The Wildest Weather in the Universe Horizon

We all love talking about the weather.

Is it too hot or is it too cold?

Is it too wet or too windy?

It's a national obsession.

But now scientists have also started looking to the heavens...

..and wondering what the weather might be like on other planets.

We are witnessing the birth of extra terrestrial meteorology,

as technology is allowing astronomers to study the weather

on other planets like never before.

Jupiter has these very long-lived storms,

but Saturn has these very violent storms.

But, incredibly, today the latest telescopes are enabling astronomers

to find and study planets beyond our solar system.

So here we have our image of ROXs 12 b,

which is pretty amazing to think we are imaging a planet

400 light years away.

And our exploration of the universe is revealing alien worlds

with weather far stranger and more extreme than anyone

could have ever imagined.

A lot of the planets that we're studying so far

are very horrible places.

You wouldn't want to go there on vacation.

On these planets you can get the most gigantic storm systems

ever witnessed by mankind.

So one side of the planet can be roasting hot while at the same time

the other side of the planet can be freezing cold.

On some exoplanets, the temperatures are such that the clouds

and the rain can be made up of liquid lava droplets.

We thought we had extreme weather on Earth,

but it turns out that it's nothing compared to what's out there.

So instead of having rain which is liquid water droplets like here

on Earth, it would be raining liquid rubies.

And the search for the weirdest weather in the universe

is only just beginning.

It's a delightfully warm spring morning in Greenwich, London.

Astronomers of all ages have gathered at the Royal Observatory,

where they're hoping to witness a rather special event.

They're waiting to glimpse another world.

And, unusually for astronomers,

they've got their telescopes out during the day.

We're here to see quite a rare astronomical event.

I'm very excited to see it. I'm just hopeful, as we all are,

that the clouds don't come along around midday and stay with us.

It's a chance to get a unique perspective

of one of our nearest neighbours.

And to take a closer look,

astronomer Tom Kerss has set up the Great Equatorial Telescope

to look at the sun for the first time since 1927.

Because at exactly 12 minutes past noon,

the planet Mercury is due to pass in front of the sun.

So here is Mercury emerging onto the face of the sun,

looking very beautiful indeed.

Over the next seven and a half hours or so,

Mercury will gradually slink across the face of the sun

as it overtakes us on the inside track in the solar system,

about 52 million miles away from the Earth right now.

Ever since we've known about the existence of other planets,

we've wondered what these mysterious alien worlds might be like.

Could they be potential homes for life?

And is there any way of finding out?

So when wondering whether other planets might be habitable or not,

the key question we need to ask to begin with is what is the atmosphere

actually like? What is the climate like?

What's the weather like? Whether it would be very extreme or whether it

would be quite pleasant and stable,

the kind of weather we think is necessary for life.

So what will the weather be like on Mercury?

If we look back at the beginning of the Mercury transit we can see

a really clean bite taken out of the sun.

And the edge is so clean because Mercury doesn't have any appreciable

atmosphere to speak of.

With no real atmosphere, Mercury is effectively a dead and barren world.

Because Mercury lacks anything that we would call an atmosphere,

there's essentially no weather on Mercury at all.

Mercury is unusual in our solar system because all the other planets

do have atmospheres and so they must also have weather.

Death Valley, California,

one of the most extreme and alien environments on Earth.

Planetary explorer Suzanne Smerkar has come here because it shares

a surprising similarity to our nearest neighbour, Venus.

Venus is the brightest object in the night sky and the reason it's so

bright is because it's covered in thick clouds.

And when you turn your telescope to it,

you can see nothing of the surface,

all you see is this bright reflection coming back at you

because of the cloud deck that's kept it shrouded in mystery.

We do know that Venus is similar in size to Earth,

is a rocky world like our own, and also relatively close to us.

So what's its climate like?

We used to think that Venus was much like the Earth,

maybe 50 degrees hotter because it's that much closer to the sun.

We thought it had an atmosphere like the Earth,

we thought it would be cool enough to have oceans.

We even thought it was covered in steamy hot swamps,

probably covered with verdant green life.

But to discover what Venus was really like, we needed to go there.

At the dawn of the space age, people started to explore.

It was the Cold War in the '60s,

and the Soviets and the US were sending spacecraft after spacecraft,

trying to be the first out there.

A huge number of spacecraft have been hurled at Venus

and there were many attempts to get to the surface.

In the late 1960s, the Russians succeeded.

The one that finally made it to the surface was Venera 7 in 1967,

and that probe fell gently through the atmosphere,

got to the surface and survived for only about two hours.

Before they died, the Venera probes revealed the true nature

of Venus's climate.

Venus has a surface temperature of 462 Celsius,

which makes it the hottest place in the solar system.

