All language subtitles for How the Universe Works - 6X08 Strange Lives of Dwarf Planets

All across our solar system,

scientists are discovering
thrilling new worlds,

dwarf planets.

They may be small,
but they're full of riddles,

oceans of subterranean water,

ice volcanoes,
and vanishing mountains.

The whole idea
that dwarf planets are small

and insignificant and boring

has just been shattered
in the last few years.

Dwarf planets defy
many of the rules

we thought governed
our solar system.

Dwarf planets are
very interesting bodies

scientifically,

but beyond that,
they tell us something

about the origin
of our own world.

Believe it or not,
they may harbor life.

Dwarf planets are
rattling the cages of scientists

and shaking up our understanding
of how the universe works.

They may have fed
the early planets

and even seeded them
with the precursors of life.

Dwarf planets just may be
the most important objects

in the solar system.

Our solar system has
eight confirmed major planets,

but we're discovering
many other small worlds

called dwarf planets.

We used to think they were just
dull lumps of rock,

but the more we study them,

the more shocking
and intriguing they become.

Naively,
I would expect these objects

to not be terribly dynamic.

They're probably just, you know,
airless, rocky, icy worlds,

and they're just sitting there,

and what we're finding out is
that that is not true at all.

There is all kinds of stuff
going on.

They're full worlds
with really interesting geology

and interesting histories

that can tell us a lot
about the solar system.

Scientists believe there may be

hundreds of dwarf planets
in our solar system.

So far,
we've only identified six.

Five of them...
Pluto, with its moon, Charon;

red-colored Sedna;

bright, distant Eris;

makemake,
and bean-shaped Haumea...

All live billions of miles
from the Sun,

out beyond Neptune
in the Kuiper belt.

They're just
the tip of the iceberg.

There are probably many,
many more dwarf worlds

that are out there
waiting to be discovered.

The sixth dwarf planet, Ceres,

lives in the inner solar system.

It orbits around
260 million miles from earth

in the asteroid belt.

The asteroid belt
is a region of the solar system

between Mars and Jupiter,

and this is where
most of the asteroids are.

This is rubble

left over from the formation
of the solar system.

In early years
of the solar system,

small rocks collided with
one another, stuck together,

and built
the rocky inner planets.

Dwarf planets grew
in the same way.

Ceres was actually
starting to get pretty big.

It was on its way
to becoming a planet

before it stopped growing,
and that makes it

stand head and shoulders
above everything else there.

So why is Ceres called
a dwarf planet and not a planet?

To be a planet, it must tick off
three cosmic boxes.

First, it needs to be a sphere.

Second, it needs to orbit
the Sun and not another body.

Third, it needs to clear its
orbital area of orbital debris.

Ceres ticks
just two of the boxes.

It is a sphere,
but a small one...

Only 600 miles across.

That's the size of Texas.

It orbits the Sun, but it hasn't
cleared its path of debris.

It's surrounded by asteroids,

so it misses out
on being a planet.

Even though we call
these objects dwarf planets,

small and dwarf
does not equal insignificant.

But being small
does have its problems.

When the molten core
of a young dwarf planet cools,

so does the heat engine
that drives geologic activity.

Ceres, we thought, would
basically be a big, dead rock.

It's a small body.

It should have
cooled off long ago.

Nothing very interesting
is going on,

and when we actually
got out to Ceres,

nothing could have been
further from the truth.

March 2015, NASA's
dawn probe arrives at Ceres.

As the dawn spacecraft
pulled up to Ceres,

we saw the craters and the
surface that we expected to see,

and then all of a sudden,

something totally mysterious
rotated into view.

One of the craters
had two bright spots,

almost like two eyes
staring right back at us.

It was such a puzzle
to the science community

because what are these
doing here?

Are they ice?
It looks very fresh.

What on earth could it be?

Scientists find over 100

of these mysterious white spots.

The largest is in a 50-mile-wide
crater called Occator.

They are, unexpectedly,

made up of a substance
we find on earth...

Sodium carbonate,
a kind of salt.

We believe the salts on Ceres

as actually very young.

We think they're as young
as 4 million years old,

and that's basically like
yesterday in terms of geology.

