All language subtitles for Universe 02

At this precise moment,

on a planet far, far away...

an alien sunrise ushers in a new day.

But will alien eyes gaze upon it?

Or will it go unseen?

Just another moment
in a vast sterile universe.

The hunt is on for the answer.

Magnificent desolation.

- Beautiful view!
- Isn't that something?

The Milky Way.

Hundreds of billions of stars...

spread across
100,000 light years of space.

Among them the Sun,

with eight planets orbiting around it,
including our home.

Until very recently, these were
the only worlds we knew of.

The only planets
we could hope to explore

for signs of life beyond Earth.

When I first got into astronomy
back in the 1970s,

we knew of no planets
beyond our solar system.

We didn't have the technology
to detect them, even if they were there.

Our neighbourhood was the only place
we could look for life.

And so the hunt for life began
in our own backyard.

Over the last few decades,

multiple missions have explored
our solar system's planets.

And even some of their moons.

But to date,
even as we continue to look,

no convincing evidence of life has
been found on any of these worlds.

Earth remains one of a kind.

The only living world around the Sun.

But as the exploration
of the solar system continued,

another search had begun...

for worlds that lie
far beyond these shores.

You know, the wonderful thing
about astronomy is

that as we develop better
and better technology

and accumulate more and more knowledge
about our universe,

we turn more and more of these points
of light in the sky into worlds.

I mean, that we've known is
a world for a long time,

because that is the planet Mars.

But just above Mars tonight is
a constellation called Pegasus.

This is the square of Pegasus.

And we now know that around there is

a star called 51 Pegasi,

which has a planet orbiting around it,
a gas giant about the size of Jupiter,

that goes round that faint point
of light every four days.

It is wonderful to think

that, in my lifetime,
in fact in my adult lifetime,

in the last 25 years,

we've gone from a universe
that could have been

devoid of planets beyond
our solar system,

to a universe that we know
is teeming with places

that we can search for life.

Over the last three decades,

some of the most powerful telescopes
on Earth have joined the hunt,

searching for planets
unimaginably far away,

hiding in the dark.

Planets like 57 Pegasi b.

The first world outside our solar system
to be detected around a Sun-like star.

57 Pegasi b is a gas giant,

around half the mass of Jupiter,

but far closer to its star.

Just imagine what that world
might be like.

A world with skies torn
by titanic winds,

where its hot interior is
bathed in rain of sapphires.

In every sense,
57 Pegasi b is an alien world.

And we soon discovered that the galaxy
is full of planets

unlike anything seen
in our solar system.

Planets enveloped by fierce radiation.

Their surfaces battered and stripped

by the high energy strobing light
of their star.

Worlds so cold
their atmospheres are frozen solid.

Or great swollen planets,

with the density of Styrofoam.

And fathomless atmospheres.

These discoveries prove
that in one sense

we really are not alone.

There are other worlds out there
waiting to be explored.

We estimate
that in the Milky Way Galaxy,

there are more planets than stars.

Hundreds of billions of them.

That's hundreds of billions
of places to look for life.

But there's a catch.

Because not all worlds,
by a long stretch,

are like this one.

The first planets we found appeared
too bizarre,

too large,

and often too close to their stars
for living things to survive.

To find worlds where life could exist,

we needed to look
for smaller, rocky planets

in orbits further from their stars.

T-minus ten,
nine, eight, seven...

We needed to look
for another earth.

...three, two,
engine start, one, zero,

and lift off
of the Delta II rocket with Kepler

on a search for planets
in some way like our own.

So the hunt moved to space

with the launch
of NASA's Kepler space telescope...

And we have separation.

...searching for Earth-like worlds

in the galaxy beyond.

Kepler crossed 94 million miles
of space,

until it arrived in a steady orbit
around the Sun,

from where it looked out
with a fixed and clear gaze,

to a single patch of sky
in the constellation of Cygnus.

Exposing its sensitive light meter

to the light of 150,000 stars,

it began to look
for Earth-like alien worlds.

Kepler doesn't detect planets directly.
They are far too small.

They're just specks of dust
relative to their parent star.

They're also very faint.

They don't emit light of their own,

so they just glow very dimly

in the reflective ambient light
of their stars.

So Kepler has
to detect planets indirectly.

Imagine that a moth just flew

across the beam of light
from the lighthouse.

