20 years ago, two astronomers made a remarkable discovery,
one which would change the way we view the universe for ever.
A planet outside our solar system, orbiting a distant star -
an exoplanet.
Since then, we have found worlds where it rains diamonds,
ones that boil at 3,000 degrees centigrade
and even a world with four suns in its sky.
But the big question is - will we ever find another Earth?
Welcome to The Sky At Night.
Our solar system is filled with an incredible variety of planets.
From small, airless and rocky worlds like Mercury
to gas giants like Saturn, with its spectacular rings of ice.
Surely our solar system couldn't be the only one to host such a wonderful variety of worlds?
And yet, until the 1990s, astronomers hadn't found
a single planet outside our solar system.
And then, in 1995, they found it -
the first really convincing evidence
for a planet orbiting another star.
Since then, we've discovered nearly 2,000, and it's involved
some pretty extraordinary science detective work.
In fact, we've found so many planets that we've reached
a remarkable conclusion.
That almost all the stars in our galaxy
have their own families of planets.
It's a really hot topic in astronomy right now.
We're all desperate to find another Earth.
Although, as we'll see later, quite what that means is open to question.
Coming up tonight, we'll be examining some of the latest
discoveries made here at the Cambridge Exoplanet Research Centre.
And finding out how the hunt for a second Earth,
a habitable planet like our own. is coming along.
We have found planets in all size, in all masses
and in many different kind of structure
from the one we have in the solar system.
Hollywood has no doubts that there are habitable planets out there.
Dallas Campbell finds out
whether exoplanets in the movies have any basis in reality.
And you can see there's the binary star, one is
a little bit whiter, one is a bit red.
Almost exactly like in Star Wars.
We'll tackle a controversial discovery that my own research
group have been involved in.
Could this really be the first discovery of an alien civilisation?
But first, how do you go about finding an exoplanet amongst
the 100 billion stars in our galaxy?
The first true exoplanet was discovered
by two astronomers in 1992.
Scientists found not just one
but at least two planetary objects orbiting around a star.
It was a stunning achievement.
These planets were four times the mass of the Earth
and orbited a type of star called a pulsar - a remnant of a supernova.
Because the pulsar permanently bathed its planets
in high-energy radiation, there was no chance for life.
So in terms of finding another Earth,
these planets were non-starters.
Nevertheless, it was a thrilling discovery,
and left us hungry for more.
What we wanted next was a planet orbiting a main sequence star
just like our own.
And, in a small observatory in France,
a young PhD student struck lucky.
Didier Queloz discovered a planet around the star 51 Pegasi.
51 Pegasi was the right kind of star,
one that wouldn't bathe its planet with deadly radiation.
But in other respects the planet in orbit around it was
spectacularly weird.
It was the size of Jupiter
yet it took just four days to orbit its star.
And its surface temperature exceeded 1,000 degrees Celsius.
It was a type of planet that has come to be known as a "hot Jupiter".
It was an extraordinary discovery that
rewrote our understanding of what exoplanets might be like.
And it was just the beginning.
I caught up with Didier to find out
more about his original discovery, and what's he's been up to since.
Tell me about the day you discovered 51 Pegasi b?
Yeah, I was a PhD student at that time
so the first time I've saw something
a bit strange going on on the series of measurements I made on 51 Peg,
I never thought it would be a planet because it was just impossible.
I thought it was a bug into the instrument,
something was wrong with the instrument, with the machinery.
And it took me a long time to figure out exactly what it was.
Did you hope that the planet you had discovered was going to be Earth-like?
Well, it was impossible because the, the, the, the instrument
we're using, the best it could do is detecting something like Jupiter.
It was designed to do that.
It was already a tremendous achievement at that time.
We can do Earth right now, but at 20 years ago it was not possible.
But it was already so much amazing to find a planet
so bizarre than this one, so I didn't care at all.
I mean, it was really a kind of game changer situation.
So what method did you use to find 51 Peg?
So at the time, the only way to find a planet was to observe
the stars, and, uh, by observing the star we are measuring
the speed of the star, looking for tiny change of that speed.
So, you have a star and you have a planet going round it.
And as the planet goes round it caused the star to
move by a little bit and you're measuring that movement, that speed.
It's a very tiny effect. You don't see it right away,
you need to use quite sophisticated algorithms to build
the average motion that tell you
at the end that there is something orbiting that star.
