Interstellar: The Journey to Proxima b The Sky at Night

Tonight on The Sky At Night, we're going interstellar.

Travelling to the stars has always seemed like an impossible dream,

but now some scientists believe

that it may be possible within our lifetimes.

And that prospect has just become even more enticing

because we have a target to aim for.

Within the last few weeks, astronomers have announced

the discovery of a hospitable planet around the Sun's nearest neighbour,

Proxima Centauri.

On tonight's programme,

we'll be finding out how this new planet was detected

and why it's such an important discovery.

And Jim Al-Khalili will be exploring the revolutionary new technology

that might take us deep into space.

Welcome to The Sky At Night.

On August 24th, astronomers made an extraordinary announcement.

Scientists are hailing a major discovery -

a new planet which they've called Proxima b.

At just four light years away, it's relatively close to us.

It's roughly the same size as Earth,

and because it's just the right distance away from its star,

it could be the right temperature to have liquid water

and possibly life.

Over the last few years,

we have identified over 3,000 planets orbiting other stars,

but this one is special.

It's already been called one of the discoveries of the century.

So what makes this planet such an enticing target

for our first interstellar mission?

Proxima b is in orbit around Proxima Centauri,

the closest star to our sun.

Only discovered in 1915, Proxima is an apparently unremarkable star.

It's the smallest of the three stars

that make up the Alpha Centauri system.

A red dwarf, like 70% of the stars in the Milky Way,

it's just 12% of the mass of the sun.

That small size means the pressure and the temperature at the core

are much less than in our sun,

and so the processes of nuclear fusion that power the star

proceed much more slowly.

And so, Proxima Centauri is cool.

Its surface temperature is only half that of the sun

and its luminosity is 500 times lower.

In fact, it's so dim that even though it's the closest star to us,

it can't be seen from Earth with the naked eye.

But the discovery of a planet around Proxima Centauri

makes it a much more exciting neighbour.

This is the paper published in Nature just last month

that announced the discovery of the planet

that the team called Proxima b.

I've come here to Queen Mary University of London

to meet Guillem Anglada-Escude,

the leader of the team that made this remarkable discovery.

Anglada was part of a project called the Pale Red Dot

that used the European Southern Observatory's telescopes

in Chile to observe the star for 60 straight nights last spring,

and it's only now, after careful analysis,

that the results have been released.

Congratulations. Thank you.

It's a wonderful, wonderful discovery,

but how on Earth do you tell that this tiny planet is there

going around the star?

Well, that... Well, that took some time.

It was not something that happened from one day to the next.

But I think the thing that's difficult for me

to get my head round is I sort of imagine you taking a picture

and looking for the planet in the image, but that's not how it works.

No. No, not in these cases.

And most of the planets don't work this way because

the planets are very faint compared to the stars.

So what you see is the star,

and we are using a method that is indirect.

So we see what the planet is doing to the star because

the planet and the star both have mass

and therefore they attract gravitationally,

and the planet going around the star moves the star itself,

and that is what we are trying to measure.

I was going to say,

because the planets are small compared to the stars,

so this motion must be very subtle.

For example, the Earth can't have much effect on the sun.

The effect of the Earth on the sun is small -

it's about 10 centimetres per second.

So you can think of just moving like this, like an ant.

For planets around stars that are much smaller, like Proxima,

the star is smaller, so the planet is making the star move more,

and in that case, the motion is about metre per second level.

Wow, so you are able to detect that a whole star is moving

at a metre per second, which is...

That's sort of walking pace.

Exactly. And it's not a trivial thing to do because what you have is

the planet going around the star periodically,

and we see this motion going up and down, up and down.

So you see a wave, like something like this.

And that's the signature that tells you that there's a planet.

When you see something like this in a star that repeats over time,

that is always consistent and a number of other things,

this is when you're convinced that you have a planet around a star.

Excellent. And then from there, the next question is,

what do we know about this planet?

What can we tell, other than the fact that it's there

and it's making the star move?

So, just from the motion that we detect, this time,

sorry, this curve, this oscillation, we know the period.

And for this planet, what is that number?

It's 11.2 days.

So it's going round pretty quickly. Yes.

From that we can infer the distance between the star and the planet.

Just from knowing how gravity works, basically.

