Original subtitles

Across the galaxies lie exotic worlds.

Some made entirely of water.

Others spewing with poisonous gas.

What kinds of creatures thrive in these places?

Will they resemble beings on Earth?

Or could life take on new, unexpected forms?

What do aliens look like?

Space, time, life itself.

The secrets of the cosmos lie through the wormhole.

♪ Through the Wormhole 2x09 ♪ What Do Aliens Look Like? Original Air Date on August 3, 2011

== sync, corrected by elderman ==

They're out there.

We can see them.

For the first time in human history,

we know the Universe is filled with planets

stranger than we could ever have imagined --

planets that might be home to extraterrestrial life.

But what will these creatures look like?

We're all products of our environment.

If I was born on a planet with carbon dioxide air

and gravity three times weaker than the Earth's,

I might look like...This.

On a planet with five times more gravity than Earth,

and a star that constantly blasted it with solar storms,

I might look like this.

[ Growls ]

We can't know the face of an alien

until we're staring at it.

But like detectives on the hunt for an unknown suspect,

biologists and planetary scientists

are beginning to piece the puzzle together.

Some of the clues are out there,

but a lot of them are right here.

To get home from school every day,

I had to cut through the yard of a scary, old house.

I never saw anyone come in or out of it,

but someone or something lived there.

I could only imagine who or what it might be.

[ Gate creaks ]

Harvard Paleontologist Andrew Knoll

has spent his life

studying creatures beyond our wildest imaginations.

Knoll: One of the things you learn

when you go through a museum like this is

that not only is it hard to imagine

what life might be on another planet,

but it's hard to imagine some of the life

that has existed on this planet.

Who would guess that there were things like dinosaurs

in the absence of their bones?

Freeman: For the past eight years,

Andrew has served as mission biologist

on NASA's Mars rovers.

It's a role he's uniquely suited for

because of his expertise in the vast array of life on Earth,

and his ability to read the history of a planet

from its rocks.

Knoll: There's a tendency for us to think about the Earth

in terms of the things we see around us today.

But the one thing that the geologic record tells us

is that there have been a series of Earths,

and that the Earth that we see around us --

all the plants and the animals

and the composition of the atmosphere --

are really an end-member,

the end state of a long series of transitions

that have happened over 4 billion years.

For example, this rock,

which formed about 3½ billion years ago,

is full of iron minerals,

which means that iron had to be able to be transported

through seawater, and it can only do that

in seawater that contains no oxygen.

Freeman: The discovery of rocks like this all over Earth

shows that for nearly the first 4 billion years

of its existence,

our atmosphere had almost no oxygen.

That Earth would have been toxic to us.

Now, there are other things that are sort of unexpected

when we actually look at deep-Earth history.

This rock was actually deposited by glacial ice

about 635 million years ago.

There are rocks like this

that formed literally all over the world at this time,

and it shows us that there was glacial ice

at sea level at the equator.

In fact, much of the Earth --

perhaps most of the Earth -- was covered with ice,

sometimes called a snowball Earth.

Freeman: These various Earths --

hotter, colder, with more or less oxygen --

were essentially alien worlds.

So, for Andrew, the best place to discover

what aliens might look like is in our own fossil records.

Knoll: These are trilobites.

Now, when you look at this,

you'll see things that are familiar.

There is a jointed, segmented body.

There are jointed, segmented legs.

And you might say,

"Well, that looks like a shrimp or an insect," and that's right.

Freeman: Biologists

call these repeated similarities of life-forms

over Earth's history "convergence."

One shape that works well

gets repeated over and over again.

This giant sea creature looks like a whale,

but it is actually an extinct lizard.

Knoll: Repeatedly over the last 250 million years,

vertebrate animals on land have re-invaded the oceans.

And every time they've done so,

they've given rise to these giant sea monsters.

Kronosaurus.

70 million years ago, there were lizards in the sea.

They were equally large.

In our own lifetimes, there's whales.

Freeman: If Earth in the past

has been as alien as planets orbiting other stars,

then aliens you've seen in movies --

lizards with two eyes, two arms, and two legs --

might be pretty close to the mark.