So that's one method,
but are there other methods for detecting these exoplanets?
People started to look for transit at that time.
What will happen is there will be for short amount of time
a slightly change of the light, a dimming of the light
of the star, like, like a cloud just hiding us the sun.
Uh, we can use that to get the size of the planet as well
as the period of the planet.
We can also combine the two, you can get the mass from the radial
velocity and you can get the size from the transit
and we doing that today to get the density of the planet.
And the density helps you to understand the structure of the planet.
And it was really the beginning.
So it was a massive trigger and that's why at that time
we were maybe 20 people in the world doing that,
now there must be 5,000 people working on this.
So we've found these hot Jupiters and now we're finding sort of
smaller and smaller planets with the better sophisticated equipment.
Do you ever think we'll find another Earth?
Oh, yes, I mean, we have found already, Earth-size planet,
or Earth-mass planet,
most of them they're not exactly Earth equivalent because very often
they're too close to the stars, much too hot.
So we don't really, really know what we're finding these days.
But we have planet in all, in all size, in all masses,
and in many different kind of structure from the one
we have in the solar system.
So we're finding more and more and hopefully, that um,
another Earth will be out there one day. Oh, yeah, there are.
Because right now we're finding an average one planet for each star.
So if you look at the star, uh, tonight,
on all the star you seeing by the naked eyes they're planets.
Some of them we are found them, other we have not yet found them.
It means that maybe they are solar system equivalent just
waiting for us to be found.
Hm, exciting times ahead. It is definitely exciting, yes.
Well, thank you, that was wonderful.
As we hunt for a second Earth,
we have to think about what it is that we are really looking for.
What is it that makes a planet earthlike?
Is it rocky? Is there liquid water on it surface?
What gravity would it have?
And would we be able to breathe if we were standing on the planet?
Astronomers are only just beginning to work out what
conditions are like on exoplanets.
But that hasn't stopped Hollywood from creating its own alien worlds.
And most of them are very much habitable.
Dallas Campbell investigates how plausible Hollywood's
exoplanets really are.
I went to see Star Wars when it first came out, aged seven,
at the ABC Cinema Haymarket, Newcastle-upon-Tyne.
It was one of those cinema experiences that will be forever etched on my brain.
It ignited my lifelong love affair with science fiction movies.
Amazingly, many of the distant worlds featured in these films were
being imagined by writers
and directors decades before the first exoplanets were found.
The planet Tatooine from Star Wars, for example, you've got this
wonderful, evocative desert landscape, and Luke Skywalker stands there contemplating his destiny,
bathed in the afternoon light from not one but two suns.
Although it has two suns,
in almost every other respect Tatooine feels reassuringly earthlike,
which makes sense, given that it was filmed in Tunisia.
But the same can't be said for this planet.
If I owned this place and hell,
I'd rent this place out and live in hell.
This is Crematoria from The Chronicles Of Riddick,
and in it the protagonist has to deal with furnace-like temperatures during the day,
so up to 372 degrees,
and then at night it gets down to minus 182 degrees -
with only a 20-minute window where the characters can actually
walk about safely on the surface.
Interstellar is about the search for a second Earth.
As food resources run out humanity must find a new home.
And en route they have a close encounter with a very strange water world called Miller,
which has 30% more gravity than Earth,
huge tidal waves and is orbiting an enormous black hole called Gargantua.
It's our only chance to save people on Earth.
If I can find a way to transmit the quantum data I'll find in there,
they might still make it.
Let's just hope there's still someone there to save.
That's just a few examples of the artistic licence used by writers and directors
in describing exoplanets.
But what about real life?
How close are we to finding anything like what we see in the movies?
I'm going to go and see exoplanet expert Ruth Angus to find out.
So how realistic are some of these exoplanets that we
see in the movies?
I guess the most famous one is Tatooine in Star Wars,
with its binary star system.
Could something like that happen?
If so, have we found anything like that? Yes, absolutely.
We've found more than one, actually.
The first one we found was Kepler-16b, we found it in 2011,
and it's the first circumbinary planet.
So that means it's a binary star, two stars that orbit each other,
and a planet on the outside that orbits both of them.
So the planet has two stars. So the stars are orbiting each other,
in a little sort of merry dance, and the planet is going round - oh, wow!
So here's a video of that system and you can see there's the binary
star in the centre, one is a little bit more massive than the other
and it's slightly whiter, and the other is a little red.
