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The Milky Way.

Hundreds of billions of stars.

Spread across a hundred thousand light
years of space.

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 51 Pegasi b.

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

51 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, 51 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.

Fathomless atmospheres.

These discoveries proved 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 10, 9, 8, 7, 6, 5... We
needed to look for another Earth.

4, 3, 2, engine start, 1, 0, and liftoff
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
reflected 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.

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.

I mean, 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 a 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.

It has company.

A gigantic, gaseous companion.

With an orbit unusually close to its
smaller sibling.

The proximity of both its star and its
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 planets 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 the universe,
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 astrobiology 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, K218b 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.

K218b 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 K218b'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,
measurement of the amount of

water vapour in the
atmosphere is pretty wide.

It's somewhere between 0.01% and 50%.

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.

K2-18b is exciting because
it is the smallest world

with an atmosphere that we
have been able to analyse.

And we have found that its mass and
density and composition of its atmosphere

and its orbits 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 far away

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 two and
a half thousand 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 the 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% of all
sun-like stars are in multiple star systems.

And for more massive stars that number is
80%.

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?

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.

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.

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 a hundred billion of them in our
galaxy.

Rogue planets might be the most common
type of planet in the Milky Way.

And although the world is full of them,
although we think most of them were torn

away from their stars 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 learned so much.

We've 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.

The 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.

Our planet seems to have largely escaped
the violence, the chaos, 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 of the Milky Way,
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?

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, engine start,
one, zero, and liftoff 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.

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
stabilize the telescope.

That was kind of good news, actually,
because it meant Kepler was going to 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 set the command to
shut everything down.

You know, it's asleep now.

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

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

It'll 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 it in the Air and Space Museum for us
all to admire.