Original subtitles

WILLIAM SHATNER: A blue oasis

formed by cosmic collisions.

Vast secrets hidden beneath hundreds of miles of rock.

And mysterious forces

with the power to either protect

or destroy life as we know it.

We walk on its surface,

breathe its air, drink its water.

But the blue marble we call Earth

remains in large part a mystery

to the eight billion people who live on it.

The story of our planet is extraordinary,

and there are many questions we still haven't answered.

How was the Earth made?

What are the secrets of its survival?

And how long

will humans be able to call it home?

Well, that's what we'll try and find out.

♪ ♪

Earth.

It is the third planet from the Sun

in a gravitationally bound system of eight planets

we call the solar system.

It has a surface area of nearly 200 million square miles,

which is covered in continental landmasses

and liquid surface water.

And it is the mysterious home

to the only known life in the universe.

(birds singing)

MICHIO KAKU: When you look at the Earth today,

it looks so calm, serene, beautiful.

These are ideal conditions for us.

And it seems like everything's normal.

But we're beginning to realize, much to our shock,

that the Earth is an oddball.

ROD PYLE: Space is a really big place,

and it's pretty clear that planets are everywhere.

We've discovered something like 5,340 planets in the universe,

and not one of them appears to be similar to Earth

to any meaningful extent.

We're not finding an atmosphere like ours.

We're not finding a planet

that can support liquid water on the surface,

oxygen, the things we think that life might need.

We've been very lucky to be where we are.

ANNA GÜLCHER: The Earth is definitely a puzzle that needs to be solved.

In that sense, you could think of-of it

as trying to get little pieces of evidence

to try to understand how the Earth has evolved over time,

especially of the early Earth.

SHATNER: While mankind has survived and even thrived

by learning to harness Earth's resources

and adapt to its ever-changing conditions,

there is surprisingly very little

we know about how and why Earth is the way it is.

Based on uranium-lead dating methods

used on a meteorite in the 1950s,

scientists have maintained

that the Earth is 4.5 billion years old.

But just how Earth was created remains a mystery.

GRAHAM LAU: The origins of Earth started 4.5 billion years ago.

So we really have to be detectives

and sleuth our way through

to look back into time to understand

the early formation of Earth.

There have been different ideas over time.

And the truth is, we really don't know for sure yet

where Earth came from.

REBECCA BOYLE: There's a lot of mysteries

that make this planet really special.

And going back to the very beginning,

we don't really know how Earth got here in its present form.

It's the only planet that can host life.

But it shows that we have a lot to learn

about the fundamental processes that built this planet.

SHATNER: How can we unravel the early history of Earth?

Curiously, a simple experiment gave scientists

an unexpected clue that might help solve this mystery.

The International Space Station, 250 miles above Earth.

While conducting a simple science experiment

for television viewers,

NASA astronaut Don Pettit makes a major

and unexpected discovery.

PYLE: In 2003,

NASA started a Saturday morning program

by which the astronauts would do citizen science for kids.

Don Pettit wanted to see what would happen

with small grains of salt if they were shaken up in zero-g.

So he thought this would be an interesting experiment.

LAU: By taking a little plastic bag of salt crystals

and shaking them up,

salt crystals started clumping together

and making larger gloms inside of this bag, basically,

showing off some of the things that happen in microgravity,

this experience of weightlessness

that the astronauts feel.

SHATNER: At first, when the grains of salt floated together

in zero gravity,

Don Pettit thought it was merely a strange space anomaly.

But when he repeated the experiment with other materials,

including sugar...

...he realized that his Saturday morning science presentation

had revealed the process

of how Earth's very first particles began to bind together

billions of years ago.

LAU: For a long time,

we weren't really sure

how all of the dust and the gas

around our Sun

coalesced together and formed our Earth.

And so, these experiments where particles

start to aggregate together

and-and grow larger clumps actually helps us to solve

a really old problem for understanding

the early evolution of our planet.

SHATNER: If, 4.5 billion years ago,

particles of space dust came together

and started to build the foundation of our planet,

then what happened next?

How did Earth become the enormous celestial body

that it is today?

On the surface, this remote countryside

may appear to be a simple valley dotted by rolling hills.

But, curiously, in July of 2005,

the area was designated as a UNESCO World Heritage Site.

So what's special about this rural landscape?

As it turns out,

Vredefort is home to an important clue

about Earth's origins that can only be seen

from space.

LAU: When we look at Vredefort from space,

what we see is an asteroid impact crater

made by an extremely large impactor.

