Nova (1974–…): Season 41, Episode 1 - Alien Planets Revealed - full transcript

From PBS - It's a golden age for planet hunters: recently, they've discovered more than 750 planets orbiting stars beyond our sun. Some of them, like a planet called Kepler-22b, might even be able to harbor life. What would that life look like? Combining startling animation with input from expert astrobiologists, Alien Planets Revealed takes viewers on a journey of the imagination as we "build" aliens from the ground up.

Are you wondering how healthy the food you are eating is? Check it -
Hundreds of billions of stars
in our galaxy.

And yet we know of only one
that shines

on a life-filled planet.

Is Earth unique?

Or are there other solar systems
and planets like ours...

out there?

Now, scientists are finding
the answer.

Thanks to this:

The Kepler Space Telescope.

The most powerful planet hunter
ever built.

It's making astonishing

The sheer numbers of planets
out there

is really quite stupendous.

From enormous gas giants...

to a land where the sun
never sets...

to worlds that may be entirely
covered in water.

Kepler is even finding planets
like our own.

This might be the first
Earth analogue

around a sun-like star
that's ever been found.

Scientists are beginning
to wonder if those planets

could be inhabited.

And if so, by what?

It's fun to speculate
about life,

but any life is going
to be subject to the laws

of chemistry and physics.

Even on another planet

we can work out how biology
is likely to adapt.

This is the story of how
one spectacular spacecraft

has brought us closer than ever

to answering mankind's
ultimate question:

Are we alone?

"Alien Planets Revealed,"
right now on NOVA.

Major funding for NOVA
is provided by the following:

Supporting NOVA and promoting
public understanding of science.

And by the Corporation
for Public Broadcasting.

And by contributions
to your PBS station from:

Additional funding is provided
by Millicent Bell through:

The most ambitious planet-hunter
ever built sits on a launch pad

at Cape Canaveral, Florida.

T minus five, four, three, two,

engines start, one, zero.

(loud explosion)

Over two decades in the making,

it is about to radically alter
what we know about our galaxy.

MAN (on radio):
Running out of the solids.

Ready for separation.

Meet the Kepler Space Telescope.

Its mission: to detect alien
worlds orbiting distant stars.

And to discover if any of them
could be a suitable home

for life as we know it.

Project scientist
Natalie Batalha

has lived with
the Kepler mission

from the very first moment.

To watch it launch
was really quite something.

This feeling that
after decades of planning

it was finally happening,
it was going up there.

That was a tremendous moment.

Launched in 2009, this
incredible optical telescope

has revealed that planets
are far more common

than we ever imagined.

What's amazing is not just
the number of planets

that Kepler has found,
but the types of planets.

Of the 3,500 potential planets
that Kepler has spotted,

some seem familiar.

Huge gas giants similar
to Jupiter or Saturn,

or smaller, rocky worlds
that could be like Earth,

Venus, Mars or Mercury.

But others are straight
out of science fiction.

Like Kepler-16b,
which orbits a double star.

Just like Luke Skywalker's
home planet in Star Wars.

Even more bizarre is Kepler-10b.

It orbits so closely to its sun
that the surface

is a vision of hell.

Kepler-10b is a scorched world.

It's got an ocean

bigger than the Pacific Ocean,

but it's an ocean not of water
but of molten lava.

That star-facing side
as it orbits

has surface temperatures
in excess of that required

to melt iron.

So this is a blow-torched world.

But for Kepler, these oddities
are just a sideshow,

because its primary mission
is to find a planet

that might have the right
conditions for alien life--

a planet like Earth.

Now, Kepler might be closing
in on just such a world.

The Kepler data suggests that
potentially habitable planets

are out there.

And applying the principles
of biology and evolution,

scientists are even beginning
to guess how ETs

could have adapted
to their environments.

The best speculation recognizes
that there are rules

to the game.

Any life that we can contemplate
is going to be subject

to the laws of chemistry
and physics.

Kepler is finding worlds

that, as far as we can tell,
have the right environment,

the correct temperatures

suitable for life
as we know it.

Kepler is unique
among space telescopes.

