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00:00Our solar system is home to giants.
00:07The gas giants Jupiter and Saturn seem to dominate.
00:12But two ice giants, Neptune and Uranus, determine the fate of the rest.
00:20Two distant planets unconnected to us.
00:24Or so we thought.
00:26We now know the fates of the ice giants are entwined with our own.
00:32They've gone from being these cold, dull worlds,
00:35to actually having in them the very secret of why you and I exist at all.
00:40Their story is of epic migration.
00:43Brutal destruction.
00:46Uranus got jabbed and then knocked on its side.
00:49Of worlds more alive than anyone imagined.
00:53They hold the key to the history of our solar system.
00:56And perhaps, to life on Earth.
01:18Uranus and Neptune.
01:22Mysterious giants that lurk in the cold outer reaches of our solar system.
01:29The furthest planets from the sun.
01:33Uranus and Neptune are sort of the sentinels of the outer solar system.
01:37They're out past Jupiter and Saturn,
01:39well over like 2 billion, 3 billion miles from the sun.
01:45Their size and location are a puzzle to planetary astronomers.
01:50Uranus and Neptune are somewhat of a mystery,
01:53because in a way, they shouldn't exist.
01:56Or at least they shouldn't exist where they are.
02:00Scientists don't understand how these giant planets grew so big,
02:04so far from the sun.
02:11The mystery began 4.5 billion years ago,
02:16with the birth of the solar system.
02:19The sun sparked into life from a disk of gas and dust.
02:29The rocky cores of the first planets then started to grow from debris in the disk.
02:37But these inner planets had a size limit.
02:41To grow into a giant planet, gas is needed.
02:47Heat from the infant sun blasted lighter gas molecules
02:51beyond the point astronomers call the frost line.
02:55Here, it's cool enough for gas molecules like hydrogen and helium to stabilise.
03:06Jupiter and Saturn took shape first,
03:09sweeping in the abundant gas and quickly becoming gas giants.
03:17But Neptune and Uranus are different.
03:21Jupiter and Saturn are about 90% hydrogen and helium,
03:24whereas Neptune and Uranus are more like 20%.
03:29So what does this difference in gas tell us about their formation?
03:34We suspect that Uranus and Neptune came a little bit later,
03:38when there was not as much gas to be swept up.
03:42Uranus and Neptune had less time to scoop up as much hydrogen and helium
03:47before these gases disappeared.
03:50But they were also forming further out,
03:53where it's cold enough for other, heavier gases to freeze.
03:58These were swept up by the growing outer planets.
04:05Out where Uranus and Neptune are, tons of ice, tons of frozen gases,
04:10we might think of them, methane, ammonia, water.
04:13And so that's what makes up their composition predominantly.
04:18They may be smaller than Jupiter and Saturn,
04:21but consisting of these different forms of ice meant that they grew dense,
04:26becoming ice giants.
04:29However, there's a problem.
04:31They are too big.
04:34The disk of gas and ice around the newborn star didn't last forever,
04:38and the material in the far reaches of the solar system was spread too thin.
04:44As you move further out in the solar system,
04:46the time scale for two bodies to find each other and collide in a crete slows down,
04:51because the periods around the sun are much longer.
04:53It just takes a very, very long time.
04:56Neptune and Uranus orbit the sun incredibly slowly,
05:00too slowly to have collided with enough icy material
05:04to grow into the giants we see today.
05:07So when we look at Neptune and at this very distant orbit,
05:10we don't have enough time in the solar system to build a planet like Neptune.
05:14We just don't think we could build Neptune where we find it today.
05:19A clue to Neptune's confusing location
05:22came when astronomers started to discover planets called exoplanets orbiting other stars.
05:29So one of the biggest surprises from studying planets around other stars
05:33is that Neptune mass planets are very, very common around other stars,
05:37but not where we find our Neptune.
05:39In fact, what we find around other stars is that Neptune mass planets are quite common,
05:44but they're common very, very close to the star.
