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[contemplative music playing]

[narrator] Life.

An unbelievable, perfect
combination of elements

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YTS.MX

that is able to create living matter.

Maybe the biggest mystery of nature.

Is life an amazing phenomenon,
exclusive to our planet?

In recent years, we have made
several amazing discoveries

that suggest that the conditions
in our solar system for life

might be more prevalent
than ever imagined.

Recent missions
are revealing strange worlds.

Moons that could have vast oceans

concealed beneath miles of ice.

Like Europa, which orbits
around the giant Jupiter.

Places where jets erupt
hundreds of miles into space,

like Enceladus, the tiny Saturn's moon.

Or moons with a very Earthlike landscape,

with mountains, valleys, clouds
and lakes of liquid methane or ethane,

like Titan, the largest moon of Saturn.

If life was ever possible or is now a fact

in any of those remote worlds
in our solar system,

with very harsh
and different conditions from Earth,

that could imply
that life could be possible

in any other remote world in outer space.

Second genesis
within the same solar system

implies that the origin of life
is a likely event.

If it happens twice
in the same solar system,

it's likely happening
everywhere in the universe.

[narrator] Now scientists
are searching for planets

far beyond the boundaries
of our solar system,

where we might detect life
in the near future.

Thanks to the NASA space telescope Kepler,
launched in 2009,

we know that in our galaxy alone,

there are billions of Earthlike exoplanets

orbiting their stars.

Given the vastness of the universe,

with more than a hundred billion galaxies,

it is hard to conceive that somewhere

there is no Earthlike planet
that can harbor life.

Unless something very unusual
happened here on Earth,

then life has developed
on thousands of millions of planets

just in our galaxy, right?

If it's not a miracle,
then it's all over the place.

That's the bottom line.

In recent years, thanks to Kepler,

we have confirmed
dozens of Earthlike exoplanets

that might harbor life

and even intelligent life
waiting to be discovered.

We don't know
if the discovery of life will happen first

on one of these moons
or planets in our solar system

or on an Earth-like exoplanet.

But what we do know is that
we are closer than ever to unveiling

one of the greatest mysteries of nature:

whether there is life in outer space.

Is there life beyond Earth?

Humankind has always
asked itself this question,

but no answer has yet been found.

It appears that we are closer
to solving this mystery.

Recent discoveries have uncovered
planets beyond our solar system

that are believed to be similar
in many ways to Earth.

[man] If I got to ride in a spaceship
to one planet that we found with Kepler,

the one that I would go to
is Kepler-186f.

It's one of the smallest ones.

Uh, it's at the right temperature,

um, that liquid water
could exist on its surface.

[narrator] Kepler-186f
is the first validated Earth-size planet

to orbit a distant star
in the habitable zone

where liquid water might pool
on the planet's surface.

The discovery of Kepler-186f
confirms that Earth-sized planets exist

in the habitable zones of other stars

and signals a significant step closer

to finding a world similar to Earth.

Kepler-186f orbits its star
once every 130 days

and receives one-third the energy
that the Earth does from the sun,

placing it near the outer edge
of the habitable zone.

If you could stand on the surface
of Kepler-186f,

the brightness of its star at nigh noon

would appear as bright as our sun is

about an hour before sunset on Earth.

[woman] Today, when we take a look
at what we know

about the origins of life on this planet,

it leads us to think

that the same things that happened here

might well have happened elsewhere,

and so life beyond this planet

is quite plausible in terms
of the science that we know today.

[narrator] We now know there are four
important candidates in our solar system

to harbor life.

Mars.

Jupiter's moon Europa.

And Saturn's moons Titan and Enceladus.

On every of them, we can find
all of the three key ingredients for life:

organic compounds, a liquid,
and an energy source.

[wind whistling]

There is water on Mars

in the form of ice at the poles
and under the surface,

but it is also flowing

from time to time at the surface
during spring and summer.

[narrator] Although there's no evidence
of any form of life on Mars yet,

scientists think it might be found soon.

On Jupiter's moon Europa,

two of those key ingredients
can also be found.

We do know there is water on Europa.

On the surface of Europa,
we have an icy crust.

This icy crust has been observed
using Voyager spacecraft

but also the Canadian spacecraft
in the '90s.

