How the Universe Works (2010) s06e03 Episode Script
Dark History of the Solar System (62 min)
Our system is a strange thing.
It's radically different from all the other planetary systems we see across the galaxy.
We are an exception rather than the rule.
To understand why, scientists peer deep into our solar system's secret history and find a dark and violent past of planetary homicide on a mass scale.
The solar system is a ghost of what it used to be.
You end up with the last survivors being a bunch of freaks.
Our home could be one of those freaks.
The rock beneath our feet could have been from long-dead planets.
And those long-dead planets could help explain why there's life on Earth.
captions paid for by discovery communications In a spiral arm of a huge galaxy called the milky way spins an extraordinary planetary system, our solar system.
For millennia, it was the only one we knew, but all that has changed.
Astronomers have discovered more than 2,600 planetary systems to date, but none of them are quite like our own.
The incredible thing about astronomy is when you look out into the universe, and you realize you have completely misinterpreted your own home.
So one important thing we've learned in discovering planets around other stars is that our system isn't the normal system.
It's not what we see everywhere.
In fact, as we discover more planets orbiting other stars, we see that ours is an oddball.
Most other solar systems look completely different than ours.
February 2017, NASA makes a huge announcement about a system in the Aquarius constellation, Trappist-1.
Trappist-1 is a little unusual.
It's a little bit smaller and cooler than the Sun, but it has seven planets orbiting it.
And you think, "well, that's not that peculiar.
We have eight.
" But these are seven roughly Earth-sized planets.
There's not a lot of variety there, and they also orbit the star very close in.
All seven planets somehow orbit closer to their star than Mercury, our innermost planet, does to our sun.
Perhaps one of the greatest puzzles that have come out of finding planets around other stars is that they typically have orbits well inside the orbit of Mercury.
It's really odd, in my view, that the solar system is hollowed out.
There's nothing inside of Mercury's orbit.
Why is that? The mystery of the missing inner planets is like a cosmic whodunit, turning scientists into detectives asking, "do we really know how the story of our solar system unfolds?" The whole process is, like, cosmic CSI.
You're trying to put together the clues to find out something that happened when nobody else was there to watch it happen.
Like detectives, scientists start with the simplest explanation.
So in some ways, the early solar system is like a pool game.
All the billiard balls represent the pieces, the building blocks, the planetesimals or the planetary embryos that are going to come together eventually to build the final system of planets.
In planetary formation, the simplest theory is that the planets all formed where we find them now.
It's called the classical model.
How does this play out? We start with a few planetesimals that collide with each other, and they grow a little bit larger in this region of the solar system.
This process continues, and you grow all the way up to planets, with each planet in each of the zone of the solar system creating material just from its neighborhood and no one's really moving around very far.
But this classical model can't explain why our inner solar system is missing all kinds of material.
The classical model has no natural explanation for why Mercury is the last thing that we know of, inward towards the Sun, that there are no planets, no asteroids, nothing inside Mercury, is still a mystery that the classical model can't easily explain.
The area close to our sun isn't just missing asteroids and small planets.
It's also missing really big ones.
The very first exoplanets, the very first alien worlds we discovered, were Jupiter-mass or bigger planets orbiting their starts very closely, even closer than Mercury orbits the Sun.
Astronomers have so far discovered around 300 gas giants scorchingly close to their suns.
They call them hot Jupiters But how they form is a mystery.
Gas giants like Jupiter should be born out of the cold, far from their suns.
It's very hard to imagine hot Jupiters forming where we see them today.
The temperatures, the distances from the star, where we find hot Jupiters are so hot, it's hard to imagine any material condensing out of the solar nebula.
This kick-started the idea that maybe these hot Jupiters, as they were called, may have actually formed farther out, like near where our Jupiter is now, and in the early solar system they started migrating inward toward their star.
So what happens when a planet the size of Jupiter moves inward? Can this help explain the inner solar system's missing mass and answer why we don't have a hot Jupiter? To find out, Kevin Walsh and colleagues simulate the first 10 million years of the solar system.
They call this model the grand tack.
The grand tack model is a scenario designed to help understand how the terrestrial planets could have formed, thinking about what the giant planets might have been doing in the early solar system.
The planets form within a thick disk of gas and debris that surrounds the newly-formed sun.
The grand tack model simulates what happens if Jupiter moves in towards the Sun through this disk.
It's pushing all of the asteroids in its path into the inner solar system.
All of that material is what is going to come together to form the rocky planets.
Jupiter's immense gravity pulls in more and more material, forming a dense wave of debris bulging out behind it.
The pressure of this bulge pushes Jupiter further inwards.
Like a wrecking ball, Jupiter should clear out all the planet-building material from the entire inner solar system and become a sun-hugging hot Jupiter, but something checks Jupiter's path of destruction.
If Jupiter had hung around much longer in the inner solar system, we wouldn't be here, so something must have drawn it out very rapidly.
And what could possibly move a big, massive planet rapidly? And the answer is another big, massive planet.
Saturn It forms just after Jupiter, and is hot on Jupiter's heels, as it, too, migrates towards the Sun.
Saturn is pretty big itself.
The combined effect of the two giant planets migrating is that once Saturn is large enough, it can actually change the way that the gas disk is interacting with both the planets.
And it can stop Jupiter's inward migration and help to turn Jupiter around and almost pulls it back to the outer solar system.
Like a sailboat switching direction, Jupiter tacks away from the Sun.
The behavior of the giant outer planets leaves our solar system with no hot Jupiter and has dramatic effect on the small inner planets, too.
As it's coming back outwards, what has Jupiter done to the inner solar system? It has removed all of the material in its path, all the way down to where we find the Earth today.
And all of that material pushed into essentially a narrow band in the inner solar system is what is going to come together to form the rocky planets as we find them today.
According to the grand tack model, without Jupiter, the rocky terrestrial planets of the inner solar system might have never formed.
One of those planets is Earth.
So as much as we owe our existence to Jupiter, we also owe it to Saturn because if Jupiter had kept moving in closer to the Sun, we almost certainly wouldn't be here now.
The grand tack may provide a vital chapter in the story of Earth's formation.
That answers why we're missing a hot Jupiter but doesn't explain why there's nothing between Mercury and the Sun, nor why we're missing one of the most common types of planet in the whole galaxy, a giant rocky world up to 10 times the size of Earth A super-Earth.
