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00:00Our solar system is a strange place, as it's radically different from all the other planetary
00:07systems we see across the galaxy.
00:10We are an exception rather than the rule.
00:13To understand why, scientists have looked into our solar system's secret history, to
00:19find a dark and violent past of planetary destruction on a mass scale.
00:28The solar system is a ghost of what it used to be.
00:32You end up with the last survivors being a bunch of freaks.
00:38Our home could be one of those freaks.
00:42The rock beneath our feet could have been from long dead planets.
00:45Those long dead planets could help explain why there's life on Earth.
01:16In a spiral arm of a huge galaxy called the Milky Way, spins an extraordinary planetary
01:22system, our solar system.
01:27For millennia, it was the only one we knew.
01:30However, all that has changed, as astronomers have discovered more than 2,600 planetary
01:37systems to date.
01:40But none of them are quite like our home.
01:46The incredible thing about astronomy is when you look out into the universe and you realize
01:51you have completely misinterpreted your own home.
01:57So one important thing we've learned in discovering planets around other stars is that our system
02:00isn't the normal system.
02:03It's not what we see everywhere.
02:05In fact, as we discover more planets orbiting other stars, we see that ours is an oddball.
02:11Most other solar systems look completely different than ours.
02:15In February 2017, NASA made a huge announcement about a system in the Aquarius constellation,
02:23Trappist-1.
02:26Trappist-1 is a little unusual.
02:28It's a little bit smaller and cooler than the sun, but it has seven planets orbiting
02:33it.
02:34You think, well, that's not that peculiar.
02:35That's not that great.
02:36But these are seven roughly Earth-sized planets.
02:38There's not a lot of variety there.
02:40And they also orbit the star very close in.
02:45All seven planets somehow orbit closer to their star than Mercury, our innermost planet,
02:50does to our sun.
02:53Perhaps one of the greatest puzzles that have come out of finding planets around other stars
03:00is that they typically have orbits well inside the orbit of Mercury.
03:05It's really odd, in my view, that the solar system is hollowed out.
03:10There's nothing inside of Mercury's orbit.
03:12Why is that?
03:16The mystery of the missing inner planets is like a cosmic whodunit, turning scientists
03:22into detectives, asking, do we really know how the formation of our solar system evolved?
03:31The whole process is like cosmic CSI.
03:35You're trying to put together the clues to find out something that happened when nobody
03:40else was there to watch it happen.
03:44Just like detectives, scientists start with the simplest explanation.
03:53So in some ways, the early solar system is like a pool game.
03:55All the billiard balls represent the pieces, the building blocks, the planetesimals or
04:00the planetary embryos that are going to come together eventually to build the final system
04:04of planets.
04:07In planetary formation, the simplest theory is that the planets all formed where we now
04:11find them.
04:13It's called the classical model.
04:16But how?
04:19You start with a few planetesimals, they collide with each other and they grow a little bit
04:23larger in this region of the solar system.
04:26This process continues and you grow all the way up to planets, with each planet in each
04:30of the zones of the solar system accreting material just from its neighborhood and no
04:34one's really moving around very far.
04:39But this classical model can't explain why our inner solar system is missing all kinds
04:44of material.
04:47The classical model has no natural explanation for why Mercury is the last thing that we
04:51know of inward towards the sun.
04:53That there are no planets, no asteroids, nothing inside Mercury is still a mystery that the
04:57classical model can't easily explain.
05:02For centuries, astronomers have been searching to find a planet between Mercury and the sun,
05:08but without success.
05:10Even before astronomers had found it, they were looking for it, they decided to name
05:14this planet Vulcan, and they looked for it really hard.
05:18They looked for where it might be on either side of the sun, you know, right after sunset
05:21or right before sunrise, and they even looked at the sun itself to see if maybe this planet
05:26would move across the face of the sun like a sunspot.
05:29And there were several times when people thought they had seen it, but follow-up observations
05:33showed, yeah, nothing there.
05:36A planet-sized object should be easy enough to spot.
05:40But what about something smaller?
05:42We don't know of anybody of any size on a closer orbit to the sun than Mercury.
