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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?

For me, the
most exciting question in the universe

is whether or not there are
inhabited planets

other than the Earth.

What are the
chances that out there in the Milky Way,

there is other complex, advanced
life like our own?

Today, our
telescopes are combing the skies,

discovering bizarre worlds...

even in places
they shouldn't be.

We didn't even
know that that kind of planet existed.

So it completely shocked
our theories of planets

and how they form.

Will we ever find a
world that resembles our home?

Earth-like
planets that orbit their stars

at the same distance
we orbit the sun

are very difficult to detect.

Looking for life is not easy,
and we're going to get

constant setbacks.

Are we finally on the
verge of answering the ultimate question:

are we alone?

I think we'll actually find the
answer while I'm still alive.

At least I hope so.

The search is on.

"Alien Worlds."

Right now, on "NOVA."

The Milky Way,

our galaxy.

As many as 400 billion stars.

Spread across 100,000
light-years.

Among these stars, the sun,

with eight planets orbiting
around it...

including our home.

Humans have been
looking up at the stars for millennia.

I think we're just so curious
about it,

yet we don't really know
what's out there.

Not only what, but who?

We've gotten to
the point in astrophysics where I think

the big question on everyone's
mind really is: are we alone?

Is all life like us

and will we recognize it
when we see it?

Today, we stand on
the brink of answering these questions.

What is different
today is that we have a chance

to actually address some of
these issues quantitatively,

scientifically.

Not just sheer speculation
and philosophically.

If and when we do find life,

it's going to completely change

the way we view ourselves,
our planet, our universe.

Over the last 50
years, humanity's robotic space probes

have found their way to every
planet in our solar system.

And even some of their moons.

But so far,

no convincing evidence of life
has been discovered.

As far as we know,
Earth remains one of a kind.

The only world where chemistry
gave rise to biology.

But even as the exploration
of the solar system continues,

another quest has begun to
discover other planets out there

orbiting distant stars:

exoplanets.

It's really only
in the last few decades that we've had

the ability to search for
exoplanets.

And from the start,
scientists dream of finding one

that might host life
like our own.

It's completely
reasonable to expect that alien life

will be quite different from
the life we find on Earth.

But Earth is the only place
in the galaxy that we know

for sure hosts life.

So we start there...
looking for an Earth twin.

And then the question becomes:

how common are Earths
across the galaxy?

Over the last three decades,

scientists have deployed
some of the most

powerful telescopes on Earth.

And used a clever technique to
detect the presence of planets

that are too dim to see next
to the bright stars they orbit.

Many teams
started to look for planets

using the same technique.

This is the Doppler technique,
or the wobble technique.

The way the wobble method works

is that you can imagine
that you've got

two dance partners, okay?

One is the star
and one is the planet,

but you don't see the planet

because it doesn't put out
much light.

And so if we look very carefully
at the star, we can see that

the star appears to be moving
away from us

and towards us,
back and forth over time.

And we can deduce just from a
basic understanding of gravity

that there must be something
else there

and even measure its mass.

But what astronomers discover

is something completely
unexpected.

The first-ever
exoplanet that was discovered

around a sun-like star

was 51 Pegasi b in 1995.

51 Pegasi b is a gas giant.

Around half the mass of Jupiter,
but so close to its star

that part of its atmosphere
may have been ripped away.

These planets are baked
by their stars' radiation.

The temperatures are in
the thousands.

Many astronomers
didn't believe it because the planet

was in the wrong place.

It was enormous,

it was massive,
and yet it was parked

right next to its star.

And a planet
has no business being there.

There's not enough material

around a forming star to make
a planet that close to the star.

Its skies
whipped into titanic winds...

and a hot interior, pelted by
raindrops of molten iron.

The discovery of 51 Peg

meant that we had to tear up
the rulebook.

We
were hoping to discover a planet,

something like what we see
in the solar system,

but instead found a planet
that had absolutely no analogue

in the solar system, and no one
could have predicted

that such planets
were even possible.

And other
discoveries were stranger still.

Planets blasted by
fierce radiation.