That impactor might have been 20 to 25 kilometers in diameter.

So this giant structure

striking the Earth...

...with so much force that, if it happened today,

it could have obliterated all of the life on Earth.

But the Vredefort impact

was slightly over two billion years ago...

...a much different time in Earth's history.

The very early Earth would have been a molten ball of rock.

It would have looked hot from space.

You would have seen all this energy coming off.

And so, our world was a much different place back then.

SHATNER: Scientists believe that the Vredefort Crater

is just one of many asteroid impacts

that occurred in Earth's early history.

And while not every one of these giant space rocks

made a mark that is still visible...

...they all contributed

to Earth's growing into the massive planet it is today.

KAKU: If you take a look at all the craters on the planet Earth,

they are reminders of the fact that we live

in the middle of a cosmic shooting gallery.

Given the fact that the Earth is 8,000 miles across,

it means that there must have been hundreds

of other cataclysmic impacts

that eventually gave rise to the planet Earth.

And it was created by the slow accumulation

of tremendous amounts of debris, rocks and asteroids

from all these collisions.

GÜLCHER: There are hints everywhere

across the surface of the Earth

that give us clues about how it has evolved over time.

We generally agree that,

ultimately, all these collisions

of bigger and bigger material

is really key for how the Earth has been formed

to what we already know today.

SHATNER: When you consider that our world was formed

by random collisions in the void of space,

it's clear that Earth's formation is a cosmic puzzle

that we're just beginning to piece together.

But the origin of Earth is only the first chapter

in the ongoing story of our planet.

In fact, the greatest enigma may lie

in how this rocky globe became...

an ocean world.

SHATNER: NASA's Apollo 8 spacecraft is halfway

through completing the first manned voyage around the Moon.

As the craft circles back from the far side of lunar orbit,

the crew glimpses a breathtaking view of Earth.

Astronaut Bill Anders documents the experience

with a stunning color photograph

that would come to be known

as Earthrise.

PYLE: The Apollo 8 Earthrise picture

was stunning.

It was amazing. It was transformative.

When you look at the picture,

in the lower part of the Earthrise photo,

you've got this dead limb of the Moon.

Dry, lifeless, gray.

Clearly not a place that anybody could live,

certainly not that life could arise.

And then in the distance,

here is the Earth rising, just this astonishing planet,

with liquid water on the surface.

Within days, this picture was on the cover of every newspaper

and every major magazine in the free world.

It's really hard to overstate the importance of that picture.

And it really moved people.

SHATNER: The iconic Earthrise photo

showed us that our world is truly a giant blue marble

floating in space.

70% of Earth's surface is covered in water,

which is remarkable when compared to most planets.

CLARK: One of the really clear things

that we see that sets Earth apart from everybody else

is that we're blue.

When we look at the planets around us,

we're not these browns of clays on the surfaces.

We're not the grays of icy worlds.

We have this very unique color.

So, water is really important

for the Earth being the planet that we see today.

LAU: The Earth is the only world we know of

to have such a deep and voluminous ocean

of liquid water, and honestly, there's still a lot of debate

as to how Earth got its water.

We don't really know how we became an ocean world.

SHATNER: Where did all of Earth's water come from?

And why does our world have so much water

when other planets don't?

Experts point to a rare and remarkable event

that may hold a clue to solving this mystery.

In this small town,

some local boys are playing basketball, when suddenly...

...a chunk of rock

slams into the ground only 40 feet away.

Incredibly, this small rock

fell to Earth from outer space.

CLARK: When this meteorite fell next to these kids playing,

one of the really interesting things is that

this was the fastest recovered meteorite that we know of.

And so, they were able to get this meteorite

really quickly into the lab,

and one of the things that was surprising at the time

is that it had small amounts of liquid water inside of it.

So, the fact that there was liquid water in space

is really mind-blowing.

It changes things,

because when you go to space, it should be dry,

and the fact that there's enough water to actually have a droplet

is really incredible.

As we look at meteorites that fall on Earth,

we're finding more and more that it's very common

to have water on some meteors and on some asteroids.

And in fact, the current theory is that this, apparently,

is where we think the water came from

to get on Earth.

SHATNER: If it's true that meteors brought water to Earth,

as scientists theorize,

then it's likely that other planets

have also received water from space rocks.

So how does the Earth retain so much liquid

when other planets can't even keep a drop?

The answer lies in our planet's distance

from the Sun.

TIMOTHY LYONS: One of the things about Earth that we perhaps take for granted

is that Earth lies within the habitable zone.