Yes. This is Kepler's law, the first Kepler's law.

And so what is that separation for this planet?

In this case, it is around 5% an astronomical unit.

OK. So that's what?

That something like 7.5 million kilometres, something like that?

You're faster than me. Yeah, OK.

But it's very close to the star.

That's much closer to the star than Mercury is to the sun.

Yes. Yeah. It's about a tenth of the distance

between Mercury and the sun.

And the other thing we get from this curve is the mass of the planet.

And what is that mass?

This mass is 1.3, 1.4 Earth masses.

So for this system, we've got a one and a third Earth mass planet

going around its star every 11 days.

So, just thinking about that,

I expect... That's much closer to the star than Mercury is to the sun,

so I'd expect that to be hot.

Yes, you would expect that to be hot if that was the sun,

but this is Proxima and it's a red star, it's a red dwarf.

And it's a small red dwarf,

so Proxima has around 12% of the mass of the sun,

so this means that if you want to keep warm,

you have to be much closer to the star.

Right. And this is when the magic happens,

where you put all the numbers together and you can estimate

how much light, how much energy is reaching the planet.

And this amount of energy is about 70%,

the amount of energy that Earth is receiving from the sun.

And so it's actually pretty warm by planetary standards.

By planetary standards.

The next calculation you can do is try to estimate

the temperature that this planet would have.

And you do the numbers and you get 240 Kelvins.

That's what? -30 centigrade.

-30, -40 Celsius, something like this.

But you would say, oh, that would be frozen,

but the same would happen to Earth.

Earth is about 255 Kelvins,

which means it's -20 Celsius. And this is not -20, right?

And what happens there is that Earth has an atmosphere and keeps it warm.

So in principle, this planet, if it has an atmosphere,

it would have a greenhouse effect, and that would keep the planet warm.

So with an atmosphere, it might be warm enough to have liquid water.

Yes. That's the...

That's also the highlight of the discovery.

Well, it's great to be talking about this.

Congratulations again.

I can't wait to see what further research comes out

and what else is there. Thanks a lot. Thank you.

The discovery of a potential earthlike planet so close to us

instantly raises another question -

could we send a spacecraft to visit it?

Everybody ready to say goodbye to our solar system?

In science fiction, interstellar travel always seems easy.

Here we go.

In the film Interstellar,

it's simply a matter of dropping through a wormhole.

Maximum warp. Punch it.

In Star Trek, a warp drive is used to bend the shape of space-time.

Compressor.

And in the Star Wars universe, you just need to throw a switch

to accelerate past light speed and into hyperspace.

But in reality, travelling to the stars

has always seemed an impossible dream.

Until now.

We asked Jim Al-Khalili to explain why it's so difficult

to travel to the stars and to investigate the technology

that might be about to make interstellar travel possible.

For decades, centuries, even,

we've been wondering what kind of fast engines would be needed

to carry us to the stars.

There's one simple overwhelming problem when it comes to

travelling across interstellar space.

As Douglas Adams once said, "Space is big. Really big."

And so far we've only been able to explore the tiniest fraction of it.

The craft that we've sent furthest into space is Voyager 1.

Launched in 1977,

it visited Jupiter and Saturn before heading for the outer edges

of the solar system. Now, nearly 40 years later,

it's escaped the solar system and has started the journey

through interstellar space.

But it has a very, very long way to go

to get as far as Proxima Centauri.

Any practical mission to the stars would need to get there

in a reasonable amount of time - say 20 years.

But that means going incredibly fast.

The distance between Earth and Proxima Centauri

is just under 4.25 light years.

Now, that works out at roughly 40 trillion kilometres,

or 4 x 10 to the 13.

Now, in order to cover this vast distance in 20 years,

a spacecraft would have to travel at 20% the speed of light.

That's roughly 64,000 kilometres per second.

If you compare this with the speed that Voyager currently travels at -

a mere 17km per second.

It is this disparity between the speed that is required

and what is commonly achievable that has always made interstellar travel

seem almost impossible.

The biggest problem in reaching the speeds needed

for interstellar travel is the sheer amount of energy required

to produce the acceleration.

The Saturn V was the largest and most powerful rocket ever built.

It weighed nearly 3,000 tonnes and almost all of that was the fuel

required to propel its meagre 44-tonne payload to the moon.