[ Woman screams ]

[ Roaring ]

Labarbera: I must admit,

I watch a lot of old monster movies from the 1950s

specifically looking at the physics

and saying, "No, no, no. That's not gonna work,"

or "Ooh, that's really good."

Freeman: University of Chicago Professor Michael Labarbera

is an expert in biomechanics.

He's trying to predict

how aliens will walk, fly, and swim

by searching for the basic rule of how animals move.

You could call it

the lowest common denominator of locomotion.

Labarbera: Things like horseshoe crabs

were crawling out on the beach and laying their eggs

when pterodactyls were flying in the sky.

One of the features that we share with these animals

is a lever-type skeleton.

I have levers in my hands.

That's what allows me to do that.

I have levers in my elbows, in my shoulders.

The basic idea is

to use a lever that has a high mechanical advantage,

that delivers a lot of the muscle force

to the output side of the lever.

Freeman: Successful designs

like jointed limbs and hard skeletons

show up again and again in the fossil record.

We see them all around us today,

and Michael expects to see them on other worlds, too.

And it doesn't matter whether the skeleton

is made out of hydroxyapatite like our bones,

made out of chitin like this animal,

or carbon nanotubes.

When a principle is easy enough

for natural selection to stumble across,

then it will evolve over and over again.

On this planet, it has evolved independently

at least half a dozen different times.

And there's every reason to believe

they will be just as common

in any other ecosystem on any other planet.

Freeman: A torso with jointed limbs acting as levers.

It's a good basic anatomy of an alien,

but can we get closer to imagining their true form?

In the 19th century,

Charles Darwin kept a series of notebooks

chronicling how the shapes of animals had evolved

to adapt to the environments they lived in.

What would a book of life on other planets look like?

What mind-bending, anatomical adaptations

might develop in alien surroundings?

The environment shapes creatures depending on their ecology.

Density of the atmosphere,

whether or not you have a world-covering ocean,

is gonna make a big difference in the history

and, thus, in the shape of the organisms.

Freeman: Which is why

to know what aliens look like,

we must learn more about the planets they live on.

Until very recently,

we had no proof other planets existed,

let alone any idea what their landscapes or atmospheres

might be like.

But now, for the first time in human history,

we can see worlds far outside our solar system.

And now that we know where E.T.s could live,

we're getting closer to revealing their hidden faces.

If we want to know what aliens look like,

we first have to know something about the places they live.

Until recently, this was impossible.

Our telescopes could only see stars,

not the planets that orbit them.

Today, alien hunters have a dedicated research ship

floating 20 million miles from Earth,

and it's discovering new worlds by the thousand.

Man: 3...2...

(Man #2) Engines start.

Man: Zero.

And liftoff of the Delta II rocket with Kepler.

Freeman: In 2009,

NASA launched its latest space telescope --

Kepler.

It's designed not to take pictures,

but to detect the tiniest changes

in the brightness of distant stars.

Its target area is a patch of our arm of the Milky Way

stretching out 3,000 light-years away from us.

Harvard Professor Dimitar Sasselov

is one of Kepler's lead scientists.

The beauty of how the Kepler telescope

discovers planets as small as the Earth

is the method,

which we call the transit method.

It's very easy to understand.

So, the planet is passing on its orbit

in front of the star.

Its shadow causes that light dip just a little bit,

and that's how we know there is a planet there.

Freeman: By the time Kepler is done with its mission,

Dimitar expects it will have found

around 100 planets the size of Earth.

But the vast majority of the planets it is finding

have almost nothing in common with our world.

Sasselov: Kepler already has a treasure chest

of weird planets, if you will --

very interesting, diverse planets.

So, we have Kepler-10, which is as hard as iron.

Then we have two or three planets

in the Kepler-11 system of six.

One or two of them are water planets --

endless ocean.

Then we have planets

almost the density of a beach ball or styrofoam.

Freeman: Perhaps the most intriguing of Kepler's discoveries

are around 300 super-sized versions of Earth --

planets made of rock, but up to five times as heavy.

If anyone can imagine the landscapes

where aliens might jog, swim, or glide,

it's Diana Valencia.

Part-time triathlete,

she's one of the first geologists

to break ground on these super Earths.

Valencia: I do not have a hammer.

I do not break up rocks.

