How the Universe Works (2010) s06e05 Episode Script
Uranus & Neptune: Rise of the Ice Giants (62 min)
Our solar system is home to giants.
Gas giants Jupiter and Saturn seem to dominate, but two ice giants determine the fate of the rest Neptune and Uranus Distant planets unconnected to us, or so we thought.
We now know the fate of the ice giants is entwined with our own.
They've gone from being these cold, dull worlds to actually having in them the very secret of why you and I exist at all.
Their story is of epic migration, brutal destruction Uranus got jabbed and then knocked on its side.
Of worlds more alive than anyone imagined.
They hold the key to the history of the solar system and perhaps to life on Earth.
captions paid for by discovery communications Uranus and Neptune Mysterious giants lurking in the cold outer reaches of the solar system The farthest planets from the Sun.
Uranus and Neptune are sort of the sentinels of the outer solar system.
They're out past Jupiter and Saturn, well over, like, 2 billion, Their size and location are a puzzle to planetary astronomers.
Uranus and Neptune are somewhat of a mystery because, in a way, they shouldn't exist, or at least they shouldn't exist where they are.
Scientists can't understand how these giant planets grew so big so far from the Sun.
The mystery starts with the birth of the solar system.
In the beginning, the Sun ignites from a disk of gas and dust.
The rocky cores of the first planets start to grow.
They collide with the debris in the disk as they orbit the Sun.
But the inner planets have a size limit.
To grow into a giant planet, you need gas.
Heat from the infant sun blasts these lighter gas molecules beyond a point astronomers call the frost line.
Out here, it's cool enough for gas molecules like hydrogen and helium to stabilize.
Jupiter and Saturn take shape first Sweeping in the abundant gas and quickly becoming gas giants.
But Neptune and Uranus are different.
Jupiter and Saturn are about whereas Neptune and Uranus are more like 20%.
So what does this difference in gas tell us about their formation? We suspect that Uranus and Neptune came a little bit later when there was not as much gas to be swept up.
Uranus and Neptune have less time to suck up as much hydrogen and helium before these gases disappear.
But they're also forming farther out, where it's cold enough for other, heavier gases to freeze.
These are swept up by the growing outer planets.
Out where Uranus and Neptune are, tons of ice, tons of frozen gases as we might think of them Methane, ammonia, water, and so that's what makes up their composition predominantly.
They may be smaller than Jupiter and Saturn, but these heavy ices mean they grow dense.
They become ice giants.
But there's a problem.
They're too big.
The disk of gas and ice around a newborn star does not last forever, and material in the far reaches of the solar system is spread thin.
As you move further out in the solar system, the time scale for two bodies to find each other and collide and accrete slows down because the periods around the Sun are much longer, and it just takes a very, very long time.
Neptune and Uranus orbit the Sun incredibly slowly, too slowly to collide with enough icy material to grow into the giants we see today.
So when we look at Neptune at this very distant orbit, we don't have enough time in the solar system to build a planet like Neptune.
We just don't think we could build Neptune where we find it today.
So what happened? It turns out, where we see them now is probably not where they started out.
One thing we've learned about solar systems is that things are in a delicate balance, and planets migrate.
They move around.
They don't form in one place and stay there forever.
So what has enough power to move a giant planet like Neptune? An even bigger one Jupiter.
One way for planets to move is by gravitationally interacting with each other, so they feel each other's gravity.
They can tug.
They can pull, and that effect can lead to planets slowly migrating around in their planetary systems.
The closer two planets are, the greater the effect, and in the early days of the solar system, the giant planets are much closer together.
On top of that, they may have orbited in a different order than we see today Jupiter, Saturn, Neptune, and Uranus.
So what causes Neptune and Uranus to swap positions? The answer lies with Jupiter and Saturn.
The two biggest giants lock into a gravitational dance.
There's always this interplay between them.
Think of Capoeira dancers, balancing and moving together in a careful, orchestrated way.
Over millions of years, a rhythm slowly builds.
These giants push and pull each other into more elliptical orbits.
But the gravitational dance reaches a climax.
The stretched orbits become unstable.
The giants move off course.
As Saturn and Jupiter twist out from the Sun, they fling Neptune out beyond Uranus.
As Neptune moves out through the solar system, it pushes debris ahead of it.
These are the leftover icy fragments from planet formation.
Neptune snowplows these bodies out.
They because the Kuiper belt, the band of thousands of small bodies of ice and rock just beyond Neptune's orbit.
You can think of the structure of that Kuiper belt as, like, blood splatter on the wall at a murder scene.
It's the record of this really violent of Neptune migrating outward across the solar system.
But Neptune's movement doesn't just fling these small icy bodies out into the Kuiper belt.
It also sends some of them crashing in towards the Sun.
Some bombard the early Earth.
It's the most violent time on our planet since the birth of the solar system itself, the Sun flared into life.
It's called the late heavy bombardment.
During the late heavy bombardment, you had rocks literally falling down from the sky constantly.
This would've been a terrible time for life.
And yet, this cascade of icy bodies also brings something essential for life.
One characteristic of the outer solar system bodies is that we often find organics.
Organics provide the basis for all living organisms we find today.
They're carbon-based molecules that form on the surface of dust grains in the early solar system.
The rocky inner planets sweep up these organics as they grow But the scorched surfaces of the young planets are too inhospitable for many of these delicate molecules to survive.
Yet organics remain intact on the small, icy bodies of the outer solar system that Neptune tosses towards the early Earth.
Neptune was the solar system's cosmic delivery service.
As far away as Uranus and Neptune are, the existence of ice giants in the outer solar system may have been critical for the existence of Earth today.
And the ice giants may have done more than give life on Earth a kickstart.
They may have prevented our planet from being completely destroyed.
The ice giants Uranus and Neptune, distant giant worlds that may have delivered the elements of life to Earth.
Without them, our planet itself might not exist at all.
These ice giants are fascinating worlds, but they may be even more important than that.
They might be the reason we're here.
Around 4 billion years ago, the young Earth is under threat from our solar system's bully, Jupiter.
Positioned between the rocky inner planets and the giant outer ones, Jupiter dominates the solar system.