05:48If there were a planet, we would have discovered it already, so it'd have to be small.
05:53So there is a proposed population that's never been observed that could orbit inside the
06:00orbit of Mercury, and these are called vulcanoids.
06:04Many astronomers question the very existence of vulcanoids.
06:08But if this theoretical population of asteroids exists, they may be impossible to observe
06:15due to being hidden by the sun's glare.
06:19Now if you were to wake me up in the middle of the night and just say really quickly,
06:22you know, do vulcanoids exist, I'd be like, no.
06:26And I hate to think this, but I suspect we're not going to find them.
06:30I would love to be proven wrong.
06:32I just have a gut feeling that we're not going to find any.
06:39The area close to our sun isn't just missing asteroids and small planets.
06:45It's also missing really big ones.
06:51The very first exoplanets, the very first alien worlds we discovered were Jupiter-mass
06:57or bigger planets orbiting their stars very closely, even closer than Mercury orbits the sun.
07:06Astronomers have so far discovered around 300 gas giants scorchingly close to their suns.
07:13They call them hot Jupiters.
07:17But how they form is a mystery.
07:20Gas giants such as Jupiter should be formed out in the cold, far from their suns.
07:29It's very hard to imagine hot Jupiters forming where we see them today.
07:34The temperatures at distances from a star where we find hot Jupiters are so hot,
07:39it's hard to imagine any material condensing out of the solar nebula.
07:44This kick-started the idea that maybe these hot Jupiters, as they were called,
07:48may have actually formed farther out, like near where our Jupiter is now.
07:52And in the early solar system, they started migrating inward toward their star.
07:58So what happens when a planet the size of Jupiter moves inward?
08:02Can this help explain the inner solar system's missing mass?
08:06And answer why we don't have a hot Jupiter?
08:10To find out, Kevin Walsh and colleagues simulated the first 10 million years of the solar system.
08:17They called this model the Grand Tac.
08:20The Grand Tac model is a scenario designed to help understand
08:24how the terrestrial planets could have formed,
08:27thinking about what the giant planets might have been doing in the early solar system.
08:34The planets form within a thick disk of gas and debris that surrounds the newly formed sun.
08:41The Grand Tac model simulates what happens if Jupiter moves in towards the sun through this disk.
08:49It's pushing all of the asteroids in its path into the inner solar system.
08:54All of that material is what is going to come together to form the rocky planets.
08:59Jupiter's immense gravity pulls in more and more material,
09:03forming a dense wave of debris bulging out behind it.
09:10The pressure of this bulge pushes Jupiter further inwards.
09:15Jupiter should clear out all the planet-building material from the entire inner solar system
09:20and become a sun-hugging hot Jupiter.
09:23But something stops Jupiter's path of destruction.
09:31If Jupiter had hung around much longer in the inner solar system, we wouldn't be here.
09:35So something must have drawn it out very rapidly.
09:38What could possibly move a big, massive planet rapidly?
09:41And the answer is another big, massive planet.
09:48And that planet is Saturn.
09:53It forms just after Jupiter and is following closely behind as it too migrates towards the sun.
10:04Saturn is pretty big itself.
10:07The combined effect of the two giant planets migrating is that once Saturn is large enough,
10:13it can actually change the way that the gas disk is interacting with both the planets.
10:18And it can stop Jupiter's inward migration and help to turn Jupiter around
10:23and almost pulls it back to the outer solar system.
10:28Similar to a sailboat switching direction, Jupiter tacks away from the sun.
10:35The behavior of the giant outer planets leaves our solar system with no hot Jupiter
10:40and has a dramatic effect on the small inner planets too.
10:46As it's coming back outwards, what has Jupiter done to the inner solar system?
10:51It has removed all of the material on its path all the way down to where we find the Earth today.
10:55And all of that material pushed into essentially a narrow band in the inner solar system
10:59is what is going to come together to form the rocky planets as we find them today.
11:06According to the Grand Tac model, without Jupiter,
11:10the rocky terrestrial planets of the inner solar system might never have formed.
11:15And one of those planets is Earth.