The habitable zone is defined as a band around a star

that could support liquid water.

And the Earth's orbit around the Sun

is just right to support liquid water.

Not too hot, like Venus;

not too cold, like planets as distant as Mars.

So, as we think about what's so special about Earth,

it's really that simple.

SHATNER: While Earth is the perfect distance

from the Sun to allow it to maintain

the life-giving liquid in various forms,

despite our ideal location,

Earth did not always appear as a big blue marble.

Earth has been many different planets

over its four and a half billion years of history.

And there's a popular idea

that the Earth may have gone into a deep cool

around 700 million years ago.

So much so that the surface oceans

may have almost completely frozen over.

And so, were you to be looking at Earth from space,

you might have seen this giant white sphere.

This is called a Snowball Earth for obvious reasons.

In the distant geologic past,

the Earth had a very different atmosphere.

It had much higher concentrations

of greenhouse gases, like carbon dioxide.

And then that cools the planet down,

freezing some of the ocean into ice.

That ice then reflects more of the light from the Sun,

leading to the formation of more ice.

And so, you have this feedback loop that leads

to the total glaciation of the planet.

SHATNER: The idea that Earth

was once covered in ice is chilling

in more ways than one.

But according to a remarkable discovery,

an even more surprising fact about our planet's water

may lie deep inside our world.

In this remote mining area,

several workers uncover a tiny diamond.

But this is no ordinary gemstone,

because when it was later analyzed in a lab

by geologist Graham Pearson,

he determined that it's a so-called "superdeep" diamond

that first formed 550 miles underground,

far deeper than most precious stones.

CLARK: This discovery is really exciting

because Graham Pearson's group found a lot of water

trapped inside this diamond.

And so, this is coming together to basically say,

"Maybe we have vast amounts of water stored

in the Earth's deep interior."

And in fact, there have been a lot of news stories

that have come out over the past decade or so

that we have somewhere between

three and five oceans' worth of water inside the Earth.

LAU: Based on our current knowledge,

some researchers have suggested there's several times more water

in the interior than we have

on the surface of our planet.

And so, Earth really is a very wet world.

And water has been important for life

throughout the history of our planet.

It's mind-blowing to think that there are literally

oceans of water in the center of the Earth.

But water's not the only part of our world

that warrants a deeper dive.

The rocky surface of our planet is also full of surprises.

For instance, a recent discovery has revealed

that Mount Everest is actually getting taller.

SHATNER: The Himalayas.

Located along the border of China and Nepal,

this sprawling chain of mountains

is home to Mount Everest, the highest summit on Earth,

which reaches over five miles into the sky.

But curiously, according to recent studies,

the world's tallest mountain is actually growing.

In 2020,

Chinese and Nepalese scientists announced

that Mount Everest stands more than two feet taller

than its previously measured height.

SAFFER: This study in 2020, it's a really interesting story

because we have a tendency to think that the land surface--

we live on it-- that it's stable.

Yet we know, uh, in detail, it's actually, uh,

constantly changing in subtle but measurable ways.

SHATNER: What are the unseen forces of nature

that subtly raised Mount Everest even higher into the sky?

Scientists call this gradual movement of the Earth

"plate tectonics."

Plate tectonics is the theory

that the Earth can be divided into distinctive plates

that are separated by very narrow plate boundaries,

along which plates can move towards each other,

which really creates all of the big landforms

we see on our planet,

such as mountains and huge volcanoes.

But plate tectonics is also very crucial

in the earthquakes that form when two plates move together

and one plate is forced underneath the other.

So, much of the surface of the Earth

is changing constantly over time.

The Himalayas are an example of this scenario where you've got

two tectonic plates colliding continuously over time.

And so, that land surface continues to get squeezed,

which played a role in the uplift of Mount Everest.

And that mountain peak is still rising right now.

SHATNER: Thanks to our understanding of plate tectonics,

we know that the Earth's surface

has been moving for billions of years.

And in fact, tectonics are yet another feature of our Earth

that makes it so different from other planets.

JACOB HAQQ-MISRA: There are no other planets in the solar system

that's this vibrant, active world

with plate tectonics the way we have it on Earth.

That is unique.

And so, it seems that this is necessary

or at least an important condition for life on Earth

in order to keep a planet from becoming stagnant.

SHATNER: As technology helps us monitor the subtle changes

happening on our world, it makes you wonder,

what did Earth look like in the distant past?

How different was it

from the planet we know today?

It's a fascinating question, and in the early 20th century,

scientists went to great lengths

to solve this mystery.

