0:00:09 > 0:00:12There are some mysteries when we look around the solar system,
0:00:12 > 0:00:15where the theories really don't match what we see.
0:00:20 > 0:00:23Science fact can be a lot weirder than science fiction.
0:00:26 > 0:00:27We started finding planets
0:00:27 > 0:00:31in places we'd never thought could possibly form a planet.
0:00:31 > 0:00:33We had to go back to the drawing board.
0:00:36 > 0:00:38How do you make solar systems? How do you make planets?
0:00:40 > 0:00:42It's as if somebody took the solar system,
0:00:42 > 0:00:45picked it up and shook it real hard.
0:00:45 > 0:00:47Our planets might have moved.
0:00:47 > 0:00:48They might have moved a lot.
0:00:48 > 0:00:50All of a sudden, everything changed.
0:00:56 > 0:00:57It's changed the way we look at
0:00:57 > 0:01:00almost every process in the solar system.
0:01:02 > 0:01:05Sometimes the blood splattered on the wall
0:01:05 > 0:01:07can tell you more about what happened
0:01:07 > 0:01:08than the body lying on the floor.
0:01:29 > 0:01:31The Royal Observatory in Greenwich
0:01:31 > 0:01:34is the historical home of British astronomy.
0:01:37 > 0:01:42Discoveries have been made here and mysteries unravelled.
0:01:42 > 0:01:46It is also home to some unique astronomical treasures.
0:01:49 > 0:01:51This is an orrery,
0:01:51 > 0:01:54a clockwork model of the solar system,
0:01:54 > 0:01:56and for most of the last four centuries
0:01:56 > 0:02:00this has been the way we think about the planets in the solar system.
0:02:01 > 0:02:04Of course, the scale is all wrong.
0:02:04 > 0:02:07But it clearly shows the traditional view of the planets
0:02:07 > 0:02:08and their fixed orbits.
0:02:08 > 0:02:11In the centre we have the sun
0:02:11 > 0:02:15and then, around it, we have the four rocky planets,
0:02:15 > 0:02:18tiny Mercury rushing around in the middle,
0:02:18 > 0:02:21Venus, the earth with the moon going around it,
0:02:21 > 0:02:23and then Mars.
0:02:23 > 0:02:25And outside of the inner planets,
0:02:25 > 0:02:27we have the gas giants,
0:02:27 > 0:02:29Jupiter, the largest planet of all,
0:02:29 > 0:02:32and then Saturn with its beautiful ring system.
0:02:32 > 0:02:35And then the two outermost planets, Uranus and Neptune.
0:02:37 > 0:02:40Astronomers always thought that the planets have been fixed
0:02:40 > 0:02:44in these orbits since they formed, more than 4 billion years ago.
0:02:45 > 0:02:49Long enough for the earth to develop into the haven it is today
0:02:49 > 0:02:51for life to evolve.
0:02:51 > 0:02:54A mechanical model like this embodies an idea
0:02:54 > 0:02:57of the solar system in which the planets all keep
0:02:57 > 0:03:00to these very neat, orderly orbits,
0:03:00 > 0:03:05moving essentially in circles and at fixed distances from the sun.
0:03:05 > 0:03:10And the natural assumption to make is that everything we see now
0:03:10 > 0:03:13formed where it is and has stayed there ever since.
0:03:15 > 0:03:19The idea that the planets are fixed in their orbits
0:03:19 > 0:03:23has been the bedrock of our understanding for hundreds of years.
0:03:23 > 0:03:26But there are some mysteries about our solar system
0:03:26 > 0:03:30that mean we may have to rethink everything we thought we knew.
0:03:33 > 0:03:36It's time for a brand-new model.
0:03:36 > 0:03:38DRAMATIC DRUMBEAT
0:03:47 > 0:03:50Recently, astronomers have started to unravel
0:03:50 > 0:03:52the mystery of how the solar system came to be.
0:03:54 > 0:03:56And to explore it
0:03:56 > 0:03:59we first need a more accurate picture of our planets.
0:04:03 > 0:04:07We need to alter the scale to reflect the huge difference in size.
0:04:07 > 0:04:11For example, Jupiter, the largest planet,
0:04:11 > 0:04:14is 11 times the radius of the Earth,
0:04:14 > 0:04:17and if you look at the masses, the difference is even greater.
0:04:17 > 0:04:21Jupiter has about 300 times the mass of planet Earth.
0:04:23 > 0:04:26The sun we've left alone.
0:04:26 > 0:04:31If we scaled that up too it would fill half the room.
0:04:31 > 0:04:34And, of course, the planets are not all bunched up.
0:04:34 > 0:04:36MECHANICAL WHIRRING
0:04:39 > 0:04:42We need to push the gas and ice giants much further away.
0:04:47 > 0:04:50To be truly accurate, with the planets this size
0:04:50 > 0:04:53we'd have to make the orbits several thousand times bigger.
0:04:59 > 0:05:02However, exactly how we ended up with this neat
0:05:02 > 0:05:04and stable arrangement of planets
0:05:04 > 0:05:07is still one of the greatest mysteries in astronomy.
0:05:14 > 0:05:16In trying to solve this mystery,
0:05:16 > 0:05:19we may discover how the earth came to inhabit
0:05:19 > 0:05:21the perfect position for life to evolve.
0:05:23 > 0:05:26Getting an earth where we have our earth today
0:05:26 > 0:05:29was not a given when this whole solar system started.
0:05:32 > 0:05:36We may be able to understand the remarkable chain of events
0:05:36 > 0:05:38that created the biggest game of pinball in the galaxy...
0:05:39 > 0:05:42The solar system could have done a lot of different things,
0:05:42 > 0:05:44it could have evolved in a lot of different ways.
0:05:44 > 0:05:47We could have ended up with our Jupiter right next to the sun.
0:05:48 > 0:05:50And it looks like it was Jupiter
0:05:50 > 0:05:53that defined the fate of the solar system.
0:05:54 > 0:05:59The giant planets' story IS the story of our solar system.
0:05:59 > 0:06:01We like to think that the earth is really important,
0:06:01 > 0:06:04but the truth is that, if you were looking from afar,
0:06:04 > 0:06:07our solar system is mainly four big planets and some debris.
0:06:09 > 0:06:11Could our place in the universe
0:06:11 > 0:06:15really be nothing more than a lucky accident?
0:06:15 > 0:06:16The question that really arises
0:06:16 > 0:06:20is how common is a solar system like ours?
0:06:23 > 0:06:27The mystery of the birth of the solar system is set to unravel.
0:06:34 > 0:06:37As they try to work out how our solar system formed,
0:06:37 > 0:06:40astronomers have noticed some baffling puzzles.
0:06:40 > 0:06:43If we look at the solar system as it is today,
0:06:43 > 0:06:46it seems quite neat and simple.
0:06:46 > 0:06:49We have four small, rocky planets close to the sun
0:06:49 > 0:06:53and then four enormous giants further out.
0:06:53 > 0:06:56But when we try to model the formation of the solar system
0:06:56 > 0:06:59on a computer, something doesn't quite add up.
0:06:59 > 0:07:01It's really hard to get the model
0:07:01 > 0:07:04to make the planets in the places where we see them today.
0:07:09 > 0:07:12Take, for instance, the curious case
0:07:12 > 0:07:14of the undersized planet Mars.
0:07:14 > 0:07:17If we look at the rocky innermost planets,
0:07:17 > 0:07:20Venus and Earth have about the same mass,
0:07:20 > 0:07:22and we'd expect Mars to have a similar mass too,
0:07:22 > 0:07:24but it actually doesn't.
0:07:24 > 0:07:27It's only about one tenth the mass of the earth or Venus,
0:07:27 > 0:07:30and that's a mystery that's very hard to explain.
