Original subtitles

the solar system our whole vast majestic

and peaceful think again once there was

nothing here but a huge thin cloud of

gas how did we get from this to this

when we look out there we're really

looking for our own origins what

happened to us piecing together our

system's history suggests a wildly

chaotic birth it's almost like a

planetary pinball game

these are exquisitely violent events

it's the ultimate survival story the

formation of our solar system

[Applause]

[Music]

throughout history the heavens have

inspired or and Wonder in human

observers our ancestors watched the

skies with amazement imagining the power

of the bright objects that we now call

planets the fact is is that for as long

as there have been people we've looked

at the sky we've looked at what we see

there we've asked the question what's

the connection between that and us the

ancients erected standing stones temples

and pyramids from which to predict

cosmic events and to worship the gods in

the sky

but the first telescopes reveal the

universe unlike anything we could have

imagined we began to see what was really

out there not gods but planets stars

immense clouds of gas in 1755 German

scientist and philosopher Immanuel Kant

turned his attention to these huge

clouds or nebulae and in the process

came up with one of the first modern

theories of how the solar system formed

all of these beautiful clouds in

interstellar space have turned out to be

very important

it's where stars are born it's where

planets form can't ski idea was that

nebulae could collapse in and condense

into planets and stars it's known as the

nebula theory

250 years later cants theory still

helped to explain the birth of our solar

system

today new observations are revealing how

the Sun planets and news of our solar

system took shape scientists like

Professor Jeff Hester are solving many

of the long-standing mysteries of the

formation process we now have answers to

questions that people have asked

themselves for as long as there have

been people and it changes the way that

we think about ourselves dr. Michelle

thaller

is an astronomer working with nasa's

spitzer space telescope it's the fourth

and last satellite of NASA's Great

observatories and it gives scientists a

totally new view of the universe what's

happening now in astronomy is we're

finally seeing the universe as it really

is not just as humans perceive it one

way to establish how our solar system

forms is to watch other distant systems

take shape but if Kant was right and new

solar systems formed deep within clouds

of interstellar gas we might never see

this happening our own senses what the

human eye can see turns out not to be

the whole story there's so much more out

there that the humans just don't have

the capability to see naturally objects

emit infrared radiation infrared has a

longer wavelength and visible light so

is not scattered by dust conventional

telescopes that use visible light cannot

see inside a dusty nebula but infrared

sensors onboard Spitzer can

the same principle is used by

firefighters

[Music]