Unlike the Hubble, which turned
its gaze far and wide

and sent back stunning images
of the cosmos,

Kepler is designed
to stare fixedly

at one small patch of sky,
taking the same snapshot

day in and day out
for years on end.

It began--
as with any new telescope--

with what astronomers call
"First Light."

Kepler's first light image
came down to us at NASA Ames

about 24 hours after
we ejected the disc cover.

As it filled my computer screen,
the image that came to my mind

was like champagne
filling a glass

with all of these stars
being the little bubbles.

It was very exciting.

Every single tiny dot
that you see is a star

that is in the field of view
of Kepler.

Kepler is focused
on a small patch of sky

near the constellation
of Cygnus the swan.

In that area of sky are
four and a half million stars

in our galaxy alone.

But Kepler isn't looking
for stars.

It's looking for planets
that orbit the stars.

But that's a problem,

because Kepler can't
directly see planets.

Planets are much smaller
and dimmer than stars.

They get lost in the glare.

So Kepler is looking
for something called a transit,

which occurs when a planet
passes in front of the star.

As it does so, it dims the light
by a fraction.

That dimming is what Kepler
is designed to detect.

It's a principle that
can be illustrated

with a distant tower
and a spotlight.

Okay, let's imagine
that there's a moth flying

around that spotlight.

Could we ever hope
to see the moth?

No way.

The moth-- although it does
reflect a tiny bit

of the spotlight's light--
it's far too feeble

for us to see that.

Since the light reflected

off a planet is typically
ten billion times fainter

than the light emitted
by its star,

detecting a planet might seem

But Kepler has a way
around that problem.

If the moth passes in front
of the bright light,

a little bit of the light
that was going to reach us

obviously gets blocked
by the moth.

Then we are able
to detect the moth

by measuring very carefully
that very subtle change

in the total brightness
of the spotlight.

The bigger the moth,
the more light it blocks,

and the more the light dims.

In exactly the same fashion,
that's how we can detect

small planets orbiting
other stars

and even measure their sizes.

That's Kepler's primary mission:

to watch thousands of stars
for signs of a transit.

Scientists plot the brightness
of each star in Kepler's view

on a graph
to see how it changes over time.

This is an actual plot
of one star over two weeks.

Sure enough, every three days

there's a tiny dip
in brightness,

revealing a planet orbiting
this star once every three days.

Just like the moth,
the bigger the planet,

the more light it blocks.

In this plot, a huge planet
transits in front of its star

causing a much larger dip.

Right from the start,
Kepler saw stars dimming.

By June 2010,
15 months after launch,

Kepler had found
over 700 potential planets.

The sheer numbers of planets
out there

is really quite stupendous.

Here you see just
a small sample of them.

And the planets range in size

from, you know, a half radius
of the Earth,

up to things that are several
times larger than Jupiter.

Already, Kepler's discoveries
are changing what we know

about alien planets.

Kepler has revolutionized
our view of planets

and planetary systems
in our galaxy.

It turns out that any
kind of planet is possible,

within the laws of physics
and chemistry.

Any planet you can conceive of
can exist in any location

in a planetary system.

from the Kepler data,

some estimates put the number
of planets in the universe

in the trillions.

But what everyone
really wants to know

is do any of these planets
have life?

To answer that,
we have to go back to the basics

and ask, is the universe full
of the same stuff everywhere?

Are the elements needed
for life as we know it


The lightest elements--
hydrogen and helium--

were made in the moments
after the Big Bang.

Other elements are made
in stars--

a product of nuclear fusion
during the star's

normal life cycle--

or produced when some stars
explode as supernovas,

scattering these essential
building blocks into space.

Every molecule in your body,
every element in your body,

was generated some time
in the distant past,

by processes within stars.

Among the most common
are the elements

that are essential to all
living things on Earth:

hydrogen, oxygen,

carbon and nitrogen.

Life could be anywhere
or everywhere in our galaxy,

but where should we look?

Since planets are proving
to be so abundant,

Chris McKay thinks
we might as well search for life

that follows rules
we already understand.

I know how to search
for organic material.

I know what the signatures
that would be important are.

So the search for life
starts off following water,

following carbon.