0:07:32 > 0:07:35This is the first of four key puzzles
0:07:35 > 0:07:39about the birth of the solar system that remain unsolved.
0:07:40 > 0:07:42And then, at the edge of the solar system,
0:07:42 > 0:07:46the two outermost planets, Uranus and Neptune,
0:07:46 > 0:07:49are much further away from the sun than we'd expect.
0:07:49 > 0:07:52It's very hard to explain how they could have formed
0:07:52 > 0:07:56and become so large at that great distance from the central star.
0:07:57 > 0:08:00If we go in and look at the asteroid belt,
0:08:00 > 0:08:03there are thousands of small, rocky objects there,
0:08:03 > 0:08:05but there are two broad types -
0:08:05 > 0:08:09some of them are very rocky and some have more of an icy content.
0:08:09 > 0:08:11And yet these two types are actually found
0:08:11 > 0:08:13relatively close together.
0:08:13 > 0:08:16It seems as though they formed under different circumstances
0:08:16 > 0:08:19but they've all ended up in roughly the same place.
0:08:19 > 0:08:21And, again, it's a mystery as to how that happened.
0:08:23 > 0:08:25And, closer to home,
0:08:25 > 0:08:28how to explain the rapid and massive bombardment
0:08:28 > 0:08:30that left the moon covered in craters.
0:08:33 > 0:08:36There are many mysteries in the solar system,
0:08:36 > 0:08:38but by unravelling these four -
0:08:38 > 0:08:39the size of Mars,
0:08:39 > 0:08:42the formation of the outer planets,
0:08:42 > 0:08:44the composition of the asteroid belt
0:08:44 > 0:08:46and the bombardment of the moon -
0:08:46 > 0:08:49we may be able to explain how our planet Earth
0:08:49 > 0:08:53found itself in a perfect position for life to evolve.
0:08:55 > 0:08:57And it all starts a long,
0:08:57 > 0:08:59long time ago.
0:09:01 > 0:09:03DRAMATIC DRUMBEAT
0:09:13 > 0:09:16Four and a half billion years ago,
0:09:16 > 0:09:18our sun burst into life
0:09:18 > 0:09:21from the collapse of a massive cloud of gas and dust.
0:09:23 > 0:09:24So, in the beginning,
0:09:24 > 0:09:26this is what you have in our early solar system.
0:09:26 > 0:09:28You have the young star just born,
0:09:28 > 0:09:33and the leftovers, just a cloud of gas,
0:09:33 > 0:09:35the nebula, the protoplanetary nebula,
0:09:35 > 0:09:37full of hydrogen and helium,
0:09:37 > 0:09:40dust and gas,
0:09:40 > 0:09:42and ice grains forming.
0:09:42 > 0:09:46And from this, eventually, you form the planets.
0:09:47 > 0:09:51We know surprisingly little about exactly how planets are formed.
0:09:51 > 0:09:55Most mysterious of all is the most important -
0:09:55 > 0:09:58the largest of all planets, Jupiter,
0:09:58 > 0:10:00which seems to have been made first.
0:10:01 > 0:10:05The first-born - giant, massive Jupiter.
0:10:05 > 0:10:08The meanest and largest of all the planets.
0:10:09 > 0:10:12It sucks up more than half of the existing nebula
0:10:12 > 0:10:15and becomes the king of the solar system.
0:10:17 > 0:10:20We know that Jupiter is made up almost entirely
0:10:20 > 0:10:24of the hydrogen and helium left over in this primordial cloud.
0:10:25 > 0:10:29Which means it must have formed incredibly quickly
0:10:29 > 0:10:33because, as the new sun heated up, it would have blasted the gas away.
0:10:36 > 0:10:38And so there's a time limit.
0:10:38 > 0:10:42Jupiter must have formed in the astronomical blink of an eye -
0:10:42 > 0:10:44just five million years.
0:10:46 > 0:10:50But exactly how it grew so fast and why it grew where it did
0:10:50 > 0:10:52remains shrouded in mystery.
0:11:01 > 0:11:05It's a mystery that Scott Bolton is hoping to shed some light on.
0:11:06 > 0:11:08He's sending a spaceship to Jupiter.
0:11:13 > 0:11:17At the Jet Propulsion Laboratory in Pasadena
0:11:17 > 0:11:20is NASA's deep space operations centre -
0:11:20 > 0:11:24control room for space flights to the moon and beyond...
0:11:28 > 0:11:32..including Scott's mission to Jupiter, Juno.
0:11:32 > 0:11:35Four...three...two...one...
0:11:35 > 0:11:41Ignition and lift-off of the Atlas V with Juno
0:11:41 > 0:11:43on a trek to Jupiter.
0:11:46 > 0:11:48Juno launched in 2011
0:11:48 > 0:11:51and is currently more than 2 million miles away,
0:11:51 > 0:11:55speeding its way to Jupiter at about 150,000mph.
0:11:58 > 0:12:01Even at that speed, it's a five-year journey.
0:12:02 > 0:12:06In 2016, if all goes according to plan,
0:12:06 > 0:12:08the probe will reach Jupiter and go into orbit
0:12:08 > 0:12:10around the king of the solar system.
0:12:14 > 0:12:18Scott and his entire team will be in this room, watching intensely.
0:12:20 > 0:12:23On the day of the Jupiter orbit insertion,
0:12:23 > 0:12:25this room will be completely full.
0:12:26 > 0:12:29The spacecraft is approaching Jupiter.
0:12:29 > 0:12:35It's moving at an incredible speed - like, 150,000mph, or even faster.
0:12:35 > 0:12:39And when it gets to Jupiter we have to slow down enough
0:12:39 > 0:12:41that Jupiter's gravity field can grab us.
0:12:43 > 0:12:44So, we have a rocket on board,
0:12:44 > 0:12:47we point it forward and we fire it,
0:12:47 > 0:12:51and that rocket has to burn at just the right time
0:12:51 > 0:12:53for just the right amount of time
0:12:53 > 0:12:58for us to slow down the perfect amount for Jupiter to grab us,
0:12:58 > 0:13:01because if it misses we fly right past Jupiter.
0:13:03 > 0:13:04There'll be a huge cheer.
0:13:04 > 0:13:09Once we get the data down that shows us we're in orbit around Jupiter
0:13:09 > 0:13:11the room will explode.
0:13:11 > 0:13:14It's the same room where all NASA's critical events are controlled from,
0:13:14 > 0:13:19including the recent landing of the Curiosity rover on Mars.
0:13:19 > 0:13:22And here you see it's right on that screen.
0:13:26 > 0:13:28We'll celebrate just like that.
0:13:29 > 0:13:31I can't wait.
0:13:35 > 0:13:39Once in orbit, Juno will spend a year circling Jupiter,
0:13:39 > 0:13:42gathering vital clues about how it formed.
0:13:44 > 0:13:46Some of the most important data
0:13:46 > 0:13:48that we really want and can't wait to get
0:13:48 > 0:13:52is things that are tied to understanding the early solar system
0:13:52 > 0:13:53and how Jupiter formed.
0:13:53 > 0:13:57So, we want to know whether there's a core in the middle of Jupiter.
0:13:57 > 0:13:59Is there a core of heavy elements,
0:13:59 > 0:14:01a concentration of materials, down in the centre?
0:14:01 > 0:14:05Or is it the same hydrogen and helium and mixture of gases,
0:14:05 > 0:14:06just squeezed down?
0:14:08 > 0:14:10Knowing what's at the centre
0:14:10 > 0:14:14is a vital clue to understanding how Jupiter was built.
0:14:24 > 0:14:28Building planets, whether rocky or gassy,
0:14:28 > 0:14:29is a tricky business.
0:14:29 > 0:14:32But it's something that Juno will shed some light on.
