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The solar system is the sun and everything that travels around it. This includes eight planets, dwarf planets, moons, asteroids, and comets. The sun holds every object in place with its strong gravity...
太阳系的形成与演化始于约46亿年前巨大分子云局部引力坍缩，中心形成太阳，剩余物质构成原行星盘并逐渐形成行星及其他天体.
太阳系的形成与演化始于约46亿年前巨大分子云局部引力坍缩，中心形成太阳，剩余物质构成原行星盘并逐渐形成行星及其他天体.
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00:00The solar system, our home, vast, majestic, and peaceful. Think again.
00:11Once there was nothing here but a huge thin cloud of gas. How did we get from this to this?
00:18When we look out there, we're really looking for our own origins. What happened to us?
00:23Piecing together our system's history suggests a wildly chaotic birth.
00:27It's almost like a planetary pinball game.
00:30These are exquisitely violent events.
00:33It's the ultimate survival story.
00:36The formation of our solar system.
01:00Throughout history, the heavens have inspired awe and wonder in human observers.
01:07Our ancestors watched the skies with amazement, imagining the power of the bright objects that we now call planets.
01:16The fact is, is that for as long as there have been people, we've looked at the sky, we've looked
01:23at what we see there, we've asked the question, what's the connection between that and us?
01:28The ancients erected standing stones, temples, and pyramids from which to predict cosmic events and to worship the gods in
01:36the sky.
01:39But the first telescopes revealed a universe unlike anything we could have imagined.
01:51We began to see what was really out there.
02:00In 1755, German scientist and philosopher Immanuel Kant turned his attention to these huge clouds, or nebulae, and in the
02:10process came up with one of the first modern theories of how the solar system formed.
02:15All of these beautiful clouds in interstellar space have turned out to be very important.
02:20It's where stars are born, it's where planets form.
02:24Kant's key idea was that nebulae could collapse in and condense into planets and stars.
02:30It's known as the nebula theory.
02:34250 years later, Kant's theory still helped to explain the birth of our solar system.
02:43Today, new observations are revealing how the sun, planets, and moons of our solar system took shape.
02:50Scientists like Professor Jeff Hester are solving many of the long-standing mysteries of the formation process.
02:58We now have answers to questions that people have asked themselves for as long as there have been people.
03:05And it changes the way that we think about ourselves.
03:11Dr. Michelle Thaler is an astronomer working with NASA's Spitzer Space Telescope.
03:16It's the fourth and last satellite of NASA's great observatory program.
03:21And it gives scientists a totally new view of the universe.
03:27What's happening now in astronomy is we're finally seeing the universe as it really is.
03:32Not just as humans perceive it.
03:37One way to establish how our solar system forms is to watch other distant systems take shape.
03:44But if Kant was right and new solar systems form deep within clouds of interstellar gas,
03:50we might never see this happening.
03:54Our own senses, what the human eye can see, turns out not to be the whole story.
03:58There's so much more out there that humans just don't have the capability to see naturally.
04:05Objects emit infrared radiation.
04:08Infrared has a longer wavelength than visible light, so is not scattered by dust.
04:14Conventional telescopes that use visible light cannot see inside a dusty nebula.
04:19But infrared sensors on board Spitzer can.
04:24The same principle is used by firefighters.
04:34This room is completely full of smoke.
04:36There is no way that you can see with your eyes or with a regular camera what's going on inside.
04:43So fire departments use infrared cameras to be able to see people inside a smoke filled room
04:48and actually see the heat of a person's body.
04:50This is exactly how Spitzer works.
04:52Spitzer actually looks through these giant clouds of smoke and dust to see planets inside.
04:57Just like these guys can find the people.
05:02Visible light is completely blocked by things like smoke and dust.
05:06But infrared light is a longer wavelength.
05:08Actually, it's just sort of a happy coincidence for us that nothing stops it in the chemistry of smoke and
05:13dust.
05:14Fire department, everything we need.
05:17Fire department!
05:18Fire department!
05:26I think you really get a sense now that we're able to see where we've never seen before using the
05:31Spitzer space telescope.
05:32A camera in a smoke filled room is one thing.
05:36A telescope in outer space is quite another.
05:46On the ground in 2003, Balor and her colleagues wait anxiously for Spitzer's first data.
05:54At stake is a unique view of the origins of our own solar system.
05:59Literally, no one had ever seen what the universe looks like in that type of light.