05:46It would be like having a Neptune inside the orbit of Mercury in our solar system.
05:51In our solar system, Neptune and Uranus are impossibly far away from the sun.
05:58In other systems, ice giant-sized planets are impossibly close to their stars.
06:05So what happened?
06:08It turns out where we see them now is probably not where they started out.
06:15One thing we've learned about solar systems is that things are on a delicate balance,
06:19and planets migrate. They move around.
06:22They don't form in one place and stay there forever.
06:27So what had enough power to move a giant planet like Neptune?
06:33An even bigger one, Jupiter.
06:37One way for planets to move is by gravitationally interacting with each other.
06:41So they feel each other's gravity, they can tug, they can pull,
06:44and that effect can lead to planets slowly migrating around in their planetary systems.
06:51In the early days of our solar system, the giant planets were much closer together,
06:57so the effect was greater.
07:01Furthermore, they may have orbited in a different order than we see today.
07:06Jupiter, Saturn, Neptune and Uranus.
07:12So what caused Neptune and Uranus to swap positions?
07:16The answer lies with Jupiter and Saturn.
07:20When these two giants locked into a gravitational dance.
07:26There's always this interplay between them.
07:28Think of capoeira dancers balancing and moving together in a careful, orchestrated way.
07:39Over millions of years, a rhythm slowly built.
07:44These giants pushed and pulled each other into more elliptical orbits.
07:51But the gravitational dance reached a climax.
07:55The stretched orbits became unstable and the giants moved off course.
08:03As Saturn and Jupiter twisted out from the Sun, they flung Neptune out beyond Uranus.
08:14As Neptune moved out through the solar system, it pushed debris ahead of it.
08:19These were the leftover icy fragments from planet formation.
08:24Neptune snowplowed these bodies out and they became the Kuiper Belt.
08:29The band of thousands of small bodies of ice and rock just beyond Neptune's orbit.
08:35You can think of the structure of that Kuiper Belt,
08:38it's like blood splattered on the wall at a murder scene.
08:41It's the record of this really violent event of Neptune migrating outward across the solar system.
08:48But Neptune's movement didn't just fling these small icy bodies out into the Kuiper Belt.
08:55It also sent some of them crashing in towards the Sun and early Earth.
09:01It was the most violent time on our planet since the birth of the solar system itself.
09:07500 million years after the Sun fled into life.
09:11It's called the Late Heavy Bombardment.
09:14During the Late Heavy Bombardment you had rocks literally falling down from the sky constantly.
09:18This would have been a terrible time for life.
09:22And yet this cascade of icy bodies also brought something essential for life.
09:29One characteristic of the outer solar system bodies is that we often find organics.
09:36Organics provided the basis for all living organisms we find today.
09:41They're carbon-based molecules that formed on the surface of dust grains in the early solar system.
09:49The rocky inner planets swept up these organics as they grew.
09:58But the scorched surfaces of the young planets were too inhospitable for many of these delicate molecules to survive.
10:06Yet organics remained intact on the small icy bodies in the outer solar system that Neptune tossed towards the early Earth.
10:17Neptune was the deliverer of life.
10:22As 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.
10:32And the ice giants may have done more than give life on Earth a start.
10:39They may have prevented our planet from being completely destroyed.
10:45NEXT EPISODE
10:57Uranus and Neptune, the distant ice giants that may have delivered the elements of life to Earth, are more important than we realized.
11:06For without them, our planet itself might not exist at all.
11:11These ice giants are fascinating worlds, but they may be even more important than that.
11:16They might be the reason we're here.
11:19Around four billion years ago, the young Earth was under threat from our solar system's tormentor, Jupiter.
11:27Positioned between the rocky inner planets and the giant outer ones, Jupiter dominates the solar system.
11:35When you have a behemoth like Jupiter in your solar system, what it does determines in part what everything else does.
11:44As Jupiter and Saturn locked into their gravitational dance, they migrated out, away from the Sun.