Their surfaces battered
and stripped

by the high-energy
strobing light of their star.

Rocky worlds large enough
to hold on to some atmosphere,

but so cold that the entire
surface has frozen over.

And great puffy planets

with an average density
like Styrofoam.

And with diamond-crushing
pressures at their centers.

These discoveries electrified
the world's astronomers.

At long last, confirmation that
there are other solar systems

out there, inhospitable
though they might be.

These worlds are truly
amazing places,

and we've learnt so much about
the physics and the chemistry

of a world like this,
but they are not places

to look for life.

Because these
are worlds too bizarre, too large,

and often too close
to their stars

for living things to survive.

Like any
major scientific revolution,

looking for life is going to be
filled with setbacks

and many false positives.

But we have to keep on.

It's a lot easier
to find something that's big

and far away rather than
something that's small

and far away.

So the fact that we've found
more large planets

is not an indication of what's
actually out there.

It's just a matter of what's
easier to find.

Finding planets like
Earth would take an extraordinary mission.

We really needed
a space mission dedicated to finding

Earth-sized planets
in our galaxy,

and ideally, enough of them
to give us real insights.

The solution?

A telescope
almost 20 years in the making.

I was working for NASA Ames.

I realized that if we were going
to find Earth-size planets,

we had to build a photometer
that was good enough to see

these small, Earth-size planets

and find out about them.

Some 17 years after I started,
NASA headquarters said, "Yes.

We will build that mission."

I was very privileged
to be at the Kepler launch,

and we were all incredibly
nervous a few days before,

because the previous mission
that had launched

on the same rocket had failed.

Their satellite went...

Went down in Antarctica.

And we were, like, "Uh-oh, no."

So we were very nervous.

T minus ten, nine, eight,

seven, six, five,

four, three, two...

Engine start, one, zero,
and lift-off

of the Delta 2 rocket with
Kepler on a search for planets

in some way like our own.

And we have separation!

It was a
spectacular night launch.

Kepler was launched without
a single flaw.

NASA's Kepler
Space Telescope takes the search

above Earth's atmosphere
into space.

Kepler travels 94 million miles
away...

until it arrives on a stable
orbit around the sun,

where it looks out
with a fixed and clear gaze

to a single patch of sky
in the constellations

of Cygnus, Lyra, and Draco.

What was special
finally was, the lid opens up

and we see the nighttime sky.

What we see is, the sky is
covered with star.

Every detector is working.

Exposing 42
incredibly sensitive light sensors

to the light of 150,000 stars...

Kepler begins its search

for Earth-like worlds.

The
problem with finding exoplanets is,

they just don't shine at all

and they're around
really bright stars.

That makes them virtually
impossible to image.

So, astronomers came up
with a different method,

and that's called
the transit method.

Kepler uses the transit method
for discovering planets

around other stars.

And it's really simple.

A star is bright,
and if a planet is orbiting it,

if the planet passes in front of
the face of the star,

the planet blocks
some of that light

and the star temporarily
appears dimmer.

So, if we were to bring that
down to Earth,

imagine this lighthouse is your
star, is your source of light.

If a moth flies in front of
that light,

even though to your naked eye
the light looks the same,

it actually has become
a little bit dimmer

because the light has been
blocked by the moth.

That's the transit method.

At the time the largest
camera ever launched into space...

Kepler is able to look for
smaller, more Earth-like worlds:

those too small to be
easily found

using terrestrial telescopes.

Soon, it starts to spot planets.

The amount of dimming

and the time between
each dimming event gave us clues

to a planet's size and its
distance from its star.

And with every world it finds,

astronomers hold their breath,

hoping for a rocky planet
that might in some way

remind them of Earth:

worlds like Kepler-36b.

Orbiting a star similar
to our own.

At first glance,

a planet that resembles ours.

So when we started finding
these rocky planets like Kepler-36b,

that were getting closer
to Earth mass,

it was just the greatest
feeling.

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

we'd found one of the first
rocky worlds

that could have an atmosphere.

A so-called Super-Earth.

What's so cool about Kepler

is that it tells us so much more
than just, the planets exist.