And that is super weird, right?

That's happening not on sort of
on a geologic era.

It's happening now, today.

What could cause patches of salt

on a world long-presumed dead?

Planetary geologist
Jani Radebaugh believes a clue

might be found at mono lake
in California.

All right, I'm here looking at
this beautiful lake

off in the distance

and standing on
massive white deposits.

These white deposits
used to be a part of this lake,

at one point.

The lake had dissolved
a lot of the materials in it,

and then as it receded,
it left behind the materials,

as it evaporated away,

and these things are,
you know, salts.

They're kind of granular
in texture,

and just to make sure,
we taste it

and yeah,
sure enough, it's salty.

The salt at lake mono

crystallizes
as the water evaporates,

the only way it can form.

The researchers believe
the same process

is taking place on Ceres.

This means there must be liquid
water beneath the surface...

...but how, out in the deep
freeze of the asteroid belt?

These bright spots are located
in the centers of craters.

They're located around
cracks in the surface,

and that is telling us
that this material

is coming from under the surface
and welling up onto it.

Absolutely nobody expected
there to be liquid water

beneath the surface of Ceres.

We cannot explain
what is keeping that water warm.

On some moons,

gravitational tugging
keeps the interiors warm,

but Ceres is not really near
anything else that's very large.

So the amazing thing is that
we may not even understand

how rocky planets work.

There may be
another source of energy,

another mechanism
for heating the interior

that we haven't even
discovered yet.

To find out how Ceres
has liquid water,

we have to rewind the clock
4.6 billion years.

Debris left over
from the formation of the Sun

slams together
to form the dwarf planets.

As they take shape,
the heavier, rocky material

sinks to the center
and forms a hot, molten core.

Slushy water-ice
floats to the top.

For a while, it stays liquid,
but once the core cools,

it freezes and forms
the solid mantel and crust.

That surface
should still be solid,

so the salt patches
remain a perplexing mystery.

We still haven't
answered the question,

"how could there actually
still be liquid water on Ceres?"

That's still
a hard question to answer.

One way this could happen is if
it's not actually pure water,

if you've mixed it
with something else.

Some scientists have proposed

that a salty ocean
lies beneath the surface.

The high concentration of salt

lowers the freezing point
of the water, keeping it liquid.

When asteroid impacts
fracture the crust,

this salty water
oozes up from below.

The liquid swiftly evaporates,
but the salt remains,

leaving a brilliant white spot
on the surface.

In fact, I'm willing to bet

there could be water
coming up now,

bringing salts up
to the surface,

evaporating away into space,

and that means liquid water
is very close

to the surface of Ceres
right now.

Ceres has an even more
startling card up its sleeve.

Recent research suggests
that it's an immigrant.

It didn't form anywhere near
the asteroid belt.

Ceres may have been born

alongside hundreds
of other dwarf planets,

many billions of miles
away from the Sun.

So how did it get here?

Most of the
dwarf planets we've discovered

lie far out in the solar system
beyond the orbit of Neptune,

but Ceres orbits
between Mars and Jupiter

in the asteroid belt.

But its location
isn't the only hint

Ceres might be an interloper.

Normally, celestial objects
are made of the same materials

as the other bodies
in their neighborhoods,

but that's not the case
with Ceres.

The asteroid belt is mostly
made up of dry, rocky bodies

composed of
the same heavy elements

that form
the rocky inner planets.

Ceres is very different.

Ceres is essentially
an icy world, right?

It's made out ices
instead of rocks,

and so that's kind of weird,
considering where it is.

The ice on Ceres
also contains chemical compounds

that in the early years
of the solar system,

didn't exist in
the asteroid belt.

The more we learned about Ceres,

the more mysterious it became.

One of the things is that Ceres
has quite a lot

of ammonia on it,
and we don't find ammonia

anywhere near the inner part
of the solar system.

But we do find it
in the outer solar system.

We've detected ammonia on Pluto,
its moon Charon,

and out in
the frozen Kuiper belt,

where we find
the other dwarf planets.

We think that the origin
of that ammonia

would've had to be in a very
cold part of the solar system,

colder than
where we find Ceres today.

