How the Universe Works - S06E08 - Strange Lives of Dwarf Planets
Category
📺
TVTranscript
00:00All across our solar system, scientists are discovering thrilling new worlds, dwarf planets.
00:12They may be small, but they're full of riddles, oceans of subterranean water, ice volcanoes and vanishing mountains.
00:22The whole idea that dwarf planets are small and insignificant and boring has just been shattered in the last few years.
00:30Dwarf planets defy many of the rules we thought governed our solar system.
00:35Dwarf planets are very interesting bodies scientifically.
00:39But beyond that, they tell us something about the origin of our own world. Believe it or not, they may harbor life.
00:47Dwarf planets are rattling the cages of scientists and shaking up our understanding of how the universe works.
00:55They may have fed the early planets and even seeded them with the precursors of life.
01:01Dwarf planets just may be the most important objects in the solar system.
01:24Our solar system has eight confirmed major planets.
01:28But we're discovering many other small worlds called dwarf planets.
01:35We used to think they were just dull lumps of rock.
01:39But the more we study them, the more shocking and intriguing they become.
01:44Naively, I would expect these objects to not be terribly dynamic.
01:49They're probably just, you know, airless, rocky, icy worlds and they're just sitting there.
01:54And what we're finding out is that that is not true at all.
01:58There is all kinds of stuff going on.
02:01They're full worlds with really interesting geology and interesting histories that can tell us a lot about the solar system.
02:08Scientists believe there may be hundreds of dwarf planets in our solar system.
02:14So far, we've only recognized six.
02:19Five of them, Pluto with its moon Charon, red-colored Sedna, bright, distant Eris, Makemake,
02:31and the bean-shaped Haumea, live billions of kilometers from the sun, out beyond Neptune in the Kuiper Belt.
02:39They're just the tip of the iceberg.
02:41There are probably many, many more dwarf worlds that are out there waiting to be discovered.
02:48The sixth dwarf planet, Ceres, lives in the inner solar system.
02:54It orbits about 410 million kilometers from Earth, in the asteroid belt.
03:03The asteroid belt is a region of the solar system between Mars and Jupiter.
03:08And this is where most of the asteroids are.
03:11This is rubble left over from the formation of the solar system.
03:14During the early years of the solar system, small rocks collided with one another, stuck together, and built the rocky inner planets.
03:24Dwarf planets grew in the same way.
03:27Ceres was actually starting to get pretty big.
03:31It was on its way to becoming a planet before it stopped growing.
03:36And that makes it stand head and shoulders above everything else there.
03:39So why is Ceres called a dwarf planet, and not a planet?
03:44To be a planet, it must fulfill three cosmic criteria.
03:49First, it needs to be a sphere.
03:52Second, it needs to orbit the sun, and not another body.
03:55Third, it needs to clear its orbital area of orbital debris.
04:02Ceres fulfills just two of these requirements.
04:04It is a sphere, although a small one, only 914 kilometers across.
04:12And it does orbit the sun.
04:16However, it hasn't cleared its path of debris.
04:20And it's surrounded by asteroids.
04:23So therefore, cannot be classed a planet.
04:27Even though we call these objects dwarf planets,
04:30small and dwarf does not equal insignificant.
04:35But being small does have its problems.
04:39When the molten core of a young dwarf planet cools,
04:43so does the heat engine that drives its geologic activity.
04:51Ceres, we thought, would basically be a big, dead rock.
04:56It's a small planet.
04:59It's a small body. It should have cooled off long ago.
05:03Nothing very interesting is going on.
05:05And when we actually got out to Ceres,
05:07nothing could have been further from the truth.
05:13In March 2015, NASA's Dawn probe arrived at Ceres.
05:19As the Dawn spacecraft pulled up to Ceres,
05:22we saw the craters and the surface that we expected to see.
05:24And then, all of a sudden,
05:26something totally mysterious rotated into view.
05:32One of the craters had two bright spots,
05:35almost like two eyes staring right back at us.
05:41It was such a puzzle to the science community,
05:44because what are these doing here? Are they ice?
05:46It looks very fresh. What on earth could it be?
05:49Scientists discover over a hundred of these mysterious white spots.
05:54The largest is in a 90-kilometre-wide crater called Ocotor.