The Kepler space telescope leads the charge in the hunt for exoplanets around other stars.
It's confirmed over 2,000 new worlds.
The single-largest finding to date, over one-third of those planets are super-Earths.
A super-Earth is a type of rocky planet that has a mass a few times the mass of the Earth, and as we look around the galaxy, we find them all over.
Our solar system doesn't have one, and you have to ask the question why not? What's different about us? In our solar system, planets range in mass, with Jupiter being the largest and Mercury the smallest.
But weirdly, we have nothing in the super-Earth-size range, which is between Earth and Uranus.
Why is there such a big gap in masses between the Earth and Uranus, which is roughly a dozen times the Earth's mass? It's a big jump from one to a dozen.
Why? In 2015, Konstantin Batygin tries to find the answer to why we have no super-Earths, and why there is nothing within the orbit of Mercury.
He reconstructs the grand tack model with one key difference The simulations start with six super-Earths in our solar system center in a tight orbit around the Sun, typical of other systems we've observed.
He calls this new model the grand attack.
One of the realizations that has come out of studying the grand tack scenario is that Jupiter's migration would have really unleashed a veritable grand attack upon the inner solar system.
The grand attack model ramps up Jupiter's action so it sends swarms of giant asteroids and planetary embryos into the inner solar system on tight-knit, overlapping orbits.
The result? Carnage.
Each big body will experience a collision with another big body once every 20 to 200 orbits.
This is exceptionally fast on cosmic-time scales.
What this means is that you take the entirety of that overlapped population of bodies, and you smash them up into smaller debris.
Jupiter is like a little kid with a hammer, you know? It just comes in and is whacking around at everything, and it's making a mess of the inner solar system.
It's a game of cosmic pool on steroids.
Let's start with our Jupiter, for our model here of the solar system.
It's causing a bunch of very violent collisions between all of this debris that it's sweeping up.
These huge collisions are making an enormous amount of really small material, which can drift really fast inward in the solar system due to the drag from the gas around the Sun.
These planetesimals collide over and over, pulverized to the size of gravel.
For the smaller debris, hitting this dense gas cloud around the Sun is like plowing into a headwind.
The swarm of rubble loses the momentum that keeps it in orbit around the Sun and starts spiraling in, but it hits roadblocks.
So as all this debris rushes inward in a big wave, it gets dragged in until it gets stuck behind the super-Earths.
Debris builds up until the super-Earths finally give way.
The super-Earths are like a dam that can't quite resist the flow of water and begins to recede and eventually gets kind of pushed onto the surface of the Sun, together with the flux of collisional debris.
It's a remarkably swift process.
In just 20,000 years, all the super-Earths crash into the Sun.
After the dramatic evolution of the inner solar system, there's only a fraction of the original mass left.
The solar system is a ghost of what it used to be.
There's nothing in the first 39 million miles from the Sun.
But slightly farther out, there's a narrow ring of rocky debris, about 10% of the original material swept in by Jupiter, just enough to rebuild the inner solar system.
A few survivors, small planetesimals start to regroup.
Over millions of years, four small, rocky planets form.
Mercury, Venus, Earth, and Mars form from this leftover debris.
The planet that we're standing on may not be an original generation solar system planet.
It's kind of like building a house with cinder blocks from a house that sat on that spot but was demolished.
Not only are we breathing the atmospheres of long-dead stars, the rock beneath our feet could have been from long-dead planets.
So Earth could be second-generation planet formed from the wreckage of the grand attack.
, But there was one thing the super-Earths took with them to their fiery grave The supply of hydrogen and helium in the inner solar system.
When you look at the Earth's atmosphere now, we don't have any hydrogen or helium in it.
There was hydrogen and helium in the disk where the inner planets formed, but that became part of the super-Earths, the first generation planets.
When Jupiter came in and dropped them into the Sun, they took their hydrogen and helium with them.
So the composition of the the air around you right now may be due to the fact that we're a second-generation planet.
Coming in second might not sound great, but maybe second place is the reason we're here.
So, the Earth we now see is Earth 2.
0.
Would Earth 1.
0 have been conducive to life? That's an interesting question.
Earth's atmosphere is a fertile blend of gases that allows life as we know it to flourish, an atmosphere that might have been completely different if Earth was a first-generation planet.
It's entirely possible that life, like us, needs to have a second-generation planet to arise in the first place.
Could our planet be more unusual than we'd ever thought? How special is the Earth in a cosmic setting? We don't really know the final answer to this question, but evidence is beginning to point to the fact that the Earth is actually kind of rare, and we should really appreciate our planet.
Finding out what happened to our solar system is like studying a cosmic crime scene.
To reveal the solar system's secret history, we need to look in unusual places, the last surviving pieces of the violence from which our home was born.
Our solar system is a celestial cold case, and it's hiding the traces of its violent past.
What you have now is a crime scene that has dried up, and you're trying to find little clues as to what happened It's a really, really difficult problem to solve.
Solid evidence could be hard to find, but sometimes we get lucky.
We don't have a time machine, so it's hard to go back in time 4 1/2 billion years and look at the solar system and see what it was doing back then.
However, sometimes nature provides, and if you don't have a time machine, sometimes a time capsule will do just as well.
And in fact, we have time capsules of the early solar system, and we call them meteorites.
Most meteorites are chunks of asteroids that fall to Earth.
Depending on their origin, they come in different shapes and sizes.
Asteroids really are like space fossils because they were formed but they've basically remained dead.
They are the leftovers, the remnants of planet formation.
They're the last little bits that haven't become planets yet.
To understand why meteorites are such useful clues, we first need to know how planets form.
It's a process called accretion.
The cloud of hot gas swirling around the Sun condenses and clumps into larger and larger bodies.
We find traces of this process inside meteorites, in tiny mineral beads called chondrules.
Chondrules are literally the seeds of all of the structure in our solar system.
Most chondrules condense out of the cloud of hot gas around the Sun as the solar system forms.
Chondrules have been described poetically as droplets of fiery rain that have solidified.
They are little globules of silicate melt that were produced in the very earliest history of our solar system.
These globules of melt solidified to form these little spheres.