06:04So, at first we were amazed at the beauty.
06:07I did not expect Spitzer's images to be as compelling visually to me as they turned out to be.
06:14They are beautiful.
06:16But these are not just pretty images.
06:19Crucially, by observing distant worlds as they form, we also learn more about the birth of our own solar system.
06:25One of the privileges of observing star and planet formation is that you're effectively looking back in time.
06:31You're looking at what we must have gone through billions of years ago.
06:35What this glimpse back in time reveals is a snapshot of the earliest stages of solar system formation.
06:42This is our best ever view inside our nebulous collapse.
06:46We've actually been able to observe with Spitzer the moment where the dust begins to collect together and then get
06:52warm and then finally young stars emerge.
06:56So it turns out Kant's 250-year-old theory is essentially correct.
07:02Our own nebula began its collapse 4.6 billion years ago.
07:06It's likely that the nebula had slowly been spinning in space ever since its formation.
07:12But as its material collapsed, it began to spin much faster.
07:16This is down to the law of physics known as conservation of angular momentum.
07:22As the nebula contracts, the same amount of energy is now concentrated in a smaller space.
07:27To keep the same momentum, rotation speed must increase.
07:33And yet the supernova delivers this in less than a day and then expands for a few months afterwards.
07:40As it expands, it's as bright as an entire galaxy. It's as bright as a billion suns.
07:46Extreme nuclear processes within the dying star fuse the elements the star created in life to make heavier elements.
07:54As the star explodes, it blasts these into space.
07:58In its death, it creates the essential elements for making planets and life elsewhere.
08:03It spews the products of millions of years of nuclear reactions out into the gas in between the stars.
08:12This is the basis for some of the clouds that eventually form new stars.
08:17Yet they're spread so thin that it's difficult to know what triggers a cloud like this to collapse.
08:22The best way to find out is to examine the material that once made up the cloud.
08:29And that means examining the two oldest types of material in the solar system.
08:35Asteroids and comets.
08:37But first, scientists need to bring back a sample from outer space.
08:50Comets are relics from the earliest days of the solar system and may hold clues to our origins.
08:57But before scientists could study them in depth, they needed samples.
09:02In 1981, this man, NASA's Dr. Peter Soh, started to work on his idea to return comet fragments to Earth.
09:11It's almost like space forensics.
09:14It's like you see some television program or crime scene, they find a piece of paint chip or a hair
09:23or some kind.
09:24Very similar to what we're trying to do.
09:26Soh and his colleagues designed the Stardust Probe to travel around 500 million kilometers from the Earth and collect comet
09:34grains from the comet Ville 2.
09:37There's just one problem.
09:39The dust particles will be moving up to six times faster than the speed of a bullet.
09:44Yet it's vital Soh gets them back to Earth undamaged.
09:48So what I need is special material to be able to contain the high-speed bullet, I'll be able to
09:55capture it without breaking up.
09:57So I need to capture the bullet without breaking up the bullet.
10:01It takes Soh over 2,000 experiments to refine his comet-catching material.
10:06It's made of the same chemical as glass, silicon dioxide, only a thousand times less dense.
10:14So calls it smooth gradient silica aerogel.
10:19If anything can stop comet grains, it's this stuff.
10:23With aerogel capture cells on board, the probe is good to launch.
10:28On February 7th, 1999, it blasts off on top of a Delta rocket.
10:34We have main engine start and liftoff of the Stardust spacecraft, returning a time capsule with the elements of the
10:42formation of our solar system.
10:44Its 3.2 billion kilometer journey takes nearly five years.
10:49But eventually, the night of the comet encounter arrives.
10:54At Mission Control, all Soh and his colleagues can do is wait.
10:58We have done many, many simulations, but still the questions, will it run into a comet?
11:07Even if it doesn't run into a comet, did it collect dust at all?
11:11It would be horrible when it comes back and no dust.
11:15Finally, in 2006, the capsule re-enters Earth's atmosphere.
11:19It makes a safe landing, and scientists rush its precious cargo to the lab.
11:24As soon as they open it, they know the mission is a success.
11:29If I capture one particle that I can see with an naked eye with a microscope, I would be pleased.
11:35But I caught more than three dozen of them.
11:38It's an incredible achievement.
11:41These are the first pristine comet particles ever to return to Earth.
11:45The grains may be tiny, but they contain a wealth of information.
11:50It will take many years, really, to understand the formation of the solar system.
