Original subtitles

Everything in the universe has a size.

Planets are big.

Insects are small.

People are somewhere in between.

Everything has a place in the grand order and we take it for granted

that things are as they should be. But are they?

Does size matter, or could things be different?

What if things were smaller?

What would be the harm in reducing the size of things here and there?

After all, being smaller has its perks.

Smaller stars burn far, far longer than big stars.

Smaller things are, relatively speaking, stronger.

And it turns out smaller people actually live longer.

Using the power of science,

we're going to do the ultimate thought experiment -

shrinking our world and everything in it, including us,

to find out if there's a reason things and people

don't exist on an even smaller scale,

or whether evolution might have taken a completely different path.

Along the way, we'll discover just how much size really matters,

and to what extent the size of something determines

the very nature of the thing itself.

You may never look at yourself, or the world around you,

in quite the same way again.

You might think this looks like an ordinary house

on an ordinary sunny morning.

But, if you watched the last programme, then you'll know

that this is, in fact, a parallel universe.

One just like our own, but with one important difference.

ALARM

In this universe, we can change the size of things

just to see what happens.

And that means our old friend, Average Joe, here,

is about to have some very strange days indeed.

In the last programme, we tried to improve Joe's universe

by making everything bigger.

The Earth, the sun and even Joe himself.

But it all turned out that bigger isn't better.

In fact, it's very often, if not always, cataclysmically worse.

So, in this programme, we're hoping we're going to have

a bit more luck by shrinking things instead, because, as it turns out,

small isn't just the opposite of big.

In fact, small throws up a whole world of other challenges.

As in the last episode, we'll be putting Joe through

three different thought experiments - resizing people, stars,

and starting with our very own planet. So, here goes.

Earth is the fifth-largest planet in our solar system.

And it's also, of course, the fourth smallest -

12,756 kilometres across,

with an atmosphere 100km deep, all the way around.

Below that is the surface, a thin layer of solid rock,

sitting on top of 5,000 kilometres of rock and molten metal.

And finally, at the core,

a 2,400km-wide ball of solid iron.

The question is - how much do those numbers really matter?

How important is it to our lives on Earth

that our world is exactly the size it is?

What would happen if we halved its width,

meaning that instead of being a shade bigger than Venus,

Earth was suddenly a shade smaller than Mars?

Well, there's only one way to find out.

And there it is, a half-sized Earth.

All made of the same stuff, same proportions,

just a little bit smaller.

Well, as it turns out, Earth's vital statistics are, well, vital,

because changing Earth's size messes with a few other things

that you don't necessarily want to be messing with.

For instance, gravity.

As we discovered when we made things big,

the gravity of a planet is in proportion to its width.

So, a half-sized planet would mean half the normal gravity at the surface.

Enough of a change to put a spring in your step.

And if you're athletic, like Joe here,

who knows what you might be capable of.

So, half gravity would mean you can jump higher...

..and only fall at half the speed.

Which would take a little bit of getting used to, but, by and large, sounds rather fun.

So, you might ask, what's the catch?

Joe's fun wouldn't last long.

The universe is a system, basically,

so size is important in the sense that if you change the size of one

single element, with respect to the others, then the whole thing breaks.

You see, gravity doesn't just affect people and objects.

It also affects the air we breathe, which would get thinner

because it would be less strongly attracted to the ground.

The air at sea level would now be as thin as it used to be

two-thirds of the way up Everest.

There would be less oxygen in every breath.

Headaches, nausea and shortness of breath would soon follow,

as Joe found himself with a nasty case of altitude sickness.

Luckily for him, humans can acclimatise to thinner air.

After a few days, Joe's red blood cell count

would increase enough to compensate for the reduced oxygen.

So, he'd be back up and about in the low gravity of a half-sized Earth,

just in time to notice that it's beginning to look a lot like Norway.

The spectacular phenomenon that is the aurora isn't usually seen

beyond the polar regions, but our new half-size Earth

would be lit up all over.

To get the inside track on how and why,

we're going to the University of Maryland.

Daniel Lathrop has spent 20 years building models

of the inside of planet Earth, to help understand

how our planet generates its magnetic field.

And that's given him a unique insight into the workings of the aurora.

Dan's model has a solid metal ball at its centre,

surrounded by a thick layer of molten metal.

Just like planet Earth.

As the Earth rotates, the currents of molten metal

generate a magnetic field.

Dan built his model to study how this happens, and along the way,

he's discovered just how important the Earth's magnetic field is.

So, the Earth's magnetic field serves as a shield

against the worst parts of bad, uh, solar weather.

So, the sun has storms that occasionally give large amounts

of radiation aimed at the Earth, and the Earth's magnetic field

inflates something like a bubble around the Earth,

the magnetosphere, that acts as a primary barrier to the worst of the radiation.