But how an icy
dwarf planet with ammonia

came to inhabit a place
where ammonia can't form...

That's a huge puzzle.

This suggests to us that Ceres

perhaps formed
in the outer solar system

and then migrated inwards

to its present location
in the asteroid belt.

We used to think

that planetary orbits
were completely immutable,

that they simply
ran like clockwork

and they didn't move around.

Now we know that
that's not the case.

In the early stages
of planet formation,

planets move around
through the gaseous disc

that encircles the young sun,

much like rafts that are
pushed around by ocean currents.

Ceres' ammonia
suggests that dwarf planets

rafted around
on the cosmic ocean

along with the young planets.

Ceres is sort of a smoking gun
that solar systems

are much more dynamic,
much more dramatic than we know.

There's mounting evidence

that Ceres formed
farther out in the solar system

and something brought
this little world in.

What could possibly
have done that?

The answer
is the planet Jupiter.

After it first formed,

the giant planet migrated in
towards the Sun.

Its massive gravity disrupted
the orbits of other bodies

in the solar system,
including that of Ceres.

The solar system formed
out of a disc of gas and dust,

and as Jupiter formed,

it would've been plowing through
this material.

And if it plows through
that material,

it's experiencing drag.

As it was losing energy,
it would start to move in

toward the Sun
relatively slowly.

Ceres formed in the
outer edges of the solar system.

It was dislodged
from the Kuiper belt

and yanked inwards
by the migration of Jupiter,

and when Jupiter
stopped migrating, so did Ceres.

It settled into a new, stable
orbit in the asteroid belt.

Once you realize

that something that strange
and dramatic can happen,

that a dwarf planet can form
far out in the solar system

and be brought in,

it makes you wonder how
many times that happened before.

Could there have been other
generations of dwarf planets

that got thrown in
towards the Sun

or maybe were thrown out
of the solar system entirely?

Scientists believe
that squadrons of rocks

and icy dwarf planets

may have hurdled into
the solar system.

Hundreds set out.

Only one survived.

If there was a population

of small dwarf planets

in the outer solar system
that migrated inwards,

Ceres might be the sole
survivor, the only one left.

So if Ceres
settled into its new home

in the asteroid belt,

where are the rest of the icy
worlds and the water on them?

The idea that Ceres
may have moved in

from the outer solar system
is interesting,

but why should it be important
to you?

And incredibly, the answer
might be inside your own body

right now.

For the longest time,
we've wondered

where did the majority
of earth's water come from?

When you think about
where the earth is,

how close it is to the Sun,

there shouldn't have been
any water here.

Understanding
the evolution of Ceres,

from where it formed
to where we find it today,

could also lead us to understand
how the earth can end up

with more water
than we would otherwise expect.

When the earth formed,

it was too hot for water
to exist on the surface.

Perhaps the squadrons
of dwarf planets

broke up on their journey,

showering earth with
water-rich lumps of rock,

enough to fill earth's oceans.

Amazingly, when we
study the chemistry of water,

the best match is that
the water in your body right now

came from asteroids themselves,

asteroids and dwarf planets
that rained down

and hit the earth
over billions of years.

Dwarf planets
may have brought something else.

February 2017,
scientists announce

the discovery
of organic materials

on the surface of Ceres.

On earth, life uses water

and organic chemistry,
carbon-based molecules.

The intriguing thing
about the dwarf planets

is that they have both of those.

Pluto and Ceres
have organic molecules.

There's liquid water
below the surface.

There's a source of energy
that warms the interior.

It is not at all impossible
that somewhere under

these cold, icy surfaces,
there could be life.

Could other
dwarf planets host life, too?

Sedna and makemake
both have red-colored patches.

The color comes from something
called tholins,

organic molecules that could be
a precursor to life.

Based on all the interesting
chemistry they're doing,

these dwarf planets
could be like

the test tubes
of the solar system.

If there are small bodies strewn
all about the solar system

that had liquid water or ice,

it could possibly serve
as an Incubator for life,

just holding on to it,

ready to crash into
another body and seed it.

The habitable zone now extends

to the entire solar system.

It really expands...
It greatly expands...