It really tells us about the process of agglomeration of smaller objects to form larger bodies.
But some chondrules tell us not only about a planet's birth, but also its death.
A meteorite called gujba contains two very different kinds of globules.
So a gujba is a type of meteorite that's made up of little spherials of silicate material as well as spherials of iron-nickel metal, and it's very unusual in that regard.
These metal spherials that we find in gujba are formed, we think, around 5 or 6 million years after the solar system forms.
At that point, there was not enough hot gas lingering in the disk to form the chondrules we see in the gujba.
So how did this globules form? The only way to really produce these globules is another process, and we think in this case it was some kind of process like collisions.
Collisions so violent they vaporized the silicates and metals.
Solid turns to gas, and then gas to liquid.
But what planetary body contains enough metal to be able to produce the droplets we see in gujba? Only something big enough to have an iron core.
When an object grows large enough, its gravity become strong enough that it differentiates, and what we mean by that is, heavy stuff is pulled down and sinks into the center, and lighter stuff floats to the top, so you have a differentiation of material.
Earth is a classic example of a body that is differentiated.
while heavy metals remain in the crust The crust and the core are separated by a molten silicate layer known as the mantle.
Gujba is a perfect example of the fact that you had large planetary bodies that were differentiated into irons and silicates, and they were colliding at velocities great enough to scatter their pieces out into the nebula again.
That is just amazing to me, that this really, really violent process in history that's captured in these tiny little fragments.
Gujba reveals the differentiated planets were commonplace as early as 5 million years after the formation of the solar system.
There were so many of these planets, they often smashed together.
If you go down this path of planet formation by giant impacts, you end up with the last survivors being a bunch of freaks.
Freaks built from the dead embers of past generations.
And this violent scenario raises the question, just how many planets did it take to build the inner solar system? Simulations say at least 30, but just four survived.
The secret history of the solar system is hard to interpret, but astronomers devise radical new solutions to connect the dots.
The grand tack model helps explain why we see no hot Jupiter planet.
The grand attack model provides an answer to the lack of material within Mercury's orbit and why our solar system has no super-Earths.
These are theories designed to crack the cosmic cold case, but to understand our red neighbor, Mars, scientists need another simulation, a 30-planet pileup.
When you look at the action in a solar system, you essentially have our smallest planet on the inside and then it gets larger and larger as you go from Venus to the Earth, so you would naturally expect Mars to be larger than it is.
It should be 10 times bigger than it is, but it's not.
Mars' size isn't its only mystery.
It's also much older than we expected.
Scientists have refined the age of Mars' mantle based on the chemical composition of a piece of martian meteorite.
The sample blew off from the planet during a violent impact and made its way to Earth.
It revealed that Mars formed rapidly, within the first 2 million years of the solar system's birth, well before the Earth.
Mars is small, and Mars formed really, really fast compared to what it should have.
The Earth is 10 times more massive.
It formed in 100 million years.
Mars formed in 2 million years.
This doesn't make sense.
It's our neighbor.
It should look just like us.
Everything about Mars feels wrong.
These two mysteries might help explain one another.
Scientists think it's possible that around 30 other similar planets formed alongside Mars within the first 2 million years of the solar system.
So what happened to this 30-planet pileup? Time for another game of cosmic pool.
So in this model, we very quickly form This is a pretty jam-packed system.
The planets are pretty close to each other, and it's just on the hairy edge of stability.
This colony of Mars-sized planets builds rapidly.
In the early days of the solar system, there's enough gas around to keep their orbits from crossing each other, but after 20 million years, the gas has gone, and their orbits start to intersect.
When it goes unstable, it's then a pretty loud and chaotic place.
As Mars-sized bodies collide with each other to build the Earth and Venus, we get a series of huge, violent collisions.
Over the next 100 million years, the Mars-sized protoplanets annihilate each other to eventually form second-generation planets, Venus and Earth.
Yet one planet stood back and watched from the sidelines.
That planet was Mars, and that is the secret to its old age, compared to Earth.
If Mars is indeed older than the Earth, that would imply that it's one of the original planetary embryos of the solar system.
Mars is essentially done early.
It is on the outside of this whole process, sitting out not accreting any more mass and watching while the Earth and Venus form out of the other big bodies that have been built.
So what prevented Mars from colliding with the rest of the planetary embryos? The answer Jupiter.
During the grand tack, Jupiter moved to the same distance from the Sun that we find Mars today, and in the process ate the red planet's lunch.
Jupiter removes all of the material that Mars otherwise would have been building on for the next tens of millions of years, essentially clears out a big chunk of the solar system and starves Mars.
If Jupiter were not there, than we would have expected Mars to have formed a fully-fledged super-Earth planet.
Earth formed from the wreckage of this pileup, but a reminder of this population lives on, every time we look to the night sky The Moon.
We were convinced we knew how the Moon formed.
Turns out, we were completely wrong.
A distance observer studying our solar system would notice something strange right away The size of Earth's moon.
Most planet's moons are tiny by comparison.
How did we get a moon so big? For a while we've realized it couldn't have formed at the same time as the Earth.
It just doesn't make sense.
The standard idea of the Moon's formation is that an object about the size of Mars collided with the early Earth.
A lot of the debris was thrown into orbit around the Earth, and it coalesced to form the Moon.
We call this Mars-sized object Thea, but exactly when and how the Moon formed remains a mystery.
Ever since the Apollo missions, we've been searching for a piece of lunar rock that can unlock this secret.
Melanie Barboni's team at UCLA is one of the few groups authorized to analyze these precious lunar samples.
But there's a problem Most moon rock is contaminated and damaged by violent events in the more recent past.
Asteroids hit the Moon, and there are geological processes that do a lot of mixing, and it's very difficult to find a pristine sample from the Moon's very formation.
Melanie and her team have come up with a novel answer to that problem.
Rather than date the entire rock, they isolate a tiny, pristine crystal within a lunar sample, known as a zircon.
Now we don't want the whole rock, we want only tiny zircon that are inside those rocks.
These zircons formed just after Thea's collision with Earth.
Once the molten crust of the Moon, it cooled and solidified.
This is much smaller than the grain of sand you find on the beach.
Zircon is the most perfect clock that nature gave us to date the Moon because it's very resistant.
Here you can see its surface is very smooth.