The shape of the planet's magnetic field funnels this cosmic radiation

towards the poles, where it hits the upper atmosphere,

causing gases to glow and giving us the beautiful,

ethereal lights of the aurora.

Of course, Dan never set out to study what would happen on a smaller Earth,

but it just so happens that he's been studying just that all along,

because over the years, he's built several versions of his model Earth at different sizes.

So these, actually, were the first three sodium experiments we built

to try to understand the Earth's magnetic field. So the first one,

a 20cm-diameter model, rapidly rotating.

Next came the 30cm experiment.

There's an inner sphere deep inside there that you can't see.

And the third experiment, at 60cm,

here's the bottom half of the outer sphere and then a solid copper model

of the inner core that independently rotates.

Then the whole thing would be filled with liquid sodium, in the experiments.

Thinking about what it would be like if the Earth were half size,

we could then examine data between the different-size experiments

to see how the magnetic fields are different.

Instead of iron and nickel like the Earth,

Dan's model is filled with sodium, because of its low melting point.

But it still takes three days before it's all molten and ready to spin.

So here we see magnetic field data from the 30cm

smaller experiment, and comparing it then to more recent data

from 3m, it's very evident that as the experiments

have gotten larger, we have much more magnetic induction,

much stronger magnetic fields overall.

A smaller Earth would have a weaker magnetosphere,

but that's not all Dan's experiments reveal.

When we go from larger to a smaller experiment,

the magnetic fields' strengths have both become weaker

and have changed pattern. And if you look at, you know,

the data of the larger model, there's kind of well-defined

north-south magnetic poles, where in the smaller experiment,

at these parameters, we had like a ring of south poles

around the equator, and then two magnetic norths at either end.

So, it is possible for the shape of the magnetic fields to change

when you change its size.

Half-sized Earth would likely have many magnetic poles

spread around its surface,

which would explain how Joe has ticked seeing the northern lights

off his bucket list without ever leaving his back garden.

The bad news, however, is that these multiple auroras

are a sign that Earth's weaker magnetic field is being overwhelmed

by the barrage of solar energy.

And from here on in for Joe, it's all going to get a bit dark.

If you have a smaller planet with a weaker magnetic field,

there will be more problems with telecommunications.

The sun still has these big, erm, anger things -

they're called coronal mass ejections, where it really sends

a burst of radiation in space.

We have systems on Earth which are so big, depending on electricity,

that when the sun is angry,

potentially you have bigger ejections and it causes problems.

And with every blast of solar wind,

a weaker magnetic field also wobbles more,

which induces surges in electrical systems down on Earth,

and generally sending us back to the Dark Ages.

SHE LIGHTS MATCH

Even on a normal-sized Earth,

sometimes the radiation from solar storms can punch through

the magnetic field and cause big problems.

On March 13th in 1989, the entire province of Quebec in Canada

suffered a power cut that lasted 12 hours.

The cause? Radiation from a solar storm tripped circuit breakers

at a hydroelectric power station.

With only a weaker magnetic field to protect us,

these things could happen all the time. Ow!

And that's just the start.

A weak magnetic field would also double down on a problem

that Joe was already struggling with.

Reduced gravity would mean gases were finding it easier to escape into space.

Increased cosmic radiation would supercharge that process.

And, for Joe, that combination would be a major problem.

Now, cosmic radiation has a particularly bad effect on oxygen.

Quite quickly, it would cause all the precious oxygen in our atmosphere

to escape off into space, leaving only an unbreathable mixture

of heavy gases like nitrogen and CO2.

And that, of course, would be a death sentence for Joe

and all other animal life.

Still, this should keep him going for a bit.

But, over time, half-sized Earth would end up as a barren,

uninhabited wasteland, just like Mars.

It's the curse of small planets.

So far, so apocalyptic.

In pursuit of a smaller world, we sentenced poor Joe to a slow,

lingering death, and turned Earth into an uninhabitable desert.

I think it's probably time to put it back to normal.

And there we are. 12,756km across, planet Earth,

a shade bigger than Venus, and just as it should be.

ALARM SOUNDS

Maybe it's time to try something a little less ambitious,

something that might work out a bit better for Joe.

What about us?

As mammals go, we're pretty big.

Maybe we could stand to lose a little.

There are about 4,000 mammal species in the world

and they come in all different shapes and sizes.

The largest, of course, are the whales.

The blue whale is absolutely enormous,

the size of several school buses put together.

And the smallest mammal is very small, two grams.

It's essentially the size of your thumb.

And the typical size of a mammal, however, erm,

is not sort of in the middle. Instead, it's much closer

to the smallest size, about 40g, which is the size of a rat.

