Are We Alone in the Universe? Horizon

Why do we humans have such a connection to the night sky?

The twinkling lights that seem like oases out there and yet we're not sure. Are there habitable worlds?

Around the world there are a group of highly intelligent,

highly trained scientists that share a surprising belief.

There are a couple of hundred billion stars just in our galaxy and

at least half of them probably have planets. That's 100 billion planetary systems.

How many planets in each system? We've got eight in ours, let's say five.

That's a half trillion, 500 billion planets out there.

Keep in mind, there are 100 billion other galaxies!

For these scientists the vastness of our universe can mean just one thing -

the existence of life.

So to think, "Look, man. This is the only place where anything interesting's happening",

you've gotta be really audacious to take that point of view.

But proving it has not been quite so simple.

I have conducted many, many searches, none of which have produced a discovery.

Until now.

In our local neighbourhood, just 200 trillion kilometres from Earth,

is a planet that we might find rather familiar.

The discovery of Gliese 581c is a marvellous discovery.

It shows how close we're getting to planets that remind us of the Earth.

Occasionally you're sitting on a plane and they guy next to you says, "What do you do for a living?".

"I look for aliens". I explain a little bit, and almost everyone is interested.

Nobody says, "That's nice. I'll go back to my magazine now".

In the desert, 300 miles north of San Francisco, Dr Seth Shostak is waiting for a message from an alien.

This is SETI, the front-line in the search for extra-terrestrial intelligence.

If a message ever comes our way, this is where it will be received.

Anybody who can build a transmitter can send messages between the stars.

If we can do that, maybe they can do it.

Here we are, the Allen Telescope Array, - designed to do one thing -

eavesdrop on any signals that might be being broadcast our way by some alien civilisation.

This vast array of telescopes is the latest in a long line

of experiments designed to eavesdrop on our nearest neighbours.

As chief astronomer of the project, Shostak is more confident than most

that he'll be on the receiving end of a close encounter.

There are 42 antennae here now, you can count them up.

But eventually the idea is to have 350 and then this thing

will be able to scan big chunks of the sky,

simultaneously observing five, six, maybe more stars at a time, looking

for the signal that somebody's out there, trying to get our attention.

It's the most powerful experiment humans have ever attempted -

to discover if intelligent life is the exception or the rule in the cosmos.

But although it's early days for Shostak and his team, the omens are not good.

The SETI project has been casting its ear out to the universe for over 50 years.

For the founding father of the search, Dr Frank Drake, the dream has never changed.

Back in the 1950s, there were many scientists interested in ET life,

but we were well aware that there were no means even to detect

planets, let alone microbes,

or any sign of non-intelligent life.

In fact, the only thing open to us was radio transmissions from intelligent civilisations.

Drake was the first scientist to believe that technology could answer the biggest question of all.

Despite widespread scepticism,

he believed that if there were intelligent life forms out there

then the least we could do is to try and listen to any radio signals they may be sending out.

In 1960, we conducted a search for radio signals from the two nearest

stars to the Earth that are like the Sun - Tau Ceti and Epsilon Eridani.

But to no-one's surprise, the search failed.

We searched for two months. We didn't find anything.

And that's actually an important result, because it showed not every star in the sky was radiating.

It also demonstrated that a search was likely to be a very long and difficult one.

Despite putting on a brave face, Drake and his ideas remained on the very fringes of astronomy.

Listening for aliens just wasn't science.

Congress people would see that they could get publicity by

attacking this project as a waste of tax-payers' money.

Using tax-payers' money to search for little green men was a common theme.

So Drake would have to wait for another generation of scientists

to bring alien-hunting in from the cold.

Being an astronomer is a bit sacrificial.

My wife is at home and she misses me.

I call her up and she says, "When are you coming home?"

It's another four, five nights, I have to tell her.

Professor Geoff Marcy is a planet hunter, an explorer of alien worlds.

Staying up all night means you don't get much sleep.

So it's a bit of a sacrifice, but I wouldn't give it up for anything. It's such a treasure.

For the last ten years, Marcy has come here to

use the planet's largest telescope in the hope of finding other worlds.

The Keck Telescope high up on Mauna Kea, Hawaii is about as close as you can get to the stars.

It's the world's largest, because the collecting area of the mirror is the largest in the world.