11:20So as much as we owe our existence to Jupiter, we also owe it to Saturn
11:24because if Jupiter had kept moving in closer to the sun, we almost certainly wouldn't be here now.
11:33The Grand Tac model may provide a vital chapter in the story of Earth's formation.
11:39It answers why we're missing a hot Jupiter,
11:42but doesn't explain why there is nothing between Mercury and the sun,
11:47nor why we're missing one of the most common types of planet in the whole galaxy,
11:52a giant, rocky world up to ten times the size of Earth, a super-Earth.
12:00The Kepler Space Telescope
12:19The Kepler Space Telescope is at the forefront in the hunt for exoplanets around other stars.
12:25It's confirmed over 2,000 new worlds, the single largest finding to date.
12:31And over one-third of those planets are super-Earths.
12:37A super-Earth is a type of rocky planet that has a mass a few times the mass of the Earth,
12:42and as we look around the galaxy, we find them all over.
12:46Our solar system doesn't have one, and you have to ask the question, why not?
12:50What's different about us?
12:54In our solar system, planets range in mass, with Jupiter being the largest and Mercury the smallest.
13:02But weirdly, we have nothing in the super-Earth size range, which is between Earth and Uranus.
13:09Why is there such a big gap in masses between the Earth and Uranus,
13:13which is roughly a dozen times the Earth's mass?
13:16It's a big jump from one to a dozen. Why?
13:24In 2015, Konstantin Batygin tried to find the answer to why we have no super-Earths,
13:30and why there's nothing within the orbit of Mercury.
13:36He reconstructed the Grand Tac model, with one key difference.
13:45His simulations start with six super-Earths in our solar system's centre,
13:50in a tight orbit around the Sun, typical of other systems we've observed.
13:56He called this new model the Grand Attack.
14:03One of the realizations that has come out of studying the Grand Tac scenario
14:07is that Jupiter's migration would have really unleashed a veritable Grand Attack upon the inner solar system.
14:16The Grand Attack model increases Jupiter's activity,
14:21so it sends swarms of giant asteroids and planetary embryos into the inner solar system,
14:27on tight-knit, overlapping orbits.
14:30The result? Carnage.
14:35Each big body will experience a collision with another big body once every 20 to 200 orbits.
14:43This is exceptionally fast on cosmic timescales.
14:49What this means is that you take the entirety of that overlapped population of bodies,
14:54and you smash them up into smaller debris.
14:57Jupiter's like a little kid with a hammer.
14:59It just comes in and it's whacking around at everything,
15:02and it's making a mess of the inner solar system.
15:06It's an utter chaotic game of pool on a cosmic scale.
15:14Let's start with our Jupiter for our model here of the solar system.
15:28It's causing a bunch of very violent collisions between all of this debris that it's sweeping up.
15:33These huge collisions are making an enormous amount of really small material,
15:38which can drift really fast inward in the solar system
15:41due to the drag from the gas around the sun.
15:46These planetesimals collide over and over, being pulverized to the size of gravel.
15:53For the smaller debris, hitting this dense gas cloud around the sun is like plowing into a headwind.
15:59The swarm of rubble loses the momentum that keeps it in orbit around the sun and starts spiraling in.
16:07But it then hits roadblocks.
16:11So as all of this debris rushes inward in a big wave,
16:14it gets dragged in until it gets stuck behind the super-Earths.
16:21Debris builds up until the super-Earths finally give way.
16:29The super-Earths are like a dam that can't quite resist the flow of water and begins to recede
16:35and eventually gets kind of pushed onto the surface of the sun together with the flux of collisional debris.
16:44It's a remarkably swift process.
16:47In just 20,000 years, all the super-Earths crash into the sun.
16:57After the dramatic evolution of the inner solar system, there's only a fraction of the original mass left.
17:02The solar system is a ghost of what it used to be.
17:08There's nothing in the first 62.7 million kilometers from the sun.
17:16But slightly further out, there's a narrow ring of rocky debris,
17:20about 10% of the original material swept in by Jupiter.
17:25This is just enough to rebuild the inner solar system.
17:30And so a few survivors, small planetesimals, start to regroup.
17:36Over millions of years, four small rocky planets form.