Humans are about 65 kilos, on average, give or take,

erm, and so, that makes us enormous.

When we look at an elephant, we may feel small. But, in fact,

humans are around 1,600 times heavier than the average mammal.

But that's not necessarily good news.

Aaron Clauset is a data scientist who studies the relationship

between size and extinction.

What we found is that the larger an animal is,

the more likely that species is to go extinct in the long run.

And there are various reasons for this.

Typically, species that are larger have smaller populations, and so,

if there happened to be a few bad years in terms of reproduction or food,

then their population could crash, and as a result, they could become extinct.

Whereas, much smaller animals typically have much larger populations,

and so they are robust to these kinds of events.

So, in general, the larger the animal is, the faster it goes extinct.

Bye-bye. And, when you think that in recent times,

the average human has been getting bigger and bigger,

that might give us cause to worry.

Does this mean then that we are accelerating towards extinction?

Well, possibly not, if you take a wider view.

Back in the Stone Age, when humans were hunter-gatherers,

the average male height wasn't that far off what it is today.

But about 12,000 years ago, during the Neolithic Revolution,

when we started farming, we quite quickly became

considerably smaller, as our new grain-based diet had a lower nutritional value.

And it's really only much more recently that we have finally

got back to hunter-gatherer size,

thanks to modern improvements in food and medicine.

Well, that's a relief. Being a big species means a long lifespan.

But not getting bigger means we don't have to worry about extinction.

A big win all round, then. Well, not necessarily.

You see, big species might have longer lives than small ones,

but within each species it seems to be the other way around.

Generally, smaller individuals of a species live longer.

For instance, smaller dogs live longer than big ones.

Could this also be true for us?

Geneticist Diana van Heemst has the answer.

I guess, you know, in a lot of species, and if we look at dogs,

horses, elephants, it's actually the smaller variance of that species

that seem to live longer, and of course, then, the real question is,

does it also apply to humans?

Well, that's not so easy to answer.

There are so many factors that influence human lifespans

that it's hard to tell what's down to size and what's down to, say,

diet or exercise. To get to the bottom of this,

Diana has been re-examining a remarkable 1970s study,

which homed in on a group of people with very similar lifestyles,

but varying heights - professional athletes.

For example, American baseball players,

there's a nice encyclopaedia which is a rich source of information,

not only for the baseball fans about, you know,

all the details about performances and nicknames, but also, actually,

it contains date of birth, date of death,

the adult's height and their weight.

If you know their height and age of death,

you can start looking for a pattern.

Wally Burnette - 1.83m.

Murry Dickson - 1.78m.

The original study used data from hundreds of players,

but we can see what they discovered by looking at just a few.

I took from the Encyclopaedia of Baseball

nine representative examples of baseball players,

and we have, you know, attached to them to the wall based on the height

and the age at death, and this mimics the original study,

which made use of the full sample of the encyclopaedia,

which found this negative correlation between height and the age at death.

The 1970s study found that size DID matter.

Being five centimetres shorter meant, on average,

you would live for two years longer.

The big question is, why?

And it's only now, four decades later,

that researchers like Diana van Heemst have come up with a plausible explanation.

In order to grow, our body makes growth hormone, which,

you know, stimulates growth, but at the same time,

it also influences lots of other processes in our body.

And if we look at the data that has been derived from work on other animals,

they stimulate the body to grow, and this is kind of a signal

that there's enough food, that - you know, there's favourable conditions,

that it would be wise to invest as much as possible energy

in growth and reproduction, and this may come at a cost,

because it means there's less energy available to invest simply

in maintaining our bodies in good shape.

And actually, when conditions get worse, or become less favourable,

like when there is a food shortage, or a lot of toxins,

then, as a consequence, as a response to that,

we don't grow, we kind of stop growth,

and we really invest the available energy in maintaining our body

and trying to kind of, you know, survive these periods of hardship

until things get better.

It seems like being big comes at a big price. But for tall people,

there is some light at the end of the tunnel.

However, size is not the only thing that matters.

There's lots of things that people can do themselves to adopt

a healthy lifestyle, like, you know, not smoking, healthy food, lots of exercise.

So, it matters, but it's not the only thing that matters.

So, just how small could we go?

Well, let's start with what we know.

The smallest adult humans known to science

are just over 50cm tall.

I used to wonder what it would be like to be a bit shorter,

mainly because being this tall, finding clothes to fit is a bit of a pain.

But today, I'm wondering what it would be like

to be substantially shorter.

Like, a third of my current height.

And that is because I'm about to meet the one person in the world

who can tell me what it's like.

23-year-old student Jyoti is on a sightseeing trip to London.