The stage for the play of life
in the entire galaxy.

Maybe we have
dwarf planets to thank

for our very existence.

All of a sudden, the smallest
bodies in our solar system

have become some of
the most interesting things

we've ever seen.

The idea
of migrating dwarf planets

opens up
some intriguing scenarios,

including one
really out-there possibility.

Some of that water
and organic material

may not be
from our solar system.

The more we learn
about dwarf planets,

the more they surprise us.

But there's one dwarf planet

whose very existence
is a mystery.

It's called Sedna,
and no one is quite sure

what it's doing
in our solar system.

Sedna may have my vote for
the single most peculiar object

in the entire solar system.

Here, we have a world which is
about 1,000 miles wide,

but it is way far out in the
solar system, way past Neptune.

Sedna is the most distant object

we've identified
in our solar system.

Standing on
the surface of Sedna,

looking back
at the solar system,

the Sun would look like
a really bright star,

but not much more than
a really bright star.

Just like Pluto,

Sedna has a strange,
elliptical orbit.

The difference is,
Sedna travels from 7 billion

to 93 billion miles
from the Sun,

and unlike Pluto,
its orbit can't be explained

by its close proximity
to Neptune.

The weird thing about Sedna
is its orbit.

How could it have gotten
that elliptical

when it's that far away
from any of the major planets?

If you have an object
that's close enough to Neptune,

Neptune's gravity
can affect its orbit

and swing it into
an elliptical orbit.

The problem is, Sedna never gets
that close to Neptune.

It doesn't get anywhere
near close enough

to be in that kind of orbit,

and that means that something
else is going on out there.

Sedna cannot be explained

using objects that we know.

Everything else,
we can understand

why its where it is
based on, you know,

the eight planets and many,
many other small bodies.

Sedna cannot be explained
by that,

and, you know, that's the sign
of a good mystery.

Something else
must have happened.

Looking at models
for how you can change

the orbits of objects,
there's almost no way

Sedna could've formed
in our solar system,

and then had its orbit change
so that it's that elliptical

and goes that far out
from the Sun.

And that means maybe...
Maybe... it didn't form here.

It may be an alien world.

How could our solar system

have snagged an alien world?

Long ago, it turns out our sun

may have rubbed cosmic shoulders
with other stars.

It was born
in a stellar nursery...

Close to many other
embryonic stars.

So if the Sun was born
in a very dense neighborhood,

whereby a lot of other stars

were forming at the same time
in the same region,

it is absolutely possible
that material could be exchanged

between these stars
as they're forming planets.

Sedna may have formed
just like any other object

around another star...
In a nice, circular orbit

out past the main planets
of that alien solar system,

but if that star
got close enough to the Sun,

our gravity may have been able
to lift Sedna out and steal it.

It's possible
that other dwarf planets

were abducted from other systems

and that these alien worlds
carried alien water

and even alien organic materials
to the inner planets.

It's so tempting to think
that we understand something

as basic as our own
solar system, our own home.

When you discover something
like Sedna,

you realize there could be a lot
out there that we haven't seen.

There are objects
that are small like Sedna

that are just so far away

that they're beyond
our limit to detect them,

so there could be hundreds,
thousands, tens of thousands

of objects out there,
new parts of our solar system

that are still waiting
to be discovered.

Our understanding
of the solar system

is changing radically.

Newly explored dwarf planets
stun us,

and even the most famous one,
Pluto, reveals new secrets...

...making us ask,

could these worlds
be as active...

And alive as our own?

Dwarf planets
all over the solar system

are revealing hidden lives.

In the asteroid belt,
salt on the surface of Ceres

suggests liquid water
beneath the surface.

Farther out,
the new horizons mission

found subsurface oceans
on Pluto.

We used to think
that water could only exist

in this Goldilocks zone,

where it's not so hot
that the water boils off

and it's not so cold
that it freezes,

but that's not the case anymore.

We look around,
and we find water

in the most unexpected places
in our own solar system.

This is kind of a revelation

of modern planetary science

that so many of these worlds
in the outer solar system

may have subsurface oceans
of liquid water.

It kind of boggles the mind
to see how far we've come

in our understanding

of the interior structures
of these worlds.