06:01And, unexpectedly, they're made up of a substance we find on Earth.
06:07Sodium carbonate, a type of salt.
06:14We believe the salt's presence in the craters
06:17We believe the salts on Ceres are actually very young.
06:20We think they're as young as four million years old,
06:23and that's basically like yesterday in terms of geology.
06:26And that is super weird, right?
06:28That's happening not on sort of a geologic era.
06:31It's happening now, today.
06:35What could cause patches of salt on a world long presumed dead?
06:42Planetary geologist Janie Radabaugh believes a clue
06:44might be found at Mono Lake in California.
06:50All right, I'm here looking at this beautiful lake off in the distance
06:54and standing on massive white deposits.
06:57These white deposits used to be a part of this lake at one point.
07:00The lake had dissolved a lot of the materials in it,
07:03and then as it receded it left behind the materials as it evaporated away.
07:08And these things are, you know, salts.
07:11They're kind of granular in texture.
07:14And just to make sure we taste it, and yeah, sure enough, it's salty.
07:21The salt at Lake Mono crystallises as the water evaporates.
07:26The only way it can form.
07:29The researchers believe the same process is taking place on Ceres.
07:34This means there must be liquid water beneath the surface.
07:41But how can liquid water exist out in the deep freeze of the asteroid belt?
07:47These bright spots are located in the centres of craters.
07:51They're located around cracks in the surface.
07:53And that is telling us that this material is coming from under the surface
07:57and welling up onto it.
08:02Absolutely nobody expected there to be liquid water beneath the surface of Ceres.
08:07We cannot explain what is keeping that water warm.
08:09On some moons, gravitational tugging keeps the interiors warm.
08:13But Ceres is not really near anything else that's very large.
08:17So the amazing thing is that we may not even understand how rocky planets work.
08:22There may be another source of energy, another mechanism for heating the interior
08:26that we haven't even discovered yet.
08:30To understand how Ceres has liquid water,
08:33we have to travel back 4.6 billion years.
08:38To when debris left over from the formation of our sun
08:41slammed together to form the dwarf planets.
08:45As they took shape, the heavier rocky material sunk to the centre
08:49and formed a hot, molten core.
08:54Slushy water ice floated to the top and for a while it stayed as a liquid.
08:58But once the core cooled, it froze, forming the solid mantle and crust.
09:03However, that surface should still be solid.
09:06So the salt patches remain a perplexing mystery.
09:11We still haven't answered the question,
09:13how could there actually still be liquid water on Ceres?
09:17That's still a hard question to answer.
09:19One way this could happen is if it's not actually pure water,
09:23if you've mixed it with something else.
09:28Some scientists have proposed that the liquid water
09:31Some scientists have proposed that a salty ocean lies beneath the surface.
09:39The high concentration of salt lowers the freezing point of the water,
09:44keeping it liquid.
09:54When asteroid impacts fracture the crust,
09:57this salty water oozes up from below.
10:01The liquid swiftly evaporates, but the salt remains,
10:06leaving a brilliant white spot on the surface.
10:10In fact, I'm willing to bet there could be water coming up now,
10:14bringing salts up to the surface, evaporating away into space,
10:17and that means liquid water is very close to the surface of Ceres right now.
10:23Ceres has an even more startling secret.
10:25Recent research suggests that it's not from our solar system,
10:29and didn't form anywhere near the asteroid belt.
10:33Ceres may have been born alongside hundreds of other dwarf planets
10:37many billions of kilometres away from the Sun.
10:40So how did it get here?
10:56NEXT EPISODE
11:03Most of the dwarf planets discovered lie far out in the solar system,
11:07beyond the orbit of Neptune.
11:12However, Ceres orbits between Mars and Jupiter in the asteroid belt.
11:18But its location isn't the only hint Ceres might be an interloper.
11:25NEXT EPISODE
11:29Normally, celestial objects are made of the same materials
11:33as the other bodies in their neighbourhoods.
11:36But that's not the case with Ceres.
11:41The asteroid belt is mostly made up of dry, rocky bodies
11:45composed of the same heavy elements that form the rocky inner planets.
11:50Ceres is very different.
11:54Ceres is essentially an icy world, right?
11:58It's made out of ices instead of rocks.
12:01And so that's kind of weird, considering where it is.