[narrator] Under an eerie water-ice crust
10- to 30-kilometers thick

that covers this tiny moon,

there's supposed to be a liquid ocean,

which may be about 100 kilometers deep.

The Saturn's moons Titan and Enceladus

have become
the search-for-life priority top spots

in the recent years.

Titan is the only moon in the solar system
that is known to have an atmosphere.

It's also the only place
in the solar system

that has an atmosphere
made primarily of nitrogen,

except for the Earth,

so Titan and Earth
are closely linked in that way.

Titan is a fascinating world.

It's the most alien place
in the solar system, so to speak,

because it, in fact, other than the Earth,

is the only place we know of

that has seas of liquid on its surface,

but those seas are not made of water,

they're made of liquid methane
and liquid ethane.

[narrator] Titan is the only place
in the solar system outside Earth

where there are stable bodies
of surface liquid,

but at minus 180° Celsius,

this liquid can't be water.

We know there are lakes

filled with super-chilled
liquid methane and ethane.

In recent years, Enceladus,
a tiny moon orbiting Saturn,

has become one of the main goals
for exobiology.

Enceladus is a small, icy moon,
quite similar to Europa

as it has a thick icy crust
and an ocean beneath.

We know there are vast jets of water ice

erupting several miles into space.

Enceladus is one of the most
interesting places in the solar system

because of the presence of this activity,

this geyser-like activity.

If we have geyser-like activity,
people expect to have water, liquid water.

[narrator] Recently, in those jets,

some of the basic chemical building blocks
of life have been detected,

so we can be sure that we have
on Enceladus

the trifecta to harbor life:

liquid, organic compounds
and an energy source.

But if life was able to emerge
in any of those remote and harsh worlds,

why couldn't it also arise
on any other planet

far beyond the boundaries
of our solar system?

Second genesis
within the same solar system

implies that the origin of life
is a likely event.

If it happens twice
in the same solar system,

it's likely happening
everywhere in the universe.

[man] If we were able to find life
within our own solar system

on another place,

and furthermore be able to say
that it developed,

you know, independently,

then you're saying,
within the same stellar system,

you had life evolve twice.

And the conclusion from that
is that life forms very easily.

[narrator] A generation ago,

just the idea of a planet
orbiting a distant star

was still in the realm of science fiction.

So, to think of the possibility
of life on a planet like that

was simply unimaginable.

In fact, the first exoplanets
weren't discovered till 1992.

That very year,
two super-Earth exoplanets were found

around pulsar PSR 1257+12

at a remote distance
of 2,300 light-years away.

This announcement shocked
the scientific community at that time,

as it was the first multi-planet,
extra-solar system ever discovered.

Could any of these
super-Earth harbor life?

Unfortunately, a pulsar
is a very different kind of star

from the sun.

In fact, it's a dead star

formed when some
of the largest stars in the universe

exploded as super novae.

The pulsar,
which is what's left after a star,

a really massive star, explodes,

then you've got this thing which is
one step away from being a black hole.

While this was really exciting,

it was hard to tell what it meant

because pulsars are
so much different from normal stars.

[narrator] These may not seem
at first to be good places

to look for habitable planets.

Super novae are, frankly,
quite apocalyptic events

that would easily vaporize
any ill-fated planets

in orbit around the exploding star.

That distant world
would be bathed in a lethal cocktail

of X-rays and charged particles

emitted by a star
so faint in visible light

that it would scarcely cast a shadow
on this world's surface.

So the chances of life arising

in such a weird and hostile environment
would be remote.

However, the real importance
of this discovery

was that for the first time ever,

the existence of planetary systems
beyond the limits of our solar system

was confirmed.

If we found two exoplanets out there,
why couldn't there be many more?

We had to wait three more years
to find an exoplanet

orbiting a sunlike star,

which was far more important,
because the conditions of such a planet

would be potentially similar
to any of the planets in the solar system.

On October 6th, 1995,
was the announcement of the discovery

of the first planet orbiting
a sunlike star in the journal Nature.

That star was 51 Pegasi, a sunlike star
located 51 light-years away,

and the exoplanet was a giant planet.

The first exoplanet found
around a star like our own

was called 51 Pegasi b.

It's very unusual.
It's a very large planet.

It's bigger than Jupiter,
it's more massive than Jupiter.