11:54It will take probably a couple of decades to milk all we can from the stardust samples.
12:01The team may have to wait decades for all the results, but in the meantime, others are trying a different
12:07approach.
12:08Comets aren't the only source of material from the early solar system.
12:12Meteorites are ancient, too, and they have one huge advantage.
12:16They come to us.
12:20The amazing thing is that any time you pick up a meteorite, with very few exceptions, you take it into
12:26the laboratory, you date it.
12:27The answer is, it's four and a half billion years old. Four and a half billion years old.
12:32These are pieces of the early solar system.
12:34And so here you have in your hand a piece of something very substantial that connects you and connects our
12:42existence and connects the history that brought us to be with this larger universe.
12:54After extensive analysis, Hester's team discover unstable isotopes of iron-60 locked inside the meteorites.
13:02It's totally unexpected.
13:05Iron-60 decays relatively quickly, so it must have come from close by.
13:10If it came from any great distance, it would have had time to decay into a different isotope before the
13:16meteorite could trap it.
13:18Only a supernova can make iron-60, so there must have been an active supernova close to the infant sun.
13:28To come along with a piece of hard evidence that says that, nope, early sun, early solar system, we're in
13:35a violent environment near a massive star, experienced a nearby supernova, it was surprising.
13:41We realized that this is going to change the way that people think about what goes on.
13:46This is going to change our understanding of the origins of the solar system.
13:50Despite the violent forces being unleashed, our solar system may have formed right alongside a supernova.
13:58These are exquisitely violent events, and they reshape everything around them.
14:05Shock waves from exploding supernovae pummel everything within 20 light-years.
14:11A galactic bomb blast.
14:14What's astonishing is that all that destructive force may have been exactly what was needed.
14:23Blast waves can push dispersed matter together, like a broom sweeping up dust.
14:29So the collapse of our giant cloud of dust may have been triggered by the blast from the nearby supernova.
14:35They'll send shock waves and compress the gas for light-years around them, and that compression triggers the birth of
14:41other stars.
14:42The blast wave could have swept together the material of our nebula.
14:46When compressed enough, gravity would start to have an effect, and the cloud would begin to collapse.
14:55Within the clouds swirling about the infant sun, the planets are beginning to take shape.
15:01And today scientists are beginning to understand the astonishing processes that formed them.
15:13The planet's not done.
15:15The planet's not done.
15:17The planet's not done.
15:18The planet's not done.
15:19The planet's not done.
15:22For years, the process of planetary formation has puzzled scientists.
15:28Some astronomers now believe planets grow within the disk of gas particles surrounding a newborn sun.
15:35but this material is spread too thin for gravity to pull it into clumps then in
15:442003 some informal experiments on board the International Space Station shed
15:49some light on this long-standing mystery astronaut Don Pettit spends his free time in space conducting
16:06a few experiments to see how particles behave in zero gravity he takes different grains such
16:18as coffee sugar and salt places them in pouches and watches as they begin to attach to each other
16:26he realizes that the friction between them as they rub together inside the pouches is creating
16:32an electrostatic charge that pulls particles together and forms them into clumps the same
16:40electrostatic charges would have been created if the experiment were carried out on earth but on
16:45earth the far stronger force of gravity would have made it impossible to observe the same results
16:52Pettit and his colleagues realize that what they're seeing in the zero-gravity conditions of the space
16:57station would have also occurred in the zero-gravity environment of the early solar system the particles
17:03in the protoplanetary disk may somehow get charged and attract each other it's a process known to
17:10planetary scientists such as Jack Lissauer as accretion we see signs of accretion in our daily lives raindrops
17:20condensed as very very tiny droplets around small little pieces of dust and they collide with one
17:29another as they fall in much the same way that grains collide in the solar nebula and accrete into bigger
17:38objects
17:40once the clumps reach the size of mountains they exert a significant gravitational force it's more
17:47powerful than the electrostatic force so the whole process accelerates fast in about 1 million years the
17:54clumps can grow into larger planetesimals or pieces of planets and can keep on growing into full-size planets
18:06gravity keeps these planets locked into their orbits around the Sun
18:11traditionally understood as a mysterious pulling force since Einstein scientists have a new understanding of gravity
18:24most people know that it's the Sun's gravity that keeps planets moving and orbits around the Sun what
18:30we're going to do here is we're going to take a little bit deeper look at what the nature of
18:34gravity really
18:35is now Newton's first law of motion law of inertia says that an object moving in a straight line at
18:41a
18:41constant speed will keep doing so so here on the overhead view we're looking down at a flat surface and
18:47if I take