The mirror is ten metres across,

1/10 the length of a football field, all to collect the starlight

coming from hundreds or thousands of light years away.

But even using the mighty Keck telescope,

astronomers can barely make out the objects in our own back yard.

Pluto, at the edge of our solar system, is a colossal 4½ billion kilometres away.

And this is the best image astronomers have achieved.

And yet Marcy wanted to look beyond our solar system,

to find the hypothetical worlds that astronomers call exoplanets, which lie around other stars.

Like our nearest star, Proxima Centauri,

a staggering 40 trillion kilometres away, or four light years.

Beyond lies the rest of our galaxy - an unimaginable 100,000 light years across.

It's our local neighbourhood of 200 billion stars.

Astronomers knew there had to be planets out there.

We saw young stars with proto-planetary disks of gas and dust

around them, surely making planets.

But we couldn't detect the planets.

And the reason is that even with the largest telescope, like this one,

the mighty Keck, the planets were lost in the glare of the host stars.

The problem is that compared to the light of a star,

the reflected light from an exoplanet is all but invisible.

The star burns a billion times more brightly.

Indeed, even with the Hubble space telescope, we can't detect planets directly around nearby stars.

Using direct observational methods, astronomers were confined within our own solar system,

unable to prove even the existence of exoplanets, let alone life.

So how do you look for something you can't see?

The answer was first proposed in an obscure paper published in 1952,

by a Russia astronomer called Otto Struve.

Struve theorised that even though the planets themselves

were invisible, there was still a way of unlocking their secrets.

He knew that each planet was held in orbit around its star by an immense gravitational force.

This force works in two directions.

The star pulls on the planet but the planet also pulls back on the star,

making the star move with the minutest wobble.

It's not much, but this wobble is just big enough

to make it theoretically detectable back here on Earth.

If you have the right technology.

The real tipping point was not in the telescopes, we've had big telescopes for several decades.

The tipping point was having digital detectors, like the digital cameras

that most of us enjoy, with the CCD light detectors at their backs - and computers.

Let's see what kind of a night we're gonna have here.

Marcy script underscore ETA underscore Earth.

-Great.

-First object on the list...

Why don't you go ahead and open the dome slit?

We're observing 80 stars, night after night, with one key goal, and that's to detect Earth-like planets.

Since the early 1990s, planet hunters like Marcy have been gazing to the stars for the tiniest wobble

that could signal the existence of another planet.

The theory seemed correct.

They had the right technology, and yet after years of searching, the exoplanets were still missing.

We were confronted with a contradiction.

On the one hand, it appeared that young stars had the right kind of planet building material,

but on the other hand, humanity had failed to find any.

All the planet hunters had to keep them going was their faith.

And the belief of the one man who has never given up hope.

It occurred to me, we need to know how often does life arise, how often does intelligence arise.

I recognised that all you had to do was multiply these factors together

and you have a very prime important equation, of basic interest,

because it tells us how many civilisations there are out there.

Without any hard evidence, back in 1960, Frank Drake went

about creating an equation that would answer the big question once and for all.

We have an equation which gives us N, the number of detectable civilisations in our galaxy.

It's based on what we know of the history of our galaxy

and particularly the history of our solar system and of life on Earth.

The equation defines all the necessary ingredients for intelligent life to arise.

There are seven factors in the equation.

Since life needs a home, it begins with a known observation.

For the rate of star formation, we know that very well.

It's about 20 stars per year.

For the fraction of planets, we didn't used to know that at all.

Everything else in the equation, from the number of stars with planets

to the number of planets per star capable of supporting life, was a total mystery.

But over the years, it hasn't stopped people from guessing.

This is our number N.

The number of technical civilisations in the galaxy.

Into the now famous, or infamous, Drake equation goes everything, from astrophysics, through

evolutionary biology to whatever it is that governs the lifetime of a detectable civilisation.

Not surprisingly, no-one's solved it yet, but anyone can have a go.

It's almost a game the whole family can play.

If you sort of take the average of people's guesses,

it gives you a total number of detectable civilisations,

which is about 10,000.

A big number.

And yet with no call from ET, and no sign of another Earth, Drake's guess seemed wildly optimistic.

When I was a child, I was living in a very small village without light.

We spent evenings with my sister laying down on the grass and looking at the sky.

And that's really good for the imagination.