Not because we can prove that
that's the only way to do it,

but because that's the only way
we know how to do it.

Sifting through the Kepler data,

scientists have made
a deliberate decision:

to look for planets
that resemble the one place

they know can sustain life...

the Earth.

We know that the Earth

is habitable; indeed, inhabited.

And so surely,
there's some drive to find

another Earth-like planet
elsewhere in the universe.

Humanity is on a quest
to find life.

Kepler's objective
is first to find out

if planets like Earth--

where we have
this one example of life--

if planets like that exist.

Earth, as we know,
is teeming with life.

Organisms have evolved
to exploit every niche.

From the beneath the waves...

to the highest mountains.

So what's so special
about our pale blue world?

For most scientists,
the key feature of Earth

is the presence of liquid water
on its surface.

Chris McKay has spent years
studying the relationship

between life and water.

The question is, can life
survive without water?

This is the place to find
the answer to that question.

This is the desert.

In this baked dry sand near
the Mojave Desert in California,

Chris is looking for critters
that can live without water.

One possible candidate
are these small black patches,

which are in fact alive.

It looks like a black flat rock,

but in fact, it's a layer of
photosynthetic microorganisms.

But Chris has discovered that
although they look bone-dry,

they actually require water.

The microorganisms have solved

the problem of the lack of water
in a very ingenious way.

The organisms secrete
organic materials,

which help to hold water.

They make little super sponges,

and so the rare moisture
that's there, they can hold it.

They can hold it longer
than in normal sand.

These organisms have created
a little tiny micro habitat

in which they live.

Chris has studied organisms
across the Earth:

in the dry valleys of Antarctica

and in the Atacama Desert
in Chile,

the driest desert in the world.

And everywhere,
he's found life using

similar survival strategies.

We have never found
an organism on Earth...

the driest, coldest places,

and we've looked--
and boy, have we looked--

we've never found an organism

that can make do
without liquid water.

So life as we know it
needs liquid water.

Fortunately, water is made

of two of the most abundant
elements in the universe:

hydrogen and oxygen.

We've even detected water vapor
in clouds of gas

in outer space.

What's harder to find
is a planet that could have

liquid water--

where the temperature
is warm enough to melt ice

and yet below the point
where water boils to steam.

The sun is like a fire.

It provides light
and to that light, warmth.

And that warmth is what keeps
a planet warm enough

to support liquid water.

If it's too far from the sun,
the water will be ice.

If it's too close,
the water will be steam.

So it needs to be
right in that zone

where the water can be liquid.

Sometimes we call it
the "Goldilocks zone."

It's not too hot,
it's not too cold,

it's just right.

Kepler's core mission

is to find Earth-size planets
in the Goldilocks zone.

Remarkably this data that shows
the timing of each transit

is also the key to finding out
how hot a planet might be.

The more frequent
the planet transits,

the hotter it is.

When a planet is very close
to the star,

it's going around
quite rapidly.

The period is very short--
can go around in weeks, even--

around the central star.

As a planet is further away,
it takes much longer.

A planet that orbits once a week

will be close to its star
and very hot.

A planet that orbits
every ten years

will be very far away
and too cold.

So scientists were looking
for a planet orbiting

a Sun-like star
roughly once a year, like Earth.

On May 12, 2009, just three days

after the spacecraft started
collecting data,

it saw the light
from the star Kepler-22

dim ever so slightly.

Ten months later,
it dimmed again.

If it happened once more
in another ten months,

it would confirm Kepler
was seeing a transit.

Sure enough,
on December 15, 2010,

the star dimmed once more.

Kepler had discovered
its first planet

in the habitable zone:

over 600 light-years away.

Kepler-22b was the first

of a planet that is at just
the right temperature,

so it's in the Goldilocks zone.

It's orbiting a star
very much like our own Sun.

The researchers can use
the amount of dimming

to measure the size
of the planet.

It works out at about 2.4 times
the radius of Earth.

So now comes the next mystery.

What kind of planet
is Kepler-22b?

What is it made of?

The key to a planet's
composition is its size.

When planets form
they start off solid,

made of rock and ice.