And I wouldn't see the moth.
But if I had a sensitive enough detector

and everything was lined up properly,
I might just see

the brightness of the light dim.

And that is how Kepler detects planets.

Imagine there's an alien astronomer
in some distant solar system

looking back at the Sun,
and everything's lined up

so they see the Earth trace across
the face of our star.

They would see
the light from the sun dim

by one hundredth of one percent.

It's a tiny amount, but it's enough.

And if they saw
that dimming was regular,

if they saw the star dim
once every year in this case,

then they would infer

that there's a planet orbiting
around the star.

With its exquisitely sensitive
light meter,

Kepler sees only
the regular dimming of pixels.

Just a few bits of information.

But from those bits, astronomers can
begin to build a picture

of the worlds that dim the starlight.

Worlds that might, in some way,
resemble our own.

Worlds like Kepler-36b.

The planet was
one of Kepler's earliest discoveries.

Orbiting a star similar to our own,

we'd found a world that at first glance,
might seem familiar.

Weighing in at around four times
the mass of our own planet,

Kepler-36b was one of the first
of a new class of planet,

a Super-Earth.

The Kepler data doesn't
just allow us to say

there's a planet around that star.

It allows us to characterise
those planets.

So, by looking at the precise way
that the light fades

and then rises again,
and the timing between the dips,

we can measure the orbit of the planets.

And if there are multiple planets
in the system,

we can even estimate their masses.

So, the Kepler data allows astronomers

to paint a picture
of the worlds it discovers.

But the more detailed our picture
of Kepler-36b became,

the less Earth-like
this super-Earth appeared to be.

It orbits very close to its star,
circling once every 14 days.

And, it has company.

A gigantic gaseous companion

with an orbit unusually close
to its smaller sibling.

The proximity of both its star
and sister planet,

allows us to imagine
the bizarre conditions

that may exist
on the surface of Kepler-36b.

The planet may be tidally-locked,

which would mean that one hemisphere
always faces the star.

On this side, the punishing heat could
turn the ground molten,

creating rivers of lava that would
criss-cross the surface.

The planet could experience
violent eruptions

as the gravitational pull
of the gas giant triggers

intense volcanism,

each time it passes by.

But Kepler-36b could
also be a planet of ice.

Because if it's tidally-locked,

the far side would face permanently
away from the star.

And we could imagine
a freezing, cold hemisphere

shrouded in eternal darkness.

For now, this is all
Just informed speculation.

But we are beginning to build
a picture of these worlds.

I mean, imagine a world

where the sun stays at the same point
in the sky forever.

So, one side of the planet is
in eternal night

and the other side in eternal day.

And even the twilight strip
between day and night,

we think, would suffer
from extreme conditions.

So, Kepler-36b just goes to show
there's so much more

to having a habitable world
than just the composition of the planet.

There's the details of its orbit,

and also the nature of the other objects
in the solar system

that are orbiting
around the star with it.

Kepler-36b is just one
of thousands of planets

that Kepler has discovered.

We now know beyond doubt

that our galaxy is home
to a diverse collection of alien worlds.

Each one of the over 4,000 planets
that we've discovered to date,

is different from all the others.

They really are an alien
and exotic bunch,

and there's certainly no planet
that's identical to the planets

that we know of in our solar system.

And I think that reveals a deep truth
about the universe.

Because although the laws of nature
that form the planets are simple

and the same everywhere,

and the fundamental ingredients
out of which the planets are made,

are simple and the same everywhere,

the nature of a planet also depends
on the history of its formation,

and the environment
around its parent star,

out of which the planet formed.

And those are all radically different.

So, each planet has
a different story to tell.

I suppose in that sense,
planets are like human beings.

And this wholly unexpected
but exciting discovery

certainly complicates
the search for life.

We needed to narrow the search

for planets further,

but not too far away
from their parent stars.

Planets at just the right distance

for their surfaces
potentially to be habitable.

Alien worlds
with one precious ingredient

that makes Earth a living planet.

Now, you might legitimately ask
can we transfer all the knowledge

we have of life here on earth
to planets elsewhere in the universe?

Well, I would answer emphatically
"Yes, we can,”

because the laws
of nature are universal.

So, the laws of physics
and chemistry that underpin biology

here, on this planet,

will apply to every planet
out there in the universe,

whether we've discovered it or not.

The chemistry of life requires
a few basic ingredients.

Carbon, nitrogen, oxygen, iron.