I'm sure there are other Earths similar to our own Earth.

And on some of them, you even may have life developing.

Professor Stephane Udry is part of a Swiss team of planet hunters

who began searching for life-bearing planets in the mid-'90s.

They had developed a new planet detector,

which had just been installed at their observatory in central France.

When you have a new instrument,

you want to check the short-term precision of the instrument.

Tests of their new detector were scheduled to last a few weeks.

Among their target stars was one similar to our own sun, called Pegasi 51.

The light from Peg 51 should remain constant, but there seemed to be a problem.

The star...

appeared to be wobbling.

The thinking was, "Oh, that should be some crazy effect of the star that could explain the observations".

They tried to reject, one after the other, all the possible explanations.

And at the end, the best explanation was the presence of a planet.

With ingenuity and a little bit of luck,

the Swiss had discovered the first planet outside our solar system.

It was massive, half the mass of Jupiter, but in a rapid orbit lasting only four days.

The planet was so close to its star, that surface temperatures exceed 1,000 degrees Centigrade.

Being part of these teams finding planets around other stars

is very exciting in that sense.

That's really a new domain that is opening in science.

The discovery of this planet opened the flood gates.

Hundreds of exoplanets have since been discovered,

but none of them have proved suitable for life.

My favourite planet is a little planet that orbits the star Gliese 756.

My favourite planetary system is called 55 Cancri.

There are two planets which I have an emotional attachment to.

It has an orbit of two days.

The planets of the star Tau Ceti.

That means its seasons occur in two days.

And the star Epsilon Eridani.

So summer-winter would alternate in two days.

It's like the solar system because it's a planet like Jupiter on a Jupiter-like orbit.

Those stars were my targets

when I first searched for the first evidence of extra terrestrial life in 1960.

It's unlike the solar system because it has three other planets that are

very close into the star, hot Jupiter-type planets.

In the last decade, astronomers have found over 260 exoplanets,

most of them searingly hot gas-giants.

As a biologist, I don't really have a favourite exoplanet at the moment,

because the astronomers keep finding me hot Jupiters and they don't do much good for biology.

What I'm looking for is something really Earth-like.

Something that's got a good chance of liquid water.

And then I'll have a favourite.

So just how rare is our blue planet?

Dr Lynn Rothschild is an astrobiologist who has studied our own solar system

in an effort to understand what makes Earth so special.

Let's pretend that this fire here is our sun and that this rock is Venus.

It's about as close to the sun you can get and still have liquid water.

So the orbit of Venus would be, say, like this.

This is getting pretty hot, cos I'm awfully close to the sun here.

Now on the other extreme, this is Mars, which is the farthest planet

from the sun that has any chance of liquid water.

There's no liquid surface water today, but we know

that there used to be in the past. So let's trace the orbit of Mars.

Now right between these two circles, where the orbit of Mars would be

and the orbit of Venus, this is where liquid water is stable.

And right in this habitable zone in our solar system is planet Earth.

Our beautiful watery world that's just covered with life.

Just 10% closer in,

and Earth would no longer be capable

of supporting liquid water.

Almost miraculously, Earth slots right into the heart of the habitable zone.

Giving this once lifeless rock just the right elements for life to take hold and flourish.

Over billions of years, microbes, plants and animals

have transformed Earth into a living, breathing world.

A world where one evolutionary line

has led to modern humankind and civilisation.

But even with our civilisation's most advanced technology,

finding other planets like Earth has proved impossible.

Rotator is vertical, angle mode zero.

The very factors that enable life, a small planet at a safe distance from

the sun, means the telltale wobbles that these planets produce are tiny.

Our Earth, when it orbits the sun, causes our sun to wobble with a speed

of 1/10th of one metre per second, a smaller motion than we can detect.

Or there could be another, more profound explanation for the missing Earth-like planets.

It's possible that other stars didn't have planets around them.

That we're just one of the freaks of nature that grew up on a rocky planet.

Either way, despite decades of searching, until 2007,

Earth remained entirely alone.

Between the Andes mountains and the Pacific Ocean, on the remote southern edge of the Atacama desert

lies one of the most extraordinary observatories on Earth.

The high elevation and the low rainfall,

just one millimetre a year, makes it the perfect place for uninterrupted views of the southern night sky.