And the atmospheric pressure on Venus is almost 100 times

that on the Earth.

With surface temperatures hot enough to melt lead,

an oppressive atmosphere of carbon dioxide, and belching clouds

made of sulphuric acid, Venus is a planetary vision of Hell.

We knew for the first time that Venus is not a swampy,

verdant region teeming with life,

but instead it's a hellish, hot inferno.

Venus is the hottest planet in the solar system,

but it's not the closest to the sun.

Sue has come to Death Valley where the unbearable temperatures

are created by the same phenomenon at work on Venus.

We are here today in Death Valley, the hottest place on Earth.

The temperature today is...

42 degrees.

Pretty balmy for Death Valley -

the hottest recorded temperature is 57 degrees

so we have it easy today.

The reason that it is so hot here is that we are at 86m below sea level

and that means that we have about 86m more atmosphere here

and that means it's higher pressure and in fact the pressure measurement

here is 1,016 bars.

And that extra bit of pressure is really what's giving us

this intense heat that we are experiencing today.

It's like adding another layer of insulation or another blanket

that's holding the heat in.

And by simply driving uphill,

Sue can reveal the tremendous insulating power of the atmosphere.

So now we are at about 1,000m and it's already

looking greener and a bit cooler up here.

At Dante's View, almost 2km above the valley floor,

the temperature is much cooler.

We're at about 1.7km above the valley floor,

where we were earlier today, and the temperature is 30 degrees Celsius,

way cooler than the 42 degrees down there.

And the reason it's so much cooler up here is that we have

that 1.7km less air.

Our pressure is 831 bars, down below it was 1,016.

So the pressure is much lower.

We have a lot less atmosphere above us,

and as a result it's much cooler and much more pleasant up here.

On Earth, the temperature typically increases

by about 6.5 degrees Celsius for every kilometre you descend.

On Venus, with its much deeper atmosphere than Earth,

this insulating effect is taken to its extreme.

It is so much hotter on Venus because the pressure is at 92 bars,

almost 100 times that on the Earth, and the atmosphere is much thicker,

much denser, and it really holds that heat in,

making Venus the incredible inferno that it is.

On top of this, Venus's atmosphere is almost entirely made up

of the greenhouse gas carbon dioxide.

And this combines with the intense pressure to make Venus

the hottest planet in the solar system.

Our next nearest neighbour couldn't be more different.

Venus and Mars are like chalk and cheese.

So Mars is the opposite extreme from Venus.

It's atmosphere is 1/100th the pressure of Earth's

and the effect of having that really low atmospheric pressure on Mars

means that it can't trap any of its heat.

So Mars is a cold,

barren desert compared to Earth or to Venus.

Because of its thin atmosphere,

Mars is home to some spectacular weather phenomena.

The rovers sent by NASA revealed that Mars is scoured

by super-size dust devils,

reaching up to a kilometre in height.

But even more impressive are the dust storms,

which dwarf those on Earth.

In the thin atmosphere of Mars,

the dust storms can get to a very high elevation,

they can get to about 20km above the surface.

And because most of Mars is a dry, dusty desert,

these dust storms can cover vast expanses.

In fact, it seems there is no limit to how far they can spread.

Every few years an enormous dust storm will grow

until the entire planet is engulfed.

Incredibly, storms like these have been shown to envelop

the whole of Mars for over two months.

WEATHER REPORT: For England it's a hot and sunny day for all, any mist and fog clearing away quickly...

Further out in the solar system, the weather gets even wilder.

It's another fine sunny day in Pasadena,

home of NASA's Jet Propulsion Laboratory.

Andrew Ingersoll is the father of extra-terrestrial meteorology.

He's come to the Deep Space Operations Centre.

It's mission control for the small fleet of spacecraft that NASA

has sent to explore the outer reaches of the solar system.

Andy has worked on all these missions.

So we've had a whole series of spacecraft

visiting the giant planets.

The first big one was Voyager in the '70s,

which zoomed past all the giant planets.

Then there was Galileo.

Then Cassini which has been in orbit around Saturn for ten years.

And now we have Juno in orbit around Jupiter.

The spacecraft have given us an unprecedented view of the weather

on these planets.

The outer planets are big balls of gas and that makes a huge difference

in the weather,

so there's lots of room for weather.

And because you don't have continents,

you don't have mountains for the winds to rub against,

and there's nothing to control the weather the way the continents

partly control our weather.

This means these planets have storms on an entirely different scale

to ours.

And the most famous storm of all has to be Jupiter's Great Red Spot.

The Great Red Spot is a huge storm in Jupiter's atmosphere.

You could put two Earths inside the Red Spot,

and the winds going around the periphery of the Red Spot

are about three times the speed of the Earth's jet streams.