When you have a behemoth like Jupiter in your solar system, what it does determines in part what everything else does.
As Jupiter and Saturn lock into their gravitational dance, they migrate out, away from the Sun.
Jupiter's immense gravity should pull Earth and Venus along with it.
Earth and Venus' orbits should stretch and overlap with each other.
It's a collision waiting to happen except It didn't.
So by the fact that we're talking about this here on Earth suggests that Earth and Venus didn't have an impact early in the solar system when Jupiter and Saturn were migrating.
Something appears to have protected us.
Scientists think something yanked Jupiter into a different orbit before it had a chance to pull Earth and Venus on a collision course.
But what could cause such a large jump in Jupiter's migration? This is where the ice giants enter the story.
Getting Jupiter to make a big jump in its migration is not easy, and so the best way that the models have been able to actually recreate this jump is to have Jupiter actually eject something the size of Neptune out of the solar system entirely.
Jupiter has a lot of gravity, and if you get too close to it, you're going to be accelerated as you fall in towards Jupiter, and it's possible that you can eject a planet completely out of the solar system this way.
It's basically slingshotting it.
But a planet the size of Neptune is heavy, even for Jupiter, and slingshotting it out of the solar system gives Jupiter a kickback.
Jupiter is knocked into a new orbit, and Earth is saved.
But which ice giant sacrificed itself for us? Neptune is still in the solar system, and so is Uranus.
If you use computer models to basically predict the behavior of the planets, what you find is that if you start with Jupiter, Saturn, Uranus and Neptune, you can't save the Earth without ejecting either Uranus or Neptune, but they're there, so we know that's not right.
However, if you add a third ice giant, a fifth giant planet out there, then that actually makes everything work.
You can save the Earth, have the planets in their present configuration, and that ice giant gets ejected from the solar system.
Imagine our solar system starting with three ice giants.
One swings too close to Jupiter.
Our solar system's bully throws its victim clean out of the playground.
Jupiter is pushed into a new orbit by the ice giant's gravity.
Earth is saved from Jupiter's deadly gravitational pull, and the solar system becomes the safe and orderly place we see today.
So we have a funny story here.
This ice giant that may have existed billions of years ago yanked Jupiter back into the outer solar system, preventing it from destroying the Earth, but in the meantime, it sacrificed itself for us, getting ejected from the solar system.
We have to thank it for our existence, but it's not there anymore.
We humans are really lucky.
Had the dinosaurs not gone extinct, we wouldn't be here.
Had this planet not been ejected out of our solar system, we wouldn't be here.
So where is this missing ice giant now? The answer is pretty amazing.
It could be clear across the other side of the milky way galaxy.
The Sun moves around the milky way galaxy at about half a million miles an hour, and in the history of the Earth, we've been around about 20 times.
We could've lost that planet anywhere across the milky way.
But is this third ice giant really lost or just hiding? January 2016 Astronomers at Caltech make an astonishing announcement.
They claim to have found evidence of a mysterious ninth planet disrupting icy bodies far out in the Kuiper belt.
Simulations suggest that if this so-called planet nine exists, it is similar in size to Neptune and Uranus.
Could this be Earth's savior? Could the solar system's primordial missing sacrificial ice giant be planet nine? Yes, it could.
Perhaps this third ice giant wasn't ejected from the solar system after all.
One type of ejection is where you just take something and you throw it out of the solar system, but another more gentle kind is when you don't quite make it all the way out, and instead you go on a very, very long period orbit around the young solar system.
Planet nine is thought to be so far out, it takes up to 20,000 years for it to travel around the Sun.
Perhaps it's been observing the dramatic, dynamical evolution of the solar system unfold from its frigid Whether planet nine is a long-lost sibling or not, ice giants played a huge role in taming Jupiter.
They made our solar system the haven it is today.
But they are not peaceful places.
Somehow, out in the deep freeze, Neptune is tormented by wild weather, mysterious superstorms and maybe even diamond rain.
Uranus and Neptune Their location at the edge of our solar system makes them very difficult to study.
The ice giants Uranus and Neptune are very mysterious to us.
They're very far away, so they're hard to observe with telescopes here at the Earth.
As a result, these planets have long been overlooked.
The only time we've glimpsed these distant giants up close was when voyager 2 flew past them in the 1980s.
The results amazed Heidi Hammel, part of the voyager 2 imaging team at the time of the Neptune flyby.
One of the most wondrous and frustrating things about planetary flybys is that you learn so much that you open a whole Pandora's box of questions.
One observation instantly intrigued scientists.
Neptune has the fastest winds in the solar system.
Here on Earth, our winds are actually driven by different temperatures from sunshine.
Neptune is so far away from the Sun that it receives almost no energy from our star.
Neptune is 3 billion miles from the Sun.
It's really cold there, so why does it have such fast winds? The less energy a planet receives from the Sun, the quieter we expect its weather to be But Neptune isn't tranquil at all.
It's covered in massive violent storms.
There are storms that are rivaling the size of the inner planets.
That's a pretty big storm.
One of the largest ever recorded on Neptune is known as the 1989 great dark spot, a single vast tempest, large enough to swallow the Earth whole, riding on a jet stream with a mind-blowing wind speed of 1,500 miles per hour.
Hands down, Neptune holds the record for the fastest wind speeds in the solar system.
The fastest tornado winds on Earth are only a few hundred miles an hour, and that does devastating destruction, so it's hard to imagine what winds on Neptune would do.
A probe entering Neptune's upper atmosphere would record freezing temperatures, minus-370 degrees Fahrenheit Too cold to generate the wind we see.
though, the probe is smashed by Neptune's relentless jet stream winds.
And the deeper you go, the warmer it becomes.
Neptune has almost three times as much heat coming from its interior than you would expect from a ball of gas out at Neptune's distance.
The strange thing about these high-speed Neptune winds is that they're not powered by heat energy from the Sun.
In fact, they're powered by heat energy from Neptune's own interior.
So where does this internal heat come from? When planets form, it's a very violent, very energetic event, and the planets are actually extremely hot, and it takes billions of years for that heat to leak away, so Neptune, it turns out, probably still has a tremendous amount of that heat that is trapped inside of it, and as that bubbles up, that's what's actually heating the atmosphere and driving this tremendous weather.