17:42Mercury, Venus, Earth and Mars formed from this leftover debris.
17:46The planet that we're standing on may not be an original generation solar system planet.
17:54It's kind of like building a house with cinder blocks from a house that sat on that spot but was demolished.
18:01Not only are we breathing the atmospheres of long-dead stars,
18:04the rock beneath our feet could have been from long-dead planets.
18:09So Earth could be a second generation planet, formed from the wreckage of the grand attack.
18:17But there was one thing the super-Earths took with them as they crashed into the sun.
18:21The supply of hydrogen and helium in the inner solar system.
18:27When you look at the Earth's atmosphere now, we don't have any hydrogen or helium in it.
18:31There was hydrogen and helium in the disk where the inner planets formed.
18:35But that became part of the super-Earths, the first generation planets.
18:39When Jupiter came in and dropped them into the sun,
18:42they took their hydrogen and helium with them.
18:44So the composition of the air around you right now may be due to the fact
18:48that we're a second generation planet.
18:52So the Earth we now consider home is in fact Earth 2.0.
18:58And being second might not sound great, but maybe being second is the reason we're here.
19:05Earth's atmosphere is a fertile blend of gases that allows life as we know it to flourish.
19:11An atmosphere that might have been completely different if Earth was a fertile planet.
19:17It's entirely possible that life like us
19:21needs to have a second generation planet to arise in the first place.
19:28Could our planet be more unusual than we had ever thought?
19:35How special is the Earth in a cosmic setting?
19:39We don't really know the final answer to this question,
19:42but evidence is beginning to point to the fact that the Earth is actually kind of rare.
19:46And we should really appreciate our planet.
19:51Finding out what happened to our solar system is like studying a cosmic crime scene.
19:57To reveal the solar system's secret history, we need to look in unusual places.
20:04The last surviving pieces of the violence from which our home,
20:08The last surviving pieces of the violence from which our home was born.
20:30Our solar system has a hidden past.
20:33One that's utterly violent.
20:36But there are only a few traces left that show it.
20:39What you have now is a crime scene that has dried up.
20:44And you're trying to find little clues as to what happened four and a half billion years ago.
20:49It's a really, really difficult problem to solve.
20:54Solid evidence can be hard to find.
20:57Yet sometimes we're fortunate.
21:00We don't have a time machine, so it's hard to go back in time four and a half billion years
21:04and look at the solar system and see what it was doing back then.
21:08However, sometimes nature provides.
21:11And if you don't have a time machine, sometimes a time capsule will do just as well.
21:16And in fact, we have time capsules of the early solar system, and we call them meteorites.
21:23Most meteorites are lumps of asteroids that have fallen to Earth.
21:28Depending on their origin, they come in different shapes and sizes.
21:33Asteroids really are like space fossils because they were formed four and a half billion years ago.
21:40But they've basically remained dead.
21:43They are the leftovers, the remnants of planet formation.
21:46They're the last little bits that haven't become planets yet.
21:51To understand why meteorites are such useful clues,
21:54we first need to know how our planets formed in a process called accretion.
22:01A cloud of hot gas swirling around the sun condensed and clumped into larger and larger bodies.
22:08We find traces of this process inside meteorites, in tiny mineral beads called chondrules.
22:17Chondrules are literally the seeds of all of the structure in our solar system.
22:22Most chondrules condensed out of the cloud of hot gas around the sun as the solar system formed.
22:30Chondrules have been described poetically as droplets of fiery rain that have solidified.
22:37They are little globules of silicate melt that were produced in the very earliest history of our solar system.
22:46These globules of melt solidified to form these little spheres.
22:50It really tells us about the process of agglomeration of smaller objects to form larger bodies.
22:58But some chondrules not only tell us about a planet's birth, but also its demise.
23:05A meteorite called gujjba contains two very different kinds of globules.
23:11So gujjba is a type of meteorite that's made up of little spherules of silicate material as well as spherules of iron, nickel, metal.
23:22And it's very unusual.
23:23These metal spherules that we find in gujjba are formed, we think, around five or six million years after the solar system forms.
