Original subtitles

Let me tell you about one of the youngest and most exciting areas

of astronomy research.

This is a field that is so riddled with diversity

and discovery that astronomers are constantly left going,

"Huh?

"Huh? Huh?"

Yes! It's finally happening, everyone!

We're doing an episode on exoplanets.

Exoplanets are planets outside the solar system -

so, typically planets orbiting any star other than the sun,

but some of them are orphaned and have no star at all,

so just outsiders really.

And, honestly, the stuff we've been discovering is just

constantly challenging everything we think we know about

how planets form and evolve.

I tell you, it is wild out there.

In fact, it is so compelling that, back in 2018,

I left my lovely, stable job as a secondary school physics teacher,

moved to the Midlands, and embarked on a PhD

in astrophysics as a mature student.

I had to learn Gen Z slang

to get the cool kids to talk to me.

And, lucky for you, tonight,

you're going to get a taste of the field I love so much

without having to leave your job -

or even your sofa, for that matter.

Welcome to The Sky At Night.

Humanity has been dreaming of finding planets out amongst

the stars since we first started staring into space,

asking questions and wondering what might be up there

waiting to be discovered.

How lucky we are to be the generation that gets

to answer those questions.

The first exoplanet around a normal star was found in 1995

by astronomers watching how a star wobbled this way and that,

back and forth,

pulled by the gravity of a Saturn-mass planet.

Such giant planets all lie far from the sun,

but this one whizzed around its star in just four-and-a-half days.

No-one had expected such a world.

And this one discovery spurred on a new generation of planet-hunters

who wanted to conduct a census of planets in our galaxy.

To do so, they used a different technique, the transit method.

Thousands of stars are monitored at once,

looking for the faint dip in brightness that happens

when a planet gets in front of its parent star.

Results from space telescopes like Kepler -

and more recently, Tess -

have told us that not only is the galaxy full of planets,

but that some of them might well be like our own Earth.

Now, exactly how Earth-like a Earth-like planet has to be

in order to count as properly Earth-like is open to debate.

If we assume that life like us needs a planet like ours -

cosy atmosphere, liquid water, the right temperature -

then results that we have on hand are close to confirming that

not only is the Milky Way full of worlds,

but lots of them are possible homes.

Are any of these worlds actually inhabited?

Well, astronomers hope to detect what's called a biosignature,

a chemical which might indicate the presence of life.

A recent paper claimed to have detected a chemical, DMS,

in the atmosphere of a Neptune-sized world

called K2-18b.

Now, on Earth, DMS is made exclusively by life,

mostly by microorganisms.

So, this is exciting.

But the picture is murky.

Some people think that K2-18b has a liquid water ocean,

others that it's a lava world.

And those details matter.

Other groups have looked at the same data and found

no trace at all of DMs.

And, even if it is there,

can we be sure that it can't be produced without life

in the chemistry of such a strange world?

People are doing lab experiments to be sure.

Basically, this stuff is hard.

But in the meantime, don't despair.

Don't worry that we haven't yet found our perfect twin Earth,

but revel instead in the diversity of worlds that we do know about.

There are hot Jupiters and hot Neptunes,

warm Jupiters and warm Neptunes,

lava worlds, Earth-like places,

Venus-like planets, comet-like planets,

stripped-core planets, diamond worlds,

planets around young stars,

planets around old stars,

planets around pulsars that make no sense at all.

There are planets...

While Chris continues his list of amazing planets

that have already been found...

..I'm in Germany, where a new mission that hopes to find

many more is being built.

Dubbed "the Planet Hunter", ESA's Plato spacecraft

is set to fly about a million miles to the L2 Lagrange point,

where it's going to unfurl its nine-metre wingspan

and settle in for about four years to observe about 200,000 stars

and the exoplanets that orbit them.

I'm meeting Industrial Prime Project Manager Pablo Jorba Coloma

by a model of the spacecraft.

At the heart of the Plato mission is the extraordinary array

of precision-engineered high-spec cameras.