And it's on a very short period orbit.
It goes around its star.

One year on this planet takes four days.

It is a very short amount of time.

[narrator] That discovery
marked a turning point

in the search for exoplanets.

From that moment on,
many new ones were found.

Nevertheless, what radically
revolutionized the search for exoplanets

was the NASA space telescope Kepler.

Before Kepler was launched,

there was hundreds of planets
that we knew of

in systems around other stars,

and now we know of thousands.

And that's why Kepler
was so revolutionary.

[narrator]
The Kepler was a space telescope

specifically designed to survey
our region of the Milky Way galaxy

to discover hundreds
of Earth-sized and smaller planets

in or near the habitable zone
of their respective stars

and determine the fraction

of the hundreds of billions
of stars in our galaxy

that might have such planets.

It works very simply.
Anybody can understand this.

It's just staring at one spot on the sky,
all the time, never blinks.

And it's looking at 150,000 stars,
and it just monitors how bright they are.

Kind of like a camera light meter, really.

And occasionally they'll see a…
this star over here, for example,

it'll get a little bit dimmer,

a very fraction of a percent dimmer,
for a few hours,

and then it will get bright again.

Well, that happens if a planet
passes in front of that star.

We've gone from finding a hundred planets
to over a thousand planets with Kepler,

those that have been confirmed.

And there's about 3,000 or 4,000 more
which we have strong evidence for

but we wouldn't consider
confirmed planets just yet.

[Seth] Kepler is on the hunt for planets.

Kepler has found literally thousands
of planets or planetary candidates.

It's… it's a planet-finding machine.

[narrator]
In 2011, for the first time ever,

Kepler provided scientists
with a census of the Milky Way,

so we could calculate
how many stars in the Milky Way

could have a planet like ours.

Around a billion.

Maybe there are a million,
maybe there are a billion,

maybe there are a hundred billion planets
in the Milky Way galaxy

that could support life,

the kind of planets
that Earthly life could survive on.

How many of them
have cooked up their own life?

And we don't know
the answer to that, okay?

Because that depends
on how hard it is to get life started.

Just 'cause I give you
all these, you know, these worlds

doesn't mean that life will get started,
but, on the other hand,

those planets are all made
out of the same stuff that Earth is.

So, again, unless something very unusual
happened here and nowhere else,

there's gonna be
biology all over the place.

[narrator] Just four years
after its launch,

on April 2013,

the Kepler team reported
one of their first great triumphs.

The discovery for the first time ever
of two exoplanets

very similar to the Earth.

Kepler-62e and Kepler-62f.

[wind whistling]

This discovery created great enthusiasm,

as it implied the confirmation
of Earthlike planets,

where life might be possible.

Each of these planets have
a radius 1.6 and 1.4 times of Earth

and orbits Kepler-62,
an orange dwarf star,

in its circumstellar habitable zone.

A modeling study also concluded
that Kepler-62e and Kepler-62f

are likely covered mostly,
perhaps completely, in water.

Kepler-62e probably has a very cloudy sky

and is warm and humid
all the way to the polar regions.

Kepler-62f would be cooler,

but still potentially life friendly.

Unfortunately, they are at a huge distance
of 1,200 light-years away

in the constellation of Lyra.

Thanks to the Kepler mission,

we now know that there are
tens of billions of planets orbiting stars

just in our galaxy, the Milky Way,

and we know there are billions
of galaxies across the universe.

So if, in just one
planetary system like ours,

life arose on one planet,

and there are
at least four more candidates,

the likelihood of finding a planet
in outer space that could harbor life

should be very high.

But Kepler has discovered for us,

not only the existence
of Earthlike planets,

but also has provided us
with amazing data about the universe,

such as the confirmation
of the existence of planets

that orbit around
not only one but two stars,

like Kepler-16b.

This planet was Kepler's
first discovery of a planet

that orbits two stars,
what is known as a circumbinary planet.

So, one of the most
exciting discoveries from Kepler

was that we were actually able
to find planets around binary stars,

and the first one
that was found was Kepler-16b.

[Fergal] Kepler-16b
is many people's favorite planet

that was discovered with Kepler.

Uh, it orbits around not one
but two stars at the same time.

This was something
which was predicted not to exist.

Two stars setting at the same time
was just a piece of science fiction.

