When I was growing up, there were nine planets in the solar system.
Then poor old Pluto got the boot, and we went down to eight.
Now scientists claim they have found a new ninth planet
orbiting far beyond Pluto.
Tonight, The Sky At Night investigates
the case for Planet Nine.
On 20th January, two astronomers from this university,
Caltech, in Pasadena, southern California,
made an extraordinary announcement.
They claimed they had compelling evidence that a new, as yet unseen, planet
was orbiting the sun, far beyond the existing planets.
So far this is a theoretical discovery. The planet's
existence has been predicted by computer models.
But if it does exist, it throws up many questions.
Where is it? What does it look like?
And most crucially, how did it get there?
We'll be talking to astronomers here in California,
and in the UK, to answer those questions and find out what
a discovery of Planet Nine would mean for our understanding of the solar system.
But first, let's put Planet Nine in context,
and see how it fits in to our understanding of the rest of the solar system.
This is what we thought was the extent of the solar system
for thousands of years.
And it's restricted to the planets we can see with the naked eye.
So in the centre, of course, we have the sun, then Mercury,
Venus, Earth, Mars,
Jupiter. And 1.5 billion kilometres from the sun sits Saturn.
And this is all we thought there was until the 18th century.
Since then, the solar system has gradually expanded.
In 1781, Uranus was discovered.
Followed in 1846 by Neptune.
Tiny Pluto was added in 1930,
pulling the edge of the solar system out to 5.9 billion kilometres
away from the sun.
Pluto was counted as a planet for over 70 years.
But now we know it is just one object in a huge doughnut-shaped
belt of billions of icy bodies
and comets called the Kuiper belt.
It was thought that the Kuiper belt marked the outer edge of the realm of the planets.
But now it seems that might not be the case.
For some of us, it's an extraordinary thought,
but many astronomers have long suspected there may be
other planets lurking in the dark expanse beyond the Kuiper belt.
And we're getting the first evidence that suggests
they may be right.
To get a feel of just how far away Planet Nine is, if it exists,
we need to plot its position in relation to the rest of the planets.
On this scale, every centimetre is about 35 million kilometres.
The Earth is 150 million kilometres from the sun.
Pluto is 40 times further away,
and the outer edge of the Kuiper belt is still further.
But Planet Nine is far beyond that.
At the point of closest approach in its orbit, it sits
200 times further from the sun than the Earth.
That's a whopping 30 billion kilometres away from the sun.
But unlike the other planets, the proposed orbit of Planet Nine is
not circular.
Its highly eccentric path takes it much, much deeper into space.
At its furthest possible point, its aphelion,
it could be as much 1,200 times further from the sun than the Earth.
On this scale, it's 54 metres away.
In reality, it would be 180 billion kilometres from the sun.
And because the orbit is inclined by 30 degrees to the
rest of the solar system, it means it's also above,
or below, the plane of the rest of the planets.
It seems almost unbelievable that there could be a planet
so far out in the solar system.
A mysterious world.
Ten times more massive than the Earth...
..on a orbit so long, it takes between 10,000
and 20,000 years to complete one lap of the sun.
But how was the discovery made?
Chris drew the...short straw and flew to California to find out.
One of the key people in the Planet Nine story is an astronomer
here at Caltech. He's got history
when it comes to changing our views of the solar system.
Ten years ago, he was instrumental in demoting poor old Pluto.
He even wrote a book about it. His name is Mike Brown.
Brown's speciality is the Kuiper belt.
In 2003, he discovered Eris, a Kuiper belt object bigger
than Pluto,
a discovery that was instrumental in Pluto's demotion.
A month later, he co-discovered Sedna.
This was the first of what became six unusual Kuiper belt objects
that have been found with highly eccentric orbits that take them
far out into space.
They were six of the most distant objects ever
found in the solar system.
It was studying their orbits that led Brown to the
idea that there might be another planet out there.
So the story starts with the outer bits of the solar system, the Kuiper belt.
What we found is that the most distant objects that are part
of the Kuiper belt, so these ones that come into the Kuiper belt
but swing out really far, the very most distant ones,
if you look at their orbits, all of their orbits are pointing off
in one direction, instead of being randomly distributed around the sky.
So how tight is that alignment?