So how does Neptune retain so much heat? The secret lies deep below the atmosphere.
As you go down and down and down, you'll just find the pressure gets more and more intense until you are eventually essentially crushed.
That atmosphere will get thicker and thicker like a fog until suddenly you would realize that instead of an atmosphere, you are in an ocean.
Neptune has a super-dense fluid mantle made up of methane, ammonia, and water.
Really, an ice giant is not a solid ball of ice, but rather a moving ocean of swirling liquid material.
This swirling liquid traps the heat, acting like a blanket, insulating the core.
This is the secret to Neptune's wild weather, and the intense interior heat and pressure in Neptune's methane-rich mantle may create another extraordinary effect.
The pressure is so intense that the methane breaks up, and methane is made of carbon and hydrogen, so if you take carbon and you compress it a lot, you could get diamond formation, and so it is entirely possible that literal diamonds are raining down in this ocean of the mantle fluid on Neptune.
From superstorms to diamond rain, Neptune is strange, dynamic beyond expectation, but Uranus is the real mystery, the victim of a cosmic one-two punch with seasons unlike anything else in the solar system.
January 1986 Voyager 2 approaches Uranus at over 40,000 miles per hour.
Astronomers have been waiting for this moment for 8 years, but on voyager's arrival, all that's revealed is a bland, pale, blue ball.
That was a little bit disappointing from my perspective as a scientist studying the atmosphere of the planet.
everything changes.
Telescopes reveal huge storms raging across the planet.
Why the enormous difference? The answer lies in the planet's extreme axial tilt.
If Neptune and Uranus are siblings, Uranus is definitely the wonky sibling.
All the planets are tilted with respect to the solar system.
The Earth is 23 degrees.
Jupiter is just a handful of degrees, but Uranus is actually on its side.
It's tipped 98 degrees.
Uranus' tilt is almost four times more extreme than any other planet in the system.
It's lying so its poles are horizontal, and its rings and moons are vertical relative to the plane of the solar system.
Earth's tilt gives it seasons.
Uranus' extreme tilt gives it extreme seasons.
Twice a year, its poles are pointed directly towards or away from the Sun, so each pole has a very intense period of midnight sun and a dark, cold polar night.
It takes Uranus 84 years to orbit the Sun, so those seasons last for a very long time.
You get, like, in the northern hemisphere as its going around the Sun and 20 years of darkness in the Southern hemisphere.
Uranus is kind of like "game of thrones.
" You're waiting ages for winter to come, and then winter lasts 20 years.
A winter's night or a summer's day that lasts for decades.
But what happens in the interim when the orbit of the planet means the Sunlight hits its spinning equator rather than one of its poles? When it's off to the side, the whole planet's lit up.
As it spins, every piece of the planet is exposed to sunlight.
Sunlight hits the equator of the spinning planet, pumping energy into the surface, warming the atmosphere and driving air currents around the planet.
The result Spring and Autumn storms.
That extreme change in how much sunlight is distributed across that planet's atmosphere probably has an important role in driving this remarkable seasonal change we see in Uranus' atmosphere.
Unlike its ice giant sibling, Neptune, Uranus has seasonal storms rather than constant ones, so why does Uranus roll around the Sun while other planets spin like tops? It goes against everything we know about planetary formation.
In some ways, forming new planets in the solar system was a lot like making cotton candy.
There was a direction that everything was coming together.
If you put a stick down in it, the material would accumulate around it in a certain direction, so that's why all the planets have roughly the same orbital axes.
If Uranus started out with a vertical orbital axis, how did it end up flipped on its side? We know there were a lot of collisions between planets or planet-sized objects in the early solar system.
It's natural to assume that Uranus probably got hit as a grazing impact from another giant object, which tipped it over on its side.
But there's a problem with this assumption.
If you hit Uranus with a single impact to knock it over to 98 degrees, then actually what you expect is that the rings left over would be orbiting in the wrong direction relative to the spin of the planet.
What event could be powerful enough to flip a planet, but gentle enough to bring everything in orbit around it along for the ride? A single big collision is probably not what happened to Uranus because that would've been too disruptive.
It's kind of like boxing.
Instead of one big knockout blow, it was the old one-two.
One theory suggests that the newly formed Uranus is hit by a protoplanet the size of Earth.
The blow is only glancing.
Uranus is knocked partway towards its current tilt.
Its ring system survives the impact and stays in orbit around the equator.
As the second object hits, Uranus is tipped all the way, and the rings follow.
Uranus got jabbed and then knocked on its side.
Uranus may orbit the Sun sideways, but Neptune's moon, Triton, has an even stranger trajectory.
It travels around Neptune in reverse.
But weirder than that, it seems to be erupting, and it could even harbor life.
Wherever we see planets, we expect to see moons.
It seems the larger the planet, the more moons orbit around it.
Jupiter has 69.
Saturn, 61.
Next come the ice giants.
Astronomers have so far detected and 13 around Neptune.
But one stands out completely Neptune's moon, Triton.
Triton is a bit of an oddball because instead of orbiting Neptune in the same direction that Neptune spins, it orbits in the opposite direction, what we call a retrograde orbit.
A giant planet and its moons form out of the same swirling disk of gas, dust, and rocky material.
The lighter gas falls into the center more easily, forming the planet, while some of the heavier rocky material is left over in the disk, forming the moons.
Typically, the moon travels in the same direction that the planet is orbiting, but in the case of Triton and Neptune, that's the complete opposite case.
We know it couldn't have formed in that orbit around Neptune.
It had to come were somewhere else, and a wonderful clue to where it came from is the nearby neighbor, Pluto.
Pluto is a dwarf planet in the nearby Kuiper belt.
It's only 200 miles smaller than Triton, but it's not just size that makes these two bodies similar.
It's their composition.
Triton is actually most similar to Pluto.
It has a similar amount of rock in its interior.
It has a similar surface composition with a lot of nitrogen and methane.
It really looks like a Pluto-like world, but it just happens to be orbiting a planet instead of orbiting the Sun.