Two fast cameras at the top are integral for guiding,

while the rest focus on monitoring the stars

to hunt for exoplanets.

So, talk me through these cameras.

They're kind of slightly offset from each other, what does that mean?

By grouping the cameras like this,

Plato can obtain a wide field of view covering 5% of the sky...

..while also getting incredible detail in smaller sections

where more cameras overlap.

But this requires precision,

and the extremes of space cause huge engineering challenges.

That's kind of specific. Why that temperature?

I-I... Honestly, I'm absolutely blown away.

So, every single camera has its own little heater

keeping it stable in temperature to, like, a thousandth of a degree?

Yes. Incredible.

What stage is the build at right now? Where have you got to?

Congratulations! I mean, what a massive milestone to hit. Yes.

Now, obviously, for an exoplaneteer like me,

this is so exciting, in terms of the data we're going to get.

But for an engineer like yourself, what's it been like?

OK, so now...

..can I see the real thing?!

SHE LAUGHS

Plato will be the culmination of the work of over 100 organisations

from across Europe.

And I don't want to be the one to mess that up.

So, first, it's time to don some PPE.

With that done, the moment has arrived.

So...

It's so beautiful.

It's Plato! SHE GIGGLES

My heart was racing when I was waiting

in the clean room to come out and like, yeah, he's a beauty.

And I was told actually that,

of the scientists who are going to use Plato data,

I'm the first one to see it,

like, mated, to see it, like, put together.

Of the exoplaneteers, I feel very privileged to be here.

This is mind-blowing.

I am absolutely obsessed.

Yeah.

Yeah, yeah, yeah.

QUIETLY: We're going into space!

My face hurts!

I'm barely holding it together from down here.

But then, I was allowed the ultimate view.

I can see all the cameras.

Are they in their positions?

Like, have they been...aligned?

DIRECTOR: George, talk to me. Huh? Talk to me.

I can't! I'm in love!

I could stay here all day.

It's incredible to think that, in about a year and a half,

Plato will be out there scanning the skies for new exoplanets.

But in the meantime, we already have

a huge sample of exoplanets to keep us busy.

..we've got icy worlds and water worlds,

and ocean worlds and piscean worlds,

super-Earths and sub-Earths,

super puffs, super...

MAGGIE: As we discover these bizarre worlds,

we've been cataloguing them

and plotting them onto graphs.

And a puzzling mystery has emerged.

So far, we've found thousands of exoplanets.

And for the majority of them, we can work out their radii.

Now, it turns out that most of them sit between Earth-size

and Neptune-size.

Now, Neptune is three-and-a-half times the size of Earth.

But there's a mystery.

To explain further, I've got a little demonstration,

and it involves sweets. SHE CHUCKLES

Now, each of these jars represents

a distribution of exoplanet size.

This one is one to one-and-a-half times the size of Earth.

This one is one-and-a-half to two times the size of Earth.

And this one is two

to three-and-a-half times the size of Earth.

Now, these sweets represent the exoplanets,

and we can fill up the jars according to their sizes.

Now, as you can see,

I think a pattern is emerging.

But let's put the other exoplanets in,

and then I'll explain.

So, now we've distributed all the exoplanets,

you can see where the mystery lies.

We have plenty in this jar.

But, in the one-and-a-half to two times the size of Earth,

there seems to be a deficit.

And the mystery is so grand, it's been given its own title.

It's called the Exoplanet Radius Valley.

Because, if you plot this on a graph,

you get a valley here in the middle.

To find out more about this gap,

I'm being joined by Larissa Palethorpe,

who's been studying this area,

and, along the way, found more than she expected.

Now, we're speaking exoplanets.

We've been doing a demonstration, looking at the Radii Valley.

And this is the area of your PhD research.

Can you tell us more?

So, my PhD thesis is called "Characterising Small Exoplanets".

Essentially, I look at these planets,

which are around Earth-size, to work out kind of why

we have this gap in the make-up of these planets,

adding new sweets into these jars, so that we can learn more

about the problem, and try and get some answers. OK.

