All language subtitles for HTUW S04x08 The First Oceans

SUB BY : DENI AUROR@
https://aurorarental.blogspot.com/

Life on Earth depends on seas, rivers and rain.

But is our blue planet unique?

Or did the universe create countless other wet worlds just like it?

Unlock the secrets of Earth's first oceans

and we'll unlock the secrets of alien life.

The Earth is the only planet we know of that has oceans of liquid water

covering its surface.

And it's the only planet we know of that has life.

If you look at every living organism on Earth,

you can see that each one has a fraction of water

that makes up the system.

We're basically bags of water

that allow chemicals to move around and do things that we call life.

On Earth, liquid water and life go hand in hand.

But how lucky are we to have a watery oasis to call home?

We have a lot of water.

Was it made here when the Earth was made?

Or was it brought here later by something from space?

Is it a fluke to have a water world like this, or is it inevitable?

For decades, scientists have been trying to establish the origins

of Earth's water and they've come to a surprising conclusion -

our planet shouldn't be wet at all.

The place where the Earth is right now seems very dry.

So if the Earth formed as a dry rock around a hot young star,

then how did this water get here?

Every possibility has problems, and we want to know the answer.

Tracing the exact source of Earth's water

is surprisingly complex.

The journey starts over 4.6 billion years ago,

during the formation of our solar system.

A vast cloud of gas and dust hangs in space...

..teeming with vast quantities of hydrogen and oxygen.

Oxygen is one of the most abundant atoms in the universe.

Hydrogen is the most abundant atom in the universe.

You're gonna get a lot of whatever it is they form.

Over millions of years, these highly reactive atoms

bind together to form H2O, water.

Water is a fairly simple molecule.

It's made of two hydrogens and one oxygen.

This newly formed water sticks to dust grains inside the gas cloud,

and freezes to form crystals of ice.

Eventually, the icy dust cloud becomes so dense

that it starts to collapse under its own gravity.

It's the start of a process that will create our entire solar system.

There's enough water here to fill the Earth's oceans

three million times over.

When we see stars that are forming right now...

and we study hundreds, thousands of them,

we see discs of material beginning to orbit around the young stars.

Gas, dust, and there's certainly quite a bit of water

in that material.

Gravity pulls more and more material into the centre of the cloud,

raising the pressure and temperature.

Eventually, the extreme forces spark nuclear fusion...

..and a protostar, our infant sun...

..bursts into life.

It's bad news for the water surrounding the newly born star.

The environment of a star when it forms is incredibly hot and violent.

Any water that was existing in that region,

because water's a volatile material, would be destroyed.

Water cannot exist near a star early on during its formation.

Astronomers believe the early sun may have sucked up

much of the dust and water surrounding it.

And then blasted this debris far out into space

in superheated jets of steam.

In the Earth's most volcanic places,

a similar process blasts hot water high into the air.

Deep underground, the water is superheated

and there's nowhere for it to go when it turns into steam,

and it's driven outward in these giant plumes.

Protostars also have lots of water around them.

And the magnetic fields around a protostar create, basically, tunnels

that the water can escape from and it's blown out along these tunnels

in giant jets that spread water out all throughout the galaxy.

In space, superheated water escapes

through the magnetic weak spots at the poles of protostars.

Pockets of water inside these vast, steamy jets

eventually solidify in the cold of space to form ice pellets.

And these speed away from the protostar,

80 times faster than machinegun bullets.

Is this really what happened in our young solar system?

It would have been so awesome to be there to see that.

But the lucky thing is that our galaxy continues to form stars,

so we can study protostars all over the galaxy

and look at things that are a lot like what the sun experienced.

In 2011, astronomers witness the formation of a star

just like our own sun.

Their telescopes reveal a central ball of gas

dragging in matter from the clouds surrounding it...

..before blasting out water at a rate equal to ten million Amazon Rivers.

It's believed a similar process ejected much of the water

in our embryonic solar system.

As the sun matures, the jets dry up

and a new threat to the remaining water emerges,

a hot stream of charged particles known as solar wind.

As the sun heats up, the ice nearby is turning into water.

And as the sun heats up more, it's turning into water vapour.

And then as the sun turns on its solar wind and becomes bright,

it starts to blow that water out.

The solar wind blows in a supersonic stream of plasma

from the sun's outer layers.

It strikes the surrounding cloud,

blasting away most of the gas and water vapour.

What's left behind is just dust with traces of water clinging to it.