12:06The ice on Ceres also contains chemical compounds
12:10that in the early years of the solar system didn't exist in the asteroid belt.
12:14The more we learned about Ceres, the more mysterious it became.
12:18One of the things is that Ceres has quite a lot of ammonia on it.
12:20And we don't find ammonia anywhere near the inner part of the solar system.
12:27But it is found on Pluto, its moon Charon,
12:31and out in the frozen Kuiper belt,
12:34where we find the other dwarf planets.
12:37We think that the origin of that ammonia
12:40would have had to be in a very cold part of the solar system.
12:44Colder than where we find Ceres today.
12:46But how an icy dwarf planet with ammonia
12:50came to inhabit a place where ammonia can't form is a huge puzzle.
12:55I'm up here in the mountains where it's rather snowy at the moment.
12:58Down a few thousand feet below us, it's rather warm and summery.
13:01And that's actually a pretty good analogy for our solar system.
13:05In the earliest days of the solar system, when the sun was just forming,
13:09of course, as it was turning on, it was quite warm in the inner solar system.
13:12When the sun burst into life 4.6 billion years ago,
13:17it was more energetic than it is today.
13:20It blasted all the lighter elements, such as ammonia and helium,
13:25out past a point called the snow line.
13:28And that's why the rocky planets, like the one we live on here in the inner solar system,
13:33are made of rock and metal.
13:35It just wasn't the ability for some of the lighter molecules
13:38in the solar system to exist.
13:39But out beyond the snow line, beyond where we find Jupiter today,
13:44the temperatures were much colder.
13:49This suggests to us that Ceres perhaps formed in the outer solar system
13:54and then migrated inwards to its present location in the asteroid belt.
14:00We used to think that planetary orbits were completely immutable,
14:05that they simply ran like clockwork and they didn't move around.
14:09Now we know that that's not the case.
14:12In the early stages of planet formation,
14:15planets move around through the gaseous disk that encircles the young sun,
14:21much like rafts that are pushed around by ocean currents.
14:26Ceres' ammonia suggests that dwarf planets moved around on the gaseous disk
14:32along with the young planets.
14:33Ceres is sort of a smoking gun, that solar systems are much more dynamic,
14:39much more dramatic than we know.
14:41There's mounting evidence that Ceres formed farther out in the solar system
14:45and something brought this little world in.
14:48What could possibly have done that?
14:51The answer is the planet Jupiter.
14:56After it first formed, the giant planet migrated in towards the sun.
15:00Its massive gravity disrupted the orbits of other bodies in the solar system,
15:05including that of Ceres.
15:09The solar system formed out of a disk of gas and dust
15:14and as Jupiter formed, it would have been ploughing through this material
15:17and if it ploughs through that material, it's experiencing drag.
15:21As it was losing energy, it would start to move in toward the sun relatively slowly.
15:26Ceres formed in the outer edges of the solar system.
15:31It was dislodged from the Kuiper belt and pulled inwards by the migration of Jupiter.
15:36And when Jupiter stopped migrating, so did Ceres.
15:41It then settled into a new, stable orbit in the asteroid belt.
15:47Once you realize that something that strange and dramatic can happen,
15:50that a dwarf planet can form far out in the solar system and be brought in,
15:54it makes you wonder how many times that happened before.
15:57Could there have been other generations of dwarf planets
16:00that got thrown in towards the sun?
16:02Or maybe were thrown out of the solar system entirely?
16:09Scientists believe that rocks and icy dwarf planets
16:13may have hurtled into the inner solar system.
16:15Hundreds began the journey. Only one survived.
16:21If there was a population of small dwarf planets in the outer solar system
16:25that migrated inwards, Ceres might be the sole survivor, the only one left.
16:31So if Ceres settled into its new home in the asteroid belt,
16:35where are the rest of the icy worlds?
16:38The idea that Ceres may have moved out of the solar system
16:41is interesting, but why should it be important to you?
16:44And, incredibly, the answer might be inside your own body right now.
16:48For the longest time, we've wondered,
16:51where did the majority of Earth's water come from?
16:54When you think about where the Earth is, how close it is to the sun,
16:57there shouldn't have been any water here.
17:02Understanding the evolution of Ceres, from where it formed
17:06to where we find it today, is a fascinating process.
