Secrets of the Solar System Horizon


Secrets of the Solar System

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There are some mysteries when we look around the solar system,

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where the theories really don't match what we see.

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Science fact can be a lot weirder than science fiction.

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We started finding planets

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in places we'd never thought could possibly form a planet.

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We had to go back to the drawing board.

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How do you make solar systems? How do you make planets?

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It's as if somebody took the solar system,

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picked it up and shook it real hard.

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Our planets might have moved.

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They might have moved a lot.

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All of a sudden, everything changed.

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It's changed the way we look at

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almost every process in the solar system.

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Sometimes the blood splattered on the wall

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can tell you more about what happened

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than the body lying on the floor.

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The Royal Observatory in Greenwich

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is the historical home of British astronomy.

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Discoveries have been made here and mysteries unravelled.

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It is also home to some unique astronomical treasures.

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This is an orrery,

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a clockwork model of the solar system,

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and for most of the last four centuries

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this has been the way we think about the planets in the solar system.

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Of course, the scale is all wrong.

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But it clearly shows the traditional view of the planets

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and their fixed orbits.

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In the centre we have the sun

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and then, around it, we have the four rocky planets,

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tiny Mercury rushing around in the middle,

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Venus, the earth with the moon going around it,

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and then Mars.

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And outside of the inner planets,

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we have the gas giants,

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Jupiter, the largest planet of all,

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and then Saturn with its beautiful ring system.

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And then the two outermost planets, Uranus and Neptune.

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Astronomers always thought that the planets have been fixed

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in these orbits since they formed, more than 4 billion years ago.

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Long enough for the earth to develop into the haven it is today

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for life to evolve.

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A mechanical model like this embodies an idea

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of the solar system in which the planets all keep

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to these very neat, orderly orbits,

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moving essentially in circles and at fixed distances from the sun.

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And the natural assumption to make is that everything we see now

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formed where it is and has stayed there ever since.

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The idea that the planets are fixed in their orbits

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has been the bedrock of our understanding for hundreds of years.

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But there are some mysteries about our solar system

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that mean we may have to rethink everything we thought we knew.

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It's time for a brand-new model.

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DRAMATIC DRUMBEAT

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Recently, astronomers have started to unravel

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the mystery of how the solar system came to be.

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And to explore it

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we first need a more accurate picture of our planets.

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We need to alter the scale to reflect the huge difference in size.

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For example, Jupiter, the largest planet,

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is 11 times the radius of the Earth,

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and if you look at the masses, the difference is even greater.

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Jupiter has about 300 times the mass of planet Earth.

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The sun we've left alone.

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If we scaled that up too it would fill half the room.

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And, of course, the planets are not all bunched up.

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MECHANICAL WHIRRING

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We need to push the gas and ice giants much further away.

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To be truly accurate, with the planets this size

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we'd have to make the orbits several thousand times bigger.

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However, exactly how we ended up with this neat

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and stable arrangement of planets

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is still one of the greatest mysteries in astronomy.

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In trying to solve this mystery,

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we may discover how the earth came to inhabit

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the perfect position for life to evolve.

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Getting an earth where we have our earth today

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was not a given when this whole solar system started.

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We may be able to understand the remarkable chain of events

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that created the biggest game of pinball in the galaxy...

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The solar system could have done a lot of different things,

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it could have evolved in a lot of different ways.

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We could have ended up with our Jupiter right next to the sun.

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And it looks like it was Jupiter

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that defined the fate of the solar system.

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The giant planets' story IS the story of our solar system.

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We like to think that the earth is really important,

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but the truth is that, if you were looking from afar,

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our solar system is mainly four big planets and some debris.

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Could our place in the universe

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really be nothing more than a lucky accident?

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The question that really arises

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is how common is a solar system like ours?

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The mystery of the birth of the solar system is set to unravel.

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As they try to work out how our solar system formed,

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astronomers have noticed some baffling puzzles.

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If we look at the solar system as it is today,

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it seems quite neat and simple.

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We have four small, rocky planets close to the sun

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and then four enormous giants further out.

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But when we try to model the formation of the solar system

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on a computer, something doesn't quite add up.

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It's really hard to get the model

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to make the planets in the places where we see them today.

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Take, for instance, the curious case

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of the undersized planet Mars.

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If we look at the rocky innermost planets,

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Venus and Earth have about the same mass,

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and we'd expect Mars to have a similar mass too,

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but it actually doesn't.

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It's only about one tenth the mass of the earth or Venus,

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and that's a mystery that's very hard to explain.

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This is the first of four key puzzles

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about the birth of the solar system that remain unsolved.

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And then, at the edge of the solar system,

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the two outermost planets, Uranus and Neptune,

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are much further away from the sun than we'd expect.

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It's very hard to explain how they could have formed

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and become so large at that great distance from the central star.

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If we go in and look at the asteroid belt,

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there are thousands of small, rocky objects there,

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but there are two broad types -

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some of them are very rocky and some have more of an icy content.

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And yet these two types are actually found

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relatively close together.

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It seems as though they formed under different circumstances

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but they've all ended up in roughly the same place.

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And, again, it's a mystery as to how that happened.

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And, closer to home,

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how to explain the rapid and massive bombardment

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that left the moon covered in craters.

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There are many mysteries in the solar system,

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but by unravelling these four -

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the size of Mars,

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the formation of the outer planets,

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the composition of the asteroid belt

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and the bombardment of the moon -

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we may be able to explain how our planet Earth

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found itself in a perfect position for life to evolve.

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And it all starts a long,

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long time ago.

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DRAMATIC DRUMBEAT

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Four and a half billion years ago,

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our sun burst into life

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from the collapse of a massive cloud of gas and dust.

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So, in the beginning,

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this is what you have in our early solar system.

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You have the young star just born,

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and the leftovers, just a cloud of gas,

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the nebula, the protoplanetary nebula,

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full of hydrogen and helium,

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dust and gas,

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and ice grains forming.

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And from this, eventually, you form the planets.