Geologist Alfred Wegener publishes a book titled

The Origin of Continents and Oceans.

In it, Wegener makes a shocking claim.

He argues that the continents we live on,

which seem solid and unchanging to us,

have actually been moving around our planet

for billions of years.

Alfred Wegener had a broad range of skills.

He was talented and bright enough to be able to understand

lots of different types of observations.

And so, he looked first and foremost at the fit

of the continents and recognized

that they fit together like a puzzle.

So, Wegener really had a sense that the continents

we see distributed throughout the world today,

at times in the past were together.

SHATNER: At first glance, Wegener's insight about

the position of the continents seems

like a perfectly logical conclusion.

However, at the time, experts

at the British Royal Geographical Society disagreed

because they felt strongly that the continents moved only

up or down, and did not shift across the Earth.

Proof of Wegener's revolutionary idea would ultimately come

from a remarkable discovery not on Earth's surface

but deep underwater.

Scientists at the Lamont Geological Observatory

of Columbia University

decide to launch an ambitious new program

to map the bottom of the ocean.

The team sends numerous ships around the world

to take sonar readings of the ocean floor.

This vast amount of data is analyzed by a young geologist

named Marie Tharp,

who was hired by the university for her map-drafting skills.

CLARK: When Marie Tharp started looking at the data

of the ocean floor,

one of the things that she noticed is that,

first of all, it-it's not flat.

There's actually giant mountain ranges

that are hiding beneath the Earth's oceans.

One of these is the mid-Atlantic Ridge that travels, basically,

like a baseball seam across the length of the Atlantic.

However, nobody believed her for quite some time.

SHATNER: Initially, Marie's colleague Bruce Heezen

dismissed her discovery of underwater mountains,

calling it "girl talk."

But ultimately he and the rest of the scientific community

had to admit that Marie was right.

In 1957, she published a map of the mid-Atlantic Ridge,

and her breakthrough proved that the continents

shift over time...

just as Alfred Wegener had theorized

more than 40 years earlier.

CLARK: One of the things that Marie Tharp was able

to demonstrate is that the mid-Atlantic Ridge,

this giant mountain range, was built from

volcanic activity that is occurring because

the Earth's plates are spreading apart.

And it's this mid-Atlantic Ridge that actually helped push

apart South America and Africa through time,

from the starting place of them being connected

during the supercontinent period of the Earth.

So that they're now separate,

and we have the Atlantic Ocean fitting in.

LAU: There's still so much that we're learning about

the processes of plate tectonics

and the evolution of our planet.

There's even still debate as to when

the continents first formed.

But we do know that our Earth is extremely unique to have

plate tectonics currently going on actively on our world.

It's intriguing to think that the ground under our feet

is alive, and that the continents

on our planet are always shifting.

But what's hidden beneath this ever-changing surface?

Perhaps the answer can be found by examining

mankind's attempts to journey

to the center of the Earth.

SHATNER: Just east of the border of Norway,

near the coast of the Barents Sea,

lies the ruins of a Soviet-era engineering facility.

Among the crumbling walls of this long-abandoned site

is a metal cap, sealed shut by 12 rusted bolts.

Beneath it is the Kola Superdeep Borehole.

It is the deepest human-made hole on Earth

and one of the 20th century's

greatest scientific accomplishments.

In the 1970s, the Soviet Union

attempted to break the world record of deep drilling,

to go well past the typical

three to four to five kilometers of deep drilling

that is normally done for oil, for example.

And their aim was to achieve a depth of about 15 kilometers.

It's not a very big hole. It's only nine inches wide,

but it's a testament to what is possible with technology.

The Kola Superdeep Borehole is just a really thin, narrow hole

straight down into the Earth.

One of the reasons why they made the hole that shape is

that the wider the space it is that you're drilling,

the more resistance that you run into.

And as you dig down deeper, strange things start to happen.

One of the biggest obstacles to digging that hole

was, the temperature increases about 60 degrees Fahrenheit

for every kilometer that you go down.

These teams found, when they were drilling down,

that the rock started to behave like plastic.

And that the solid rock was actually gumming up

the works of their drills.

ROBERT SCHOCH: There's so much pressure there,

there's so much heat there,

eventually they could drill no deeper

and they had to abandon it.

And since then, there has been no project

that has gotten deeper.

It's actually the deepest hole that has ever been drilled

to this day by humans.

SHATNER: By the end of the project, in 1995,

scientists had recorded reaching an unprecedented 12,000 meters,

or 7.6 miles, deep into the Earth's crust.