17:08It could also lead us to understand how the Earth can end up
17:12with more water than we would otherwise expect.
17:15When the Earth formed, it was too hot for water to exist on the surface.
17:21Perhaps the hundreds of dwarf planets broke up on their journey,
17:26showering Earth with water-rich lumps of rock.
17:31Amazingly, when we study the chemistry of water,
17:34the best match is that the water in your body right now
17:37came from asteroids themselves.
17:40Asteroids and dwarf planets that rained down and hit the Earth
17:43over billions of years.
17:46Dwarf planets may have brought something else, too.
17:50In February 2017, scientists announced the discovery
17:54of organic materials on the surface of Ceres.
17:58On Earth, life uses water and rocks,
18:00on the surface of Ceres.
18:03On Earth, life uses water and organic chemistry,
18:06carbon-based molecules.
18:09The intriguing thing about the dwarf planets is that they have both of those.
18:12Pluto and Ceres have organic molecules.
18:15There's liquid water below the surface.
18:18There's a source of energy that warms the interior.
18:21It is not at all impossible that somewhere under these cold, icy surfaces,
18:24there could be life.
18:27Could other dwarf planets host life, too?
18:30Jupiter and Makemake both have red-colored patches
18:33from something called tholins,
18:36organic molecules that could be a precursor to life.
18:40Based on all the interesting chemistry they're doing,
18:43these dwarf planets could be like the test tubes of the solar system.
18:50If there were small bodies strewn all about the solar system
18:54that had liquid water or ice,
18:57it could possibly serve as an incubator for life,
19:00ready to crash into another body and seed it.
19:03The habitable zone now extends to the entire solar system.
19:07It really expands, it greatly expands the stage
19:10for the play of life in the entire galaxy.
19:13Maybe we have dwarf planets to thank for our very existence.
19:18All of a sudden, the smallest bodies in our solar system
19:21have become some of the most interesting things we've ever seen.
19:25The idea of migrating dwarf planets
19:27opens up some intriguing scenarios,
19:30including one really amazing possibility.
19:34Some of that water and organic material
19:38may not actually be from our solar system.
19:58The more we learn about dwarf planets,
20:01the more they surprise us.
20:13But there's one dwarf planet whose very existence is a mystery.
20:18It's called Sedna,
20:21and no one is quite sure why it's in our solar system.
20:24Sedna may have my vote
20:27for the single most peculiar object in the entire solar system.
20:31Here we have a world which is about 1,000 miles wide,
20:35but it is way far out in the solar system, way past Neptune.
20:42Sedna is the most distant object identified in our solar system.
20:47Standing on the surface of Sedna,
20:50looking back at the sky,
20:52standing on the surface of Sedna, looking back at the solar system,
20:56the Sun would look like a really bright star,
20:59but not much more than a really bright star.
21:04Just like Pluto, Sedna has a strange elliptical orbit.
21:09The difference is Sedna travels from 11 billion
21:13to 150 billion kilometers from the Sun.
21:17But unlike Pluto, its orbit can't be explained
21:20by its close proximity to Neptune.
21:26The weird thing about Sedna is its orbit.
21:29How could it have gotten that elliptical
21:32when it's that far away from any of the major planets?
21:35If you have an object that's close enough to Neptune,
21:38Neptune's gravity can affect its orbit and swing it into an elliptical orbit.
21:42The problem is Sedna never gets that close to Neptune.
21:45It doesn't get anywhere near close enough to be in that kind of orbit.
21:47And that means that something else is going on out there.
21:55Sedna cannot be explained using objects that we know.
21:59Everything else we can understand why it's where it is
22:02based on the eight planets and many, many other small bodies.
22:06Sedna cannot be explained by that.
22:08And that's the sign of a good mystery.
22:10Something else must have happened.
22:13Looking at models for how you can change the orbits of objects,
22:16there's almost no way Sedna could have formed in our solar system
22:20and then had its orbit change so that it's that elliptical
22:24and goes that far out from the Sun.
22:26And that means maybe, maybe it didn't form here.
22:30It may be an alien world.
22:43How could our solar system have captured an alien world?
22:49Long ago, it turns out that our Sun may have passed close to other stars.
22:56It was born in a stellar nursery,
22:59so close to many other embryonic stars.
