[narrator] We have uncovered
a remarkable planet,
a world of extreme weather,
alien landscapes,
and bizarre phenomena.
It's the most complex system
we know of in the universe.
That planet
is Earth,
and we are only beginning
to unlock its secrets.
The planet Earth
is a haven for life.
[man] Conditions are perfect.
I mean, they really are perfect.
[narrator] Oceans of liquid
water make life possible.
But that's not enough.
[narrator] A complex secret
gives Earth its life,
while hundreds of other
known planets fall short.
[man] Not a single one
of these planets
look just like the Earth.
[narrator] What makes Earth
so perfect?
Why does it depend on
the moon, volcanoes,
even the planet Jupiter?
What puts Earth in the ultimate
Goldilocks Zone for life?
...on Secrets of the Earth.
May 19, 2012.
In the Arizona desert,
a 35-year-old visitor
from Germany is overcome by
105-degree temperatures,
the first of two hikers
to die this weekend
from early-season heat.
Water makes up 60%
of the human body.
[man] As little as two to three
percent decrease in body water
can onset the symptoms
of dehydration.
[narrator] Humans on Earth
exist on a knife's edge
between life and death,
surviving only in a narrow
range of temperatures.
So, too, in the solar system,
there is only a narrow range
in which planets
can support complex life.
This place--not too hot,
not too cold--
is called the Goldilocks Zone.
Estimates put
the Goldilocks Zone
between 67 million
and 279 million miles
from the sun,
and it's just right
for the one thing
life seems to need,
liquid water.
[man] You're not so close
that water boils away.
You're not so far away
that water turns to ice.
Perfect Goldilocks.
[Stephenson]
We're in the Ka'u Desert
on the island of Hawaii.
When you look at
dry places like this,
it really makes you appreciate
how vital water is
to animals and plants.
[narrator] Hawaii's Big Island
is also home to rainforests,
waterfalls,
seacoast,
and even snowcapped mountains,
where the secrets
of life's success on Earth
play out.
Among those secrets,
a recipe for life
that needs far more
than just liquid water.
[Ward] One of the biggest
surprises of the last 20 years
is the large number of
conditions the Earth needs
to maintain its state
as being habitable.
[bird screeches]
[narrator] Besides Earth's
distance from the sun,
life also depends
on the planet's volcanoes,
its large moon,
and even the planet Jupiter.
Liquid water is important,
but that's possible
at a wide range
of temperatures,
not all of them good for life.
[boiling]
[Ward] The highest temperature
that complex life can live at
is very, very different
from that of microbes.
We animals really can't live
much above 113
to 115 degrees Fahrenheit.
[narrator] The Nahuku
rainforest on Hawaii
is a Goldilocks Zone
on a Goldilocks planet.
[Ward] There are more species
packed around me right now
than any other place
I could go to.
[narrator] This rainforest
and others like it
represent the apex of species
diversity on planet Earth.
The diversity
springs from conditions
that are not only wet
but also warm
without being too hot.
[Ward] Life likes to be warm.
Seventy to 80 to 90 degrees,
that's ideal.
That's where you want to be.
[narrator] The Earth's volcanoes
hold the secret
of the planetary thermostat.
If it weren't for these
mountains of fire,
this entire planet
would freeze.
January 3, 1983.
Hawaii's Kilauea volcano
begins its longest eruption
in recorded history
as lava flows through
residential areas.
[man] The first homes
destroyed there were 16 homes
shortly after that
initial eruption,
a vivid reminder
of the destructive power
of the volcano.
[narrator] The volcano's fury
fuels mankind's fear.
But the erupting mountains
hold a secret.
They are essential to life
and play a central role
in making Earth
a Goldilocks planet.
Volcanoes are commonplace
in the solar system.
They have erupted on Mars,
on Jupiter's moon Io,
and may number
in the thousands on Venus.
But unlike other planets,
Earth's volcanoes are coupled
to plate tectonics,
the planet's method
of resurfacing its crust.
Earth is the only known planet
whose crust
actively moves around
with many distinct plates.
Many scientists now believe
plate tectonics
is the most important secret
other than water
in making Earth
just right for life.
It all has to do
with carbon dioxide.
Right now, it may be
making the planet too warm,
but life actually needs
some of that warming.
Without CO2, the Earth
would be very cold.
[Ward] It would be
zero degrees Fahrenheit,
32 degrees below freezing.