Finding liquid water
so far from the Sun

left scientists stunned...

...and the surprises
keep on coming

as we study these distant worlds
up close.

Makemake, 2/3 the size of Pluto.

Its surface is covered
in ethane and methane ice.

The methane is frozen into
1/2-inch-sized ice grains.

It reacts with sunlight, forming
organic molecules call tholins.

These color the planet
red-brown.

Farther out lies Eris.

It's 9 billion miles
from the Sun,

and its surface temperature...

About 400 degrees Fahrenheit
below zero.

Eris is an absolutely
tantalizing object.

We really don't know
much about it at all.

It should be
very similar to Pluto,

but one of the things we notice

is that Pluto's surface
is kind of patchy.

There are areas
that are very bright,

but also areas
that are quite dark.

Eris, on the other hand, seems
to be almost entirely bright.

Eris is one of
the shiniest objects

in the solar system,

reflecting 96%
of the light that hits it.

Scientists wondered why.

A clue comes from
its near neighbor, Pluto.

When new horizons flew past,
it spotted something strange.

One of the funny little details
is that as we flew over Pluto,

we realized
that there were things

that looked a lot like
sand dunes down there.

Now, that may not sound
incredibly exotic.

You know, what's very
interesting about a sand dune?

Sand dunes may sound dull,

but they can reveal a lot
about the mechanics of a planet.

Unlike the dunes
we know and love on the earth

that are made of sand,
the dunes on Pluto

are made entirely
of particles of ice.

There's only one thing
that can build dunes... wind.

Dunes are like a visual
representation of the wind

that's moving across the valley
and carrying the sands with it

and depositing it into these
big, beautiful dune forms.

Our planet is large enough

to hold on to an atmosphere.

Air, warmed by the Sun, rises.

Fresh air rushes in underneath,
generating winds.

Pluto is so small
and so far from the Sun,

it shouldn't have an atmosphere
or wind or dunes.

The problem Pluto has,

like other small bodies
in the solar system,

is that it's really hard for it
to hold on to an atmosphere.

It's just too small.

The very thin, light atmospheric
gases basically just escape.

Neither Haumea nor makemake

have detectable atmospheres,

but when new horizons
looked back at Pluto

as the dwarf planet
passed in front of the Sun,

scientists spotted
a thin haze of gas.

Turns out,
Pluto has an atmosphere.

But this atmosphere is temporary

because Pluto's orbit
is elliptical.

Some of the time,
it's far from the Sun.

Other times,
it's much closer and warmer,

creating a kind of
winter and summer.

Pluto's atmosphere
depends on the season.

In the summer, it's warm enough
to have an atmosphere,

and in the winter,
that atmosphere freezes out.

Over the course of just
a single orbit around the Sun,

the surfaces
of these dwarf planets

may change significantly,

condensing and coding out
atmosphere

when they're far from the Sun,

having that atmosphere
revolatilize

and redistribute the surface
when they're closer to the Sun.

Pluto is currently
in its summer phase.

The extra heat
during the long super summer

evaporates some of
the nitrogen ice on the surface,

creating a thin,
wispy atmosphere.

It turns out that even though

the atmosphere of Pluto
is very thin, there is wind.

It's really light,
but there's just enough wind

to be able to carry particles
with it once they start moving.

The seasonal cycle

could help explain
Eris' brightness.

Eris is three times
further away from the Sun

than Pluto is,

but when you put
a nitrogen atmosphere

three times further away,

that nitrogen freezes solid
to the surface.

Eris could be an indicator

of what Pluto looks like
when it enters its winter.

The gases will freeze, and it'll
become even more reflective.

In winter,

Pluto's dunes
will be locked in place...

Frozen on the surface,

unlike the icy features
of another dwarf planet

and the case of
the vanishing volcanoes.

From afar,
the dwarf planet Ceres

looks uniform and dull...

...but up close, one huge
feature comes into view.

One of the strangest objects
that we saw

when we began to map
the surface of Ceres

was something called Ahuna Mons.

Now, this was a strange, jutting
hill, very, very sharp sides,

and it didn't match any of
the other terrain on Ceres.