The ones that we know for sure that are aligned are six objects.
And they are aligned within about a 107-degree swath of the sky.
And that's what? Something like this.
OK, and it's not just the alignment in direction as well,
the orbits have other things in common.
So, first you see the alignment in direction.
Then you realise that these six very most distant objects are all
tilted in the same direction downward and a little bit to the side.
They are within about seven degrees of being in the same
skewed plane of each other.
And the combination of those two is what really shows
you that there is something strange going on.
You could get the six of them lined up, just due to chance,
it's about a 1% chance you would get it due to chance. 1%, you know,
it's a small number, but it's not that small of a number.
But if you take also the fact that they're tilted
like that, that's another approximately 1% chance.
So it's about a 0.01% chance that this is all just randomly aligned.
So is that what got your attention and got you working on the problem?
That was the big clue that something is going on in the outer solar system.
This was an extraordinary finding.
The tight clustering of the six orbits might have been caused by the
gravitational influence of something massive in the outer solar system.
For Mike Brown, the question now became, what was out there
and how could he find it?
Working down the corridor from Mike is theoretical astronomer
Konstantin Batygin. Mike and Konstantin have worked together for ages,
and it always helps to have a friendly theorist on hand.
And so Mike popped in to see if Konstantin could help with his problem.
Konstantin's expertise is building computer models of planetary systems.
Hey, how's it going? How are you? Yeah, nice to see you.
'And so Mike challenged him to build a computer model of the solar system
'that would replicate the strange orbits of the six Kuiper belt objects.'
When you started looking into this problem of these strange
orbits, I know a planet wasn't the first thing you considered.
Indeed, yes. What else could have caused this effect?
We had a whole slew of explanations
that we decided to look into first. You know, so gravity of
the Kuiper belt, so the mass of the disc itself
holding itself together in a coherent fashion.
All the bodies pulling on each other, essentially.
That's right, to create almost a fluid-like effect.
As it turns out, the Kuiper belt doesn't have enough material
for this to be relevant. If it was 1,000 times more massive,
that model would have some pull.
We looked into whether or not a passing star could perturb
the structures, leave this long tail of orbits that we are now
here to observe.
Such encounters don't happen very often,
and the alignment would go away over time,
so this would have had to be recent.
And there is no sign of a very nearby star that has just done this.
That's right. So I think that's a very uncomfortable route to go.
And so with all of that out the way, you start to think about planets.
We were careful to ensure that we're not chasing a red herring here.
That this data is indeed real, the alignment is not some observational bias,
it's not...it can't be a coincidence. All of that.
Before going down the path of introducing additional
planets into the solar system.
When he started adding planets to his models, Batygin experimented
with thousands of possible orbits - until he found one that
replicated the strange orbits of the six unusual Kuiper belt objects.
Suppose the orbit of Neptune is about this big.
OK. Most conventional Kuiper belt objects,
Pluto included...
..have orbits that
sort of hug the orbit of Neptune and are oriented every which way.
But importantly, if you look at the whole Kuiper belt from afar,
it looks like a more or less axisymmetric disc.
This rule of thumb is broken when you look at the most distant orbits.
So these are the famous, the now famous six?
That's right. If you look at the now famous six,
they all tend to swing out into the same overall direction.
So your simulation's predicted a nice elongated,
distant Planet Nine orbit.
Planet Nine has a sort of similar sized orbit,
but it is facing the wrong way.
You might as well draw the planet on. Go on. We haven't seen it yet, but just for satisfaction.
The planet, I am pretty sure, is in this part of its orbit.
Cos otherwise we would have found it. We would have found it. Right.
So there's our planet. There's our planet. It's fascinating.
Most of the time, the objects are on opposite sides of the solar system.
They are completely unaware of each other throughout most of their orbits.
It is only when they come around at
perihelion... When they are closest to the sun. When they are closest to the sun,
they temporarily interact. And the mechanism, of course,
is just gravitational interactions, and that's what maintains this lock step.
So every time they come around, they get this kick? Or every two times...
No, it gets complicated. For some orbits, it's every
33 times of this orbit per 19 times of this orbit.
But, fundamentally, it's not too different from tugging on something
repeatedly in a coherent way.
Batygin's model had passed the first test. It had successfully
replicated the orbits of the famous six.