Please come in, I have something to show you in here.

Professor Stephane Udry is the proud owner of a machine which could change the course of human history.

Inside this big box is an enclosure and inside there is a vacuum tank

with the instrument, that is the most sensitive in the world now for planet detection.

-With this instrument we can detect low mass planet five, ten times the mass of the Earth.

-Can we go in?

No. Of course not, because just opening the door will destroy the measurement for a few days.

Because we need to have a very stable instrument to be able to repeat the measurement with the same precision,

day after day, month after month, years after years.

And that's exactly what they've been doing.

They drew up a list of a thousand targets taken from the Gliese Catalogue of Nearby Stars

and began measuring and re-measuring each candidate,

hunting for wobbles that had previously been too small to detect.

But one star caught Stephane's attention.

Gliese 581 was in our target list since the beginning.

Categorised as Gliese 581a, it's a red dwarf star, a third of the mass of our own sun.

When the wobble was plotted it revealed 581b, a massive planet the size of Neptune,

close into the star

and orbiting once every 5½ days.

It was no Earth,

but the star's wobble held some fine detail that intrigued Stephane.

We noticed that there was something else in the system.

There seemed to another, smaller planet lurking in the detail.

That something else could be a five Earth mass planet very close to the star.

If Stephan's hunch was right, it would be the smallest planet ever detected around a distant sun.

And this planet seemed to be habitable.

We got excited because the distance was just right for the planet to possibly be in the habitable zone.

After years of hunting, the search

for the first "Second Earth" was over.

European astronomers have spotted a new planet outside our solar system

which closely resembles the planet Earth.

The probability that there is life somewhere else in the Universe goes up a bit.

This latest find has set the world of astronomy alight.

For the Swiss team, the breakthrough was a triumph.

It is always very exciting to be the first one to know.

The discovery of Gliese 581c is a marvellous discovery.

It shows how close we are were getting to planets that remind us of the Earth.

It shows that potential life-bearing planets exist.

When you know, when you realise it, and you are the only one, it's like being in the spaceship

coming to a planet and being the first one to see the landscape.

For those tempted to make the journey, pick a clear night

and look for the constellation Libra.

Invisible to the naked eye,

Gliese 581 lies just north of the brightest star in the constellation.

Remarkably, it's one of our closest neighbours, a shade over 20 light years distant.

At the heart of the system is the parent star.

Close by is 581b, 16 times more massive than Earth and too hot for life to survive.

Beyond, just on the inner warm edge of the habitable zone,

lies Gliese 581c -

the smallest and most Earth-like exoplanet yet detected.

At last, scientists have found another planet that may just be capable of supporting life.

Not much out here.

See if there's any under the rock.

Nope. A lot of UV radiation.

Nothing green, nothing coloured I can see. Very dry.

For astrobiologists like Dr Lynn Rothschild,

its discovery means they can begin to imagine what it would be like to spend a day on a Super-Earth.

We're up here on the edge of the Atacama desert in Chile right near the Bolivian border.

You can see it's very dry, in fact, one of the driest places on Earth.

This is a great place to get an idea of what an extra-solar planet,

for example Gliese 581c, might be like.

Let's imagine that we're on Gliese 581c.

There's an awful lot of rocks around. It's dry.

The planet's mass is five times that of Earth.

This means that gravity will pull twice as hard.

Whereas on the moon, the astronauts could jump with no effort,

on this planet you would be suffering from extra gravity.

If you took a rock and you threw it, it would come crashing down,

much faster than that of the Earth.

High gravity will affect the look of the planet. No mountains.

Just low hills and vast plains.

And the last thing is it's close to the parent star,

and so the radiation from the sun would be much stronger than on Earth.

Here we're getting burned, there we would probably be fried.

The planet's red dwarf star will dominate the sky -

a fiery ball five times larger than our own sun back home.

And a few hours into their trip,

interstellar visitors will discover that this sun never moves.

The planet is so close to its star

that immense gravitational forces have united the two.

They're tidally locked, with the planet presenting just one face to the light.

On the Earth, we're used to getting up in the morning, the sun rises.

We have our midday meal, in the evening we have dinner, if we're lucky, we get a nice sunset.

But on something like Gliese 581c it would be totally different.

If I wanted to see the equivalent of a sunset, I'd be the one who'd have to get into the car and move.