Further out from the sun,

the solar wind has less impact and it's also much colder.

The result is a boundary of water ice half a billion miles from the sun,

known as the snow line.

When astronomers talk about the snow line what they mean is

how far away from the young sun was water able to condense.

Where did it get cool enough for water

to finally condense into droplets, get onto objects, become ice?

Any closer than that and you're just gas.

The solar system's first and biggest planet

is born at the snow line.

Where the ice is thickest, clumps form.

And then, attracted to each other by gravity, join up.

A colossal snowball builds.

It draws in all the matter around it,

eventually creating the gas-giant planet Jupiter.

Where the snow line is thinner,

a similar process forms the other gassy planets...

..Saturn, Uranus and Neptune.

On the inner, dry side of the snow line, dust clumps together,

forming a family of small, rocky planets, including...

..Earth.

Many astronomers believe our planet is fashioned from little more

than arid rocks with microscopic droplets of water sticking to them.

But even this precious reserve of water is about to be threatened

by the most violent event in our planet's history.

*

*

Our solar system, 4.6 billion years ago.

Earth is just one of many large, rocky balls forming around the sun.

And our young planet's gravity continues to pull in chunks of debris

from the surrounding dust cloud.

These rocks hitting the Earth hold tiny amounts of water,

remnants of a time before the sun sparked into life.

Planetary scientist Dan Durda believes this water

had little chance of survival on the newborn Earth,

thanks to the heating effects of multiple high-speed asteroid impacts.

Building a planet is a very violent process.

We can demonstrate pretty easily here with the high-speed impact

of a bullet.

OK. So that's a single impact.

When the bullet punches into the target,

some of its kinetic energy is converted into heat.

You can see this sudden hot burst using a thermal imaging camera.

In the case of a real impact, a large asteroid impact,

the energy is a lot greater. You're actually melting rock.

And four-and-a-half billion years ago,

impacts like that were happening once a month.

Let's go see what we got up there.

A sub-machine-gun demonstrates how this heat would have built up

after multiple asteroid impacts.

The cumulative effect of all these impacts is

to heat the surface of the Earth to near magma, lava-like temperatures.

The combined energy of the impacts boils the surface of the young Earth.

There's a lot of impact and it's a very high-temperature activity,

so any water that would be present, it would be hard to hold onto it.

Because of the heat and the energy, the water probably escaped.

After 60 million years, the planet-building stops

and Earth's surface cools enough to form a crust,

potentially trapping any remaining water inside it.

But not for long.

The crowded early solar system is home to more planets

than exist today,

and one, known as Theia, hurtles towards Earth.

Smashing into Earth,

Theia gouges out a huge chunk of our planet's crust.

The rocky fragments create a colossal ring of debris

that will eventually coalesce to form the Moon.

Reeling from the impact, Earth reverts to a ball of lava,

and the heat drives off yet more water.

The collision with Theia leaves the crust of the Earth bone-dry.

So where does the water that we see today come from?

There are only two possibilities.

In order for the water to survive, it either has to be embedded

deeply enough in rocks that it isn't melted and evaporated,

or it has to come to Earth after it forms.

Was our planet originally formed from much wetter rock

than the scientists had believed?

Or were the oceans delivered to the Earth much later...

..from somewhere else?

Initially, delivery seems the most likely possibility.

Far out beyond the orbit of Neptune,

lies a vast band of icy material called the Kuiper Belt.

It's made up of the leftover building-blocks

of the gas-giant planets.

Occasionally, chunks of this icy debris, known as comets,

tumble into the inner solar system.

Did such marauding comets bring water to the early Earth?

Comets are basically big, dirty snowballs.

They're giant balls of ice that have rock, pebbles, gravel, dust,

embedded in them.

We think that comets are about 50% made of ice.

So if you looked at everything in the solar system,

trying to find a source for water on Earth

there's seemingly an obvious answer, and you'd look at comets.

Passing comets present some of the most spectacular sights

in the night sky.

As they approach the sun, the solar wind blasts water from the surface

of these dusty snowballs,

generating a bright tail that can stretch for millions of kilometres.

Today, comets are relatively rare visitors to the inner solar system,

but four billion years ago, they were common

and Earth was in the firing line.

It's completely reasonable to expect

that icy bodies from the outer solar system

came inward and hit the Earth.

How much of a contribution is the question?

Rocks dating to soon after the Moon formed

prove the early Earth had vast oceans.