But there was another, unexpected effect.
The model also predicted a second set of Kuiper belt objects,
with even stranger orbits.
Trouble was, they didn't seem to exist.
The biggest twist to the whole story came when our
numerical models consistently would generate orbits
that looked more or less like this.
So sort of coming in and out of the blackboard here. That's right. They are on their sides.
They are perpendicular to the plane of the solar system.
This is just so weird. You know, the fact that the simulation is producing these
orbits that should be perpendicular, they should be readily
observable. And the fact that we don't know of them is big trouble.
So it's not just that they might be there. They have to be there.
We initially thought of this as counter evidence to the existence of the planet.
Fine, it explains alignment,
but it also predicts stuff that's clearly not there.
This was the key moment where we looked at it and said, "This makes no sense."
But then I started looking more carefully at the other
objects in the dataset that I had not been paying much
attention to in the last couple of years.
There are five of these objects on perpendicular orbits that I didn't...
I remember when one of them was discovered a few years ago
and thinking, "That's just weird." Nobody had any explanation for them.
And I said, "Konstantin, I'm going to go and plot these right now
"and we're going to see where they are. And if they're sitting at these two spots right here,
"my head's going to explode." And we plotted them.
One of them is right here,
four of them are exactly right here where we predicted. And we both sat
there and stared at that, and I think my jaw hit the floor.
It really was an honest, blind prediction of something we didn't know was there.
And that...that... When I look at those object on those wings,
and then I see those aligned objects, too,
I...think I'm really pretty convinced
it's really out there.
If Planet Nine does exist, it would be a remarkable discovery.
The first new planet to be found in our solar system in 170 years.
That isn't to say it was completely unexpected.
Because there are places we have found planets
that are similar to Planet Nine.
I went to University College London to meet exoplanet expert
Professor Giovanna Tinetti.
Hello, Giovanna.
Giovanna, if Planet Nine does exist,
it seems to have some very exotic characteristics, compared with the rest of our solar system.
In your work in exoplanets, have you seen anything similar to
these characteristics out there?
We're talking about an object that is about tens times
the mass of the Earth.
These king of objects are classified as super-Earths, which is
a very exotic name. The NASA Kepler satellite tells us
basically that these are among the most frequent objects in our galaxy.
The fact that we didn't have any objects of this particular size was a bit puzzling,
in a certain sense. But if Planet Nine does exist, then of course we are ticking the box
and we also have our own super-Earth.
So we had one missing, and now it looks as if we've got one. Exactly, exactly.
But Planet Nine, if it's out there, is a whopping 200 astronomical units
away from Earth - at its closest approach.
So, that just seems an awful long way away.
Have you got examples of planets that far away from their sun?
Actually we do. And what you see now is
GU Psc b, which is located at 2,000 astronomical units from its star.
So that's the further reaches of Planet Nine, potentially.
This is a real image, by the way. I know. The fact you can actually take an image like this is amazing.
It's really very cool. OK.
So, we've looked at the size, we've looked at the distance
from its local star.
But now it's also a very eccentric orbit.
And that seems quite exotic for our solar system. It definitely is.
Most of the planets in our own solar system are orbiting in orbits
which are almost circular.
But actually, out there, we are seeing planets that are orbiting in extremely
elliptical orbits. And we have some examples, like
this planet that is called HD 80606b.
And, as you can see from this picture, we have planets
out there that are orbiting in very elliptical,
what we call eccentric, sort of orbits.
So, if Planet Nine does exist, it is not as exotic as we perceive it to be.
Because there are lots of other examples out there that are similar to the
characteristics of Planet Nine. You're right, Maggie, and if Planet Nine were proposed
and discovered before extra-solar planets were found,
probably we would dismiss this case, saying, "This is way too eccentric,
"this is way too odd.
"We can't have a planet like this."
And the fact that, on the contrary, now we know that
planets like this can indeed exist is really making
the case for Planet Nine even more interesting.
Well, thank you very much, it's been fascinating speaking to you. It was a pleasure.
In theory, there is no reason why Planet Nine should not be out there.
But there is only one way to prove its existence.
And that is to directly observe the planet.
Mike Brown took Chris to the famous Griffith Observatory to look
at the skies above LA and to point out
where he thinks Planet Nine is.