With winds whipping round at about 650km per hour,

and releasing so much energy that it heats the atmosphere above it

to around 1,400 degrees Celsius,

the Red Spot has been raging for as long as we on Earth

have been able to observe Jupiter.

Shortly after Galileo built the first telescope,

people were using these primitive telescopes to look at Jupiter

and they saw this storm.

And it's apparently been there ever since, which is remarkable,

compared with Earth storms.

The Red Spot has been there for over 350 years and that makes it

the longest-living storm that we know of.

Jupiter may have the longest-lasting storm,

but it's Saturn, the next gas giant,

that is home to the largest and most powerful storm ever seen

in the solar system.

And in 2010, the Cassini spacecraft was there to see it.

Saturn, of course, is a spectacular sight because of the rings,

and it's also rather boring as far as the weather is concerned.

It's a bland thing.

But every now and then - 20, 30 years -

Saturn erupts with a giant storm,

and Cassini was fortunate to be orbiting Saturn at the time

of one of these eruptions.

What happened was, on December 5th 2010,

the radio receiver on Cassini started picking up the radio signal

of lightning.

And on the same day the camera saw a little storm up in

the northern hemisphere of Saturn.

By January it had developed into a fair-sized thing,

and then we watched it for six months.

During that time a huge storm grew and wrapped itself

around the entire planet, covering 4 billion square kilometres

until its head caught up with its tail.

Driven by winds going at around 1,800km per hour, with huge

lightning flashes 10,000 times stronger than those we get on Earth.

It's very funny, Jupiter has these very long-lived storms,

but Saturn has these very violent storms.

We don't fully understand why there is this difference in the weather

between Jupiter and Saturn.

Whether it's duration or size,

the storms on both these planets dwarf those on Earth.

However, because they receive far less heat from the sun

than the Earth does, something else is also powering their weather.

The weather on Jupiter and Saturn comes from two sources.

One is the sun, as on Earth,

and the other is the internal heat left over

from when the planets formed.

It's this internal heat trying to escape through

their deep atmospheres that makes the gas planets so tumultuous.

This zone dry and sunny throughout the day.

Later this afternoon there will be more in the way of clouds...

The storms on the gas planets are certainly

weirder and wilder than any we have on Earth,

but when it comes to the clouds and the rain, things get even stranger.

It's a typical June morning in Southern California...

..and a local weather phenomenon known as the June gloom makes it

the perfect day for taking a closer look at the clouds.

Dr Kevin Baines has a passion for the skies, on Earth and beyond,

and he's been studying the clouds on the gas planets.

Coming out of SoCal,

we're going to make a left turn on alpha

and taxi over to the run-up area for runway one.

So on a planet you'll get clouds at different levels,

depending on the local temperature and the local pressure.

On Earth, all clouds are made of water.

So in southern California we have this marine layer.

What happens is it actually forms over the water.

The Pacific Ocean, of course, has a lot of water,

so during the day it heats up and releases water into the air

as water vapour. As this water vapour rises,

it cools then it condenses out as water droplets in the air,

and when you get millions and millions of water droplets,

it forms a cloud.

Clouds will form wherever it gets too cold for water to stay

as a vapour in the air.

And if there's enough moisture,

the cloud droplets grow in size until they are big and heavy enough

to fall as rain.

Jupiter and Saturn also have a layer of water clouds.

If we were to transport ourselves magically to Jupiter or Saturn,

we could find a water layer like this.

But other substances form clouds at colder temperatures,

so above the layer of water clouds,

higher up in the atmospheres of Jupiter and Saturn,

there are two more cloud layers.

As you climb up out of the water layer,

it gets so cold that first you get ammonia hydrosulphide,

which is very exotic cloud made of both ammonia

and sulphur put together,

and then as you climb up even higher into the atmosphere,

when it gets down to about minus 130 Celsius,

there ammonia gas in the atmosphere condenses out and forms clouds.

So on these planets it doesn't just rain water,

there could also be a light rain of liquid ammonia.

Now if you go out to Uranus and Neptune, it is so cold out there,

about -300 degrees,

that you even have methane gas come out as clouds.

And so on Uranus and Neptune, liquid methane could fall from the sky.

Bizarre as they are, ammonia and methane aren't

the weirdest rains of all,

because back on Saturn, in the depths of its atmosphere,

Kevin believes that an astonishing process is at work that creates

what could be the strangest rain in the solar system.

This process can be witnessed an on idyllic summer day in Oxfordshire.

Inside this unremarkable office building,

a manufacturing company is replicating the conditions deep

in Saturn's atmosphere, not to study it, but for industrial purposes.

Using these massive presses,

they're turning carbon graphite into something far more valuable...