this room is completely full smoke there

is no way that you can see with your

eyes or with a regular camera what's

going on inside

so fire departments use infrared cameras

to be able to see people inside a

smoke-filled room and actually see the

heat of a person's body this is exactly

how Spitzer works Spitzer actually looks

through these giant clouds of smoke and

dust to see planets inside just like

these guys can find the people

visible light is completely blocked by

things like smoke and dust but infrared

light is a longer wavelength actually

it's a sort of a happy coincidence for

us that nothing stops it in the

chemistry of smoke and dust

I think you really get a sense now that

we're able to see where we've never seen

before using the Spitzer Space Telescope

a camera in a smoke-filled room is one

thing a telescope in outer space is

quite another

on the ground in 2003 Balor and her

colleagues wait anxiously for spitz's

first data at stake is a unique view of

the origins of our own solar system

literally no one had ever seen what the

universe looks like in that type of

light so at first we were amazed at the

beauty I did not expect

Spitzer's images to be as compelling

visually to me as they turned out to be

they are beautiful but these are not

just pretty images crucially by

observing distant worlds as they form we

also learn more about the birth of our

own solar system one of the privileges

of observing star and plant formation is

that you're effectively looking back in

time you're looking at what we must have

gone through billions of years ago what

this glimpse back in time reveals is a

snapshot of the earliest stages of solar

system formation this is our best ever

view inside our nebulous collapse we've

actually been able to observe with

Spitzer the moment where the dust begins

to collect together and then get warm

and then finally young stars emerge so

it turns out cants 250 year-old theory

is essentially correct our own nebula

began its collapse 4.6 billion years ago

it's likely that the nebula had slowly

been spinning in space ever since its

formation but as its material collapsed

it began to spin much faster this is

down to the law of physics known as

conservation of angular momentum as the

nebula contracts the same amount of

energy is now concentrated in a smaller

space to keep the same momentum rotation

speed must increase

as our nebula began to collapse under

its own gravity and spin faster a dense

clump of matter formed in the center

it's called a protostar at first its

heat was due to friction but when the

clump reached 10 million degrees Celsius

nuclear fusion kicked in 4.5 billion

years ago our star was born

hydrogen atoms fuse together to form

helium and in the process release

photons it's the first light made by our

Sun and it will keep on shining for four

billion years a vast nuclear furnace

nearly a million miles across from a

distance this process seems almost

peaceful with the sun's power gently

radiating out into the early solar

system there was all sorts of activity

going on things were changing rapidly

violence explosions intense radiation we

had a pretty spectacular origin I think

recent research suggests the solar

system's birth was far from peaceful our

Sun may in fact have been born out of

one of the most violent events in the

cosmos the explosive death of another

star

and even before that it seems there was

an explosion so huge that it created an

entire universe the Big Bang no one

knows what caused it but we think that

the Bang created space-time and all the

matter in the universe yet for

astrophysicists such as professor Stan

Woosley there's a puzzle the Big Bang

produced almost none of the 88 naturally

occurring chemical elements when we look

around we don't just see hydrogen and

helium

we see planets and people and we're made

out of carbon and oxygen and planets are

made out of iron and silicon and that

didn't all come from the Big Bang it had

to come from somewhere else here on

earth we have learned how to create new

elements for ourselves

nuclear technology the same thing

happens inside stars hydrogen and helium

from the Big Bang fuse to form heavier

elements and if the stars big enough at

the end of its life it does something

spectacular it explodes

the energy of a supernova explosion is

essentially equal to all of the energy

that the Sun will put out in its ten

billion year lifetime and yet the

supernova delivers this in less than a

day and then expands for a few months

afterwards as it expands it's as bright

as an entire galaxy it's as bright as a

billion suns extreme nuclear processes

within the dying star fuse the elements

the star created in life to make heavier

elements as the star explodes it blasts

these into space and it's death it

creates the essential elements for

making planets and life elsewhere it

spews the products of millions of years

of nuclear reactions out into the gas in

between the stars this is the basis for

some of the clouds that eventually form

new stars yet they're spread so thin

that it's difficult to know what

triggers our cloud like this to collapse

the best way to find out is to examine

the material that once made up the cloud

and that means examining the two oldest

types of material in the solar system

asteroids and comets but first

scientists need to bring back a sample

from outer space

comets are relics from the earliest days

of the solar system and may hold clues

to our origins but before scientists