OK, well, clear sky. Sort of.
Where is it? Where is this planet?
Well, we know its path across the sky, we know its orbit,
so we know the path. And the path across the sky
goes from somewhere over here, up across here, right through the middle of Orion and
Taurus, and then down south across this way, and then of course all the way back around.
We actually also know that around Orion and Taurus is where
it is the most distant from the sun.
And we think that that is where it is. We think the places that are closer
to the sun, it would be too bright, we would have seen it already.
OK. So, in some sense, you're already getting some observational
data on the star. You know where it isn't.
And you know other things as well.
The other thing that we would know is that if it is too big,
if it is the size of Saturn or Jupiter, it would have been
detected by the WISE spacecraft a couple of years ago.
They did a survey of the whole sky, looking for things that move,
and we now know that there is nothing as big as Jupiter or Saturn
out to many, many times the distance of where we think Planet Nine is.
So it has to be small, small meaning the size of Neptune
or maybe a little bit bigger.
And it has to be pretty far away still, and somewhere off from here.
And pretty faint. We're not talking amateur telescopes here to find this.
No, in the end, it is going to take us
some time on the biggest telescopes in the world.
But with the biggest telescopes in the world, it is well within
range, and we'll be able to find it.
It will be exciting. Yes. So, I know you're looking for it.
Lots of other people will as well, I'm sure they will now.
Does it bother you who is going to find it?
I would love to find it myself. I would love to have that moment which
I've gotten to have before on these big objects in the Kuiper belt,
like Eris, where you see it and for a moment you are the only person
in the history of humanity who has seen this object so far away.
I would love that, I will not lie.
At the same time, I want it to be found.
And so if someone else finds it first
because they were searching hard for it and helping us out on the search,
yes, I will feel, my heart will sink a little bit, but I think
that will be overcome by the fact I'm so excited it was found.
It's up there somewhere, probably. Right there.
One of the big questions about Planet Nine that we will have to
answer if people are to take the claims seriously is,
how did it get to its current orbit?
To get to the bottom of that question, I've come to
Queen Mary University of London to meet Professor Richard Nelson.
Richard, what's the problem with the proposed orbit of Planet Nine?
Well, it's very difficult to form a planet on an orbit like that.
We believe, according to current models, that it would take
almost the age of the solar system itself to form
a planet all that far out. Because it has to accrete onto the body.
That's right. The accretion process which brings small
agglomerations of material together and builds larger and larger objects,
leading eventually to a planet all the way out there just takes far too
long because it's moving around very, very slowly and there's a very low density
of material there as well. So, if it didn't form out there,
at the outer limits of the solar system, where did it form?
Well, our best guess is that it probably formed among the existing giant
planets - Jupiter, Saturn, Uranus and Neptune. So, there
are theoretical models already which indicate that if you run
computer simulations of the early solar system and its evolution, and
you have a dynamical instability, having a fifth planet there actually
helps those models reproduce what we see in the solar system today.
So that fifth planet could be Planet Nine? It could be Planet Nine, which is no longer
there but orbiting further out. So, what are we seeing here?
What we are seeing here is a computer model of the early stages of the solar system.
But with five planets. So the white planet there is Planet Nine.
The other planets are Jupiter, Saturn, Uranus and Neptune.
You'll see the planets are going around on circular orbit. As you'd expect. As you'd expect.
They're all interacting, they're all perturbing one another.
But Planet Nine, in this simulation, is a bit too close to Saturn when it starts.
And at some point, we're going to see it have an encounter which gives it
quite a dramatic little kick, and you will see it change.
There it goes. That's right, so you see that. And this keeps going, on and on.
Jump forward, you can see the planet's orbit has expanded a lot,
and there you have a very eccentric and a very inclined orbit.
Does that do it? Does this give us the orbit of Planet Nine?
Unfortunately not, because Planet Nine has an orbit where the closest approach to the sun
is about 200 or 300 astronomical units away.
So, as you will have seen here, Planet Nine keeps coming
back into the inner solar system
and continues to interact with the giant planets there.
Eventually, one of those encounters will be strong enough to throw it out of the solar system.
How can you stop it from pinging out into the galaxy?