And I suppose, that's it - the sort of more sweets we have,

the better we understand the distribution.

And you have the proud position of detecting an exoplanet yourself.

Tell us more about your exoplanet.

Yes, so Gliese 12 b,

I co-led the discovery of that planet last year.

It's an Earth-size, very temperate planet,

so it's about 42 degrees Celsius on the surface,

which makes it a very exciting candidate for follow-up

to see whether... Mm, yes!

So, almost Earth-like? I mean, 42 is a bit warm, but...

Yeah, so we would classify that as kind of Earth-like,

kind of looking at, can liquid water exist on the surface? Yes.

And obviously, at 42 degrees Celsius,

it could, but it's hard to say whether the planet

is Earth-like right now from the information we have.

Right now, we currently only know the radius of the planet,

so the size of it.

In future, we're going to learn more about the mass -

that's currently being worked on right now. Yeah.

But what we really want to understand is,

does it have an atmosphere?

So, I have to bring it up - life.

We don't know about if it has an atmosphere or anything like that,

but...it just feels quite exciting that, you know,

potentially there could be life, maybe? Yeah, potentially.

It's kind of hard to make a claim like that with kind of

the way we analyse data right now,

but it's definitely a good candidate for looking at kind of

a temperate, Earth-size planet,

and what ends up evolving on the surface of that.

So hopefully, maybe it's habitable, but we don't know right now. Yes.

So hopefully, we will know in the future.

And I suppose, the other exciting thing is, it's not that far away?

So, it's actually our nearest Earth-sized, temperate,

transiting planet found today. A lovely description!

Yeah, it's a bit of a mouthful.

So, near - it's 40 light years - it's Earth-size,

it's about one Earth radii.

It's temperate, it's 42 degrees Celsius on its surface,

and it's transiting, so it passes in front of a star,

which makes it helpful for observations.

Orbiting a red dwarf star that is just 27% the size of our sun,

Gliese 12 b is a fascinating planet.

But we still have the radius value mystery to solve.

Are there theories out there that might explain

why we have this lull? Yes, there are theories.

So, it's to do with how planets form and evolve.

There are a few different mechanisms as to how we think this might work,

but the key, base theory is that planets start off

with atmospheres and, through some process -

whether it's to do with the star or the way they've formed -

it's that they have their atmospheres stripped from them.

And hence, they become super-Earths.

So, there are different mechanisms.

We haven't been able to nail down the mechanism yet,

but essentially, it's atmospheric loss.

OK. Yes. That's what we think.

And this is sort of the transition zone. Yes.

And so, they start off here, they end up here, and then

they just pass through this. Yeah.

And so there might not be many out there? Yeah.

So theoretically, we should see planets moving through the valley.

And, depending on the exact theory you choose is

how long it would take them to move through the valley.

So, that's how we're going to narrow things down. Yes!

So, again, it's adding more sweets to this jar

and seeing how long they stay in that jar for,

before they jump over.

Hopefully it will help us nail it down.

Well, you can't argue with more sweets. Exactly, exactly.

..we've got planets with clear atmospheres,

planets where it rains glass,

planets where it rains iron,

hazy planets, clou... cloudy planets,

planets with rings...

But the planet we really want to find is one exactly like ours.

And maybe Plato will be the one to find it.

George has torn herself away from the spaceship,

and is sitting down with Thomas Walloschek,

the ESA project manager of the mission,

to find out what makes it so special.

So, talk to me about Plato's aims.

What's Plato going to achieve for us?

So, we are really looking for Earth-like planets

around sun-like stars

in what people call the habitable zone -

so, meaning there might be a possibility of liquid water.

What's special about this mission?

I would say we are a multi-telescope mission,

which is quite different to the missions beforehand.

Beforehand, we had, let's say, single telescopes.

We have 26 cameras on board.

But also, there, we have a blue filter and a red filter

on the fast cameras, which could give us a hint about

already the atmospheres of these planets.