Accelerating a spacecraft to the speeds needed to reach the stars

would require much more energy than you could ever produce

with a conventional rocket.

It would need a completely new type of propulsion system.

In the 1970s, the British Interplanetary Society

set out to see if it was possible to design a spacecraft

that could travel at 12% the speed of light.

Such a craft would reach Proxima Centauri in about 40 years.

They called it Project Daedalus.

And here it is.

It was to be huge craft - 200 metres along -

and to save on the energy of getting it off the Earth's surface,

it was to be built in orbit.

Now, it would be powered by a nuclear pulse engine

using nuclear fusion, a technology that hasn't even been invented yet,

but that was seen to provide much more energy than chemical rockets

that we use today. Still, to get it up to speed,

it would need 50,000 tonnes of deuterium helium-3 fuel

that would be stored in these vast tanks.

Now, there's not enough helium on Earth for this,

so they suggested that helium could be harvested

from the surface of Jupiter.

Easy, really.

Perhaps, unsurprisingly,

Project Daedalus never made it off the drawing board.

But, more than 40 years later, there's another suggestion.

In April, Stephen Hawking and Internet billionaire Yuri Milner

announced that they were putting up $100 million to develop

a new interstellar project called Breakthrough Starshot.

For the first time in human history,

we can do more than just gaze at the stars.

We can actually reach them.

There are two key features to this new system.

The first is that the spacecraft won't be carrying its own engines.

Instead, it will have a sail that is propelled by the force of light.

Released from a launcher in orbit,

the spacecraft will be accelerated by the second new concept -

a vast array of lasers fired from Earth.

Theoretically, the planned 100 gigawatt laser

that has about the same power output as 100 nuclear power stations

could accelerate a spacecraft to nearly a quarter

of the speed of light in about two minutes.

It would reach Mars in just half an hour.

It would overtake Voyager in about four days

and it would get to Proxima Centauri in little over 20 years.

There's only one problem -

to reach those speeds, the spacecraft will have to be

incredibly light, probably weighing no more than one gram.

It's not exactly the Starship Enterprise,

but what could you achieve with a one-gram spacecraft?

I called up Harvard cosmologist Avi Loeb,

one of the scientists behind the project, to find out more.

Avi, this is a hugely ambitious project.

Do you really think it's possible?

Yes, we hope that we can achieve the goals

of this very ambitious project

within the lifetime of our generation.

This project is as ambitious as was building the pyramids

or building cathedrals in ancient times.

You can think of it as the cathedral of our generation.

The only difference from past cathedrals is that it reaches

all the way out the stars.

And what about the cost?

Presumably this is going to be hugely expensive.

The cost is up to $10 billion -

of the order of the biggest science projects that we encountered so far,

such as Cern or the James Webb Space Telescope.

A critic will say that's a lot of money to send a one-gram spacecraft

through space. How much science can you do with a one-gram payload?

Fortunately, these days we can pack a lot of smart electronics

into a single gram. If you look at a cellphone and strip it

from the protective case

and strip it from the human interface,

you're left roughly with a gram, and that includes a camera,

a communication device, navigation -

all of the ingredients we need in the Starshot spacecraft.

And presumably, if the technology is successful,

it could be used for other than just interstellar travel.

Yes, this technology can be used to explore the space in between us

and the nearest star.

For example, we could search for life within the solar system.

It would take us only a few days to reach Pluto,

instead of about a decade that it took New Horizons to get there.

And so, in principle, the technology that we develop

will allow us to probe the edge of the solar system

within a relatively short time.

And what's the timescale for the project?

What happens next?

The first five to ten years will be dedicated to a feasibility study,

where we will demonstrate the technology of reaching a speed

far larger than previously reached with chemical rocketry

in a laboratory set-up.

And after demonstrating that, we hope to expand the system

until we reach the final design

within about 20 to 30 years from now.

Following that, we hope to launch the spacecrafts,

and it will take them about 20 years to reach Alpha Centauri,

and another four years for the signal from them to teach us.

And so, altogether,

we hope to get those signals while we are still alive.

I'm the same age as you,

so I just hope we're both around to see this project completed

and successful in our lifetime.

I wish you the very best of luck.

Thank you so much.

Before the system becomes a reality,

there are many other technical problems to solve...