What I do is I do numerical models

to understand how the Earth works

and use that to understand

how bigger Earths and similar planets work, as well.

Freeman: To understand

whether the super Earths could harbor life,

Diana is zeroing in on the basic geological engine

that powers rocky planets --

plate tectonics.

The movement of a planet's hard outer crust

is driven by a hot and viscous layer

of semi-molten rock below it

moving much like a jar of bubbling honey.

This experiment here shows us in broad lines what happens.

The mantle is a very viscous fluid,

and both fluids are very sensitive to temperature.

So, as we turn this heat up to simulate Earth's engine,

you will start seeing motion underneath the surface.

Now you see the overturn.

Now you start seeing things

that are moving all sorts of directions.

It's not just moving up.

Freeman: As heat rises,

it forms convective cells in the mantle,

which cause the plates on the surface to shift.

These shifts trigger volcanic eruptions and earthquakes --

events we associate more with death than life.

[ Rumbling ]

But that's just the short-term view.

From Diana's geological perspective,

this cycling of material from the inside of our planet

to the atmosphere has been vital to the evolution of life.

Valencia: Thanks to this process,

the surface temperature of the Earth

has not swung very much,

and it has been around that of liquid water

for over billions of years.

Freeman: Super Earths are bigger

and therefore hotter on the inside.

And when you turn up the heat,

plate tectonics kicks into a higher gear.

That may mean more volcanoes and more earthquakes.

But also, a planet with a much more stable temperature.

Valencia: On super Earths,

because convection would be much faster,

this cycle could respond much quicker --

perhaps an order of magnitude quicker.

And then we can speculate

that that has enabled the evolution of complex life.

Freeman: Think about how a super Earth

would have dealt with the impact

of the meteorite that wiped out the dinosaurs.

On Earth, this event triggered an extended global winter

that spelled the demise of those cold-blooded giants.

But on a bigger planet,

better able to control its temperature,

dinosaurs might survive

and have the chance to evolve bigger brains.

However, there is one major downside

to living on a giant version of Earth.

The core of our world is a spinning ball of liquid metal

generating a powerful magnetic field.

That field deflects a torrent of dangerous radiation from the sun

and forms a protective cocoon for all life here.

Diana's models predict

that super Earths may not have these force fields.

Valencia: It's very possible

that these planets do not have a molten core,

because their interiors are under so much pressure.

So, if you are a creature in a planet

that doesn't have a geomagnetic field,

you are being bombarded by high-energy particles,

and those are interacting with your cells,

causing mutations, probably.

So, you have to be clever, as an organism,

to adapt to those conditions.

Freeman: What kind of alien could survive

on a radiation-soaked super Earth?

It would need a protective shell,

perhaps laced with heavy metals like lead.

It would have powerful limbs and sharp claws

to let it burrow under the ground

during heavy radiation bursts.

Most important, it would need

effective genetic repair mechanisms

to fix the inevitable radiation damage to its cells.

Pure fantasy? Maybe not.

Similar life-forms,

albeit much smaller, called water bears,

survive in boiling-hot,

radiation-blasted regions on Earth.

Inhabitants of rocky super Earths

might look surprisingly familiar.

But imagine a world where there is no rock,

and where creatures living in the ocean

also fly through the sky.

On Earth,

evolution has produced countless variations on life --

animals that glide through the water

and soar through the sky.

Beings that slither, crawl, walk, and run.

If life on other worlds

follows the evolutionary pattern of life here,

what other mind-bending features might arise?

Okay. So, you got the planet, you've got the atmosphere.

Exaggerated.

Yeah.

At M.I.T. in Cambridge,

astrophysicist Sara Seager and biochemist William Bains

are beginning to imagine

what these distant worlds will be like.

The atmosphere's gonna come from somewhere,

so you're gonna have volcanoes producing atmosphere.

They're trying to predict how a planet's size and composition

will shape its biosphere.

Before the discovery of exoplanets,

people thought that all planetary systems

would be like our solar system.

And since that time,

discoveries of exoplanets and exoplanetary systems

have surprised us over and over and over again.

So, what has changed? Everything has changed.

Most science fiction assumes

that aliens are gonna be walking around,

they're gonna be breathing air.

You know, they landed a starship,

and they shared dinner with the Captain.