23:32At that point, there wasn't enough hot gas lingering in the disk to form the chondrules we see in gujjba.
23:39And the chondrules that we see in gujjba are the ones that were formed in the first half of the 20th century.
23:46At that point, there wasn't enough hot gas lingering in the disk to form the chondrules we see in gujjba.
23:53So how did these globules form?
23:58The only way to really produce these globules is another process.
24:03And we think in this case it was some kind of process like collisions.
24:09Collisions that were so violent they vaporized the silicates and metals.
24:15Solid turned to gas and then back to liquid.
24:21But what planetary body contained enough metal to be able to produce the droplets we see in gujjba?
24:28Only something big enough to have an iron core.
24:34When an object grows large enough, its gravity becomes strong enough that it differentiates.
24:38And what we mean by that is heavy stuff is pulled down and sinks into the center,
24:43and lighter stuff floats to the top, so you have a differentiation of material.
24:48Earth is a classic example of a body that has differentiated.
24:53Heavy metals like iron and nickel sink to the core, while lighter rocky silicates remain in the crust.
25:01The crust and the core are separated by a molten silicate layer known as the mantle.
25:09Gujjba is a perfect example of the fact that you had large planetary bodies
25:16that were differentiated into irons and silicates,
25:20and they were colliding at velocities great enough to scatter their pieces out into the nebula again.
25:26That is just amazing to me, this really, really violent process in history
25:32that's captured in these tiny little fragments.
25:38Gujjba reveals that differentiated planets were commonplace
25:43as early as 5 million years after the formation of the solar system.
25:48And out in the asteroid belt, there's an even bigger clue to the early formation of differentiated planets.
25:56The asteroid Psyche.
25:59The asteroid Psyche has always been so mysterious because it's a giant hunk of almost pure metal.
26:06Psyche is unlike any other asteroid in the belt.
26:10So what is it? And where did it come from?
26:14Well, everyone's got their pet theory for Psyche.
26:18My pet theory is a little weird. I think Psyche was the remnant of a hit-and-run collision.
26:24A hit-and-run collision which removed the outer layers, leaving behind only an iron-nickel core.
26:30Psyche is literally part of the core of a planet that was smashed apart. There's nothing else it can be.
26:36To really reduce something to its core, you have to hit something twice, maybe three times.
26:45Scientists think Psyche could once have been a proto-planet
26:49that existed a few million years after the birth of the Sun.
26:55A planet large enough to differentiate.
26:58All that's left now is a remnant iron core, with the crust and mantle having been destroyed.
27:08There were so many of these planets, they often smashed together.
27:15If you go down this path of planet formation by giant impacts,
27:20you end up with the last survivors being a bunch of freaks.
27:23And one such freak is our near neighbour, the red planet, Mars.
27:29But to better understand it, scientists needed another simulation.
27:34This one involving a 30-planet pile-up.
27:38When you look at the architecture of the inner solar system,
27:41you essentially have a smallest planet on the inside,
27:44and then it gets larger and larger as you go from Venus to the Earth.
27:47So you would naturally expect Mars to be larger than it is.
27:50It should be ten times bigger than it is, but it's not.
27:56Mars' small size isn't its only mystery.
28:00It's also much older than we expected.
28:03Scientists have refined the age of Mars' mantle
28:07based on the chemical composition of a piece of Martian meteorite.
28:12The sample blew off from the planet during a violent impact,
28:16and made its way to Earth.
28:18It revealed that Mars formed rapidly,
28:21within the first two million years of the solar system's birth,
28:25and well before the Earth.
28:29Mars is small, and Mars formed really, really fast,
28:33compared to what it should have.
28:36The Earth is ten times more massive.
28:38It formed in a hundred million years.
28:40Mars formed in two million years.
28:42This doesn't make sense. It's our neighbour.
28:44It should look just like us.
28:45Everything about Mars feels wrong.
28:49These two mysteries might help explain one another.
28:54Scientists think it's possible that around 30 other similar planets
28:58formed alongside Mars within the first two million years of the solar system.
29:03So what happened to Mars and these 30 planets?
29:09Time for another game of Cosmic Pool.