..like building a material that can withstand a 100 gigawatt laser

without burning up,

and how to get a signal back from a tiny spacecraft

hurtling away from us at 20% the speed of light.

It's an exciting prospect.

There are still many practical problems to solve and, you know,

it's still hard to believe that it would succeed.

But looking at this project makes me realise that something I always

thought was unreachable may actually be possible.

If it succeeds, it wouldn't just revolutionise space travel,

it would vastly increase our knowledge of the universe around us.

And who knows? With the will and the money,

it may actually happen in my lifetime.

If we do develop the means to travel to the stars,

Proxima Centauri won't be our only destination.

There are other nearby stars we could visit.

Pete has been identifying some of the other potential targets.

Within 15 light years of the sun, there are approximately 58 stars

in 39 separate stellar systems, each being very different.

This group of stars are the closest to being within reach

of an interstellar mission.

Many of them are cool red dwarfs like Proxima Centauri,

and the more optimistic studies place at least one planet

in the habitable zone around each one.

One of the closest red dwarfs is Barnard's Star.

Just six light years away,

it has the highest proper motion of any star in the sky.

At present, it's well placed in the west-southwest at around 9pm,

positioned off the eastern shoulder of Ophiuchus.

To find it, look for a faint V in the sky known as Poniatovski's Bull.

Barnard's Star sits to the top right.

The closest sunlike star to ours is Tau Ceti.

11.9 light years away, it sits low in the constellation of Cetus,

rising in the east-southeast,

and is one of my favourite stars to observe.

It's at its highest from 3am.

Locate the Great Square of Pegasus,

then follow the left-hand side down

to a bright star known as Deneb Kaitos.

Off to the left is a quadrilateral of fainter stars,

Tau being the southernmost of these four.

With a possible system of five planets in orbit,

including one in the habitable zone,

Tau Ceti would make an exciting target for an interstellar mission.

There are likely to be unique and bizarre planets in orbit

around almost all of the stars in the sky.

And if we could send a tiny probe to just one of these stars,

imagine how amazing it would be to be able to look at

another solar system at close quarters.

Proxima b is undoubtedly an exciting discovery,

but just because it could have liquid water on the surface

doesn't mean it's going to turn out like Earth.

And it certainly doesn't mean that it will be habitable,

because planets in very similar environments

can develop in very different ways.

Just look at Earth and Venus -

twin planets of about the same size

and at a similar distance from the sun.

But they've developed very differently.

Where the Earth became a warm and temperate world, a haven for life,

Venus lost its water and succumbed to a runaway greenhouse effect.

Its sulphurous atmosphere heated its surface

to more than 450 degrees centigrade.

It is completely unsuitable for life.

Proxima b might be like Earth,

but it could equally well be like Venus,

or like something else entirely.

And so, what can we say about conditions on the planet

and about its chances of being hospitable to life?

Maggie has been talking to expert on planetary atmospheres Jo Barstow.

Yes.

So, Joanna, can you tell me how excited you are about

the discovery of this new exoplanet?

Well, incredibly excited.

I think this is pretty much going to transform the field that I work in.

How is it similar to Earth, or how is it different?

Well, one of the things we think is the same based on the mass

that's been measured with these new results

is that it's likely to be rocky, and that's a good sign.

That means that it should have a solid surface,

that means that there should be potential, maybe,

for something to live on that surface.

The major difference is driven by the fact that it's orbiting

much closer to that star,

and that introduces all sorts of potential problems.

And one of those is that we think the planet

is something we call tidally locked.

And I have here a very small star.

Oh, yes.

And a very large planet. And a very large, not to scale, planet at all.

So what's happening, because the planet is so close to the star,

tidal forces mean that the same side of the planet

is always facing the star.

So as it goes round the star, it's rotating like this.

Its day is actually the same length as its year.

So that's like the moon? Exactly like the moon.

From Earth we can only see one side of the moon because that's the side

that always faces the Earth.

And so what that means is that one side is getting all of the light

from the star and therefore getting much hotter

than the other side of the planet.

And that could potentially produce

very extreme temperature differences.

So, looking at life, how does that impact?

Is there any way of evening out that temperature or do you always have

that sort of dichotomy - the hot side and the cold side?