0:14:34 > 0:14:36What Juno's about is actually trying to discover
0:14:36 > 0:14:39the recipe for the solar system.
0:14:39 > 0:14:42How do you make solar systems? How do you make planets?
0:14:42 > 0:14:46And the stage that we're at is we're collecting the ingredient list
0:14:46 > 0:14:48and that's really an important part of any recipe -
0:14:48 > 0:14:51first you gather up the ingredients, figure out what they are,
0:14:51 > 0:14:53then there's some process that you have to do
0:14:53 > 0:14:55in order to bake your cake.
0:14:56 > 0:15:00But the exact nature of that process is not entirely clear.
0:15:01 > 0:15:07The recipe for a rocky planet, like the earth or Mars, is a slow one.
0:15:07 > 0:15:10It can take up to 100 million years.
0:15:10 > 0:15:13But the ingredient list is simple -
0:15:13 > 0:15:15dust.
0:15:15 > 0:15:18Dust starts out in the early solar system
0:15:18 > 0:15:21in a very fine grain, like this - even finer.
0:15:21 > 0:15:25Then, eventually, they start to stick together through electrostatic forces
0:15:25 > 0:15:27and they build bigger and bigger pieces.
0:15:27 > 0:15:30Eventually, the rocks got big enough, they started to stick together
0:15:30 > 0:15:33to the point where they started to form their own gravity.
0:15:34 > 0:15:37But there's a big leap from dust grains
0:15:37 > 0:15:40to rocks that are large enough to clump together
0:15:40 > 0:15:41through their own gravity.
0:15:41 > 0:15:44These rocks, even being so large as they are
0:15:44 > 0:15:46that none of us could lift them,
0:15:46 > 0:15:49they still don't have important gravity.
0:15:50 > 0:15:53Even larger rocks are needed to really start to get enough gravity
0:15:53 > 0:15:57to start to attract the rest of the material for it to collapse
0:15:57 > 0:16:01and start to form a planet as large as Earth.
0:16:04 > 0:16:06It's a slow process,
0:16:06 > 0:16:09but there's no rush when it comes to building a rocky planet.
0:16:11 > 0:16:14Gas giants, on the other hand, are trickier.
0:16:14 > 0:16:15You have to make them fast.
0:16:18 > 0:16:21Because Jupiter and the other gas giants
0:16:21 > 0:16:23are mostly hydrogen and helium,
0:16:23 > 0:16:26and the sun is mostly hydrogen and helium,
0:16:26 > 0:16:30that tells us right away that those planets had to have formed
0:16:30 > 0:16:33while that nebula of hydrogen and helium was still around.
0:16:35 > 0:16:40There are two ways to build a gas giant like Jupiter that fast.
0:16:40 > 0:16:44We don't know exactly how Jupiter formed.
0:16:44 > 0:16:45The two main theories
0:16:45 > 0:16:48are either it has a direct gravitational collapse,
0:16:48 > 0:16:51like we think the sun had, from the nebula
0:16:51 > 0:16:55and sort of builds, um, from the outside in
0:16:55 > 0:16:58and formed Jupiter pretty quickly,
0:16:58 > 0:17:01or it starts to build from the inside out.
0:17:02 > 0:17:05If it collapsed from the cloud of gas,
0:17:05 > 0:17:08then it will be gas all the way through to the centre.
0:17:09 > 0:17:13But if the second theory is right, then it first built a rocky core
0:17:13 > 0:17:15up to ten times the mass of the Earth,
0:17:15 > 0:17:17which then drew in a blanket of gas.
0:17:18 > 0:17:21Either way, it had to happen fast.
0:17:23 > 0:17:26But if Jupiter was going to build a heavy core that quickly,
0:17:26 > 0:17:28it couldn't be done with dust alone.
0:17:29 > 0:17:32There was another crucial ingredient.
0:17:33 > 0:17:35Ice.
0:17:43 > 0:17:45Kevin Walsh is a planet builder.
0:17:47 > 0:17:49His job is to create theoretical models
0:17:49 > 0:17:53of how the planets in the solar system formed -
0:17:53 > 0:17:57models that can best explain the evidence and the clues.
0:17:57 > 0:18:00I think that the most likely way that Jupiter formed
0:18:00 > 0:18:03was by building a solid core of material
0:18:03 > 0:18:05and then hauling the gas down on top of it.
0:18:05 > 0:18:07Either way that you form Jupiter,
0:18:07 > 0:18:09either from accreting straight from the gas
0:18:09 > 0:18:12or building up a rocky core, it has to be done
0:18:12 > 0:18:14in four or five million years,
0:18:14 > 0:18:17before all of the gas is gone from the disc around the sun.
0:18:19 > 0:18:21That's a lot quicker than the time it took
0:18:21 > 0:18:24to build a rocky planet from dust alone.
0:18:26 > 0:18:29But Jupiter had the help of that extra icy ingredient.
0:18:32 > 0:18:33So, we think that the key ingredient
0:18:33 > 0:18:36that allowed Jupiter and Saturn to form so fast,
0:18:36 > 0:18:37compared to the rocky planets,
0:18:37 > 0:18:40is that they formed far enough from the sun
0:18:40 > 0:18:43that water could condense from the gas around the sun
0:18:43 > 0:18:47and form ice, and increase the density of material
0:18:47 > 0:18:51and give you more material to build a larger, rockier core faster.
0:18:54 > 0:18:57That could explain how Jupiter built a rocky core so quickly.
0:18:59 > 0:19:03But it doesn't explain why it grew where it did.
0:19:04 > 0:19:06It's not unreasonable to think
0:19:06 > 0:19:09it would form at the place with the most ice.
0:19:11 > 0:19:14That's a place called the ice line.
0:19:14 > 0:19:16But it's not where Jupiter is today.
0:19:18 > 0:19:21So, right now when we look at our solar system, we look at Jupiter
0:19:21 > 0:19:25and it's beyond the ice line by a fair bit,
0:19:25 > 0:19:28whereas we think it was really advantageous to form Jupiter
0:19:28 > 0:19:30right at the ice line.
0:19:30 > 0:19:32So already that's suspicious.
0:19:37 > 0:19:40If Jupiter was built from a collapsing cloud,
0:19:40 > 0:19:42we'd expect it to be further out.
0:19:44 > 0:19:48If, on the other hand, it was built from a rocky core,
0:19:48 > 0:19:50we'd expect it to be closer to the sun.
0:19:51 > 0:19:54But it's not in either of these two places.
0:19:54 > 0:19:56So the big question is,
0:19:56 > 0:19:59is Jupiter in the wrong place?
0:20:08 > 0:20:12To even ask that question has, until recently, been a heresy.
0:20:14 > 0:20:19At the historic Chamberlin Telescope in Denver, Colorado,
0:20:19 > 0:20:21Kevin Walsh is following in the footsteps
0:20:21 > 0:20:23of some famous astronomers.
0:20:29 > 0:20:32He's taking a closer look at Jupiter.
0:20:34 > 0:20:36You can see Jupiter with the naked eye,
0:20:36 > 0:20:39but looking at it through a telescope like this
0:20:39 > 0:20:40makes it a lot more fun.
0:20:41 > 0:20:45The bands of colour are really clear and crisp
0:20:45 > 0:20:46and the moons are real bright.
0:20:46 > 0:20:50It comes alive. It becomes a real planet when you look at it through a telescope.
0:20:52 > 0:20:56Galileo was the first astronomer to point a telescope at Jupiter,
0:20:56 > 0:20:59more than 400 years ago,
0:20:59 > 0:21:02and no-one ever questioned that Jupiter will always be,
0:21:02 > 0:21:04and has always been, in that same orbit.
0:21:06 > 0:21:07Jupiter, right now...
0:21:09 > 0:21:11..looks the same as it would have looked for Galileo.