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We know surprisingly little about exactly how planets are formed.

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Most mysterious of all is the most important -

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the largest of all planets, Jupiter,

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which seems to have been made first.

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The first-born - giant, massive Jupiter.

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The meanest and largest of all the planets.

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It sucks up more than half of the existing nebula

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and becomes the king of the solar system.

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We know that Jupiter is made up almost entirely

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of the hydrogen and helium left over in this primordial cloud.

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Which means it must have formed incredibly quickly

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because, as the new sun heated up, it would have blasted the gas away.

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And so there's a time limit.

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Jupiter must have formed in the astronomical blink of an eye -

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just five million years.

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But exactly how it grew so fast and why it grew where it did

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remains shrouded in mystery.

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It's a mystery that Scott Bolton is hoping to shed some light on.

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He's sending a spaceship to Jupiter.

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At the Jet Propulsion Laboratory in Pasadena

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is NASA's deep space operations centre -

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control room for space flights to the moon and beyond...

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..including Scott's mission to Jupiter, Juno.

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Four...three...two...one...

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Ignition and lift-off of the Atlas V with Juno

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on a trek to Jupiter.

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Juno launched in 2011

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and is currently more than 2 million miles away,

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speeding its way to Jupiter at about 150,000mph.

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Even at that speed, it's a five-year journey.

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In 2016, if all goes according to plan,

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the probe will reach Jupiter and go into orbit

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around the king of the solar system.

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Scott and his entire team will be in this room, watching intensely.

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On the day of the Jupiter orbit insertion,

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this room will be completely full.

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The spacecraft is approaching Jupiter.

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It's moving at an incredible speed - like, 150,000mph, or even faster.

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And when it gets to Jupiter we have to slow down enough

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that Jupiter's gravity field can grab us.

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So, we have a rocket on board,

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we point it forward and we fire it,

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and that rocket has to burn at just the right time

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for just the right amount of time

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for us to slow down the perfect amount for Jupiter to grab us,

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because if it misses we fly right past Jupiter.

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There'll be a huge cheer.

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Once we get the data down that shows us we're in orbit around Jupiter

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the room will explode.

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It's the same room where all NASA's critical events are controlled from,

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including the recent landing of the Curiosity rover on Mars.

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And here you see it's right on that screen.

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We'll celebrate just like that.

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I can't wait.

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Once in orbit, Juno will spend a year circling Jupiter,

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gathering vital clues about how it formed.

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Some of the most important data

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that we really want and can't wait to get

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is things that are tied to understanding the early solar system

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and how Jupiter formed.

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So, we want to know whether there's a core in the middle of Jupiter.

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Is there a core of heavy elements,

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a concentration of materials, down in the centre?

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Or is it the same hydrogen and helium and mixture of gases,

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just squeezed down?

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Knowing what's at the centre

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is a vital clue to understanding how Jupiter was built.

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Building planets, whether rocky or gassy,

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is a tricky business.

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But it's something that Juno will shed some light on.

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What Juno's about is actually trying to discover

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the recipe for the solar system.

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How do you make solar systems? How do you make planets?

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And the stage that we're at is we're collecting the ingredient list

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and that's really an important part of any recipe -

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first you gather up the ingredients, figure out what they are,

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then there's some process that you have to do

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in order to bake your cake.

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But the exact nature of that process is not entirely clear.

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The recipe for a rocky planet, like the earth or Mars, is a slow one.

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It can take up to 100 million years.

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But the ingredient list is simple -

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dust.

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Dust starts out in the early solar system

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in a very fine grain, like this - even finer.

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Then, eventually, they start to stick together through electrostatic forces

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and they build bigger and bigger pieces.

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Eventually, the rocks got big enough, they started to stick together

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to the point where they started to form their own gravity.

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But there's a big leap from dust grains

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to rocks that are large enough to clump together

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through their own gravity.

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These rocks, even being so large as they are

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that none of us could lift them,

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they still don't have important gravity.

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Even larger rocks are needed to really start to get enough gravity

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to start to attract the rest of the material for it to collapse

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and start to form a planet as large as Earth.

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It's a slow process,

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but there's no rush when it comes to building a rocky planet.

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Gas giants, on the other hand, are trickier.

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You have to make them fast.

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Because Jupiter and the other gas giants

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are mostly hydrogen and helium,

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and the sun is mostly hydrogen and helium,

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that tells us right away that those planets had to have formed

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while that nebula of hydrogen and helium was still around.

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There are two ways to build a gas giant like Jupiter that fast.

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We don't know exactly how Jupiter formed.

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The two main theories

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are either it has a direct gravitational collapse,

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like we think the sun had, from the nebula

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and sort of builds, um, from the outside in

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and formed Jupiter pretty quickly,

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or it starts to build from the inside out.

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If it collapsed from the cloud of gas,

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then it will be gas all the way through to the centre.

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But if the second theory is right, then it first built a rocky core

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up to ten times the mass of the Earth,

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which then drew in a blanket of gas.

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Either way, it had to happen fast.

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But if Jupiter was going to build a heavy core that quickly,

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it couldn't be done with dust alone.

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There was another crucial ingredient.

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Ice.

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Kevin Walsh is a planet builder.

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His job is to create theoretical models

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of how the planets in the solar system formed -

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models that can best explain the evidence and the clues.

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I think that the most likely way that Jupiter formed

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was by building a solid core of material

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and then hauling the gas down on top of it.

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Either way that you form Jupiter,

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either from accreting straight from the gas

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or building up a rocky core, it has to be done

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in four or five million years,

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before all of the gas is gone from the disc around the sun.

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That's a lot quicker than the time it took

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to build a rocky planet from dust alone.

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But Jupiter had the help of that extra icy ingredient.

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So, we think that the key ingredient

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that allowed Jupiter and Saturn to form so fast,

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compared to the rocky planets,

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is that they formed far enough from the sun

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that water could condense from the gas around the sun

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and form ice, and increase the density of material

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and give you more material to build a larger, rockier core faster.