Almost exactly like in Star Wars.
It is, it is almost exactly like in Star Wars, yeah.
And is the planet that's orbiting this system, would it be a rocky
planet looking a little bit like Tunisia by any chance?
We should be so lucky!
No, unfortunately, this planet is a Saturn-like planet.
The reason why this is such a benchmark system is
because before we found Kepler-16b, we didn't actually know
whether you could form an exoplanet in a binary system.
And that's because the gravitational pull acting on the planet
varies all the time because these two stars are moving.
So that means that the planet is kind of, like,
walking drunkenly around in space, and if that kind of drunken walk
gets too extreme it's going to spiral inwards and plunge into
the surface of one of the stars or it will be flung outwards completely. Right.
So we didn't know whether we would find any planets in stable
orbits around binary stars.
Kepler-16b showed us that they do exist and we can find them.
What about some of these exotic exoplanets that we
see in the movies?
Something like Crematoria, which is
the aptly named planet in The Chronicles Of Riddick.
This sort of extremely hot during the day, extremely cold at night.
Is that just complete fantasy or is that possible?
Well, we have managed to map the temperature on the planet WASP-43b,
so we actually know what the temperature of the day side is
and we know what the temperature of the night side is.
And in this graphic,
we can see in the day it's really hovering up around 1,500 Kelvin,
and in the night it drops all the way back down to, almost to zero.
I find it amazing that we have the technology that enables us
to do that.
One of the things about the planet Miller in Interstellar,
is that it's orbiting a black hole and you get into all
kinds of exotic physics - time dilation and all this sort of stuff.
But the fact that its orbiting a black hole, is,
is a really interesting idea.
Is that at all feasible?
Yeah.
There's no reason why you can't have a planet orbiting a black hole.
And in fact there is some evidence to suggest that we've even
seen a super Jupiter being eaten by a black hole.
Uh, and so this, this graphic shows you.
So this is the planet here
and the black hole is somewhere in here, you can't quite see it,
but material is streaming from the planet onto the black hole.
I'm trying to imagine what you might see,
because obviously a black hole, it's not actually emitting anything.
So what would the view be like?
Well, in this example, you'd see this very bizarre
kind of arc of light stretching from you to the black hole.
So what's that? What is that light?
Where is that light coming from?
That's, that's the dust and gas falling off the planet into
the black hole and crossing the event horizon. So the raw materials that we find that makes up planets.
The raw materials, yeah, exactly.
So this material as it's falling into the black hole gets
extremely hot and it starts to give off radiation, it glows.
So you would see the kind of glowing signature
of this accreting material.
Do science fiction writers, are they creative enough, do you think?
Do you think they are slightly limited by their own imagination?
And actually what's really interesting is the stuff that's actually out there?
Things like, uh, you know, a super Jupiter being eaten by a black hole
and a planet made of diamonds and this sort of stuff?
I'm sure that's true. The old expression "stranger than fiction"
is absolutely applicable to exoplanets.
There are lots of things that we've found that are stranger than things we can imagine
and that's the beauty of the exoplanet world - we are discovering things we didn't even
know were possible and new physics is being discovered all the time.
Thank you very much, that was fascinating.
One of the greatest success stories in exoplanet research has been
Nasa's Kepler space telescope.
It has found over half of the exoplanets we know about today.
Five, four, three, two...
engine start.
One, zero, and lift off of the Delta Two rocket with Kepler,
on a search for planets in some way like our own.
Kepler was launched in March 2009.
Its mission was to survey our region of the Milky Way,
to try and find another Earth.
And it's discovered more than a thousand planets.
One of most interesting findings of the Kepler space mission
to date has been the discovery of a class of planet
known as "Super Earths".
They seem to be fairly common throughout the galaxy
and yet none exist within our own solar system.
They're larger than Earth but smaller than an icy giant.
We don't know what they're made of. Are they rocky or icy?
It remains a mystery, at least for now.
Perhaps even more exciting is the recent discovery of
Kepler-452b, 1,400 light years away.
It's probably the most earthlike planet we've found so far.
It's a rocky world that's orbiting in the habitable zone
of its parent star, where there could be liquid water.
But there's still a lot we don't know about the planet,
for example what its atmosphere is made of.
Kepler has done a tremendous job in finding
exoplanets virtually everywhere it's looked, but it's not designed to
give us the detailed information to tell us what those planets would be like.