23:05So if the Sun was born in a very dense,
23:09So if the Sun was born in a very dense neighborhood,
23:13whereby a lot of other stars were forming at the same time in the same region,
23:17it is absolutely possible that material could be exchanged between these stars
23:21as they're forming planets.
23:23Sedna may have formed just like any other object around another star
23:27in a nice circular orbit out past the main planets of that alien solar system.
23:32But if that star got close enough to the Sun,
23:34our gravity may have been able to lift Sedna out and steal it.
23:43It's possible that other dwarf planets were abducted from other systems
23:49and that these alien worlds carried alien water
23:53and even alien organic materials to the inner planets.
24:04Then again, there could be a completely different explanation for Sedna's weird orbit.
24:11The elusive, theoretical Planet Nine.
24:17This distant, hidden planet is thought to be massive.
24:21Its gravity could have dragged Sedna into its strange, stretched-out orbit.
24:28Sedna was the first object that pointed towards the existence of Planet Nine.
24:34This is the Planet Nine family prototype that we're using
24:38to try to then track down this planet out there.
24:43The tracking of other distant objects revealed a family of objects,
24:48all with the same strange, elliptical orbits.
24:53There are a group of objects out there that all have their orbits
24:57that are roughly aligned with each other.
25:00Now, these are all elliptical orbits and they could be this way or this way or this way or that way,
25:04they all have about the same tip and they're all sort of rotated in the same direction.
25:08Not exactly, but close enough that it's peculiar
25:12and statistically unlikely that that would happen by random chance.
25:16However, they could have had their orbits aligned this way
25:19if there is a massive planet, a Planet Nine, if you will,
25:23orbiting the Sun way out past Neptune.
25:28We haven't found Planet Nine.
25:30And until we do, we won't know from where Sedna really came.
25:36But this dwarf planet is forcing us to reassess our own cosmic neighbourhood.
25:45It's so tempting to think that we understand something as basic as our own solar system, our own home.
25:51When you discover something like Sedna,
25:54you realise there could be a lot out there that we haven't seen.
25:56There are objects that are small like Sedna that are just so far away
26:00that they are beyond our limit to detect them.
26:03So there could be hundreds, thousands, tens of thousands of objects out there,
26:07new parts of our solar system that are still waiting to be discovered.
26:13And these dwarf planets all over the solar system are revealing hidden lives.
26:19Salt on the surface of Ceres suggests liquid water beneath the surface.
26:24And further out, the New Horizons mission found subsurface oceans on Pluto.
26:32We used to think that water could only exist in this Goldilocks zone
26:36where it's not so hot that the water boils off and it's not so cold that it freezes.
26:40But that's not the case anymore.
26:43We look around and we find water in the most unexpected places in our own solar system.
26:48This is kind of a revelation of modern planetary science
26:50that so many of these worlds in the outer solar system
26:53may have subsurface oceans of liquid water.
26:56It kind of boggles the mind to see how far we've come
26:59in our understanding of the interior structures of these worlds.
27:04Finding liquid water so far from the sun left scientists stunned.
27:10And the surprises keep on coming as we study these distant worlds in detail.
27:15Makemake, a dwarf planet that's two-thirds the size of Pluto,
27:20has a surface covered in ethane and methane ice.
27:27The ice reacts with sunlight, forming organic molecules called tholins,
27:33which colour the planet red-brown.
27:39Further out, the ice is covered by a layer of ice.
27:43Further out lies Eris.
27:46It's 14 billion kilometres from the sun,
27:49and its surface temperature is about minus 240 degrees Celsius.
27:56Eris is an absolutely tantalising object.
27:59We really don't know much about it at all.
28:02It should be very similar to Pluto.
28:04But one of the things we notice is that Pluto's surface is kind of patchy.
28:07There are areas that are very bright, but also areas that are quite dark.
28:11Eris, on the other hand, seems to be almost entirely bright.
28:19Eris is one of the shiniest objects in the solar system,
28:22reflecting 96% of the light that hits it.
28:26Scientists wondered why.
28:30A clue came from its near neighbour, Pluto.
28:34When New Horizons flew past, it spotted something strange.
28:40One of the funny little details is that as we flew over Pluto,
28:44we realised that there were things that looked a lot like sand dunes down there.
