All language subtitles for How.The.Universe.Works.S09E06.Mystery.Of.Alien.Worlds.1080p.AMZN.WEB-DL.DDP2.0.H.264-alfaHD_track4_[eng]

NARRATOR: We're discovering
the cosmos is full of alien planets.

It's so exciting. In my lifetime,
we didn't even know
if exoplanets existed

and now they're everywhere.
It's incredible.

Exoplanets, strange worlds
outside our solar system.

Hellishly hot worlds,
violently colliding worlds,

worlds getting eaten by their stars.

There's much, much more out there
than we had ever imagined.

Exoplanets are rewriting
what makes a planet a planet.

It's a bit of a mystery
how these planets can even exist.

It seems to defy
the laws of physics.

Alien worlds that challenge our
understanding of planetary systems.

It's actually been a bit
of a wake-up call.

JESSIE: We think
we understand what's happening

and then the universe surprises us
with something completely different.

Exoplanets are shaking up our
understanding of the universe.

The cosmos is a chaotic array
of the odd, the weird
and the wonderful.

The more we find, the less we know.

We've now found
over 4,000 exoplanets,

a rapidly increasing array
of strange alien worlds.

and the more we uncover,
the weirder they get.

They don't act at all like what
we're seeing in our solar system.

There are planets
out there interacting.

There are planets dive-bombing
their sun,

gigantic planets orbiting
really close in.

Everything, in every
kind of combination
you can possibly imagine.

One alien world truly stands out.

This is the planet from hell.

When we examined
the atmosphere of this planet,
what we find is liquid iron.

The iron is heated up so much,
it's been vaporised and it's
falling out of the sky like rain.

But why is this planet
so much more extreme
than the ones in our solar system?

What this is telling us is that
the universe is really good

at making lots of planets
that are wildly different
than the one we live on.

AOMAWA: Outside of our apparently
stable and calm solar system,

it is the Wild Wild West out in
the cosmos

where crazy stuff is happening,
completely unchecked.

WASP-76b is 640 light years away,
in the Pisces constellation.

At first this planet looks
like nothing out of the ordinary.

WASP-76b orbits a star
just like our Sun,

which is really reassuring
in a universe
which is full of the unfamiliar.

WASP-76b is a gas giant,

a bit like Jupiter
in our solar system

but it's location
makes a big difference.

You take a star similar to ours,

you take a planet
similar to Jupiter,

but instead of parking it
in the outer solar system,

you put it really,
really close to the star.

Jupiter is almost 500 million
miles away from the Sun.

WASP-76b is just 3,000,000 miles
from its star.

And that's what makes this planet
a hot Jupiter.

Temperatures on WASP-76b
exceed 4,000 degree F,

creating one of the most extreme
environments in the universe.

If I were fortunate enough to be
able to go and visit this world,

I would have to take a lot
of precautions

because it is essentially a
hellscape on that planet.

JANI: The heat is absolutely insane.

There is nothing like it
in our solar system.

The fact that it's so close
to its star has another consequence.

WASP-76b's spin is
locked to its star.

The gravity from the star will grip
onto the planet,

slow its rotation over time
if it had any to start with

and lock it so that one face
always faces towards the star.

This gravitational grip
is called tidal locking.

We're used to the idea of tidal
locking with our own moon.

We only ever see
one side of the moon,

the far side of the moon
continues to face space.

There are consequences for being
a tidally locked planet and not
all of them are good.

That can set up some pretty extreme
weather conditions,

very hot on the daytime side

and extremely cold
on the nighttime side.

In 2020, we took a closer look

at the atmosphere between the day
and night side of the planet.

This twilight zone has plenty of
rain, but here it rains molten iron.

I mean, this thing has iron rain.

If you were writing a sci-fi novel,

"Oh, let's make some iron rain?"

But this is reality.

It is hot enough to vaporize iron
and make it rain.

When I grew up watching sci-fi on TV
and reading novels,

there'd always be a planet where
there was a strange condition,

"Oh, iron rains out of the sky,"
and I'd be like,
"That's ridiculous,"

and now what I found out is that

nature is way nuttier than anything
we could have thought of.

How do you get vaporized iron?

Well, most materials can exist
in different states.

So think about water, right?

Water can be a solid when it's ice

and then when you heat it up,
it becomes

water, the liquid part of water

and then if you heat it up more,
it becomes steam,
like out of a kettle.