But the universe is stranger
than what scientists can imagine,

and it turns out that
this sort of thing is true.

[narrator] Since 1992,

over 2,000 exoplanets
have been discovered.

Thanks to future space telescope
missions planned for launch,

the number of observed exoplanets
is expected to increase greatly

in the coming years.

Despite having discovered
just a tiny fraction

of all of those billions
of exoplanets that we think that exist,

how could we know
how many of them could harbor life?

In astronomy and astrobiology,

the region around a star where a planet
with sufficient atmospheric pressure

can maintain liquid water on its surface

is known as
the circumstellar habitable zone.

[Fergal] The habitable zone is a place,

it's kind of a way of thinking
about the right way to go look

for planets like our own.

[narrator] The Earth is obviously
in the circumstellar habitable zone

of our solar system.

[wind whistling]

A potentially habitable planet
implies a terrestrial planet

with conditions
roughly comparable to those of Earth,

and thus potentially favorable to life.

There's a sweet spot,

an area where it's not too hot
and not too cold,

and we call that the habitable zone,

or some people call that
the Goldilocks zone.

If you've got a planet in that region,
if it's small, if it's rocky enough,

and it had water, that water would be
in a liquid state, more than likely,

so that would be
a good place to go looking.

[narrator] On November 2013,
astronomers reported,

based on Kepler space mission data,

that there could be as many
as 40 billion Earth-sized planets

orbiting in the habitable zones
of sunlike stars and red dwarfs,

just in our galaxy, the Milky Way,

11 billion of which
may be orbiting sunlike stars.

Those 11 billion exoplanets
orbiting stars like our sun

really are a huge number
of potential Earthlike worlds.

However, now we know that stars
very different from our sun

may be a good place to look for life.

In May 2016,
a team of astronomers announced,

for the first time ever,

the finding of three habitable planets
orbiting a star

completely different from ours,
an ultracool dwarf star.

It's the first planetary system
found around a star like this.

The star, named Trappist-1,

is just 40 light-years away

and is much cooler and redder
than the sun

and barely larger than Jupiter.

In fact, stars like this
are very common in the Milky Way,

and they are very long-lived.

The three planets are very similar
in size to the Earth

and might have habitable regions
on their surfaces.

So the answer to what
the best place in the universe is

to find life nowadays

has radically changed.

To date, among all of the more
than 1,000 confirmed exoplanets,

there are around 50

that are in the circumstellar
habitable zone

of the star they orbit around.

And therefore they could be
potentially considered Earthlike planets,

which does not imply
these distant worlds may harbor life.

However, could life be possible
outside those habitable zones?

[wind whistling]

The discovery of hydrocarbon lakes
on Saturn's moon Titan

has begun to call into question
the carbon chauvinism

that underpins
circumstellar habitable zone theory.

Liquid water environments
have been found to exist

in the absence of atmospheric pressure

and at temperatures outside

the circumstellar habitable zone
temperature range.

Just because a planet
is outside the habitable zone

doesn't mean that it couldn't have life.

[narrator] For example,
Saturn's moon Enceladus

and Jupiter's Europa,

both outside the habitable zone
of our solar system,

may hold large volumes of liquid water

in subsurface oceans.

If we are considering
the possibility that life could arise

in such harsh environments,

which are not
in the circumstellar habitable zone,

that implies that
we should look for exoplanets

far beyond those zones.

So the chances of finding an exoplanet

with conditions to support life
are much higher.

We know that it's not an easy task

to study and analyze the planets
and moons of our own solar system,

as they are millions
of kilometers away from Earth.

However, exoplanets are
not just millions of kilometers away

but many light-years away from Earth.

How can astronomers manage
to study exoplanets

if they are trillions of kilometers away?

There are several methods used
by astronomers and astrobiologists

to discover and to study

these extremely distant,
mysterious worlds.

Before the launching
of the Kepler mission in 2009,

the most successful technique
for detecting exoplanets

was the Doppler spectroscopy,

also known as the radial velocity method.

The radial velocity method
relies on the fact

that a star does not remain
completely stationary

when it is orbited by a planet.

The planet is much smaller than its star,

but it still exerts
a tiny gravitational pull

or tug on the star as it orbits.

When a planet is behind the star,
from our point of view,

it pulls the star slightly away from us.