If Triton is like Pluto, maybe it also started life in the Kuiper belt.
Could it have been captured by Neptune's gravity, pulled into the gas giant's orbit? It's not easy to capture a moon into orbit around a planet.
It's not natural for a body to come close to another world and just spiral in.
You've somehow got to put on the brakes when it's close in.
For Neptune to capture Triton, Triton had to be slowed down, but how? Again, Kuiper belt objects hold a clue.
Many of the largest are binary pairs, two worlds orbiting each other, like Pluto and its large moon Charon.
Perhaps Triton was one of a pair as well.
If Triton was in orbit with a partner, each of the pair would travel at different speeds.
This speed difference is key.
Triton's velocity is just a little bit slower.
As it's orbiting its companion, it's slow enough that it could actually get captured by Neptune, while that other one would speed off across the solar system.
Triton's original dance partner is flung out and away.
Triton has a new, much bigger companion, Neptune.
But these unlikely partners are dancing out of sync.
Capture explains the backwards orbit, but there's an even stranger mystery to solve.
On a moon like Triton, we'd expect to see heavily cratered terrain, the hallmark of geologically dead worlds.
Instead, voyager 2 reveals a world that's startlingly alive.
I think one of the most amazing discoveries of the entire voyager mission is that when we flew past Triton, we saw these jets of liquid nitrogen coming up out of the surface.
We had no idea that this tiny, little cold world out there would still be alive.
Smooth, icy planes cover the surface.
Geysers of nitrogen punch up through the crust and spew black dust We thought it was too far from the Sun, too cold, too dead.
It was just going to be an ice ball like all the other moons that had tended to be, but, no.
It's a fresh, young surface.
Triton's surface is a frosty 200 degrees below freezing.
Where's the heat coming from to drive these surface features? The answer lies with Neptune's capture of Triton.
Triton's orbit is circular, but it was once elliptical.
As Triton moved closer in and farther away, it would have been repeatedly squashed and stretched by Neptune's gravity.
That would generate massive amounts of friction inside of Triton.
Might well have completely melted Triton due to those forces, and as that happened, it would have acted as a brake.
All that friction would have circularized Triton's orbit and left it in the orbit that we see today.
This change in orbit is what gives Triton its heat.
The surface of Triton froze over, but the moon still retains some of this warmth deep below the icy shell.
Astronomers think there's enough heat to melt ice into water, forming an underground liquid ocean on a world 3 billion miles from the Sun.
Liquid water, heat There is only one question, and it's impossible not to ask it.
Could there be life? If there's a source of energy on Triton, then perhaps there's a form of life that figured out how to take advantage of that energy source.
An incredible thought.
If life has carved out a niche, this frozen ball could be the most distant habitable world from the Sun.
Neptune's moon, Triton, might be alive, but Uranus' moons are even stranger.
They have a neat trick to cheat death entirely.
Uranus An ice giant with beautiful, shimmering rings and 27 moons And their position is a mystery.
Half exist within a tightly packed orbit.
It shouldn't be possible for this many moons to be in such close proximity to each other.
The environment around Uranus is very busy, and the system appears unstable, and the moons should be colliding, but yet, we see these nice, well-formed moons.
And what's even more surprising is that in 2003, the Hubble space telescope revealed two new rings and two new moons Cupid and Mab.
Question is, where did these moons come from? A clue lies in Uranus' rings.
In addition to the very packed moon system, you also have rings around Uranus, which is also somewhat unexpected, but these two unexpected qualities might actually explain one another.
Anytime you see a ring system, you're seeing part of a process.
There was probably a time in Earth's history when it had a ring, you know, when our moon was being formed.
It seems that the new moons are made up of material from a previous ring system.
But when scientists model Cupid's future, they discover it's dangerously close to another moon, Belinda.
In a few thousand years, two of the moons in particular, Cupid and Belinda, are likely to collide together as their trajectories intersect.
And this collision will create a domino effect.
When there is a moon collision, that sets up a very delicate gravitational balance, and it also creates a lot of debris, so one collision sets off a string of collisions.
We think Cupid and Belinda will destroy not only each other, but all of Uranus' inner moons.
A runaway cascade of destruction grinds Uranus' moons to pebbles.
All of this will happen in only a few thousand years.
The fact that we are predicting to have a collision within a few thousand years feels kind of contrary because this system has been here for billions of years, so it seems a little bit lucky that we're looking just now, and we think there's going to be a collision in the near future.
What if this cycle of destruction is a case of Uranian groundhog day? When you look at the Uranian system, I like to think of it sort of like a violent hockey game.
Guys are taking big hits.
They can't continue, so what must you do? You have to sub them in and out.
When you look at this game that's that violent, you know that the guys you see on the ice are not the guys that started the game.
Whoo! It's a whole new set of players.
Like hockey subs, Uranus keeps its moons fresh by recycling them.
Every time these moons collide, the debris forms a ring system around Uranus, and then over time, that ring system starts to spawn new moons.
Over time, the rings of debris that surround Uranus form into new moons, which in turn collide and grind each other to dust.
Uranus, it turns out, is actually very Eco-conscious.
Its moons shatter.
They hit each other, form all this debris, and then the moons reform from that material, and then the whole pattern, the whole cycle starts up again, and the moons hit, shatter, and new moons form all over again.
Ultimately, this eternal cycle of life and death that we see with these moons might be a story that is actually kind of true of many things in the universe, when it comes to stars, planets, and maybe even the universe itself.
Uranus and Neptune, no longer forgotten outposts of the solar system.
Suddenly, these planets are revealed to you as worlds.
It's breathtaking.
It's it's awe-inspiring.
It's humbling, and it makes you proud that you're part of a species that could actually do that, to get out there and to see these places.
Dynamic worlds with dramatic, often violent histories, stolen moons, giants flung out into the cold, saving the Earth from destruction.
Uranus and Neptune are puzzle pieces in the solar system.
They are giant planets.
They have a lot to tell us, not just about themselves but about everything in our solar system, including our own planet, Earth.
Somewhere in the story of the ice giants is the reason you and I are actually here to talk about it at all, the reason the Earth was able to form and stabilize and become an environment for life.