But just how many comet impacts would it have taken to fill these seas?

The answer is staggering.

Comets come in different sizes,

and if you take the run-of-the-mill average comet,

it might take 20 or 30 million comets to make the Earth's oceans.

If millions of comets did bring water to the early Earth,

they must have done it in a very short period

and just after the Moon formed.

Astronomers scour the solar system for evidence

of this rapid-fire icy attack.

And they find it in the most unlikely of places -

our seemingly waterless moon.

Hey, I've got a picture. Yeah?

Pick up that little rock. Atta boy.

In the 1970s, Apollo astronauts collect rocks

from the Moon's largest craters to determine when they formed.

They brought them back to our laboratories and we could date them.

When were those rocks actually made?

Planetary geologists had assumed these craters had been

blasted out around the time the Moon formed.

They were in for a big surprise.

We found that many of the big impact basins on the Moon

were formed not in the earliest days of the accretion of the Moon,

but several hundred million years later

During the period we call the Late Heavy Bombardment.

Late because it happened several hundred million years

after the Earth and Moon had formed.

The Late Heavy Bombardment begins when the gas giants align.

Their combined gravity disrupts a vast belt of asteroids

lying close to Mars.

Sending a shower of rocks towards Earth,

the Moon and the inner planets.

Then Neptune swings outwards,

smashing into the comets of the Kuiper Belt

and sending many of them hurdling inward, too.

All hell breaks loose.

99% of the Kuiper Belt and the asteroid belt disappear,

lots of bodies get thrown every which way.

We look at the Moon, we see that it is scarred,

it is covered with craters.

The Earth didn't somehow magically escape that same bombardment.

For every crater you see on the Moon,

the Earth is a bigger target out there in space,

there were probably 20 or 30 craters formed on the Earth.

We don't necessarily see them everywhere today

because it's a lively planet with geologic processes

that erase those craters.

The Earth is pummelled by the Late Heavy Bombardment.

But how many of these impacts were delivered by water-rich comets?

We don't know.

And that's one of the forefront science questions is,

could the comets have come in to deliver ocean water at that time?

If comets made our oceans,

they should have left a unique chemical signature behind,

because not all water is the same.

On Earth, for every 10,000 drops of ordinary water,

there exists three drops of semi-heavy water,

a rare molecule made from deuterium instead of hydrogen.

Deuterium is a normal hydrogen nucleus,

except instead of just being one proton by itself,

it's a proton and a neutron connected together.

The extra neutron in deuterium adds weight,

and that's why water made from these atoms is called heavy.

Semi-heavy water forms more easily in cold conditions.

So the edges of the solar system have more of it

than regions closer to the sun.

The ratio of deuterium to hydrogen is a very sensitive probe

of where water formed in our solar system.

And therefore we can look at the abundance of heavy water

to determine where that water formed and how.

In 1986, scientists get their first opportunity

to test the chemistry of cometary water...

when Halley's Comet makes a fleeting return to the night sky.

Astronomers look for the telltale signature of semi-heavy water...

..but the result is not what they're expecting.

They measured the water for the first time

and found it was about twice as heavy as Earth oceans.

Then in the 1990s,

comets Hyakutake and Hale-Bopp pay a visit.

Just like Halley's, these comets are as old as the ones

that smashed into the Earth during the Late Heavy Bombardment.

But both Hyakutake and Hale-Bopp

also turn out to have way more semi-heavy water

than the Earth's oceans.

People started to get worried

because the entire Earth water budget, as measured by oceans,

could not be made of just these comets.

In 2015, the Rosetta space probe analyses a comet up close,

and this time, the data is indisputable.

A semi-heavy water content

three times greater than the water on Earth.

For the most part, the chemistry of Earth's oceans and atmospheres...

..is actually a very poor match for the chemistry of comets.

When you look at the flavour of hydrogen in the water molecules

that make up the comets that we've measured so far,

it doesn't exactly match

the flavour of the water that we find on our planet.

It's clear in my mind

that comets could not have brought all of Earth's water.

But if the dirty snowballs weren't to blame, what was?

An unexpected candidate begins to emerge.

In 2011,

the Dawn space probe flies by the giant asteroid Vesta.

We used to think rocky objects like Vesta were completely dry,

but the scientists see evidence of water on the surface.

And Vesta's water turns out to be a perfect chemical match

for Earth's oceans.

The type of water is exactly what we think

is contributing to the water on the Earth.