0:21:11 > 0:21:14It's a little bigger and brighter through this great telescope,
0:21:14 > 0:21:15but it's the same Jupiter,
0:21:15 > 0:21:18so if I came back tomorrow night, it'd look the same.
0:21:18 > 0:21:20So that's the view -
0:21:20 > 0:21:23the planets that we look at now seem like they never change.
0:21:23 > 0:21:25And why would they change?
0:21:27 > 0:21:30This was the bedrock of our understanding -
0:21:30 > 0:21:32that the planets' orbits are fixed.
0:21:37 > 0:21:41The first hint of something odd came 35 years ago
0:21:41 > 0:21:44from astronomers trying to calculate how the solar system formed.
0:21:46 > 0:21:49They kept getting a strange result.
0:21:49 > 0:21:53Some of those calculations were suggesting that it was possible
0:21:53 > 0:21:56that a planet like Jupiter could have been moved around.
0:21:57 > 0:22:01It was a result so crazy that it was totally ignored.
0:22:01 > 0:22:03So if you built a model to try to understand
0:22:03 > 0:22:05some of the events of the early solar system
0:22:05 > 0:22:09and your model is telling you that planets could have migrated or moved,
0:22:09 > 0:22:10that Jupiter could have moved,
0:22:10 > 0:22:13then it was telling you that you probably made a mistake.
0:22:13 > 0:22:17So the idea of planet migration, it was just never possible.
0:22:17 > 0:22:18Just didn't seem possible.
0:22:23 > 0:22:29But in 1995, astronomers were forced to face up to the impossible.
0:22:31 > 0:22:35The mystery began to unravel when dramatic evidence was uncovered
0:22:35 > 0:22:37from somewhere completely unexpected.
0:22:54 > 0:22:56Astronomer Chris Watson
0:22:56 > 0:22:59is searching for the weirdest places in the galaxy.
0:23:01 > 0:23:05He's a planet hunter, one of a growing band of astronomers
0:23:05 > 0:23:08involved in the hunt for exoplanets -
0:23:08 > 0:23:11alien worlds circling around other stars.
0:23:13 > 0:23:15It's one of the hottest fields in astronomy.
0:23:17 > 0:23:22Up until now, or until recently, we've only had one planetary system
0:23:22 > 0:23:24that we could study, and that was the solar system,
0:23:24 > 0:23:26the planets around the sun.
0:23:26 > 0:23:29And there are about 100 billion stars in our galaxy
0:23:29 > 0:23:32and there are about 100 billion galaxies in the universe,
0:23:32 > 0:23:34and we could only study one.
0:23:36 > 0:23:39But the study of planets and planetary systems exploded
0:23:39 > 0:23:42with the amazing discovery in 1995
0:23:42 > 0:23:45of a planet orbiting around another star.
0:23:47 > 0:23:50Less than 20 years ago, we first found another planet
0:23:50 > 0:23:52around another star that's like our sun
0:23:52 > 0:23:55and that was a dramatic breakthrough.
0:23:55 > 0:23:59And we now know of over 1,000 planets.
0:23:59 > 0:24:00And they're very strange -
0:24:00 > 0:24:03these are nothing like our solar system
0:24:03 > 0:24:06and, in some cases, I think really science fact
0:24:06 > 0:24:09could be a lot weirder than science fiction.
0:24:11 > 0:24:15The planets they have been finding look stranger than anyone imagined.
0:24:21 > 0:24:24We've found planets around binary stars,
0:24:24 > 0:24:27where you actually have two stars orbiting each other,
0:24:27 > 0:24:29a bit like Tatooine off of Star Wars.
0:24:29 > 0:24:33That would be a magical world if you had a habitable planet there,
0:24:33 > 0:24:35because you would imagine
0:24:35 > 0:24:37you'd actually have two shadows on everything
0:24:37 > 0:24:39and two stars in the sky, as well.
0:24:46 > 0:24:50None of the planets found have been remotely like home.
0:24:52 > 0:24:56Recently, a planet was discovered a little bit bigger than Earth,
0:24:56 > 0:24:58but incredibly close to its star.
0:24:58 > 0:25:02It's probably a rocky world, but it's so hot
0:25:02 > 0:25:04it would actually be molten,
0:25:04 > 0:25:07so...it'd be this, but the actual lava on it.
0:25:08 > 0:25:12And most puzzling of all are the largest planets,
0:25:12 > 0:25:14so weird as to seem impossible.
0:25:16 > 0:25:19What was strange when we first discovered these planets
0:25:19 > 0:25:21is they were massive worlds,
0:25:21 > 0:25:24they were gas giants much like Jupiter.
0:25:24 > 0:25:27But these were much, much closer to their parent stars.
0:25:30 > 0:25:35Jupiter lives out in the cold outer reaches of our solar system,
0:25:35 > 0:25:37taking 12 years to orbit the sun.
0:25:38 > 0:25:42But these alien giants were found in the fiery heat,
0:25:42 > 0:25:43right next to their star,
0:25:43 > 0:25:47hurtling around in crazy orbits of just a few days.
0:25:48 > 0:25:51They were nicknamed "hot Jupiters".
0:25:55 > 0:25:57They're right up against the host star,
0:25:57 > 0:25:59and it's amazing, they really...
0:25:59 > 0:26:01At the time, we thought, "How did they get there?
0:26:01 > 0:26:03"They really shouldn't be there."
0:26:03 > 0:26:06Other scientists were thinking, "Well, you're a bit crazy,
0:26:06 > 0:26:09"these Jupiters should not be that close to the star."
0:26:12 > 0:26:15Everyone was baffled by the existence of hot Jupiters.
0:26:15 > 0:26:19They were planets that, quite simply, shouldn't exist.
0:26:22 > 0:26:26In theory, the only place you could build a gas giant
0:26:26 > 0:26:30would be out in the cold, far away from a star,
0:26:30 > 0:26:32because that's the only place
0:26:32 > 0:26:34you can find the necessary ingredients.
0:26:36 > 0:26:39What I'm holding in my hand is a lump of dry ice,
0:26:39 > 0:26:42and this represents the building blocks
0:26:42 > 0:26:44of planets like Jupiter.
0:26:44 > 0:26:48And this is fine - it's quite happy out here,
0:26:48 > 0:26:51far away from the fire that represents our sun
0:26:51 > 0:26:55or any other star that one of these gas giants might be forming round.
0:26:55 > 0:27:01But look at what happens when I bring it closer to the fire.
0:27:04 > 0:27:07Too close to the star and the ice just turns to gas.
0:27:08 > 0:27:12And without ice you can't build a gas giant.
0:27:12 > 0:27:16So there's very little left after just a few minutes.
0:27:16 > 0:27:19And what this means
0:27:19 > 0:27:24is that gas giants can't form close to the star.
0:27:24 > 0:27:28The building blocks just cannot exist that close.
0:27:28 > 0:27:30They have to have formed further away
0:27:30 > 0:27:32where the raw materials can exist.
0:27:35 > 0:27:39If these hot Jupiters couldn't have formed where we find them,
0:27:39 > 0:27:41it could only mean one thing.
0:27:41 > 0:27:44So we think that, in actual fact,
0:27:44 > 0:27:48these gas giants form further out,
0:27:48 > 0:27:50then they actually move towards the star,
0:27:50 > 0:27:53they actually migrate inwards.
0:27:56 > 0:27:59The planets are on the move.
0:28:02 > 0:28:05The discovery that planets could change orbit
0:28:05 > 0:28:07was a shocking revelation.
0:28:07 > 0:28:11It turned the world of planetary science on its head.
0:28:12 > 0:28:15The implications of a planet the size of Jupiter
0:28:15 > 0:28:18roaming freely around a planetary system
0:28:18 > 0:28:20could be devastating.