We get their sizes, we get their
orbital characteristics,

we can get their masses.

And so what we're learning

is not just that the planets
are there,

but what these planets are like.

But the more
closely we examine Kepler-36b,

the clearer it becomes

that this is not a world like
our own.

Finding
exoplanets in the Kepler data,

you really have to do some
detective work.

You're trying to find a signal
that's less than one percent

of all the other things that are
going on in that data.

The light from
the star dims every 14 days,

the length of a year
for the orbiting planet.

This rocky world orbits
incredibly close.

And it has company...

a gigantic gassy companion
planet.

The discovery of
the Kepler-36 system is really interesting

because the first planet found
was Kepler-36c.

That's a gas giant planet.

The two planets
have unusually close orbits,

creating bizarre conditions

on the surface of Kepler-36b.

Kepler-36b,
in many ways, is Earth-like,

but it also is very
un-Earth-like

in pretty profound ways

that mean it's unlikely to have
life on its surface.

Thanks to its close-by star,

the planet's rotation becomes
tidally locked.

One face rests permanently

under its star's bright glare.

Punishing heat likely turns
the ground molten...

creating rivers of lava
that crisscross the surface.

On top of that,

every 97 days,
the immense gravitational pull

from the gas giant may trigger

intense volcanism
on the smaller planet

each time it passes between
the giant and the star...

throwing lava high
in violent eruptions.

But Kepler-36b is a planet
of molten rock,

fire, and ice,

because its far side permanently
faces away from the star...

creating a frigid hemisphere
shrouded in eternal darkness...

where magma freezes into a
strange, desolate landscape.

The Kepler-36 system

is a perfect example of how
exoplanets are completely

different to what we know
in the solar system.

Kepler-36b is just
one of the first among hundreds

of Super-Earth-size planets

that the Kepler telescope
discovers.

The more scientists look,
the more they find.

At the time
that the Kepler mission launched,

the team was hoping they might
find a few exciting planets

and they were very prepared
to analyze those data.

Within the first few months, we
realized that we did not have

enough people to handle
the deluge of planets.

The mission shows
beyond doubt that our galaxy is home

to a multitude of diverse
alien worlds.

Kepler was a
game-changer for exoplanets.

It was a game-changer for us
here on Earth,

understanding that there are
planets in their thousands,

in their millions, in their
billions in our galaxy alone.

We
suddenly had this new problem.

We had gone from just having
a handful of planets

to hundreds of planets
to look through.

So, we needed to tighten
the net, focusing on planets

that didn't just resemble
Earth's size,

but its habitability.

Now that they have
discovered that planets are plentiful,

astronomers must narrow
their search to home in on

hard-to-find planets.

Rocky worlds that hold the one
precious ingredient that makes

Earth a living world.

Everything we know about life

we've discovered by looking
at ourselves and our world.

Life on
Earth is extremely diverse.

Millions and millions of species
of all kinds.

And these
endless beautiful forms

occupy just about every
available niche on the planet.

We've had the
opportunity to explore a variety

of extreme environments,

and in pretty much all of them,
we find life.

Earth has taught us that,

given the right conditions,

life is remarkably adaptable...

and resilient.

Life.

Here on Earth, it is absolutely
everywhere.

Once life got started,
it has held on tenaciously.

But, where can we find life
elsewhere in the universe?

If it's done so well here
on Earth,

certainly this can't be
the only place.

But if we're going to find life
on other worlds,

we're going to need to know
where to look,

because there's a lot of worlds
out there, and man,

is the volume of space huge!

So you can't just go looking
willy-nilly.

You need to find a marker

that guides you to where life
most likely can be found.

For life to get started
on a planet,

it needs a few specific things.

You have to have energy
to power that life

and you have to have
the right chemistry.

But even if you have
both of those,

there is still one more thing
that we don't know

is necessarily going to be found
on exoplanets.

Water,
and it has to be liquid water.

Every single living
thing on Earth requires liquid water

to exist.

To uncover life that resembles
anything like what we have here,

finding liquid water will
be crucial.

There's a concept:

follow the water.