11:52Jupiter's immense gravity should have pulled Earth and Venus along with it.
11:58And their orbits should have stretched and overlapped with each other.
12:06A collision was an inevitability.
12:15Except it didn't happen.
12:18It didn't happen.
12:19So 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.
12:28Something appears to have protected us.
12:34Scientists think something ripped Jupiter into a different orbit, before it had a chance to pull Earth and Venus on a collision course.
12:42But what caused such a large jump in Jupiter's migration?
12:48This is where the ice giants enter the story.
12:52Getting Jupiter to make a big jump in its migration is not easy.
12:57And 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.
13:10Jupiter has a lot of gravity.
13:12And if you get too close to it, you're going to be accelerated as you fall in towards Jupiter.
13:17And it's possible that you can eject a planet completely out of the solar system this way.
13:22It's basically slingshotting it.
13:27But a planet the size of Neptune is heavy, even for Jupiter.
13:32And slingshooting it out of the solar system had an impact on Jupiter.
13:37It was knocked into a new orbit, and Earth was saved.
13:43But which ice giant sacrificed itself for us?
13:46Because Neptune is still in the solar system.
13:50And so is Uranus.
13:53If you use computer models to basically predict the behaviour of the planets,
13:57what 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.
14:05But they're there, so we know that's not right.
14:07However, if you add a third ice giant, a fifth giant planet out there, then that actually makes everything work.
14:14You can save the Earth, have the planets in their present configuration, and that ice giant gets ejected from the solar system.
14:22Imagine our solar system starting with three ice giants.
14:27One then swings too close to Jupiter.
14:31And our solar system's bully throws its victim clean out of the playground.
14:37Jupiter is pushed into a new orbit by this third ice giant's gravity.
14:42Earth is saved from Jupiter's deadly gravitational pull, and the solar system becomes the safe and orderly place we see today.
14:51So we have a funny story here.
14:53This ice giant that may have existed billions of years ago yanked Jupiter back into the outer solar system, preventing it from destroying the Earth.
15:00But in the meantime, it sacrificed itself for us, getting ejected from the solar system.
15:04We have to thank it for our existence, but it's not there anymore.
15:08We humans are really lucky.
15:10Had the dinosaurs not gone extinct, we wouldn't be here.
15:13Had this planet not been ejected out of our solar system, we wouldn't be here.
15:19So where is this missing ice giant now?
15:22The answer is pretty amazing.
15:24It could be clear across the other side of the Milky Way galaxy.
15:27The 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.
15:36We could have lost that planet anywhere across the Milky Way.
15:43But is this third ice giant really lost?
15:48Or just hiding?
15:54In January 2016, astronomers at Caltech made an astonishing announcement.
16:00They claimed to have found evidence of a mysterious ninth planet, disrupting icy bodies far out in the Kuiper belt.
16:10Simulations suggested that if this so-called planet nine exists, it is similar in size to Neptune and Uranus.
16:18Could this be Earth's saviour?
16:21Could the solar system's primordial, missing, sacrificial ice giant be planet nine?
16:28Yes, it could.
16:31Perhaps this third ice giant wasn't ejected from the solar system after all.
16:36One type of ejection is when you just take something and you throw it out of the solar system.
16:40But another, more gentle kind, is when you don't quite make it all the way out,
16:44and instead you go on a very, very long period orbit around the young solar system.
16:51Planet nine is thought to be so far out that it's impossible to see it.
16:55But because it is thought to be so far out, it takes up to 20,000 years for it to travel around the sun.
17:02Perhaps it's been observing the dramatic dynamical evolution of the solar system unfold from its frigid 20,000-year orbit.
17:13Whether planet nine is a long-lost sibling or not, ice giants played a huge role in taming Jupiter.
17:21They made our solar system the haven it is today.
17:26But because Uranus and Neptune are at the very edge of our solar system, it makes them very difficult to study.