29:16So in this model, we very quickly form 20 or 30 Mars-sized planets.
29:23This is a pretty jam-packed system.
29:25The planets are pretty close to each other,
29:27and it's just on the hairy edge of stability.
29:31This colony of Mars-sized planets builds rapidly.
29:37In the early days of the solar system,
29:40there's enough gas to keep their orbits from crossing each other.
29:43But after 20 million years, the gas has gone,
29:47and their orbits start to intersect.
29:52When it goes unstable, it's then a pretty loud and chaotic place.
29:57As Mars-sized bodies collide with each other to build the Earth and Venus,
30:01we get a series of huge violent collisions.
30:07Over the next 100 million years,
30:09the Mars-sized protoplanets annihilate each other
30:13to eventually form second-generation planets, Venus and Earth.
30:19Yet one world managed to avoid all the chaos and destruction.
30:25That planet was Mars,
30:28and that is the secret to its old age compared to the Earth.
30:33If Mars is indeed older than the Earth,
30:35that would imply that it's one of the original planetary embryos of the solar system.
30:41Mars is essentially done early.
30:43It is on the outside of this whole process,
30:46sitting out, not accreting any more mass,
30:49and watching while the Earth and Venus form out of the other big bodies that have been built.
30:54So what prevented Mars from colliding with the rest of the planetary embryos?
31:01The answer is Jupiter.
31:03During the Grand Tac,
31:06Jupiter moved to the same distance from the Sun that we find Mars today,
31:10and in the process, consumed the red planet's material.
31:16Jupiter removes all of the material that Mars otherwise would have been building on
31:21for the next tens of millions of years,
31:24essentially clears out a big chunk of the solar system and starves Mars.
31:28If Jupiter were not there,
31:29then we would have expected Mars to have formed a fully-fledged super-Earth planet.
31:35Earth formed from the wreckage of this pileup.
31:39But there's a reminder of these early proto-planets up in our night sky.
31:44The Moon.
31:47We were convinced we knew how the Moon formed.
31:50It transpired.
31:52We were completely wrong.
31:59EARTH'S MOON
32:17A distant observer studying our solar system would notice something strange right away.
32:24The size of Earth's Moon.
32:27Most planets' moons are tiny by comparison.
32:30So how did we get a moon that's so big?
32:36For a while we realised it couldn't have formed at the same time as the Earth.
32:40It just doesn't make sense.
32:42The standard idea of the Moon's formation
32:44is that an object about the size of Mars collided with the early Earth.
32:48A lot of the debris was thrown into orbit around the Earth,
32:51and it coalesced to form the Moon.
32:53We call this Mars-sized object Theia.
32:57But exactly when and how the Moon formed remains a mystery.
33:03Ever since the Apollo missions, we've been searching for a piece of lunar rock
33:07that can unlock this secret.
33:13Melanie Barboni's team at UCLA
33:16is one of the few groups authorised to analyse these precious lunar samples.
33:20But there's a problem.
33:23Most Moon rock is contaminated and damaged by violent events in the more recent past.
33:30Asteroids hit the Moon and there are geological processes that do a lot of mixing,
33:34and it's very difficult to find a pristine sample from the Moon's very formation.
33:40Melanie and her team have developed a novel answer to that problem.
33:44Rather than date the entire Moon's formation,
33:46they isolate a tiny, pristine crystal within a lunar sample,
33:50known as a zircon.
33:54Now we don't want the whole rock,
33:57we want only tiny zircons that are inside those rocks.
34:02These zircons formed just after Theia's collision with Earth,
34:06once the molten crust of the Moon had cooled and solidified.
34:11This is much smaller than the Earth's crust.
34:13Zircon is the most perfect clock that nature gave us to date the Moon
34:17because it's very resistant.
34:20You can see its surface is very smooth.
34:23There are no fractures on it.
34:26Zircons tick off time like clocks.
34:29They contain large radioactive elements that decay into smaller ones.
34:33Scientists can tell how old a rock is by looking at its surface.
34:37It's a very interesting discovery.
34:39Zircons decay into smaller ones.
34:42Scientists can tell how old a crystal is
34:45by measuring the radioactive decay.