And this is true
of every chemical element.

So for iron, if you heat it
even more up, it becomes a gas.

So you really can have clouds
of iron vapour,

condensing and raining liquid iron.

These nightmare weather conditions

are a direct result
of WASP-76b's proximity to its star.

WASP-76b is so close to its star
that its star is superheating
its atmosphere.

So the upper atmosphere
is heated and rises.

The atmosphere on the day side
reaches over 4,000 degree F.

The planet's night side
is cooler at 2,730 degree F.

This difference in temperature sets
up spectacular wind streams.

One of the really cool things
about this brand new class
of planets we've found

is discovering weather
we've never seen before.

These hot Jupiters have
these equatorial jets of wind

that are supersonic, traveling
at thousands of miles per hour

and that wind is pulling the rain
around to the night side.

The air on the hot side expands
because it's being heated

and will flow over to the other side
so you get these torrential winds

blowing that hot air
to the cooler side.

If there is vaporized iron,
gaseous iron in the atmosphere
on the hot side,

it will blow over
to the cooler side.

On Earth, the fastest recorded winds
have reached speeds in excess
of 250 miles an hour.

On WASP-76b, winds hit speeds
in excess of 11,000 miles an hour,

strong enough to move
millions of tons of iron vapor

to the planet's night side
where it undergoes
a dramatic change.

It's cooler there,
can't be maintained as a gas

so it condenses and becomes a liquid
and then rains out.

There are clouds coming up
and forming and then rain is falling

but it's iron, it's iron vapor,
it's iron rain.

It would be spectacular to see
in that brief moment you have
before you vaporize too. (LAUGHS)

(INTENSE MUSIC PLAYING)

You might think
that having clouds of iron rain

is WASP-76b's strangest feature.

But astronomers are even
more puzzled by the location
of this gas giant.

When you look at the planets
in our solar system,

you can divide them
into gas giants like Jupiter

and rocky planets like the Earth.

In our solar system, rocky planets
are close to the sun

and gas giants are farther away.

But in exosystems, the positions
of different kinds of planets
are all messed up.

KEVIN: We're finding Jupiter-sized
planets super close to their stars,

instead of in the outer
parts of the solar system.

And we're finding rocky planets
really close to stars

and packed in really tight
in weird configurations.

These planets orbiting
close to their stars

survive being blasted
with intense radiation.

They're taking part
in the ultimate endurance challenge.

But not all worlds are so tough.

Some are so light and delicate,
they're barely there at all.

Are these weird puffballs
even planets?

The more exoplanets we find,
the more we realize

how weird
these new worlds really are.

Some planets are so unlikely,
so odd and so bizarre,

scientists wonder,
how can they even exist.

Before we discovered exoplanets,

we thought that our solar system
would be representative,

that other solar systems
would look like ours,

rocky planets, gas giants,
ice giants.

When we went out there
and found them,

they don't look
anything like our solar system.

In 2012, we discovered
three gas giants

orbiting the sun-like star
Kepler-51,

located 2,615 light years away
in the constellation of Cygnus.

At first there seemed
to be little to distinguish
these planets from Jupiter.

Then, in 2019,
we took a closer look.

You go into a system like Kepler-51,
a Sun-like star

and you kind of go in expecting
or hoping to find,

you know, Earth-like planets,
planets familiar from our own
solar system.

And then you find
something like this

and you're kinda like, you know,
"What the heck is that?"

We've been hunting for exoplanets

and we've gotten used to some
weird things but this is
truly out there.

This is a truly alien scenario.

According to how we think
planets are formed,

the worlds orbiting Kepler-51
shouldn't exist.

These three objects
orbiting Kepler-51

are kind of like cosmic conmen

because they appear
to be like Jupiter

but in fact their masses
are just a few times that of Earth.

These are planets that have, like,
a tenth of the density of water.

If you could throw these things
into a giant ocean,
they would float.

The super puff planets form
from helium and hydrogen,
just like Jupiter.

But unlike Jupiter,
gas on the super puffs
is not densely packed together.

It's loose,
creating big fluffy balls.

So even though they're
the same size as Jupiter,

their mass is just one percent

That's like a heavyweight fighter
with the mass of a prairie dog.

The super puff planet,
which is the greatest name
ever for a planet,

is really a very low-density planet.

Really what it means is that it's
very, very fluffy and light.

It's almost like it
has a light snow-like consistency.