When it's in front,
it pulls the star slightly toward us.

This causes the star
to wobble back and forth.

Astronomers look
for this wobbling to find planets.

They use something called
a spectrograph and powerful telescopes

to examine the light coming from a star.

A spectrograph, like a prism,
splits the light from the star

into its component colors,
producing a spectrum.

Some of the starlight gets absorbed

as it passes through
the star's atmosphere,

and this produces small dark gaps,
or lines, in the spectrum.

As the star moves closer to us,

these lines shift toward
the blue end of the spectrum.

As the star moves away,
the lines shift back

toward the red end of the spectrum.

So the spectrum appears
first slightly blue shifted

and then slightly red shifted.

Therefore, astronomers
can look for orbiting planets

by looking for these back and forth
motions of the lines

in a star's spectrum.

From the speed
it's being pulled toward you

and how long it takes
to go around in that circle,

you can work out the mass of the planet,

or the companion thing
which is pulling it around.

If that mass is very small,
the thing is a planet.

That method has been very successful.

It found most of the planets
which were discovered early on.

[narrator] However, the Kepler technique
was based on the planet's transit.

The transit method is based
on the observation

of a star's small drop
in brightness that occurs

when the orbit
of one of the star's planets passes,

"transits," in front of the star.

The amount of light lost depends
on the sizes of the star and the planet,

and the duration of the transit depends
on the planet's distance from the star

and the star's mass.

With the Kepler space telescope,
when we look at a star,

we don't see the planet directly.

All we see is a tiny dip
in the brightness of the star

when the planet passes in front of it.

From the size of the dip, we work out
the relative size of the planet.

Is it a big planet or a small planet?

[narrator] The combination
of transit photometry

and Doppler velocimetry

reveals planetary radius,
mass and density,

which are some of the main parameters

to evaluate
the potentiality of the exoplanet

as an Earthlike candidate,
and so to harbor life.

Once you know the distance from the star,

you know how hot the star is,

you can estimate what the temperature
on the surface of the planet

would be like.

[narrator] Another of the main parameters

to analyze the Earthlike potential
of an exoplanet

is its atmospheric composition.

Atmospheric studies of exoplanets
might be performed

with spectroscopy
during planetary transit.

During the transit, the stellar light

passes through the atmospheric limb
of the planet.

Spectral analysis of this filtered light

reveals the structure
and composition of the atmosphere.

Astronomers could identify
the most Earthlike exoplanets

by detecting the biomarkers,
which are the imprints

that life forms have
on their host planet atmosphere.

For instance, the molecular oxygen
that we are breathing

results from the presence
of life on Earth.

Unfortunately,
with present-day technology,

it's extremely hard to closely study

the atmospheric composition
of those remote planets.

The main thing that's keeping us from
being able to do that now is technology.

We don't have the technology,
you know, in functioning instruments

at the moment to be able
to do that effectively.

But that's something that will change
with other missions in the pipeline,

like Jack Webb Space Telescope
and other missions that are happening.

[narrator] Kepler continuously monitors
over 100,000 stars similar to our sun

for brightness changes
produced by planetary transits.

Thanks to this ingenious technique,

Kepler has confirmed to date
more than 1,000 exoplanets.

Scientists think that
about a few dozen of them

can be labeled as Earthlike.

Considering the possibilities
of finding an Earthlike exoplanet

are much higher
in the circumstellar habitable zone

of its planetary system,

the planet hunting then started
to focus on those areas.

One of the first discoveries
was 70 Virginis b,

an exoplanet located
approximately 60 light-years away

in the constellation of Virgo.

70 Virginis b was located
exactly in the middle

of the circumstellar habitable zone
of its planetary system,

so it was supposed not to be
too hot or too cold.

Unfortunately, further studies
reported that this remote world

was a gas giant
with very high temperatures,

which ruled out any potential
for liquid water,

and therefore of life.

The early findings were discouraging
in terms of detecting an Earth analog.

But this was just the beginning.

In 1998, a discovery made
in the star Gliese 876,

a red dwarf located
in the constellation of Aquarius

at a distance of 15 light-years
away from Earth,

really encouraged astronomers.

A gas giant was detected
in its habitable zone.

Gliese 876 b.

Three years later, another gas giant
closer to this one was found.