There is no fractures on it.
Zircons tick off time like clocks.
They contain large radioactive elements that decay into smaller ones.
Scientists can tell how old the crystal is by measuring the radioactive decay.
The zircons Melanie found rewrite the history of the Moon.
The Moon is around than what we thought.
This means the Moon formed no later than 60 million years after the birth of the Sun.
This places the formation of the Moon right in the middle of the destruction of the 30-planet pileup.
It's entirely possible that Thea was once a member of this colony of Mars-sized objects.
It wasn't just some Mars object that was out in the outer solar system and came careening in and smashed into us.
It was one of these no-longer-existing planetesimals that slammed into us and formed the Moon.
But scientists looking for traces of Thea on the Moon draw blanks.
One of the intriguing things about moon rocks is how similar they are, chemically, to rocks on Earth.
It has the same geochemical fingerprints, the oxygen isotopes, of the Earth, and all the other chemical isotopes of the Earth.
It looks just like Earth rock.
If the Moon really is the product of one giant collision, well, whatever hit the Earth, there should be different proportions of that on the Earth as opposed to the Moon, but we don't find that.
The Moon is identical to material from the Earth, except it's missing heavy elements, iron and nickel, found in the Earth's core.
Instead, it mainly contains lighter rocky elements found in the Earth's crust and mantle.
Why? It wasn't a head-on collision.
It was a grazing collision.
Now, that's important because the heavy material was starting to sink into the center of the Earth, and the lighter stuff was floating to the top.
And if this were a grazing collision, then that lighter material would have been splashed out, and that's what would have formed the Moon.
And the Moon is, in fact, less dense than the Earth, which makes sense if it formed from this lighter material that was near the top.
It looks like you took a blob of the Earth's mantle and just put it into space around the Earth.
A single head-on collision would leave traces of both Thea and Earth's core on the Moon, but a glancing blow wouldn't knock off enough material to form a moon as big as ours.
One way we can end up with the Earth-moon system that we see today and solve all these problems, is that instead of having one big collision, there were a series of several smaller collisions.
Each impact grazes off a section of Earth's crust that forms a ring around our planet.
With each small collision, material would have been thrown into orbit around the Earth.
Eventually this collisional debris merges to form a small, new moon, a moonlet.
Now after several of these collisions, you'll have debris from each collision circling the Earth.
Some of it is still in the form of debris, some of it is in the form of moonlets.
Eventually they coalesce to form our current moon.
It seems our moon may well be the product of a series of cosmic collisions in the early solar system.
What we see when we look up in the sky now isn't the moon, but it's basically the last moon that survived.
It was just the one that happened to be there when all of these impacts stopped.
The closer we come to understanding our violent past, the more we appreciate the calm of the present, but as we try to predict our future, it seems we are destined for chaos once again, as a distant mystery planet in the outer solar system moves in from the cold.
The birth of our solar system Violent, chaotic, catastrophic.
When we look at the solar system when it was very young, all of our models pretty much say the same thing.
It was not nice and orderly.
It was a disaster.
And then things settled down.
Life had a chance to take hold and evolve under very stable, very friendly conditions.
So when you look around right now, you're seeing the story of an ancient, violent past that has smoothed out into the wonderful environment we know today.
Our solar system might seem stable, but there is still something very strange about it.
There is still one enduring mystery, and that is why the solar system tilted? The eight planets orbit in roughly the same flat plane, but compared to the spin axis of the Sun, that plane is tilted, making the Sun look lopsided.
And it turns out, when you look at the Sun's tilt, it's actually tipped by 6 degrees, the plane of the solar system.
And that may not sound like a lot, but it's actually quite a bit compared to the tilts of all the planets of the solar system, and this is an outlier.
It's strange.
What could have done that? The tilt contradicts what we know about how the solar system formed, a spinning cloud collapses into a disk.
The spinning disk then becomes the Sun and all the planets.
It should all be spinning on the same axis.
So one possible way that you could change the orientation of the pull of the Sun relative to the plane the planets are in, is if there was something out there tugging on the planets for a very long time.
Konstantin Batygin and Mike Brown claim they've found the missing something Planet 9, a theoretical giant orbiting off-kilter in the far reaches of the solar system.
Planet 9 resides on a long and substantial orbit, and it itself is pretty massive, about 10 Earth masses or so.
If it's orbiting the Sun on a highly elliptical tilted orbit, every time it gets close to the Sun, it's going to tug on the planets just a little bit.
But over hundreds and thousands of orbits, it can actually tip the orbits of all the planets in the solar system, but it won't tip the Sun.
Over billions of years, the planetary system slowly twists out of alignment with its original plane.
Planet 9's distant reach may solve the mystery of the solar system's tilt, but it might also have a disastrous effect on the outer planets as the Sun starts to die.
the Sun, just like every other star in the universe, has a life cycle.
It was born.
It is currently living its life, and it will die.
As it dies, it will bloat up into a red giant star, and then the outer layers will begin to drift away.
Now what that means is that the Sun will be losing mass very quickly.
The thing that holds us in orbit around the Sun is the gravitational pull of the Sun.
Jupiter, Saturn, Uranus, and Neptune will move away from the Sun as its outer layers expand, but that's not the case for distant Planet 9.
Scientists think that Planet 9 orbits so far out that as the Sun dies, it will loosen its gravitational grip on the planet.
Planet 9 starts to feel the influence of other objects more than the Sun.
It turns out a passing star, for example, could affect its orbit, or even tides from the galaxy itself, our galaxy's gravitational field can affect this planet, and drop it into the inner solar system.
This change in Planet 9's orbits could be disastrous for the solar system.
And if that happens, it could actually wreak havoc on the gas giants Jupiter, Saturn, Uranus, and Neptune and distort their orbits, maybe dropping them into the Sun or flinging them out of the solar system, as well.
Planet 9 could make the death of the solar system just as violent as its birth.
Right now we're in this wonderful sweet spot where life can evolve and take hold between two eras of almost unimaginable violence.
If there is a Planet 9, then it's kind of a rehash of what happened in the early solar system, when everything was really chaotic because of a giant planet moving inward.
The same thing could happen again.
Born in chaos.
Perhaps ending in worse.