That where there's water,
there might be life.

The universe
itself is awash with water.

Great reservoirs have been
detected throughout the galaxy

in vast, interstellar clouds.

But just because water
is plentiful,

that doesn't mean that planets
are necessarily

the places where it ends up.

So, while we've detected water

on the moons of Jupiter
and Saturn...

only one of the eight planets
in our solar system

has liquid water flowing on
its surface.

An ocean world where, long ago,
life began to thrive.

So the challenge, then,

was to find an Earth-mass planet
in a habitable zone.

Meaning it's the right distance
from its host star

to support liquid water on
its surface.

Earth-like planets that orbit
their stars

at the same distance we orbit
the sun,

who are right on the limit
of what Kepler could find.

We need to see
the planet cross in front of its star,

and it takes us multiple orbits
to observe this.

And so it's really, really
difficult to find these planets.

And while
Kepler can detect planets,

it cannot tell what
they're made of.

To detect water, the most
powerful space telescope yet

must join the quest.

Kepler and Hubble
worked well in coordination.

Kepler would find planets around
distant stars,

and then Hubble would follow
them up in more detail,

to teach us more about
the atmospheres

of the planets found by Kepler.

When a planet
goes in front of the star,

some of the starlight shines
through the atmosphere.

But just like shining a
flashlight through the fog,

some light doesn't make it
through and some does.

And by piecing together which
parts of the starlight

make it through the planet
atmosphere and which don't,

we can actually tell something
about the gases

in the atmosphere of the planet
that are absorbing.

And then we can tell something
about the makeup of the planet.

In 2015,
Kepler finds Hubble a target.

A world within
its star's habitable zone.

More than eight times the mass
of Earth,

K2-18b is enormous.

With a powerful
gravitational pull

that allows it to form, and hang
on to, an atmosphere.

Hubble examines light from
K2-18b's star

as the planet passes
in front of it.

And discovers
a long-awaited signature.

Water vapor.

K2-18b is exciting because,

with the Hubble observations,

we found water vapor in its
atmosphere.

And this is the smallest planet
to date

which sits in the habitable zone
of its host star

and in whose atmosphere
water has been detected.

Earth has tiny but
environmentally significant

amounts of water vapor in our
atmosphere,

ranging from four percent to
less than one percent.

So far, the
data we have has shown

that there's anywhere between
0.01% and about 15% water vapor

in the atmosphere of K2-18b.

And that might seem like quite
a broad constraint,

but we have to remember that

this is a planet that's
many, many light-years away.

So the fact that we can detect
this water at all

is already really exciting.

124 light-years from Earth,

we may have found evidence
of water on another world.

K2-18b is a huge milestone.

It's what we call
a sub-Neptune-sized exoplanet.

Now, what does that mean?

Well, it could be, it's a planet
sort of like Jupiter,

heavy, massive hydrogen
atmosphere, no life.

That's like Jupiter.

Or it could be that water
represents an ocean.

It doesn't have
a heavy atmosphere.

Instead, it has an ocean
and the water is evaporating.

So it's a wonderful discovery.

Within the data
lies a tantalizing possibility.

If K2-18b's vast mass
is formed from water,

its skies could be full
of clouds,

where water vapor collects,

forming droplets

and eventually falling.

A cycle, potentially feeding
vast oceans...

that could stretch across
the entire planet,

making it a water world.

This finding is remarkable

because, 20 years ago,
no one would have dreamt

that we would be here today,

talking about detecting
the atmosphere of a planet

and trying to understand
the conditions on its surface.

For me, K2-18b
has been a real turning point

because it really
opens up the field

to looking for life in these
slightly bigger planets.

After the
astonishing breakthrough of K2-18b,

Kepler goes on to make
many more discoveries.

So, Kepler
was a really simple mission.

It's just this minivan-sized
spacecraft

that stared at this field
of stars.

But that one mission discovered
so many alien worlds.

Worlds that astound astronomers

and dramatically expand our
understanding of the universe.

As we look out into the galaxy,

we can start
to answer the question

of what properties do planets
and planetary systems need

in order to support the
evolution,

and continued existence,
of life.