0:28:23 > 0:28:28Over recent years, the search for exoplanets has exploded.
0:28:30 > 0:28:33So here we are, nearly 2,400 metres up
0:28:33 > 0:28:37on the volcanic island of La Palma.
0:28:37 > 0:28:42What you can see before you are suites of professional telescopes
0:28:42 > 0:28:46and what we're going to do is we're going to use one of these
0:28:46 > 0:28:50telescopes to actually look at planets orbiting another star.
0:28:51 > 0:28:54So, this cloud can be a bit of a problem.
0:28:54 > 0:28:57Normally it's not so cloudy, but we are in the depths of winter
0:28:57 > 0:28:59and this is actually quite local cloud.
0:29:01 > 0:29:0620 years ago, all these telescopes were busy looking at stars.
0:29:06 > 0:29:10Now, increasingly, many are focusing on planets.
0:29:11 > 0:29:16There's quite a few telescopes here, and probably every night
0:29:16 > 0:29:20there's some project related to extrasolar planets going on.
0:29:20 > 0:29:25It is a really rich, blossoming field of astronomy.
0:29:29 > 0:29:32And, provided the clouds clear,
0:29:32 > 0:29:35tonight Chris will be pointing his telescope
0:29:35 > 0:29:37at an exoplanet called WASP-84 b.
0:29:39 > 0:29:41These clouds will clear.
0:29:47 > 0:29:49But, even with clear skies,
0:29:49 > 0:29:52spotting alien planets is no easy matter.
0:30:04 > 0:30:09To see other planets in our solar system from Earth is pretty easy.
0:30:09 > 0:30:11So this candle represents our sun
0:30:11 > 0:30:15and if I pop down this little rock, representing a planet,
0:30:15 > 0:30:17you can clearly see the reflected sunlight.
0:30:19 > 0:30:22But even the nearest stars are so far away
0:30:22 > 0:30:25that the reflected light from the planets gets completely lost.
0:30:27 > 0:30:30So, now we have our star, much further away,
0:30:30 > 0:30:32and if I put my planet down,
0:30:32 > 0:30:35while it's still reflecting the starlight,
0:30:35 > 0:30:36because you're so far away,
0:30:36 > 0:30:39the reflected light is actually drowned out
0:30:39 > 0:30:41in the glare of the star itself.
0:30:45 > 0:30:47Because the planets are so hard to see,
0:30:47 > 0:30:50astronomers have found other ways to detect them.
0:30:50 > 0:30:54One of the best ways is actually to watch and see
0:30:54 > 0:30:58if the planet actually crosses in front of the star.
0:30:58 > 0:31:03So, if we were an alien civilisation looking back at our solar system,
0:31:03 > 0:31:07we happen to catch Jupiter transiting the face of our sun,
0:31:07 > 0:31:11we would see a 1% dip in the sunlight.
0:31:11 > 0:31:15For a planet a lot smaller, like the Earth,
0:31:15 > 0:31:19that dip is much, much smaller - it's minuscule.
0:31:19 > 0:31:22And that's why it's so, so difficult to detect these.
0:31:28 > 0:31:30But techniques have improved dramatically
0:31:30 > 0:31:33and now, for astronomers like Chris Watson,
0:31:33 > 0:31:36planet-hunting is all part of a night's work.
0:31:44 > 0:31:48This is our telescope, Telescopio Nazionale Galileo,
0:31:48 > 0:31:50and this will be our baby for the night.
0:31:54 > 0:31:56The skies are clearing beautifully,
0:31:56 > 0:31:59so I think we're in for a really nice night ahead.
0:32:01 > 0:32:04Thanks to ground-based telescopes like this,
0:32:04 > 0:32:07as well as space telescopes like NASA's Kepler mission,
0:32:07 > 0:32:10thousands of planets have now been found.
0:32:12 > 0:32:15And not just planets, but entire planetary systems.
0:32:23 > 0:32:25So this is the Kepler Orrery,
0:32:25 > 0:32:29which shows the orbits and the sizes of planets.
0:32:29 > 0:32:33So these are candidates that the Kepler space mission has found.
0:32:33 > 0:32:36So these are transiting planets.
0:32:36 > 0:32:39They don't, however, look much like we'd expect.
0:32:39 > 0:32:41And up there, on the top left,
0:32:41 > 0:32:47you can see the orbits of the four innermost planets of our solar system
0:32:47 > 0:32:49from Mercury out to Mars.
0:32:49 > 0:32:54What you can see is the huge diversity
0:32:54 > 0:32:56of all the different planetary systems.
0:32:59 > 0:33:03Each set of rings shows a different planetary system
0:33:03 > 0:33:06and each blob, a different planet, with its size and orbit.
0:33:08 > 0:33:10They break every rule in the book
0:33:10 > 0:33:12and make us look like the odd one out.
0:33:14 > 0:33:18So we have large gas giant planets in there,
0:33:18 > 0:33:22and then you can see the really short period,
0:33:22 > 0:33:25really weird solar systems.
0:33:26 > 0:33:29They really don't look anything like our own solar system.
0:33:29 > 0:33:35Some of these planets actually have orbits of just a few hours.
0:33:36 > 0:33:40There's even systems spiralling around multiple planets in here.
0:33:43 > 0:33:45That one's weird. What's going on here?
0:33:57 > 0:34:01Who knows what we might discover in this rich smorgasbord of planets?
0:34:04 > 0:34:05It is ridiculous, actually.
0:34:05 > 0:34:07HE CHUCKLES
0:34:07 > 0:34:09What is going on with that?
0:34:12 > 0:34:13Extraordinary worlds.
0:34:16 > 0:34:18Some may host life.
0:34:21 > 0:34:23Our exploration of these alien worlds
0:34:23 > 0:34:26is only just beginning,
0:34:26 > 0:34:29but already they're revealing some incredible secrets.
0:34:36 > 0:34:39Tonight, Chris and his team are training the telescope
0:34:39 > 0:34:42on a star they known has a hot Jupiter orbiting it.
0:34:42 > 0:34:45INDISTINCT CONVERSATION
0:34:48 > 0:34:51They hope to reveal just how devastating
0:34:51 > 0:34:53a migrating gas giant could be.
0:34:55 > 0:34:58So this star that we're looking at, WASP-84,
0:34:58 > 0:35:01was actually discovered to have a transiting planet around it.
0:35:01 > 0:35:04That transiting planet, we know at the moment,
0:35:04 > 0:35:08is about a little bit less massive than Jupiter.
0:35:08 > 0:35:10And we know its orbital period,
0:35:10 > 0:35:12so its year is about eight-and-a-half days,
0:35:12 > 0:35:16and we're going to follow it as it transits the star.
0:35:16 > 0:35:19A planet the size of Jupiter
0:35:19 > 0:35:22orbiting its star once every eight days
0:35:22 > 0:35:24is already pretty weird.
0:35:24 > 0:35:28But some of these alien worlds have even weirder orbits than that.
0:35:28 > 0:35:32- What's the air mass with that, then? - About 1.25?
0:35:32 > 0:35:35'We would expect the planet and the star'
0:35:35 > 0:35:37to be spinning in the same way.
0:35:37 > 0:35:41But we see quite a few systems where that is just not the case.
0:35:42 > 0:35:46Some of these planets are going completely the wrong way.
0:35:47 > 0:35:49If the star is spinning clockwise,
0:35:49 > 0:35:52the planet is spinning anti-clockwise.
0:35:52 > 0:35:55- 169, we're talking about. - Yeah, it's about...
0:35:55 > 0:35:57A planet orbiting in the wrong direction
0:35:57 > 0:35:59is a sign of some truly cataclysmic event.
0:36:01 > 0:36:04And tonight, as it passes in front of its star,
0:36:04 > 0:36:07Chris will be able to analyse the orbit of WASP-84 b.