And it also requires a ready supply
of high quality energy

from heat within the planets
or perhaps from starlight.

But life here on earth also requires

one very important fundamental
extra ingredient, which is liquid water.

Liquid water is
a deceptively complicated substance.

It's a very powerful solvent.
But it also has structures

which are constantly forming
and disappearing within it,

which act as a kind of scaffolding
around which biology happens.

Organic molecules are
orientated by that scaffolding

so they can react together.

Now, it is certain
that every living thing

here on Earth requires
liquid water to survive.

And I would say it is
a very good assumption

that every living thing anywhere
out there in the universe

will require it, too.

The universe is filled with water.

Great reservoirs have been detected
throughout the galaxy,

amongst the gas clouds of giant nebulae.

But just because water is plentiful,

that doesn't mean
that it necessarily ends up

in oceans on planetary surfaces.

Of the eight planets
in our solar system,

only one has liquid water flowing
permanently on its surface today.

An ocean world where,
long ago, life began.

Around four billion years ago,
life on Earth would have begun

probably in places
not dissimilar to this,

where there's geothermal activity,
a source of energy,

in contact with rich concentrations
of reactive chemical elements

and minerals, but also crucially, that.

The magical solvent, liquid water.

Now, many rocky planets out there
in the galaxy will probably have this,

but far fewer, we think, will have that.

Large bodies of liquid water
on the surface.

So, that's why
there's a kind of a catchphrase

in the astro-biology community,

which is if you want to search for life,

follow the water.

Whilst life on Earth was evolving,

124 light years away,

amidst a collapsing cloud
of gas, dust and ice,

a small star was born.

And the cloud's
swirling leftovers condensed

to form a brand new world.

In 2015, Kepler found a planet
orbiting comfortably

within its star's habitable zone.

More than eight times
the mass of the Earth,

K2-18b is a giant,

with a powerful gravitational pull.

If the planet is rocky,

this may have allowed it to hang on
to a thick atmosphere.

K2-18b might have
all the makings of a water world.

And a legendary space telescope had
Kepler's new discovery in its sights.

The most powerful space telescope
of them all

had joined the hunt.

Hubble examined the light
from K2-18b's parent star,

as the planet passed in front of it,

and detected, what may be,
a faint signature...

of water vapour.

124 light years from Earth,

we may have, at last, found
the evidence of water

on an alien world.

This was the first observation

of water vapour in the atmosphere

of a planet orbiting
in the habitable zone around its star.

Now, admittedly, measurements
of the amount of water vapour

in the atmosphere is pretty wide.

It's somewhere between 0.01 percent
and 50 percent.

I mean, this is a planet
that's a long way away.

But, for comparison,

our planet has a few percent
water vapour in its atmosphere.

So, that observation is important
for two reasons.

One is, it is not zero.

There is water vapour in the atmosphere.

But, secondly, if the measurement
is at the lower end,

a few percent of water vapour
in the atmosphere,

then that is consistent
with this world being a planet

with oceans on its surface.

The nature of this planet is
currently the subject

of intense scientific debate.

The planet may be
more like a mini-Neptune,

a gas planet.

But, it is possible to dream
of a rocky alien world

with skies full of clouds,

where water droplets collect,

and eventually fall,

feeding vast oceans...

that cover the surface
of a massive planet.

A water world,

where the elixir of life...

is in plentiful supply.

K218-b is exciting

because it's the smallest world

with an atmosphere
that we've been able to analyse.

And we've found
that its mass and density,

and composition of its atmosphere,

and it's orbit are consistent
with it being a world with water.

And it might be a world
with oceans on its surface.

We don't know for sure,

but just imagine
what that small, faraway world

around a faint red star might be like.

Kepler went on to make
many more discoveries...

until, in October 2018,

it finally ran out of fuel.

After nine years, it had found
over 2500 alien worlds...

showing us just how common
potentially Earth-like planets might be.

We estimate that there may
be around 20 billion

potentially Earth-like worlds,

that's rocky planets
in the habitable zone around a star

that may support liquid water
on the surface, in our galaxy.

That is 20 billion
potential homes for life.

Now, we don't know the probability

that given the right conditions,
life will begin on a planet.

But we do have evidence, from our world.

What we know is that here on Earth,

life began pretty much
as soon as it could

after the Earth had formed
and cooled down,

and the oceans formed on its surface.