If they become big enough,
they're able to hold on

to huge amounts of gas--

hydrogen and helium
the most abundant.

These become the gas giants,
like Jupiter and Neptune.

Smaller planets like Earth
are primarily rocky

and can only hold on
to a thin atmosphere.

Kepler-22b is in the middle,

bigger than all the
rocky planets in our system,

but smaller than the gas giants.

So is it mainly gas or rock?

Astronomer Geoff Marcy
is attempting to find out.

Tonight, he's using
the Keck Telescope in Hawaii,

which he's operating via video
link from Berkeley, California,

over 2,000 miles away.

MAN (on screen):
So Geoff, I just opened up
and did an autofocus.

We've got about .85 parts...

That's great, fantastic.

And we're set up and ready to go
on your first target.

Tonight we're going
to be observing Kepler-22b,

the planet which is still
somewhat mysterious.

We don't know whether the planet
is rocky, like the Earth,

or gaseous,
like Jupiter and Saturn.

To answer this question,
Geoff is using

a very different way
of detecting planets.

Nicknamed the wobble method,
this technique was used

to find some
of the first exoplanets,

and it's also used
to find out more

about the planets
discovered by Kepler.

We can detect planets
around other stars,

even without seeing the planets,
by watching the star.

Let's create a star.

And, of course,

we'd like a planet
orbiting that star.

Now what's interesting
is that the planet

not only orbits the star,
but the star is yanked on

gravitationally by the planet.

So we see the star
wobble around,

and we can detect this with
our telescopes here on Earth.

The Keck Telescope can see
if a star is wobbling

backwards and forwards
in response to a planet

we can't actually see.

But even more beautifully,
we can measure

the mass of the planet,

because the more massive
the planet orbiting the star,

the more violently that star
is yanked on gravitationally.

If the telescope detects
a large wobble,

it means a massive planet
is orbiting the star.

We're heading
to the Kepler field now.

So I'm going to set
the exposure meter...

Geoff takes aim at Kepler-22. that gives us
Doppler shift precision

of about 1.5 meters per second.

So we'll be able to measure
the speed of the star

to within plus or minus
human walking speed.

Here we go.

And there it is, Kepler-22;
looks beautiful.

Even with this high level
of precision,

Geoff fails to find
a measurable wobble,

which means that the planet
is not extremely massive.

So far what we can tell

is that the planet
is probably not purely solid;

it's not a big ball of rock.

Kepler-22B could still be
a gas world,

a kind of mini Neptune.

But there's another possibility
that's even more intriguing.

This planet might be unlike
anything in our solar system:

a water world.

We do know that as long
as the planet

is over seven times
the Earth's mass,

it could possibly have
liquid water in its surface.

So Kepler-22b, despite being
a relatively big planet,

is still in the potentially
habitable category.

From the study of other planets,

all of those somewhat larger
than Earth planets

have lots of water and also,
perhaps, some atmosphere.

So, by association,
the bet is that Kepler-22b

will be a rocky planet with
a very dense and thick ocean.

If Geoff is right,
this would be a planet

completely covered in water.

It's the perfect planet
for a paddle.

Geoff can imagine
what it would be like to kayak

on Kepler-22b.

If you were kayaking
on Kepler-22b,

you'd look overhead
and you'd see a host star

that would remind you
of our own Sun here,

a yellowish star,
about the same size as our Sun.

The gravity of the planet
might be a little higher

or a little lower
than the Earth,

but not so different.

But look beneath the surface

and this ocean
is unlike anything on Earth.

The ocean would be

thousands of kilometers thick.

There would be no ocean floor
to dive down and visit

because it would simply be
too far away.

Kepler-22b could have all the
ingredients for Earth-like life.

Energy is plentiful in sunlight
from the nearby star.

The necessary elements
like hydrogen, oxygen, carbon

and nitrogen are commonplace
throughout the galaxy.

But for life to occur,
these elements would have

to form into a molecule
with a very special property,

a molecule that can
self-replicate like DNA.

Life is a chemical system
capable of Darwinian evolution.

We need molecules for life
that contain information.

