Original subtitles

(intense orchestral music)

Everything in the universe has its size.

Planets are big.

Insects are small.

People are somewhere in between.

Everything has its place in the grand order.

Things are as they should be.

But does it have to be that way?

Does size really matter or could things be different?

After all, short people live longer than tall people.

Small things are stronger than big things.

And really tiny creatures can do things

we can only dream of.

So why are things the size they are

and what if we could change that?

Using the latest science,

we are going to do the ultimate thought experiment.

We are going to shrink everything in our world

including us

to see whether a smaller world really is more beautiful.

Along the way, we're going to discover

just how much size matters,

how it defines everything.

You'll never look at yourself or your world

the same way again.

(explosion blasting)

You might think this looks like an ordinary house

on an ordinary sunny morning.

If you watched the last program,

then you'll know that this is a parallel universe,

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

In this universe, we can change the size of things

just to see what happens.

(alarm beeping) (man sighing)

We're going to change the size of stars,

planets, and living things,

and see the surprising effect it has

on a normal guy going about his normal day.

We'll find out if the way things are

is the only way they can be or if size is just an accident.

In our last grand thought experiment,

we made everything big, but it all went wrong, badly wrong.

So now, we're going to go the other way

to see if small really could be beautiful.

(intense orchestral music)

So where do we start?

Since this is a thought experiment,

we can start anywhere we like.

So how about here?

Home, our planet.

Earth is the fifth largest planet in the solar system

or the fourth smallest.

Maybe we could move it down the scale

towards Venus, Mars, or Mercury.

This is the Earth you're used to,

12,756 kilometers across

with a circumference of 40,000 kilometers

which takes an airliner about two days to fly around.

There's an atmosphere

100 kilometers deep all the way around,

then a thin layer of solid rock,

five thousand kilometers of rock and molten metal,

and finally, a 2,600 kilometer ball

of solid iron at the core.

But these are just numbers.

The question is do they matter?

Is it important that our world

is exactly the size it is now?

Surely, it's simple enough to imagine our Earth

but say half the size across.

All made of the same stuff, same proportions,

just a bit smaller.

Problem is changing Earth's size

changes things you don't want to be changing

and one of those is gravity.

Gravity, basically a very,

very important force in the universe.

Whenever you have an object,

it will attract everything that you have around you.

So this works for planets,

but it also works for everything in the universe.

For instance, galaxies or stars.

Everything attracts everything.

A half-sized planet

means half the normal gravity at the surface,

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

And if you're athletic,

who knows what you would be capable of.

Half gravity means you can jump higher

and fall slower.

So far, so good, but then the fun would stop.

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.

For starters, half gravity

means air pressure is half what it was

as the planet pulls the atmosphere less strongly towards it.

Pressure at sea level is now

the same as it used to be 5,500 meters up a mountain.

There is less oxygen in every breath.

Within a few hours, humanity would have oxygen deficiency,

otherwise known as altitude sickness.

Humans can adapt to thinner air.

After a few days, red blood cell counts

would begin to increase

to compensate for the reduced oxygen.

Just in time to notice the next strange change.

The aurora is an amazing display of light

sometimes visible near the North and South Poles,

but why does the aurora exist

and what has made it turn up

far from where it's supposed to be?

It's a puzzle, and to solve it,

we need to look not to outer space,

but to the University of Maryland.

Daniel Lathrop has spent 20 years

building models of the inside of planet Earth

to help him understand just how our planet

generates a magnetic field,

work that is vital for understanding the aurora.

Dan's model has a solid metal ball at its center

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.

You may wonder why he bothered.

The fact is the Earth's magnetic field

is very important indeed.

So the Earth's magnetic field serves as a shield

against the worst parts of bad 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.

It's this solar radiation

that causes the aurora.

The shape of the planet's protective magnetic field

tunnels the sun's radiation towards the poles

where it hits the upper atmosphere.

This makes the gases glow, giving us a beautiful light show.

But Dan's machine can also help us find out

why a smaller Earth has auroras in unexpected places

because over the years,

he's built several smaller versions of his model Earth.

So these actually are the first three sodium experiments

we built to try to understand the Earth's magnetic field.

So the first one,

20 centimeter diameter model rapidly rotating.

Next came a 30 centimeter experiment.

There's an inner sphere deep inside there

that you can't see.

And the third experiment at 60 centimeters.