Well, thankfully, if the planet has an atmosphere, then it might help

to even out that temperature difference.

The atmosphere actually sort of lets the heat be distributed

around the planet? Yes.

Basically, it enables the heat to be distributed from

what we call the day side, the side that's receiving all the light,

round to the night side, and it evens everything out.

What is the likelihood of having an atmosphere?

I mean, because it's closer to that star.

Yes, and that is also a bit of a problem.

I mean, we want it to have an atmosphere quite apart from the fact

that it can even out temperature differences to give any life there

something to breathe. And if you were going to have an ocean

or liquid water, then you also need to have an atmosphere.

But because it's so close to the star, it's possible it may no longer

have an atmosphere, even if it did once.

So this star doesn't give out as much light as the sun,

but what it does do is it gives out about the same amount of X-rays

as the sun does. And for us out at Earth,

the sun's X-rays are not an enormous problem,

but if you imagine being 20 times closer,

then suddenly those X-rays do become a bit of a problem.

X-rays are not great for life.

Also, when this star experiences what we call coronal mass ejections,

which are events where some of the material actually leaves the star

and goes out into space,

that causes on Earth beautiful auroral displays,

but for a planet like Proxima Cen b...

That much closer. ..then you're going to have problems, potentially,

because those coronal mass ejections could actually start

to eat away at the atmosphere of that planet.

And if it experiences enough of those,

then eventually the atmosphere could potentially

get physically stripped away.

Let's assume that this planet has an atmosphere

and it's a benign atmosphere, it has liquid water -

what sort of life do you think could possibly live on this planet?

Well, I think we can fairly safely say it isn't going to look

exactly like life on Earth.

And one of the things that I think you're very unlikely to see

are lots of beautiful green, leafy plants.

If there is any kind of plant life,

it's likely to be a different colour.

And the reason for that is that plant life on Earth has evolved

to take advantage of exactly the kind of light

that we receive from the sun.

Now, the star Proxima Centauri is a much redder star than the sun,

so it puts out much more light in the red part of the spectrum.

It also puts out quite a lot of infrared radiation

that we can't even perceive.

And so that means plant life on that planet, if there is any,

it could look red or it could even look black or grey.

What do you think the probability is of going there?

I mean, there are really exciting projects like Starshot.

Do you think we'll ever get there within our lifetime?

I think, actually, it's possible,

and that's the first time I've ever thought it's possible,

which is why I'm so excited about this.

The thing about Starshot is that, unlike most of the ideas

that are thrown around about interstellar travel,

there aren't actually any hard theoretical barriers to doing that.

It is theoretically possible.

It's a technological challenge, but it's perhaps of a magnitude

similar to challenges we've already overcome as a species.

I can see why you're excited. Yes!

Well, thank you. That's been fascinating. Thank you.

Well, Maggie, you're the engineer here.

Do you really think this idea of an interstellar probe is possible?

I'd like to think so,

but the problem is the stars are so far away,

so the technical challenge is quite huge.

But looking at the theory, it does seem viable.

It's an exciting solution as well.

It's like something out of science fiction -

we have a giant laser pushing this probe towards the stars.

It's a wonderful story to tell.

It is. And I think it's going to be expensive,

but I think it might be worth the effort.

Space science is great at doing miniaturisation,

and this space probe is going to have to be tiny,

have an onboard camera, a transmitter

to send information back, and so the technology

that goes into that can help us all.

Yeah. I suppose if we've got one of these things

to go to Proxima Centauri we can send them to other stars

with other planets, we could shoot around the solar system as well.

I do find the cost difficult, though.

From a scientific point of view, I think there's probably

other places to spend the money.

But the inspirational value is great.

Knowing that that planet's there,

it would be sad if we weren't trying to get there, don't you think?

I think so. It's our next-door neighbour star,

it's got something that looks fairly earthlike -

we've just got to go there, and this seems like a good way of doing it.

Yeah. Just knowing the probe is on the way would be so exciting.

Well, that's all we've got time for this month,

but do make sure you check out the star guide, which is on the website.

We'll be back next month with a final update on the Rosetta mission,

including the latest exciting images that reveal the fate

of the Philae lander that disappeared on the surface

of a comet nearly two years ago.

But, in the meantime, get outside and...

get looking up. Goodnight.

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