Our best bet for finding another Earth is to build
new instruments that can take a closer look.
That's what Nasa is doing.
Its Tess planet finder launches in just two years' time.
Its mission is to detect small planets orbiting bright host stars.
Bright stars will reveal more detailed
information about their planets, enabling us
to better identify those that are truly earthlike.
Of course, the ultimate prize isn't just
finding a planet that is like Earth, it's finding intelligent life.
We don't really know how that would manifest itself,
but Chris has recently been involved in a discovery that has got
some people very excited.
I don't normally talk about my own research on the programme,
but in the last few weeks we've announced a remarkable, strange discovery,
probably the strangest thing we've seen in the universe in the last 20 years.
And according to the internet and the newspapers we've discovered
alien spaceships in orbit around a star 1,500 light years away.
That's not quite what happened.
We've been running a project for the last few years called Planet Hunters
which has invited hundreds of thousands of people to go online
and to look through this data from Kepler and try and find planets.
And we've found planets, but a few of the volunteers pointed us
to this one particular star which is behaving really oddly.
This is its brightness over the course of three or four years.
You can see most of the time it's fairly stable,
but there are these dips.
Just like you'd expect if there was a planet,
but the dips happen randomly,
some of them are huge,
so at one point something blocks out 20% of the star's light.
That's way too much for this to be a normal planet
and the fact that they're irregular means this can't be something
on a circular orbit that blocks out the star's light again and again and again.
Something really odd is going on here.
In fact, this is the only star amongst the 150,000
that's doing anything like this.
So the obvious question is what's blocking the light from the star?
We think this might be not exoplanets,
but a family of exocomets.
If you have a comet on an elliptical orbit about the star
AND that comet has broken up, so that you have a string of comets -
rather like what happened to Shoemaker-Levy 9
when it hit Jupiter a couple of decades ago -
then, as each of those pieces of comet pass by the star,
we'd see a dip.
And so you see there's one here, there's one here,
and then this sudden flurry of large bits that block light out.
But we don't really understand what's going on and
so we should consider everything,
including the idea that this is some sort of alien civilisation.
In fact, people are taking the idea so seriously,
that astronomers have already pointed radio telescopes at
this star just to listen out, Seti-style, for any signals
that might be coming our way from what's become the most interesting star in the galaxy.
When it comes to finding exoplanets,
you need more than even the best home telescope.
But Pete's here to show us
another way we can all connect with the thrill of exoplanet discovery.
It's actually not too bad an evening,
it started off rather cloudy today but the skies have cleared
and I can see some stars shining away up there,
there's a bit of haze in the west, but that was forecast to come in.
I'm just glad we can see some stars.
What I'm looking for is a huge pattern in the sky that
looks like a giant square, and that's the square of Pegasus.
And early evening about this time of year, it's quite high up
in the sky towards the south-east direction.
It's pretty prominent, just look for this giant square.
Once you've located it, look to the right-hand side of the square.
Close to the mid point there is a faint star,
and that's called 51 Pegasi.
OK, well, I think I have a photograph of 51 Pegasi there,
and of course on the back of my camera it'll look just like a regular star.
The planet is there, we know it's there, it's been detected
and confirmed by a number of different sources.
The planet going around 51 Pegasi has a name, it's formal name is
51 Pegasi b, but there is an unofficial name as well, which is Bellerophon.
Now Bellerophon in mythology was the person that tamed the flying horse
Pegasus, so it's rather nice being in the constellation of Pegasus itself.
So, I think with exoplanets and regular telescopes it's really
a journey of the imagination, you can see the star there
through the eyepiece or on the back of the camera, but it's your
mind that takes you into that system to imagine what it must look like.
Absolutely fantastic.
Go to the website to find out more about how to locate 51 Pegasi,
to see this month's Star Guide, and also to learn about an exciting
new project The Sky At Night has been involved in.
This month, we're trying out a brand-new BBC messaging tool
called Whispering Stars.
It's a prototype at the moment but we'd love you to have a go.
On the star map, you can click on any star with a circle round it.
Not only will you see information from Pete about that star,
but you can also leave your own message.
Then I can share a link to that message via e-mail or social media.
The recipient can find your message using a computer or a smartphone.
I've got a new message, just have to click to find it
and it points me up into the sky towards whichever star
the message is attached to,
so apparently I should keep going.
