrogen turns into a liquid.
34:12But when it reaches tens of millions of times the pressure, more like at Jupiter's core,
34:18something really weird happens to the hydrogen.
34:23The pressure is so great that it actually rearranges the hydrogen,
34:28which is a very basic molecule, until it is able to conduct.
34:33So it changes the structure of H2 into a metallic form.
34:39Scientists think this is what's happening inside Jupiter.
34:44Pressure and heat have transformed the planet's core into metallic hydrogen.
34:53Jupiter's metallic core works like the iron core in the Earth.
34:57It generates the gas planet's gigantic magnetic field.
35:06Gravity and heat shape how planets evolve from their inner cores to their outer atmospheres.
35:13They're the great creative forces in planet building.
35:20But there's another ingredient that has a lot to do with how planets turn out.
35:28And that ingredient is water.
35:37Planets may seem fixed and unchanging, but they never stop evolving.
35:43In our own solar system, one lost its atmosphere and became a barren wasteland.
35:51Another heated up and became the planet from hell.
35:58The Earth has changed as well, and the game changer was water.
36:07When you look at Earth from space, you see a lot of water.
36:12We are the blue planet after all, so it must be really wet, right?
36:19It looks at first glance that our Earth, of course, covered three quarters by oceans.
36:25It's a very water-rich world. Not true.
36:29The Earth by mass is only 0.06% water.
36:36There's some water on the surface in the form of oceans, some water trapped in the mantle.
36:40But actually, the Earth is a relatively dry rock.
36:46All of the inner rocky planets formed very close to the sun, so they started off dry.
36:57Any water they might have had evaporated away or was blown away by impacts.
37:06These massive collisions that formed the Earth were so energetic
37:12that any water that had been here would have been vaporized and lost from the Earth.
37:23So where did Earth get all the new water we have today?
37:28It moved here.
37:32When you look farther out, and you look at Jupiter, Saturn, Uranus and Neptune,
37:37those planets have enormous amounts of water locked up inside them.
37:46And even more dramatically are the moons.
37:50The moons of Jupiter, Saturn, Uranus and Neptune are at least 50% water.
37:58There was a lot of water out there, so how did some of it get to planet Earth?
38:04And the answer, almost certainly, is that left farther out in our solar system
38:10were some asteroids and some comets, far enough from the sun that they could retain their water.
38:21Millions of these watery comets and asteroids came flying into the inner solar system.
38:28And some of them smashed into Earth.
38:34Over the eons, the Earth acquired the water that had been a part of the asteroids.
38:42And that indeed makes up the mass of water that nearly covers the Earth today.
38:51But the amount of water that was delivered, that was the luck of the draw.
39:00Couldn't it have been the case that the Earth would have acquired maybe half as much water as it did?
39:06If so, the Earth would be nearly dry on its surface, if not completely dry,
39:11the sponge of the interior soaking up the rest of the water.
39:16No surface water would have meant no life.
39:21And what about too much water?
39:24We would be a water world.
39:27The oceans much deeper, covering the continents, even Mount Everest.
39:33And so you can ask then, if the Earth were covered by water,
39:37only having twice as much as it currently has,
39:40only having twice as much as it currently has,
39:43would we have had a planet that was suitable for technological life?
39:52Technology requires dry land.
39:57And it's quite likely that the precise amount of water that the Earth just happens to have
40:04has allowed a technological species like we Homo sapiens to spring forth.
40:13The world as we know it exists because a blizzard of comets and asteroids
40:18delivered just the right amount of water about 4 billion years ago.
40:26And just maybe the same thing is happening right now somewhere else in the universe.
40:35One thing's for sure, there is plenty of water out there.
40:41Hydrogen, the most common atom in the universe,
40:44and oxygen, one of the next most common atoms in the universe, H2O,
40:49is certainly going to be a very popular molecule, and indeed it is, within our universe.
40:55So, water is everywhere in the universe, and we're discovering that planets are too.
41:01But we still haven't found another planet with liquid water.
41:05Scientists have discovered more than 400 new planets.
41:10None of them look like our world.
41:14What we have not yet found is a planet that is about the same size and mass
41:20and chemical composition as the Earth orbiting another star.
41:25So it remains an extraordinary holy grail for humanity
41:30to find other abodes that remind us of home.
41:35But we'll keep looking.
41:37We know that there are around 200 billion stars in our galaxy alone.
41:44And as many as 40 billion of them could have planets.
41:50We're still hopeful that when we discover terrestrial-style planets,
41:55that will help us tremendously in understanding how our own inner solar system planets
42:00and the Earth evolved in comparison to the outer solar system planets.
42:08We are entering into what is going to be thought of in the future
42:12as the golden age of planetary discovery.
42:16We will really, for the first time, begin to truly understand
42:20the actual diversity that lies out there.
42:22I think it's going to be a fantastically exciting time.
42:28Planets form according to the laws of physics and chemistry.
42:32What they become, that has a lot more to do with luck.
42:37Many scientists believe it's only a matter of time
42:42Many scientists believe it's only a matter of time
42:46before we find another planet like Earth.
42:50One that formed from the same ingredients, in the right place,
42:54with just the right amount of water.
42:57One thing's for sure, there are billions of planets out there
43:01waiting to be discovered.