We need molecules
that can mutate

so that that information changes
and that can be inherited.

I suspect that carbon
is going to be the stuff of life

and there will be information
containing molecules

such as DNA.

Even here on Earth we don't know
how DNA first came to exist.

But even if the creation

of a molecule that works
like DNA is unlikely,

life may have sheer numbers
on its side.

It may be that only one planet
in a million

will give rise to life,
but on the basis of Kepler,

some people have suggested
that there might be

ten to the 19th planets
in our galaxy

or in nearby galaxies.

That's a very big number.

And so the probability
that life exists elsewhere

is pretty high.

It's this emerging
statistical fact

that leads many scientists
to be confident

that extraterrestrial life
does exist.

Is there life elsewhere?

Well, our galaxy alone
has 100 billion stars.

If DNA-- or a similar
self-replicating molecule--

has formed on
any of those planets,

life could have been around
for some time.

If we go back to the very oldest
sedimentary rocks on our planet

and look for the signature
of life, it's already there.

And that tells us that life
arose early.

But early life was limited to
simple, single-celled organisms.

Microbes had the Earth to
themselves for billions of years

before complex life evolved.

It may be that microbes
rule our galaxy as well.

The most probable life
on other planets

would be something
much more similar to bacteria

than to you or me.

Of course, we don't know
how likely it is

that complex life would evolve
on an alien world.

It's still very much
an open question

as to whether life could get
started on such a water world,

but over billions of years,
if the climate is stable enough,

evolution will have had
the chance

to get beyond microbial life

to forests of complex,
advanced trees.

Could plants adapt to life
in an endless ocean?

On a water world,

the main challenge these plants
are going to face

is remaining buoyant;
you can't start sinking down.

You want to float, perhaps by
having bladders filled with gas

that help you to buoy up
in the water.

While scientists can speculate
about life on Kepler-22B,

for now, there is no way
to confirm

that there is water
on its surface.

Fortunately, it isn't the only
potential life-supporting planet

Kepler has found.

On February 28, 2012,
nearly three years after launch,

NASA announced over 1,000
potential new planets.

One of these was orbiting
the star KOI 2626.

This potential planet

is about one and a half times
the size of Earth,

small enough that
it's likely to be rocky.

But there's a problem:

it doesn't orbit a star
like our Sun.

It orbits
a very different kind of star.

When I was a kid,

I was told in school that
the Sun is an average star,

which is a complete lie.

The Sun is not an average star.

Most stars in the galaxy
are much less massive

and put out much less energy
than the Sun.

And we call these very
low-mass stars M-dwarfs.

Our Sun is yellow in color,
very massive, very large,

and compared to it, these low
mass M-Dwarf stars are tiny.

They are only
about 10% or 20% the size,

10% or 20% the mass,

and they only put out 1/1,000th
the amount of energy.

M-dwarf stars,
also known as red dwarfs,

make up over three quarters
of all stars in the universe.

Because the star is so small,

it's easier to spot
a small planet orbiting one.

Even though
an Earth-sized planet

covers only about 0.1%
of a red dwarf,

it typically dims the light
by an easy-to-see one percent.

Dave Charbonneau and his team

have searched
through the Kepler data

and they've found that
more than half of all such stars

have planets Earth-sized
or a little larger.

But could a planet
orbiting such a dim star

be warm enough to have oceans
of water and even life?

An M-type star provides much
less sunlight than our star.

To get the same amount
of warmth,

just like this dimmer fire,
you have to move in closer.

There's still a habitable zone:

you can get close enough
to the star

that it's warm enough
for liquid water.

KOI 2626.01 orbits its red dwarf
star once every 38 days.

But being so near to a star
can create a strange situation.

Gravity from the star
pulls more strongly

on the closest side
of the planet:

a slight drag
that can cause one side

to always face the star.

Astronomers call this
tidal locking.

This alien world would have
a light side

on which the sun never sets,

and a dark side
on which the sun never rises.

At first, scientists thought

that this would make the planet
impossible for life

because it wouldn't have
an atmosphere.

When we first conceived
of the notion

of a tidally locked planet,
even in the habitable zone,

the thought was,
"Well, on the dark side,

"it's going to be
very, very cold,

"so cold that the atmosphere
could condense out completely.