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That could explain how Jupiter built a rocky core so quickly.

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But it doesn't explain why it grew where it did.

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It's not unreasonable to think

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it would form at the place with the most ice.

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That's a place called the ice line.

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But it's not where Jupiter is today.

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So, right now when we look at our solar system, we look at Jupiter

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and it's beyond the ice line by a fair bit,

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whereas we think it was really advantageous to form Jupiter

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right at the ice line.

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So already that's suspicious.

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If Jupiter was built from a collapsing cloud,

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we'd expect it to be further out.

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If, on the other hand, it was built from a rocky core,

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we'd expect it to be closer to the sun.

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But it's not in either of these two places.

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So the big question is,

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is Jupiter in the wrong place?

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To even ask that question has, until recently, been a heresy.

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At the historic Chamberlin Telescope in Denver, Colorado,

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Kevin Walsh is following in the footsteps

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of some famous astronomers.

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He's taking a closer look at Jupiter.

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You can see Jupiter with the naked eye,

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but looking at it through a telescope like this

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makes it a lot more fun.

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The bands of colour are really clear and crisp

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and the moons are real bright.

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It comes alive. It becomes a real planet when you look at it through a telescope.

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Galileo was the first astronomer to point a telescope at Jupiter,

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more than 400 years ago,

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and no-one ever questioned that Jupiter will always be,

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and has always been, in that same orbit.

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Jupiter, right now...

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..looks the same as it would have looked for Galileo.

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It's a little bigger and brighter through this great telescope,

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but it's the same Jupiter,

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so if I came back tomorrow night, it'd look the same.

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So that's the view -

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the planets that we look at now seem like they never change.

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And why would they change?

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This was the bedrock of our understanding -

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that the planets' orbits are fixed.

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The first hint of something odd came 35 years ago

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from astronomers trying to calculate how the solar system formed.

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They kept getting a strange result.

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Some of those calculations were suggesting that it was possible

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that a planet like Jupiter could have been moved around.

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It was a result so crazy that it was totally ignored.

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So if you built a model to try to understand

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some of the events of the early solar system

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and your model is telling you that planets could have migrated or moved,

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that Jupiter could have moved,

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then it was telling you that you probably made a mistake.

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So the idea of planet migration, it was just never possible.

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Just didn't seem possible.

0:22:170:22:18

But in 1995, astronomers were forced to face up to the impossible.

0:22:230:22:29

The mystery began to unravel when dramatic evidence was uncovered

0:22:310:22:35

from somewhere completely unexpected.

0:22:350:22:37

Astronomer Chris Watson

0:22:540:22:56

is searching for the weirdest places in the galaxy.

0:22:560:22:59

He's a planet hunter, one of a growing band of astronomers

0:23:010:23:05

involved in the hunt for exoplanets -

0:23:050:23:08

alien worlds circling around other stars.

0:23:080:23:11

It's one of the hottest fields in astronomy.

0:23:130:23:15

Up until now, or until recently, we've only had one planetary system

0:23:170:23:22

that we could study, and that was the solar system,

0:23:220:23:24

the planets around the sun.

0:23:240:23:26

And there are about 100 billion stars in our galaxy

0:23:260:23:29

and there are about 100 billion galaxies in the universe,

0:23:290:23:32

and we could only study one.

0:23:320:23:34

But the study of planets and planetary systems exploded

0:23:360:23:39

with the amazing discovery in 1995

0:23:390:23:42

of a planet orbiting around another star.

0:23:420:23:45

Less than 20 years ago, we first found another planet

0:23:470:23:50

around another star that's like our sun

0:23:500:23:52

and that was a dramatic breakthrough.

0:23:520:23:55

And we now know of over 1,000 planets.

0:23:550:23:59

And they're very strange -

0:23:590:24:00

these are nothing like our solar system

0:24:000:24:03

and, in some cases, I think really science fact

0:24:030:24:06

could be a lot weirder than science fiction.

0:24:060:24:09

The planets they have been finding look stranger than anyone imagined.

0:24:110:24:15

We've found planets around binary stars,

0:24:210:24:24

where you actually have two stars orbiting each other,

0:24:240:24:27

a bit like Tatooine off of Star Wars.

0:24:270:24:29

That would be a magical world if you had a habitable planet there,

0:24:290:24:33

because you would imagine

0:24:330:24:35

you'd actually have two shadows on everything

0:24:350:24:37

and two stars in the sky, as well.

0:24:370:24:39

None of the planets found have been remotely like home.

0:24:460:24:50

Recently, a planet was discovered a little bit bigger than Earth,

0:24:520:24:56

but incredibly close to its star.

0:24:560:24:58

It's probably a rocky world, but it's so hot

0:24:580:25:02

it would actually be molten,

0:25:020:25:04

so...it'd be this, but the actual lava on it.

0:25:040:25:07

And most puzzling of all are the largest planets,

0:25:080:25:12

so weird as to seem impossible.

0:25:120:25:14

What was strange when we first discovered these planets

0:25:160:25:19

is they were massive worlds,

0:25:190:25:21

they were gas giants much like Jupiter.

0:25:210:25:24

But these were much, much closer to their parent stars.

0:25:240:25:27

Jupiter lives out in the cold outer reaches of our solar system,

0:25:300:25:35

taking 12 years to orbit the sun.

0:25:350:25:37

But these alien giants were found in the fiery heat,

0:25:380:25:42

right next to their star,

0:25:420:25:43

hurtling around in crazy orbits of just a few days.

0:25:430:25:47

They were nicknamed "hot Jupiters".

0:25:480:25:51

They're right up against the host star,

0:25:550:25:57

and it's amazing, they really...

0:25:570:25:59

At the time, we thought, "How did they get there?

0:25:590:26:01

"They really shouldn't be there."

0:26:010:26:03

Other scientists were thinking, "Well, you're a bit crazy,

0:26:030:26:06

"these Jupiters should not be that close to the star."

0:26:060:26:09

Everyone was baffled by the existence of hot Jupiters.