So it looks like a really solid deal

that those types of asteroids were putting the water on the Earth.

Scientists turn their focus from comets to asteroids.

But how could these dry-looking rocks have provided enough water

to fill the Earth's oceans?

The explanation could lie within the haunting remains

of a failed planet.

Today our solar system hosts four rocky planets.

Mercury,

Venus,

Earth...

..and Mars.

But there should have been five.

Billions of years ago, planets were forming all over our solar system.

But there was an area in-between Mars and Jupiter

where the gravity of Jupiter pretty much pulled apart anything

that tried to form.

The scattered remains of that failed rocky planet

now fill this gravitational battleground.

Its debris forms a vast band of rubble around the sun,

called the asteroid belt.

Rocks inside the asteroid belt range in size from grains of sand

to giant boulders hundreds of kilometres wide.

When I first started studying astronomy, we called them rocks,

dry rocks.

Now we understand that there may be a lot of water,

maybe even liquid water on some of the larger asteroids.

Our new-found understanding of asteroid water

comes from a study of meteorites,

tiny fragments from the asteroid belt that occasionally fall to Earth.

I've got a sample of a meteorite called a Carbonaceous chondrite.

And it looks and feels rather dry to the touch,

but I can tell you that that sample actually contains

about 20%, by weight, water.

Even crushing doesn't release the hidden moisture,

because the water is chemically bound to the minerals

that make up the rock.

Yeah, let's see if we can get some heat going here on our...

on our burner.

Heat allows the water molecules to break their chemical bonds

and escape as vapour.

Look at all that water coming out.

Just this small sample of meteorite is driving off all of this water.

So here is direct tangible evidence of the amount of water,

the astonishing amount of water that can be delivered to the Earth

from the impact of asteroids.

Four billion years ago, countless asteroids

smash into Earth during the Late Heavy Bombardment.

Each impact generates an intense burst of heat

that releases the water trapped inside the asteroid.

This water vapour then falls back to the ground as rain.

And this same water remains with us to this day, in our oceans...

..our rivers...

..and even in our coffee cups.

When we look at this fingerprint of deuterium on the Earth's water,

it better matches meteorites and asteroids than it does comets.

So, yes, certainly some of the water came from comets,

but the majority of water in your body right now,

amazingly, may have come from the asteroid belt.

But water-bearing asteroids may not completely solve

the mystery of Earth's first oceans.

A remarkable new geological discovery suggests these impacts

only tell part of the story.

There's an amazing amount of water on the surface of the Earth.

The Pacific Ocean has an area of roughly

half the surface of the Earth.

Millions and millions of cubic miles of water

and yet that's not where all the water on Earth is.

There's quite a bit of it under the surface.

In recent years, geologists have made a stunning discovery -

a layer of heated rock lying deep below the Earth's crust,

which holds vast quantities of water.

Seismologists stumble on the layer

while analysing the rumble of earthquakes.

When a big earthquake strikes, low-frequency sound waves

travel through the difference layers of Earth's interior

before reaching the crust on the other side of the planet.

Studies of these long-range rumbles show some of the sound waves

slow down when they reach a scorching layer of rock,

sitting 480km below the crust.

And there's only one thing known to delay the passage of sound

through rock -

water.

Now, it's not like an ocean of water.

It's water molecules bound up in minerals and with other molecules.

But if you take all that water and put it all together,

we think it actually would add up to more than all the water

in all the oceans on the Earth combined.

This vast underground reserve of water is a genuine puzzle...

..because there's no way comets or asteroids

could have penetrated so deeply below the Earth's crust.

That's actually inside the Earth.

It doesn't seem that there's an easy way to get it from the surface

down hundreds of miles into the mantle.

So it seems far more likely that that water that exists,

that was discovered, came with the Earth when it formed.

Was the Earth born wet?

It's a controversial idea, but the evidence is mounting.

Steve Mojzsis believes this grey dust provides

the most conclusive proof to date of the wet-birth theory.

This is a vial filled with little zircon minerals.

These zircon minerals are amongst the oldest known substances

that we have from our planet.

And this sample here formed a mere 150 million years

after our planet formed,

and it's the very best record of the earliest Earth.

Until recently, scientists believed Earth was a scorched, dry ball

this early in its history.

But ancient zircon samples paint a very different picture...

..because the zircon contains the chemical signature

of the Earth's first water oceans.

The amazing find from samples such as these

is that liquid water on our planet is a primordial phenomenon.