Wherever she goes, she gets as much attention as the biggest attractions.

TRANSLATION: When I go outside,

then everyone gathers together and stares at me.

Then I feel a bit strange.

And at the same time,

it feels good that they all look at me.

Did you have this one specially made, then, or did you just happen

to find a small spoon somewhere?

TRANSLATION: I haven't had anything specially made for me.

I can easily find things in India in the market, in the baby section.

In our last episode, we met Sultan Kosen, the world's tallest man.

Sultan's incredible stature is down to his body producing

too much growth hormone, a condition which has gone on to cause him

considerable health problems in his adult life.

For Jyoti, though, the story is a happier one.

TRANSLATION: The doctors told me I have hormone deficiency.

This is the reason I can't grow taller.

I don't have any other health problems.

The only complaint that Jyoti has is a practical one.

The world is simply too big for her.

TRANSLATION: One thing which I can't do because of my height is drive cars.

And when I want to go out, I can't go out alone.

I always have to have help from my family,

like my sisters and brothers. I always need help.

These are the problems I face.

But in an environment that's made to measure, she fares much better.

TRANSLATION: In my house, everything is specially made for me.

In my bedroom, I have a small bed,

cupboard, chair, table,

and everything made in my size, all my furniture.

I don't have any problems in my house.

So, what if we were all smaller?

In fact, there's a group of scientists who think

that smaller humans would solve some of humanity's biggest problems.

Food would go further, we'd all live longer, and what's more,

they reckon there'd be less disease and fewer wars.

A tiny utopia.

So, let's just say they're right.

How much further could we shrink Joe?

Let's go from the size of the smallest human...

..down to the size of the smallest mammal.

This is an Etruscan shrew.

In the wild, they weigh just two grams,

which makes them the smallest mammal, by weight, in the world.

Potentially a good role model, then, for a miniature human being.

Professor Michael Brecht has studied them for years.

Ah, here they are.

Cool. And I want to chase them into this.

So, now, here we have them.

Let me show you what we do for gender determination.

So, the sexes, they look quite similar.

The really foolproof sex testing is what I'm going to do now.

So we actually use this box here, and what you do is, carefully,

you put the shrew into the little box and you carefully sniff on it.

Now, if it's very, very stinky, it turns out it's a female.

If you sniff on it and you pass out, it's a male.

So, let me do this here.

Female.

OK. Now, let's figure out how much she weighs.

This is on the higher side for these animals.

Many of the adults are just two grams.

They have perfectly the same mammalian equipment, it's all there,

it's just very tiny.

Like, uh, it's very difficult to circulate blood

through such a small body.

The circulation system of mammals is much more suitable

for bigger bodies, and both the respiration and the blood supply

are a huge challenge for such a small body.

So what we would see is they have a giant heart, yeah,

5% of the body weight or so, a really big heart.

What we also see is that they have unheard of respiration rates.

one would see a breath per minute go up to about 1,000 breaths

per minute, an absolutely unheard of rate in mammals.

It's really also difficult to understand how a mammalian brain

and lung could do that.

1,000 breaths a minute is hard to fathom,

but their hearts push things even further,

beating up to 1,500 times a minute.

That's 20 beats for every beat of a human heart.

It's clearly hard work for a mammal to be so small.

The question is, why bother?

The idea that ecologists have about these animals

is that they are specialists for small spaces, yeah?

For tunnels, and they go into small spaces where no other predator

can go, and then, paradoxically, they are, again, big predators.

Matching your size to your environment is an important part

of evolution, but filling this particular niche

has its difficulties.

The biggest problem they face is heat loss.

You see him in a thermal camera and you see how much heat he gives off,

how much he lights up.

And this is actually a central problem of their life.

Uh, the immense heat loss they have, or energy loss they have,

is a result of their unfavourable surface-to-volume ratio.

If you peeled me, and please don't, by the way,

but if you did, and then measured my skin,

you would find that I have around a quarter of a square centimetre

of skin for every gram of my body weight.

But if you peeled an Etruscan shrew, and likewise, don't,

you would find that he's got a lot more,

around 20 times more for every gram.

Now, this is all down to the square-cube law,

which states that as a shape grows or shrinks in size,

its volume changes much faster than its surface area,

and one consequence of having a proportionally larger surface area

is smaller animals lose heat much faster.

Losing heat is particularly bad news if you're a mammal like Joe,

because unlike insects and reptiles,

mammals have to keep their bodies at a constant temperature

of around 37 degrees centigrade.

And if you think these furry little fellows have problems keeping their

body temperature up, try being one of their babies.

But somehow, with the help of their parents, they survive.

The newborn shrews are incredibly small, inconceivably small.