Ahuna Mons is a very
peculiar feature on Ceres.

This is a mountain that is
standing three miles high,

and there's nothing else like it

on the entire surface of Ceres.

Ahuna Mons dominates
the landscape of Ceres.

With its steep sides
and enormous height,

it looks a lot
like volcanoes on earth,

but earth is still
geologically active.

Ceres is so small, its molten
core should be frozen solid.

Planetary scientist Nina Lanza

heads to one of the most
volcanically active places

on earth... Iceland.

She has a drone's-eye view
of mount Helgafell,

a volcano similar in shape
to Ahuna Mons.

So this volcano is
what's called a rhyolitic dome,

and so it's a type of lava

that kind of gets squeezed out
through fissures,

and then forms
this kind of blobby dome feature

that gets pushed up by the magma
coming up from beneath.

On earth,
red-hot magma bubbles slowly

out of cracks in the surface,
building a steep-sided volcano.

But dwarf planets like Ceres
are too small

to have a hot core of
molten rock to power volcanism.

There isn't molten rock
on these smaller worlds

that have a lot of ice on them.

Instead, what's molten
is water under the surface,

and if the water can work
its way up through cracks

and erupt out in the surface,
you get a volcano.

But it's a cold-water volcano.

We call these cryovolcanoes.

Liquid water squeezes up

through fissures in the surface.

It quickly freezes,
building the mountain.

This volcano, you can see that
it's a pretty young feature,

and it's not very eroded.

We expect on earth
that wind and water

will slowly erode
this mountain away.

With no wind or weather

to erode Ceres' cryovolcanoes,
once created,

they should remain
on the surface

for billions of years.

Ahuna Mons is very strange

because it's the only
tall mountain on Ceres.

Why should that be?

You don't typically get
just one of something.

You should have dozens of them,
and, in fact,

Ceres may have had quite a few
cryovolcanoes in the past,

but they're all gone today.

Just to put that
into perspective,

imagine if this is
the only mountain on earth.

Why would there only be
one mountain?

What would that mean?

This leads us to ask
the question, you know,

are the volcanoes on Ceres
disappearing?

The idea that volcanoes
are vanishing...

It just sounds
totally science fiction,

and really,
not realistic at all.

Of course,
volcanoes can't just vanish,

but actually,
in the right context,

in certain scenarios,
they actually can.

The key to this magic trick...
Gravity.

It can flatten solid matter.

How quickly depends on

the structural composition
of the material.

If you want to build
a sand castle on the beach,

you can't use dry sand.

It doesn't stick together,

so you want to mix
a little bit of water in there

so that when you make
the structure,

it holds together,

but if you mix in too much
water, it just dribbles away.

It viscously relaxes.

It slumps.

Even Iceland's rock volcanoes

are slowly slumping
under their own weight.

Strange as it is
to imagine this,

it turns out
the mountain behind me

is actually slowly
relaxing back down.

It's just happening very slowly,

not on a time scale
that we can directly observe.

It may be that
there were many cryovolcanoes

on the surface of Ceres.

They no longer show
any trace of their existence...

So if we waited around
a little bit longer

until all of Ahuna Mons
had slowly relaxed

back into the planet,
we'd see no trace of it, either.

Maybe Ahuna Mons
hasn't always stood alone.

Maybe it's just
the last of its kind.

Ceres continues
to confound our expectations,

and there are still
many mysteries

with the other dwarf planets
to be solved,

such as how they got their moons

and why Pluto lies on its side.

Just like their larger cousins,

dwarf planets often
have orbiting satellites.

We now realize that all of
the largest dwarf planets

have moons around them,
have a moon.

Most of them have one.
Haumea has two.

Pluto has five.

Four billion years ago,

the young solar system
was chaotic,

filled with small bodies
orbiting the Sun.

One hit the infant earth,
forming the moon.

Smash-ups like these happened
throughout the solar system.

The dwarf planet Haumea formed
from an explosive collision

between two larger objects...

Which may account for
its unusual bean-like shape.

All the dwarf planets
suffered huge impacts.

Haumea had this big one
that left it spinning.

Eris has a tiny moon,
presumably from a giant impact.