28:47Now, that may not sound incredibly exotic,
28:50you know, what's very interesting about a sand dune.
28:52Sand dunes may sound dull,
28:55but they can reveal a lot about the mechanics of a planet.
29:00Unlike the dunes we know and love on the Earth that are made of sand,
29:04the dunes on Pluto are made entirely of particles of ice.
29:07And there's only one thing that can build dunes.
29:12Wind.
29:15Dunes are like a visual representation of the wind
29:19that's moving across the valley and carrying the sands with it
29:23and depositing it into these big, beautiful dune forms.
29:28Our planet is large enough to hold on to an atmosphere.
29:32Air warmed by the sun rises,
29:34and fresh air rushes in underneath, generating winds.
29:40Pluto is so small and so far from the sun,
29:44it shouldn't have an atmosphere, wind or dunes.
29:49The problem Pluto has, like other small bodies in the solar system,
29:52is that it's really hard for it to hold on to an atmosphere.
29:55It's just too small.
29:57The very thin, light atmospheric gases basically just escape.
30:00Neither Haumea nor Makemake have detectable atmospheres.
30:07But when New Horizons looked back at Pluto
30:10as the dwarf planet passed in front of the sun,
30:13scientists spotted a thin haze of gas.
30:18It transpired that Pluto does have an atmosphere.
30:25However, this atmosphere is temporary
30:27because of Pluto's elliptical orbit.
30:30Some of the time it's far from the sun,
30:33and at others it's much closer and warmer,
30:36creating a kind of winter and summer.
30:41Pluto's atmosphere depends on the season.
30:44In the summer it's warm enough to have an atmosphere,
30:47and in the winter that atmosphere freezes out.
30:50Over the course of just a single orbit around the sun,
30:53the surfaces of these dwarf planets may change significantly.
30:55Condensing and coating out atmosphere when they're far from the sun,
30:59having that atmosphere re-volatilize and redistribute the surface
31:03when they're closer to the sun.
31:06Pluto is currently in its summer phase.
31:09The extra heat during the long super-summer
31:12evaporates some of the nitrogen ice on the surface,
31:15creating a thin, wispy atmosphere.
31:20It turns out that even though the atmosphere of Pluto is very thin,
31:23there is wind.
31:26It's really light, but there's just enough wind
31:29to be able to carry particles with it once they start moving.
31:33The seasonal cycle could help explain Eris's brightness.
31:38Eris is three times further away from the sun than Pluto is.
31:43But when you put a nitrogen atmosphere three times further away,
31:46that nitrogen freezes solid to the surface.
31:49Eris could be an indicator of what Pluto looks like when it enters its winter.
31:53The gases will freeze, and it'll become even more reflective.
31:59In winter, Pluto's dunes will be locked in place,
32:03frozen on the surface.
32:06Unlike the icy features of another dwarf planet,
32:10where the volcanoes just simply vanish.
32:15From afar, the dwarf planet Ceres looks uniform and dull.
32:21But up close, it's not.
32:25It's a little bit more than a dwarf planet.
32:29It's a dwarf planet that's been around for a long time.
32:33It's a dwarf planet that's been around for a long time.
32:37It's a dwarf planet that's been around for a long time.
32:41But up close, one huge feature comes into view.
32:46One of the strangest objects that we saw when we began to map the surface of Ceres
32:51was something called Ahuna Manz.
32:54Now this was a strange jutting hill.
32:59Very, very sharp sides.
33:02And it didn't match any of the other terrain on Ceres.
33:06This is a mountain that is standing three miles high,
33:10and there's nothing else like it on the entire surface of Ceres.
33:16Ahuna Mons dominates the landscape of Ceres.
33:20With its steep sides and enormous height,
33:23it looks a lot like volcanoes on Earth.
33:26But Earth is still geologically active.
33:30Ceres is so small, its molten core should be frozen solid.
33:37Planetary scientist Nina Lanza
33:40heads to one of the most volcanically active places on Earth...
33:44Iceland.
33:48She has a drone's-eye view of Mount Hilderflot,
33:51a volcano similar in shape to Ahuna Mons.
33:57So this volcano is what's called a rhyolitic dome,
34:00and so it's a type of lava
34:03that kind of gets squeezed out through fissures
34:06and then forms this kind of blobby dome feature
34:11that gets pushed up by the magma coming up from beneath.