0.2 grams is just absolutely incredible,

and they look kind of unreal.

I mean, their whole body is totally transparent.

They huddle together very heavily.

The mother is very protective and, obviously,

also supplies a lot of energy.

So, it's feasible, then, that a human could exist

at just five centimetres tall.

But, of course, we'd face all the same problems as the shrew,

an insane heart rate and a constant battle to keep warm.

Life at their size requires a totally different lifestyle.

It's not just about huddling together for warmth.

If you're losing energy fast, you need to be very good

at replacing it.

In fact, scientists have discovered that small animals

have to have a completely different relationship to food

than big animals.

More than 100 years ago,

a scientist named Kleiber observed empirically that the amount of food

that an animal requires increases, of course,

with how big the animal is, so, an elephant eats more than a deer does,

but that the relationship doesn't go up proportionally.

So, an elephant eats a little bit less than you'd expect

than an equal number of deer would.

An Asian elephant weighs about 5,000kg,

but how much does it eat?

Just ask a zoo keeper.

This is the amount of hay one of our male elephants gets every day,

43 kilos to 45 kilos.

About 1% of its body weight.

For the dik-dik, their mass is about seven kilos.

This is the amount of alfalfa our dik-dik get on a daily basis,

0.5 kilos.

Which works out as 7% of its body weight.

In fact, if you map out the amount of energy animals need to eat

relative to their body weight,

a very clear pattern emerges that holds true

for just about all creatures.

It's called Kleiber's law and it shows that as you get smaller,

the amount of food you need, relative to your size,

increases rapidly.

Or, to put that another way, the smaller you are,

the hungrier you get.

Applied to five-centimetre-tall Joe here,

Kleiber's law tells us that he'd have to eat his own body weight

every day. Most of his life would be spent looking for, and eating food.

Just like the Etruscan shrew.

But what if we went smaller still?

So, if you were to take a mammal and, um,

to make it smaller than the smallest current mammal,

then it would cease to be a mammal as we know it,

because the rate at which it would lose heat into the environment

would be so great that it couldn't maintain its internal temperature

to be warm-blooded, and so it would have to change its physiology,

it would have to become cold-blooded and use different strategies

in order to regulate its temperature.

Going smaller means saying goodbye to being a mammal.

From here on in, we'll need to be cold-blooded

with organs more like an insect. But it's worth it,

because incredible things start happening

once you get down to the size of a wasp.

At Cambridge University, they're finding

that for very small creatures,

the world is a completely different place to the one we experience.

It's almost as if they're ruled by different laws of physics.

In terms of their relative strength,

you might almost say that insects are superheroes.

So, some of the strongest ants can easily carry four or five times

their own body weight, which, for us,

is the equivalent of almost a small car,

if you're a relatively big human.

This is all because volume, area and length change by different amounts

when you make things smaller.

The overall effect is to make small creatures much stronger

than big ones, relatively speaking.

An ant supports the weight of a paintbrush,

which is roughly 2.5 grams...

That corresponds to around 500 times its own weight,

which would be the equivalent of me for 40 tonnes,

which is probably about six, seven lorries.

So that's really quite impressive.

But there's more to being small than strength.

Now, it turns out that our old friend, the square-cube law,

which means that small creatures lose heat more easily

because of their large-surface-area-to-weight ratio,

also has an upside if you find yourself falling from height.

A five-millimetre human falling from a countertop is equivalent

to a normal-size human falling 300m.

So, Joe can be forgiven for thinking that it's not going to end well.

But it does.

So, one of the very first studies that thought about the question,

how size matters and what the right size for an animal is,

thought about a problem of why you can drop an ant down a shaft

and the ant just falls on the ground and walks away,

but if you would do the same to a human, the human would break.

Because the ant is so small,

air resistance is much more important for the ant,

so the velocity with which the ant hits the ground

is much slower than what would happen to a human.

Taken to an extreme, it's why rock dust floats in the air,

but rocks don't, even though they're made of the same stuff.

And there are even more advantages to being so small.

So, one of the things we're interested in

is how well insects stick,

and one of the techniques we use to measure that is a centrifuge.

So, we take a little ant, put it on a centrifuge,

and start spinning it around.

And we then try and measure at what acceleration these ants

actually fall off the centrifuge.

Wow.

G-force, or G, is the name for the feeling

you might get on a roller-coaster.

In the tightest turn, you might experience 6G,

but even with much higher G-forces, somehow the ants hang on.

And during these measurements, we've seen ants withstand 500G,

1,000G for 10, 20, even 30 seconds,

and then they fall off, and walk off as if nothing happens.

It's this stickiness that allows insects to walk up the smoothest of

walls, even hang on to the ceiling.