These tiny zircon crystals are compelling evidence

that the Earth was bathed in liquid water,

millions of years before the Late Heavy Bombardment

brought comets and asteroids to the Earth.

Vast quantities of water must have been in the mix

when the Earth was created.

But this simple fact

means everything we think we know about the birth of our planet...

is wrong.

The heat of the sun evaporates water

from the surface of the Earth's hottest places.

Our vast, parched deserts are almost liquid-less.

Five billion years ago, the great cosmic desert

that stretches from the young sun to the snow line

is just as dry.

How could the wet interior of our planet

form out of this rocky, arid dust?

We think that the materials that were forming in the solar system

right where the Earth is today, would have been much more dry

than the Earth actually is.

So we think that the Earth had to get an extra contribution

of water-rich material.

Where did all this extra cosmic water come from?

Something must have transported it in bulk

from the wet side of the snow line.

A clue comes from observing distant exoplanets,

being cooked alive by their parent stars.

What we see a lot of are Jupiter-sized planets

sitting really close to their star, sometimes extremely close,

sometimes much closer than Mercury is to the sun.

Initially, these star-grazing giants were a mystery.

How did they grow so big so far away

from the icy riches of the snow line?

The only possibility is that these planets must have formed

far out from their parent stars

and then later, migrated in.

We know that those kind of planets can't form there,

they're simply too big.

They must have formed farther out and moved inward,

migrated towards their star.

And that is interesting because that makes you wonder,

was our solar system always the configuration it is today?

Or have our planets moved back and forth?

Exoplanet observations have forced astronomers

to devise a radical new theory

about the formation of our own solar system.

Known as the Grand Tack Hypothesis,

this theory suggests Jupiter radically altered its course.

There was a time when the disc of dust and gas

was very thick around the young sun.

And that actually put a drag on planets as they orbited around.

In the Grand Tack model,

Jupiter forms on the outer, wet side of the snow line.

But, slowed down by the matter around it,

the gas giant's orbit spirals in closer to the sun.

There's amazing evidence that Jupiter may have moved in

as far as the orbit of Mars.

As Jupiter moves in, it brings with it

massive quantities of water from beyond the snow line.

This is a chance to push material

from much further out in the solar system,

and throw it into the region where the Earth is forming.

A chance to add a bunch of water-rich material

to an otherwise dry Earth.

It's kind of like a huge snowplough,

just blasting this material and pushing it inwards.

So that while the Earth was forming,

Jupiter could have been scattering a bunch of icy bodies

from the outer part of the solar system in to where the Earth was forming,

while the Earth was still being put together.

Jupiter's inward spiral stops after 100,000 years,

when Saturn forms.

As the gravity of these two massive planets interact,

they change tack, heading away from the sun.

The water Jupiter leaves behind clumps together with dust

to form Earth and its neighbouring rocky planets.

But how did this water trapped inside the Earth,

turn into the first oceans?

Volcanoes may have played a crucial role.

Think about the very young Earth as a blister of volcanic activity.

You see these giants clouds of ash and dust falling down,

but in there, there also would have been water vapour.

Water vapour that could have cooled and condensed in the atmosphere,

built up clouds over hundreds, or maybe even thousands of years.

Until there was a moment

when there was enough water in the atmosphere to begin to rain.

There really was a first rain, billions of years ago.

As this volcanic water rains down on the surface of the Earth,

the first rivers and oceans develop.

This happens long before the Late Heavy Bombardment

that brings comets and asteroids to Earth.

Based on evidence from the rocks, it appears that that liquid water

is indigenous, native to our planet.

Comets and asteroids brought some water to Earth.

But if the Grand Tack Hypothesis is correct,

then Jupiter delivered most of the water

we see filling our oceans today.

And Earth wasn't the only planet watered by Jupiter's foray

into the inner solar system.

Both Mars and Venus may have once had oceans, too.

To truly appreciate how remarkable our living blue planet is...

..we need to find out why we remained watery,

whilst our planetary neighbours dried up.

Water defines the sights and sounds of our planet.

As vapour, it paints the sky with rolling clouds.

And as a liquid, it sculpts and shapes the Earth's surface.

Water fills every cell of every living thing.

And seen from space, our brilliant blue oceans are unique,

a stark contrast to our drab planetary neighbours.

Looking at our nearest neighbours,

we see the catastrophe that happens when you lose water.

Not only is water important for biological life,

but the evolution of a planet really changes

when you lose this particular molecule.