17:35The ice giants Uranus and Neptune are very mysterious to us.
17:39They're very far away, so they're hard to observe with telescopes here at the Earth.
17:46Because of which, both have long been overlooked.
17:49The only time we've glimpsed these distant giants up close was when Voyager 2 flew past them in the 1980s.
17:59The results amazed Heidi Hammel, part of the Voyager 2 imaging team at the time of the Neptune flyby.
18:09One of the most wondrous and frustrating things about planetary flybys is that you learn so much
18:18that you open a whole Pandora's box of questions.
18:24One observation instantly intrigued scientists.
18:30Neptune has the fastest winds in the solar system.
18:36Here on Earth, our winds are actually driven by different temperatures from sunshine.
18:42Neptune is so far away from the sun that it receives almost no energy from our star.
18:48Neptune is 3 billion miles from the sun. It's really cold there. So why does it have such fast winds?
18:56The less energy a planet receives from the sun, the quieter we expect its weather to be.
19:05But Neptune isn't tranquil at all.
19:08It's covered in massive, violent storms.
19:13There are storms that are rivaling the size of the inner planets. That's a pretty big storm.
19:21One of the largest ever recorded on Neptune is known as the 1989 Great Dark Spot.
19:28A single vast tempest, large enough to engulf the Earth.
19:33Riding on a jet stream with a mind-blowing wind speed of 2,000 kilometers per hour.
19:40Hands down, Neptune holds the record for the fastest wind speeds in the solar system.
19:45The fastest tornado winds on Earth are only a few hundred miles an hour, and that does devastating destruction.
19:53So it's hard to imagine what winds on Neptune would do.
19:56If a probe entered Neptune's upper atmosphere, it would record temperatures of minus 220 degrees Celsius.
20:04Too cold to generate the wind we see.
20:08But a thousand kilometers down, though, and the probe would be smashed by Neptune's relentless jet stream winds.
20:15And the deeper you go, the warmer it becomes.
20:22Neptune 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.
20:33The strange thing about these high-speed Neptune winds is that they're not powered by heat.
20:39The strange thing about these high-speed Neptune winds is that they're not powered by heat energy from the sun.
20:46In fact, they're powered by heat energy from Neptune's own interior.
20:53But where does this internal heat come from?
20:57When planets form, it's very violent, very energetic events.
21:01And the planets are actually extremely hot, and it takes billions of years for that heat to leak away.
21:06So Neptune, it turns out, probably still has a tremendous amount of that heat that is trapped inside of it.
21:12And as that bubbles up, that's what's actually heating the atmosphere and driving this tremendous weather.
21:19So how does Neptune retain so much heat?
21:24The secret lies deep below the atmosphere.
21:28As you go down and down and down, you just find the pressure gets more and more intense until you're eventually essentially crushed.
21:35And that atmosphere would get thicker and thicker like a fog,
21:38until suddenly you would realise that instead of an atmosphere, you were in an ocean.
21:45Neptune has a super-dense fluid mantle, consisting of methane, ammonia and water.
21:54Really, an ice giant is not a solid ball of ice, but rather a moving ocean of swirling liquid material.
22:04This swirling liquid traps the heat, acting like a blanket, insulating the core.
22:11And this is the secret to Neptune's wild weather.
22:16Yet the intense interior heat and pressure in Neptune's methane-rich mantle may create another extraordinary effect.
22:26The pressure is so intense that the methane breaks up, and methane is made of carbon and hydrogen.
22:31So if you take carbon and you compress it a lot, you could get diamond formation.
22:37And so it is entirely possible that literal diamonds are raining down in this ocean of the mantle fluid on Neptune.
22:49From super storms to diamond rain, Neptune is strange, dynamic and beyond expectation.
22:57But Uranus is the real mystery.
23:00The victim of a cosmic one-two punch, and with seasons unlike anything else in the solar system.
23:08In January 1986, Voyager 2 approached Uranus at over 70,000 km per hour.