So, how does it work?

Insects, or geckos, or any animal that climbs with adhesive feet,

they can't use a glue,

because it will take a long time to activate and deactivate.

So, as far as we know,

climbing animals use intermolecular forces to stick to surfaces.

It's still under debate what exactly these intermolecular forces are.

If you have two molecules and they attract each other,

then you have to convince them to split apart,

and that's what helps these animals to stick.

We experience these intermolecular sticking forces, too,

but at our normal human size

we're not even aware of them because they're tiny compared to gravity.

But once more, being small changes the rules.

A five-millimetre human could climb a wall just like an ant.

But it's not all good news for small creatures.

A five-millimetre human may be good at climbing,

but he might not understand why he's doing it.

It's a problem that affects all very small creatures, brain size.

Our brains rely on our neurons and it looks like that neurons remain

relatively constant in size across different animals.

So, whether you're a very small animal, or a very big animal,

the neurons are approximately the same size.

Now, this immediately means that if you're very small,

you have fewer neurons, and that might present you with a problem

regarding your cognitive abilities.

The bottom line is, if you're going to get really small,

you're going to lose brainpower.

A five-millimetre-tall Joe would only have around

two million neurons, which would put his ability to do crosswords

somewhere between a cockroach and a small fish.

He'd be smart enough to spot food, but probably not smart enough

to worry about the puddle of coffee in the way.

For normal-sized people, surface tension is barely noticeable,

but when you're tiny, it's suddenly deadly.

Surface tension is a force that becomes very, very powerful

if you're very small, and really unimportant if you're really big.

So, for very small animals, a droplet of water

can be very dangerous,

while very large animals will hardly notice the droplet.

So, great - tiny humans might be able to climb walls,

and carry things 100 times their own body weight,

but what's the point if you're too stupid to tie your own shoelaces,

constantly looking for food, and if the wasps don't kill you,

the coffee will?

Because if that's what being a human is, you can count me out,

and I think that's the point.

There is a sweet spot for human sizes that works

for the way the world is now, and it's no coincidence

that we are all in it.

Other conditions would have provoked other sizes,

because that's how evolution works.

So, having established that, we should probably put Joe

out of his misery. So, back to normal with you.

In fact, it turns out that the size we are now is a perfect fit

for the way we live, and the world we live in.

Lifespan, health, food, society, resources,

it all goes hand in hand with our size.

We've tried shrinking the planet, and even ourselves, but so far,

smaller has not proved to be any more beautiful.

But there's one thing we haven't tried...

..something so big that surely we could make it a little smaller,

without ending life as we know it...

..the sun.

Perhaps a smaller sun would be a good idea.

Our current sun is a kind of star known to astronomers

as a yellow dwarf.

But, of course, it's not really much of a dwarf by Earth's standards.

In fact, it's 1.4 million kilometres wide.

That's 109 times wider than Earth.

Just to put that in perspective,

flying around the world nonstop in a commercial airliner

would take round about two days.

But if you were circumnavigating the sun nonstop,

apart from the whole being burned alive thing,

just to do one loop would take you six months.

Guess I'd better settle in, then!

But does it have to be so large,

or would everything work out fine with a smaller, gentler sun?

One that wouldn't damage our skin.

One that we could safely look at with the naked eye.

Well, the key to this question is understanding what makes a star

shine in the first place.

A star is actually just a big ball of gas,

which is pulled together by gravity.

At a certain point, the mass of the ball gets so large

that superheated gas at the centre begins to fuse together.

This is, of course, nuclear fusion,

which generates such enormous amounts of energy

that we've been trying to replicate the process on Earth

ever since it was discovered.

The problem is that it's incredibly hard to do.

This is one of the world's best attempts...

..JET - Joint European Torus. The first place on Earth they managed to

achieve controlled nuclear fusion.

It's a phenomenally complicated and expensive facility,

which uses as much power as a small town,

in very short bursts, to superheat gases until they fuse.

The fusion takes place inside this chamber.

We actually create a highly ionised gas, or plasma.

The centre of the plasma, which is probably roughly just above my head,

would be where the temperature and the density are at the maximum.

The temperature there could be something in the region

of between 100 and 150 million degrees centigrade.

Conditions inside the machine are so extreme

that they can only run it for 30 seconds at a time

before it becomes unstable.

Today they're running the fusion test at even higher power levels

than they've tried before.

Ten, nine, eight, seven, six, five, four, three, two, one, zero!

SIRENS BLARING

What you are looking at right now is fusion,

the very same process that happens at the heart of a star.

The challenge of making fusion happen on Earth

is far greater than the challenge of making it happen inside a star,

because we can't resort to the simple technique that stars use,

being massive.