34:18On Earth, red-hot magma bubbles slowly out of cracks in the surface,
34:23building a steep-sided volcano.
34:27But dwarf planets such as Ceres are too small
34:30to have a hot core of molten rock to power volcanism.
34:36There isn't molten rock on these smaller worlds
34:39that have a lot of ice in them.
34:41Instead, what's molten is water under the surface.
34:44And if the water can work its way up through cracks
34:47and erupt out in the surface,
34:49you get a volcano, but it's a cold-water volcano.
34:53We call these cryovolcanoes.
34:57Liquid water squeezes up through fissures in the surface,
35:01which quickly freezes, thus building the mountain.
35:06This volcano, you can see that it's a pretty young feature,
35:10and it's not very eroded.
35:13We expect on Earth that wind and water
35:16will slowly erode this mountain away.
35:21With no wind or weather to erode Ceres' cryovolcanoes,
35:25once created, they should remain on the surface
35:28for billions of years.
35:31Ahuna Mons is very strange
35:33because it's the only tall mountain on Ceres.
35:36Why should that be?
35:38You don't typically get just one of something,
35:40you should have dozens of them.
35:42And in fact, Ceres may have had
35:44quite a few cryovolcanoes in the past,
35:46but they're all gone today.
35:48Just to put that into perspective,
35:50imagine if this is the only mountain on Earth.
35:53Why would there only be one mountain?
35:55What would that mean?
35:56This leads us to ask the question,
35:58you know, are the volcanoes on Ceres disappearing?
36:02The idea that volcanoes are vanishing,
36:05it just sounds totally science fiction
36:08and really not realistic at all.
36:10Of course, volcanoes can't just vanish.
36:12But actually, in the right context,
36:15in certain scenarios, they actually can.
36:19But how is due to gravity,
36:22how does it flatten solid matter?
36:24How quickly depends on the structural composition
36:27of the material.
36:30If you want to build a sandcastle on the beach,
36:32you can't use dry sand, it doesn't stick together.
36:34So you want to mix a little bit of water in there
36:36so that when you make the structure,
36:38it holds together.
36:40But if you mix in too much water,
36:42it just dribbles away, it viscously relaxes, it slumps.
36:47Even the rock volcanoes on Iceland
36:49are slowly slumping under their own weight.
36:53Strange as it is to imagine this,
36:55it turns out the mountain behind me
36:57is actually slowly relaxing back down.
37:01It's just happening very slowly,
37:03not on a time scale that we can directly observe.
37:07It may be that there were many cryovolcanoes
37:10on the surface of Ceres.
37:12They no longer show any trace of their existence.
37:16So if we waited around a little bit longer
37:18until all of Ahuna Mons had slowly relaxed back into the planet,
37:21we'd see no trace of it either.
37:26Maybe Ahuna Mons hasn't always stood alone.
37:30Maybe it's just the last of its kind.
37:35This is just one among a number of mysteries
37:37surrounding dwarf planets that need to be solved.
37:45And more arise each day,
37:47including how, just like their larger cousins,
37:50they often have orbiting satellites.
37:55We now realise that all of the largest dwarf planets
38:00have moons around them, have a moon.
38:03Most of them have one, Haumea has two, Pluto has five.
38:11Four billion years ago, the young solar system was chaotic,
38:16filled with small bodies orbiting the Sun.
38:21One hid the infant Earth,
38:25forming the Moon.
38:29Collisions such as these happened throughout the solar system.
38:34The dwarf planet Haumea formed from an explosive impact
38:37between two larger objects,
38:39which may account for its unusual bean-like shape.
38:43All the dwarf planets suffered huge collisions.
38:46Haumea had this big one that left it spinning.
38:48Eris has a tiny moon, presumably from a giant impact.
38:51Makemake has one.
38:53All these biggest objects have these tiny fragments of moons
38:56showing us their history of just getting battered
38:59and pieces being knocked off everywhere.
39:04Most dwarf planets' moons are tiny,
39:07not much bigger than asteroids.
39:09But one moon is very different.
39:12Pluto's moon, Charon.
39:15Pluto's moon is, if anything, weirder than Pluto itself.
39:20It's Frankenstein's moon.