23:18Astronomers had been waiting for this moment for eight years.
23:22But Uranus had not yet been discovered.
23:25It had not yet been discovered.
23:28It had not yet been discovered.
23:31It had not yet been discovered.
23:34Astronomers had been waiting for this moment for eight years.
23:38But on Voyager's arrival, all that was revealed was a bland, pale blue ball.
23:45That was a little bit disappointing from my perspective as a scientist studying the atmosphere of the planet.
23:54Twenty years later, everything changed.
23:58Telescopes revealed huge storms raging across the planet.
24:03Why the enormous difference?
24:06The answer lies in the planet's extreme axial tilt.
24:12If Neptune and Uranus are siblings, Uranus is definitely the wonky sibling.
24:18All the planets are tilted with respect to the solar system.
24:21The Earth is 23 degrees, Jupiter is just a handful of degrees, but Uranus is actually on its side.
24:27It's tipped 98 degrees.
24:30Uranus' tilt is almost four times more extreme than any other planet in the system.
24:36Relative to the plane of the solar system, it's lying so its poles are horizontal and its rings and moons are vertical.
24:45Earth's tilt is responsible for its seasons.
24:49Uranus' extreme tilt is responsible for its extreme seasons.
24:53Twice a year, its poles are pointed directly towards or directly away from the sun.
24:59So each pole has a very intense period of midnight sun and a dark, cold, polar night.
25:07It takes Uranus 84 Earth years to orbit the sun, so these seasons last for a very long time.
25:15You get like 20 years of sunlight in the northern hemisphere every year.
25:20You get like 20 years of sunlight in the northern hemisphere as it's going around the sun,
25:25and 20 years of darkness in the southern hemisphere.
25:32Uranus is kind of like Game of Thrones.
25:35You're waiting ages for winter to come, and then winter lasts 20 years.
25:40But what happens in the interim, when the orbit of the planet means that sunlight hits its spinning equator,
25:46rather than one of its poles?
25:49A winter's night or a summer's day that lasts for decades.
25:55When it's off to the side, the whole planet's lit up.
26:00As it spins, every piece of the planet is exposed to sunlight.
26:04Sunlight hits the equator of the spinning planet, pumping energy into the surface,
26:10warming the atmosphere and driving air currents around the planet.
26:16The result? Spring and autumn storms.
26:21That extreme change in how much sunlight is distributed across that planet's atmosphere
26:28probably has an important role in driving this remarkable seasonal change we see in Uranus' atmosphere.
26:37Unlike its ice giant sibling, Neptune, Uranus has seasonal storms, rather than constant ones.
26:46So why does Uranus roll around the sun, while other planets spin?
26:52It goes against everything we know about planetary formation.
26:57In some ways, forming new planets in the solar system was a lot like making cotton candy.
27:03There was a direction that everything was coming together.
27:07If you put a stick down in it, the material would accumulate around it in a certain direction.
27:12So that's why all the planets have roughly the same orbital axes.
27:18So if Uranus started out with a vertical orbital axis, how did it end up flipped on its side?
27:27We know there were a lot of collisions between planets or planet-sized objects in the early solar system.
27:41It's natural to assume that Uranus probably got hit as a grazing impact from another giant object, which tipped it over on its side.
27:51But there's a problem with this assumption.
27:53If you hit Uranus with a single impact to knock it over to 98 degrees,
27:58then actually what you expect is that the rings left over will be orbiting in the wrong direction relative to the spin of the planet.
28:09What event could be powerful enough to flip a planet,
28:14but gentle enough so that everything orbiting around it is brought along too?
28:18A single big collision is probably not what happened to Uranus, because that would have been too disruptive.
28:26It's kind of like boxing. Instead of one big knockout blow, it was the old one-two.
28:34One theory suggests that the newly formed Uranus was hit by a protoplanet the size of Earth.
28:40The blow was only glancing,
28:44so Uranus was knocked only part way towards its current tilt.