34:48The zircons Melanie found rewrote the history of the Moon.
34:53The Moon is around 140 million years older than when we saw it.
35:00This means the Moon formed no later than 60 million years
35:04after the birth of the Sun.
35:06And it places the formation of the Moon
35:09right in the middle of the destruction of the 30-planet Pileup.
35:13It's entirely possible that Theia
35:16was once a member of this colony of Mars-sized objects.
35:20It wasn't just some Mars object that was out in the outer solar system
35:25and came careening in and smashed into us.
35:28It was one of these no longer existing planetesimals
35:31that slammed into us and formed the Moon.
35:34But scientists looking for traces of Theia on the Moon
35:38have found nothing.
35:40One of the intriguing things about Moon rocks
35:43is how similar they are chemically to rocks on Earth.
35:46It has the same geochemical fingerprints,
35:48the oxygen isotopes of the Earth,
35:50and all the other chemical isotopes of the Earth.
35:52It looks just like Earth rock.
35:54If the Moon really is the product of one giant collision,
35:57well, whatever hit the Earth,
35:59there should be different proportions of that on the Earth
36:01as opposed to the Moon.
36:03We'll find that.
36:06The Moon is identical to material from the Earth,
36:10except it's missing the heavy elements iron and nickel
36:13found in the Earth's core.
36:15Instead, it mainly contains lighter rocky elements
36:19found in the Earth's crust and mantle.
36:21Why?
36:24It wasn't a head-on collision.
36:26It was a grazing collision.
36:28Now that's important because the heavy material
36:30was starting to sink into the centre of the Earth
36:31and the lighter stuff was floating to the top.
36:33And if this were a grazing collision,
36:35then that lighter material would have been splashed out
36:38and that's what would have formed the Moon.
36:40And the Moon is, in fact, less dense than the Earth,
36:42which makes sense if it formed from this lighter material
36:45that was near the top.
36:47It looks like you took a blob of the Earth's mantle
36:49and just put it into space around the Earth.
36:51A single head-on collision would have left traces
36:54of both Theia and Earth's core on the Moon.
36:57However, just one glancing blow
36:59wouldn't have knocked off enough material
37:01to form a Moon as big as ours.
37:04One way we could end up with the Earth-Moon system
37:07that we see today and solve all these problems
37:09is that instead of having one big collision,
37:12there were a series of several smaller collisions.
37:18Each impact grazed off a section of Earth's crust
37:21that formed a ring around our planet.
37:25With each small collision,
37:26material would have been thrown into orbit around the Earth.
37:29Eventually, this collisional debris merged
37:32to form a small new Moon, a Moonlet.
37:36Now, after several of these collisions,
37:39you'll have debris from each collision circling the Earth.
37:42Some of it is still in the form of debris,
37:44some of it is in the form of Moonlets.
37:46Eventually, they coalesce to form our current Moon.
37:50It seems our Moon may well be the product
37:53of a series of collisions.
37:54Our Moon may well be the product
37:56of a series of cosmic collisions in the early Solar System.
38:01What we see when we look up in the sky now
38:04isn't the Moon, but is basically the last Moon that survived.
38:09It was just the one that happened to be there
38:12when all of these impacts stopped.
38:19The birth of our Moon and the rest of our Solar System
38:21was violent, chaotic, catastrophic.
38:26When we look at the Solar System when it was very young,
38:29all of our models pretty much say the same thing.
38:32It was not nice and orderly. It was a disaster.
38:35And then things settled down.
38:37Life had a chance to take hold and evolve
38:40under very stable, very friendly conditions.
38:42So when you look around right now,
38:44you're seeing the story of an ancient, violent past
38:47that has smoothed out into the wonderful environment
38:49we know today.
38:51Our Solar System might seem stable,
38:54but there's still something very strange about it.
38:57There is still one enduring mystery,
39:00and that is, why is the Solar System tilted?
39:04The eight planets orbit in roughly the same flat plane.
39:08But compared to the spin axis of the Sun,
39:11that plane is tilted,
39:13making the Sun look lopsided.