0:36:08 > 0:36:11'The purpose of these observations is actually'
0:36:11 > 0:36:15to see whether we have a nicely aligned system -
0:36:15 > 0:36:17a bit like the planets we have in our solar system,
0:36:17 > 0:36:22where the star spins in the same direction as the planet orbits.
0:36:22 > 0:36:26Or do we have something that would be the smoking gun
0:36:26 > 0:36:29of a really violent interaction
0:36:29 > 0:36:34which has maybe scattered that planet into one of these weird orbits?
0:36:34 > 0:36:35So, perhaps over the poles,
0:36:35 > 0:36:40or actually spinning in the opposite direction to that of the star.
0:36:41 > 0:36:43But the big question is
0:36:43 > 0:36:46what could be the cause of such planetary upheaval?
0:36:46 > 0:36:48Whoa!
0:36:52 > 0:36:54After following the transit through the night,
0:36:54 > 0:36:58Chris has the verdict on Planet WASP-84 b.
0:37:00 > 0:37:04So, the transit's finished. We've had a quick look at the data
0:37:04 > 0:37:09and what we've found has actually taken us a bit by surprise.
0:37:09 > 0:37:12We thought that this planet system would be misaligned.
0:37:12 > 0:37:14Now that we've had a look at the data,
0:37:14 > 0:37:16it looks as though it's actually aligned.
0:37:18 > 0:37:21WASP-84 b turns out to be orbiting the right way.
0:37:23 > 0:37:25But Chris has found many of these hot Jupiters
0:37:25 > 0:37:28that are travelling in completely the wrong direction.
0:37:30 > 0:37:34It's evidence of how, in migrating, they must have caused havoc.
0:37:36 > 0:37:38With these very strange orbits,
0:37:38 > 0:37:42it looks as though it's been a very violent process.
0:37:42 > 0:37:44To actually take one of these planets
0:37:44 > 0:37:48and just chuck it into a different orbit,
0:37:48 > 0:37:50that's very violent.
0:37:50 > 0:37:54One of the easiest ways to do that is to have a collision.
0:37:54 > 0:37:58Take two planets, interaction between them,
0:37:58 > 0:38:00and you can eject one planet
0:38:00 > 0:38:04and fling the other planet really close in to the star.
0:38:09 > 0:38:13These giant gas planets are the bully of the playground.
0:38:14 > 0:38:17They have the power to throw other planets around
0:38:17 > 0:38:19like a game of cosmic pinball.
0:38:23 > 0:38:26Beasts the size of Jupiter are so vast
0:38:26 > 0:38:28they can eject entire planets from the system.
0:38:32 > 0:38:35They can launch them into crazy polar orbits.
0:38:39 > 0:38:42They even have the power to destroy entire worlds.
0:38:49 > 0:38:53A planet like Jupiter, the mass of Jupiter, the size of it,
0:38:53 > 0:38:55just dominates planetary systems,
0:38:55 > 0:38:58and it's got the power to really decide
0:38:58 > 0:39:01the fate of the other planets.
0:39:03 > 0:39:07I think we'd be quite glad there's not a hot Jupiter in our system.
0:39:07 > 0:39:09We wouldn't be seeing this.
0:39:18 > 0:39:21We've discovered other systems where planets migrate
0:39:21 > 0:39:23and hot Jupiters cause havoc.
0:39:24 > 0:39:26But what about our own solar system?
0:39:28 > 0:39:33Our planets certainly seem fixed in their rigid, clockwork orbits.
0:39:43 > 0:39:49Our earth has been the same distance from the sun for 4.5 billion years.
0:39:50 > 0:39:52Long enough to create an atmosphere,
0:39:52 > 0:39:54build mountains,
0:39:54 > 0:39:55and for life to evolve.
0:39:57 > 0:40:01But the evidence from other planetary systems now means
0:40:01 > 0:40:05a complete rethink on how and where our planets formed.
0:40:05 > 0:40:09When we started discovering planets around other stars,
0:40:09 > 0:40:12we started finding planets in completely unexpected places,
0:40:12 > 0:40:15places we never thought could possibly form a planet.
0:40:15 > 0:40:18We had to go back to the drawing board and say,
0:40:18 > 0:40:19"Wow, planets can move.
0:40:19 > 0:40:22"Planets can really move. Maybe that happened here."
0:40:26 > 0:40:29It's a big leap, and to make that leap
0:40:29 > 0:40:33and say things might have been completely unstable,
0:40:33 > 0:40:38totally chaotic for a time period, that's really hard to imagine.
0:40:38 > 0:40:41But that's the leap that we need to take.
0:40:43 > 0:40:47The crazy results that suggest Jupiter might have changed orbit
0:40:47 > 0:40:50might not be mistakes after all.
0:40:51 > 0:40:55Instead, migration could be the key that unlocks
0:40:55 > 0:40:58many of the mysteries of how our solar system came to be.
0:40:58 > 0:41:01Now that we've taken this tool of planetary migration that we started
0:41:01 > 0:41:04to understand by looking at planets around other stars,
0:41:04 > 0:41:07we've realised that it's absolutely critical to understand
0:41:07 > 0:41:10how our solar system formed and evolved.
0:41:10 > 0:41:13And central to it all is mighty Jupiter.
0:41:13 > 0:41:16Certainly in our planetary system, Jupiter is the key.
0:41:16 > 0:41:19It's over three hundred times more massive than the Earth,
0:41:19 > 0:41:22so Jupiter wins.
0:41:22 > 0:41:23Jupiter decides what happens.
0:41:26 > 0:41:31The inescapable truth seems to be that planets move.
0:41:31 > 0:41:34And, if it can happen in exoplanetary systems,
0:41:34 > 0:41:36it can happen in ours.
0:41:36 > 0:41:38If we want to make a model that explains
0:41:38 > 0:41:40how our solar system came to be,
0:41:40 > 0:41:44we have to break the brass rods and set the planets free.
0:41:48 > 0:41:53Once we accept the idea that the planets can move,
0:41:53 > 0:41:57we can begin to explain some of the unsolved mysteries of the solar system.
0:42:02 > 0:42:06In particular, why Mars is so small
0:42:06 > 0:42:09and the curious composition of the asteroid belt.
0:42:28 > 0:42:32Kevin Walsh has developed a model of the early solar system
0:42:32 > 0:42:34that involves a wild dance of the planets.
0:42:38 > 0:42:40It's an intricate and chaotic dance,
0:42:40 > 0:42:43and if it had gone slightly differently
0:42:43 > 0:42:46it could have stopped our developing solar system in its tracks.
0:42:48 > 0:42:52In his model, Jupiter takes a wild ride through the solar system.
0:42:58 > 0:43:01It takes us right back to the moment of birth,
0:43:01 > 0:43:04when Jupiter had just formed from the cloud of gas.
0:43:06 > 0:43:08The key is that, though Jupiter is really big,
0:43:08 > 0:43:10it's 300 times the mass of the earth,
0:43:10 > 0:43:13the gas disc around the sun was much more massive,
0:43:13 > 0:43:16so the gas can actually push Jupiter in towards the sun.
0:43:17 > 0:43:22As soon as it was born, Jupiter began to migrate inwards.
0:43:22 > 0:43:25Over the course of half a million years,
0:43:25 > 0:43:27it spiralled in towards the sun.
0:43:28 > 0:43:31It was on its way to becoming a hot Jupiter.
0:43:33 > 0:43:36So, the idea that you could form something as big as Jupiter
0:43:36 > 0:43:38and have it pushed inward by the gas disc
0:43:38 > 0:43:40actually makes a fair amount of sense,
0:43:40 > 0:43:42because we see it, we see it all over.