Makemake has one.

All these biggest objects have
these tiny fragments of moons

showing us their history
of just getting battered

and pieces
being knocked off everywhere.

Most dwarf planets'
moons are tiny,

not much bigger than asteroids,

but one moon
is very different...

Pluto's moon, Charon.

Pluto's moon is, if anything,
weirder than Pluto itself.

It's Frankenstein's moon.

It looks like somebody
tore a moon apart

and then just kind of
slapdashed it back together.

One hemisphere is smooth.

One is very rugged.

It's got a canyon that's like a
notched carved out of the side.

It is really bizarre.

An impact may have formed Charon

and left it tied to Pluto

in an oddly
codependent relationship.

In some ways, you can think of

the Pluto-Charon system
as almost a binary planet.

There is no other planet
in the solar system

where the moon is so large in
proportion to it and so close.

Like other binary objects,

Pluto and Charon orbit around
a central gravitational point.

Locked in
this gravitational dance,

Pluto and Charon always
show each other the same face.

One of the really interesting
things about Pluto and Charon

is that they're
what we call tidally locked.

When Pluto and Charon formed,

they were probably both rotating
on their own axes,

but the two worlds actually
slowed down their rotation

and locked together,

with one side constantly facing
the other as they orbit around.

But Pluto's rotation
is tipped over

like a top spinning on its side,

so Charon's orbit around Pluto
is also tipped over.

Almost every planet in the
solar system has an orbital axis

that points in roughly
the same direction.

Pluto's is tilted down
about 120 degrees.

Scientists have long wondered

what caused this disparity.

Did Charon pull Pluto over?

Or is the tilt a result of
the impact that formed Charon?

A clue was revealed
when new horizons

sent back images
of Pluto's heart.

One of the more endearing
features of Pluto

as the new horizon's probe
approached it

was a gigantic
heart-shaped region

on the side of Pluto
facing the spacecraft.

Sputnik Planitia,
a bright, white heart

against Pluto's dark,
pockmarked surface.

When we got closeups of this,
it was completely fascinating.

I gasped out loud.

This is how shocking this was,
and I remember saying,

"oh, my gosh.
There are no craters there!"

It is smooth,
like it's a frozen-over lake.

This is indicative
of something liquid,

something flowing
under the surface of Pluto,

and what we're seeing
is the top, frozen layer of it.

There are even convection cells

where the ice appears to be
warming and spreading out.

That suggests that underneath,
there's a source of energy,

and amazingly, there may even be
a huge basin of liquid water

under that ice.

Sputnik Planitia
may hide a giant,

subterranean ocean
of liquid water.

It's also a gigantic scar
on Pluto's surface.

Most likely,
given the shape and size,

Sputnik Planitia was formed
in a giant impact.

Something smacked into Pluto.

Could the combination
of subsurface water

and an impact account for
Pluto's unusual tilt?

One theory suggests
that an object

smashed into the top of Pluto.

The impact
shattered the surface,

and water oozed up
to fill the crater.

The liquid water
knocked Pluto off balance,

and the gravitational dance
with Charon

spun this heavy heart
out to the opposite side.

One idea is that Sputnik
Planitia formed where it is

because ices
can accumulate in the floor

of a giant impact base.

But it's not yet certain whether
that's actually the case or not.

Dwarf planets,
once thought to be dead lumps,

have come alive with mysteries.

They've challenged
all our assumptions,

and yet, we've barely
scratched the surface

of these perplexing worlds.

There are many more
dwarf planets to discover,

and who knows what surprises
they may have in store?

We don't know the final count
of dwarf planets

because we're still
finding them,

but I think that
there are probably somewhere

between 100 and 200
dwarf planets out past Pluto.

There are probably many,
many more as you go

even further out
in the solar system.

Dwarf planets are, perhaps,
the most interesting objects

we've found in the solar system.

They're diverse.
They're geologically active.

They contain liquid water.

Just because they're small,

that doesn't mean
they're insignificant

or they should be ignored.

They are where it's at.

For me, that is just
the best, the most exciting.

We have all of
these new worlds to study

that we didn't even dream
existed just a few years ago.

That's science.