Back this way, it's this one here,
which looks like it is...Vega.
And here's the message from Maggie, which says,
"Dear Chris, I know you like astrophotography
"but did you know that Vega was the first star to be photographed?"
So a message sent through the stars - sort of - and you can go to our
website at bbc.co.uk/skyatnight to use Whispering Stars for yourself.
We'll be using it to try and send you information over the next few months.
Even if we find another rocky planet, just like ours,
orbiting a star similar to ours, even orbiting at the right distance,
there's another piece of information
we need before we can confirm we've found another Earth.
And that information is the atmospheric composition of that exoplanet.
And rather astonishingly, we're doing just that.
Chris has been talking to Nikku Madhusudhan, who is leading
the research into exoplanet atmospheres here at Cambridge.
So we've discovered all of these planets, we know, I dunno,
what size they are, what mass they are, but it's not really enough.
I want to know what they're like.
Do we have any prospect of trying to understand that?
To, to investigating these planets properly? No, absolutely.
So the way you do that is, you know, transiting planets, for example,
what the transit method is
is when the planet's star system is aligned
in such a way that you can infer the planet going in front of the star.
Yeah, we see these dips in the star light. Exactly.
And we say, OK, there's a planet there.
As it turns out, the dip in the starlight is just a little
bit more if you have an atmosphere on top of the planet.
Because the atmosphere blocks the light from the star as well.
Yeah, yeah, but the key point is that the atmosphere blocks
the light in some wavelengths and not in others.
And so that means if you have a transiting planet you can say
something about what its atmosphere is made of?
Exactly, you can infer its chemical composition.
OK, so Earth is mostly nitrogen, bit of oxygen,
bit of carbon dioxide, what do we see on these exoplanets?
Yep, so the exoplanets, um, that we are most
able to study today are the big and hot ones.
It's because the bigger the planet,
the bigger the starlight you're blocking.
Yeah, the bigger the dip.
The bigger the dip, so it's just easier to find them.
And hot because the hotter the atmosphere is the more
puffier it is. Hm. Right, so, so the bigger it is.
So the hotter the atmosphere, the bigger the atmosphere, and the more light it blocks.
Right, so most of the results to date have come from these
objects called hot Jupiters.
What do we know about the conditions?
We know they're large, that means they must be made of gas, mostly.
What do we know about the atmosphere? We are finding less water than we would expect
if the planets formed in a solar-like environment
Oh, OK. And they could be less by even a factor of ten.
One explanation is that maybe these objects have clouds in their atmospheres.
So you're, you're saying the water would be hidden? By the clouds?
Yeah, exactly. So you only see the bit of the atmosphere above the clouds?
Exactly, so, so that's one way. So it's a straightforward explanation -
astronomers being confused by clouds is common here on Earth as well.
It's not only on Earth, not only a terrestrial experience,
even when you go to exoplanets clouds are a headache.
But there is a subtlety.
Clouds are not the same clouds we have here.
Right, so on Earth they are water clouds.
But these are planets at a few thousand degrees. Of course, so...
So you can't have water clouds.
So you may have to invoke iron clouds,
or silicate clouds, you see it in rocks,
on Earth rocks are made of silicates,
so you might be able to make
clouds out of those same silicates, hanging up in the atmosphere.
Wow, suddenly it's a place to me,
because we are talking about what they're like.
I can, I sort of feel they're real.
Yeah. Remarkable stuff, and we'll come back
and see how you're getting on in a few years perhaps.
Thank you very much. Thank you, thank you.
It's amazing we've come so far in just 20 years.
Going from no exoplanets to about 2,000 now!
And turning them from just points on a graph, from just data,
to real places with atmospheres, and rain,
and even clouds to annoy the astronomers!
What I'm fascinated by, is if we did find an earthlike planet,
I mean, even signs of life, what would we do then?
We'd have to go, wouldn't we?
If you looked at the sky and you could say that star has an earthlike planet,
the right temperature, with life in its atmosphere - you'd have to send a probe.
It'd take thousands of years to get there! We still should send something, I mean, maybe not us,
but we should send something just so we know we're on our way. I have to agree, you're right!
That's it for this month. Next month we have a Christmas special!
We'll be taking an astronomical look at the mystery surrounding
the Star of Bethlehem.
Was it just a conjunction of planets
or something more exciting like a comet,
or a supernova?
In the meanwhile, get outside and get looking up.
Goodnight.