You look at some of the planetary environments

out there,

and that is not gonna happen.

It's gonna be very different.

Freeman: Recently, Sara and William

have been studying GJ 1214b,

a planet about 40 light-years away

that's more than twice the size of Earth

and shows signs of having an atmosphere.

Together they are working to discover

what it might be like

to descend beneath the clouds of 1214b.

Now, this planet --

we're not totally sure what it's made of,

but it could be a water planet with a steam atmosphere.

And depending on the temperature of the planet,

the clean division between liquid water

and air with water vapor in it may not exist.

Freeman: What sort of life could possibly emerge

on a boiling-hot, steam world?

Bains: So, on Earth,

an environment like this with boiling water and steam

is inimicable to nearly all life.

But we're trying to imagine an alien world

in which this is the normal environment,

and we can now start to model a planet

that has a huge ocean covering it

and nevertheless is incredibly hot.

That makes us think about,

"could there be life in the ocean?

"Can the chemistry work?

And if it can, what would it look like?"

A molecule like DNA wouldn't survive these conditions,

but William believes

more heat-tolerant genetic material

would likely evolve.

And he's beginning to imagine

what entries might fill the pages of a book of life

for GJ 1214b.

The atmosphere of this planet would be mostly water.

It would be steam.

It would be very dense and be very hot.

So, as you go down through it,

you'll find drifting plants, flying plant life,

and a whole range of organisms that eats that plant life.

Organisms would be sort of flying fish

or swimming birds, depending on how you look at it.

So, they'll be able to actually fly through

or swim through this zone almost as if it was ocean.

Freeman: Earth's oceans gave rise to creatures of all sizes,

but the kings of the deep are the giant filter feeders --

whales.

1214b could have them, too.

So, the organism we're imagining here

works in a very similar way.

It might have a very different shape.

But it moves through the ocean

and then can move up into this interfacial zone.

They can spend much longer in the interfacial zone

and move much further up into it than, say, a whale breaching

because the density is greater.

Freeman: This aquatic world is a vision

of what Earth might have been like

if it were larger and wetter.

Humans couldn't survive here, but could life find a way?

We don't know...Yet.

There are many important things in science,

and one of the most important ones is imagination.

So, what is so fascinating so far --

in exoplanets, anything is possible

within the laws of physics and chemistry,

and anything we imagine will exist somewhere.

Follow the water. There, you'll find life.

That's what the astrobiologists like to say.

But what if there is no water?

What about planets enveloped in toxic air

where the building blocks of life

are completely different from our own?

Could they also be alive?

Life is tenacious.

Everywhere on Earth,

from the coldest depths of the sea

to the boiling fissures of volcanoes,

living things find a way to thrive.

But the conditions on alien planets

could be even more extreme.

We're discovering worlds of fire and ice,

worlds of permanent night,

worlds where hurricanes are constant and global.

What kind of alien could live in these hellish places?

Gliese 581d floats 20 light-years away from Earth

in the constellation Libra.

It's one of the small group of planets we have spotted

that might harbor alien life.

Its red star burns

with only half the heat of our Sun,

but because the planet spins very slowly,

one side is much hotter than the other.

And its rocky surface is blasted by constant wind --

a great place to fly a kite.

Biomechanics expert Michael Labarbera

believes the thick atmosphere on Gliese 581d

would shroud the surface in darkness,

so life would have to climb up in search of light.

He imagines kite-shaped plants that rise above the storm clouds

to get their daily dose of solar energy.

These kite plants have to be able to get up

into the higher regions of the atmosphere

in order to get enough light,

and the way they do that is

to utilize the shear in the atmosphere.

Freeman: Michael's kite requires two forces

to stay aloft and stable --

wind to lift the kite,

and an anchor to keep it from blowing ever upward.

The alien kite plant works much the same way.

So, what we've posited for this particular plant is

a lifting surface on one end of the string,

and at the other end of the string,

something that functions like a parachute

that produces a drag force.

And because the wind changes with altitude,

they're moving at different speeds.

You then get a lift force that keeps the kite up

and it pulls on the drag chute,

but that keeps the tension on the string

and the whole system is stable.

Freeman: Sounds unlikely?

Michael doesn't think so.