18:48this ball which represents a planet and I roll it across the surface and sure enough it moves in a
18:53straight line at a
18:54constant speed now Einstein said there's actually more to it than that Einstein said that if you took a mass
19:01for example the
19:01mass of the Sun represented here by this plunger that that mass had the effect of distorting the shape of
19:09space time now to us it
19:11doesn't look distorted if you look on the overhead shot it still looks as flat as it did before just
19:18like a chessboard but in fact it is distorted and so now when I take my planet and I move
19:23it through this distorted
19:24space-time it tries to follow a straight line but there's no straight line available for it to follow and
19:32so
19:32instead what it winds up doing is it winds up moving around the central object on an orbit it's that
19:38effect of
19:39that curved space-time that we usually call gravity gravity exerts an equal force in every direction so
19:49its effect is always to attract matter evenly towards the center of a body therefore when it's strong
19:55enough gravity will restart any body into a sphere this is why all planets in the solar system form nearly
20:04perfect spheres with a diameter of over 480 kilometers no other shape is possible all the planets are roughly spherical
20:15and
20:16all come from the same original disk but they all end up as radically different worlds from rocky Mars to
20:24gassy Jupiter
20:27the differences are primarily down to one basic law of physics different kinds of material condense at
20:34different temperatures it has to do with given a given temperature what is a gas and what is a solid
20:41and
20:42that's it at first all the matter around our young star is so hot it's in a gaseous form then
20:49further out as
20:50things cool material begins to condense out of the cloud like snowflakes forming on earth
21:00in the inner solar system closest to the Sun it stays hot far too hot for gases or ices to
21:06condense at
21:08these temperatures they'd evaporate instantly but metallic particles can condense here and the planet that
21:15forms in this zone is highly metallic mercury astrophysicist steve dash is an authority on planetary
21:24formation his research has shown him just how hostile a world mercury must be it has no atmosphere it has
21:33no
21:33oceans it's just rock and the part that faces the Sun tends to get hot enough to melt lead for
21:44example it's
21:45many hundreds of degrees Fahrenheit the fierce heat is thanks to mercury's proximity to the Sun it's
21:52almost three times closer than the Earth move further out and the temperature eases a little by around 500
21:59degrees Celsius now more rocky particles can condense so the rockier terrestrial planets Venus Earth and Mars
22:10form in this region since Venus orbits around 50 million kilometers further from the Sun than mercury we
22:21could expect it to be a more hospitable place Venus is probably the closest thing we have in the solar
22:29system
22:29to your conception of health the pressures are immense the temperatures are high and it rains sulfuric acid
22:39around 40 million kilometers further our home planet the earth has been nicknamed the Goldilocks planet it's not
22:48too hot it's not too cold it's orbiting at just the right distance from the Sun and that makes it
22:55very conducive to life
22:57earth has a protective ozone layer to shield us from harmful UV radiation and a magnetic field that
23:05deflects lethal cosmic radiation earth is also the only inner planet to have a large moon today scientists
23:16like Gary Lofgren still study the lunar samples first brought back by the Apollo astronauts over 30 years ago
23:34Lofgren works in the secure lunar storage facility where NASA keeps most of the 380 kilos of rock brought
23:42back from the moon for years now scientists have been studying the samples to discover how the moon and the
23:49wider solar system may have been formed it's an invaluable resource for geologists and the Apollo
23:59astronauts made it a top priority to bring samples back even if it meant ignoring orders Neil Armstrong
24:10personally made sure that we got a bunch of rocks by basically sort of disobeying the rules as I understand
24:16it and he kind of snuck out of the view of the TV camera which he wasn't supposed to do
24:19and filled
24:20up a box full of rocks when the samples are analyzed back in the lab they soon give up some
24:26surprising
24:27information we found out quickly that the chemistry was very different from the earth and it wasn't the
24:32kind of chemistry we expected from our previous ideas about the formation of the moon the lunar rocks and
24:38therefore the moon itself turn out to be younger than previously thought all the theories we thought of
24:43before we went there kind of went out the window really quick and it took a few years before good
24:48ideas began
24:49to come forth according to nebula theory there should not be enough of the right material left over from
24:56planetary formation for moons to form in the inner solar system it remained a scientific conundrum until after decades of
25:05painstaking work scientists came up with a new theory a violent and devastating scenario that involved the
25:14earth experiencing a head-on interplanetary smash in 1969 Neil Armstrong becomes the first man to walk
25:26on the moon only when the Apollo astronauts bring back lunar samples are scientists finally able to
25:34understand how our moon may have formed their study of chemical isotopes in the samples suggests the moon
25:42contains material that was once part of the earth it is a totally unexpected find scientists are at a loss
25:50to