One thing is clear What we thought we knew of our cosmic home grows more intriguing with each new clue to this once cold case, now a very hot one.
It's radically different from all the other planetary systems we see across the galaxy.
We are an exception rather than the rule.
To understand why, scientists peer deep into our solar system's secret history and find a dark and violent past of planetary homicide on a mass scale.
The solar system is a ghost of what it used to be.
You end up with the last survivors being a bunch of freaks.
Our home could be one of those freaks.
The rock beneath our feet could have been from long-dead planets.
And those long-dead planets could help explain why there's life on Earth.
captions paid for by discovery communications In a spiral arm of a huge galaxy called the milky way spins an extraordinary planetary system, our solar system.
For millennia, it was the only one we knew, but all that has changed.
Astronomers have discovered more than 2,600 planetary systems to date, but none of them are quite like our own.
The incredible thing about astronomy is when you look out into the universe, and you realize you have completely misinterpreted your own home.
So one important thing we've learned in discovering planets around other stars is that our system isn't the normal system.
It's not what we see everywhere.
In fact, as we discover more planets orbiting other stars, we see that ours is an oddball.
Most other solar systems look completely different than ours.
February 2017, NASA makes a huge announcement about a system in the Aquarius constellation, Trappist-1.
Trappist-1 is a little unusual.
It's a little bit smaller and cooler than the Sun, but it has seven planets orbiting it.
And you think, "well, that's not that peculiar.
We have eight.
" But these are seven roughly Earth-sized planets.
There's not a lot of variety there, and they also orbit the star very close in.
All seven planets somehow orbit closer to their star than Mercury, our innermost planet, does to our sun.
Perhaps one of the greatest puzzles that have come out of finding planets around other stars is that they typically have orbits well inside the orbit of Mercury.
It's really odd, in my view, that the solar system is hollowed out.
There's nothing inside of Mercury's orbit.
Why is that? The mystery of the missing inner planets is like a cosmic whodunit, turning scientists into detectives asking, "do we really know how the story of our solar system unfolds?" The whole process is, like, cosmic CSI.
You're trying to put together the clues to find out something that happened when nobody else was there to watch it happen.
Like detectives, scientists start with the simplest explanation.
So in some ways, the early solar system is like a pool game.
All the billiard balls represent the pieces, the building blocks, the planetesimals or the planetary embryos that are going to come together eventually to build the final system of planets.
In planetary formation, the simplest theory is that the planets all formed where we find them now.
It's called the classical model.
How does this play out? We start with a few planetesimals that collide with each other, and they grow a little bit larger in this region of the solar system.
This process continues, and you grow all the way up to planets, with each planet in each of the zone of the solar system creating material just from its neighborhood and no one's really moving around very far.
But this classical model can't explain why our inner solar system is missing all kinds of material.
The classical model has no natural explanation for why Mercury is the last thing that we know of, inward towards the Sun, that there are no planets, no asteroids, nothing inside Mercury, is still a mystery that the classical model can't easily explain.
The area close to our sun isn't just missing asteroids and small planets.
It's also missing really big ones.
The very first exoplanets, the very first alien worlds we discovered, were Jupiter-mass or bigger planets orbiting their starts very closely, even closer than Mercury orbits the Sun.
Astronomers have so far discovered around 300 gas giants scorchingly close to their suns.
They call them hot Jupiters But how they form is a mystery.
Gas giants like Jupiter should be born out of the cold, far from their suns.
It's very hard to imagine hot Jupiters forming where we see them today.
The temperatures, the distances from the star, where we find hot Jupiters are so hot, it's hard to imagine any material condensing out of the solar nebula.
This kick-started the idea that maybe these hot Jupiters, as they were called, may have actually formed farther out, like near where our Jupiter is now, and in the early solar system they started migrating inward toward their star.
So what happens when a planet the size of Jupiter moves inward? Can this help explain the inner solar system's missing mass and answer why we don't have a hot Jupiter? To find out, Kevin Walsh and colleagues simulate the first 10 million years of the solar system.
They call this model the grand tack.
The grand tack model is a scenario designed to help understand how the terrestrial planets could have formed, thinking about what the giant planets might have been doing in the early solar system.
The planets form within a thick disk of gas and debris that surrounds the newly-formed sun.
The grand tack model simulates what happens if Jupiter moves in towards the Sun through this disk.
It's pushing all of the asteroids in its path into the inner solar system.
All of that material is what is going to come together to form the rocky planets.
Jupiter's immense gravity pulls in more and more material, forming a dense wave of debris bulging out behind it.
The pressure of this bulge pushes Jupiter further inwards.
Like a wrecking ball, Jupiter should clear out all the planet-building material from the entire inner solar system and become a sun-hugging hot Jupiter, but something checks Jupiter's path of destruction.
If Jupiter had hung around much longer in the inner solar system, we wouldn't be here, so something must have drawn it out very rapidly.
And what could possibly move a big, massive planet rapidly? And the answer is another big, massive planet.
Saturn It forms just after Jupiter, and is hot on Jupiter's heels, as it, too, migrates towards the Sun.
Saturn is pretty big itself.
The combined effect of the two giant planets migrating is that once Saturn is large enough, it can actually change the way that the gas disk is interacting with both the planets.
And it can stop Jupiter's inward migration and help to turn Jupiter around and almost pulls it back to the outer solar system.
Like a sailboat switching direction, Jupiter tacks away from the Sun.
The behavior of the giant outer planets leaves our solar system with no hot Jupiter and has dramatic effect on the small inner planets, too.
As it's coming back outwards, what has Jupiter done to the inner solar system? It has removed all of the material in its path, all the way down to where we find the Earth today.
And all of that material pushed into essentially a narrow band in the inner solar system is what is going to come together to form the rocky planets as we find them today.
According to the grand tack model, without Jupiter, the rocky terrestrial planets of the inner solar system might have never formed.
One of those planets is Earth.
So as much as we owe our existence to Jupiter, we also owe it to Saturn because if Jupiter had kept moving in closer to the Sun, we almost certainly wouldn't be here now.
The grand tack may provide a vital chapter in the story of Earth's formation.
That answers why we're missing a hot Jupiter but doesn't explain why there's nothing between Mercury and the Sun, nor why we're missing one of the most common types of planet in the whole galaxy, a giant rocky world up to 10 times the size of Earth A super-Earth.