Gliese 876 c.

We know that life as we know it

is not possible on gas giant planets
such as Jupiter or Saturn.

But the big surprise
was that both exoplanets

may have habitable moons
orbiting around them,

as Jupiter and Saturn have.

[Fergal] This was one of the first planets
to be discovered in the habitable zone,

and people theorized that,
if it had a moon around it,

the moon would be rocky

and the moon would be at the right
temperature to have liquid water.

[narrator] Why couldn't
any of these hypothetical moons

around Gliese 876 b and c harbor life,

as we hope Jupiter's moon Europa

or Saturn's moons
Titan and Enceladus might?

So it's an exciting place to think about
and maybe to look for in the future.

[narrator] After the discovery
of these exoplanets

with potential Earthlike moons,

several similar exoplanets with moons
orbiting around them were discovered.

Maybe on any of those remote moons,
life arose in the past,

or exists in the present,
or might appear in the future.

After all these early discoveries,
we started to approach to the main goal,

to find the most Earthlike world.

An Earth analog,
also referred to as a twin Earth,

or Earthlike planet,

is a planet or moon
with environmental conditions

similar to those found
on the planet Earth.

If life could arise on Earth
millions of years ago

and if we look for exoplanets
similar to our planet,

the chances of finding
habitable planets skyrocket.

Recent discoveries have uncovered planets

that are believed to be similar
in many ways to Earth,

with relatively high
Earth similarity indexes.

The size is often thought
to be a significant factor,

as planets of Earth size are thought
more likely to be terrestrial in nature

and be capable of retaining
an Earthlike atmosphere.

From the point of view of Kepler,
a planet is Earthlike

if it's small enough
that it's probably rocky,

it's not a gas giant
like Jupiter or Saturn.

[narrator] But size alone
is a poor measure,

particularly in terms of habitability,

because next to us, there is a planet
with a very similar size and mass,

Venus, where it is almost
impossible for life to arise.

There are other criteria to be considered,

like the surface gravity
or the star size and type.

A planet is Earthlike if it's
the right distance away from a star

that it's in the habitable zone,
that it's not too close that it's too hot

and all the water
it would have has boiled away,

and not so cold that if there was
any water, it would all freeze to ice.

[narrator] If we are able to examine
all of these parameters of an exoplanet,

we would be able to know
if it is or not a real twin Earth.

So, when we say Earthlike for Kepler,
we usually just mean

that it's small enough
that we think that it's solid enough

that you could stand on it,
but that doesn't mean

that it has an atmosphere
or that it has an ocean.

[narrator] It's also often cited that
an Earth analog must be terrestrial,

that is, it should possess
a planetary surface

composed of materials similar to Earth's.

The conclusion would be
that extrasolar planets or moons

in the center
of its circumstellar habitable zone,

the so-called Goldilocks position,

with substantial atmospheres,

may possess oceans
and water clouds like those on Earth.

In addition to surface water,
a true Earth analog

would require a mix of oceans or lakes

and areas not covered by water.

[Fergal]
We believe that water is essential,

but just about everything else you can
think about which is important for life,

uh, there seems to be life
on the Earth that doesn't need it.

There is life that survives
without sunlight.

There's life that
survives without oxygen.

There's life that survives deep down
underneath the ice in Antarctica.

[narrator] Unfortunately,
with the present technology,

we can't properly evaluate
most of the parameters,

like the temperature,
the atmospheric composition,

or the surface of the exoplanets.

Nevertheless, considering we have already
discovered hundreds of exoplanets,

we can't help but wonder

if a real Earth analog
has already been discovered.

On 18th April 2013,

astronomers from the Kepler team
announced a discovery

that created great expectation.

For the first time ever,
two very Earthlike exoplanets were found.

They were the Kepler-62e

and the Kepler-62f.

And orbits Kepler-62,
an orange dwarf star,

in its circumstellar habitable zone.

They immediately became
prime candidates to host alien life.

A modeling study also concluded
that Kepler-62e and Kepler-62f

are likely covered mostly,
perhaps completely, in water.

Kepler-62e probably has a very cloudy sky
and is warm and humid

all the way to the polar regions.

Kepler-62f would be cooler,
but still potentially life-friendly.