..and Kevin has come over from California to see how this process

can help explain what's happening deep inside Saturn.

We know that on Saturn there's carbon soot.

We know that by looking at these dark clouds that we saw with

our camera onboard the Cassini spacecraft orbiting Saturn,

and we see the spectroscopic signature of carbon soot there.

The carbon soot is created by lightning,

lightning actually zapping methane in the atmosphere.

Something very strange then happens to the soot as it falls

through Saturn's atmosphere.

It is transformed into something remarkable,

a process that is actually being replicated here.

What we do, effectively,

is we take this carbon graphite and mix it with several other materials

and we assemble what we call a capsule.

We take that capsule, the graphite material inside it,

and we place it inside the actual press itself.

The press is then closed up and the carbon graphite is exposed

to extreme temperatures and pressures.

High pressures are generated by these anvils that can press down

onto the graphite, pressures of around 50,000 atmospheres.

The graphite's then going to be heated to about 2,000 Celsius -

that heating happens by large electrical currents.

This process mimics what's happening inside Saturn.

We know we have carbon, which is very much like the graphite

that we just put into the machine over here.

The carbon soot precipitates or falls through the atmosphere,

and eventually it will get to the 7,000km level.

At that point, it will be experiencing the pressures

and temperatures that we experience in the press over here.

Inside the press,

the intense heat and crushing pressure transform the carbon

from graphite into diamond.

All right.

So this may not look like diamonds,

but we will then take this rubble and process it further

and we'll extract the diamond.

Great. So, the diamonds are in there somewhere?

Somewhere inside this rubble, Kevin, there are diamonds.

This high-temperature,

high-pressure process can make a variety of diamonds

which are used in industry.

Here we have tiny triangles -

effectively, these are used for wire drawing dies.

You get square-shaped diamonds which, in this case,

are used as single crystal cutting tools.

These diamonds are yellow because they contain nitrogen.

So what we were making earlier today, effectively,

are these diamond grits, tiny little stones of single crystal diamonds,

normally about 100 microns in size.

Kevin believes the same thing is happening on Saturn.

So we really think it's very similar conditions,

very similar process that's happening.

At the 7,000km level in Saturn,

carbon soot will transform itself into diamonds,

creating a diamond rain.

As the carbon soot falls from the clouds,

the extreme temperature and pressure deep in the atmosphere

turn it into diamonds.

So inside Saturn we have a huge region of diamond rain.

Our exploration of the other planets in our solar system has revealed

weather stranger and more powerful than anything we have here on Earth.

But what about beyond our solar system?

What is the weather like in the rest of the universe?

Some of those showers could be quite heavy. They'll be some dry spells

in between, but limited brightness and it will be a cool one...

Perched at the top of Mauna Kea in Hawaii,

4,000m above sea level, is the Keck Observatory,

one of the pre-eminent earthbound telescopes for finding planets

orbiting other stars, known as exoplanets.

Brendan Bowler has one of the best jobs in the world -

he's an exoplanet explorer.

I feel incredibly lucky studying astronomy and contributing

to exoplanetary science.

It's humbling, in many ways,

to be able to contribute and answer some of the questions

that we've been thinking about as humans for millennia.

The aim of my job is to find planets orbiting other stars,

which is a very difficult task to do.

Planets are both much smaller,

much lower mass and much fainter than the stars that they orbit,

so trying to find planets in the glare of their stars

is very difficult.

So it's as if we're trying to find a firefly buzzing around a spotlight

that's 10 billion times brighter than that firefly,

from a distance of New York all the way to London.

So how do you find a planet orbiting a distant star,

let alone study its weather?

There are two indirect methods that we primarily use to find planets.

The first is the radial velocity technique.

Let's say this is the planet and the post is the star that it's orbiting.

As a planet orbits its star,

the planet exerts gravitational influence on the host star,

which causes the host star to wobble.

We can search for planets by looking for the back and forth wobble

that planets induce on their host stars.

And that's exactly how the first exoplanet orbiting a sun-like star

was discovered, in 1995, changing the face of astronomy for ever.

And since then, planet hunters have discovered thousands more

orbiting distant stars.

But planetary explorers aren't satisfied

with simply finding planets.

What we really want to do is to be able to characterise the planet

in more detail.

The radial velocity method only tells us about

the mass of the planet.

The bigger the wobble, the bigger the mass,

but luckily there's a second technique for finding planets

known as the transit method, which reveals a whole lot more.

The transit method you can think of as a planet crossing between us,

our line of sight, and the star that it orbits, just like this.

Now when that crossing event occurs,

it will cause a dip in the brightness of that host star.

So we can use that to find planets,

by searching for periodic dips in the brightness of that star.