"All the carbon dioxide

"that would normally be
in the atmosphere

"would be present as ice,

forming a giant polar cap
on the dark side."

Ice like this.

This is a chunk of carbon

But atmosphere expert Kevin Heng
wasn't so sure.

He builds computer simulations
of alien atmospheres.

He's a kind of interplanetary
weather forecaster.

Heng used what we know
about the red dwarf planet

to simulate the atmosphere.

And this is what
he came up with:

a map of the whole planet
laid out flat,

the colors representing
the predicted temperature.

What you're seeing

is a temperature map
near the surface.

Right in the middle of the map

is the point
where the temperatures

are expected to be the highest.

And this temperature difference

between the hot side
and cold side

has a profound effect
on the atmosphere.

Here on the dark side,
the air cools and sinks,

spreading out
around the planet.

That creates a wind

that blows along the surface
to the hot side.

Here, the air heats up and rises

before returning
to the cold side.

So if this planet
has an atmosphere,

it would have a permanent,
planet-wide wind system.

This reduces the temperature

and keeps the atmosphere
from freezing,

which means that the planet
might be habitable after all

with a stable atmosphere

and in some places,
a pleasant climate.

You probably want to live close

to where the day side
transitions into the night side,

the so-called day/night

Temperatures are between 10
and 20 degrees Celsius.

So a good spot for real estate.

If there is plant life here,

it would have to adapt
to an endless deep red sunset.

Here on Earth, most plants
capture energy with chlorophyll,

a green pigment
adapted to harness

the yellow light from our Sun.

But on a red dwarf planet
with its red light,

an alien plant-like species

might adapt to produce
different pigments.

Nancy Kiang
is trying to work out

what those alien plants
could be like.

One clue is to be found
in the marshes of New England.

The sandy areas you might think

are just dead areas
with no plants,

but actually there's life
teeming underneath.

Living in the sand are layers
of photosynthesizing bacteria.

What we see here
are different kinds of bacteria

that are photosynthetic.

Each layer
has different pigments

that absorb different
wavelengths of light.

The bacteria in the top layer
are green.

They use the same chlorophyll
that plants do

because they receive
the full spectrum of sunlight.

But many of those frequencies
get absorbed,

leaving the lower layers with
a different color of light.

So they have evolved
new photosynthesizing molecules

adapted to that different

Studying these different
molecules allows Nancy

to imagine what sort of pigments
would work best on alien worlds

bathed in the dim red light
of a red dwarf.

It leads to an intriguing

Very likely, these plants
will try to absorb

as much visible light
as they can.

So these plants to our eye
might look black.

Black because the plants

would need to absorb
as much energy as possible

from the weak sun.

Plants might find the situation
actually easier to contend with

because the sun isn't moving
through the sky.

You can just point yourself like
a solar panel towards the sun.

But one of the other features
of a tidally locked planet

would be the fearsome winds

blowing constantly
in the same direction.

So maybe a flat panel

wouldn't actually end up
being the optimal design

and you want to be
a bit more flexible and giving

to allow the wind

to kind of blow through
your perhaps fan-like shape.

If there are plants,

could there also be alien
animals on this planet?

From the wide variety of
creatures that exist on Earth,

we know that many seemingly
bizarre creatures are possible.

It's fun to speculate

about what life might look like
on another planet,

and I think it's
legitimate science

insofar as it tells us that
there are rules to the game.

Any life is going to be subject

to the laws of chemistry
and physics.

Based on what we know
about life on our world

and from what we know
about the environment

on this distant planet,

scientists are beginning
to imagine

what alien animal life
might look like.

On Earth, we find
filter feeders:

life that filters particles
or little grains of food

out of the water
as it wafts past them.

On a planet like this,

we might find a similar strategy
on the land.

So maybe we'd find something
like land whales

that aren't filtering out
plankton from the sea

like the whales in our oceans,

but are great big static animals
that try to filter out

particle matter from the wind
to feed off.