28:48Its ring system survived the impact and stayed in orbit around the equator.
28:54As the second object hit, Uranus was then tipped all the way,
28:59and the whole planet was flipped over.
29:01Uranus got jabbed and then knocked on its side.
29:07Uranus may orbit around the Sun sideways,
29:10but there is an object within our solar system with an even stranger trajectory.
29:16And it's not a planet, but it's a moon.
29:21It's a planet orbiting around the Sun.
29:23With an even stranger trajectory.
29:26And it's not a planet, but it's a moon.
29:32Wherever we see planets, we expect to see moons.
29:35It seems the larger the planet,
29:39the more moons orbit around it.
29:44Jupiter has 69.
29:47Saturn, 61.
29:50Next are the ice giants.
29:54So far, astronomers have detected 27 moons around Uranus
29:59and 13 around Neptune.
30:03But one stands out more than any other.
30:07Neptune's moon, Triton.
30:11Triton is a bit of an oddball,
30:14because instead of orbiting Neptune in the same direction that Neptune spins,
30:17it orbits in the opposite direction.
30:20What we call a retrograde orbit.
30:22A giant planet and its moons
30:25form from the same swirling disk of gas, dust and rocky material.
30:30The lighter gas falls into the center more easily,
30:34forming the planet.
30:36While some of the heavier, rocky material is left over in the disk,
30:40forming the moons.
30:41Typically, the moon travels in the same direction that the planet is orbiting.
30:45But in the case of Triton and Neptune,
30:48that's the complete opposite case.
30:51We know it couldn't have formed in that orbit around Neptune.
30:55It had to come from somewhere else.
30:57And a wonderful clue to where it came from
31:00is the nearby neighbor, Pluto.
31:03Pluto is a dwarf planet in the nearby Kuiper Belt.
31:06And it's only 330 kilometers smaller than the Earth.
31:09And it's only 330 kilometers smaller than Triton.
31:13But it's not just the size that makes these two bodies similar.
31:17It's their composition.
31:19Triton is actually most similar to Pluto.
31:23It has a similar amount of rock in its interior.
31:26It has a similar surface composition with a lot of nitrogen and methane.
31:30It really looks like a Pluto-like world,
31:33but it just happens to be orbiting a planet instead of orbiting the Sun.
31:37If Triton is like Pluto,
31:40maybe it also started life in the Kuiper Belt.
31:45Could it have been captured by Neptune's gravity
31:48and pulled into the gas giant's orbit?
31:52It's not easy to capture a moon into orbit around a planet.
31:56It's not natural for a body to come close to another world and just spiral in.
32:01You've somehow got to put on the brakes when it's close in.
32:04For Neptune to capture Triton,
32:07Triton had to be slowed down.
32:10But how?
32:12Again, Kuiper Belt objects hold a clue.
32:16Many of the largest are binary pairs,
32:19two worlds orbiting each other,
32:22like Pluto and its large moon, Charon.
32:26Perhaps Triton was one of a pair as well.
32:30If Triton was in orbit,
32:32if Triton was in orbit with a partner,
32:35each of the pair would travel at different speeds.
32:38This speed difference is key.
32:42Triton's velocity is just a little bit slower as it's orbiting its companion.
32:47It's slow enough that it could actually get captured by Neptune
32:51while that other one would speed off across the solar system.
32:57Triton's original companion was flung out and away.
33:01Triton now had a new, much bigger companion,
33:05Neptune.
33:07But these unlikely partners were dancing out of sync.
33:12This capture explains the backwards orbit,
33:16but there's an even stranger mystery to solve.
33:20On a moon such as Triton, we'd expect to see heavily cratered terrain,
33:25the hallmark of geologically dead worlds.
33:27Instead, Voyager 2 revealed a world that's startlingly alive.
33:34I think one of the most amazing discoveries of the entire Voyager mission
33:38is that when we flew past Triton,
33:40we saw these jets of liquid nitrogen coming up out of the surface.