Crucially, the transit method also tells us how big the planet is,

because the bigger the planet,

the greater the dip in the light from the star.

The transit method tells us about the size of the planet,

while the radial velocity tells us about the mass of the planet,

so we can use both of those together to measure the density of planets,

because density is mass over volume.

And this reveals what the planet is made of.

Small dense planets are rocky,

whereas large planets that are not dense are gas giants.

The first planets discovered were huge gas giants like Jupiter,

and since then, planet hunters have found all sorts of combinations

of size and mass, including small, dense planets,

rocky worlds that could have atmospheres.

But finding out any more detail about an exoplanet's atmosphere,

climate and, ultimately, its weather, is extremely difficult.

But not impossible.

The key to doing this is the fact that different gases absorb light

at different wavelengths.

So we can study the composition of the atmospheres of exoplanets

by breaking up the light that we receive at Earth

into its constituent colours.

This is what we're doing here with the projector which emits

white light - we're dispersing it with a prism,

spreading out the light. We can see the various wavelengths and colours

it's split into.

If we have a gas intervening between the projector

and the prism, then the different colours will be blocked out,

depending on the nature of the gas.

So what I'm going to do is put a gas in this beam of light

by burning baking soda, which is sodium bicarbonate.

So here we have our baking soda.

We'll drop a little bit of that into the flame.

And so what we're doing is making the equivalent of an atmosphere

of sodium atoms in the beam, which is absorbing some of the light.

What we see on the spectrum is a narrow dark line that comes and goes

as I drop it in, in the yellow part of the spectrum,

which corresponds to the wavelength that sodium absorbs.

Every chemical has its own unique pattern of absorption lines.

So astronomers can use this information to detect

the different substances in the atmospheres of planets.

And that's exactly what Brendan is going to try to do tonight.

He's pointing the Keck telescope's awesome light gathering power

at a newly discovered planet.

I'll be using the Keck telescope to study a planet

about 400 light years away.

Its name is ROXs 12 b,

it has a mass between 10-15 times that of Jupiter,

and we know it's a gas giant, but we don't know what it's made out of,

which is the goal of our observations.

By studying the light that is emitted from this planet,

we'll be able to learn about the chemical composition and physical

properties of its atmosphere.

ROXs 12 is about to rise.

We have two more minutes.

It's below the telescope limits, but it is about to go up,

and then we can slew to it.

We're trying to look in the infrared,

to both image the planet and get a spectrum of it.

When we can get a spectrum of the planet,

we can learn what's in its atmosphere.

Direct imaging of distant planets like this

is at the very cutting edge of astronomy.

It's incredibly difficult to image planets,

but for the most massive planets, like ROXs 12 b,

it emits enough light that we can actually detect the photons,

so we can see the planet and take pictures of the planet.

And for this, we need our very best telescopes.

Keck is the biggest telescope in the world,

so we need the size of the mirror, which is ten metres in diameter,

to gather enough photons.

Incredibly, the Keck telescope also compensates for interference

from our own atmosphere.

Stars twinkle because of turbulence in the Earth's atmosphere.

We don't like that twinkling, we want it to stop,

so we use adaptive optics to actively compensate, in real time,

thousands of times per second, for that turbulence.

It's as if we're putting these big telescopes that we have

on the ground in space.

So far, only a handful of planets have ever been

directly imaged like this.

Martha, what are the coordinates?

16, 26, 28.1.

Can you go to ROXs 12 and the moon and look how it looks in the tracks?

Yeah.

That's it finished, and we're ready.

This is our sixth attempt to get this target.

We've been weathered out, we've had instrument issues,

and we think we're finally going to get it tonight.

OK, so I think we have the target centred up

in the field of view here.

I think we can start...

exposing.

So, here we have our image of ROXs 12 b.

So this is an infrared image of this planet,

which is pretty amazing to think that we are imaging a planet

400 light years away.

ROXs 12 b is one of only 15 exoplanets

to have ever been directly imaged.

And, incredibly, the faint light captured in this picture

will reveal the secrets of ROXs 12 b's atmosphere,

the first step towards understanding its weather.

We're looking at the infrared light from this planet.

This is light that's emitted in the interior of the planet

and passed through its atmosphere, and whatever chemicals, molecules,

atoms are in the atmosphere,

will induce absorption features in the spectrum,

and that's what we're looking for.

So here we can actually get a spectrum in real-time,

and let's go ahead and do it.

So here's our spectrum of ROXs 12 b in the infrared.

What we're looking for are absorption features

from carbon monoxide, CO.

So we can see these two dips here in the spectrum,

which correspond to the wavelengths where CO absorbs,

and that means that this planet really does have carbon monoxide

in its atmosphere.