Even though scientists speculate
about how life might survive

on truly alien worlds like this,

most still believe that

our best hope of finding life
that we might recognize

lies in finding a rocky planet
a similar size to Earth

that orbits a sun
more like our own.

It's a challenge
of astronomical proportions.

A planet as tiny as an Earth

transiting across the disc
of its star

is going to produce
a change in brightness

of just one part per 10,000.

Imagine the tallest hotel
in New York City,

and everybody has
their light on,

and one person in this hotel

lowers the blinds
by about two centimeters.

That's the change in brightness
that we are trying to detect

from the transit of a planet
as small as an Earth

passing in front of a star
the size of our Sun.

So Kepler scientists
scour the data

for signs of a truly tiny dip:

evidence of an Earth-like

And they find
something promising

around star KOI 701,
1,200 light-years away.

The Keck Telescope confirms that

there are at least three planets
orbiting the star.

The third planet out,

that planet is
in the habitable zone.

But it's also large, like 22b.

It's like Kepler-22b,
a water world.

The team was about to go public

when something
extraordinary happened.

So 701.03 was done--

in the bag,
nice ribbon tied around it,

ready to go off to the publisher
for publication--

when we got an email
from one of our team members

telling us that he had spotted
what looks to be

another interesting signal
in the data.

What Natalie's team member
had spotted at the last moment

could turn out to be

one of Kepler's
most exciting discoveries.

It looked like KOI 701

didn't just have three planets
orbiting it.

There were four.

And this new potential planet

was in the right place
and the right size.

We examined the data
more carefully,

and there is another planet that
is smack in the habitable zone,

and it's only a little bigger
than the Earth.

It was exactly what Kepler
had been searching for:

a planet in the habitable zone,

small enough that it could be
made of water and rock,

like the Earth.

And so this is something
you could stand on,

something that is rocky.

This might be the first
truly Earth analogue

around a Sun-like star
that's ever been found.

The planet, today confirmed
as Kepler-62f,

is over 1,200 light-years away.

Even our fastest spacecraft

would take many millions
of years to reach it.

But this could be
the most Earth-like world

Kepler has found.

And that's the amazing planet

that has the hairs standing up
on the back of my neck.

If this planet has oceans
of water and rocky land masses,

then the possibilities
for life here

could be as numerous as they are
on our own planet.

On our planet,

we began with simple bacteria,

and today, we have a planet
that has ten million species

of animals, plants, fungi,
kelps, all sorts of things.

In the oceans,
the environment could be

surprisingly similar
to that on Earth.

The density and properties
of water are dictated

by the laws of physics,
which are universal.

What we might expect is

for the aquatic or marine life
to be actually pretty similar,

pretty familiar
to what we know on Earth,

because the main consideration
is being able

to move through the water
as efficiently as possible.

So you want to be hydrodynamic,

you want to be streamlined
in your shape.

And a very good streamlined
shape is known as fusiform:

you'd be kind of bullet shaped,
just like most fish on Earth.

This ocean could be home
to a complex ecosystem

with different sizes of organism

filling different
ecological niches.

But because this planet
is 40% larger than Earth,

it's likely to have
stronger gravity.

So what might we find on land?

Animals will respond

to the increased gravitational
pull on a larger planet,

and so they would want
to have body plans

which are also very strong,

so with column-like legs to
support their increased weight.

On Earth, all vertebrates
have a maximum of four legs

because all animals
with a backbone

evolved from a fish that had
four fleshy, lobed fins.

But could aliens have more legs?

That's what Bill Sellers
wants to find out.

He usually looks at extinct
species like dinosaurs

by recreating their structure
in a computer.

The same technique can be
applied to an alien body plan.

When we have an alien life form,

we treat the shape of the animal
as the framework of the robot.

We add motors
to actually drive the creature,

and then the trick is that
we get the computer to learn

how to be the most
effective driver

for this particular
shaped animal.

Bill is investigating

how life might move
in a high-gravity environment.

The computer model he's using
is called a genetic algorithm.

First, he creates
an eight-legged alien.

Then, the computer generates

hundreds of random movement
patterns and tests them out.