25:50explain how this material got there but eventually a bold new theory emerges a theory suggesting that in
26:00the early solar system there were not eight but around 20 planet-sized objects orbiting the Sun when
26:07you think about the early stages of planet formation you can think of it as a sort of a pinball
26:12game where
26:13there are these objects that are careening all over the place occasionally running into one another
26:21four point five three billion years ago the young earth is on a direct collision course with an object
26:27named fair a body the size of Mars if we had that collision today it would probably be the end
26:36of
26:36mankind it would be the equivalent of many many hydrogen bombs exploding on the surface of the earth
26:47it's a world-shattering impact blasting material with such force that billions of tons of rock
26:55escape earth's gravity and eject into space this material is captured by the earth's gravitational pull
27:03and enters into orbit over a few million years it amalgamates and becomes our moon
27:15the collision is further evidence of a violent early solar system a period when countless bodies
27:21followed chaotic orbits often they'd wreak havoc but some objects became permanent features of our system
27:29the two Martian satellites for example Phobos and Deimos are both believed to be former asteroids
27:36captured by the gravity of Mars these two moons are relatively tiny even the larger body Phobos is just
27:44one-tenth the size of our moon although Mars itself is only one-third the size of Earth it has
27:51some of the
27:51most extraordinary features in the solar system Mars is a spectacular planet a geologist wonderland for
27:59example it has the solar system's largest canyon Valles Marineris which has side tributaries which
28:07would dwarf the Grand Canyon on Earth it's about the length of the United States it has the solar
28:13system's largest volcano Olympus Mons which would dwarf Everest
28:28Mars is the last of the four inner terrestrial planets the rocky bodies orbiting inside the asteroid belt
28:35that these planets stopped growing at their present-day size was largely down to the question of supply
28:44rock and metal was scarce in the early solar system making up only around 0.6 percent of the total
28:50material available also the inner planets couldn't accumulate gas compounds because it was too hot for
28:57them to condense here all of the gas compounds condense much further out before we reach this zone in
29:07the outer planets there's the asteroid belt to contend with science fiction paints this as a crowded
29:15zone difficult to cross we now know that there are at least tens of thousands of asteroids in the
29:23asteroid belt that are a mile or larger in size nevertheless it would be very unlikely if you were piloting
29:29your spaceship through the asteroid belt to run into one of these in fact we have to aim very carefully
29:36to get space probes to visit asteroids the average distance between asteroids in the asteroid belt is
29:42about 1 million miles as we traverse the asteroid belt we hit an invisible but critical border it's called
29:52the frost line cross this boundary in space around 450 kilometers out from the Sun and we find totally
30:03different kinds of planets there was a frost line somewhere in the asteroid belt beyond which it was cold
30:12enough for water to condense and this was a gradual transition but a very important one beyond the frost
30:21line the outer planets take shape these are very different worlds to the inner planets and that's
30:27due to the kinds of substance that can condense in this cooler region colder than minus seventy degrees
30:35Celsius hydrogen compounds like water methane and ammonia are able to condense and there are
30:42trillions of tons of this material available in the early solar system but it's only once the outer
30:48planets reach ten times the mass of earth that their growth really takes off at that critical mass they
30:58have enough gravitational influence to start sucking up trillions of tons of gas there's a snowball effect
31:05as they get bigger the planets attract ever more gas it was a race about how fast these cores could
31:15grow
31:15compare it to how long the gas discs hung around Jupiter and Saturn just happen to be at the right
31:20place in the solar system
31:26they quickly become gigantic super planets and since gas now makes up 90 percent of their mass they're also
31:34known as the gas giants largest by far is Jupiter this giant planet has a mass of over 300 times
31:43that of earth
31:44while its volume is more than 1000 times greater according to Hal Leveson we're fortunate that Jupiter did reach this
31:53size the planet's gravity is so powerful it often changes the trajectory of comets that could otherwise enter the inner
32:01solar system
32:02Jupiter actually plays the role of a protective big brother that protects us from the hostile environment that exists beyond
32:10its orbit and in particular there are a lot of these small icy comets that could come in and hit
32:17us that Jupiter deflects away throwing them out of the solar system
32:25Saturn is the second largest planet in the solar system it turns out we're lucky it never got any bigger
32:32according to our simulations if you have two planets the size of Jupiter in one solar system they will eventually
32:38knock out all the other planets and we were close Saturn is not that much smaller than Jupiter so we
32:44were right on the border of being a stable solar system had Saturn been a little bit bigger we probably
32:49wouldn't
32:49be here talking about it
32:54it's a terrifying thought the gravity of a larger Saturn acting in concert with Jupiter could have sucked in and