0:26:120:26:15

They were planets that, quite simply, shouldn't exist.

0:26:150:26:19

In theory, the only place you could build a gas giant

0:26:220:26:26

would be out in the cold, far away from a star,

0:26:260:26:30

because that's the only place

0:26:300:26:32

you can find the necessary ingredients.

0:26:320:26:34

What I'm holding in my hand is a lump of dry ice,

0:26:360:26:39

and this represents the building blocks

0:26:390:26:42

of planets like Jupiter.

0:26:420:26:44

And this is fine - it's quite happy out here,

0:26:440:26:48

far away from the fire that represents our sun

0:26:480:26:51

or any other star that one of these gas giants might be forming round.

0:26:510:26:55

But look at what happens when I bring it closer to the fire.

0:26:550:27:01

Too close to the star and the ice just turns to gas.

0:27:040:27:07

And without ice you can't build a gas giant.

0:27:080:27:12

So there's very little left after just a few minutes.

0:27:120:27:16

And what this means

0:27:160:27:19

is that gas giants can't form close to the star.

0:27:190:27:24

The building blocks just cannot exist that close.

0:27:240:27:28

They have to have formed further away

0:27:280:27:30

where the raw materials can exist.

0:27:300:27:32

If these hot Jupiters couldn't have formed where we find them,

0:27:350:27:39

it could only mean one thing.

0:27:390:27:41

So we think that, in actual fact,

0:27:410:27:44

these gas giants form further out,

0:27:440:27:48

then they actually move towards the star,

0:27:480:27:50

they actually migrate inwards.

0:27:500:27:53

The planets are on the move.

0:27:560:27:59

The discovery that planets could change orbit

0:28:020:28:05

was a shocking revelation.

0:28:050:28:07

It turned the world of planetary science on its head.

0:28:070:28:11

The implications of a planet the size of Jupiter

0:28:120:28:15

roaming freely around a planetary system

0:28:150:28:18

could be devastating.

0:28:180:28:20

Over recent years, the search for exoplanets has exploded.

0:28:230:28:28

So here we are, nearly 2,400 metres up

0:28:300:28:33

on the volcanic island of La Palma.

0:28:330:28:37

What you can see before you are suites of professional telescopes

0:28:370:28:42

and what we're going to do is we're going to use one of these

0:28:420:28:46

telescopes to actually look at planets orbiting another star.

0:28:460:28:50

So, this cloud can be a bit of a problem.

0:28:510:28:54

Normally it's not so cloudy, but we are in the depths of winter

0:28:540:28:57

and this is actually quite local cloud.

0:28:570:28:59

20 years ago, all these telescopes were busy looking at stars.

0:29:010:29:06

Now, increasingly, many are focusing on planets.

0:29:060:29:10

There's quite a few telescopes here, and probably every night

0:29:110:29:16

there's some project related to extrasolar planets going on.

0:29:160:29:20

It is a really rich, blossoming field of astronomy.

0:29:200:29:25

And, provided the clouds clear,

0:29:290:29:32

tonight Chris will be pointing his telescope

0:29:320:29:35

at an exoplanet called WASP-84 b.

0:29:350:29:37

These clouds will clear.

0:29:390:29:41

But, even with clear skies,

0:29:470:29:49

spotting alien planets is no easy matter.

0:29:490:29:52

To see other planets in our solar system from Earth is pretty easy.

0:30:040:30:09

So this candle represents our sun

0:30:090:30:11

and if I pop down this little rock, representing a planet,

0:30:110:30:15

you can clearly see the reflected sunlight.

0:30:150:30:17

But even the nearest stars are so far away

0:30:190:30:22

that the reflected light from the planets gets completely lost.

0:30:220:30:25

So, now we have our star, much further away,

0:30:270:30:30

and if I put my planet down,

0:30:300:30:32

while it's still reflecting the starlight,

0:30:320:30:35

because you're so far away,

0:30:350:30:36

the reflected light is actually drowned out

0:30:360:30:39

in the glare of the star itself.

0:30:390:30:41

Because the planets are so hard to see,

0:30:450:30:47

astronomers have found other ways to detect them.

0:30:470:30:50

One of the best ways is actually to watch and see

0:30:500:30:54

if the planet actually crosses in front of the star.

0:30:540:30:58

So, if we were an alien civilisation looking back at our solar system,

0:30:580:31:03

we happen to catch Jupiter transiting the face of our sun,

0:31:030:31:07

we would see a 1% dip in the sunlight.

0:31:070:31:11

For a planet a lot smaller, like the Earth,

0:31:110:31:15

that dip is much, much smaller - it's minuscule.

0:31:150:31:19

And that's why it's so, so difficult to detect these.

0:31:190:31:22

But techniques have improved dramatically

0:31:280:31:30

and now, for astronomers like Chris Watson,

0:31:300:31:33

planet-hunting is all part of a night's work.

0:31:330:31:36

This is our telescope, Telescopio Nazionale Galileo,

0:31:440:31:48

and this will be our baby for the night.

0:31:480:31:50

The skies are clearing beautifully,

0:31:540:31:56

so I think we're in for a really nice night ahead.

0:31:560:31:59

Thanks to ground-based telescopes like this,

0:32:010:32:04

as well as space telescopes like NASA's Kepler mission,

0:32:040:32:07

thousands of planets have now been found.

0:32:070:32:10

And not just planets, but entire planetary systems.

0:32:120:32:15

So this is the Kepler Orrery,

0:32:230:32:25

which shows the orbits and the sizes of planets.

0:32:250:32:29

So these are candidates that the Kepler space mission has found.

0:32:290:32:33

So these are transiting planets.

0:32:330:32:36

They don't, however, look much like we'd expect.

0:32:360:32:39

And up there, on the top left,

0:32:390:32:41

you can see the orbits of the four innermost planets of our solar system

0:32:410:32:47

from Mercury out to Mars.

0:32:470:32:49

What you can see is the huge diversity

0:32:490:32:54

of all the different planetary systems.