39:22It looks like somebody tore a moon apart
39:24and then just kind of slapped, dashed it back together.
39:27One hemisphere is smooth, one is very rugged.
39:30It's got a canyon that's like a notch carved out of the side.
39:33It is really bizarre.
39:36An impact may have formed Charon
39:39and left it tied to Pluto in an oddly codependent relationship.
39:44In some ways, you can think of the Pluto-Charon system
39:47as almost a binary planet.
39:49There is no other planet in the solar system
39:51where the moon is so large in proportion to it and so close.
39:56Like other binary objects,
39:58Pluto and Charon orbit around a central gravitational point.
40:03Locked in this gravitational dance,
40:06Pluto and Charon always show each other the same side.
40:10One of the really interesting things about Pluto and Charon
40:13is that they're what we call tidally locked.
40:15When Pluto and Charon formed,
40:17they were probably both rotating on their own axes.
40:19But the two worlds actually slowed down their rotation
40:22and locked together,
40:24with one side constantly facing the other as they orbit around.
40:28But Pluto's rotation is tipped over,
40:31like a top spinning on its side.
40:34So Charon's orbit around Pluto is also tipped over.
40:39Almost every planet in the solar system
40:41has an orbital axis that points in roughly the same direction.
40:44Pluto's is tilted down about 120 degrees.
40:49Scientists have long wondered what caused this disparity.
40:53Did Charon pull Pluto over,
40:55or is the tilt a result of the impact that formed Charon?
41:01A clue was revealed when New Horizons sent back images of Pluto.
41:06One of the more endearing features of Pluto
41:08as the New Horizons probe approached it,
41:10was a gigantic heart-shaped region
41:13on the side of Pluto facing the spacecraft.
41:18Sputnik Planitia.
41:20A bright white heart against Pluto's dark, pockmarked surface.
41:25When we got close-ups of this, it was completely fascinating.
41:32I gasped out loud. This is how shocking this was.
41:35And I remember saying, oh my gosh, there are no craters there.
41:39It is smooth, like it's a frozen over lake.
41:42This is indicative of something liquid,
41:44something flowing under the surface of Pluto.
41:46And what we're seeing is the top frozen layer of it.
41:51There are even convection cells,
41:52where the ice appears to be warming and spreading out.
41:55That suggests that underneath there's a source of energy.
41:58And amazingly, there may even be a huge basin of liquid water under that ice.
42:04Sputnik Planitia may hide a giant subterranean ocean of liquid water.
42:10It's also a gigantic scar on Pluto's surface.
42:15Most likely, given the shape and size,
42:18Sputnik Planitia was formed in a giant impact,
42:21something smacked into Pluto.
42:24Could the combination of subsurface water
42:27and an impact account for Pluto's unusual tilt?
42:33One theory suggests that an object smashed into the top of Pluto.
42:38The impact shattered the surface,
42:40and water oozed up to fill the crater.
42:44The liquid water knocked Pluto off balance,
42:47and a gravitational dance with Charon
42:50spun Sputnik Planitia to the opposite side.
42:53One idea is that Sputnik Planitia formed where it is
42:56because ices could accumulate in the floor of a giant impact basin.
43:01But it's not yet certain whether that's actually the case or not.
43:07Dwarf planets, once thought to be dead lumps,
43:10have come alive with mysteries.
43:13They've challenged all our assumptions,
43:15and yet we've barely scratched the surface of these perplexing worlds.
43:19There are many more dwarf planets waiting to be discovered,
43:23and who knows just what surprises we might find.
43:28We don't know the final count of dwarf planets
43:30because we're still finding them,
43:32but I think that there are probably somewhere between
43:35one and two hundred dwarf planets out past Pluto.
43:39There are probably many, many more
43:41as you go even further out in the solar system.
43:44Dwarf planets are perhaps the most important
43:47Dwarf planets are perhaps the most interesting objects
43:50we've found in the solar system.
43:52They're diverse, they're geologically active,
43:54they contain liquid water.
43:56Just because they're small, that doesn't mean they're insignificant
43:59or they should be ignored.
44:01They are where it's at.
44:03For me, that is just the best, the most exciting.
44:05We have all of these new worlds to study
44:08that we didn't even dream existed just a few years ago.
44:11That's science.