But nothing lasts forever.

In October 2018,
Kepler finally runs out of fuel.

It was sad
to see an old friend leave

when they sent the command
to shut everything down.

It's asleep now.

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

To date, Kepler has given us

over two and a half thousand
planets.

And we're still analyzing
the Kepler data,

so there could be many more
to come.

A galactic census,

showing us just how common
planets like K2-18b might be.

We know
there's about, on average,

two or more planets per star.

Maybe ten percent of those are
small planets,

ten percent of those in
the habitable zone.

So that means there is about
20 billion planets

about the size of the Earth

in the habitable zone
of their star.

We don't know about it, and we
might never know about it,

but it is absolutely possible
that of the panoply of planets,

of alien worlds
that Kepler has observed,

there's actually been some
planets that host alien life.

Maybe they have
their own forests and lakes

filled with fish,
and skies filled with birds.

It's so incredible to think
about the endless possibilities

that are out there for life
to take.

These
tantalizing glimpses of worlds

give us a foundation
to build upon

for the next,
even more ambitious mission.

One, zero.

Lift-off, the SpaceX Falcon 9

carrying TESS,
a planet-hunting spacecraft

that will search for new worlds
beyond our solar system.

Kepler
taught us about statistics

and how common planets are.

But the planets found by Kepler
were often orbiting stars

that were too distant
and too faint for further study.

With TESS, we're looking at all
of the nearby stars.

Vehicle has passed
maximum aerodynamic pressure.

Because they are close

and the stars appear bright
to us,

we can go and learn whether they
have atmospheres,

what the atmospheres
are made of,

whether they have moons
and rings...

all the stuff that is far too
difficult to study

for the Kepler stars themselves.

There are
hundreds of people involved

in making the TESS mission
a success,

and now the world
is analyzing the data.

TESS is finding the planets

that will make the legacy
catalogue

that will be used
for generations to come.

Among this vast
array of worlds, what kind of life

are we hoping to find?

When we say
we're looking for life,

we usually don't say exactly
what we mean by that.

And in fact, it turns out
defining life

in a way that satisfies
everybody hasn't been done yet.

We have to
distinguish between simple life

and more complex,
advanced life like ourselves.

And there's quite a difference
there, between the two.

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

for over a quarter of the age
of the universe itself.

From microbes

to a global technological
civilization.

With our telescopes,
we stand on the cusp

of finding signs of at least

simple life-forms on other
planets.

But we have also been
calling out,

sending out transmissions into
the galaxy,

in the hope of connecting

with another
intelligent species.

For millennia,
we've looked out at the skies.

And it's only in the last
century,

really, the blink of an eye
for humanity,

that we've actually been able
to send messages

and probes beyond the Earth.

And as of yet, we're
still waiting for a response.

What we're hearing is
just silence.

But with millions
of potentially habitable worlds

in the Milky Way,
why is no one calling to us?

In 2020, scientists make an
unsettling find:

a planet, almost the size of
Earth,

floating freely
through the galaxy, all alone.

Did it form there by itself

or has it been thrown out
of another planetary system?

A planet that shows
us that the galaxy is more chaotic,

and perhaps more hostile,
than we thought.

As we look
out through the galaxy,

there are many cases

where planets would experience
much more violent histories,

and life might not be able
to survive

more than a billion years.

The rogue
planet may be a refugee,

telling us about systems
far more chaotic than our own.

Because not all stars
exist alone.

Some have company,

orbiting one another in
a careful dance.

So when
we look up at the night sky

and we see single points
of light,

which we assume are stars,

often what's actually there are
two stars orbiting each other.

But they are so close together
that they appear as one.

We're finding that
these binary and multiple-star systems

are actually amongst the most
common types in the universe.

Sometimes planets orbit both of
these stars together,

and other times, the planet
will orbit one of the two stars,

but not the other star.

Dawn.

Ushered in by not one star,

but two.

Perhaps the rogue world
began its life

in a binary system, like this.

On a planet
that's orbiting two stars,

there would always be
two shadows

and you would see two sunsets.