could study them in depth they needed

samples in 1981 this man

NASA's dr. Peter Sohn started to work on

his idea to return comet fragments to

earth it's almost like a space forensics

it's like you see some television

program or crime scene they found a

piece of paint chip or hair or some

carpet summer affair while we're trying

to do so and his colleagues designed the

Stardust probe to travel around 500

million kilometres from the earth and

collect comet grains from the comet

ville - there's just one problem the

dust particles will be moving up to six

times faster than the speed of a bullet

yet it's vital so gets them back to

earth undamaged so what I need a special

material to be able to continue the

high-speed bullet I'd be able to capture

it without breaking up so I need to

capture the bullet without breaking out

the bullet it takes so over 2,000

experiments to refine his comet catching

material it's made of the same chemical

as glass silicon dioxide only a thousand

times less dense so calls it's smooth

gradient silica aerogel

if anything can stop comet grains it's

this stuff with aerogel capture cells on

board the probe is good to launch on

February 7th 1999 it blasts off on top

of a Delta rocket

we have main engine start and liftoff of

the Stardust spacecraft returning a time

capsule of the elements the formation of

our solar system

it's 3.2 billion kilometers Ernie takes

nearly five years but eventually the

night of the comet encounter arrives

admission control also and his

colleagues can do is wait

we had been many many simulations but uh

still the questions will it run into a

comment even if it does run into the

comet did it collect us at all you'd be

horrible it comes back and no does

finally in 2006 the capsule re-enters

Earth's atmosphere it makes a safe

landing and scientists rush its precious

cargo to the lab as soon as they open it

they know the mission is a success if I

capture one particle that I can see with

a naked eye with a microscope I will be

pleased but I caught more than three

dozen of them it's an incredible

achievement these are the first pristine

comet particles ever to return to Earth

the grains may be tiny but they contain

a wealth of information it will take

many years really to understand the

formation of the solar system it will

take probably a couple of decades to to

milk all we can from the starter samples

the team may have to wait decades for

all the results but in the meantime

others are trying a different approach

comets aren't the only source of

material from the early solar system

meteorites are ancient too and they have

one huge advantage

come to us the amazing thing is that any

time you pick up a meteorite with very

few exceptions you take it into the

laboratory you date it

the answer is it's four and a half

billion years old four and a half

billion years old these are pieces of

the early solar system and so here you

have in your hand a piece of something

very substantial that connects you and

connects our existence and connects the

history that brought us to be with this

larger universe after extensive analysis

Hester's team discover unstable isotopes

of iron 60 locked inside the meteorites

it's totally unexpected iron 60 decays

relatively quickly so it must have come

from close by if it came from any great

distance it would have had time to decay

into a different isotope before the

meteorite could trap it only a supernova

can make iron 60 so there must have been

an active supernova close to the infant

son to come along with a piece of hard

evidence that says that nope early son

early solar system were in a violent

environment near a massive star

experienced a nearby supernova it was

surprising we realized that this is

going to change the way that people

think about what goes on this is going

to change our understanding of the

origins of the solar system despite the

violent forces being unleashed our solar

system may have formed right alongside a

supernova

these are exquisitely violent events and

they reshape everything around them

shockwaves from exploding supernova

pummel everything within 20 light-years

a galactic bomb blast what's astonishing

is that all that destructive force may

have been exactly what was needed blast

waves can push dispersed matter together

like a broom sweeping up dust so the

collapse of our giant cloud of dust may

have been triggered by the blasts from

the nearby supernova they'll send

shockwaves and compress the gas were

light years around them and that

compression triggers the birth of other

stars the blast wave could have swept

together the material of our nebula when

compressed enough gravity would start to

have an effect and the cloud would begin

to collapse

within the cloud swirling about the

infant Sun the planets are beginning to

take shape and today scientists are

beginning to understand the astonishing

processes that formed them

for years the process of planetary

formation has puzzled scientists some

astronomers now believe planets grow

within the disk of gas particles

surrounding a newborn Sun but this

material is spread too thin for gravity

to pull it into clumps then in 2003 some

informal experiments onboard the

International Space Station shed some

light on this long-standing mystery

astronaut Don Pettit spends his free

time in space conducting a few

experiments to see how particles behave

in zero-gravity

he takes different grains such as coffee

sugar and salt places them in pouches

and watches as they begin to attach to