It would not be an exaggeration to say that the ice giants are the coolest planets in the solar system.
Gas giants Jupiter and Saturn seem to dominate, but two ice giants determine the fate of the rest Neptune and Uranus Distant planets unconnected to us, or so we thought.
We now know the fate of the ice giants is entwined with our own.
They've gone from being these cold, dull worlds to actually having in them the very secret of why you and I exist at all.
Their story is of epic migration, brutal destruction Uranus got jabbed and then knocked on its side.
Of worlds more alive than anyone imagined.
They hold the key to the history of the solar system and perhaps to life on Earth.
captions paid for by discovery communications Uranus and Neptune Mysterious giants lurking in the cold outer reaches of the solar system The farthest planets from the Sun.
Uranus and Neptune are sort of the sentinels of the outer solar system.
They're out past Jupiter and Saturn, well over, like, 2 billion, Their size and location are a puzzle to planetary astronomers.
Uranus and Neptune are somewhat of a mystery because, in a way, they shouldn't exist, or at least they shouldn't exist where they are.
Scientists can't understand how these giant planets grew so big so far from the Sun.
The mystery starts with the birth of the solar system.
In the beginning, the Sun ignites from a disk of gas and dust.
The rocky cores of the first planets start to grow.
They collide with the debris in the disk as they orbit the Sun.
But the inner planets have a size limit.
To grow into a giant planet, you need gas.
Heat from the infant sun blasts these lighter gas molecules beyond a point astronomers call the frost line.
Out here, it's cool enough for gas molecules like hydrogen and helium to stabilize.
Jupiter and Saturn take shape first Sweeping in the abundant gas and quickly becoming gas giants.
But Neptune and Uranus are different.
Jupiter and Saturn are about whereas Neptune and Uranus are more like 20%.
So what does this difference in gas tell us about their formation? We suspect that Uranus and Neptune came a little bit later when there was not as much gas to be swept up.
Uranus and Neptune have less time to suck up as much hydrogen and helium before these gases disappear.
But they're also forming farther out, where it's cold enough for other, heavier gases to freeze.
These are swept up by the growing outer planets.
Out where Uranus and Neptune are, tons of ice, tons of frozen gases as we might think of them Methane, ammonia, water, and so that's what makes up their composition predominantly.
They may be smaller than Jupiter and Saturn, but these heavy ices mean they grow dense.
They become ice giants.
But there's a problem.
They're too big.
The disk of gas and ice around a newborn star does not last forever, and material in the far reaches of the solar system is spread thin.
As you move further out in the solar system, the time scale for two bodies to find each other and collide and accrete slows down because the periods around the Sun are much longer, and it just takes a very, very long time.
Neptune and Uranus orbit the Sun incredibly slowly, too slowly to collide with enough icy material to grow into the giants we see today.
So when we look at Neptune at this very distant orbit, we don't have enough time in the solar system to build a planet like Neptune.
We just don't think we could build Neptune where we find it today.
So what happened? It turns out, where we see them now is probably not where they started out.
One thing we've learned about solar systems is that things are in a delicate balance, and planets migrate.
They move around.
They don't form in one place and stay there forever.
So what has enough power to move a giant planet like Neptune? An even bigger one Jupiter.
One way for planets to move is by gravitationally interacting with each other, so they feel each other's gravity.
They can tug.
They can pull, and that effect can lead to planets slowly migrating around in their planetary systems.
The closer two planets are, the greater the effect, and in the early days of the solar system, the giant planets are much closer together.
On top of that, they may have orbited in a different order than we see today Jupiter, Saturn, Neptune, and Uranus.
So what causes Neptune and Uranus to swap positions? The answer lies with Jupiter and Saturn.
The two biggest giants lock into a gravitational dance.
There's always this interplay between them.
Think of Capoeira dancers, balancing and moving together in a careful, orchestrated way.
Over millions of years, a rhythm slowly builds.
These giants push and pull each other into more elliptical orbits.
But the gravitational dance reaches a climax.
The stretched orbits become unstable.
The giants move off course.
As Saturn and Jupiter twist out from the Sun, they fling Neptune out beyond Uranus.
As Neptune moves out through the solar system, it pushes debris ahead of it.
These are the leftover icy fragments from planet formation.
Neptune snowplows these bodies out.
They because the Kuiper belt, the band of thousands of small bodies of ice and rock just beyond Neptune's orbit.
You can think of the structure of that Kuiper belt as, like, blood splatter on the wall at a murder scene.
It's the record of this really violent of Neptune migrating outward across the solar system.
But Neptune's movement doesn't just fling these small icy bodies out into the Kuiper belt.
It also sends some of them crashing in towards the Sun.
Some bombard the early Earth.
It's the most violent time on our planet since the birth of the solar system itself, the Sun flared into life.
It's called the late heavy bombardment.
During the late heavy bombardment, you had rocks literally falling down from the sky constantly.
This would've been a terrible time for life.
And yet, this cascade of icy bodies also brings something essential for life.
One characteristic of the outer solar system bodies is that we often find organics.
Organics provide the basis for all living organisms we find today.
They're carbon-based molecules that form on the surface of dust grains in the early solar system.
The rocky inner planets sweep up these organics as they grow But the scorched surfaces of the young planets are too inhospitable for many of these delicate molecules to survive.
Yet organics remain intact on the small, icy bodies of the outer solar system that Neptune tosses towards the early Earth.
Neptune was the solar system's cosmic delivery service.
As far away as Uranus and Neptune are, the existence of ice giants in the outer solar system may have been critical for the existence of Earth today.
And the ice giants may have done more than give life on Earth a kickstart.
They may have prevented our planet from being completely destroyed.
The ice giants Uranus and Neptune, distant giant worlds that may have delivered the elements of life to Earth.
Without them, our planet itself might not exist at all.
These ice giants are fascinating worlds, but they may be even more important than that.
They might be the reason we're here.
Around 4 billion years ago, the young Earth is under threat from our solar system's bully, Jupiter.
Positioned between the rocky inner planets and the giant outer ones, Jupiter dominates the solar system.
When you have a behemoth like Jupiter in your solar system, what it does determines in part what everything else does.