0:32:540:32:56

Each set of rings shows a different planetary system

0:32:590:33:03

and each blob, a different planet, with its size and orbit.

0:33:030:33:06

They break every rule in the book

0:33:080:33:10

and make us look like the odd one out.

0:33:100:33:12

So we have large gas giant planets in there,

0:33:140:33:18

and then you can see the really short period,

0:33:180:33:22

really weird solar systems.

0:33:220:33:25

They really don't look anything like our own solar system.

0:33:260:33:29

Some of these planets actually have orbits of just a few hours.

0:33:290:33:35

There's even systems spiralling around multiple planets in here.

0:33:360:33:40

That one's weird. What's going on here?

0:33:430:33:45

Who knows what we might discover in this rich smorgasbord of planets?

0:33:570:34:01

It is ridiculous, actually.

0:34:040:34:05

HE CHUCKLES

0:34:050:34:07

What is going on with that?

0:34:070:34:09

Extraordinary worlds.

0:34:120:34:13

Some may host life.

0:34:160:34:18

Our exploration of these alien worlds

0:34:210:34:23

is only just beginning,

0:34:230:34:26

but already they're revealing some incredible secrets.

0:34:260:34:29

Tonight, Chris and his team are training the telescope

0:34:360:34:39

on a star they known has a hot Jupiter orbiting it.

0:34:390:34:42

INDISTINCT CONVERSATION

0:34:420:34:45

They hope to reveal just how devastating

0:34:480:34:51

a migrating gas giant could be.

0:34:510:34:53

So this star that we're looking at, WASP-84,

0:34:550:34:58

was actually discovered to have a transiting planet around it.

0:34:580:35:01

That transiting planet, we know at the moment,

0:35:010:35:04

is about a little bit less massive than Jupiter.

0:35:040:35:08

And we know its orbital period,

0:35:080:35:10

so its year is about eight-and-a-half days,

0:35:100:35:12

and we're going to follow it as it transits the star.

0:35:120:35:16

A planet the size of Jupiter

0:35:160:35:19

orbiting its star once every eight days

0:35:190:35:22

is already pretty weird.

0:35:220:35:24

But some of these alien worlds have even weirder orbits than that.

0:35:240:35:28

-What's the air mass with that, then?

-About 1.25?

0:35:280:35:32

'We would expect the planet and the star'

0:35:320:35:35

to be spinning in the same way.

0:35:350:35:37

But we see quite a few systems where that is just not the case.

0:35:370:35:41

Some of these planets are going completely the wrong way.

0:35:420:35:46

If the star is spinning clockwise,

0:35:470:35:49

the planet is spinning anti-clockwise.

0:35:490:35:52

-169, we're talking about.

-Yeah, it's about...

0:35:520:35:55

A planet orbiting in the wrong direction

0:35:550:35:57

is a sign of some truly cataclysmic event.

0:35:570:35:59

And tonight, as it passes in front of its star,

0:36:010:36:04

Chris will be able to analyse the orbit of WASP-84 b.

0:36:040:36:07

'The purpose of these observations is actually'

0:36:080:36:11

to see whether we have a nicely aligned system -

0:36:110:36:15

a bit like the planets we have in our solar system,

0:36:150:36:17

where the star spins in the same direction as the planet orbits.

0:36:170:36:22

Or do we have something that would be the smoking gun

0:36:220:36:26

of a really violent interaction

0:36:260:36:29

which has maybe scattered that planet into one of these weird orbits?

0:36:290:36:34

So, perhaps over the poles,

0:36:340:36:35

or actually spinning in the opposite direction to that of the star.

0:36:350:36:40

But the big question is

0:36:410:36:43

what could be the cause of such planetary upheaval?

0:36:430:36:46

Whoa!

0:36:460:36:48

After following the transit through the night,

0:36:520:36:54

Chris has the verdict on Planet WASP-84 b.

0:36:540:36:58

So, the transit's finished. We've had a quick look at the data

0:37:000:37:04

and what we've found has actually taken us a bit by surprise.

0:37:040:37:09

We thought that this planet system would be misaligned.

0:37:090:37:12

Now that we've had a look at the data,

0:37:120:37:14

it looks as though it's actually aligned.

0:37:140:37:16

WASP-84 b turns out to be orbiting the right way.

0:37:180:37:21

But Chris has found many of these hot Jupiters

0:37:230:37:25

that are travelling in completely the wrong direction.

0:37:250:37:28

It's evidence of how, in migrating, they must have caused havoc.

0:37:300:37:34

With these very strange orbits,

0:37:360:37:38

it looks as though it's been a very violent process.

0:37:380:37:42

To actually take one of these planets

0:37:420:37:44

and just chuck it into a different orbit,

0:37:440:37:48

that's very violent.

0:37:480:37:50

One of the easiest ways to do that is to have a collision.

0:37:500:37:54

Take two planets, interaction between them,

0:37:540:37:58

and you can eject one planet

0:37:580:38:00

and fling the other planet really close in to the star.

0:38:000:38:04

These giant gas planets are the bully of the playground.

0:38:090:38:13

They have the power to throw other planets around

0:38:140:38:17

like a game of cosmic pinball.

0:38:170:38:19

Beasts the size of Jupiter are so vast

0:38:230:38:26

they can eject entire planets from the system.

0:38:260:38:28

They can launch them into crazy polar orbits.

0:38:320:38:35

They even have the power to destroy entire worlds.

0:38:390:38:42

A planet like Jupiter, the mass of Jupiter, the size of it,

0:38:490:38:53

just dominates planetary systems,

0:38:530:38:55

and it's got the power to really decide

0:38:550:38:58

the fate of the other planets.

0:38:580:39:01

I think we'd be quite glad there's not a hot Jupiter in our system.

0:39:030:39:07

We wouldn't be seeing this.

0:39:070:39:09

We've discovered other systems where planets migrate

0:39:180:39:21

and hot Jupiters cause havoc.

0:39:210:39:23

But what about our own solar system?