Years of studying organisms on Earth has convinced him

that life would evolve to suit any environment.

Labarbera: Evolution goes through very strange pathways

to get to an endpoint.

This particular one,

we don't have an example here on Earth,

but on the planet posited here

with low solar input for the ground level

and a high wind shear,

it's entirely possible that it could function.

Freeman: Closer to the surface of Gliese 581d,

the once bright sunlight dims

as this exoplanet enters a permanent, hazy twilight.

The atmosphere is thick and murky,

but warm enough to sustain life.

In fact, Michael Labarbera speculates

that it could host a thriving ecosystem

of hunters and prey.

What kind of predator would evolve here?

An aerial hunter -- thin-winged and bat-like,

but able to soar and glide for days like an albatross.

A...Bat-atross?

Labarbera: Now, this animal,

because the atmosphere is relatively opaque,

has to be able to travel long distances at minimal cost

in order to find their prey.

It's got long wings.

It's got relatively narrow wings because they're more efficient.

It has a big wing area relative to its body.

Freeman: On Earth,

albatrosses use a technique called dynamic soaring

to travel thousands of miles

while barely flapping their wings.

Gliding in long loops,

the bat-atross would also conserve energy

by letting air currents carry it along.

Labarbera: The animal actually covers many times the distance

in these loops that it's covering on the ground,

but it doesn't matter.

It doesn't cost it anything. It's free.

It's energy that's supplied by the environment,

not by the organism.

Freeman: But how, in a world of permanent twilight,

does this hunter find its prey?

In the absence of light,

there's got to be some other way of locating prey.

One way is just to sit and listen

and wait for your prey to make noise.

The other way is for you to make noise

and listen for echoes -- what we call sonar.

So that you send a sound beam out

and you wait for a reflection.

I can get a lot of information

from the response of the ball as it comes back.

So, the delay between when I throw and when it returns

tells me how far away the object is.

If it comes back faster than I threw it out,

then the object is coming towards me.

If it's going in the other direction,

it will come back slower.

If you're looking for prey, this is a wonderful idea,

unless your prey, of course, can detect the sound.

Freeman: The bat-atross would be an effective killer,

so its prey would need to develop effective defenses.

William Bains imagines an animal

similar to the hard-shelled marine life

that evolved on Earth

hundreds of millions of years ago.

Bains: The nautilus is natural prey for the hunters,

and they'll have three defense mechanisms.

First is, of course, they have a shell.

The second is if you're being hunted by sonar,

then you develop very good ears so you can hear sonar.

When you hear the ping of a sonar, you run for it.

And it has a jet propulsion system

that can squirt itself forward in emergencies.

These guys will be able to

jet themselves through the atmosphere

in short bursts, moving very quickly.

So, at the last minute,

they'll jet to one side and escape being eaten.

Freeman: But even with these defenses,

the bat-atross would be a fearsome opponent,

and the nautilus won't always get away.

Labarbera: It's life on the edge.

There always is a top predator.

It's the rarest animal,

but it's not the guy you want to meet in a dark alley.

Freeman: Brutal conditions breed brutal life-forms.

Here on Earth, over hundreds of millions of years,

billions of different creatures competed for survival,

but eventually, a special mutation enabled one animal

to become the planet's top predator.

That mutation was the human brain.

Somewhere out in space,

alien evolution should have created beings

at least as smart as we are.

What do intelligent extraterrestrials look like?

This man thinks he knows,

and the answer could be bad news for life on Earth.

With each new world we discover,

we come one step closer to finding evidence

of life beyond Earth

and perhaps to fulfilling our dreams

of communicating with alien life-forms.

But if that day ever comes,

we'd better brace ourselves for a shock,

because many scientists think

they may not look like living beings at all.

For the past 50 years,

the search for extra-terrestrial intelligence, SETI,

has attempted to capture any glimmer of communication

from alien worlds.

For Seth Shostack, SETI's senior astronomer,

it's a search for our distant cosmic image,

for a species with a brain at least as smart as ours.

When it comes to intelligent life,

we haven't found it.

So, there are people on all sides of the issue.

But the one thing that can convince you --

I think can convince anybody -- is that even if you think

the processes that could lead to life,

lead to intelligent life,

are not going to occur very often,

there's so many chances for it to happen in the cosmos,

it would be miraculous

if we were the only world with intelligent beings.