33:02destroyed all the smaller planets
33:07thankfully Saturn never follows this fateful path instead it grows into one of the crowning glories of our solar system
33:17whenever I show people planets in my telescope I always save Saturn for last it's beautiful it's so stunning it's
33:25definitely a gorgeous planet because of its large rings
33:31the rings are made up of billions of fragments these fragments continually grow and disintegrate due to the conflicting gravitational
33:40forces of Saturn and its moons
33:51Uranus
33:51Uranus lies nearly twice as far out from the Sun as Saturn both it and Neptune are icy worlds with
33:58much less gas than the other gas giants
34:00in a sense they got to the feast too late
34:05Uranus is a plain looking place a flat green color thanks to a layer of methane high in its atmosphere
34:12since it forms further out it becomes an ice giant rather than a gas giant temperatures average minus two hundred
34:21and twelve degrees Celsius
34:25since Pluto has been officially downgraded to dwarf planet status blue Neptune is now our outermost planet
34:34until recently these bodies were thought to mark the very edge of our solar system
34:39our idea of the solar system changed drastically really in 1992 when this structure that we call the Kuiper belt
34:46today was discovered
34:48around 45 billion kilometers out from the Sun the Kuiper belt home to an estimated 70,000 large icy objects
34:57there's a whole population of leftover dregs of planets formation you can think of it as right where these things
35:04haven't quite come together
35:05then a thousand times more distant from the Sun lies the mysterious Oort cloud comprised of up to a trillion
35:13comets
35:16most of these probably formed much closer to the Sun as part of the same process that formed the planets
35:22but the gravitational influence of the young Jupiter and Saturn forced most of these comets onto very long orbits
35:29carrying them to the far reaches of the solar system
35:33the outer edges of the Oort cloud really are the final frontiers almost a full light year out from the
35:41Sun
35:43beyond our solar system lie billions of other stars many with planets of their own
35:52for a long time it's been one of the holy grails of astronomy to actually find a planet around another
35:57star
35:57the only solar system the only planets we knew about were our own
36:01so what was the first plant going to be like around another star
36:06then in 1992 comes the discovery they've been waiting for
36:11the first ever detection of an exosolar planet a planet orbiting a distant star
36:18and then the very first planet we find outside our solar system is bigger than Jupiter way up against the
36:24star
36:25it completely threw out everything that we've been taught about solar systems
36:29there is absolutely no way that an object that big could form close to their star
36:35there's just not enough stuff there
36:38the nature of these planets simply doesn't fit within standard models of solar system formation
36:44and it's not just a problem for distant worlds
36:48when we look back at our own solar system it becomes clear there are problems with the standard theory of
36:54how our planets formed
36:56it's interesting when we try to apply our models our understanding of planet formation to Uranus and Neptune
37:02they fail utterly you just can't build those planets where we see them today
37:09there simply should not have been enough material available at this distance from the sun
37:14for planets this size to grow it's a mystery Uranus and Neptune seem to be in the wrong place
37:23and this is just one of several mysteries that standard nebula theory cannot explain
37:29there's also the enigma of the Kuiper belt according to the model it shouldn't really be there
37:34even more puzzling our moon's craters all seem to date from the same time
37:40together these problems threaten to undermine scientific understanding of how the solar system forms
37:56the moon may be our closest neighbor
38:00but it still harbors secrets
38:05with no substantial atmosphere or weather
38:08the lunar landscape experiences little change
38:12so it was believed that one impact strike every few thousand years would be enough to explain its crated surface
38:19but then the Apollo missions brought back lunar rock samples
38:23including crater debris thrown out by ancient impacts
38:27and these samples tell a very different story
38:30it became clear as we studied the moon that a large number of the impact craters formed around 3.9
38:37billion years ago
38:38and clearly there was a high concentration of impacts on the moon during that time
38:43Lunar geologists use radiometric dating to assess the age of the samples
38:48it came as a big surprise to find out that a high proportion of moon craters
38:54formed during one relatively brief period of bombardment
39:00Dr. David Kring is an expert on this phase
39:03known as the late heavy bombardment
39:06or lunar cataclysm
39:08this evidence from the moon is actually telling us something about the entire origin and architecture of the solar system
39:16and how it evolved
39:19tens of thousands of large meteorites must have hit the moon during the lunar cataclysm
39:24Earth too must have taken a pounding
39:27today only younger craters like this 20,000 to 50,000 year old example in Arizona remain on Earth
39:35most older craters have disappeared
39:37but three and a half billion years back there'd be no chance of missing them
39:45we're talking about 20 to 40,000 impact craters that would have had enough energy to change global conditions
39:55where could such a vast number of impact objects come from?