0:39:240:39:26

Our planets certainly seem fixed in their rigid, clockwork orbits.

0:39:280:39:33

Our earth has been the same distance from the sun for 4.5 billion years.

0:39:430:39:49

Long enough to create an atmosphere,

0:39:500:39:52

build mountains,

0:39:520:39:54

and for life to evolve.

0:39:540:39:55

But the evidence from other planetary systems now means

0:39:570:40:01

a complete rethink on how and where our planets formed.

0:40:010:40:05

When we started discovering planets around other stars,

0:40:050:40:09

we started finding planets in completely unexpected places,

0:40:090:40:12

places we never thought could possibly form a planet.

0:40:120:40:15

We had to go back to the drawing board and say,

0:40:150:40:18

"Wow, planets can move.

0:40:180:40:19

"Planets can really move. Maybe that happened here."

0:40:190:40:22

It's a big leap, and to make that leap

0:40:260:40:29

and say things might have been completely unstable,

0:40:290:40:33

totally chaotic for a time period, that's really hard to imagine.

0:40:330:40:38

But that's the leap that we need to take.

0:40:380:40:41

The crazy results that suggest Jupiter might have changed orbit

0:40:430:40:47

might not be mistakes after all.

0:40:470:40:50

Instead, migration could be the key that unlocks

0:40:510:40:55

many of the mysteries of how our solar system came to be.

0:40:550:40:58

Now that we've taken this tool of planetary migration that we started

0:40:580:41:01

to understand by looking at planets around other stars,

0:41:010:41:04

we've realised that it's absolutely critical to understand

0:41:040:41:07

how our solar system formed and evolved.

0:41:070:41:10

And central to it all is mighty Jupiter.

0:41:100:41:13

Certainly in our planetary system, Jupiter is the key.

0:41:130:41:16

It's over three hundred times more massive than the Earth,

0:41:160:41:19

so Jupiter wins.

0:41:190:41:22

Jupiter decides what happens.

0:41:220:41:23

The inescapable truth seems to be that planets move.

0:41:260:41:31

And, if it can happen in exoplanetary systems,

0:41:310:41:34

it can happen in ours.

0:41:340:41:36

If we want to make a model that explains

0:41:360:41:38

how our solar system came to be,

0:41:380:41:40

we have to break the brass rods and set the planets free.

0:41:400:41:44

Once we accept the idea that the planets can move,

0:41:480:41:53

we can begin to explain some of the unsolved mysteries of the solar system.

0:41:530:41:57

In particular, why Mars is so small

0:42:020:42:06

and the curious composition of the asteroid belt.

0:42:060:42:09

Kevin Walsh has developed a model of the early solar system

0:42:280:42:32

that involves a wild dance of the planets.

0:42:320:42:34

It's an intricate and chaotic dance,

0:42:380:42:40

and if it had gone slightly differently

0:42:400:42:43

it could have stopped our developing solar system in its tracks.

0:42:430:42:46

In his model, Jupiter takes a wild ride through the solar system.

0:42:480:42:52

It takes us right back to the moment of birth,

0:42:580:43:01

when Jupiter had just formed from the cloud of gas.

0:43:010:43:04

The key is that, though Jupiter is really big,

0:43:060:43:08

it's 300 times the mass of the earth,

0:43:080:43:10

the gas disc around the sun was much more massive,

0:43:100:43:13

so the gas can actually push Jupiter in towards the sun.

0:43:130:43:16

As soon as it was born, Jupiter began to migrate inwards.

0:43:170:43:22

Over the course of half a million years,

0:43:220:43:25

it spiralled in towards the sun.

0:43:250:43:27

It was on its way to becoming a hot Jupiter.

0:43:280:43:31

So, the idea that you could form something as big as Jupiter

0:43:330:43:36

and have it pushed inward by the gas disc

0:43:360:43:38

actually makes a fair amount of sense,

0:43:380:43:40

because we see it, we see it all over.

0:43:400:43:42

But something stopped Jupiter from crashing into the sun

0:43:460:43:49

or ending up as a hot Jupiter.

0:43:490:43:51

So, if it formed and started migrating inwards,

0:43:520:43:55

there must have been a mechanism to stop it

0:43:550:43:57

and bring it back out to the outer part of the solar system.

0:43:570:44:00

We think the key to stop its inward migration, to keep it

0:44:000:44:03

from going all the way in towards the sun is the presence of Saturn.

0:44:030:44:08

While Jupiter was on its wild ride, Saturn was born.

0:44:080:44:12

Saturn is also growing. It's going through the same process Jupiter did.

0:44:120:44:16

It's building a big core and it's getting really massive,

0:44:160:44:19

and once it gets really massive as well it can move in the disc also.

0:44:190:44:23

And it too began spiralling in towards the sun.

0:44:230:44:26

So as Saturn is racing inwards, it gets very close to Jupiter

0:44:260:44:30

and they actually get close enough that they get locked in a resonance

0:44:300:44:33

where their orbital periods are closely aligned

0:44:330:44:35

and they interact very closely gravitationally.

0:44:350:44:38

Now, when these two get really close

0:44:380:44:41

it actually stops Jupiter's inward migration.

0:44:410:44:44

The two planets were involved in a kind of gravitational dance.

0:44:440:44:48

And, as they came close, Jupiter changed direction

0:44:480:44:52

and was flung back to the outer solar system...

0:44:520:44:54

..just like a sailing ship changing course in a grand tack.

0:44:560:44:59

So this theory called the grand tack is called that

0:45:030:45:06

because our planets are moving inwards

0:45:060:45:09

and they get really close and they stop and they turn and they go back

0:45:090:45:12

outwards, and it's kind of like a sailboat tacking across the wind.

0:45:120:45:15

Jupiter's wild ride could explain two key mysteries -

0:45:150:45:19

first, why Mars is so small.

0:45:190:45:22

So, when Jupiter migrates inwards

0:45:220:45:25

it kind of snowploughs all the rocky material it sees,

0:45:250:45:29

it snowploughs it and pushes it inwards.

