The far north of our planet
is a land of extremes.
When winter takes hold…
…arctic winds cause temperatures to plummet.
Even the hardy Alaskan brown bear must take refuge.
Unable to endure the freezing conditions,
they are forced to embark on an extraordinary journey.
As the North Pole tilts into the darkness and cold of winter…
the bears begin climbing
high into the mountains…
to find shelter in caves or dens
dug into the steep slopes.
Hundreds of meters up.
Bears living at the ends of the Earth
must contend with the incredible challenges each season brings.
To do it,
they rely on a very special connection to the universe,
because written into a bear's biology
is every critical moment…
…of Earth's four-and-a-half-billion-year history.
Events of unimaginable power…
…and destruction
that have created a world like nowhere else in our universe.
This is Luna, a six-year-old female.
Having reached the safety of her den…
this will be her home for the next six months.
A cozy spot
to see out the winter in hibernation.
As the days get shorter…
her body responds to the rhythm of our planet.
Her heart rate begins to slow…
…to just eight beats per minute.
As she settles down…
her body temperature drops.
And she falls into a deep, dreamful sleep.
The powerful connection between bear and universe…
compels her to spend the harsh winter months underground.
Blissfully unaware of the long journey ahead.
Because outside the den…
…Earth is on the move.
Every day, the planet spins once on its axis.
Each month, the Moon completes one full circuit around the Earth.
And as the months pass,
the Earth and the Moon orbit the Sun.
During the time she'll spend asleep,
Luna will travel 470 million kilometers.
An epic cosmic voyage
halfway around the Sun.
And this year,
she has some new traveling companions.
While in and out of sleep…
…Luna has given birth…
to two little bears.
The cubs will eat and sleep in the den until springtime.
But two more mouths to feed
is taking its toll on Luna's body.
Fat cells beneath her skin begin to shrink
as her energy stores are used up producing milk.
After five months underground,
Luna has already lost more than a quarter of her body weight.
And with outside temperatures still locked below zero…
Luna urgently needs spring to arrive
if she is to continue feeding her newborn cubs.
Luckily,
Earth has a happy quirk.
It's tilted on its side.
And so, as it moves through space,
at different times of the year,
different parts of the planet lean toward the Sun,
causing the seasons to turn.
Which means, in Alaska,
something remarkable is happening.
With each passing day,
the Sun gets higher in the sky.
Its warming rays transforming the landscape.
Just when Luna and her cubs need it most.
Nerve endings in her skin detect the warmth of the Sun.
And as the days get longer…
…specialized cells in the back of her eye
sense the tiny increases in light levels.
These secret messages,
direct from our nearest star…
tell Luna it's time to wake from hibernation.
As they signal the gentle turning of our planet's seasons.
But out in the solar system…
seasons like Earth's
are far from typical.
Our nearest neighbor, Venus…
…is much closer to the Sun
and so is much hotter than Earth.
And because the planet has no tilt,
it has virtually no seasons.
Temperatures remain searingly hot all year round.
Further away,
out past Earth…
…is Mars,
whose orbit causes its distance from the Sun
to vary dramatically through the year,
making seasons on Mars…
…far more extreme than our own.
During winter, over a quarter of the planet's atmosphere freezes solid.
While in summer,
when Mars passes closer to the Sun,
the increased heat creates dust storms
so severe they can engulf the entire planet.
But beyond Mars,
past Jupiter…
…and Saturn…
…three billion kilometers from the Sun,
Uranus has the most extreme seasons in the solar system.
This planet spins on its side.
So as it orbits,
the summer side faces the Sun…
…while the winter side won't see daylight for 42 years.
Only Earth has seasons
that, even at the extremes of our planet,
allow life a fighting chance.
With the arrival of spring,
Luna's family emerge from the den.
The cubs can now enjoy their first moments in the Sun.
But the turning seasons mean the cubs must now set off on a daunting journey.
Because to find food,
Luna must lead them back to the lowlands
on the other side of the mountain.
It's a grueling climb for little legs.
With hundreds of meters of ice and rock to overcome.
Once they're over the summit, the hardest part of the trek is over.
But survival is far from guaranteed.
To make it through the rest of the year,
the family will rely…
…on Luna's instinctive understanding
of the seasons ahead.
Each one…
a result of Earth's yearly journey through space.
A journey that began…
…before the Earth even existed.
A time when our solar system was little more than ash.
No Sun,
no planets,
and no moons.
Just a vast cloud of gas and dust.
Then, in a distant solar system…
…a star explodes.
A supernova that sends a shock wave across the galaxy.
On impact, the shockwave causes the cloud to fall in on itself.
And begin to rotate.
As it spins faster and faster,
the cloud flattens out into a disc.
A maelstrom of gas and rock.
In the center, under intense pressure…
…the Sun is born.
While further out, rock collides with rock…
and our planet begins to grow.
Until after 20 million years,
Earth is fully formed.
Still circling the Sun like the cloud from which it grew.