Without that carbon dioxide,
the entire planet would be
a frozen wasteland.
[narrator] The CO2 emerges
from Earth's volcanoes
along with their
destructive lava.
The essential gas cycles both
in and out of the atmosphere,
acting like a thermostat
to keep the planet
not too hot, not too cold.
It may be one of the best kept
secrets of Earth's existence,
a series of
interconnected reactions
that puts the entire planet
to work,
a chain of events
in which one weak link
would make life as we
know it impossible.
Fiery volcanoes seem
unlikely players in the drama
that makes Earth
so perfect for life.
And yet, without them,
the planet would be
covered with ice,
and life would have
little chance of success.
The secret is
in the carbon dioxide
that volcanoes supply
to the planet's atmosphere.
[Ward] The volcanic vent
you see here is one of many
in the Hawaiian Islands.
It's from vents like these
that carbon dioxide
comes out of the deep Earth
and goes into
the Earth's atmosphere.
[narrator] As a greenhouse gas,
CO2 turns
Earth's temperature up.
But in doing so,
it becomes the core component
of the planetary thermostat.
It is a delicate global
balancing act.
If the temperature is cool,
the planet makes carbon dioxide
go up,
so the air gets warmer.
If the air gets warmer,
the planet makes carbon dioxide
go down to turn things cooler.
One way a hotter Earth
makes CO2 go down
is by ramping up the chemical
weathering of rocks.
Carbon dioxide
mixed with rainwater
turns into a weak acid that
eats away at Earth's rocks.
When temperatures go up,
the process naturally
goes faster.
The acid reactions
absorb carbon dioxide
and suck it out of the air.
Earth's weathered rocks get
help from another process
to draw down carbon dioxide
in the atmosphere.
[Ward] More heat also speeds up
the growth of untold numbers
of plants and animals
in the ocean.
Many of these draw
carbon dioxide
right out of the air and use it
to make their shells.
All of these shells slowly sink
onto the sea bottom
to produce a limestone layer
on the sea floor.
[narrator] With both
the formation of seashells
and the weathered rocks,
carbon dioxide dips down
in the atmosphere.
If it goes too far,
the planet could freeze,
so it needs a way to make CO2
go back up.
That happens deep underwater.
In the middle of the ocean,
massive ridges on the seabed
erupt with lava
as plate tectonics
drives the motion
of the Earth's crust,
making the ocean floor
spread apart.
Miles away,
the spreading sea floor
crashes into
a continental plate.
The sea plate dives down
into the Earth's interior.
The limestone melts,
liberating its carbon dioxide.
[Ward] The carbon dioxide
that was set free underground
eventually comes out here,
back where we started
at a volcanic vent.
So now the Earth becomes warmer,
and the cycle begins again.
[narrator] The finely-tuned
system has adjusted
the atmosphere's
carbon dioxide up and down
to keep the Goldilocks Zone
on Earth intact
for billions of years.
Plate tectonics also makes
Earth's land surfaces move.
The giant Pacific Plate,
for instance,
slowly moves to the northwest
and carries with it
the Big Island of Hawaii.
[Gallagher] I like to think
of the ground beneath my feet
as stable,
but geologically speaking,
we're in constant motion.
Right where I'm standing
on the mountain of Kilauea,
I'm moving two to four inches
a year annually north-northwest.
So if you came back to see me
next year, I wouldn't be here.
I'd be here.
[narrator]
When continents move apart,
species are isolated
and evolve into new ones.
Life's wild variety grows.
The complex diversity is
an essential survival tool...
...especially when Earth
comes under attack
from outer space.
The secret ingredients
for thriving life abound
on planet Earth.
But its wealth of rich climates
is also home to environments
on the edge of habitability.
One of them is Mauna Kea,
an inactive volcano
on the Big Island of Hawaii.
It's an alien landscape home
to a dozen giant telescopes.
But at high altitude,
astronomers are working
on the margins of
the Goldilocks Zone for humans.
[man] I've been up here
at 13,500 feet
many times, and sometimes
I feel a little bit
of altitude sickness.
My head isn't working right.
I feel nauseous.
The oxygen content up here
is only about 60% that
of sea level.
[narrator] Astronomers here
have discovered that the recipe
for life's success on Earth
includes many ingredients
from space.
The moon, unusually large
for a planet of Earth's size,
has a secret impact on life.
[Filippenko]
It provides stability
for Earth's axis of rotation.