0:45:290:45:31

Much of the dust and rocky debris

0:45:320:45:34

that would have gone on to build Mars got pushed out of the way.

0:45:340:45:38

So, by Jupiter coming in and clearing out all of this

0:45:390:45:42

material on its way, it kind of reduces the total

0:45:420:45:45

amount of material that Mars can feed on to grow,

0:45:450:45:49

and so Mars ends up kind of being starved of rocky material

0:45:490:45:52

and only grows to be a tenth the mass of the earth.

0:45:520:45:55

And this explains why Mars is the planetary runt we see today.

0:45:570:46:02

The theory also explains why the asteroid belt has an icy ring

0:46:060:46:10

and a rocky ring so close together.

0:46:100:46:13

During its travels, Jupiter scattered everything in its path.

0:46:220:46:25

It threw rocks from the inner part of the solar system outwards...

0:46:280:46:31

..and ice from the outer reaches inwards...

0:46:350:46:38

..leaving the two distinct bands we see today.

0:46:400:46:42

That's how we end up with two different types of material

0:46:440:46:47

sitting on top of each other in the middle of the asteroid belt

0:46:470:46:49

in a very small region.

0:46:490:46:51

So Jupiter's wild ride could explain two key mysteries -

0:46:510:46:55

the size of Mars and the composition of the asteroid belt.

0:46:550:46:58

And if it had travelled any further in

0:47:000:47:03

the earth itself may have become a very different type of planet.

0:47:030:47:07

But the birth of the solar system

0:47:140:47:16

wasn't the only turbulent time in its history.

0:47:160:47:19

About 500 million years later, 4 billion years ago,

0:47:210:47:25

the solar system entered its teenage years -

0:47:250:47:28

an intense period of trouble, chaos and uncertainty.

0:47:280:47:32

It's a period of turbulence that could explain

0:47:340:47:37

two further mysteries...

0:47:370:47:39

..the craters on the moon

0:47:410:47:43

and the birth of Uranus and Neptune.

0:47:430:47:45

There are some mysteries when we look around the solar system,

0:47:510:47:54

where the theories really don't match what we see.

0:47:540:47:57

If we just take a bunch of small icy objects

0:47:590:48:02

from which Uranus and Neptune were made,

0:48:020:48:05

put them out in the outer part of the solar system in our computer

0:48:050:48:08

models and watch how they grow, it turns out they can't grow at all.

0:48:080:48:13

A couple of billion miles away from these ice giants, on the moon,

0:48:150:48:19

there's a clue that Hal Levison believes

0:48:190:48:21

could help solve the puzzle.

0:48:210:48:24

In fact, the moon is covered in clues.

0:48:240:48:27

So, when you look at the moon,

0:48:280:48:30

some of these biggest crater impact basins, like here and here,

0:48:300:48:34

all formed in a very short period of time

0:48:340:48:37

that we call the Late Heavy Bombardment.

0:48:370:48:40

It came roughly 500 million years after the birth of the solar system,

0:48:400:48:45

when all the planets had long formed.

0:48:450:48:47

And all of a sudden, out of the blue,

0:48:490:48:51

the moon got clobbered by big objects coming in and hitting it.

0:48:510:48:57

And that indicates that you had this very violent upheaval

0:48:570:49:02

and the only way that we can form an influx like this, stuff raining

0:49:020:49:07

down in onto the moon, is through changing the orbits of the planets.

0:49:070:49:14

To account for this violent upheaval,

0:49:150:49:17

Hal and some colleagues devised a new model.

0:49:170:49:20

It explains why we now see Uranus and Neptune

0:49:210:49:24

in places they can't possibly have formed.

0:49:240:49:27

We think what happened is they formed closer to the sun

0:49:280:49:31

and got delivered to where we see them today.

0:49:310:49:34

Uranus and Neptune must have formed much closer in,

0:49:360:49:40

where there was plenty of icy material,

0:49:400:49:42

and beyond them was a neat disc of icy, comet-like objects.

0:49:420:49:46

But this was not a stable system,

0:49:480:49:51

and a series of small changes led to a period of utter chaos.

0:49:510:49:56

These objects leak out of this disc,

0:49:560:49:59

get gravitationally scattered by all these planets,

0:49:590:50:03

like billiard balls going around,

0:50:030:50:05

and get eventually ejected to interstellar space by Jupiter.

0:50:050:50:08

That causes the planets' orbits to slightly spread over time

0:50:120:50:16

and what we think happened is that Jupiter

0:50:160:50:19

and Saturn got to the point where Jupiter goes around the sun

0:50:190:50:24

exactly twice for every time Saturn moves around the sun.

0:50:240:50:28

And that allows their tugs on one another to become much stronger

0:50:280:50:33

and as a result, Jupiter and Saturn get a little excited,

0:50:330:50:37

their orbits become less circular and more inclined

0:50:370:50:40

and they start getting... sort of tugging on one another.

0:50:400:50:43

Uranus and Neptune, which are much smaller than Jupiter and Saturn, feel

0:50:430:50:47

that fight, feel that tension and as a result, their orbits just go nuts.

0:50:470:50:52

In a sudden period of chaos, Uranus and Neptune

0:50:540:50:57

were flung out into the orbits we see today.

0:50:570:51:00

The ice giants, Uranus and Neptune, get scattered into this disc

0:51:020:51:05

that existed outside their orbits, and that thing went kaplooie.

0:51:050:51:08

Vast lumps of ice were scattered everywhere,

0:51:130:51:17

raining into the inner solar system

0:51:170:51:19

and bombarding the earth and the moon.

0:51:190:51:21

Every square inch of the earth at one time got hit

0:51:250:51:29

due to this instability, so it was not a very safe place to be.

0:51:290:51:33

We had this view that the solar system was this nice clock

0:51:350:51:39

and things just moved around in nice regular ways.

0:51:390:51:42

What this new model shows is a real paradigm shift.