Here's the bottom half of the outer sphere

and then a solid copper model of the inner core

that independently rotates.

And 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 sized,

we could then examine data

between the different size experiments

to see how the magnetic fields are different.

Dan's model is filled with the metal sodium

because of its low melting point,

but it still takes three days

before it's fully melted and ready to spin.

(intense orchestral music)

So here, we see magnetic field data

from the 30 centimeter smaller experiment

and comparing it then to more recent data from three meter,

it's very evident

that as our experiments have gotten larger,

we have much more magnetic induction,

much stronger magnetic fields overall.

So smaller Earth

would have a weaker magnetosphere.

But that's not all the experiments reveal.

When we go from larger to a smaller experiment,

the magnetic field strengths have both become weaker

and have changed pattern.

If you look at the data in the larger model,

there's a kind of well-defined north, south magnetic poles,

where in the smaller experiment at these parameters,

we have 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.

A smaller Earth would be likely

to have many magnetic poles spread around its surface

meaning auroras could show up where you least expect them

and they'd be stronger than ever,

but that's a bad thing, a very bad thing.

It's a sign that our magnetic field is being overwhelmed.

If you have a smaller planet

with a weaker magnetic field,

there would be more problems with telecommunications.

The sun still has these big angry things

that could (mumbling) 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 (mumbling) and it causes problems.

The sun's outbursts

would cause a weaker magnetic field to wobble violently

inducing surges in electrical systems.

The result is havoc to power supplies,

communications, and in fact, just about all electronics.

Then things go from bad to worse.

If we had no magnetic field

around the Earth at all,

then we would be directly hit

by the radiation from the sun

and the Earth would become a very, very toxic environment.

Without the protective effect

of a strong magnetic field, Earth is in real trouble.

Lower gravity means gases can escape into space.

Cosmic radiation supercharges the process.

Eventually, there would be no air left to breathe,

its precious atmosphere stripped and blasted away.

(dramatic orchestral music)

The Earth would end up like Mars.

It's the curse of small planets.

Small planets are not really up to supporting human life,

so it's time to put things back to how they were.

A planet 12,756 kilometers across

with an atmosphere 100 kilometers deep all the way around

and a circumference of 40,000 kilometers.

Planet Earth just as it should be.

Not too big, not too small.

(alarm beeping)

So far, so not so good.

In pursuit of a smaller, more beautiful world,

we turned it into an inhabitable desert.

Maybe it's time to try something a little less risky

like human beings.

So how big are people anyway?

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,

it not sort of in the middle.

Instead, it's much closer to the smallest size,

about 40 grams which is the size of a rat.

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

and so that makes us enormous.

When we look at an elephant,

we may feel small, but in fact,

human beings 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've 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 happen to be a few bad years

in terms of reproduction or food,

then their population could crash,

and as a result, they can become extinct.

Whereas much smaller animals

typically have much larger populations

and so they are robust to these kinds of events.

In the extinction that wiped out the dinosaurs,

every land creature that weighed over 25 kilograms died

which is why the only dinosaurs we have today are the birds.

So in general, the larger the animal is,

the faster it goes extinct.

Bye-bye.

For living things,

size is a matter of life or death,

but human size is not fixed.

It has changed a lot over the course of history.

Back in the Stone Age when humans lived as hunter-gatherers,

the average male height was similar to now.

During the Neolithic Revolution when we started farming,

we shrunk hugely as our new grain-based diet

had a lower nutritional value.

Humans stayed short for several thousand years.

It's only in the last 200 years

that we finally got back to hunter-gatherer size

and beyond thanks to modern improvements

in food and medicine.

But paradoxically, getting taller isn't always a good thing.

Although big animals

have longer lifespans than smaller ones,

within each species, the story is different.

It's been observed that in many species,

it's the shorter individuals that actually live longer

like in dogs.

Could the same also be true for humans?

I guess in a lot of species

and if we look at dogs, horses, elephants,

it's actually the smaller variants of that species

that seem to live longer.

And of course, then the real question is

does it also apply to humans?

But that's not so easy to answer.

There are so many influences on our lives.

How can you tell what's due to size

and what's due to, say, diet or exercise?

Geneticist Diana van Heemst has been reexamining

a remarkable 1970's study that found a solution.

It honed in on a group of people

with very similar lifestyles, but varying heights.

Professional athletes.