So that might suggest
that whilst there isn't

a sense of inevitability
about the origin of life

given the right conditions,

it might, at least,
be reasonably probable.

So, I think that there is
at least a chance

that life may have begun
on some, perhaps many,

of those 20 billion Earth-like worlds
out there in our galaxy.

But I think there are
two questions about life.

One question is about the origin,
and the existence of microbes,

but often, when we speak about aliens,

what we really mean is not microbes,
but complex creatures.

Indeed, things that we can
speak to, civilisations.

What is the probability there will be
other civilisations out there

in the Milky Way?

Well, again, the answer is,
we don't know.

But there are observations we can make.

Patterns we can see in the Milky Way

that might allow us
to make an educated guess.

We don't know precisely

how we highly-intelligent
complex creatures came to be here.

But we do know, for certain,

that life on Earth didn't
begin this way.

We are the product of a story
that has been playing out

for over a quarter
of the age of the Universe.

From microbes...

to a global technological civilisation,
reaching out for others.

For now, at least,
we remain surrounded by silence.

The messages we've sent out
into the cosmos remain unanswered,

and the telescopes we use
to scan the skies

for alien signals remain quiet.

Now, that's not to say, of course,

that there aren't
other civilisations out there.

We may have been looking
for the wrong thing, in the wrong place.

But I think the answer to the question
of the great silence can be found here,

on Earth.

Because, here, it took
four billion years of stability

for a civilisation to emerge.

That is a vast amount of time.

And when we look to the other worlds
out there in the Milky Way,

it's those two things,
stability and time,

that appear to be
very rare commodities indeed.

In 2013, the European Space Agency
launched the Gaia Space Telescope.

Its mission, to survey the stars
of our galaxy, the Milky Way.

Billions of stars have been mapped.

Each star, a potential host
for alien worlds.

And patterns are
already beginning to emerge.

Not all stars exist alone.

Some have company.

And, bizarre as they seem,

Gaia has discovered around a million

of these binary
or multiple star systems.

We've known for a long time
that binary star systems,

and indeed, multiple star systems exist,

but we didn't know
precisely how common they are.

But now, we have
a huge amount of high-precision data,

including the Gaia data, which tells us

that around 50 percent
of all sun-like stars are

in multiple star systems.

And for more massive stars,
that number is 80 percent.

So, how does the prevalence
of multiple star systems in the galaxy

shift the odds in the hunt
for another Earth?

Could Earth-like planets exist
in multiple star systems?

And, if so, what might their fate be?

In 2020, we may have found a clue.

A planet the size of Mars, floating
freely through the galaxy.

A so-called rogue world.

But planets can't form alone
in interstellar space.

So, where did it come from?

Dawn...

ushered in not by one star...

but two.

Perhaps the rogue world grew up
in a close binary system.

Subject to the gravitational pull
of two stars...

its orbit may have been unstable...

as its parent stars fought
to control its destiny.

But, even in single star systems,

the weak gravitational interactions
between the planets

can change their orbits.

Now, in a double star system,
the planets are not only subjected

to the gravitational pulls
of each other,

they're subjected
to the stronger gravitational pull

of another star.

So, even if a planet gets
into a stable orbit,

it's very likely that it won't
stay in that orbit for long.

So, in double star systems,

the line between order
and chaos is very thin indeed.

Even subtle changes
in a planet's orbit can lead

to dramatic changes in climate.

And that's why the surface conditions
on planets in double star systems may

be unlikely to remain
stable enough for long enough

for intelligent life to evolve.

And the changes
in the orbits of planets can

sometimes be anything but subtle.

A close encounter...

may have given the rogue world
a final gravitational Kick...

flinging it outwards...

and releasing it
from the grip of its parent stars,

setting it loose...

on a journey through the galaxy.

Far from the warmth of its stars,

any liquid water the rogue world
might once have had...

Would have frozen solid.

Any atmosphere that once protected it...

would have frozen out onto the surface.

The rogue would have become a world...

with conditions
that no living thing could endure.

An entire planet alone and adrift,

only to be detected by us,
millions of years later.

A small Earth-like rogue planet...

roaming the darkness of space

for eternity.

This lonely, wandering planet is
not a unique world.

Although rogue planets are
very difficult to detect,

it's estimated that there may be
over 100 billion of them in our galaxy.

Rogue planets might be
the most common type of planet

in the Milky Way.