Now, of course, these will be very impressive cameras,

but can you put it in terms I'll understand?

How many megapixels, how does it compare to

my phone camera, for example?

You know, roughly the size of your phone.

And then, we can talk maybe about the size of one sensor

of one of the cameras, which is 20 megapixels.

But we have four of them per camera,

which makes it 80 megapixels already per camera.

And, if you put it to 26 cameras,

you have 2.1 gigapixels -

so 2.1 billion pixels, really, that we have at hand

to do our observations.

So, a slight improvement on my 12 megapixels?

I would say so.

So, how important is the stability of the configuration?

Yeah, that's really one of the main drivers of the mission.

So, we are looking at the southern hemisphere

as one of our observation fields,

and we want to look at this for two years.

Within these two years, what we are trying to achieve is

that we have a variation of a target,

not more than plus or minus one pixel

over the camera's...the camera's sensors.

The aim is to have the same star roughly on the same pixel

for the full two years?

Yeah, that's, in principle, the idea.

That's astonishing! Yeah.

Do you think Plato will do it?

Will it find Earth 2.0?

I definitely hope so.

And, let's say statistics show that we have a chance.

It is thrilling to think that, one day,

Plato may detect other planets exactly like our own.

But, while we wait to find out,

an unexpected gas giant is challenging our understanding

of how planets form.

TOI 694 is a faint red dwarf.

Nothing to write home about, and normally,

a star no-one would pay attention to.

But a paper out last month revealed that this star

has a planet all of its own,

and it's one that shouldn't exist.

I'm at the University of Warwick

meeting Edward Bryant -

who discovered this planet - to find out more.

So, Ed, what have you found?

So, what I found is a new planet called TOI-6894 b.

And what's really exciting about this planet is,

although the planet itself is just the size of Saturn,

the star it orbits is only 20% the size of our sun.

I think I've worked out what these are for.

This football here shows the size of our sun in our own solar system.

With strange sunspots...

With some sunspots and solar activity going on.

And then, the red snooker ball is showing the size of the star,

TOI-6894, relative to the sun.

And then the small bouncy ball there is showing

the size of the planet, both TOI-6894 b and Saturn.

Saturn's the second-largest planet in our solar system,

and, though it's a gas giant,

it's less than a tenth the size of the sun's diameter.

TOI-6894 b, on the other hand,

is almost half the size of its red dwarf host star.

So, is it unusual to have such a massive planet

around such a small star?

It is unusual, yes.

And the reason it's so unusual is because we wouldn't have expected

that a star this small could have formed a planet this large.

Why not? So, when we think these planets form protoplanetary disks,

these are huge disks of gas and rock and dust

that surround the young star.

I've always thought of it as the leftover material from the star.

That's absolutely right. It's everything that's left from

the cloud that collapses to form the star.

And, within these disks, the solid materials -

so the rock and the dust - collides together, and,

over about a few million years or so, builds up

a very massive core that then will accrete gas

and become the planet.

But the problem around these very low-mass stars is we think

that these less massive stars have less massive disks.

And so we wouldn't have thought that there would be enough material

to form a planet this massive.

So, what's happening?

Is this some different form of planet formation?

Or is there something else going on?

So, it could be a different form of planet formation,

or it could be that we just don't understand the disks very well.

So, there's a lot of work going on currently to trying

to understand these disks.

And these disks have not been studied in large numbers,

and these planets are very rare.

So, this may just be the star that got lucky.

It could just be, yes, that this was a star that got lucky,

and, for some reason, had a disk that was a different composition

than what we expected, either more massive or a higher percentage of it

was this rocky material that could form the core of the planet.

Now, what about the planet itself? So, we don't know much yet,

because all we have currently is a mass and a radius,

and an estimate of what the temperature might be like.

So, how will we find out more about it?

By observing its atmosphere using transmission spectroscopy,

and using telescopes such as JWST.

So, what will we learn from these JWST observations?

So, as well as learning what gases are in the atmosphere,

one thing that we may be able to work out is

the exact mass of the core.