33:44We had no idea that this tiny little cold world out there would still be alive.
33:50Smooth, icy plains cover the surface.
33:53Geysers of nitrogen burst up through the crust
33:56and spew black dust eight kilometres into the sky.
34:01We thought it was too far from the sun, too cold, too dead.
34:06It was just going to be an ice ball, like all the other moons had tended to be.
34:11But no, it was a fresh, young surface.
34:15Triton's surface is a frosty minus 230 degrees Celsius.
34:21So from where is the heat coming to drive these surface features?
34:26The answer lies with Neptune's capture of Triton.
34:33Triton's orbit is circular,
34:36but it was one of the first moons to have a surface like this.
34:40Triton's orbit is circular,
34:43but it was once elliptical.
34:46And as Triton moved closer in and further away,
34:50it would have been repeatedly squashed and stretched by Neptune's gravity.
34:56That would generate massive amounts of friction inside of Triton.
35:00It might well have completely melted Triton due to those forces.
35:04And as that happened, it would have acted as a brake,
35:06or that friction would have circularised Triton's orbit
35:10and left it in the orbit that we see today.
35:14It was this change in orbit that gave Triton its heat.
35:18The surface of Triton froze,
35:21but the moon still retains some of this warmth deep below the icy shell.
35:28Astronomers think there's enough heat to melt ice into water,
35:32forming an underground liquid ocean.
35:36On a world four and a half billion kilometres from the sun.
35:42Liquid water.
35:44Heat.
35:46There's only one question, and it's impossible not to ask it.
35:50Could there be life?
35:53If there's a source of energy on Triton,
35:56then perhaps there's a form of life that figured out how to take advantage of that energy source.
36:03It's an incredible thought.
36:06If life has carved out a niche,
36:09this frozen ball could be the most distant, habitable world from the sun.
36:16Neptune's moon, Triton, might be alive,
36:19but Uranus's moons are even stranger.
36:23They have a trick that enables them to cheat death entirely.
36:37Uranus.
36:39An ice giant with beautiful shimmering rings,
36:43and 27 moons,
36:46whose position is a mystery.
36:49Half exist within a tightly packed orbit,
36:53but it shouldn't be possible for this many moons
36:56to be in such close proximity to each other.
37:00The environment around Uranus is very busy,
37:03and a system appears unstable, and the moons should be colliding.
37:07But yet, we see these nice, well-formed moons.
37:12And what's even more surprising is that in 2003,
37:16the Hubble Space Telescope revealed two new rings,
37:20and two new moons,
37:23Cupid and Mab.
37:27The question is, where did these moons come from?
37:30A clue lies in Uranus's rings.
37:34In addition to the very packed moon system,
37:37you also have rings around Uranus, which is also somewhat unexpected.
37:41But these two unexpected qualities might actually explain one another.
37:46Any time you see a ring system, you're seeing part of a process.
37:51There was probably a time in Earth's history when it had a ring,
37:55you know, when our moon was being formed.
37:59It seems that the new moons are made up of material
38:03from a previous ring system.
38:06But when scientists modelled Cupid's future,
38:10they discovered it's dangerously close to another moon,
38:14Belinda.
38:16In a few thousand years, two of the moons in particular,
38:20Cupid and Belinda, are likely to collide together
38:24as their trajectories intersect.
38:27And this collision will create a domino effect.
38:31When there is a moon collision,
38:34that sets up a very delicate gravitational balance,
38:38and it also creates a lot of debris.
38:41So one collision sets off a string of collisions.
38:43It's thought that Cupid and Belinda will destroy not only each other,
38:47but all of Uranus' inner moons.
38:50A runaway cascade of destruction
38:53that will grind Uranus' moons to pebbles.
38:57All of this will happen in only a few thousand years.
39:02The fact that we are predicting to have a collision within a few thousand years
39:06feels kind of contrived,
39:09because this system has been here for billions of years.