This is a moving forward start,
but as you can see,

what happens
if it gets the pattern wrong,

then although it's still moving
its legs,

it's lost all forward velocity,

and actually it's starting
to go backwards

in a very unstable way.

Other examples of failures,
for example, this one,

what happens is the front pair
of legs trip over

and then the whole thing

and because of where
the weight is,

the back legs are up in the air
and spinning round.

So again,
this is a complete failure,

and we wouldn't work
from those.

But the computer can take the
best of those random patterns

and combine them
to produce new variations.

It echoes the process of natural
selection that drives evolution.

After the evolutionary process
is finished,

we end up with these very
stable, very efficient gaits.

What the work has shown is that
this eight-legged creature

mechanically could cope
with higher gravity.

Bill's work shows that

it is mechanically plausible
that this new Earth-like planet

could have animals with eight
legs walking across its surface.

If this planet
does have complex animals,

could it also have
intelligent life?

For astronomer Geoff Marcy,
this is the ultimate question.

We really don't know.

First of all, we don't know how
commonly life gets kick-started.

And secondly, we don't know
how commonly it evolves

into intelligent life.

The probability of intelligent
life evolving on another planet

is perhaps the greatest
unanswered question.

But there are hints on Earth

that intelligence might be
the exception, not the rule.

Sharks like the ones we see here

have ruled the seas
for 400 million years,

and yet their brains
are no larger than peas.

How could it be that
after 400 million years,

sharks have not developed
a higher intelligence?

And of course
the answer must be

that high-IQ sharks compete
no more successfully

than the less-intelligent sharks
among us,

and that tells us
something frightening

about life elsewhere
in the universe:

perhaps the intelligence
of which we humans are so proud

is not an attribute
that is strongly favored

in Darwinian evolution.

If Geoff is right, human
intelligence may be the result

of a very unusual
set of circumstances

that just happened to make our
ancestors develop bigger brains.

But others, like Sara Seager,
are more optimistic.

It's a tough call about whether
or not there's intelligent life

around any of the nearest stars.

I believe there is
intelligent life out there

somewhere in the galaxy

and that a future generation may
be able to establish contact.

While the chances of intelligent
life evolving may be small,

the number of places
it could be is huge.

About 900 of these
planet candidates

are twice the Earth's radius
or smaller.

So what this collection

of exoplanet candidates
is telling us

is that small planets,
planets like Earth,

might be common.

Based on the Kepler data,
scientists estimate

that at least one in six stars
has an Earth-sized planet.

There could be upwards

of 17 billion Earth-sized
planets in our galaxy alone.

Well, we are at a remarkable
moment, actually.

The Kepler Space Telescope

has identified
several thousand planets.

But what's unexpected
and remarkable

is that we're finding planets

that are a little bit larger
than the Earth

that as far as we can tell
have the correct composition--

rock and water--
suitable for life as we know it.

Kepler has shown us
how commonplace

Kepler has shown us
how commonplace

planets outside
our solar system are,

suggesting our galaxy

may have other places
suitable for life.

Kepler continued to collect data
until May 2013,

when the guidance system failed,

and it lost its steady lock

on the stars
it had watched so keenly.

But the hunt for alien planets
is far from over.

Kepler may have a second life,

using pressure from sunlight
to keep it oriented.

And in coming years, new planet
hunters will take up the quest,

exploring the age-old question:
"Are we alone?"

The exploration continues
on NOVA's website.

Could there be extraterrestrial
life in our own solar system?

Planetary scientist
Carolyn Porco

thinks a tiny moon of Saturn's
may be the best place to look.

And meet Seth Shostak,
a SETI astronomer

who uses radio telescopes

to listen for communications
from outer space.

Watch original video shorts,
explore in-depth reporting,

and dive into interactives.

Follow us on Facebook
and Twitter.

Major funding for NOVA
is provided by:
On NOVA, a weapon
of mass destruction

Supporting NOVA and promoting
public understanding of science.

And by the Corporation
for Public Broadcasting.

And by contributions
to your PBS station from:

Additional funding is provided
by Millicent Bell through:

This NOVA program
is available on DVD.

NOVA is also available
for download on iTunes.

Captioned by
Media Access Group at WGBH