As Jupiter and Saturn lock into their gravitational dance, they migrate out, away from the Sun.
Jupiter's immense gravity should pull Earth and Venus along with it.
Earth and Venus' orbits should stretch and overlap with each other.
It's a collision waiting to happen except It didn't.
So by the fact that we're talking about this here on Earth suggests that Earth and Venus didn't have an impact early in the solar system when Jupiter and Saturn were migrating.
Something appears to have protected us.
Scientists think something yanked Jupiter into a different orbit before it had a chance to pull Earth and Venus on a collision course.
But what could cause such a large jump in Jupiter's migration? This is where the ice giants enter the story.
Getting Jupiter to make a big jump in its migration is not easy, and so the best way that the models have been able to actually recreate this jump is to have Jupiter actually eject something the size of Neptune out of the solar system entirely.
Jupiter has a lot of gravity, and if you get too close to it, you're going to be accelerated as you fall in towards Jupiter, and it's possible that you can eject a planet completely out of the solar system this way.
It's basically slingshotting it.
But a planet the size of Neptune is heavy, even for Jupiter, and slingshotting it out of the solar system gives Jupiter a kickback.
Jupiter is knocked into a new orbit, and Earth is saved.
But which ice giant sacrificed itself for us? Neptune is still in the solar system, and so is Uranus.
If you use computer models to basically predict the behavior of the planets, what you find is that if you start with Jupiter, Saturn, Uranus and Neptune, you can't save the Earth without ejecting either Uranus or Neptune, but they're there, so we know that's not right.
However, if you add a third ice giant, a fifth giant planet out there, then that actually makes everything work.
You can save the Earth, have the planets in their present configuration, and that ice giant gets ejected from the solar system.
Imagine our solar system starting with three ice giants.
One swings too close to Jupiter.
Our solar system's bully throws its victim clean out of the playground.
Jupiter is pushed into a new orbit by the ice giant's gravity.
Earth is saved from Jupiter's deadly gravitational pull, and the solar system becomes the safe and orderly place we see today.
So we have a funny story here.
This ice giant that may have existed billions of years ago yanked Jupiter back into the outer solar system, preventing it from destroying the Earth, but in the meantime, it sacrificed itself for us, getting ejected from the solar system.
We have to thank it for our existence, but it's not there anymore.
We humans are really lucky.
Had the dinosaurs not gone extinct, we wouldn't be here.
Had this planet not been ejected out of our solar system, we wouldn't be here.
So where is this missing ice giant now? The answer is pretty amazing.
It could be clear across the other side of the milky way galaxy.
The Sun moves around the milky way galaxy at about half a million miles an hour, and in the history of the Earth, we've been around about 20 times.
We could've lost that planet anywhere across the milky way.
But is this third ice giant really lost or just hiding? January 2016 Astronomers at Caltech make an astonishing announcement.
They claim to have found evidence of a mysterious ninth planet disrupting icy bodies far out in the Kuiper belt.
Simulations suggest that if this so-called planet nine exists, it is similar in size to Neptune and Uranus.
Could this be Earth's savior? Could the solar system's primordial missing sacrificial ice giant be planet nine? Yes, it could.
Perhaps this third ice giant wasn't ejected from the solar system after all.
One type of ejection is where you just take something and you throw it out of the solar system, but another more gentle kind is when you don't quite make it all the way out, and instead you go on a very, very long period orbit around the young solar system.
Planet nine is thought to be so far out, it takes up to 20,000 years for it to travel around the Sun.
Perhaps it's been observing the dramatic, dynamical evolution of the solar system unfold from its frigid Whether planet nine is a long-lost sibling or not, ice giants played a huge role in taming Jupiter.
They made our solar system the haven it is today.
But they are not peaceful places.
Somehow, out in the deep freeze, Neptune is tormented by wild weather, mysterious superstorms and maybe even diamond rain.
Uranus and Neptune Their location at the edge of our solar system makes them very difficult to study.
The ice giants Uranus and Neptune are very mysterious to us.
They're very far away, so they're hard to observe with telescopes here at the Earth.
As a result, these planets have long been overlooked.
The only time we've glimpsed these distant giants up close was when voyager 2 flew past them in the 1980s.
The results amazed Heidi Hammel, part of the voyager 2 imaging team at the time of the Neptune flyby.
One of the most wondrous and frustrating things about planetary flybys is that you learn so much that you open a whole Pandora's box of questions.
One observation instantly intrigued scientists.
Neptune has the fastest winds in the solar system.
Here on Earth, our winds are actually driven by different temperatures from sunshine.
Neptune is so far away from the Sun that it receives almost no energy from our star.
Neptune is 3 billion miles from the Sun.
It's really cold there, so why does it have such fast winds? The less energy a planet receives from the Sun, the quieter we expect its weather to be But Neptune isn't tranquil at all.
It's covered in massive violent storms.
There are storms that are rivaling the size of the inner planets.
That's a pretty big storm.
One of the largest ever recorded on Neptune is known as the 1989 great dark spot, a single vast tempest, large enough to swallow the Earth whole, riding on a jet stream with a mind-blowing wind speed of 1,500 miles per hour.
Hands down, Neptune holds the record for the fastest wind speeds in the solar system.
The fastest tornado winds on Earth are only a few hundred miles an hour, and that does devastating destruction, so it's hard to imagine what winds on Neptune would do.
A probe entering Neptune's upper atmosphere would record freezing temperatures, minus-370 degrees Fahrenheit Too cold to generate the wind we see.
though, the probe is smashed by Neptune's relentless jet stream winds.
And the deeper you go, the warmer it becomes.
Neptune has almost three times as much heat coming from its interior than you would expect from a ball of gas out at Neptune's distance.
The strange thing about these high-speed Neptune winds is that they're not powered by heat energy from the Sun.
In fact, they're powered by heat energy from Neptune's own interior.
So where does this internal heat come from? When planets form, it's a very violent, very energetic event, and the planets are actually extremely hot, and it takes billions of years for that heat to leak away, so Neptune, it turns out, probably still has a tremendous amount of that heat that is trapped inside of it, and as that bubbles up, that's what's actually heating the atmosphere and driving this tremendous weather.