For example, American baseball players,

there's a nice encyclopedia

which is actually a rich source of information

not only for the baseball fans

about all the details of batting performances and nicknames,

but also actually, it contains date of birth,

date of death, their adult height, and their weight.

The study took this information

on height and age of death and looked for a pattern.

Wally Burnette, one meter 83.

Murry Dickson, one meter 78.

The original study used data

from thousands of players,

but we can see what they discovered

by looking at just a few.

I took from the encyclopedia of baseball

nine representative examples of baseball players,

and we have, you know, attached them to the wall

based on the height and the age at death.

And this kindly mimics the original study

which made use of the full sample of the encyclopedia

which found there's negative correlation

between height and the age at death.

The 1970s study found that size did matter.

Being five centimeters shorter meant on average

you would live two years longer.

The big question is why.

Although the original work was conducted in the 1970s,

it's only now researchers like Diana

have come up with a possible explanation.

In order to grow, our body makes growth hormone

which stimulates growth,

but at the same time, it also influences

lots of other processes in our body.

And if you look at the data

that have been derived from work on other animals,

we can see that those growth hormones,

they stimulate the body to grow

and this is kind of a signal

that there's enough food,

that there's favorable 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 adverse

or become less favorable like when there is a food shortage

or lots 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 survive this period of hardship

until things get better.

It seems being big comes at a price.

But for tall people,

there is some light at the end of the tunnel.

However, height is not the only thing that matters.

There's lots of things that people can do themselves

to adopt a healthy lifestyle

like not smoking, healthy food, lots of exercise.

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

So just how small can we go?

Well, let's start with what we know.

The smallest adult humans known to science

are just over 50 centimeters tall

like Jyoti Amge, the world's smallest woman.

24 year old student Jyoti

is on a sight seeing trip to London.

Wherever she goes, she gets as much attention

as the biggest attractions.

When I go outside,

then everyone gathers together and stares at me.

Then I feel a bit strange, but at the same time,

it feels good that they all look at me.

In our last episode,

we met Sultan Kosen, the world's tallest man.

Sultan is so tall because his body

produces too much growth hormone.

For Jyoti, the opposite is true.

While Sultan has various health issues caused by being tall,

for Jyoti, the story is different.

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.

Jyoti's main complaint is simple.

The world is just too big for her.

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 a specially adapted environment,

it's a different story.

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 the size of Jyoti?

We could adapt our environment to suit our smaller size.

We would need less food and energy,

which has to be a good thing.

But surely, we can go further than this.

Could a mammal this small survive?

The answer is yes.

In the wild, Etruscan shrews weigh just two grams

which makes them one of the smallest mammals in the world.

So it's a good role model for a miniature human being.

Each one has a heart, lungs,

and all the organs you would expect,

but the similarities with human beings stop there.

This tiny mammal lives its life

right at the edge of what is possible.

Professor Michael Brecht has studied them for years.

Here they are.

And I want to chase them into this.

So now, here we have him.

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 gonna do now.

So we actually use this box here

and what you do is you 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.

Okay, 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.

It's very difficult to circulate blood

through such a small body.

The circulation system of mammals,

it's 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 (mumbling) heart

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

What we also see

is that they have unheard of respiration rates.

So when they are very excited, very nervous,

one would see breaths per minute

go up to about 1,000 breaths per minute,

an absolutely unheard of rate in mammals.

Really, also difficult to understand

how a mammalian brain and lung could do that.

Almost a thousand breaths a minute is certainly fast,

but their hearts push things even further,

beating up to 1,500 times a minute.

That's 20 beat 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 that they go into small spaces

where no other predator can go

and where they're then paradoxically,

are again big predators.

Matching your size to your environment

is an important part of evolution,

but filling this niche comes at an incredible cost.

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.

Their immense heat loss they have or energy loss they have

as a result of their unfavorable surface to volume ratio.

For every gram of body mass,

small creatures like the shrew

have more skin than bigger creatures like us.

A human has a quarter of a square centimeter

of skin for every gram, but a shrew has much more,

almost 20 times more, in fact.

So a shrew loses heat much more easily,

which is particularly bad news for a mammal.

Unlike insects and reptiles,

mammals have to keep their bodies at a temperature

of around 37 degrees centigrade to survive.

Only one mammal has the heat loss problem

worse than Etruscan shrews, their babies.

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

The newborn shrews are incredibly small,

inconceivably small, 0.2 grams.

It's just absolutely incredible.