The spectrum also revealed that this exoplanet has water vapour,

iron hydride, vanadium oxide, potassium and sodium

in its atmosphere - fairly typical for an exoplanet.

So by studying the light from exoplanets

hundreds of light years away,

astronomers are able to detect what's in their atmosphere...

..a key ingredient that goes into creating their weather.

Later today, a mixture of brighter spells

and showers for the majority...

As well as being able to detect the gases in a planet's atmosphere,

scientists can also use the infrared light to work out

just how hot a planet is.

It's a blustery day in California

and exoplanet meteorologist Heather Knutson

is visiting Santa Monica Pier.

So the main thing that determines the temperature of a planet

is the distance that it is from its host star.

Planets that are really close in are going to be boiling hot.

Planets that are further away will be a little bit cooler

by comparison.

Most of the exoplanets discovered so far are close to their stars,

so scientists expected them to be hot,

but they didn't know how hot.

So we can actually go and measure the temperature of these planets

by measuring their brightness in infrared light.

Hotter things are going to glow more brightly in infrared wavelengths,

cooler things are going to be a little bit dimmer and fainter.

So probably the hottest planet that we know of is a planet

called WASP-33 b.

WASP-33 b is the hottest planet discovered so far

in the entire universe.

It's a gas giant, four and a half times the size of Jupiter.

Its atmosphere is a scorching 3,200 Celsius.

So this planet is hot for two reasons -

one is it's very close to its host star.

The other is it orbits a star that is bigger and hotter than the sun.

Both those things together combine to make this

one of the hottest planets we've discovered.

Planets like WASP-33 b are nicknamed Hot Jupiters,

and they don't just have extreme temperatures...

..because being close to their star has another important effect

on the weather.

So all planets spin on their axis, just like I am now.

The Earth spins once every 24 hours,

but not all planets spin at the same speed.

There are some planets which we're discovering which are very,

very close to their stars.

They're so close that the star tugs on the planet as it spins around

on its axis, and the tugging of that star actually slows

the planet's rotation down,

keeps slowing it down and keeps slowing it down until the planet

rotates at exactly the same speed that it orbits.

So the same side of the planet always faces towards the star,

just like I'm always facing the centre of this ride here.

We call this tidal locking,

and it means the planet has a permanent day side

and a permanent night side.

And being tidally locked has a dramatic impact.

So whenever you have one part of a planet that's hot and another part

that's cold, the natural result is that you get a wind moving

from one part to another.

Here at the beach during the day the land heats up but the sea stays

relatively cold, and so you get this nice wind moving from the ocean

towards the land that's trying to even out the temperatures.

So when we first discovered these very close-in planets,

we realised they were probably close enough to be tidally locked.

And one of the very first things we wanted to know is what that meant

for the planet's atmosphere.

Did it mean these planets had a boiling hot day side

and a freezing cold night side?

Or were there winds in the atmosphere that were able to carry

some of that heat around to the night side?

To find out, Heather mapped the temperature on a Hot Jupiter...

..which scientists think is blue in colour.

The particular planet we decided to look at was a Hot Jupiter

called HD 189733.

That's kind of a mouthful, but I can tell you that this is actually

my favourite Hot Jupiter, this was one of the very first planets

that I looked at when I was a grad student.

By looking at it in infrared,

Heather was able to measure its temperature.

So this is the map we made.

So the colour tells you the temperature of different parts

of the atmosphere. So here on the day side things are relatively hot,

so the day side is about 900 degrees Centigrade.

Here, on the edges, that's the night side,

and that's a relatively cool part of the atmosphere,

it's only 700 degrees Centigrade, which is still really hot.

That difference is actually much smaller than we expected,

and the fact it's so small suggested to us that this planet must have

strong winds circulating through its atmosphere and carrying that hot air

from the day side around to the night side.

Incredibly, these winds have now been measured directly,

and it turns out that HD 189733 b is home

to the fastest winds in the universe

which rage around it at about 8,700km per hour,

seven times the speed of sound,

and 20 times faster than the fastest winds ever experienced on Earth.

Showers, some of them of sleet and snow.

Elsewhere fewer showers and here the showers will be of rain...

It's a beautiful tropical morning on the Big Island in Hawaii.

Exoplanet expert Hannah Wakeford is taking to the skies

to explore another bizarre effect the extreme heat on exoplanets

has on their weather.

The strangest thing about exoplanets is the clouds and the rain -

they're nothing like we have here on Earth.

Spectroscopy has revealed that exoplanets have clouds,

and also what these clouds might be made of.

We know that exoplanets have clouds.

If we have a planet that we know should be gaseous because of

its density but we don't detect any spectral signatures from that gas,

then we think there must be clouds in the way which are blocking that

light and obscuring our view.