The Kepler space telescope leads the charge in the hunt for exoplanets around other stars.
It's confirmed over 2,000 new worlds.
The single-largest finding to date, over one-third of those planets are super-Earths.
A super-Earth is a type of rocky planet that has a mass a few times the mass of the Earth, and as we look around the galaxy, we find them all over.
Our solar system doesn't have one, and you have to ask the question why not? What's different about us? In our solar system, planets range in mass, with Jupiter being the largest and Mercury the smallest.
But weirdly, we have nothing in the super-Earth-size range, which is between Earth and Uranus.
Why is there such a big gap in masses between the Earth and Uranus, which is roughly a dozen times the Earth's mass? It's a big jump from one to a dozen.
Why? In 2015, Konstantin Batygin tries to find the answer to why we have no super-Earths, and why there is nothing within the orbit of Mercury.
He reconstructs the grand tack model with one key difference The simulations start with six super-Earths in our solar system center in a tight orbit around the Sun, typical of other systems we've observed.
He calls this new model the grand attack.
One of the realizations that has come out of studying the grand tack scenario is that Jupiter's migration would have really unleashed a veritable grand attack upon the inner solar system.
The grand attack model ramps up Jupiter's action so it sends swarms of giant asteroids and planetary embryos into the inner solar system on tight-knit, overlapping orbits.
The result? Carnage.
Each big body will experience a collision with another big body once every 20 to 200 orbits.
This is exceptionally fast on cosmic-time scales.
What this means is that you take the entirety of that overlapped population of bodies, and you smash them up into smaller debris.
Jupiter is like a little kid with a hammer, you know? It just comes in and is whacking around at everything, and it's making a mess of the inner solar system.
It's a game of cosmic pool on steroids.
Let's start with our Jupiter, for our model here of the solar system.
It's causing a bunch of very violent collisions between all of this debris that it's sweeping up.
These huge collisions are making an enormous amount of really small material, which can drift really fast inward in the solar system due to the drag from the gas around the Sun.
These planetesimals collide over and over, pulverized to the size of gravel.
For the smaller debris, hitting this dense gas cloud around the Sun is like plowing into a headwind.
The swarm of rubble loses the momentum that keeps it in orbit around the Sun and starts spiraling in, but it hits roadblocks.
So as all this debris rushes inward in a big wave, it gets dragged in until it gets stuck behind the super-Earths.
Debris builds up until the super-Earths finally give way.
The super-Earths are like a dam that can't quite resist the flow of water and begins to recede and eventually gets kind of pushed onto the surface of the Sun, together with the flux of collisional debris.
It's a remarkably swift process.
In just 20,000 years, all the super-Earths crash into the Sun.
After the dramatic evolution of the inner solar system, there's only a fraction of the original mass left.
The solar system is a ghost of what it used to be.
There's nothing in the first 39 million miles from the Sun.
But slightly farther out, there's a narrow ring of rocky debris, about 10% of the original material swept in by Jupiter, just enough to rebuild the inner solar system.
A few survivors, small planetesimals start to regroup.
Over millions of years, four small, rocky planets form.
Mercury, Venus, Earth, and Mars form from this leftover debris.
The planet that we're standing on may not be an original generation solar system planet.
It's kind of like building a house with cinder blocks from a house that sat on that spot but was demolished.
Not only are we breathing the atmospheres of long-dead stars, the rock beneath our feet could have been from long-dead planets.
So Earth could be second-generation planet formed from the wreckage of the grand attack.
, But there was one thing the super-Earths took with them to their fiery grave The supply of hydrogen and helium in the inner solar system.
When you look at the Earth's atmosphere now, we don't have any hydrogen or helium in it.
There was hydrogen and helium in the disk where the inner planets formed, but that became part of the super-Earths, the first generation planets.
When Jupiter came in and dropped them into the Sun, they took their hydrogen and helium with them.
So the composition of the the air around you right now may be due to the fact that we're a second-generation planet.
Coming in second might not sound great, but maybe second place is the reason we're here.
So, the Earth we now see is Earth 2.
0.
Would Earth 1.
0 have been conducive to life? That's an interesting question.
Earth's atmosphere is a fertile blend of gases that allows life as we know it to flourish, an atmosphere that might have been completely different if Earth was a first-generation planet.
It's entirely possible that life, like us, needs to have a second-generation planet to arise in the first place.
Could our planet be more unusual than we'd ever thought? How special is the Earth in a cosmic setting? We don't really know the final answer to this question, but evidence is beginning to point to the fact that the Earth is actually kind of rare, and we should really appreciate our planet.
Finding out what happened to our solar system is like studying a cosmic crime scene.
To reveal the solar system's secret history, we need to look in unusual places, the last surviving pieces of the violence from which our home was born.
Our solar system is a celestial cold case, and it's hiding the traces of its violent past.
What you have now is a crime scene that has dried up, and you're trying to find little clues as to what happened It's a really, really difficult problem to solve.
Solid evidence could be hard to find, but sometimes we get lucky.
We don't have a time machine, so it's hard to go back in time 4 1/2 billion years and look at the solar system and see what it was doing back then.
However, sometimes nature provides, and if you don't have a time machine, sometimes a time capsule will do just as well.
And in fact, we have time capsules of the early solar system, and we call them meteorites.
Most meteorites are chunks of asteroids that fall to Earth.
Depending on their origin, they come in different shapes and sizes.
Asteroids really are like space fossils because they were formed but they've basically remained dead.
They are the leftovers, the remnants of planet formation.
They're the last little bits that haven't become planets yet.
To understand why meteorites are such useful clues, we first need to know how planets form.
It's a process called accretion.
The cloud of hot gas swirling around the Sun condenses and clumps into larger and larger bodies.
We find traces of this process inside meteorites, in tiny mineral beads called chondrules.
Chondrules are literally the seeds of all of the structure in our solar system.
Most chondrules condense out of the cloud of hot gas around the Sun as the solar system forms.
Chondrules have been described poetically as droplets of fiery rain that have solidified.
They are little globules of silicate melt that were produced in the very earliest history of our solar system.
These globules of melt solidified to form these little spheres.
It really tells us about the process of agglomeration of smaller objects to form larger bodies.