I like to say that science
fiction got some things right.

What does that feel like,
the heat of two suns?

Is that a nice heat,
or is that too much?

There's so many questions

around binary star systems.

For a time, the
planet may enjoy a stable period

in the habitable zone of one
of these stars.

But the peace does not last.

The moment there's
more than two bodies involved

in a gravitational dance, things
get complicated really fast.

Because each body
gravitationally affects

every other body.

On Earth, the most obvious
manifestation of gravity

from a body that's not Earth
is the tides.

The moon affects our tides.

What can happen in these
binary systems

is that this strong gravity
between these two stars

can shift orbits drastically.

This huge,
combined gravitational force...

interferes with
the planet's orbit,

creating chaos.

There's a very thin line

between stability
and instability.

And when instability kicks in,

it goes from a stable system
to a chaotic system.

And at that point,
anything can happen.

As each star and
every planet in the system pulls the rest,

the orbits of each world
constantly shift.

Further adding to the turmoil.

Until a close encounter with
another world

gives the planet a huge
gravitational kick.

Flinging it outwards.

And releasing it from the grip
of its parent stars.

Setting it loose.

A rogue planet,
wandering the galaxy.

The planet
will slowly start to cool down

until it is just a rock with
no source of heat,

traveling alone through
the universe.

Far from the
warm embrace of its parent stars.

Any liquid water the rogue world
might have once had

freezes solid.

You would
have the night sky all the time

if you were on one of these
planets,

with no hope of a sun ever
rising.

Any atmosphere
that once protected it

freezes.

Leaving the surface

exposed to cosmic radiation,

conditions which no living thing
could endure.

It effectively becomes

something of a dead planet.

Almost a fossil of itself
flying through space.

Alone and adrift.

Only to be detected by us
millions of years later.

The way we find rogue worlds

is through a process called
gravitational microlensing.

We look at a distant star,

a star that has nothing to do
with the planets

that we're hoping to discover,

and between the telescope
that we're using

and that distant star,
a rogue planet

wanders through.

The gravity
of the planet is really small,

but it's enough to bend that
distant star's light

so that we can see that it
exists and measure its mass.

One of billions of
planets on which the spark of life

may have begun,
only to be extinguished.

Worlds where any civilizations

could not have survived.

Our planet is a wonderful place
to exist.

We have been sheltered
through history.

We've experienced some
large impacts,

but we've had a relatively
benign existence.

It could be that the reason

we haven't found any alien
civilizations yet

is because we live in a
particularly hospitable corner

of our galaxy, with just the
right conditions

for life to arise.

Or maybe we just haven't
looked hard enough.

Our quest for another
living planet has only just begun,

and the only way to know if we
are alone is to keep looking.

I still feel very amazed

that we have so much knowledge
about these objects

that are just so far away
from us.

We have
found our first rocky worlds.

Some in the habitable zone
around their stars.

And even worlds with water.

Kepler has found
thousands of new planets.

But for every question that
it's answered,

it's raised at least five
new ones.

Candidate
worlds for future missions

to search for the evidence
of life itself.

I am actually very optimistic,

personally, that we might find
life within the next ten years.

But we have also
found hordes of bizarre, tortured worlds

around violent stars.

And rogue planets,

where life as we understand it
seems impossible.

What we see is
a menagerie rife with diversity

and worlds that we could never
have imagined.

It's like looking at those
snowflakes under the microscope

and seeing the wondrous
structure

that we never knew about before,
but is beautiful

and amazing to be a part of.

Perhaps it is these
worlds that will help us understand

why, for now, one planet
continues to remain apart.

You might ask,

why do we keep searching for
these alien worlds?

And I think the answer is
profoundly human:

we want to know what else
is out there.

I think
that it's not at all a guarantee

that there's life elsewhere
in the galaxy.

And what we need to do is

to go out and make measurements
and to see,

which universe do we live in?

Do we live in the "Star Trek"
universe,

where every star has lots
of planets

with life and civilizations?

Or are we in fact, to the best
of our ability to measure,

truly alone?