So how does Neptune retain so much heat? The secret lies deep below the atmosphere.
As you go down and down and down, you'll just find the pressure gets more and more intense until you are eventually essentially crushed.
That atmosphere will get thicker and thicker like a fog until suddenly you would realize that instead of an atmosphere, you are in an ocean.
Neptune has a super-dense fluid mantle made up of methane, ammonia, and water.
Really, an ice giant is not a solid ball of ice, but rather a moving ocean of swirling liquid material.
This swirling liquid traps the heat, acting like a blanket, insulating the core.
This is the secret to Neptune's wild weather, and the intense interior heat and pressure in Neptune's methane-rich mantle may create another extraordinary effect.
The pressure is so intense that the methane breaks up, and methane is made of carbon and hydrogen, so if you take carbon and you compress it a lot, you could get diamond formation, and so it is entirely possible that literal diamonds are raining down in this ocean of the mantle fluid on Neptune.
From superstorms to diamond rain, Neptune is strange, dynamic beyond expectation, but Uranus is the real mystery, the victim of a cosmic one-two punch with seasons unlike anything else in the solar system.
January 1986 Voyager 2 approaches Uranus at over 40,000 miles per hour.
Astronomers have been waiting for this moment for 8 years, but on voyager's arrival, all that's revealed is a bland, pale, blue ball.
That was a little bit disappointing from my perspective as a scientist studying the atmosphere of the planet.
everything changes.
Telescopes reveal huge storms raging across the planet.
Why the enormous difference? The answer lies in the planet's extreme axial tilt.
If Neptune and Uranus are siblings, Uranus is definitely the wonky sibling.
All the planets are tilted with respect to the solar system.
The Earth is 23 degrees.
Jupiter is just a handful of degrees, but Uranus is actually on its side.
It's tipped 98 degrees.
Uranus' tilt is almost four times more extreme than any other planet in the system.
It's lying so its poles are horizontal, and its rings and moons are vertical relative to the plane of the solar system.
Earth's tilt gives it seasons.
Uranus' extreme tilt gives it extreme seasons.
Twice a year, its poles are pointed directly towards or away from the Sun, so each pole has a very intense period of midnight sun and a dark, cold polar night.
It takes Uranus 84 years to orbit the Sun, so those seasons last for a very long time.
You get, like, in the northern hemisphere as its going around the Sun and 20 years of darkness in the Southern hemisphere.
Uranus is kind of like "game of thrones.
" You're waiting ages for winter to come, and then winter lasts 20 years.
A winter's night or a summer's day that lasts for decades.
But what happens in the interim when the orbit of the planet means the Sunlight hits its spinning equator rather than one of its poles? When it's off to the side, the whole planet's lit up.
As it spins, every piece of the planet is exposed to sunlight.
Sunlight hits the equator of the spinning planet, pumping energy into the surface, warming the atmosphere and driving air currents around the planet.
The result Spring and Autumn storms.
That extreme change in how much sunlight is distributed across that planet's atmosphere probably has an important role in driving this remarkable seasonal change we see in Uranus' atmosphere.
Unlike its ice giant sibling, Neptune, Uranus has seasonal storms rather than constant ones, so why does Uranus roll around the Sun while other planets spin like tops? It goes against everything we know about planetary formation.
In some ways, forming new planets in the solar system was a lot like making cotton candy.
There was a direction that everything was coming together.
If you put a stick down in it, the material would accumulate around it in a certain direction, so that's why all the planets have roughly the same orbital axes.
If Uranus started out with a vertical orbital axis, how did it end up flipped on its side? We know there were a lot of collisions between planets or planet-sized objects in the early solar system.
It's natural to assume that Uranus probably got hit as a grazing impact from another giant object, which tipped it over on its side.
But there's a problem with this assumption.
If you hit Uranus with a single impact to knock it over to 98 degrees, then actually what you expect is that the rings left over would be orbiting in the wrong direction relative to the spin of the planet.
What event could be powerful enough to flip a planet, but gentle enough to bring everything in orbit around it along for the ride? A single big collision is probably not what happened to Uranus because that would've been too disruptive.
It's kind of like boxing.
Instead of one big knockout blow, it was the old one-two.
One theory suggests that the newly formed Uranus is hit by a protoplanet the size of Earth.
The blow is only glancing.
Uranus is knocked partway towards its current tilt.
Its ring system survives the impact and stays in orbit around the equator.
As the second object hits, Uranus is tipped all the way, and the rings follow.
Uranus got jabbed and then knocked on its side.
Uranus may orbit the Sun sideways, but Neptune's moon, Triton, has an even stranger trajectory.
It travels around Neptune in reverse.
But weirder than that, it seems to be erupting, and it could even harbor life.
Wherever we see planets, we expect to see moons.
It seems the larger the planet, the more moons orbit around it.
Jupiter has 69.
Saturn, 61.
Next come the ice giants.
Astronomers have so far detected and 13 around Neptune.
But one stands out completely Neptune's moon, Triton.
Triton is a bit of an oddball because instead of orbiting Neptune in the same direction that Neptune spins, it orbits in the opposite direction, what we call a retrograde orbit.
A giant planet and its moons form out of the same swirling disk of gas, dust, and rocky material.
The lighter gas falls into the center more easily, forming the planet, while some of the heavier rocky material is left over in the disk, forming the moons.
Typically, the moon travels in the same direction that the planet is orbiting, but in the case of Triton and Neptune, that's the complete opposite case.
We know it couldn't have formed in that orbit around Neptune.
It had to come were somewhere else, and a wonderful clue to where it came from is the nearby neighbor, Pluto.
Pluto is a dwarf planet in the nearby Kuiper belt.
It's only 200 miles smaller than Triton, but it's not just size that makes these two bodies similar.
It's their composition.
Triton is actually most similar to Pluto.
It has a similar amount of rock in its interior.
It has a similar surface composition with a lot of nitrogen and methane.
It really looks like a Pluto-like world, but it just happens to be orbiting a planet instead of orbiting the Sun.
If Triton is like Pluto, maybe it also started life in the Kuiper belt.