And, although we think
most of them were torn away

from their star soon after formation,

this does suggest that star systems are
not always stable places,

where complex life could evolve
over billions of years.

Our hunt for another living planet has
only just begun.

Yet, we've already learnt so much.

We found our first rocky worlds.

Some in the habitable zone
around their stars.

Some, potentially,
with liquid water on the surface.

Candidate worlds for future missions
to search for evidence of life.

But we've also found hordes
of bizarre, tortured worlds,

orbiting around violent stars,

and a multitude of rogue planets...

where complex life,
as we understand it seems impossible.

Perhaps it's these worlds
that hint at the reason why,

for now, one planet stands apart...

alone.

Our planet seems to have
largely escaped the violence,

the chaos, and the constant change

that seems to characterise
a galaxy like the Milky Way.

Yes, there's been
the odd mass extinction,

but there's been
an unbroken chain of life here on Earth,

stretching back four billion years.

And if that's what you need

to go from the origin of life
to a civilisation,

then, although there may be billions
of worlds out there where life began,

there may be very few civilisations.

But that's just an opinion.
It's an educated guess.

And, given the profound nature
of the question,

no matter how educated the guess,

I think it would be ridiculous
for us to stop looking

both inside our galaxy and beyond.

For, we may have just received
the first glimpse...

of a world beyond the Milky Way...

around 30 million light years away,

nestled in the spiral arms
of the Whirlpool Galaxy.

A world the size of Saturn.

A find that marks
an expansion of our horizons.

The beginning of the hunt
for extra-galactic planets.

The potential discovery
of a planet orbiting around a star,

in another galaxy is something
that I never thought I'd see.

And it opens up
the intriguing possibility

that we might be able to explore
not only the question,

"Are we alone in our galaxy?"
but, "Are we alone in the universe?"”

And the answer to that question
may lie far in the future.

We might never answer that question.

But I said, the question
"Are we alone?" is profound,

because answering it would teach us

much more
about what it means to be human.

Well, I think we become
a little bit more human

with every world that we explore,

because that ability
to lay the foundations,

to explore questions to which we may
never receive answers in our lifetime,

questions for our children
or our grandchildren to answer,

Is a fundamental part
of what it means to be human.

It's a fundamental part
of what makes us so special,

here on this little world,

looking up at the stars,
whether we're alone or not.

Five. Four. Three. Two.

Engines start. One. Zero.

And, lift off
of the Delta II rocket with Kepler,

on a search for planets,

in some way, like our own.

We had worked together, thousands
of people worked together,

and it's all coming together.

And we have separation.

It was so emotional to see
the project they had worked on

for so many years or decades
finally go to space,

and all that hope and promise,
all bundled up in the machinery.

Kepler was an immediate success,

discovering over 2,000 new planets
in its first four years of operation.

But in the summer of 2012,

the team faced a challenge
that threatened the entire mission.

One of the things that
the Kepler Mission needs to operate

are reaction wheels
that spin and hold it on target.

So, it always points
at the same stars, and doesn't jiggle.

Well, we had four wheels that did that.

And we knew that we only
had a couple of spare gyroscopes,

and we knew that spacecraft tend
to have gyros fail.

And, after a while, it failed.

Three months later,
the second one failed.

And, since we needed three,

we could no longer look
at the Kepler field of view.

I had hope that they'll figure out
a way to work with two gyros,

and, indeed, they did.

So, the very clever people,
the engineers and scientists said,

"What we can use, is we'll use
the sunshine for the third wheel.

"We'll make this thing
reflect sunlight off it,

"we'll use the other two wheels,
and now, we can point in the sky.”

The faint pressure of sunlight
helped stabilise the telescope.

That was, kind of,
good news, actually,

because it meant Kepler was gonna
have to go off the Kepler field now,

and we could get all kinds
of other stars and observe them,

and so, it actually was a boon
for stellar astronomy.

After another four years
of discoveries,

in total it had found
over 2,600 planets,

making it, by far, our most successful
planet-hunter to date.

It was sad when they sent
the command to shut everything down.

You know, it's asleep now.

It's in orbit around the sun.
It will continue that orbit.

But since it launched from Earth,
it will come back to Earth.

It will come and visit us again
in about 40 years,

and my hope is people will say,

"This is a historic telescope.
It told us about all these planets”,

and they will go up
and pick up this telescope

and bring it back to Earth,

and put in the Air and Space Museum
for us all to admire.