And they look kind of unreal.

I mean, the whole body is totally transparent.

They huddle together very heavenly.

The mother is very protective

and obviously also supplies a lot of energy.

If mammals can get this small,

then why not a human?

It's feasible that the human body

could work at just five centimeters 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 this 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 the relationship doesn't go up proportionately.

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

than an equal number of deers would.

An Asian elephant weighs about 5,000 kilograms,

but how much does it eat?

Just ask a zookeeper.

This is the amount of hay

one of our male elephants get every day.

43 kilos to 45 kilos.

Which is about 1% of its body weight.

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

This is amount of alfalfa

our dik-dik get on a daily basis.

0.5 kilos.

Around 7% of its body mass.

Repeat this for all kinds of different animals

and a clear pattern emerges.

The smaller the animal,

the more food energy they need per kilo of body mass.

And for the smallest animals,

the amount of food they need goes through the roof.

Applied to our five centimeter human,

Kleiber's Law reveals

we'd have to eat our own body weight every day.

Most of our life would be spent looking for food

just like the Etruscan shrew.

But what if we went smaller still?

So if you were to take a mammal

and 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,

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.

So going smaller

means saying goodbye to being a mammal.

From here on, 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 as if they're ruled by separate 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 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 small.

The overall effect is to make small creatures

relatively much stronger than big ones.

Professor David Labonte carries out tests on ants

to see just how strong.

Ants support 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 it's really quite impressive.

But there's more to being small

than just strength.

(insect buzzing)

As we know, small creatures

have bigger surface area for their weight,

a fact that can be a life saver.

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 the problem

of why you can drop an ant down a shaft

and the ant just falls down 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 of the centrifuge.

Wow.

G-force is the name

for the feeling you might get on a steep rollercoaster.

In the tightest turn, you might experience six G.

But even with much, much higher g-forces,

somehow, the ants hang on.

And during these measurements,

we've seen ants withstand 500 G, 1,000 G

for 10, 20, even 30 seconds,

and then they fall off and walk off as if nothing happened.

It's this same ability to hang on

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 would 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 when helps these animals to stick.

We experience

these intramolecular 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 millimeter human could climb a wall just like an ant.

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

A five millimeter 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 or a very big animal,

the neuron is 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 brain power.

A five millimeter human

would only have around two million neurons

which puts him somewhere between a cockroach

and a small fish.

Smart enough to spot food,

but probably not smart enough

to worry about the puddle of coffee in the way.

For normal size 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.

Super powers or not,

being small has too many downsides

like tiny brains, constantly looking for food,

and of course, if you're that small,

almost everything in the world wants to eat you.

So it's for the best that we return things to how they were

because the size we are now

is a perfect fit for the way we live

and the world we live in today.

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, small has not proved to be 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.

Less skin cancer, a cooler climate.

The sun we have is a kind of star

known to astronomers as a G2 drawf,

but it's not really much of a dwarf by human standards.

In fact, it's 1.4 million kilometers wide.

That's 109 times wider than the Earth.

But does it have to be so preposterously large

or would everything work out fine with a smaller,

gentler sun that wouldn't damage our skin

and we can look at without sunglasses?

The key to this question

is understanding what makes a star shine.

Stars are big balls of gas

which have been pulled together by gravity.

At some point, the gases literally fuse.

Subatomic hydrogen particles

get squashed together to make helium.

This is 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 turns out it's incredibly hard to do.

This is one of the world's best attempts,

JET, Joint European Torus,

based at Culham in the U.K.

The first place on Earth

that they managed to achieve controlled nuclear fusion,

a phenomenally complicated and expensive facility

where they use as much power as a small (mumbling)

to superheat the gases until they fuse.

Jet physicist Ivor Coffey knows just how intense things get.

The fusion takes place inside this chamber.

We actually create a highly ionized gas or plasma.

Sort of the center of the plasma

which is probably roughly just above my head

would be where the temperature

and density are at the maximum temperature.

There could be something in the reads

in between 100 and 150 million degrees centigrade.

Conditions in here are so extreme

that they can only run the machine for 30 seconds at a time.

Today, they're running the fusion test

at even higher power levels than they've tried before.

10, nine, eight, seven, six,

five, four, three, two, one, zero.

(intense orchestral music)

What you are looking at right now

is actual fusion.

This is the very same process

that happens at the heart of a star.