Unfortunately, they are at a huge distance
of 1,200 light-years away

in the constellation of Lyra.

Soon after, it was discovered,
an exoplanet even more similar to Earth,

Kepler-186f.

This finding was a milestone
as it was the first rocky planet

found in the habitable zone of its system.

It is 492 light-years away from the Earth.

[Fergal] Kepler-186f
is possibly my favorite planet

to come out of the Kepler mission.

It's a small planet.

It's maybe 10% to 20%
bigger than the Earth.

Based on everything we know,
it's almost certain to be rocky,

and it's the right distance
away from its parent star

that, if the atmosphere is right,
if the greenhouse effect is right,

it could have liquid water on the surface.

[narrator] After that discovery,

several more Earthlike candidates
started to arise.

Like Kepler-438b,

Kepler-442b,

or Kepler-440b.

All of them were
very similar to our planet,

but none of them was a real twin Earth.

But everything changed on July 23rd, 2015.

That day, NASA's Kepler
space telescope science team

shocked the scientific community
with an amazing finding.

The most Earthlike planet ever
was discovered.

Its name, Kepler-452b.

What made different this one

to the other previous
Earth analog candidates?

Kepler-452b is the very first
apparently rocky planet

that orbits a G-type star like our sun.

It's a planet
in a habitable zone around a star

which is almost a clone of our own sun.

[narrator] After this discovery,
the Earth is a little less lonely

in the universe.

Kepler-452b circles its star,

which is about as hot as our sun,

10% brighter, and 20% larger,

at an orbital radius
just 5% larger than that of the Earth.

A year on this planet
is 385 Earth days long,

just 20 days longer than Earth's.

What makes this slightly less exciting

from the point of view
of could it be habitable is its size.

Our best guess at the size
is that it's about 60% bigger

than our own Earth.

[narrator] It is the smallest
Earth analog planet ever found

in the habitable zone
of a G-type star like our sun.

Previous research on
super-Earth size planets like 452b

suggests this one
has a good chance of being rocky.

If it is a rocky world,

it would weigh in
at about five Earth masses,

giving it a surface gravity
of roughly two grams,

which would mean that our weight
would be double on its surface.

Kepler-452b could have
a thick, cloudy atmosphere

and volcanic activity.

Even more exciting
than Kepler-452b's Earthlike demeanor

is the fact that this world
has spent around six billion years

in the habitable zone of its star.

That's considerable time
for life to arise

somewhere on its surface or in its oceans

should the conditions for life exist.

Kepler-452b is about
1.5 billion years older than the Earth.

If it was Earth-sized,
the planet and its aging, brightening star

might be at a point in their evolution

where liquid water would be rapidly
evaporating from the surface.

[thunder rumbling]

But because of its higher mass,
astronomers believe Kepler-452b

could continue to hold liquid water
for the next 500 million years or so.

So far, it's the only known world
in the system

which lies some 1,400 light-years away
in the Cygnus constellation.

Obviously, we're not going
to get there anytime soon,

but it's fascinating to imagine

that far off
in the distant reaches of space,

a world very much like our own
might already exist.

If this twin Earth exists,
why couldn't thousands more like it exist?

Less than a year after
the amazing finding of Kepler-452b,

on May 2016,

a new discovery shocked
the scientific community.

Astronomers using telescopes

at European Southern Observatory in Chile

discovered three planets
around a dim dwarf star

just 40 light-years from Earth
in the constellation of Aquarius.

These worlds may be the best targets
so far found in the hunt for life

elsewhere in the universe.

They used the Trappist telescope
to monitor the brightness

of an ultracool dwarf star
in the constellation of Aquarius,

which has been named Trappist-1.

Trappist-1 is much cooler
and redder than the sun

and barely larger than Jupiter.

Stars like this are very common
in the Milky Way

and they are very long lived.

This was the first time that planets
have been found around one of them.

The three planets are very similar
in size to the Earth

and might have habitable regions
on their surfaces.

But the really exciting result is that
these are the first Earth-like planets

that are well suited
for the detection of life.

The ultracool dwarf stars
are the only places

where life could be detected
on an Earth-sized exoplanet

using our current technology.

The light from a much brighter star,
like the sun for example,

would swamp vital measurements
of the atmospheres

of any candidate planets.