39:11So it seems a little bit lucky that we're looking just now
39:14when we think there's going to be a collision in the near future.
39:19What if this cycle of destruction is a case of Uranian Groundhog Day?
39:33When you look at the Uranian system,
39:36I like to think of it sort of like a violent hockey game.
39:40Guys are taking big hits.
39:45They can't continue.
39:48So what must you do?
39:51You have to sub them in and out.
39:54When you look at this game that's that violent,
39:57you know that the guys you see on the ice
40:00are not the guys that started the game.
40:03It's a whole new set of players.
40:05Like ice hockey substitutes,
40:08Uranus keeps its moons fresh by recycling them.
40:12Every time these moons collide,
40:15the debris forms a ring system around Uranus.
40:18And then over time, that ring system starts to spawn new moons.
40:24Over time, the rings of debris that surround Uranus form into new moons,
40:29which in turn collide and grind each other to dust.
40:32Uranus, it turns out, is actually very eco-conscious.
40:36Its moons shatter, they hit each other,
40:39form all this debris, and then the moons reform from that material.
40:43And then the whole pattern, the whole cycle starts up again.
40:47And the moons hit, shatter, and new moons form all over again.
40:51And one moon in particular shows evidence of past recycling.
40:55If you look at the moon Miranda, one of Uranus' moons,
40:58it actually looks like Frankenstein's moon.
41:01It looks like somebody ripped a moon apart and slapped it all back together.
41:05This may be evidence of the moon forming from the debris of previous collisions.
41:10If Uranus' moons have been colliding and reforming every million years
41:15since Uranus first formed,
41:18we could be observing the 4000th generation of moons.
41:22The system never settles into a stable configuration.
41:25Moons collide and are destroyed, and the remnants are recycled.
41:30Birth, life, death, rebirth.
41:35Ultimately, this eternal cycle of life and death that we see with these moons
41:40might be a story that is actually kind of true of many things in the universe
41:44when it comes to stars, planets, and maybe even the universe itself.
41:49Uranus and Neptune.
41:51Uranus and Neptune.
41:54No longer forgotten outposts of our solar system.
41:59Suddenly, these planets are revealed to you as worlds.
42:05It's breathtaking, it's awe-inspiring, it's humbling.
42:10And it makes you proud that you're part of a species that could actually do that,
42:16to get out there and to see these places.
42:18But our brief encounter with these planets has left tantalizing gaps in our knowledge,
42:23waiting to be filled.
42:26To me, sort of speaking on behalf of the human species,
42:29I think it's kind of criminal that we haven't spent more time studying these fascinating worlds.
42:33Voyager 2 flew past Uranus and Neptune decades ago,
42:37and it was only in their systems for a few days.
42:40So it was literally a drive-by.
42:43There's so much we learned, but there is so much more we still don't understand.
42:48Right now, we just have a brief snapshot here, a brief snapshot there,
42:53and we try to fill in the in-between with theory.
42:56Scientists agree the only way we can know more is with dedicated missions
43:01to visit both Uranus and Neptune.
43:04Not just, like, a 24-hour flyby,
43:07but actually study them for days and weeks and months and even years.
43:11We need to go there and orbit these planets and stay there for a while
43:15so that we can figure out what's making them tick.
43:18Dynamic worlds.
43:21With dramatic, often violent histories.
43:24Stolen moons.
43:26And giants flung out into the cold,
43:29saving the Earth from destruction.
43:32Uranus and Neptune are puzzle pieces in the solar system.
43:36They are giant planets.
43:39They have a lot to tell us, not just about themselves,
43:42but about everything in our solar system, including our own planet Earth.
43:46Somewhere in the story of the ice giants
43:49is the reason you and I are actually here to talk about it at all.
43:52The reason the Earth was able to form and stabilize
43:55and become an environment for life.
43:57It would not be an exaggeration to say that the ice giants
44:01are the coolest planets in the solar system.