But despite this remarkable feat of engineering,

the Kola Superdeep Borehole penetrated less than one percent

of the 4,000 miles between the planet's surface

and inner core.

If we've only been able to explore a tiny portion

of Earth's crust, what do we really know

about the inner workings of our planet?

Since time immemorial, people have created mythologies

and stories to explain what was underneath our feet.

However, the first person to use mathematics and physics

to decode the mystery of the Earth was Edmond Halley,

famous for discovering and understanding Halley's Comet.

He thought that the Earth might be hollow.

CLARK: Edmond Halley was really interested in the idea that

there were basically these concentric shells

inside the Earth

and that each one of these shells had different properties.

They could spin independent of each other inside of the planet.

When we think about something like Halley's theory,

we forget that things like the Hollow Earth

were valid scientific thought.

It wasn't just a science fiction idea.

It was one that was still the best explanation at the time

to try and fit the data and information they had.

And the reality is different from that,

but it's still an exciting idea.

SHATNER: While we now know our planet is not hollow,

Halley's concept of concentric layers has become

an accepted theory.

For decades, experts have proposed that our world

is comprised of four parts: the inner core,

the outer core, the mantle and the crust.

But how did we determine this

if we can barely scratch Earth's surface?

LAU: Today, we analyze the way that waves from earthquakes

move through the Earth

and we see how they bounce off of certain rocks

or are deflected deep underground.

And that tells us about the structure,

what the interior of the Earth looks like.

But at the same time, it's interesting that we've

never seen the inside of our world in person.

SHATNER: While it may be impossible for humans to physically reach

the center of the Earth, we have made some

fascinating discoveries

in the world's deepest mine.

The deepest mine is located

in South Africa, and this is deep.

It's about four kilometers deep,

But the excitement is all about the unintended discoveries.

For example, about two and a half kilometers down,

a whole new species of bacteria was discovered.

And this bacteria is the only living thing down there.

And so, you don't know what you're gonna find

until you get down there and take a good look.

SHATNER: Will we ever really know what lies beneath our feet?

Our understanding of what our planet is made of

is ever-changing.

For example, in February 2023,

geologists at Australian National University

made a stunning announcement

when they detected a surprising fifth layer that lies

within Earth's core.

There is now a innermost inner core,

inside what we thought of as being the core.

Part of why this is exciting is that this is very, very new.

So we don't know where this is gonna go.

We're still in this age of discovery,

and really, there's some very fundamental information

that we still need to learn about the Earth's interior.

KLINGER: The more we're able to explore the depths of the Earth,

the more we're able to understand

about the planet itself,

about the cosmos

and our place in it.

Will we ever be able to reach the center of the Earth?

(chuckles) It's fun to think about.

But experts warn there are

more pressing mysteries to explore,

such as our planet's relationship with the Sun

because, while the Sun is vital to our existence,

it also has the power

to destroy all life as we know it.

SHATNER: While excavating a construction site,

workers unearth a massive kauri tree

that has been buried in the soil of an ancient swamp

for thousands of years.

Fused into the preserved tree rings

are clues to solving a 42,000-year-old mystery

of one of the deadliest solar events

ever recorded on our planet, called the Adams Event.

BEN McGEE: The Adams Event is

what scientists think is a small magnetic pole reversal

around 42,000 years ago.

And why is this important?

Well, when a magnetic field flips on our planet...

...it effectively goes away for a little while.

And this lets solar radiation come in

and, basically, wreak havoc around the globe.

HAQQ-MISRA: Earth has a magnetic field with two poles.

The way you think of a bar magnet--

there's a north pole and a south pole,

and it's roughly aligned

with our geographic north and south pole.

Earth's magnetic field is like a protective blanket

that protects our planet from dangerous solar flares

coming from the Sun.

In order to power a magnetic field,

a planet needs a molten hot core

that is full of electrically charged particles

that are spinning around.

And so, this is how the Earth generates its magnetic field.

Honestly, when it comes to planets,

we've got it pretty good.

The Earth's magnetic field

is our force field.

SHATNER: But what happens if our force field is unstable?

When geoscientists

at the University of New South Wales

in Sydney, Australia

found unusually high levels of radiation from the Sun

baked into the rings of the ancient kauri tree,

it revealed that Earth's last magnetic field disruption--

42,000 years ago--

caused drastic and deadly consequences.

McGEE: Without the Earth's magnetic field,

you could have had intense charging of the atmosphere...