The main requirement, if you want to trigger fusion,

anywhere in the universe, inside of stars or in a laboratory on Earth,

is that you have to create conditions of very high temperature,

and the problem then is that if you have high temperature,

you also basically have the problem that this high-temperature ball

of material wants to be pushed out by the pressure.

So you have to somehow overcome the pressure,

and the stars do this by having all the gravity

of the overlying material.

The gravity of the star is confining the pressure of the hot material.

For stars, then, size very definitely matters.

If there's not enough material creating pressure

to contain and superheat the gas inside, fusion can't happen.

If we make stars smaller, less massive, the temperature of the star

in the centre will also go down, and at some point,

the temperature is not sufficient any longer to ignite nuclear fusion.

And this really is the fundamental limit, if you like, for stardom.

So, how small a dwarf still packs enough punch?

Well, before we get to that...

Here's a quick bit of star terminology for you.

Now, a star that can't do fusion is called a brown dwarf,

although it's not always brown and technically,

it's not actually a star.

A red dwarf is bigger than a brown dwarf,

although still small by star standards,

and is red, unless, of course, it's yellow. So that's easy to remember!

Then we have the white dwarfs.

Now, these are collapsed stars, the size of a planet,

and they no longer do nuclear fusion.

So they're very faint and should probably be called really small,

dark-grey dwarfs. Finally, there's black dwarfs.

Now, these are white dwarfs that have completely run out of energy,

but since that takes longer than the current age of the universe

to happen, they aren't technically even possible yet.

And that's just the dwarfs.

Don't even get me started on the supergiants.

Anyway, the main thing to remember is that stars behave completely

differently depending on their size,

and that their names don't always make sense.

Anyway, if we were changing the size of our sun,

there's a limit to how small we could go before fusion ceased.

In fact, the very smallest is just under one tenth of the width

of our current sun.

ALARM

So, with a sun this size, what kind of Earth would Joe wake up to?

Well, for starters, he'd be seeing red.

The reason that this lunar star would be red,

is that this star would have a much-reduced gravity,

and therefore, also, it would have a much-reduced temperature.

It would shift the peak wavelength of your photons

that the star is emitting to a longer and longer wavelength.

This means that we've shifted from the yellow, that our sun has,

into the red, that those red dwarf stars would have.

The smallest stars, like this one,

give off much less light, and less light means less heat.

Now, to put that into some kind of perspective,

the energy that our current sun sends us on average is equivalent

to 24 60-watt light bulbs per square metre,

which keeps Earth nice and toasty.

But if we swapped our sun for a tiny star like this one,

it would only be sending the equivalent of half a glow-worm

of light for every square metre.

And I don't much fancy trying to stay warm

huddling around half of him!

Within hours Joe, would find that things were getting a bit nippy,

to say the least.

What would happen to a planet around such a red dwarf central star?

The most dramatic thing is that because of the very, very much

reduced temperature, we would basically experience a deep freeze.

Within a week, temperatures would plummet...

..because a star this size gives off just one six thousandth of the heat

of our sun.

All the liquid water would be converted into ice.

Even our atmosphere would begin to freeze out.

We would enter into a state of complete cold,

deep, desperate freeze.

All water on the planet would freeze.

As temperatures dropped further, the air itself would turn solid,

causing the atmosphere to collapse.

So, how would we save the world?

The answer may seem obvious,

move the planet closer to the sun, so that things warm up again.

But would that actually solve things?

Earth's normal orbit is about 150 million kilometres from the sun,

which is in the middle of what's known as the Goldilocks Zone,

the habitable belt around the sun,

where it's not too hot and not too cold.

With the sun ten times smaller, that zone would now be 100 times closer.

So that's where we'd need to move the Earth.

Now Joe would be getting the same energy from the sun

as he was used to, but there'd be a few changes.

The new sun may be much smaller,

but we'd be so close to it that it would look much bigger in the sky,

ten times bigger than he's used to.

But other than that, would it be business as usual?

What kind of a world have we made?

Well, that's actually a question that scientists have been trying to

answer, because it may have huge implications

for the future of our species.

Looking up at the night sky,

you probably wouldn't even notice Proxima Centauri,

but it's actually our nearest neighbour,

the closest star to us outside of our solar system.

The reason you might not notice it is because it's very small.

just one-seventh the size of the sun, but it's up there

if you know where to look.

And at Queen Mary University of London,

astronomers have been looking very hard at the faint light it gives off,

to see what they can discover about the sun's tiny neighbour.

Proxima Centauri is the nearest star to the sun.

This is where astronomy begins.

So it's really the first spot in the next frontier.

So the first place to go when we go beyond our solar system.

So that makes it very special.