[narrator] The pull
of the moon's gravity
keeps the Earth
from wobbling uncontrollably.
[Ward] Without our moon,
we'd be flopping around
like a dead fish.
This isn't stability.
Life needs two things.
It needs conditions to live,
but it needs those conditions
to exist over
long periods of time.
You need conditions to remain
very, very stable.
[narrator] Having a large moon
is part of the secret,
but stability also comes
from unlikely sources.
[Filippenko] Many people
are astonished to learn
that the planet Jupiter,
500 million miles away,
has a huge effect
on life on Earth.
[narrator] A major impact
was a rare event on Earth
when an asteroid may have
wiped out the dinosaurs
65 million years ago.
It remains rare today
because Jupiter
acts as a cosmic
vacuum cleaner,
as the planet's overwhelming
gravity grabs comets
and asteroids, sweeping them
out of the solar system.
[Ward] Jupiter in our lifetime
swept a comet
right out of existence.
Shoemaker-Levy 9
went crashing into Jupiter.
That particular comet
was on an Earth-crossing orbit.
Jupiter sucked it out
of existence.
[man] If you do not have
a gigantic Jupiter,
the solar system would be
full of debris
that would hit the Earth
all the time,
meaning that life
would be impossible.
So, thank you, Jupiter,
for cleaning out
the solar system.
[narrator] Earth's astronomers
are now engaged
in one of history's
greatest quests:
a search for other planets
suitable for life.
In early 2013, they identified
a possible planet
more like Earth
than any yet found.
At 2,070 light-years distant,
it is 1.5 times Earth's mass
and orbits within
its star's Goldilocks Zone.
[Filippenko] There's thought
to be about 50 billion planets
in our Milky Way galaxy,
of which perhaps 500 million
are in the habitable zone.
But that's not enough.
[narrator] Mars, for instance,
is in the habitable zone,
but its weak gravity
and irregular magnetic field
left it unable to hold onto
most of its atmosphere.
Future outgassing
might reverse the process,
but today it cannot support
complex life.
It also turns out
that the Goldilocks Zone
for any such planet
doesn't last forever.
Even Earth's time
as a just right planet
may run out
sooner than you think.
Planet Earth's secret set
of special factors
make it perfect for life,
most important--
its place around the sun
in the Goldilocks Zone.
[Filippenko] A planet is said
to be in the Goldilocks Zone
when its distance from the star
that it orbits
leads to temperatures
that allow liquid water
to exist on its surface.
[narrator] But the deepest
secret of Earth's
Goldilocks existence
is not one of place
and temperature.
It is one of time.
For complex life to evolve,
Goldilocks temperatures
must remain stable
for long periods.
[Ward] We have been in what
we call the habitable zone now
since the formation
of life on Earth
at least four billion years ago.
[narrator]
And that depends on a sun
that does not burn out quickly.
[Kaku] If the sun were
a gigantic star,
it would burn its hydrogen
very rapidly
and only last for maybe
a billion years or so,
and that's too short to get life
off the ground.
[narrator] But even a long-lived
star like Earth's sun
changes over time,
and so does its
Goldilocks Zone.
[Ward] As our sun
burns this fuel, in fact,
it gives off more and more heat.
It's actually going to expand.
[narrator] Although a hotter sun
could incinerate complex life,
there's a secret killer
that may strike millions
perhaps a billion years sooner.
Today carbon dioxide is
temporarily on the rise,
making the Earth too warm.
But in the long term,
the mechanisms that
take it out of the atmosphere
will dominate,
making CO2 gradually disappear.
[Ward] We can predict
that perhaps in as little
as a billion years from now,
there will no longer be
enough carbon dioxide for plants
to undertake photosynthesis.
When that happens,
we very rapidly lose oxygen
in our atmosphere,
and when that happens,
that spells the end
of all animals on planet Earth.
[narrator] But would
the increased heat from the sun
result in new Goldilocks Zones
migrating into more distant
parts of the solar system?
[Filippenko] It could be
that Mars will become
temporarily habitable.
Or maybe one of the moons
of Jupiter or Saturn
that has an icy crust will have
liquid water on its surface.
[narrator] Life's future
will be at stake.
While microbes may survive
the extreme conditions
on a superheated Earth,
could its intelligent
life forms escape
to seek refuge elsewhere?
If identical conditions
aren't found,
will successor worlds
be good enough?
Can they evolve
their own set of factors
that have made the success
of complex life
one of the greatest
secrets of the Earth?