40:00within the impact craters we have chemical traces of the objects that produce them and those point to the asteroid
40:07belt
40:10it seems to match
40:11the objects that once hit the moon are chemically consistent with the rocks found in the asteroid belt
40:17so this is almost certainly where they came from
40:19but today most asteroids are in stable orbits
40:23there's nothing obvious that could force thousands of them out of the asteroid belt and into the inner solar system
40:30so for decades the bombardment was a scientific puzzle
40:36in 2004 Hal Levinson had a Eureka moment
40:42working with an international team he came up with a new model of solar system formation
40:47it's like a light going off
40:50you sit in your office and you say
40:52I've solved this problem
40:53this problem people have been working on for decades
40:56and I'm the only one in human history
40:59that understands this little part of the problem we're trying to solve
41:05Levinson's idea assumes that the planets might have formed in very different orbits and then moved
41:12most scientists had believed the solar system was stable and relatively predictable
41:17but many experts now believe this is totally wrong
41:25this flies in the face of conventional wisdom
41:28astronomers probably always thought the planets formed where we find them today
41:32these new numerical simulations which show that they migrated a substantial amount
41:37and even had close encounters that caused them to switch their order is startling and new
41:43in Levinson's simulations Jupiter and Saturn tugged on each other whenever their orbits came closest together
41:49but overall these pulls cancelled out
41:53only when Jupiter circled the Sun exactly twice for every time Saturn circled once did something dramatic seem to occur
42:02it's called a 2 to 1 resonance and it happened 3.9 billion years ago
42:10and that configuration actually makes the planets jiggle with one another
42:14they perturb one another
42:15instead of cancelling out the two planets gravitational tugs then work in the same direction
42:22to Levinson it's just like pushing a swing
42:25each well-timed push nudges the swing higher
42:29for Jupiter and Saturn each gravitational nudge stretched their orbits until they reached their present-day patterns
42:37but the process had a fateful impact on other planets
42:41Uranus and Neptune can't stand that at all
42:44and they just go totally bonkers
42:46the orbits cross one another
42:48they gravitationally get close to one another
42:50one of them gets thrown out
42:53the other one gets thrown in
42:54they rattle around off the Jupiter and Saturn
42:57according to the theory Uranus and Neptune may even switch places
43:01ripping surrounding swarms of asteroids out of their orbits
43:05it's a recipe for disaster
43:09some asteroids are thrown out into space
43:12while others are pushed into the Sun
43:14thousands slam into the inner planets and Earth's moon
43:22it sounds like an improbably chaotic transformation
43:25but the evidence now suggests this did happen
43:30it could never happen today
43:32Jupiter and Saturn are now too far apart
43:34but this event may have been a key moment in the formation of our solar system
43:40but the really incredible thing is
43:42that it could so easily have turned out very very different
43:46you could have the wrong type of star
43:48you could have too many giant planets
43:49there are many ways things can go wrong
43:52and we're right there on that thin lucky line of existence
43:58instead of the diverse solar system we now see
44:01we could have remained forever as ashes, gas and dust
44:06and what of our future
44:09five billion years from now
44:10as our Sun runs out of fuel
44:13it will expand
44:15consuming some of the inner planets
44:16other planets may survive as ghost worlds
44:19still orbiting their dead Sun
44:22there will be no life of any kind
44:25but for now
44:26we're lucky to be alive
44:29we're lucky to be alive
44:30we're lucky to be alive
44:31we're lucky to be able toarsa
44:44stutter
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