0:51:430:51:47

It says that the solar system is not this nice, safe, quiescent place,

0:51:470:51:53

but can go through periods of intense violence.

0:51:530:51:56

This new model of the solar system is now dynamic and turbulent.

0:51:580:52:02

The prime mover in all this upheaval is our playground bully, Jupiter.

0:52:040:52:09

Such a bully, in fact, that David Nesvorny believes that Jupiter

0:52:160:52:21

may have been responsible for the ultimate planetary crime.

0:52:210:52:24

He ran the new model over and over again

0:52:260:52:29

with slightly different starting conditions.

0:52:290:52:31

I ran about 3,000, 4,000 models like this,

0:52:330:52:38

just playing with the initial state

0:52:380:52:41

and at a time, I considered the standard theory,

0:52:410:52:44

which was that the outer solar system had four planets.

0:52:440:52:48

His results were alarming -

0:52:490:52:52

change the starting conditions even slightly

0:52:520:52:55

and the solar system looks very different.

0:52:550:52:57

Frequently, what happened in my simulation was that Jupiter

0:52:590:53:02

just slingshots Uranus and Neptune from the solar system

0:53:020:53:05

and they ended somewhere in interstellar space.

0:53:050:53:09

So that wasn't right.

0:53:090:53:11

Obviously not right.

0:53:110:53:13

Then David had a radical idea.

0:53:150:53:18

If Neptune and Uranus didn't get flung out of the solar system,

0:53:180:53:21

maybe something else did.

0:53:210:53:23

I couldn't quite fit the solar system, how it looks like today.

0:53:250:53:29

So I was thinking and thinking and thinking, and then I thought,

0:53:300:53:34

"How about if the solar system had an extra planet?"

0:53:340:53:37

He started investigating the possibility

0:53:410:53:44

that an entire planet might have gone missing.

0:53:440:53:47

As ever, the prime suspect was Jupiter.

0:53:490:53:53

So, now I am pointing at Jupiter,

0:53:560:53:59

so I can see the disc of Jupiter,

0:53:590:54:02

and then, nicely aligned, four giant moons.

0:54:020:54:07

It has a huge influence...

0:54:080:54:10

..and could have had an even bigger influence in the past.

0:54:110:54:14

It may even have been able to eject an entire planet

0:54:190:54:22

from our solar system.

0:54:220:54:24

This is the solar system.

0:54:250:54:27

The sun is in the middle...

0:54:270:54:29

..then we have the terrestrial planets.

0:54:300:54:32

Then there's the asteroid belt and the outer planets.

0:54:320:54:36

To get the arrangement of planets we see today, David thinks

0:54:370:54:41

we once had an extra ice giant but it was thrown out by Jupiter.

0:54:410:54:45

I start playing with the possibility that we had an additional planet.

0:54:460:54:53

So the best case I have found was when I placed

0:54:530:54:57

this third ice giant between Saturn and Uranus initially,

0:54:570:55:02

somewhere here.

0:55:020:55:04

What happens in this case is that during the instability,

0:55:040:55:07

this planet evolves, has close encounters with Jupiter

0:55:070:55:11

and Saturn and gets ejected from the solar system.

0:55:110:55:15

The ejected planet may have been a sacrificial lamb

0:55:270:55:31

that saved us from Jupiter's destructive powers

0:55:310:55:34

and allowed our planets to settle in the pattern we see today.

0:55:340:55:37

So, what became of our missing lonely planet?

0:55:400:55:43

In the simulations I have, the planet is ejected from the solar system

0:55:440:55:49

with a speed of about 1km per second.

0:55:490:55:52

But this happened about four billion years ago, so do your math.

0:55:540:55:59

It will end up very far from the solar system,

0:56:010:56:03

so today it can be almost anywhere in the galaxy.

0:56:030:56:07

20 years ago, the mystery of the solar system began to unravel.

0:56:200:56:24

Evidence from alien worlds shattered the long-held view

0:56:260:56:29

that our planets have fixed orbits.

0:56:290:56:32

It led to a whole new understanding of a turbulent and dynamic past...

0:56:340:56:37

..which makes us wonder, might things have turned out differently?

0:56:390:56:43

The solar system could have done a lot of different things,

0:56:430:56:46

it could have evolved in a lot of different ways.

0:56:460:56:49

What we see in our own solar system

0:56:490:56:51

is the result of a lot of unlikely or random events,

0:56:510:56:57

and so our solar system is unique.

0:56:570:56:59

Ending up with a stable system of planets was just a fluke,

0:57:030:57:07

a lucky roll of the dice.

0:57:070:57:09

It's amazing we survived at all.

0:57:100:57:12

Getting an earth where we have our earth today was not a given

0:57:120:57:17

when this whole solar system started.

0:57:170:57:19

It took all these series of events to get a rocky planet

0:57:190:57:22

of this size at this distance with this amount of water

0:57:220:57:25

to build the earth that we live on today.

0:57:250:57:28

The fate of the entire solar system, including the earth,

0:57:300:57:34

was defined above all by the movements of our gas giant, Jupiter.

0:57:340:57:38

If Jupiter's orbit moved differently,

0:57:400:57:43

if Jupiter moved into the inner solar system,

0:57:430:57:46

then it's unlikely that the earth would be here.

0:57:460:57:49

Of all the planetary systems so far discovered,

0:57:510:57:54

it seems we are the only one with the lucky roll of the dice.

0:57:540:57:58

You might think that maybe the solar system that we have here

0:57:590:58:02

is actually the oddball and that the natural order

0:58:020:58:05

of things are these other systems that we think of as weird.

0:58:050:58:09

And, if we are so unusual,

0:58:110:58:13

will we ever find anywhere else in the universe so welcoming to life?

0:58:130:58:17

Even though our solar system might be...let's say one in a million -

0:58:190:58:23

that may seem like a really small number -

0:58:230:58:26

there are 100 billion stars in the galaxy.

0:58:260:58:29

So even something as unlikely as our solar system,

0:58:290:58:32

there may be lots of them around.

0:58:320:58:35

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