But some chondrules tell us not only about a planet's birth, but also its death.
A meteorite called gujba contains two very different kinds of globules.
So a gujba is a type of meteorite that's made up of little spherials of silicate material as well as spherials of iron-nickel metal, and it's very unusual in that regard.
These metal spherials that we find in gujba are formed, we think, around 5 or 6 million years after the solar system forms.
At that point, there was not enough hot gas lingering in the disk to form the chondrules we see in the gujba.
So how did this globules form? The only way to really produce these globules is another process, and we think in this case it was some kind of process like collisions.
Collisions so violent they vaporized the silicates and metals.
Solid turns to gas, and then gas to liquid.
But what planetary body contains enough metal to be able to produce the droplets we see in gujba? Only something big enough to have an iron core.
When an object grows large enough, its gravity become strong enough that it differentiates, and what we mean by that is, heavy stuff is pulled down and sinks into the center, and lighter stuff floats to the top, so you have a differentiation of material.
Earth is a classic example of a body that is differentiated.
while heavy metals remain in the crust The crust and the core are separated by a molten silicate layer known as the mantle.
Gujba is a perfect example of the fact that you had large planetary bodies that were differentiated into irons and silicates, and they were colliding at velocities great enough to scatter their pieces out into the nebula again.
That is just amazing to me, that this really, really violent process in history that's captured in these tiny little fragments.
Gujba reveals the differentiated planets were commonplace as early as 5 million years after the formation of the solar system.
There were so many of these planets, they often smashed together.
If you go down this path of planet formation by giant impacts, you end up with the last survivors being a bunch of freaks.
Freaks built from the dead embers of past generations.
And this violent scenario raises the question, just how many planets did it take to build the inner solar system? Simulations say at least 30, but just four survived.
The secret history of the solar system is hard to interpret, but astronomers devise radical new solutions to connect the dots.
The grand tack model helps explain why we see no hot Jupiter planet.
The grand attack model provides an answer to the lack of material within Mercury's orbit and why our solar system has no super-Earths.
These are theories designed to crack the cosmic cold case, but to understand our red neighbor, Mars, scientists need another simulation, a 30-planet pileup.
When you look at the action in a solar system, you essentially have our smallest planet on the inside and then it gets larger and larger as you go from Venus to the Earth, so you would naturally expect Mars to be larger than it is.
It should be 10 times bigger than it is, but it's not.
Mars' size isn't its only mystery.
It's also much older than we expected.
Scientists have refined the age of Mars' mantle based on the chemical composition of a piece of martian meteorite.
The sample blew off from the planet during a violent impact and made its way to Earth.
It revealed that Mars formed rapidly, within the first 2 million years of the solar system's birth, well before the Earth.
Mars is small, and Mars formed really, really fast compared to what it should have.
The Earth is 10 times more massive.
It formed in 100 million years.
Mars formed in 2 million years.
This doesn't make sense.
It's our neighbor.
It should look just like us.
Everything about Mars feels wrong.
These two mysteries might help explain one another.
Scientists think it's possible that around 30 other similar planets formed alongside Mars within the first 2 million years of the solar system.
So what happened to this 30-planet pileup? Time for another game of cosmic pool.
So in this model, we very quickly form This is a pretty jam-packed system.
The planets are pretty close to each other, and it's just on the hairy edge of stability.
This colony of Mars-sized planets builds rapidly.
In the early days of the solar system, there's enough gas around to keep their orbits from crossing each other, but after 20 million years, the gas has gone, and their orbits start to intersect.
When it goes unstable, it's then a pretty loud and chaotic place.
As Mars-sized bodies collide with each other to build the Earth and Venus, we get a series of huge, violent collisions.
Over the next 100 million years, the Mars-sized protoplanets annihilate each other to eventually form second-generation planets, Venus and Earth.
Yet one planet stood back and watched from the sidelines.
That planet was Mars, and that is the secret to its old age, compared to Earth.
If Mars is indeed older than the Earth, that would imply that it's one of the original planetary embryos of the solar system.
Mars is essentially done early.
It is on the outside of this whole process, sitting out not accreting any more mass and watching while the Earth and Venus form out of the other big bodies that have been built.
So what prevented Mars from colliding with the rest of the planetary embryos? The answer Jupiter.
During the grand tack, Jupiter moved to the same distance from the Sun that we find Mars today, and in the process ate the red planet's lunch.
Jupiter removes all of the material that Mars otherwise would have been building on for the next tens of millions of years, essentially clears out a big chunk of the solar system and starves Mars.
If Jupiter were not there, than we would have expected Mars to have formed a fully-fledged super-Earth planet.
Earth formed from the wreckage of this pileup, but a reminder of this population lives on, every time we look to the night sky The Moon.
We were convinced we knew how the Moon formed.
Turns out, we were completely wrong.
A distance observer studying our solar system would notice something strange right away The size of Earth's moon.
Most planet's moons are tiny by comparison.
How did we get a moon so big? For a while we've realized it couldn't have formed at the same time as the Earth.
It just doesn't make sense.
The standard idea of the Moon's formation is that an object about the size of Mars collided with the early Earth.
A lot of the debris was thrown into orbit around the Earth, and it coalesced to form the Moon.
We call this Mars-sized object Thea, but exactly when and how the Moon formed remains a mystery.
Ever since the Apollo missions, we've been searching for a piece of lunar rock that can unlock this secret.
Melanie Barboni's team at UCLA is one of the few groups authorized to analyze these precious lunar samples.
But there's a problem Most moon rock is contaminated and damaged by violent events in the more recent past.
Asteroids hit the Moon, and there are geological processes that do a lot of mixing, and it's very difficult to find a pristine sample from the Moon's very formation.
Melanie and her team have come up with a novel answer to that problem.
Rather than date the entire rock, they isolate a tiny, pristine crystal within a lunar sample, known as a zircon.
Now we don't want the whole rock, we want only tiny zircon that are inside those rocks.
These zircons formed just after Thea's collision with Earth.
Once the molten crust of the Moon, it cooled and solidified.
This is much smaller than the grain of sand you find on the beach.
Zircon is the most perfect clock that nature gave us to date the Moon because it's very resistant.
Here you can see its surface is very smooth.
There is no fractures on it.
Zircons tick off time like clocks.