The extremely low mass
of these planets presents
a problem for planetary scientists.

This is an incredibly
unlikely situation.

How can these super puff worlds
even exist?

Gas giants like Jupiter start
with an ice and rock core.

This core grows until it generates
enough gravity to pull in gas,

building an atmosphere
almost 2,000 miles deep.

Do the super puff planets
form the same way?

It's a bit of a mystery how these
planets can even exist,

just based on what we know about
planet formation.

It's really an unusual thing
to have something that is so light

because that's not how planets
that we recognize typically form.

Scientists have a theory about
how the super puff planets formed.

DAN: We see these three planets
relatively close
to their parent star today

but very likely,
given their composition,

they probably formed
a lot farther away,
beyond the snow line as we call it.

Star systems split into two regions,

a warm inner region
close to the star,

and a colder outer region
farther away.

The snow line separates
the two zones.

Gas planets only form
outside the snow line,

far from the star where gas
can clump together.

You're actually able to grab onto
a lot of hydrogen and helium
and, and build up an atmosphere.

Beyond the snow line,
water condenses to a solid form.

This process greatly boosts
the formation of minute planets.

These icy planetesimals jumpstart
the rapid growth

of what will become gas giants.

The super puffs formed even
farther out than Jupiter,
in a far colder area.

Compared to Jupiter, the super puffs

had a relatively small core

but because they developed
in this colder region,

they still pulled in a huge quantity
of hydrogen and helium.

You end up with something
that is kind of

large in size but still really
low density, low in mass.

These planets have been growing in
size for around 500 million years

and as they've been growing,
they've also been moving.

The gravity of their parent star can
pull these planets closer

so we can see chains
of super puff planets,

like, like cars on a train,
all marching inwards
towards the star.

The closer they get to the star,

the more stellar winds batter
the super puffs.

The winds blast off the loose
puffy atmosphere

in a process
called photo evaporation.

This process of photo evaporation

results in these planets losing
their atmospheres,

literally losing billions of tons
of atmosphere every second.

These super puffs
are like orbiting dandelions

that are getting blown away
in the wind.

Scientists predict that over the
next four and a half billion years,

the super puff planet
closest to its star

will lose all its atmosphere,

leaving a planet with a radius
smaller than Neptune.

The other two super puff planets
will escape largely unscathed.

We are rapidly discovering a wide
range of weird oddball worlds.

Planet hunters are also searching
for something more familiar.

For all these treasures
that we've been digging up,

we haven't found the crown jewels,

a planet similar to Earth.

But just what are our chances
of finding an Earth-like planet?

Finding a world
that replicates the Earth

is really the holy grail
of searching for exoplanets,

and I think it's out there,

we just need to keep looking
and keep developing new technology
and ways of exploring.

PHIL: The big question, are we going
to find another Earth?

The answer is, "Yes, we will."

And the reason I say that is because

there are a lot of planets
in our galaxy,

and just looking
at really rough numbers,

there are probably billions
of planets similar to Earth,

and mixing and matching
all of those conditions,

it seems to me that the way to bet
is that some other planet,

at least one, if not a lot,
are gonna look a lot like our own.

The numbers are so big out
there in the universe.

There's gotta be
something like Earth.

There's gotta be
a few things like Earth.

There probably is an exact replica
of Earth somewhere.

Will we find it in our galaxy?
Probably not.

We're gonna have to accept something
that's a bit different from
what we picture as perfect.

I look at that variety thinking
how much more is possible

and how many more worlds
are there out there with life,
not as we know it.

So for me I think it's glorious.

There's so much more
possibility for life than
we previously imagined.

Finding an exoplanet
with conditions suitable for life
takes a lot of luck.

Sifting through these exoplanets,
looking for something
that's habitable for life

is like an interstellar dating app.

If we have molten iron rain,
that's definitely out.

You see toxic atmosphere,
and you swipe.

And you see red giant,
and you swipe.

So it's like, "Oh, too hot,
too cold, too small,
too thick an atmosphere."

No UV rays, no, no, no, doesn't even
have a star, but, like,

it's just not working again
and again and again.

When it comes to finding life,
there is one basic element
that everyone agrees is necessary.

There is a phrase that we use
whenever we talk about
the search for life elsewhere.

Follow the water.

And now we think there could be
lots of worlds out there
that do contain water.

But is there a catch?
Could they hold too much water?