each other he realizes that the friction

between them as they rub together inside

the pouches is creating an electrostatic

charge that pulls particles together and

forms them into claps the same

electrostatic charges would have been

created if the experiment were carried

out on earth but on earth the fast

stronger force of gravity would have

made it impossible to observe the same

results Pettit and his colleagues

realized that what they're seeing in the

zero gravity conditions of the space

station would have also occurred in the

zero gravity environment of the early

solar system the particles in the proto

planetary disc may somehow get charged

and attract each other

it's a process known to planetary

scientists such as Jack lisara as

accretion we see signs of accretion in

our daily lives raindrops condensed as

very very tiny droplets around small

little pieces of dust and they collide

with one another as they fall in much

the same way that grains collide in the

solar nebula and accrete into bigger

objects once the clumps reach the size

of mountains they exert a significant

gravitational force it's more powerful

than the electrostatic force so the

whole process accelerates fast in about

1 million years the clumps can grow into

larger planetesimals or pieces of

planets and can keep on growing into

full-sized planets

gravity keeps these planets locked into

their orbits around the Sun

traditionally understood as a mysterious

pulling force since Einstein scientists

have a new understanding of gravity most

people know that it's the Sun's gravity

that keeps planets moving in orbits

around the Sun what we're going to do

here is we're going to take a little bit

deeper look at what the nature of

gravity really is now Newton's first law

of motion law of inertia says that an

object moving in a straight line of the

constant speed will keep doing so so

here on the overhead view we're looking

down at a flat surface and if I take

this ball which represents a planet and

I roll it across the surface and sure

enough it moves in a straight line at a

constant speed now Einstein said there's

actually more to it than that

Einstein said that if you took a mass

for example the mass of the Sun

represented here by this plunger that

that mass had the effect of distorting

the shape of space-time now to us it

doesn't look distorted if you look on

the overhead shot it still looks as flat

as it did before just like a chessboard

but in fact it is distorted and so now

when I take my planet and I move it

through this distorted space-time it

tries to follow a straight line but

there's no straight line available for

it to follow and so instead what it

winds up doing is it winds up moving

around the central object on an orbit

it's that effect of that curved

space-time that we usually call gravity

gravity exerts an equal force in every

direction so its effect is always to

attract matter evenly towards the center

of a body therefore when it's strong

enough gravity will restart any body

into a sphere this is why all planets in

the solar system form nearly perfect

spheres with the diameter of over 480

kilometers no other shape is possible

all the planets are roughly spherical

and all come from the same original disk

but they all end up as radically

different worlds from rocky Mars to gasy

Jupiter the differences are primarily

down to one basic law of physics

different kinds of material condense at

different temperatures it has to do with

given a given temperature what is a gas

and what is a solid and that's it at

first

all the matter around our young star is

so hot it's in a gaseous form then

further out as things cool material

begins to condense out of the cloud like

snowflakes forming on earth in the inner

solar system closest to the Sun it stays

hot far too hot for gases or ices to

condense at these temperatures they'd

evaporate instantly but metallic

particles can condense here and the

planet that forms in this zone is highly

metallic mercury

astrophysicist Steve desh is an

authority on planetary formation his

research has shown him just how hostile

world Mercury must be it has no

atmosphere and has no oceans it's just

rock and the part that faces the Sun

tends to get hot enough to melt lead for

example it's many hundreds of degrees

Fahrenheit the fierce heat is thanks to

Mercury's proximity to the Sun it's

almost three times closer than the earth

move further out and the temperature

eases a little by around five hundred

degrees Celsius now more rocky particles

can condense

so the rockier terrestrial planets Venus

Earth and Mars form in this region

since Venus orbits around 50 million

kilometres further from the Sun than

mercury we could expect it to be a more

hospitable place Venus is probably the

closest thing we have in the solar

system to your conception of health the

pressures are immense the temperatures

are high and it rained sulfuric acid

around 40 million kilometers further our

home planet the earth has been nicknamed

the Goldilocks planet it's not too hot

it's not too cold

it's orbiting at just the right distance

from the Sun and that makes it very

conducive to life Earth has a protective

ozone layer to shield us from harmful UV

radiation and a magnetic field that

deflects lethal cosmic radiation earth

is also the only inner planet to have a

large moon today scientists like Garry

Lofgren still study the lunar samples

first brought back by the Apollo

astronauts over 30 years ago

[Music]