Orbiting once every 365 days,
which is how Earth got its year.
But at this point in the planet's story,
it's a year without seasons.
Because the chaos that created them…
is still to come.
In Alaska,
the vibrancy of spring…
…has given way to lazy summer days.
The cubs are now five months old.
And still learning…
how to be bears.
Because they're growing so quickly,
the cubs are constantly hungry.
And while it might look idyllic…
summer in Alaska is a time of hardship.
Without a decent meal to be found in the meadows,
the bears must make do with grass.
Which means the cubs still rely on their mother's milk
for the bulk of their food.
But Luna hasn't eaten properly since before she went into hibernation,
so she's running on empty.
But nearby, out in space,
help is on the way.
Because today, our planet has a constant companion.
The Moon.
Locked in orbit around the Earth,
it's our nearest celestial neighbor.
The largest and brightest object in the night sky.
And because the Moon is very big and very close…
it exerts a powerful gravitational pull.
Strong enough to influence our world.
It lifts the oceans away from the planet's surface…
…creating Earth's tides.
And on a few days around the full and new Moon…
the lowest summer tides reveal an entirely new landscape.
Mudflats extend hundreds of meters from the shore.
Allowing hungry bears access to buried treasure.
At just the right time for Luna and her young family.
But finding what's hidden in the mud
is a job best left to the grown-ups.
Luna's sense of smell is 2,000 times better than our own.
Perfect for detecting tasty morsels.
Soft-shelled clams.
She's so skilled at prizing them apart,
she can get through almost one a minute.
The clams are a welcome supplement to her meager summer diet.
Vital energy she can pass along to the cubs.
These gifts, delivered to her by the Moon,
will help Luna sustain her family through summer.
But the bear's relationship with the Moon…
…runs much, much deeper.
Because it was during the Moon's formation that Earth got its seasons.
Not long after it formed,
our planet gained a companion.
But it wasn't the Moon.
It was another planet that formed in Earth's orbit.
A sibling called Theia.
It was roughly the size of Mars.
-A vast ball of rock and lava.
Throughout their childhood,
Earth and Theia lived in harmony.
Until gravitational disturbances in the young solar system…
set the two planets on a collision course.
In this moment…
…Earth's future hangs in the balance.
Theia is obliterated…
as the impact melts both worlds.
For an instant in cosmic time…
our planet had two moons.
But then gravity dragged one of them back down to be consumed by Earth.
Leaving only the moon we know today.
And the collision transformed our planet forever.
The impact knocked it over to one side
so that today it spins with a tilt of 23 degrees.
So, as it moves around in its orbit,
first one hemisphere
then the other is angled toward the Sun…
giving Earth its seasons.
As summer draws to a close,
the longer nights are a sign
that the harsh Alaskan winter is fast approaching.
The cubs are now seven months old.
And Luna knows that if they are to survive their long winter sleep,
the family must build up their fat reserves.
But the Moon is a constant reminder…
…help is on the way.
A thousand kilometers off the coast of Alaska…
another creature is tuned in to Earth's seasonal rhythm.
Sockeye salmon.
After two years roaming the Pacific Ocean…
…they've reached maturity.
And now, the changing day length…
…triggers hormones to be released into their blood.
Chemical signals that alter their behavior.
Calling them home
to breed in the rivers where they were born.
Luna's exhausted and hungry,
but still she leads her cubs to the river's edge.
Because she senses something magical is about to happen.
Today, the unique clockwork of the seasons
will deliver a vital lifeline to the bears.
The Earth has reached a pivotal position in its journey around the Sun.
And now the Moon raises the tide.
Providing the final push that drives the salmon into the rivers.
At long last, the cubs get their first taste of salmon.
And although a single fish will not see the bears through winter,
it's the cubs' first real meal in nearly eight months of life.
The arrival of the first salmon is a relief to Luna.
Because experience has told her
there will soon be much richer pickings upstream.
As the days grow ever shorter,
the very same hormones that set the salmon off on their journey
now dramatically transform them.
Their heads change shape.
And their bodies blush bright red.
As in their millions,
they surge upriver into their spawning grounds.
And the final piece of the bears' story falls into place.
A story as old as the planet itself.
The formation of the Earth…
the collision that created the Moon…
and our planet's unique relationship with its moon and star…
have combined to create a world like nowhere else in the universe.
A Goldilocks planet.
The only place we know that's just right for life.
And for Luna,
four and a half billion years of cosmic history
is now distilled into this moment.
The annual convergence of salmon and bear.
And the precious opportunity
to feed her family.
Luna and her cubs now have what they need to make it through winter.
And while ultimately, their future on our changing planet is uncertain,
for this year, the salmon will sustain them…
as our planet continues on this relentless voyage…
…around the Sun.
Next in the story of our universe,
the ocean wanderer whose life cycle reveals
how everything in the universe was made.
Because in every turtle's egg
are atoms from The Big Bang…
…forged as stars die.
But how are stars and planets created?
And how do the same ingredients transform into living things?