And sometimes we can detect signatures directly from

those clouds by the way that they scatter or reflect the light.

But these aren't clouds we'd recognise.

A lot of the exoplanets that we've been able to follow up are very hot,

over 1,000 degrees,

so we know that water can't exist as a liquid at those temperatures,

so they're not going to be clouds like we have here on Earth.

Woo, we're in a cloud!

So, on some exoplanets,

the clouds will be made of far more exotic substances.

The temperatures are such that substances that we think of

as solids on Earth can actually exist as liquids or gas

in exoplanet atmospheres.

We can get a glimpse of this on Earth in volcanoes,

where temperatures can reach over 1,000 degrees Celsius.

Down there is the crater of Pu'u 'O'o,

and you can see the lava bubbling away.

The temperature of this lava lake is around 1,000 degrees

and all of the rock has melted.

The metals and minerals and the silicates that make up

the Earth's crust have all become molten.

It's amazing - you can really feel the temperature from the lava lake

all the way up here. It's really very hot.

It's 1,000 degrees melting the Earth's crust down there,

so it's no surprise.

And it's these substances that are thought to make up the clouds

on some exoplanets.

There's a planet called 55 Cancri e

that we think is rocky because of its density,

but it orbits very close to its parent star and is tidally locked,

so temperatures on the day side should be high enough to melt the rock,

making it a lava planet.

55 Cancri e is a lava planet.

While its night side will be relatively cool and solid rock,

its day side is an ocean of permanently molten lava.

On the day side, the temperatures go over 2,500 degrees.

This is hot enough to vaporise the rock at the surface.

This can then be lifted into the atmosphere and condensed

to form clouds of liquid lava droplets that then can be

transported to colder parts of the planet,

where they will rain down as pebbles on the surface.

So on some planets, it rains rock

rather than water like it does here in Hawaii.

Back on the ground on Kilauea,

Hannah has an example of what rock rain might be like.

Right here was the site of a massive eruption.

All along this fissure,

fountains of lava shot into the air nearly 100m high.

The liquid lava droplets then cooled and solidified in the air

before raining down onto the surface as these tiny pebbles,

and sometimes we get these perfect little droplets called Pele's tears.

This is what we think the rain might be like an planets like 55 Cancri e.

But perhaps the strangest rain in the entire universe has been discovered

on a giant gas planet which orbits a star hundreds of light years away.

We've been able to study the exoplanet WASP-12b

and the way that it scatters light suggests that there are clouds

high up in the atmosphere.

At this part of the atmosphere, the temperature is around 2,000 degrees,

so the most likely substance forming these clouds

is an aluminium oxide called corundum,

which forms the basis of rubies.

So instead of having rain which is liquid water droplets

like here on Earth, it would be raining rubies.

We are only just witnessing the birth of exoplanet meteorology.

But so far, what astronomers have discovered on exoplanets

is even more extreme and bizarre than anything anyone had imagined.

Compared to what's out there,

the most extreme weather on Earth - our hurricanes and tornadoes,

our rain and our snow - all seem pretty mild.

Our climate and weather is actually very hospitable.

The Earth is a nice place and that's all because of the weather.

We've got warm temperatures - not too hot, not too cold.

It's a great place.

Ultimately, the planet hunters of the world are hoping to find one thing -

another Earth.

A small, rocky planet with a thin blue line encircling it.

A planet with a nice climate,

a climate that could be hospitable to life.

What we're really looking for is rocky terrestrial-type planets

with an atmosphere around them which is habitable.

We'd like that planet to be at the right temperature to have liquid water,

so that means being at just the right distance from your stars

and having exactly the right kind of atmosphere.

But so far, astronomers have mainly found planets

with extreme environments -

planets with ruby rain or lava clouds.

So the planets that we've found so far aren't particularly nice places to go.

They're not somewhere you would put on your vacation list any time soon.

Because at the moment, it's easier to both find and study

the bigger planets...

When you're looking at other stars,

it's easy to find the large planets like Jupiter and Saturn,

where you don't have the right kind of atmosphere.

..or ones that are close to their stars.

Our surveys are really good at finding planets that are very close

to their stars, which means mostly the planets that we've discovered

are much too hot to host life as we know it on Earth.

Just over 20 years ago, astronomers began finding exoplanets.

The first were giant Hot Jupiters orbiting close to their stars,

because they were the easiest to spot.

And now hundreds of smaller rocky planets have also been found,

but most of these are still larger than Earth

and still too close to their stars.

But the search for another Earth is still in its infancy.

Dr Brice Demory is a planet hunter

and he may have found the promised land of planetary exploration -

a planet that could have warm, mild weather,

weather just like a lovely summer evening in Cambridge.