The next step is to make
more detailed observations

using the next generation of telescopes,

such as ESO's
European Extremely Large Telescope

and the James Webb Space Telescope.

That will allow astronomers to study
the atmospheres of planets like this

and to search for molecules
related to biological activity,

like ozone, methane or water.

Although there is not yet
any proof of the existence of life

on all of these exoplanets
that we've already found,

even in the most Earthlike of them,
like Kepler-452b,

we can't help but wonder
if any of those potential forms of life

that might have arisen there
were, or will be, able to evolve

into intelligent life.

If any of those extremely remote worlds
was formed billions of years ago,

as Earth did, and it became
into a habitable planet,

then the organic compound had time enough

to mix up and organize into living forms.

Perhaps any of those living forms
might have evolved

into complex forms of life,

and some of those complex forms of life

might evolve into intelligent beings
with consciousness.

Thanks to Kepler mission research,

it's known that just in our galaxy,
the Milky Way,

there might be millions of Earth analogs,

and there are billions
of galaxies across the universe.

So the chances are much higher
than we could have ever imagined.

For that reason, science is carrying out
an intense search for intelligent life.

The SETI Institute
in California is nowadays

the main world institution
devoted to the search

for extraterrestrial intelligence.

Its name, "SETI,"

stands for "search
for extraterrestrial intelligence."

This search is based
on the use of radio telescopes.

Radio telescopes receive radio waves.

As we can't go to space aboard
spacecraft to find that intelligent life,

what we look for are radio signals.

What we're looking for is a signal
that's at one spot on the radio dial.

Just like when you're listening
to the radio in your car,

you know, you tune across the dial,
you hear static everywhere,

and then at one spot you hear…
[imitates signal]

…and there's a station.

Okay, that's the signal that's produced
by a transmitter somewhere.

It's not natural static.

It's not like a quasar or a pulsar
or galaxies or hot gas/cold gas.

All those things in space
make radio noise,

but it's all over the dial.

So we look for signals
that are at one spot on the dial,

and, of course, the source of the signal
has to be up in the sky.

Those are the kinds of criteria
we use to know that,

even if we don't know what it means,
we at least know they're there,

they're on the air.

[narrator] Unfortunately, till now,
we haven't received a signal

that can be really attributed
to extraterrestrial intelligence.

[dramatic music playing]

In the coming years, NASA and ESA,
the European Space Agency,

have planned to launch
several space telescopes

that will surely help to unveil
the mystery of life

beyond the boundaries
of our solar system.

The most ambitious one
is the James Webb Space Telescope.

This project
is an international collaboration

between NASA, ESA
and the Canadian Space Agency, CSA.

It will be the premier observatory
of the next decade,

serving thousands
of astronomers worldwide.

It will study every phase
in the history of our universe,

ranging from the first luminous glows
after the big bang

to the formation of solar systems
capable of supporting life

on planets like Earth,

to the evolution of our own solar system.

This new telescope,
three times more powerful than Hubble,

will be able to analyze starlight
passing through the atmospheres

of the closest Earthlike worlds,

looking for the telltale signs
of life itself,

like, for instance,
detecting gasses in its atmosphere

usually linked to life processes,
such as oxygen, methane, carbon dioxide,

or nitrogen.

JWST is really going to help us understand

what atmospheres of planets are like
under different conditions,

and that's going to be
a really exciting result.

[narrator] There is another mission.
The CHEOPS mission.

"CHEOPS" comes from
"characterizing exoplanet satellite."

This is an ESA mission, dedicated
to searching for exoplanetary transits

by performing
ultra-high-precision photometry

on bright stars
already known to host planets.

With all of these new space telescopes

programmed to be launched
in the coming years,

we are sure
that finding a habitable exoplanet

and any consistent biosignature
will be just a matter of time.

[peaceful music playing]

If we are ever able
to find evidence of the existence

of any form of life among
one of those billions of exoplanets

we know are across
the observable universe,

it would be undoubtedly shocking.

If we did find life somewhere else
in the solar system

and we were able to do, for example,
a genetic analysis to determine

that it was distinct from life on Earth,

that would be
a really earth-shaking discovery.

[narrator] Living generation
might be witness

of a finding that would
undoubtedly be a turning point

in the history of humankind,

the discovery of life in outer space.

[contemplative symphonic music playing]