Beyond this point is the dark side

of the planet, perpetually turned outwards to the cold of space.

I wouldn't want to live here, I wouldn't want to be a colonist

on another world that was barren like this.

I'd take even a year-long field trip, but I wouldn't sign up for the rest of my life.

Comfortable as Gliese 581c may be for a day trip,

for life to exist there, for it truly to be second Earth, it must have one other vital ingredient.

Water is the one thing life on Earth has in common, so we think

looking for water on other planets

is a way to look for life on those planets.

For astrophysicists like Sean Raymond, finding water

on other worlds is the key to finding life.

Every day in his laboratory, he makes new solar systems from scratch.

So the way we do this is computer simulations of a disc of rocks orbiting

a star, and we let them collide and let their orbits evolve, and such.

And it turns out these take quite a long time to do.

Over the months, Sean's computer calculates

how alien planetary systems evolve over millions of years.

Here's a movie of one of these simulations. You can see everything

on the inner disc starts off red, meaning quite dry.

All these guys start off being the size of the moon, or actually a little smaller.

And then the number of bodies is going down as they collide and grow into larger things.

And by about 10 million years or so, a planet almost the size of the Earth is formed right there.

And you can see it's still red.

These new planets are all dry. Only far out from the star

are temperatures low enough for water to collect.

It's not until a little later... you'll see in a second, it gets collided by something that's

blue and turns - right there, it went from being completely dry to having some water.

And that process of water delivery continues over the next 100 million years or so.

Over this time, icy comets and asteroids from

the outer solar system are drawn inwards towards the young planets.

Shaun's theory is they bring with them vast amounts of water,

transforming dead worlds into blue planets.

That's a pretty good Earth analogue.

And we think this is how the solar system terrestrial planets formed.

Sean has run hundreds of simulations.

And each time, something happens to the planets in the habitable zone -

-they nearly all have water.

-Water is very abundant.

In the solar system, water is two to four times more abundant than rock and iron.

It looks like Earth might, on average, be a little bit water-poor.

And many planets may end up with a lot more water than the Earth.

Including the newly discovered 581c.

Gliese 581c especially, is very exciting, a very big discovery.

These planets would have acquired some water-rich material, so they

probably have water contents comparable to Earth at least.

Far from being a barren rock,

this new planet may be awash with liquid water.

But in their rush to tell the world of another world,

the Swiss had overlooked one thing - the planet's atmosphere.

We got very excited about Gliese 581c

when we realised that it was just at the right distance from the star.

But then, talking with specialists of the evolution of atmospheres

on the planet, they told us that maybe the greenhouse effect could be big.

And so the temperature could be too high for the development of life.

If the planet's atmosphere contains too much water vapour or carbon dioxide...

a runaway greenhouse effect could take hold.

Rather than resembling Earth, 581c could be a super-Venus.

Instead of liquid water,

steam would shroud a searingly hot world, incapable of supporting life.

It's probably too hot to be habitable.

If it has water at all, which is doubtful, that water would be boiled off, evaporated and gone.

581c may, after all, lie on the hot side of the habitable zone,

but the light the Swiss team were collecting from the planet's star held another surprise.

After decades of fruitless searching for habitable worlds, out popped another one.

We had to wait for one more year

before being able to actually find another planet a bit further out.

There is a third planet in the system.

They'd discovered a second super-Earth in the same system -

Gliese 581d.

This world lies on the far, cold edge of the habitable zone.

On first calculations, this would make it a giant frozen world.

But if it too enjoys a greenhouse effect, then it could be just warm enough for liquid water.

If there is some atmosphere, and a greenhouse effect, then the temperature could

be even better on that planet for the development of life.

Perched on opposite edges of the habitable zone,

the conditions on the planets in this system will be harsh.

Perhaps too harsh for life to survive.

Here on Earth, Dr Lynn Rothschild is investigating places where conditions mirror the extreme

environments found on both the G581 planets.

We're up here in the altiplano in Bolivia. Up at about...

well over 4,000m, or 15,000ft.

In the winter it's frozen - it's not a whole lot warmer in the summer,

and yet life lives up here.

Every place we've gone that's cold - the Antarctic, the ice caps, we've found life.

And even under here, there's plenty that's growing. It's just amazing.

So life in the freezing conditions of the outer planet is a possibility.