0:43:46 > 0:43:49But something stopped Jupiter from crashing into the sun
0:43:49 > 0:43:51or ending up as a hot Jupiter.
0:43:52 > 0:43:55So, if it formed and started migrating inwards,
0:43:55 > 0:43:57there must have been a mechanism to stop it
0:43:57 > 0:44:00and bring it back out to the outer part of the solar system.
0:44:00 > 0:44:03We think the key to stop its inward migration, to keep it
0:44:03 > 0:44:08from going all the way in towards the sun is the presence of Saturn.
0:44:08 > 0:44:12While Jupiter was on its wild ride, Saturn was born.
0:44:12 > 0:44:16Saturn is also growing. It's going through the same process Jupiter did.
0:44:16 > 0:44:19It's building a big core and it's getting really massive,
0:44:19 > 0:44:23and once it gets really massive as well it can move in the disc also.
0:44:23 > 0:44:26And it too began spiralling in towards the sun.
0:44:26 > 0:44:30So as Saturn is racing inwards, it gets very close to Jupiter
0:44:30 > 0:44:33and they actually get close enough that they get locked in a resonance
0:44:33 > 0:44:35where their orbital periods are closely aligned
0:44:35 > 0:44:38and they interact very closely gravitationally.
0:44:38 > 0:44:41Now, when these two get really close
0:44:41 > 0:44:44it actually stops Jupiter's inward migration.
0:44:44 > 0:44:48The two planets were involved in a kind of gravitational dance.
0:44:48 > 0:44:52And, as they came close, Jupiter changed direction
0:44:52 > 0:44:54and was flung back to the outer solar system...
0:44:56 > 0:44:59..just like a sailing ship changing course in a grand tack.
0:45:03 > 0:45:06So this theory called the grand tack is called that
0:45:06 > 0:45:09because our planets are moving inwards
0:45:09 > 0:45:12and they get really close and they stop and they turn and they go back
0:45:12 > 0:45:15outwards, and it's kind of like a sailboat tacking across the wind.
0:45:15 > 0:45:19Jupiter's wild ride could explain two key mysteries -
0:45:19 > 0:45:22first, why Mars is so small.
0:45:22 > 0:45:25So, when Jupiter migrates inwards
0:45:25 > 0:45:29it kind of snowploughs all the rocky material it sees,
0:45:29 > 0:45:31it snowploughs it and pushes it inwards.
0:45:32 > 0:45:34Much of the dust and rocky debris
0:45:34 > 0:45:38that would have gone on to build Mars got pushed out of the way.
0:45:39 > 0:45:42So, by Jupiter coming in and clearing out all of this
0:45:42 > 0:45:45material on its way, it kind of reduces the total
0:45:45 > 0:45:49amount of material that Mars can feed on to grow,
0:45:49 > 0:45:52and so Mars ends up kind of being starved of rocky material
0:45:52 > 0:45:55and only grows to be a tenth the mass of the earth.
0:45:57 > 0:46:02And this explains why Mars is the planetary runt we see today.
0:46:06 > 0:46:10The theory also explains why the asteroid belt has an icy ring
0:46:10 > 0:46:13and a rocky ring so close together.
0:46:22 > 0:46:25During its travels, Jupiter scattered everything in its path.
0:46:28 > 0:46:31It threw rocks from the inner part of the solar system outwards...
0:46:35 > 0:46:38..and ice from the outer reaches inwards...
0:46:40 > 0:46:42..leaving the two distinct bands we see today.
0:46:44 > 0:46:47That's how we end up with two different types of material
0:46:47 > 0:46:49sitting on top of each other in the middle of the asteroid belt
0:46:49 > 0:46:51in a very small region.
0:46:51 > 0:46:55So Jupiter's wild ride could explain two key mysteries -
0:46:55 > 0:46:58the size of Mars and the composition of the asteroid belt.
0:47:00 > 0:47:03And if it had travelled any further in
0:47:03 > 0:47:07the earth itself may have become a very different type of planet.
0:47:14 > 0:47:16But the birth of the solar system
0:47:16 > 0:47:19wasn't the only turbulent time in its history.
0:47:21 > 0:47:25About 500 million years later, 4 billion years ago,
0:47:25 > 0:47:28the solar system entered its teenage years -
0:47:28 > 0:47:32an intense period of trouble, chaos and uncertainty.
0:47:34 > 0:47:37It's a period of turbulence that could explain
0:47:37 > 0:47:39two further mysteries...
0:47:41 > 0:47:43..the craters on the moon
0:47:43 > 0:47:45and the birth of Uranus and Neptune.
0:47:51 > 0:47:54There are some mysteries when we look around the solar system,
0:47:54 > 0:47:57where the theories really don't match what we see.
0:47:59 > 0:48:02If we just take a bunch of small icy objects
0:48:02 > 0:48:05from which Uranus and Neptune were made,
0:48:05 > 0:48:08put them out in the outer part of the solar system in our computer
0:48:08 > 0:48:13models and watch how they grow, it turns out they can't grow at all.
0:48:15 > 0:48:19A couple of billion miles away from these ice giants, on the moon,
0:48:19 > 0:48:21there's a clue that Hal Levison believes
0:48:21 > 0:48:24could help solve the puzzle.
0:48:24 > 0:48:27In fact, the moon is covered in clues.
0:48:28 > 0:48:30So, when you look at the moon,
0:48:30 > 0:48:34some of these biggest crater impact basins, like here and here,
0:48:34 > 0:48:37all formed in a very short period of time
0:48:37 > 0:48:40that we call the Late Heavy Bombardment.
0:48:40 > 0:48:45It came roughly 500 million years after the birth of the solar system,
0:48:45 > 0:48:47when all the planets had long formed.
0:48:49 > 0:48:51And all of a sudden, out of the blue,
0:48:51 > 0:48:57the moon got clobbered by big objects coming in and hitting it.
0:48:57 > 0:49:02And that indicates that you had this very violent upheaval
0:49:02 > 0:49:07and the only way that we can form an influx like this, stuff raining
0:49:07 > 0:49:14down in onto the moon, is through changing the orbits of the planets.
0:49:15 > 0:49:17To account for this violent upheaval,
0:49:17 > 0:49:20Hal and some colleagues devised a new model.
0:49:21 > 0:49:24It explains why we now see Uranus and Neptune
0:49:24 > 0:49:27in places they can't possibly have formed.
0:49:28 > 0:49:31We think what happened is they formed closer to the sun
0:49:31 > 0:49:34and got delivered to where we see them today.
0:49:36 > 0:49:40Uranus and Neptune must have formed much closer in,
0:49:40 > 0:49:42where there was plenty of icy material,
0:49:42 > 0:49:46and beyond them was a neat disc of icy, comet-like objects.
0:49:48 > 0:49:51But this was not a stable system,
0:49:51 > 0:49:56and a series of small changes led to a period of utter chaos.
0:49:56 > 0:49:59These objects leak out of this disc,
0:49:59 > 0:50:03get gravitationally scattered by all these planets,
0:50:03 > 0:50:05like billiard balls going around,
0:50:05 > 0:50:08and get eventually ejected to interstellar space by Jupiter.
0:50:12 > 0:50:16That causes the planets' orbits to slightly spread over time
0:50:16 > 0:50:19and what we think happened is that Jupiter
0:50:19 > 0:50:24and Saturn got to the point where Jupiter goes around the sun
0:50:24 > 0:50:28exactly twice for every time Saturn moves around the sun.
0:50:28 > 0:50:33And that allows their tugs on one another to become much stronger
0:50:33 > 0:50:37and as a result, Jupiter and Saturn get a little excited,
0:50:37 > 0:50:40their orbits become less circular and more inclined
0:50:40 > 0:50:43and they start getting... sort of tugging on one another.