Lovgren works in the secure lunar

storage facility where NASA keeps most

of the 380 kilos of rock brought back

from the moon for years now scientists

have been studying the samples to

discover how the moon and the wider

solar system may have been formed it's

an invaluable resource for geologists

and the Apollo astronauts made it a top

priority to bring samples back even if

it meant ignoring orders Neil Armstrong

personally made sure that we got a bunch

of rocks by basically sort of disobeying

the rules as I understand it he kind of

snuck out of the view of the TV camera

which he wasn't supposed to do and

filled up a box full of rocks when the

samples are analyzed back in the lab

they soon give up some surprising

information we found out quickly that

the chemistry was very different from

the earth and it wasn't the kind of

chemistry we expected from previous

ideas about the formation of the moon

the lunar rocks and therefore the moon

itself turn out to be younger than

previously thought all the theories we

thought up before we went there kind of

went out the window really quick and it

took a few years before good ideas began

to come forth

according to nebula theory there should

not be enough of the right material left

over from planetary formation for moons

to form in the inner solar system it

remained a scientific conundrum until

after decades of painstaking work

scientists came up with a new theory a

violent and devastating scenario that

involved the earth experiencing a

head-on interplanetary smash

in 1969 Neil Armstrong becomes the first

man to walk on the moon only when the

Apollo astronauts bring back lunar

samples as scientists finally able to

understand how our moon may have formed

their study of chemical isotopes in the

samples suggests the moon contains

material that was once part of the earth

it is a totally unexpected find

scientists are at a loss to explain how

this material got there but eventually a

bold new theory emerges a theory

suggesting that in the early solar

system there were not eight

but around 20 planet sized objects

orbiting the Sun when you think about

the early stages of planet formation you

can think of it as a sort of a pinball

game where there are these objects that

are careening all over the place

occasionally running into one another

4.5 3 billion years ago the young earth

is on a direct collision course with an

object named Theia

a body the size of Mars if we had that

collision today it would probably be the

end of mankind it would be the

equivalent of many many hydrogen bombs

exploding on the surface of the earth

it's a world shattering impact blasting

material with such force that billions

of tons of rock escape Earth's gravity

and eject into space this material is

captured by the Earth's gravitational

pull and enters into orbit over a few

million years it amalgamates and becomes

our moon

the collision is further evidence of a

violent early solar system a period when

countless bodies followed chaotic orbits

often they'd wreak havoc but some

objects became permanent features of our

system the two Martian satellites for

example Phobos and Deimos are both

believed to be former asteroids captured

by the gravity of Mars these two moons

are relatively tiny even the larger body

Phobos is just one-tenth the size of our

moon although Mars itself is only

one-third the size of Earth it has some

of the most extraordinary features in

the solar system Mars is a spectacular

planet a geologists wonderland for

example it has the solar system's

largest canyon Valles Marineris which

has side tributaries which would dwarf

the Grand Canyon on earth it's about the

length of the United States it has the

solar system's largest volcano Olympus

Mons which with dwarf Everest

Mars is the last of the four inner

terrestrial planets the rocky bodies

orbiting inside the asteroid belt that

these planets stopped growing at their

present-day size was largely down to the

question of supply rock and metal was

scarce in the early solar system making

up only around 0.6 percent of the total

material available also the inner

planets couldn't accumulate gas

compounds because it was too hot for

them to condense here

all of the gas compounds condense much

further out before we reach this zone in

the outer planets

there's the asteroid belt to contend

with

science fiction paints this is a crowded

zone difficult to cross we now know that

there are at least tens of thousands of

asteroids in the asteroid belt that are

a mile or larger in size nevertheless it

would be very unlikely if you were

piloting your spaceship through the

asteroid belt to run into one of these

in fact we have to aim very carefully to

get space probes to visit asteroids the

average distance between asteroids in

the asteroid belt is about 1 million

miles as we traverse the asteroid belt

we hit an invisible but critical border

it's called the frost line cross this

boundary in space around 450 kilometers

out from the Sun and we find totally

different kinds of planets there was a

frost line somewhere in the asteroid

belt beyond which it was cold enough for

water to condense and this was a gradual

transition but a very important one

beyond the frost line the outer planets

take shape these are very different

worlds to the inner planets and that's

due to the kinds of substance that can

condense in this cooler region colder

than minus 70 degrees Celsius hydrogen

compounds like water methane and ammonia