A 2019 study suggests the Milky Way
might contain many worlds

with thousands of times more
water than Earth.

Many of these planets
are a bit smaller than Neptune.

We call them sub-Neptunes.

What they found were
these sub-Neptunes,

planets smaller than Neptune
but bigger than Earth,

unlike any planets we'd seen before.

We think we've found such a planet,

just 40 light years from Earth
in the constellation Ophiuchus.

Scientists have nicknamed the planet
the Water World.

GJ1214b could be one
of these sub-Neptunes

with more water than we would know
what to do with.

So far, we're not too sure
what GJ1214b looks like.

Though Earth is called
the Blue Planet,

it's only 0.05 percent
water by mass.

As much as 70 percent
of GJ1214b's mass could be water.

The planet is thought to have
a rocky core, strange oceans

and a hot steamy atmosphere
of water vapor.

We spend a lot of time looking
for very small amounts
of water to establish

whether or not a planet
could even be habitable

and so it's kind of amazing
that we just found this planet

that was essentially nothing
but water.

Unlike Earth, GJ1214b

most likely has no complex
arrangement of water
and land masses.

The lack of interaction between
stable land masses

and a, and a healthy long-term
stable ocean might
really be a killer

and you might need that land
interacting with that water

to have a good location for life.

We think life began in the oceans
but it needed chemicals
from rocks to start.

Without the interaction between
land and oceans,

life might not have evolved.

Not only is there no land sea
relationship on GJ1214b,

evolution here may be limited
in another way.

Earth's oceans are replenished with
chemicals from hydrothermal vents

thousands of feet down
on the seabed.

GJ1214b's ocean floors
are thousands of miles deep.

Right at the bottom of these
incredibly deep oceans,

you've got very high pressures,
you've got so much water above you

and you've got
very cold temperatures.

You're really being sheltered
from any incoming solar
radiation or sunlight.

So, the water itself
could turn to ice.

Most ice on Earth is called ice 1.

When ice is subject
to increasing pressure,

its categorization number goes up.

We think the ice
on GJ1214b is ice 7,

the type of ice we believe to be
on moons like Enceladus and Europa.

On GJ1214b, we believe ice 7
seals off the seabed,

preventing potential nutrients
from the rocky core
from passing into the ocean.

We've been following the water.
That's been the, the key to trying
to understand astrobiology

and then we, we find these
worlds where it's too much
of a good thing,

there's too much water for perhaps
for life to exist.

So it's certainly
one of those things

that a little you need but maybe
too much is bad too.

We need to find worlds with
just the right amount of water
and land for life to evolve.

GJ1214b looks like a dead end

but the hunt goes on.

Space is big and I like the idea
that it's not just for us.

So I'm hopeful. Whether it'll be in
my lifetime or my daughter's,
I don't know, but I'm hopeful.

As we continue to probe the cosmos,
we've discovered one
hopeful distant object.

A moon. But this exo-moon
is a monster.

It's four times larger than Earth.

So how did it get so big?

Each time we find new stars
and their weird worlds,

we have to rethink the rules
of our own planetary system.

We think we understand
what's happening

and then the universe surprises us
with something completely different.

Our search for exoplanets
has been remarkably successful

but we've yet to spot those
highly familiar objects that orbit
many planets in the solar system.

It's been an incredibly
exciting time,
finding over 4,000 exoplanets

but there's still something
we haven't found that we're
really excited by, exo-moons.

We expect to see exo-moons
around exoplanets

because our own solar system
is full of moons.

Almost all the planets in our solar
system have moons around them.

In fact Earth is the only planet
that only has one moon.
Most have more.

So the question is, are moons
unusual in general for planets

or are we just not seeing
the moons that are out there.

Astronomers find exoplanets
when they pass in front of a star.

It's called a transit

and creates a dip or a wobble
in the light from the star.

But moons pose a problem.

They're incredibly small,
so to find even one
we'd have to be very lucky.

In October 2017,

astronomers took a closer look
at a star 8,000 light years away.

The light dipped
as a Jupiter-sized exoplanet
passed in front of the star,

then three and a half hours later,
they saw the light dip again.

There was actually evidence
as this planet transited

and went across the star
blocking out a little bit of light
of the star

that there was another large object
rotating around the planet.

The planet, Kepler-1625b,
appeared to have a companion
orbiting around it.

By looking at the light
that was coming from the system

and how it was changing,
they thought they'd discovered
the first exo-moon.