39:15And it turns out, when you look at the Sun's tilt,
39:18it's actually tipped by six degrees,
39:20the plane of the Solar System.
39:22And that may not sound like a lot,
39:24but it's actually quite a bit compared to the tilts
39:27of all the planets of the Solar System.
39:29And this is an outlier. It's strange.
39:31What could have done that?
39:33The tilt contradicts what we know
39:36about how the Solar System formed.
39:38A spinning cloud collapsed into a disk.
39:41The spinning disk then became the Sun.
39:43And all the planets.
39:45So it should all be spinning on the same axis.
39:50So one possible way that you could change
39:53the orientation of the pole of the Sun
39:55relative to the plane the planets are in,
39:57is if there was something out there
39:59tugging on the planets for a very long time.
40:03In 2016, Caltech astronomers Konstantin Batygin and Mike Brown
40:08claimed they'd found the missing something.
40:10Planet Nine.
40:14A theoretical giant orbiting off-kilter
40:17in the far reaches of the Solar System.
40:21Planet Nine resides on a long and substantial orbit.
40:26And it itself is pretty massive.
40:29About ten Earth masses or so.
40:31If it's orbiting the Sun on a highly elliptical, tilted orbit,
40:34every time it gets close to the Sun,
40:36it's going to tug on the planets,
40:37just a little bit.
40:39But over hundreds and thousands of orbits,
40:41it can actually tip the orbits
40:43of all the planets in the Solar System.
40:45But it won't tip the Sun.
40:48Over billions of years,
40:50the planetary system slowly twists
40:52out of alignment with its original plane.
40:56Planet Nine's distant reach
40:58may solve the mystery of the Solar System's tilt.
41:02But it might also have a disastrous effect
41:04on the outer planets.
41:05As the Sun starts to die.
41:16The Sun, just like every other star in the Universe,
41:19has a life cycle.
41:21It was born, it is currently living its life,
41:23and it will die.
41:25As it dies, it will bloat up into a red giant star,
41:28and then the outer layers will begin to drift away.
41:31Now, what that means is that the Sun will be lost
41:33mass very quickly.
41:35The thing that holds us in an orbit around the Sun
41:38is the gravitational pull of the Sun.
41:41So what does this mean for the planets?
41:44So it's not like the Sun is going to go gently
41:47into that good night.
41:49It's going to engulf Mercury for sure,
41:51and it's also going to expand and eat Venus.
41:53The Earth, eh, maybe.
41:55We're not sure.
41:57Either way, we're going to get cooked,
41:59and all the inner planets are going to get cooked.
42:01But what about the outer planets?
42:04Jupiter, Saturn, Uranus and Neptune
42:07will move away from the Sun
42:09as its outer layers expand.
42:11But that's not the case for distant Planet 9.
42:14Scientists think that Planet 9 orbits so far out
42:17that as the Sun dies,
42:19it will loosen its gravitational grip on the planet.
42:22Planet 9 will then start to feel the influence
42:24of other objects more than the Sun.
42:27It turns out a passing star, for example,
42:30could affect its orbit.
42:31Or even tides from the galaxy itself.
42:34Our galaxy's gravitational field can affect this planet
42:37and drop it into the inner solar system.
42:40This change in Planet 9's orbit
42:42could be disastrous for the solar system.
42:45And if that happens, it could actually wreak havoc
42:48on the gas giants, Jupiter, Saturn, Uranus and Neptune,
42:51and distort their orbits,
42:53maybe dropping them into the Sun
42:55or flinging them out of the solar system as well.
42:58Planet 9 could make the death of the solar system
43:01just as violent as its birth.
43:05Right now we're in this wonderful sweet spot
43:08where life can evolve and take hold
43:10between two eras of almost unimaginable violence.
43:13If there is a Planet 9,
43:15then it's kind of a rehash
43:17of what happened in the early solar system.
43:19When everything was really chaotic
43:21because of a giant planet moving inward,
43:23the same thing could happen again.
43:25Born in chaos,
43:27perhaps ending in far worse,
43:30one thing is clear.
43:33What we thought we knew of our cosmic home
43:36grows more intriguing with each new clue
43:39to this once cold case.