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.

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,