And that was really exciting.

Known as Kepler-1625b I,
this exo-moon candidate

caused a significant dip
in the light

and that could only mean one thing.

When we analysed the signal
caused by this potential moon,

it must have been
caused by something
four times the width of Earth,

so something
like the size of Neptune

and we have no moons
in our solar system
that are Neptune-sized.

In our solar system,
objects the size of Neptune
are planets, not moons.

Neptune and planets
of a similar mass
are ice and gas giants.

Moons in our solar system
don't have this composition.

They're all solid.

Just when we thought
we understood moons
and how they worked,

now here comes an exoplanet
to tell us, "Not so fast."

One problem with this system
is we don't have many good ideas
for how it formed.

Everything we know
about moon formation

comes from solid moons.

There are two main ways
that we think moons can form.

The first is you have a rocky world,
something comes in and smacks it.

And the thing that smacks it
plus the debris that's ejected
from that world

then go on to form a new moon
which is how we think
the Earth's moon formed.

Another way potentially
is that when that planet was forming

and there was a big cloud of dust
and it was swirling around

that the moons formed
out of that dust at the same time
as the planet.

But there may be another way
the moon orbiting Kepler-1525b
could have formed.

An exo-moon doesn't have to form
around the planet itself,

like we see around
Jupiter or Saturn,

but instead let's imagine
there's some kind of rogue planet

wandering by a larger planet
and it gets captured

and becomes a moon.

Perhaps, billions of years ago,

the planetary core of Kepler-1625b

grows in a disc of gas and dust.

It's not alone.

Nearby another protoplanet forms.

It's a little bit like twins.

Each twin is going to argue
for their own amount of resources
in the womb

and that's sort of
the same thing happening here.
It's a battle for resources.

Kepler-1625b grabs more gas
and dust than its twin,

growing larger and larger.

The now huge exoplanet slowly drags
its smaller sibling closer,

eventually pulling it into orbit.

The smaller protoplanet
becomes Kepler-1625b's moon.

The one thing that exoplanets
have taught us

is that we have no idea how systems
in our universe have to evolve

and so it's completely feasible
that this...

there is a really large
Neptune-sized moon
around a host planet

and we just need more evidence
in order to,

to make sure that that's true.

Scientists are confident
that such evidence will be found

when new technology comes online.

But sometimes, astronomers spot
things that make them

doubt their own instruments

events like a planet disappearing.

We've discovered some
extraordinary exoplanets.

Super-hot worlds
with molten iron rain.

Super puff planets so fragile
they might blow away.

Exoplanets that defy physics.

But stranger still is the case
of the disappearing planet.

Over a decade ago,
the Hubble telescope spotted
a planet orbiting Fomalhaut,

one of the brightest stars
in the night sky.

Fomalhaut is a very nearby,
very young star and the images
of this system are incredible

because what you see is the central
star surrounded by a bright ring.

It looks just like
the eye of Sauron.

We observed the new planet,
called Fomalhaut b, for six years.

Then something surprising happened.

All of a sudden,
it just wasn't there anymore.
Where did this planet go?

For it to suddenly be gone,
it was amazing.

It was astounding.
It was terrifying.

If Fomalhaut b can suddenly vanish,
what could that mean
for other planets?

And us?

We live on a planet, so we have
a vested interest in understanding

how planets could disappear,
if that's a phenomenon that exists.

October 2019...

Astronomers investigate the idea
of a vanishing planet

by looking at BD+20 307,

a star system straight
out of the movies.

Just like that iconic image
from Star Wars,

from Tatooine where you look up
and there's two stars in the sky,

that's actually not that crazy.

Out in the Wild Wild West
of the universe,

you have lots of different
star systems.

In fact it's more common to have
pairs of stars orbiting each other

than to have stars by themselves.

If two star systems are the norm,

what makes BD+20 307 different?

The two stars lie within
a bright disc of gas and dust,
like Fomalhaut.

But Fomalhaut is a young system,

less than 500 million years old.

BD+20 307 is a billion years old

and that's weird because
the material in the disc is so old,

it should have formed new planets
long ago.

So what's going on?

Rings of dust are characteristic
of young planetary systems.

What does it mean when we see
a disc of material surrounding
an older star,

a star over a billion years old?

Well, one thing could be
the collisions of planets.

We think that in this system,
planets collided and
that formed the disc that we see.

When planets collide,
they don't just spew out
masses of material.