Freeman: Humans aren't the largest

or the fastest

or the most agile animals on Earth,

but we are the smartest.

Our brains have put us on top.

There is, however, plenty of room for improvement.

There's an unavoidable tendency to think

that we're kind of the crown of creation.

This is it.

You know, 4 billion years of evolution

from the beginnings of life to us.

You know, I think if you asked the dinosaurs

the same question --

"Do you think you're the crown of creation?"

I bet they would have said "yes,"

if they could have talked.

"This is it. This is the end of evolution."

Well, they were wrong.

And it would be wrong for us to think

we're the end of evolution, too, obviously.

Freeman: So, where will evolution take us next?

And where is it likely to have taken alien civilizations?

Seth thinks we need to look at our computers for the answer.

Since the 1970s,

when floppy disks were the gold standard,

this speed at which computers process instructions

has increased more than 100,000 times.

Today, for $1,000, you can buy a computer

that has, if you will, the thinking capability --

or at least the computational capability of a lizard.

Not so interesting.

But by 2020 or 2025, $1,000 will buy you a laptop

that has the same computational power as a human Brian.

Freeman: The I.Q.s of artificial brains

are going from zero to 200

in the historic blink of an eye.

How would a similar trajectory play out

on a planet that is a mere 500 years ahead of us?

The interesting thing about artificial intelligence,

of course, is its pace of evolution.

I mean, we're stuck with Darwinian evolution,

but the machines wouldn't be.

What it means is that if you develop a thinking machine,

it's going to improve itself very, very quickly.

Freeman: In 1948, mathematician John von Neumann

imagined a machine so intelligent

it could make copies of itself.

Each copy would improve on the previous model,

much as nature continually improves on its designs.

But this machine's evolution would take place much faster

than biological evolution.

Today, von Neumann machines exist in crude form.

On a planet more advanced than our own,

could they be the most intelligent life-form,

the dominant life-form?

Will our first contact be

with a race of super-intelligent machines?

You're only gonna hear from a species

that's at least as clever as we are.

So, what are the odds that they're within 50 or 100 years

of our level of development?

Pretty slim.

They're likely to be thousands, millions,

maybe even more years ahead of us.

So, if you think about that for a moment,

you recognize that if we do find a signal,

the odds are pretty good

that that signal's coming from artificial intelligence,

not some soft, squishy, little gray guy with big eyeballs.

Freeman: On some distant planet,

the book of life may no longer contain any biological forms.

And if mechanical life has enough power,

there's no limit to how large or complex it can become.

Shostack: Or maybe they've reorganized themselves

so that they can share the thinking load

amongst many members of the species,

like distributed processing with computers.

I mean, why should the aliens be content

to be stuck with a kind of intelligence

that can fit inside their heads?

Freeman: Alien evolution

could produce a living machine planet

throbbing with the combined intelligence

of billions of alien minds.

If such advanced life exists, how would we spot it?

And should we even want to?

Will aliens welcome us as friends

or view us as threats?

Or perhaps see Earth as a world to conquer?

We wonder what aliens look like,

but what do we look like to them?

This woman has put herself inside their heads,

and she believes she has the answer.

As long as humans have looked up at the night sky,

we have wondered whether something or someone out there

is looking back.

We want to know what aliens look like.

What do we look like to aliens?

If there is intelligent life out there,

does the Earth look like a place worth visiting?

May 29, 2008.

31 million miles out in space,

the eyes of a technologically advanced race

scan our planet for the signatures of life.

Not aliens, but this was still a close encounter

of an extraordinary kind.

It was the NASA space probe EPOXI.

Sent out to get closeups of comets,

EPOXI briefly turned its lens back to its mother planet.

And for the first time,

we saw the Earth as aliens might see us.

Astrophysicist Sara Seager was part of the EPOXI team.

Sara normally studies exoplanets,

looking for clues

about alien atmospheres and ecosystems.

The EPOXI probe gave her the chance to find out

what Earth might look like to an alien astronomer.

If you pretend you know nothing about Earth,

what could you learn about Earth?

An alien would be able to pick out Earth's rotation rate.

