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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.