The violence of the event shakes up
the whole planetary neighbourhood.

Planetary objects come in with
such energy and such speed

that essentially they are
vaporising each other.

Observing a planet
essentially being destroyed

tells us something about what might
happen in our own solar system.

Our planets feel very stable
in their orbits

but we don't realize
that in the future,

those orbits might be very different
than they are today.

Early in its existence,
our solar system
was a demolition derby...

with many, many collisions.

It's how rocky planets
like ours formed.

Back then there were more than eight
or nine planets in our solar system.
There were hundreds.

and planets were running
and interacting with each other
all the time.

Eventually, the planets
we see today formed

and our solar system settled into
a nice regular arrangement.

Finding strange systems
like BD+20 307

makes us question that narrative.

One of the really valuable lessons
that astronomers have learned

from studying planets
around other stars is

that it appears very clear now

that planets don't necessarily
stay where they are
in a solar system.

Over time the orbits of planets in
our solar system slowly shift.

The repercussions of these orbital
fluctuations could shatter
our cosmic neighbourhood.

The odds are slim but billions
of years from now,

Mercury could be pulled out
of its orbit by gravitational
interactions with Jupiter.

This action would
set Mercury on a fateful course.

One potential future that our solar
system may have is actually

that Mercury could
collide with Earth,

which sounds crazy but would
also be a real bummer.

So what we see in BD+20 307
is theoretically possible
in our own solar system.

It's actually been a bit of
a wake-up call.

It's a transformation in our
understanding of how
our solar system works.

Studying other systems shows us
just how vulnerable planets can be.

Things can change at any time
in a planetary system,

that we could be watching a planet
on its orbit one day and poof,

it could suffer that
really big collision the next.

But could this all explain the case
of the Fomalhaut b system?

Could it have collided
with another planet,
wiping it out completely?

Well, maybe.

April 2020, astronomers
at the University of Arizona

come up with a new
theory about Fomalhaut.

Every good mystery needs
a shocking twist at the end

and the twist in this tale

could be that the planet disappeared
before Hubble's eyes

because it never was there
to begin with.

Instead of a planet that we thought
we'd captured inside the ring,

it was actually a collision
between two smaller objects
called planetesimals.

Planetesimals are infant planets,

bodies that measure from a few miles
to hundreds of miles across.

They smashed together
and created a huge dust cloud

which we caught with Hubble.

All we saw was a bright blob
of light that looked like a planet.

They didn't spot a planet
but they did learn
a very important lesson.

We're actually observing a process,
part of the way that solar systems
grow and are born

and in many ways, that's,
I think, more important
and more useful to us

than having spotted yet
another planet.

So to observe those characteristics
in this system

mean that we got really lucky
to observe it right now

because if we came back in hundreds
or tens of thousands of years,

that dust would have gone away,
it would have eliminated itself
and we wouldn't see it anymore.

So we got lucky
to see it right now
when it was still around.

That was still
a pretty neat thing to find,

the evidence of two small
unseen planets colliding

and that should remind you
that solar systems

are not necessarily all that safe,
the same thing could happen here.

For life to exist on another planet,

you'd typically wanna look
for a nice calm safe neighbourhood
for that life to exist.

And so if we have systems
where planets are disappearing
without a trace,

that's not a great sign.

So when we look at a system
that's a billion years old

and has apparently had
a huge collision between planets,

it's a good time to step back
and look at our solar system

and say, you know,
we're four and a half billion
years in

and so far our planets
are pretty stable.

Some things have moved around
here and there

but we're not seemingly at risk
of having two planets collide.

It's a scary lesson to learn,
the possibility
of ongoing planetary collisions.

Exoplanets are opening our eyes
to the way the universe works.

We must question
some long-held assumptions.

One standard text predicts the Sun
will eventually engulf the Earth.

But could there be a way out?
Do some planets cheat death?

In four and a half billion years,
our Sun will expand
to become a red giant.

When our own star turns
into a red giant

in four and a half billion years
from now,

then it will expand
and it will engulf

Mercury and Venus
and the Earth and the Moon

and it will cook the surfaces
of all of those bodies.

But is there a way
of escaping this apocalypse?

When we look beyond our solar system
to the Aquarius constellation,
we find hope.

Planet HD 203949 b
is living on borrowed time.

It orbits a red giant star.

Red giant stars have burned up
all the hydrogen in the middle

and they've moved to
the next stage of their development.