...leading to auroras...

...blue sky lightning...

... and increased radiation doses.

And it's possible that the Adams Event caused

widespread ecosystem disruption

and even extinction as the Earth adjusted

to, suddenly, a world whose deflector shield

against all the radiation from the rest of the universe

suddenly went down.

SHATNER: It's estimated that during the Adams Event,

the Earth's magnetic field shrank to as low as five percent

of its current strength,

exposing the planet to powerful solar winds,

galactic cosmic rays, and unfiltered radiation...

...that destroyed the ozone layer.

McGEE: Some have hypothesized

that the Adams Event contributed

to the extinction of the Neanderthals,

possibly because their pale skin may not have been able

to withstand increased levels of ultraviolet radiation.

Or maybe they were just not as adept at adapting

to changes in the ecosystem.

But in either case, it looks like this had something to do

with taking out the Neanderthals.

BETH SHAPIRO: We don't really know why it is

that Neanderthals became extinct

by around 39,000 years ago.

But perhaps they were just simply not as well-adapted

to the extremely dramatic shifts

that were happening to their environment at the time.

SHATNER: According to the U.S. Geological Survey,

the reversal of Earth's magnetic fields are random

and can occur as often as every 10,000 years

or as infrequently as every 50 million years.

It's hard to comprehend how such an event

would alter life as we know it

and transform our source of life on this planet

to a reaper of global destruction.

The Sun's energy is the source of all life on Earth.

The Sun, though, doesn't entirely help life on Earth.

It can also threaten it.

The Sun gives off ultraviolet radiation,

and that can be extremely damaging to life on Earth.

It can destroy DNA.

As the Adam's Event showed,

the Sun is at once the source of light

and heat and warmth

and, ultimately, of life on the Earth,

but the Sun is also

this raging monster...

...in that when we drop our planetary defenses,

we are in danger.

SHATNER: Will a solar disaster like the Adams Event

strike the Earth again?

It's among the many possibilities

that could change life as we know it.

And as we seek to understand the history of our world,

many have turned to the stars

to see if there could be another planet

just like Earth somewhere in the universe.

SHATNER: Aerospace company Blue Origin prepares

to launch a spacecraft

that will take four civilians on an 11-minute trip

into orbit around Earth, and I had the privilege

of being one of those passengers.

After we blasted into space

and entered zero gravity,

looking down upon our planet was perhaps

the most exhilarating experience of my life.

Oh, wow.

When we returned to Earth...

(group cheering)

...I felt overwhelmed by a deep sense

of awe and wonder.

(crying): It was the most profound experience

I can imagine.

I-I'm so filled with emotion about what just happened.

I-I just...

It's extraordinary.

Extraordinary.

Now that I've seen our world from space,

it's hard for me to imagine that there could be a planet

as extraordinary as Earth.

But on the other hand, the universe is a vast expanse

filled with a countless number of planets.

So it begs the question,

could there actually be another Earth?

In 2014, scientists got one step closer to the answer,

when they identified the first potential Earth-like planet,

which is 582 light-years away from Earth

and has been named Kepler-186f.

Kepler-186 was one of the first rocky planets

detected orbiting another star.

This is a planet

that's in the outer region around its star,

so it doesn't receive as much light

as the Earth does from the Sun...

...but enough that

the surface of that planet could be habitable.

SHATNER: In addition to Kepler-186f,

scientists have discovered 54 more Earth-like planets.

But while these worlds may bear some similarity to ours,

that doesn't mean we've found another Earth just yet.

PYLE: How truly unique Earth is is still kind of a mystery.

We've got tools that could help us

to try and figure out how many other

Earth-like planets might be out there.

But, ultimately, we're not gonna know

until we move further out.

We're gonna have to send probes there

to really understand what these planets are all about.

SHATNER: While we look to find

other planets like Earth out in the vast universe,

our blue marble offers plenty of mysteries

just waiting for humanity to solve.

CLARK: The mystery in understanding the Earth is,

on one hand, we know that

the fundamental building blocks that made the Earth are

spread throughout the universe.

On the other hand,

we don't see life in other places.

The exciting thing for us as scientists is when

the Earth does something new.

We're able to take that evidence

and add a new piece of the puzzle

of trying to understand what makes the Earth tick.

It seems that, in spite of everything we've learned,

Earth still holds many mysteries.

If we continue to investigate the origins of our world,

will we be able to fully understand

what makes it so special?

Let's hope so because our future may depend on it.

But for now,

Earth's place in the universe will remain...

unexplained.

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