They would be able to notice

that we have surfaces of very different reflectivity --

that's cloud, land, and ocean.

And they could also see that we have weather.

They would see variability

that isn't related to the rotation rate of Earth.

The second thing EPOXI did was look at a spectrum of Earth --

that is, take the white light

and split it up into the different colors

and to check and see if any of those colors were missing.

We call that a spectrum.

Freeman: The spectrum of Earth's colors

are like a flag

announcing the presence of life on our planet.

The blue of the oceans, the white of the clouds,

the green of the land

are all markers of an active ecosystem.

Seager: If an alien is looking back at us from far away,

the aliens would see

that we have oxygen in the atmosphere.

In fact, our atmosphere has 20% oxygen by volume.

What's so fascinating is that, without life,

our Earth would have basically 10 billion times less oxygen.

So, oxygen would be essentially non-existent on Earth.

And oxygen on Earth is created by life,

so those aliens would know that oxygen in such large quantities

should not be in our atmosphere

unless it is being continually produced by something.

And nothing that we know of in geophysics

can produce so much oxygen.

And that's why we attribute it to life.

Freeman: Aliens might see that our planet supports life,

but they might not see

that Earth is technologically advanced.

They would have to look carefully

to detect things like atmospheric pollution

or the heat signatures of our cities.

Reading the colors of our world

and the millions of others like it out in the Universe

would be easy for an advanced alien civilization.

Unfortunately, it is not yet easy for us.

Spotting exoplanets

pushes the limits of current technology.

If we want to see colors, we need a new set of tools.

Astrophysicist Dimitar Sasselov

wants to do something about that.

Sheila: These are little round planets.

I'm gonna just drop a few on to show transiting planets.

I guess there's two transiting.

Dimitar's wife, Sheila, paints scenes of deep space

inspired by his research on the Kepler planet-finding probe.

This is the kind of thing we want to discover with Kepler.

A transiting planet, and there is a moon around it.

That would be great.

So, there it is.

That's the planet with life on it -- right here.

We have a big problem.

This challenge relates to our inability to measure

the colors of the star or the planets separately

to very high precision.

And the challenge is about the factor of 10 to 100

beyond what current technology works.

Freeman: The biggest barrier we have

to seeing the colors of other planets

is something every photographer has run into --

camera shake.

If you take a picture in the dark,

you need as much light as possible,

which means you can't move the camera

or you'll get a blurry image.

Earth-like planets are so small and so far away

that their images only fill one thousandth of a single pixel

of a digital camera.

If that pixel moves even slightly,

the camera shake will ruin the picture.

But how can you possibly keep one pixel perfectly still

over the days and years it takes

to track an object in distant space?

Dimitar's solution is the astro-comb.

It uses lasers to keep a telescope's camera sensor

precisely calibrated over a period of decades.

Sasselov: The astro-comb that you see here

is the technological breakthrough

which was needed to bridge that gap.

Freeman: When we see the true colors of other worlds,

we will know where and how life is distributed

across the Universe.

And the next phase of our quest for alien life will begin.

Where will it take us?

What exciting, new worlds will we see?

What new and unexpected creatures might live on them?

Biologists think that life out there might look Earth-like,

but it won't look human.

With so many planets out there, so many chances at life,

we could have human-like relatives on a far-away Earth.

Creatures like us,

perhaps as anxious as we are to know

if they are alone in the Universe.

As our tools improve, so do our odds of finding them.

Sasselov: It is clear that we're in a new age

of exploration and discovery.

It hasn't been for 500 years

that people have tried to discover planets

around other stars.

Now we have them.

We have much more to explore, and the best is yet to come.

1,000 years from now,

when people look back at our generation and ask,

"What are the biggest accomplishments?"

I like to think of these people making interstellar journeys

and looking back and thinking

we were the ones who started it all.

Freeman: What do aliens look like?

What are the limits of our imagination?

The true face of an alien

will probably defy our scientific speculations.

But our efforts won't be wasted,

even if we do get all the details wrong.

Our eternal intrigue about alien life

and our persistent fear of it

both rise from the same source --

the quest to understand our place

in the family of life-forms that populate the cosmos.

Know that,

and we'll know the destiny of humankind.

== sync, corrected by elderman ==