A stage that's terminal
for a planet orbiting this star.

If you're a planet
and you've been orbiting

fairly close to your star
for billions of years,

you might feel like you've got
a good relationship,

that it's pretty safe
but in fact you would be wrong.

In fact this star that has been
taking care of you

for billions of years
is now going to destroy you.

After billions of years
of generating heat and light,

a star's hydrogen fuel runs out.

The star's core
becomes unstable and contracts.

Gravity just pulls
everything to the centre.

And then there's a rebound.
Everything comes back again

and that creates this big envelope
of gas around the star.

The outer layers of gas blow off
and expand outwards.

As the gas envelope gets bigger,

the surface cools
to under 10,000 degree F.

The cooler stars appear red

so later in stars' lives,
they're big bloated red giants.

When a star goes red giant,
it expands and it expands outwards

and it's likely
that it's going to come

and engulf some of the planets
that orbit that star.

The surface has cooled
but temperatures
still exceed 8,000 degree F.

If you're in the red giant
expansion zone,
you're going to get cooked.

Exoplanet HD 203949 b orbits
within this zone.

So, is this planet toast?

September 2019,

we take a closer look
at the red giant

threatening the planet using
a technique called asteroseismology.

Asteroseismology measures
the vibrations of stars.

Asteroseismology applied to these stars

is a really useful way
to get at more information

than we might get
by just looking at their brightness
or their temperatures.

Vibrations go back and forth
within the stars

and we can see those
by monitoring the surface.

One of the things that happens
when these stars get to their
red giant phase

is they start ringing like a bell.

When we hear these stars ringing,

it actually gives us
the most precise information
we have about any stars.

We can measure their mass,
their radius, their density,

much more exquisitely
than any other star.

The vibrations
from the red giant star reveal
something highly surprising.

When we analysed
the way this star was ringing,

we realized it was actually
less massive than we'd determined
from other methods.

It told us that star probably has
already gone through
its red giant phase.

The star we see today is a little
smaller than it should have been
quite a while back.

This star has lost some of its outer
layers and has started to shrink.

If this star has already gone
through its red giant phase
and is shrinking again,

that means at one point
it was bigger than
the orbit of this planet.

If the planet was within
the red giant zone,
it should have been destroyed

but somehow it remained intact.

By all accounts,
this planet shouldn't exist

but somehow we see it there today,
cheating death. What a survivor.

So how can we explain
this escape act?

Could it be this planet
changed its orbital position
to allow it to cheat death?

Or maybe HD 203949 b was never even
in the kill zone.

Perhaps this planet originally
formed further out

and migrated in after the red
giant phase was completed.

Maybe some of the clouds of gas shed
from the star reached the planet.

This gas dragged on the planet,
slowing its orbit down.

Gradually the planet migrated
inwards after the star reached
its maximum size.

And we then evolve down
to the system that we see today,

a post red giant star with a planet
that shouldn't be there,
so to speak.

This exoplanet may have escaped
oblivion but its future
doesn't look bright.

Its star will shrink down to a cool,
dim white dwarf.

If I were a planet, you know,
I would be sad at the existence

that I would live afterward,
just because it
would be so different.

It would be cold and dark
and I would still be bound

with a star that is no longer there
in the same way that it was.

This is our future.
But it won't happen for another
five billion years.

In the meantime, we can be thankful
we live on Earth

rather than one of the weird worlds
we've discovered in our galaxy.

The more and more
exoplanets we find,

more we realize
how lucky we really are.

We see planets that are too big,
too small, too much atmosphere,

too little atmosphere,
too close to their star,

too far from their star,
too little water, too much water.

Everything on Earth is just right.

Compared to our home world,
exoplanets push
and twist and stretch

the boundaries of planetary science.

But every new world we discover
expands our knowledge and moves us

closer to understanding
our place in the universe.

If we can understand
how planets form

and why they form the way they do
and how they evolve,

then we can know our past, present
and future, even better.

I think as we find more
and more of these planets,

we're going to find out more about
our own solar system
and our place in this menagerie.

A lot of times we think
about other planets

and even life in the universe
as resembling very much our own

but these weird worlds
open the possibility

that there's much more out
there than we had ever imagined.

We've found so many different
kinds of crazy worlds
in crazy places doing crazy things.

It's so interesting.

Imagine how boring it would be
if we only found our solar system
everywhere else.

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