And, using that mass of the core, that feeds back into what

formation process may have caused this planet in the first place,

whether it has a very massive core or a less massive core,

that could have formed through a different mechanism.

While powerful cameras in space hope to reveal the secrets

of planets orbiting distant stars...

..it's by pointing a camera at the moon orbiting our Earth

that you can capture a rather special image.

Pete is on hand to explain.

While short nights and not particularly dark skies

can make stargazing difficult during the summer months,

the moon can always be relied on to delight.

And this is a good time to look out for an effect

known as the moon illusion.

This is where the moon appears huge against the horizon -

and that is the key word,

because it is an optical illusion that only occurs when the moon

is seen near the horizon.

And the reason why it is currently a great time to look out

for this illusion is that we're at a point in an 18.6-year cycle,

which means the fuller phases of the moon appear low to the horizon

at this time of year.

You may have already seen July's full moon,

which barely scraped ten degrees above the southern horizon -

that's less than the width of your clenched fist at arm's length.

The shallow angle of rising and setting for the fuller phase of

the moon means it appears closer to the horizon for longer than usual.

And that's great for looking out for the moon illusion.

Good nights to look for it in mid-July will be

on the 14th or 15th of July,

at around 2340 BST,

when a waning gibbous moon will appear above

the east-southeast horizon.

Then again, on the 8th and 9th of August,

you can see the same effect with the full moon rising

over the south-east horizon from around 2115 BST.

However, photographing these moons can be disappointing,

and there have been many people who've seen a huge moon

on the horizon, taken a photograph of it with their phone,

looked at the result,

and been disappointed how small the moon looks.

But for amateur photographers, it's all about the framing.

The key to getting a great picture simulating the effect

is to ensure you have included something on the horizon

to create the perspective your eye perceives.

To do this, you ideally want to use a long focal length lens,

or a telescope,

and you want to frame the image to include some interesting -

but distant - foreground object in the field of view.

Get it right, and the effect can be very impressive.

Despite the long daytime periods,

there's plenty more to see at this time of year.

As always, you can check out my more detailed star guide,

which is available at...

Ever since I started out in research,

I have just fallen deeper and deeper in love with exoplanets,

because we're not just discovering these incredible,

strange new worlds,

we're also gaining a deeper understanding

of how the universe works.

And I cannot wait to see what my guy Plato

and exoplaneteers in general just go on to discover.

..surprising egg-shaped planets.

There are eyeball planets.

There are marshmallow planets, candyfloss planets,

and even popcorn...

But before we go, there is one more thing.

This month marks 25 years since Chris Lintott

first appeared on The Sky At Night.

And, in celebration of that,

here's some of his best bits.

And now, on to our main theme.

Here we go!

Ready for take-off.

And with me, Chris Lintott,

welcome to The Sky At Night, Chris. Thank you.

Tonight's programme - we want to talk about the Saturnian moons.

An annular eclipse.

Cosmic ghouls.

Galactic cannibalism.

And I can't wait!

It's going to be really exciting.

We're watching the team at Mission Control,

and they look pretty calm -

calmer than I feel, anyway.

And that was the annular eclipse! CHEERING

Chris, where are you?

I'm at the Institute of Astronomy,

in the dome of my favourite telescope.

This is the Cam, in Cambridge,

and we're just coming under the mathematical bridge.

I'm here on the Isidis Planitia.

I think you'd find Mars a pretty pleasant place to be.

Well, it's flat and red.

One of the problems in exploring the solar system

are the sheer distances involved.

Maybe we just need to think bigger.

Seriously, this is too many Chrises!

I don't think I expected that.

Well, there it is! Everyone cheering...

LOUD CHEERING Yes! So, so...

It's been absolutely incredible.

APPLAUSE I think people are quite happy.

So, I'm not sure what any of that means. What do you reckon?

I don't understand it at all.

Thank you very much, Chris.

And, from The Sky At Night, goodnight.

Goodnight.