So we are looking for rocky planets

similar to the Earth in size and located at the right distance from its star.

It is a bit like cooking a marshmallow -

If the marshmallow is too close to the fire, then it will burn,

if it is too far away, it will never cook.

So we want the planet to be at the distance that is just right

for habitable conditions to happen.

We have just found three Earth-sized planets that are orbiting

a very cool star called Trappist-1,

and these planets are remarkable.

The first one is located here and receives about four times the level

of radiation that the Earth does.

The second one, look at it here,

receives twice the level of radiation that the Earth does.

Those planets are probably too hot to be habitable.

The third one is the most interesting one.

We're not exactly sure of its location right now,

but we believe it is located just here,

where it would receive about the same level of radiation

as the Earth does.

So this is our best candidate to date for habitability prospects.

Could this planet really be another Earth?

As ever, this will depend on its atmosphere.

The atmosphere dramatically affects the habitability of a planet.

In the solar system, Venus,

the Earth and Mars are all within a very close habitable zone,

but the atmosphere of Venus and Mars make them completely un-habitable.

Even if this planet has the right type of atmosphere,

it could still be very different to Earth.

These three planets are tidally locked to their star,

meaning that they show permanent day side that would too hot for

habitability and a permanent night side that would be too cold,

while still having hospitable temperatures between the two.

It is a bit like this marshmallow -

if I put it in the fire and I don't rotate it,

one side will be completely burnt while the other will be uncooked.

But in the middle, it would be just right.

These planets could have a barren, frozen wasteland on the night side,

a baking inferno on their day side and yet have a temperate

and yet have a potentially habitable strip down the middle

where it is permanent twilight.

But if you have a thick atmosphere surrounding this planet

then all the heat coming from the star on the day side

will recirculate to the night side,

making the day side cooler and the night side warmer.

To really know if any of these planets could be habitable,

we need to study their atmospheres.

But they are too small for even our best telescopes.

So the atmospheres in small planets actually are very thin.

So it is very difficult to detect them,

even with the state-of-the-art telescopes that we have today.

We have reached the limits of our current technology,

but NASA is building a new space telescope called the James Webb,

which will enable us to study the atmospheres of exoplanets

in far more detail than is possible today.

It will have far greater light-gathering abilities

than its predecessor, the Hubble Space Telescope.

James Webb will have a mirror diameter of 6.5m,

which compared to the 2.4m of Hubble means that James Webb

will collect seven times more photons than Hubble does,

which means it will have more signal to study these planets.

The James Webb will also be able to look at a far greater range

of wavelengths.

So the James Webb will have the possibility to go

much further in infrared than what Hubble is able to do today,

which means that it will give us the ability to probe for many more

chemical compounds than what we are able to do with Hubble.

When James Webb launches in 2018,

astronomers are going to be able to study Earth-sized planets

and discover if they are potentially habitable.

I'm convinced that we will find a habitable planet maybe in the next

five or ten years.

There are so many planets in our galaxy,

and this is the result of the last 20 years of planet hunting,

that based on sheer probability we will definitely find

at least another habitable planet.

But, for the time being, our planet remains unique.

Our exploration of other worlds so far suggests that it is a fairly rare

combination of factors that make our climate and weather so hospitable.

What makes the Earth so perfect for life is that first off it is rocky.

It has also got an atmosphere around it.

And our atmosphere is just right.

The size and the mass of our atmosphere is critical.

And, in addition, the composition of the atmosphere is just right.

Carbon dioxide and water in our atmosphere gives us just the right

greenhouse effect.

Venus has too much greenhouse effect.

Mars doesn't have enough.

We're also the right distance from our star.

We're just far enough away from the sun that we are not too hot,

but we're close enough that we're not too cold.

The temperature is perfect for water to exist in all three conditions

and that is so vital for life to have developed

and evolved on this planet.

And the Earth even spins in the right way.

So the other thing that makes Earth such a great place to live

is that it spins on its axis every 24 hours,

so the day and night temperatures never get super extreme.

So our planet has the right unique combination of things to make it

just the right place for life to have developed and maintained itself

for billions of years.

For thousands of years, we've gazed up at the night sky,

wondering what other planets might be like.

Astronomers began by studying our own solar system.

Now they are exploring the wider universe and can even study

the weather on planets hundreds of light years away.

They have discovered climates and weather stranger than fiction -

alien worlds with extreme temperatures,

bizarre clouds and even ruby rain.

But they've yet to find another planet like Earth,

with weather that is suitable for life,

that's not too hot or too cold.

So, for the time being,

it looks like a warm and pleasant day on Earth with a gentle breeze

and a slight risk of rain

might actually be the weirdest weather of them all.