0:50:43 > 0:50:47Uranus and Neptune, which are much smaller than Jupiter and Saturn, feel
0:50:47 > 0:50:52that fight, feel that tension and as a result, their orbits just go nuts.
0:50:54 > 0:50:57In a sudden period of chaos, Uranus and Neptune
0:50:57 > 0:51:00were flung out into the orbits we see today.
0:51:02 > 0:51:05The ice giants, Uranus and Neptune, get scattered into this disc
0:51:05 > 0:51:08that existed outside their orbits, and that thing went kaplooie.
0:51:13 > 0:51:17Vast lumps of ice were scattered everywhere,
0:51:17 > 0:51:19raining into the inner solar system
0:51:19 > 0:51:21and bombarding the earth and the moon.
0:51:25 > 0:51:29Every square inch of the earth at one time got hit
0:51:29 > 0:51:33due to this instability, so it was not a very safe place to be.
0:51:35 > 0:51:39We had this view that the solar system was this nice clock
0:51:39 > 0:51:42and things just moved around in nice regular ways.
0:51:43 > 0:51:47What this new model shows is a real paradigm shift.
0:51:47 > 0:51:53It says that the solar system is not this nice, safe, quiescent place,
0:51:53 > 0:51:56but can go through periods of intense violence.
0:51:58 > 0:52:02This new model of the solar system is now dynamic and turbulent.
0:52:04 > 0:52:09The prime mover in all this upheaval is our playground bully, Jupiter.
0:52:16 > 0:52:21Such a bully, in fact, that David Nesvorny believes that Jupiter
0:52:21 > 0:52:24may have been responsible for the ultimate planetary crime.
0:52:26 > 0:52:29He ran the new model over and over again
0:52:29 > 0:52:31with slightly different starting conditions.
0:52:33 > 0:52:38I ran about 3,000, 4,000 models like this,
0:52:38 > 0:52:41just playing with the initial state
0:52:41 > 0:52:44and at a time, I considered the standard theory,
0:52:44 > 0:52:48which was that the outer solar system had four planets.
0:52:49 > 0:52:52His results were alarming -
0:52:52 > 0:52:55change the starting conditions even slightly
0:52:55 > 0:52:57and the solar system looks very different.
0:52:59 > 0:53:02Frequently, what happened in my simulation was that Jupiter
0:53:02 > 0:53:05just slingshots Uranus and Neptune from the solar system
0:53:05 > 0:53:09and they ended somewhere in interstellar space.
0:53:09 > 0:53:11So that wasn't right.
0:53:11 > 0:53:13Obviously not right.
0:53:15 > 0:53:18Then David had a radical idea.
0:53:18 > 0:53:21If Neptune and Uranus didn't get flung out of the solar system,
0:53:21 > 0:53:23maybe something else did.
0:53:25 > 0:53:29I couldn't quite fit the solar system, how it looks like today.
0:53:30 > 0:53:34So I was thinking and thinking and thinking, and then I thought,
0:53:34 > 0:53:37"How about if the solar system had an extra planet?"
0:53:41 > 0:53:44He started investigating the possibility
0:53:44 > 0:53:47that an entire planet might have gone missing.
0:53:49 > 0:53:53As ever, the prime suspect was Jupiter.
0:53:56 > 0:53:59So, now I am pointing at Jupiter,
0:53:59 > 0:54:02so I can see the disc of Jupiter,
0:54:02 > 0:54:07and then, nicely aligned, four giant moons.
0:54:08 > 0:54:10It has a huge influence...
0:54:11 > 0:54:14..and could have had an even bigger influence in the past.
0:54:19 > 0:54:22It may even have been able to eject an entire planet
0:54:22 > 0:54:24from our solar system.
0:54:25 > 0:54:27This is the solar system.
0:54:27 > 0:54:29The sun is in the middle...
0:54:30 > 0:54:32..then we have the terrestrial planets.
0:54:32 > 0:54:36Then there's the asteroid belt and the outer planets.
0:54:37 > 0:54:41To get the arrangement of planets we see today, David thinks
0:54:41 > 0:54:45we once had an extra ice giant but it was thrown out by Jupiter.
0:54:46 > 0:54:53I start playing with the possibility that we had an additional planet.
0:54:53 > 0:54:57So the best case I have found was when I placed
0:54:57 > 0:55:02this third ice giant between Saturn and Uranus initially,
0:55:02 > 0:55:04somewhere here.
0:55:04 > 0:55:07What happens in this case is that during the instability,
0:55:07 > 0:55:11this planet evolves, has close encounters with Jupiter
0:55:11 > 0:55:15and Saturn and gets ejected from the solar system.
0:55:27 > 0:55:31The ejected planet may have been a sacrificial lamb
0:55:31 > 0:55:34that saved us from Jupiter's destructive powers
0:55:34 > 0:55:37and allowed our planets to settle in the pattern we see today.
0:55:40 > 0:55:43So, what became of our missing lonely planet?
0:55:44 > 0:55:49In the simulations I have, the planet is ejected from the solar system
0:55:49 > 0:55:52with a speed of about 1km per second.
0:55:54 > 0:55:59But this happened about four billion years ago, so do your math.
0:56:01 > 0:56:03It will end up very far from the solar system,
0:56:03 > 0:56:07so today it can be almost anywhere in the galaxy.
0:56:20 > 0:56:2420 years ago, the mystery of the solar system began to unravel.
0:56:26 > 0:56:29Evidence from alien worlds shattered the long-held view
0:56:29 > 0:56:32that our planets have fixed orbits.
0:56:34 > 0:56:37It led to a whole new understanding of a turbulent and dynamic past...
0:56:39 > 0:56:43..which makes us wonder, might things have turned out differently?
0:56:43 > 0:56:46The solar system could have done a lot of different things,
0:56:46 > 0:56:49it could have evolved in a lot of different ways.
0:56:49 > 0:56:51What we see in our own solar system
0:56:51 > 0:56:57is the result of a lot of unlikely or random events,
0:56:57 > 0:56:59and so our solar system is unique.
0:57:03 > 0:57:07Ending up with a stable system of planets was just a fluke,
0:57:07 > 0:57:09a lucky roll of the dice.
0:57:10 > 0:57:12It's amazing we survived at all.
0:57:12 > 0:57:17Getting an earth where we have our earth today was not a given
0:57:17 > 0:57:19when this whole solar system started.
0:57:19 > 0:57:22It took all these series of events to get a rocky planet
0:57:22 > 0:57:25of this size at this distance with this amount of water
0:57:25 > 0:57:28to build the earth that we live on today.
0:57:30 > 0:57:34The fate of the entire solar system, including the earth,
0:57:34 > 0:57:38was defined above all by the movements of our gas giant, Jupiter.
0:57:40 > 0:57:43If Jupiter's orbit moved differently,
0:57:43 > 0:57:46if Jupiter moved into the inner solar system,
0:57:46 > 0:57:49then it's unlikely that the earth would be here.
0:57:51 > 0:57:54Of all the planetary systems so far discovered,
0:57:54 > 0:57:58it seems we are the only one with the lucky roll of the dice.
0:57:59 > 0:58:02You might think that maybe the solar system that we have here
0:58:02 > 0:58:05is actually the oddball and that the natural order
0:58:05 > 0:58:09of things are these other systems that we think of as weird.
0:58:11 > 0:58:13And, if we are so unusual,
0:58:13 > 0:58:17will we ever find anywhere else in the universe so welcoming to life?
0:58:19 > 0:58:23Even though our solar system might be...let's say one in a million -
0:58:23 > 0:58:26that may seem like a really small number -
0:58:26 > 0:58:29there are 100 billion stars in the galaxy.
0:58:29 > 0:58:32So even something as unlikely as our solar system,
0:58:32 > 0:58:35there may be lots of them around.