are able to condense and there are

trillions of tons of this material

available in the early solar system

but it's only once the outer planets

reached ten times the mass of Earth that

their growth really takes off at that

critical mass they have enough

gravitational influence to start sucking

up trillions of tons of gas there's a

snowball effect as they get bigger the

planets attract ever more gas it was a

race about how fast these cores could

grow compared to how long the gas disks

hung around Jupiter Saturn just happened

to be at the right place in the solar

system they quickly become gigantic

super planets and since gas now makes up

90% of their mass they're also known as

the gas giants largest by far is Jupiter

this giant planet has a mass of over 300

times that of Earth while it's volume is

more than 1,000 times greater according

to how Leverson we're fortunate that

Jupiter did reach this size the planet's

gravity is so powerful it often changes

the trajectory of comets that could

otherwise enter the inner solar system

Jupiter actually plays the role of a

protective big brother that protects us

from the hospital environment that

exists beyond its orbit and in

particular there are a lot of these

small icy comets that could come in and

hit us that Jupiter deflects away

throwing them out of the solar system

Saten is the second largest planet in

the Solar System it turns out we're

lucky it never got any bigger according

to our simulations if you have two

planets the size of Jupiter in one solar

system they will eventually knock out

all the other planets now we were close

Saturn is not that much smaller than

Jupiter so we were right on the border

of being a stable solar system had

Saturn been a little bit bigger we

probably wouldn't be here talking about

it it's a terrifying thought the gravity

of a larger Saturn acting in concert

with Jupiter could have sucked in and

destroyed all the smaller planets

thankfully Saten never follows this

fateful path instead it grows into one

of the crowning glories of our solar

system

whenever I show people planets in my

telescope I always say Saturn for last

it's beautiful it's so stunning it's

definitely a gorgeous planet because of

its large rings the Rings are made up of

billions of fragments these fragments

continually grow and disintegrate due to

the conflicting gravitational forces of

Saturn and its moons

Uranus lies nearly twice as far out from

the Sun as Saturn both it and Neptune

are icy worlds with much less gas than

the other gas giants in a sense they got

to the feast Thule Uranus is a

plain-looking place a flat green color

thanks to a layer of methane high in its

atmosphere since it forms further out it

becomes an ice giant rather than a gas

giant temperatures average minus 212

degrees Celsius

since Pluto has been officially

downgraded to dwarf planet status blue

Neptune is now our outermost planet

until recently these bodies were thought

to mark the very edge of our solar

system our idea the solar system changed

drastically really in 1992 when this

structure that we call the copper belt

today was discovered around 45 billion

kilometres out from the Sun the Kuiper

belt home to an estimated 70,000 large

icy objects there's a whole population

of leftover dregs of planets formation

you can think of it as right where these

things haven't quite come together then

a thousand times more distant from the

Sun lies the mysterious Oort cloud

comprised of up to a trillion comets

most of these probably formed much

closer to the Sun as part of the same

process that formed the planets but the

gravitational influence of the young

Jupiter and Saturn forced most of these

comets onto very long orbits carrying

them to the far reaches of the solar

system the outer edges of the Oort cloud

really are the final frontiers almost a

full Lightyear out from the Sun beyond

our solar system lie billions of other

stars many with planets of their own for

a long time it's been one of the Holy

Grails of astronomy to actually find a

planet around another star the only

solar system the only planets we knew

about were our own so what was the first

plant going to be like around another

star

then in 1992 comes the discovery they've

been waiting for the first-ever

detection of an EXO solar planet a

planet orbiting a distant star and in

the very first planet we find outside

our solar system is bigger than Jupiter

way up against the star it completely

threw out everything that we'd been

taught about solar systems there is

absolutely no way that an object that

big could form close to their star

there's just not enough stuff there the

nature of these planets simply doesn't

fit within standard models of solar

system formation and it's not just a

problem for distant worlds when we look

back at our own solar system it becomes

clear there are problems with the

standard theory of how our planets

formed it's interesting when we try to

apply our models our understanding of

planet formation to Uranus and Neptune

they fail utterly you just can't build

those planets where we see them today

there simply should not have been enough

material available at this distance from

the Sun for planets this size to grow

it's a mystery Uranus and Neptune seem

to be in the wrong place and this is

just one of several mysteries that

standard nebula Theory cannot explain

there's also the enigma of the Kuiper

belt according to the model it shouldn't