In August 2016, they made an astonishing discovery

by analysing the light that Proxima Centauri gives off.

So, basically, what we do, we go to a telescope,

the telescope has an optical fibre sitting at the focus,

and then the light from the star goes through the optical fibre

to the basement of the observatory, where there's a spectrometer,

and what the spectrometer does,

it takes the light coming from the optical fibre

and these two elements here, a prism and a grating,

separate the light into wavelengths.

And we see that there are these dark spots in the middle of the traces.

These are the footprints of molecules and atoms

of the atmosphere of the star.

As they continued to observe the star,

they saw that the spectrum of Proxima Centauri was changing.

So, we come here to the telescope two months later

and we take more data

and we see that the measurements start to trend.

Something is happening, but we don't know what.

We get more measurements, more measurements,

more measurements, and after two years,

then we see that it reaches a peak, and then you have this signal.

And it's repeating also.

If we keep observing the star, we see the same thing

over and over again.

These wobbles in the spectrum reveal that Proxima Centauri

is being pulled backwards and forwards.

It's the telltale sign of a planet orbiting close by.

The newly discovered planet was given the name Proxima b,

but what got the scientists excited is that this planet

has a lot of similarities to our own.

It is roughly Earth-sized, and mostly made of rock.

However, unlike Earth, which takes 365 days to go around the sun,

the spectrum patterns reveal that Proxima b takes a mere 11 days,

meaning it must be very close to its star.

In fact, the maths shows that Proxima b is also at the perfect distance

for supporting life, not too hot, and not too cold.

So could this planet one day be a viable next step,

as we branch out of our own solar system?

Might life even exist there already?

It's only right with a solar system that we can expect

to actually start to search for evidence of life in planets

like this one in Proxima Centauri and also some very nearby stars.

If the locals proved to be friendly,

could we one day, in the distant future, make Proxima b our new home?

What can our thought experiment tell us

about this newly discovered world?

Well, it wouldn't just be the red light and the huge sun in the sky

that would make things look different.

Plants on Earth are green

because they use specific wavelengths of light.

But if our sun was red like Proxima Centauri,

those crucial wavelengths would be missing,

and our green plants couldn't survive.

To stand any chance of absorbing enough sunlight,

they'd have to be black, so it would all start to look seriously alien.

And there is another surprising side effect of orbiting

so close to a star that literally stops the world going round...

..tidal locking.

So, basically, you have a small star,

the small star makes very little energy, it's also faint.

So you need to be warm, you need to be close to it,

and the fact of being close to it means that you always have

very strong tidal forces then, and most likely,

what will happen is like what happens with the moon to the Earth,

the rotation of the planet synchronised to your width

of the planet, so the same side faces the star.

In a fully tidally-locked planet, one side is frozen

in perpetual night, but the other basks in 24-hour sunlight.

Hence they're sometimes called eyeball planets.

So, have we finally done a thought experiment Joe can at least survive?

Probably not. Don't rush out to Proxima b,

because there are some big downsides to this small-star scenario.

Most of the planet is uninhabitable, too hot on the light side,

too cold on the dark side.

Oh, and I should mention that close to a star

there are huge levels of radiation.

So, at the very least, you'd have to live underground.

OK, that didn't go so well after all,

much like it didn't go well expanding the sun,

or making the planet bigger, or smaller,

or shrinking Joe, or making him into a giant.

And it all goes to show just how narrow and fragile

the universal balance is, that allowed us to exist

in the first place.

If our sun, or our planet had been a different size, there'd be no us.

And if we were a different size, well, we just wouldn't be us.

Size does matter.

Size determines on the one hand, you know, the height we will achieve,

but on the other hand, size determines our lifespan,

because size determines how much energy we invest

in maintaining our body in good shape.

A very large animal and a very small animal are from completely different

worlds. So, they face completely different problems.

Evolution has produced completely different solutions

to these problems and for scientists, it's really interesting

to try and understand how things work at small scales and big scales.

How big you are determines the scale of the world around you

and how you interact with it. The smaller you become,

the kinds of things that are dangerous to you change.

But size is also a very mutable variable, it's flexible,

and mammals have found a way to live very successfully

at all different sizes.

I think size is important in the universe.

It's a clockwork, really. You have your clock and everything works

perfectly, but if you change the size of one of the cogs,

then it doesn't fit with the rest any more

and the whole system will collapse.

We set out to examine the nature of our universe

by changing the size of things in it, but in doing so,

we've also examined the very nature of size itself.

Size isn't like colour or pattern, it's not arbitrary.

It's absolutely intrinsic to the nature of a thing itself.

Change the size of something, and what you end up with

is something else. Planets are big, insects are small,

and people are somewhere in between.

Mess with that at your peril.