Could it have been captured by Neptune's gravity, pulled into the gas giant's orbit? It's not easy to capture a moon into orbit around a planet.
It's not natural for a body to come close to another world and just spiral in.
You've somehow got to put on the brakes when it's close in.
For Neptune to capture Triton, Triton had to be slowed down, but how? Again, Kuiper belt objects hold a clue.
Many of the largest are binary pairs, two worlds orbiting each other, like Pluto and its large moon Charon.
Perhaps Triton was one of a pair as well.
If Triton was in orbit with a partner, each of the pair would travel at different speeds.
This speed difference is key.
Triton's velocity is just a little bit slower.
As it's orbiting its companion, it's slow enough that it could actually get captured by Neptune, while that other one would speed off across the solar system.
Triton's original dance partner is flung out and away.
Triton has a new, much bigger companion, Neptune.
But these unlikely partners are dancing out of sync.
Capture explains the backwards orbit, but there's an even stranger mystery to solve.
On a moon like Triton, we'd expect to see heavily cratered terrain, the hallmark of geologically dead worlds.
Instead, voyager 2 reveals a world that's startlingly alive.
I think one of the most amazing discoveries of the entire voyager mission is that when we flew past Triton, we saw these jets of liquid nitrogen coming up out of the surface.
We had no idea that this tiny, little cold world out there would still be alive.
Smooth, icy planes cover the surface.
Geysers of nitrogen punch up through the crust and spew black dust We thought it was too far from the Sun, too cold, too dead.
It was just going to be an ice ball like all the other moons that had tended to be, but, no.
It's a fresh, young surface.
Triton's surface is a frosty 200 degrees below freezing.
Where's the heat coming from to drive these surface features? The answer lies with Neptune's capture of Triton.
Triton's orbit is circular, but it was once elliptical.
As Triton moved closer in and farther away, it would have been repeatedly squashed and stretched by Neptune's gravity.
That would generate massive amounts of friction inside of Triton.
Might well have completely melted Triton due to those forces, and as that happened, it would have acted as a brake.
All that friction would have circularized Triton's orbit and left it in the orbit that we see today.
This change in orbit is what gives Triton its heat.
The surface of Triton froze over, but the moon still retains some of this warmth deep below the icy shell.
Astronomers think there's enough heat to melt ice into water, forming an underground liquid ocean on a world 3 billion miles from the Sun.
Liquid water, heat There is only one question, and it's impossible not to ask it.
Could there be life? If there's a source of energy on Triton, then perhaps there's a form of life that figured out how to take advantage of that energy source.
An incredible thought.
If life has carved out a niche, this frozen ball could be the most distant habitable world from the Sun.
Neptune's moon, Triton, might be alive, but Uranus' moons are even stranger.
They have a neat trick to cheat death entirely.
Uranus An ice giant with beautiful, shimmering rings and 27 moons And their position is a mystery.
Half exist within a tightly packed orbit.
It shouldn't be possible for this many moons to be in such close proximity to each other.
The environment around Uranus is very busy, and the system appears unstable, and the moons should be colliding, but yet, we see these nice, well-formed moons.
And what's even more surprising is that in 2003, the Hubble space telescope revealed two new rings and two new moons Cupid and Mab.
Question is, where did these moons come from? A clue lies in Uranus' rings.
In addition to the very packed moon system, you also have rings around Uranus, which is also somewhat unexpected, but these two unexpected qualities might actually explain one another.
Anytime you see a ring system, you're seeing part of a process.
There was probably a time in Earth's history when it had a ring, you know, when our moon was being formed.
It seems that the new moons are made up of material from a previous ring system.
But when scientists model Cupid's future, they discover it's dangerously close to another moon, Belinda.
In a few thousand years, two of the moons in particular, Cupid and Belinda, are likely to collide together as their trajectories intersect.
And this collision will create a domino effect.
When there is a moon collision, that sets up a very delicate gravitational balance, and it also creates a lot of debris, so one collision sets off a string of collisions.
We think Cupid and Belinda will destroy not only each other, but all of Uranus' inner moons.
A runaway cascade of destruction grinds Uranus' moons to pebbles.
All of this will happen in only a few thousand years.
The fact that we are predicting to have a collision within a few thousand years feels kind of contrary because this system has been here for billions of years, so it seems a little bit lucky that we're looking just now, and we think there's going to be a collision in the near future.
What if this cycle of destruction is a case of Uranian groundhog day? When you look at the Uranian system, I like to think of it sort of like a violent hockey game.
Guys are taking big hits.
They can't continue, so what must you do? You have to sub them in and out.
When you look at this game that's that violent, you know that the guys you see on the ice are not the guys that started the game.
Whoo! It's a whole new set of players.
Like hockey subs, Uranus keeps its moons fresh by recycling them.
Every time these moons collide, the debris forms a ring system around Uranus, and then over time, that ring system starts to spawn new moons.
Over time, the rings of debris that surround Uranus form into new moons, which in turn collide and grind each other to dust.
Uranus, it turns out, is actually very Eco-conscious.
Its moons shatter.
They hit each other, form all this debris, and then the moons reform from that material, and then the whole pattern, the whole cycle starts up again, and the moons hit, shatter, and new moons form all over again.
Ultimately, this eternal cycle of life and death that we see with these moons might be a story that is actually kind of true of many things in the universe, when it comes to stars, planets, and maybe even the universe itself.
Uranus and Neptune, no longer forgotten outposts of the solar system.
Suddenly, these planets are revealed to you as worlds.
It's breathtaking.
It's it's awe-inspiring.
It's humbling, and it makes you proud that you're part of a species that could actually do that, to get out there and to see these places.
Dynamic worlds with dramatic, often violent histories, stolen moons, giants flung out into the cold, saving the Earth from destruction.
Uranus and Neptune are puzzle pieces in the solar system.
They are giant planets.
They have a lot to tell us, not just about themselves but about everything in our solar system, including our own planet, Earth.
Somewhere in the story of the ice giants is the reason you and I are actually here to talk about it at all, the reason the Earth was able to form and stabilize and become an environment for life.
It would not be an exaggeration to say that the ice giants are the coolest planets in the solar system.