And even the on the inner, hotter world, where temperatures could exceed the boiling point of water,

scientists are beginning to understand how life could survive.

We don't know how life actually got started on Earth.

But we do know that when we look at modern organisms,

and at their evolution, the most ancient ones

seem to be the ones that live at extremely high temperatures,

just like these areas around here.

Indeed, the more scientists look,

the wider the range of habitats they find in which living organisms can thrive.

So this gives us hope, this gives us optimism that when we go elsewhere

to other worlds, that there might be life.

For now, no-one knows for sure if life could survive in the massive,

strange worlds of the G581 system.

And Earth-bound planet hunting may have reached the end of the line.

Because to find true Earth-sized planets,

the hunt is moving into space.

This is a spaceship factory.

In these category A clean rooms, machines are built

that their designers hope will unlock the secrets of the universe.

That's the interferometer.

-There's the focus mechanisms right here.

-Here's one focus mechanism.

-This is the actual focus mechanism.

-This is the flight hardware.

-Wonderful.

Today the team are midway through assembling their latest mission -

the giant Kepler space telescope.

But it's not scheduled to fly until 2009, so currently the spaceship is in bits.

This is where the primary mirror is gonna sit, on top of this.

So that measures how well you've got the optics aligned?

That's right, you can measure how well it's working.

Leading the NASA team assembling the space telescope is Bill Borucki.

It's magnificent, it's just wonderful to see it come together.

We've been planning this for years and years.

So to actually see it here...

This is the flight equipment, this will go into space.

It's this that will make our discovery.

I'm delighted to see all the details that seem to be right.

When Kepler flies, it will undertake a four-year mission to seek out new worlds.

But it won't be looking for wobbles.

Instead, Kepler will be hunting for planets that pass in front of their stars, creating a tell-tale wink.

Looking at the star, it seems to wink, it gets dimmer for a while.

Like it closed its eye for a second and then opened it.

This is because the planet moved in front of it and blocked some of its light.

It happens in our solar system too.

We had a transit, Mercury going in front of the sun fairly recently, we could see that with a telescope.

For the wink technique to work,

a space telescope is essential.

Free from the interference of Earth's atmosphere, it gives

Kepler an uninterrupted view of a very special part of the galaxy.

Kepler only looks at one area of the sky.

It's a good area for us, in that it has a huge number of stars.

Kepler will scan the same 100,000 stars

over its entire four-year mission,

constantly measuring the brightness of each one.

And from day one, it will be sensitive enough to detect the wink

of an Earth-sized planet crossing its sun, tens of light years away.

It's always very exciting, because we've always wanted to know - are there lots of Earths out there?

Geoff Marcy and Stephane Udry and all these other people are extremely competitive.

They want to find planets, they want the answers too.

Well, we all do, and the best way to do that is to co-operate.

There's a bit of a race going on, but it's a delightful race.

The competition is lovely, and it makes us get up in the morning, go to work, and work a little harder.

So who's gonna find the first Earth-sized object?

We are. Kepler's going to find the first Earths in the habitable zone.

Between them, the planet hunters are beginning to define the first galactic map of Earth-like worlds.

At last, a phone directory for those listening for a message from ET.

They're gonna allow us to sharpen our gaze of the heavens,

where we're pointing these antennas,

trying to pick up a signal, they're gonna tell us, "You don't have to look at every star,

"these ones have planets", and eventually they'll be able to say,

"These are the ones that have planets the same size as Earth."

And ten years after that, they'll be able to say,

"These are the ones with oxygen or methane in their atmosphere.

"So they have some biology, and it's up to you to find out if any of that biology is smart or not."

Rather than the entire galaxy of 200 billion stars, in the future, SETI

need only tune into the handful of star systems that Kepler discovers.

Everything has caused us to become more optimistic.

We really believe in the next 20 years or so, we're going to learn

a great deal more about life beyond Earth

and very likely we'll have detected that life

and perhaps even intelligent life elsewhere in our galaxy.

Remember, there's a flip side to this - it could be that advanced

technological civilisations, species, are a rarity, one in a million, maybe one in a billion.

If so, we humans could be quite a precious rarity in the Milky Way galaxy.

Maybe, in fact, they're not out there watching us.

We may be the ones to be the first to go out and explore the galaxy.

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