They contain large radioactive elements that decay into smaller ones.
Scientists can tell how old the crystal is by measuring the radioactive decay.
The zircons Melanie found rewrite the history of the Moon.
The Moon is around than what we thought.
This means the Moon formed no later than 60 million years after the birth of the Sun.
This places the formation of the Moon right in the middle of the destruction of the 30-planet pileup.
It's entirely possible that Thea was once a member of this colony of Mars-sized objects.
It wasn't just some Mars object that was out in the outer solar system and came careening in and smashed into us.
It was one of these no-longer-existing planetesimals that slammed into us and formed the Moon.
But scientists looking for traces of Thea on the Moon draw blanks.
One of the intriguing things about moon rocks is how similar they are, chemically, to rocks on Earth.
It has the same geochemical fingerprints, the oxygen isotopes, of the Earth, and all the other chemical isotopes of the Earth.
It looks just like Earth rock.
If the Moon really is the product of one giant collision, well, whatever hit the Earth, there should be different proportions of that on the Earth as opposed to the Moon, but we don't find that.
The Moon is identical to material from the Earth, except it's missing heavy elements, iron and nickel, found in the Earth's core.
Instead, it mainly contains lighter rocky elements found in the Earth's crust and mantle.
Why? It wasn't a head-on collision.
It was a grazing collision.
Now, that's important because the heavy material was starting to sink into the center of the Earth, and the lighter stuff was floating to the top.
And if this were a grazing collision, then that lighter material would have been splashed out, and that's what would have formed the Moon.
And the Moon is, in fact, less dense than the Earth, which makes sense if it formed from this lighter material that was near the top.
It looks like you took a blob of the Earth's mantle and just put it into space around the Earth.
A single head-on collision would leave traces of both Thea and Earth's core on the Moon, but a glancing blow wouldn't knock off enough material to form a moon as big as ours.
One way we can end up with the Earth-moon system that we see today and solve all these problems, is that instead of having one big collision, there were a series of several smaller collisions.
Each impact grazes off a section of Earth's crust that forms a ring around our planet.
With each small collision, material would have been thrown into orbit around the Earth.
Eventually this collisional debris merges to form a small, new moon, a moonlet.
Now after several of these collisions, you'll have debris from each collision circling the Earth.
Some of it is still in the form of debris, some of it is in the form of moonlets.
Eventually they coalesce to form our current moon.
It seems our moon may well be the product of a series of cosmic collisions in the early solar system.
What we see when we look up in the sky now isn't the moon, but it's basically the last moon that survived.
It was just the one that happened to be there when all of these impacts stopped.
The closer we come to understanding our violent past, the more we appreciate the calm of the present, but as we try to predict our future, it seems we are destined for chaos once again, as a distant mystery planet in the outer solar system moves in from the cold.
The birth of our solar system Violent, chaotic, catastrophic.
When we look at the solar system when it was very young, all of our models pretty much say the same thing.
It was not nice and orderly.
It was a disaster.
And then things settled down.
Life had a chance to take hold and evolve under very stable, very friendly conditions.
So when you look around right now, you're seeing the story of an ancient, violent past that has smoothed out into the wonderful environment we know today.
Our solar system might seem stable, but there is still something very strange about it.
There is still one enduring mystery, and that is why the solar system tilted? The eight planets orbit in roughly the same flat plane, but compared to the spin axis of the Sun, that plane is tilted, making the Sun look lopsided.
And it turns out, when you look at the Sun's tilt, it's actually tipped by 6 degrees, the plane of the solar system.
And that may not sound like a lot, but it's actually quite a bit compared to the tilts of all the planets of the solar system, and this is an outlier.
It's strange.
What could have done that? The tilt contradicts what we know about how the solar system formed, a spinning cloud collapses into a disk.
The spinning disk then becomes the Sun and all the planets.
It should all be spinning on the same axis.
So one possible way that you could change the orientation of the pull of the Sun relative to the plane the planets are in, is if there was something out there tugging on the planets for a very long time.
Konstantin Batygin and Mike Brown claim they've found the missing something Planet 9, a theoretical giant orbiting off-kilter in the far reaches of the solar system.
Planet 9 resides on a long and substantial orbit, and it itself is pretty massive, about 10 Earth masses or so.
If it's orbiting the Sun on a highly elliptical tilted orbit, every time it gets close to the Sun, it's going to tug on the planets just a little bit.
But over hundreds and thousands of orbits, it can actually tip the orbits of all the planets in the solar system, but it won't tip the Sun.
Over billions of years, the planetary system slowly twists out of alignment with its original plane.
Planet 9's distant reach may solve the mystery of the solar system's tilt, but it might also have a disastrous effect on the outer planets as the Sun starts to die.
the Sun, just like every other star in the universe, has a life cycle.
It was born.
It is currently living its life, and it will die.
As it dies, it will bloat up into a red giant star, and then the outer layers will begin to drift away.
Now what that means is that the Sun will be losing mass very quickly.
The thing that holds us in orbit around the Sun is the gravitational pull of the Sun.
Jupiter, Saturn, Uranus, and Neptune will move away from the Sun as its outer layers expand, but that's not the case for distant Planet 9.
Scientists think that Planet 9 orbits so far out that as the Sun dies, it will loosen its gravitational grip on the planet.
Planet 9 starts to feel the influence of other objects more than the Sun.
It turns out a passing star, for example, could affect its orbit, or even tides from the galaxy itself, our galaxy's gravitational field can affect this planet, and drop it into the inner solar system.
This change in Planet 9's orbits could be disastrous for the solar system.
And if that happens, it could actually wreak havoc on the gas giants Jupiter, Saturn, Uranus, and Neptune and distort their orbits, maybe dropping them into the Sun or flinging them out of the solar system, as well.
Planet 9 could make the death of the solar system just as violent as its birth.
Right now we're in this wonderful sweet spot where life can evolve and take hold between two eras of almost unimaginable violence.
If there is a Planet 9, then it's kind of a rehash of what happened in the early solar system, when everything was really chaotic because of a giant planet moving inward.
The same thing could happen again.
Born in chaos.
Perhaps ending in worse.
One thing is clear What we thought we knew of our cosmic home grows more intriguing with each new clue to this once cold case, now a very hot one.