really be there even more puzzling our

moon's craters all seem to date from the

same time together these problems

threaten to undermine scientific

understanding of how the solar system

forms

the moon maybe our closest neighbor but

it still harbors secrets with no

substantial atmosphere or weather the

lunar landscape experiences little

change so it was believed that one

impact strike every few thousand years

would be enough to explain its cratered

surface but then the Apollo missions

brought back lunar rock samples

including crater debris thrown out by

ancient impacts and these samples tell a

very different story it became clear as

we studied the moon that a large number

of the impact craters formed around 3.9

billion years ago and clearly there was

a high concentration of impacts on the

moon during that time Luna geologists

use radiometric dating to assess the age

of the samples it came as a big surprise

to find out that a high proportion of

moon craters formed during one

relatively brief period of bombardment

dr. David kring is an expert on this

phase known as the late heavy

bombardment or lunar Cataclysm this

evidence from the moon is actually

telling us something about the entire

origin and architecture of the solar

system and how it evolved tens of

thousands of large meteorites must have

hit the moon during the lunar Cataclysm

earth too must have taken a pounding

today only younger craters like this

20,000 to 50,000 year old example in

Arizona remain on earth most older

craters have disappeared but three and a

half billion years back there'd be no

chance of missing them

we're talking about twenty to forty

thousand impact craters that would have

had enough energy to change global

conditions where could such a vast

number of impact objects come from

within the impact craters we have

chemical traces of the objects that

produce them and those point to the

asteroid belt it seems to match the

objects that once hit the moon are

chemically consistent with the rocks

found in the asteroid belt so this is

almost certainly where they came from

but today most asteroids are in stable

orbits there's nothing obvious that

could force thousands of them out of the

asteroid belt and into the inner solar

system so for decades the bombardment

was a scientific puzzle in 2004 how

Levinson

had a Eureka moment working with an

international team he came up with a new

model of solar system formation it's

like a light going off you sit in your

office and you say I've solved this

problem this problem people have been

working on for decades and I'm the only

one in human history that understands

this little part of the problem we're

trying to solve

Levinson's idea assumes that the planets

might have formed in very different

orbits and then moved most scientists

had believed the solar system was stable

and relatively predictable but many

experts now believe this is totally

wrong this flies in the face of

conventional wisdom astronomers probably

always thought the planets formed where

we find them today these new numerical

simulations which show that they

migrated a substantial amount and even

had close encounters that caused them to

switch their order is startling and new

in Levenson simulations Jupiter and

Saturn tugged on each other whenever

their orbits came closest together but

overall these poles cancelled out only

when Jupiter circled the Sun exactly

twice for every time Saturn circled once

did something dramatic seemed to occur

it's called a two-to-one resonance and

it happened 3.9 billion years ago Mac

configuration actually makes the planets

jiggle with one they perturb one instead

of cancelling out the two planets

gravitational tugs then work in the same

direction to Levenson it's just like

pushing a swing each well-timed push

nudges the swing higher for Jupiter and

Saturn each gravitational nudge

stretched their orbits until they

reached their present-day patterns but

the process had a fateful impact on

other planets Uranus and Neptune can't

stand that at all and they just go

totally bonkers the orbits cross one

another they gravitationally get close

to one another one of them gets thrown

out the other one gets thrown in they

rattle around off the Jupiter and Saturn

according to the theory Uranus and

Neptune may even switch places

ripping surrounding swarms of asteroids

out of their orbits it's a recipe for

disaster

some asteroids are thrown out into space

while others are pushed into the Sun

thousands slam into the inner planets

and Earth's moon

it sounds like an improbably chaotic

transformation but the evidence now

suggests this did happen it could never

happen today Jupiter and Saturn are now

too far apart but this event may have

been a key moment in the formation of

our solar system but the really

incredible thing is that it could so

easily have turned out very very

different it could have the wrong type

of star you could have too many giant

planets there are many ways things can

go wrong

and we're right there on that thin lucky

line of existence instead of the diverse

solar system we now see we could have

remained forever

as ashes gas and dust and what of our

future 5 billion years from now as our

Sun runs out of fuel it will expand

consuming some of the inner planets

other planets may survive as ghost

worlds still orbiting their dead son

there will be no life of any kind but

for now we're lucky to be alive

you

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