(intense orchestral music)

The problem of making fusion happen on Earth

is no different to the problem of making it happen in space,

but stars manage to solve this problem

by being incredibly big.

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 will 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, size matters.

If they're not big enough, fusion can't happen.

And without fusion, it's not a star at all.

If we make stars smaller, less massive,

and the temperature of the star

in the center 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.

The very smallest we could make our sun

or indeed any star

is around 10 times the width of the Earth.

But with a sun this size,

what kind of Earth would we wake up to?

(alarm beeping)

For starters, you'd be seeing red.

The reason that this low mass star would be red

is that this star would have much reduced gravity,

and therefore also it would have much reduced temperature.

It would shift the peak wavelength of your photons

that the star is emitting

into longer and longer wavelengths.

This means that we shifted from the yellow that our sun has

into the red that those red dwarf stars would have.

A star this size

would be no brighter than the moon,

but light isn't the only casualty.

Within hours, we'd feel a bigger problem.

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.

Temperatures would plummet.

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.

As the Earth cools further,

all the gases in the air solidify

causing the atmosphere to collapse.

So how do we save the world?

The answer may seem obvious.

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

But would that actually work?

Earth's normal orbit

is about 150 million kilometers from the sun.

This is the middle of the habitable zone,

the region of the solar system where it's not too hot

and not too cold.

Now, we've moved the Earth

nearly 100 times closer to the tiny sun.

(intense orchestral music)

At this distance, our planet gets the same energy

from the sun as we're used to,

but this is a different world.

The sun may be tiny, but we are so close

that it would look big in the sky.

Around 10 times bigger than we're used to.

But there are problems to face,

problems that the best scientists in the world

are trying to solve

because they've recently discovered a planet just like this

and it turns out it's our nearest neighbor.

If you look at the many stars in the night sky,

you probably won't even notice Proxima Centauri.

It's actually the closest of them all,

but it's very small,

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

And at Queen Mary University 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 neighbor.

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 analyzing the light that Proxima Centauri gives off.

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

The telescope has an optical fiber sitting at the focus

and then the light from the star

goes through the optical fiber

to the basement of the observatory

where there's a spectrometer.

And what the spectrometer does.

Takes the light coming from the optical fiber

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 in the atmosphere of the star.

When they observed the star again,

they saw that the spectrum of Proxima Centauri was changing.

So we come here to the telescope two months later.

We take more data

and then we see that the measurements start to trend.

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

We got 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 the star has been pulled backward and forwards.

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

Further study shows that this planet called Proxima b

has a lot of similarities to our own.

It's roughly Earth-sized and mostly made of rock,

but unlike Earth, which takes 365 days to go around its sun,

the spectrum patterns reveal that Proxima b

takes a mere 11 days

meaning it must be very close to its star.

When the scientists did the maths,

they realized it is at a perfect distance

for the possibility of life.

Not too hot and not too cold.

There are many more factors to consider

before they know if life is possible.

It's only right to the solar system

that we can expect to actually

start to search for evidence of life

in the planets like this one is Proxima Centauri

and also some very nearby stars.

As the search for life on Proxima b begins,

what can we learn from it to help our thought experiment?

Could life survive this close to a small sun?

For a start, plants would have to be a different color.

Plants are green because of the specific wavelength

of light they use.

But if our sun was red like Proxima Centauri,

crucial wavelengths would be missing

and green plants wouldn't work.

They'd have to be black

to absorb as much sunlight as possible.

Our planet would look very different,

but there's a much bigger problem.

A possible side effect of being so close to a star.

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 also have a 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

is synchronized to the orbit of the planet

so the same side faces the star.

With one side frozen in perpetual night

and the other in never-ending sun,

planets that are fully tidally locked

are sometimes called eyeball planets.

The world we are used to isn't possible

around a smaller sun.

Even if we move close enough to the sun to be warm,

Earth would be a very difficult place to live.

(intense orchestral music)

It turns out that the size of our sun is everything.

Any smaller and we can't have a green world,

can't have night and day,

and most of our world becomes deadly.

It seems we are better off with the sun the size it was,

us the size we are, living on a planet that is just right.

Size does matter.

Size determines on the one hand

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

live in 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 anymore

and the whole system will collapse.

Size isn't like a color or clothing.

It's not arbitrary.

It goes hand in hand with so many things.

Gravity, intelligence, the evolution of life itself.

Our size is who we are and what we always will be.

(intense orchestral music)