All the inner planets were sculpted from the same materials,

and there's good evidence that both Venus and Mars

once had oceans, too.

Mars had a lot of water in its past.

The whole surface is covered with these incredible rivers

and stream beds that are empty now,

but really looked just like what we see on the Earth.

And with Venus we think we see evidence

that it was also a water world when it was young, too.

Although we still haven't explored Venus

as well as we've explored Mars.

So the best evidence we have suggests that all of these planets

started out wet and went through a watery phase.

Clearly something is happening in the intervening billions of years

that is erasing the water away from these planets.

Why did our neighbours dry up?

Around four-and-a-half billion years ago, Mercury forms.

The closest to the sun of all the planets in our solar system,

it's also the smallest.

Tiny Mercury barely outsizes our moon.

And when it comes to holding onto surface water,

size matters.

If it's a small planet,

it's actually going to lose water to space

because it doesn't have the gravity to hold onto it.

Next in line from the sun sits Venus.

Earth-sized Venus holds onto its water, at least for a while.

Four billion years ago, Venus and Earth looked like twins.

Both have oceans of liquid water

and both are cloaked in thick atmospheres.

But Venus takes up residence closer to the sun

and grows hotter.

Its oceans evaporate,

pumping the atmosphere full of water vapour,

a powerful greenhouse gas.

And Venus heats up even more.

Venus got itself into a terrible vicious cycle.

Water got baked out of rocks.

The minerals themselves were baked to such high temperatures,

they released their water vapour.

There was no way for the water to condense.

It was too hot so there were no rains

and so the water moved higher and higher

into the atmosphere over time, where it got blown away.

And now Venus is this hellish landscape,

cooked under a heavy atmosphere.

Of all the rocky planets,

Mars sits furthest from the sun.

Billions of years ago, an ocean a mile deep

covered half its northern hemisphere.

But it wasn't heat or a lack of gravity

that caused Mars to lose all its liquid water.

Mars's oceans were blasted away...

by radiation.

Mars does not have a magnetic field and you need a magnetic field

to protect yourself from the solar wind,

these sub-atomic particles blasting away from the sun.

Earth has a magnetic field,

generated by its spinning molten-iron core.

This field protects our atmosphere from the solar wind.

Mars, however, is smaller than Earth, and its core cooled,

shutting down its magnetic field

and exposing its atmosphere to the savagery of solar radiation.

Mars's water didn't stand a chance.

Incoming radiation split apart the hydrogen from the oxygen.

So the hydrogen's very light

and just went to space and it was gone.

So then we were left on Mars with a lot of oxygen.

This is why we hypothesise Mars is a red planet

because it's very rusty, and that's because

all the oxygen that used to be in the water is now in the rocks.

Mars's oceans evaporated, leaving behind traces of ice

and staining its landscape a vivid red.

Four rocky planets created at the same time

from the same building materials...

..but only one got lucky.

Water is fundamental to life on Earth.

It's the perfect solvent for organic molecules

to let the machinery of life do what it does.

You and I could not survive without water.

Water enables the geology, it enables the climate

and it enables the biology of Earth.

So I think that that marks it as a pretty special substance.

It took 14 billion years

and a great deal of luck for our watery Earth to form,

and then to stay watery long enough for life to evolve.

And the more we learn about water,

the more we'll discover just how many other worlds

in the universe got lucky, too.

And as we discover more and more exoplanets,

and more and more planetary systems,

we're gonna get a better lay of the land.

And have a better view of whether or not our system

is something that could be common,

or whether something like the Earth is actually rare.

It's going to require tomorrow's technology to get a better view.

For now, our telescopes don't have the power

to see exoplanets clearly enough to identify water.

These places are still so far away

that we're not gonna be able to resolve pictures of oceans

and continents and little clouds whipping around.

But chemically, we could detect the signs, not only of water vapour,

but organic molecules.

Scientists hope the next generation of telescopes

will detect water on Earth-sized exoplanets

by analysing the chemical signatures of light

passing through the atmospheres of these distant worlds.

And when that happens,

there's a good chance we'll discover a Milky Way,

packed full of watery worlds.

And we've already discovered planets

where, in principle, liquid water could exist.

We don't yet know for certain. We will find out.

I think in the next few decades, we will know.

We will be able to identify a planet where we can say,

"Yes, there's liquid water on the surface of that planet."

That will be when the universe changes

and we really grow up and realise we have brothers and sisters

in the Milky Way galaxy.