If Planet Nine is actually proven to be there, it's going to tell us
something very interesting and important about the origin of our solar system.
So, what we've looked at so far is a solar system forming in isolation.
But now let's imagine that the solar system formed within a star cluster.
If we look out within the galaxy, we see that stars do tend to
form within groups and clusters of stars.
If Planet Nine was thrown out onto one of these very eccentric orbits
while the star cluster was still there, then gravitational
perturbations from the star cluster could have caused Planet Nine to
change its orbit when it was its most distant position from the sun.
So, I have a simulation which shows that here.
When it goes out on this orbit this time round, it's not
going to come back in any more. There's going to be
an interaction with a nearby star that changes the shape of the orbit.
Woo! So, suddenly it expands outward
and stops going back into the inner solar system.
So, that's what saves it from being ejected from the solar system.
So, a lot of your model does seem to tie in with
the evidence we're seeing for Planet Nine. It can explain the huge orbit, the inclined
orbit and why it's still there and not out there in the galaxy.
That's right. That's why I said at the beginning there, I think proving that
Planet Nine is present within our solar system, with the orbit suggested, would also indicate
very strongly that our solar system didn't form in a lonely place,
but was part of an earlier star cluster.
Pretty fantastic stuff. Wonderful. Thank you, it's been fascinating.
A pleasure.
And that leaves one final question.
What sort of planet is Planet Nine?
And what are the conditions like so far out in space?
At the far reaches of its orbit, the sun would appear tiny.
Just a point of light.
But it would still be the brightest star in the sky.
Although the sun would be a million times dimmer than it is here
on Earth, it would be 1,200 times brighter than Venus appears to us.
That far from the sun is very cold.
Some estimates have suggested it could be below -250 Celsius.
As for the qualities of the planet, we don't really know.
The computer models suggest it should
be ten times more massive than Earth.
But it could take different forms.
One possibility is that Planet Nine could have been ejected
from the inner solar system early in its development,
when it was just the core of a giant planet.
This would make it predominantly rocky and icy.
Now, with ten times the mass of Earth, it would be about twice
the diameter. And it would probably appear to be a dark,
pinkish red in colour.
But if the planet was ejected when it was more fully formed,
then it would have a chance to build up the gassy layers,
like Neptune and Uranus.
Then it would be much less dense, and so bigger.
Maybe four times the diameter of Earth.
It's thought the planet may have an atmosphere rich in hydrogen
and helium.
In these cold temperatures,
it's likely it will appear as a bland, featureless sphere.
Unless the planet is still producing some internal heat, in which
case we may see structure in the atmosphere.
Clouds of methane ice, ammonia and even water ice.
It all seems so exciting, and yet despite the possibilities,
so far Planet Nine is no more than a prediction in a computer model.
And they have been proven wrong before.
So, very unscientific question.
But the one that keeps coming up.
How sure are you that this exists?
Oh, one quadrillion percent.
HE LAUGHS
You've got an unscientific question, but a scientific answer. CHRIS LAUGHS
Right back at you. But as a theory, you believe your models?
Actually, we try very hard not to believe our models.
But look, if this thing is not out there,
if the search comes up empty, then the outer solar system
has some really, really substantial explaining to do.
And you've ruled out the other explanations, right?
At least for me, it's difficult at this point to argue for a
solar system that makes sense without Planet Nine.
Mike, what about you?
So, I'm slightly lower than a quadrillion percent.
The human brain is really good at seeing patterns.
When I can't sleep at night, it's because I am worried
that we saw a pattern and made up an explanation.
And then as more observations...
come in, the pattern disappears. I don't think it's going to happen,
I'm actually very convinced. But that doesn't mean I sleep all through the night.
I sleep all through the night.
I am really eager to see it, to confirm it, to learn what it is like,
to see if it has moons and rings.
I just want to know all these things.
And do you have any sense of what that will feel like?
You know, I bought a cigar... CHRIS LAUGHS
..about a year and a half ago and it's still sitting in my desk.
You know, every time something dramatic happens, I always forget to smoke it.
So maybe I won't forget...that time.
I'm going to go and buy a good bottle of champagne, and I'm going to put
it in my office, put it in the fridge in my office, so keep it
cold. And when it happens, we're just going to drink it right then.
And between now and the cigars and champagne,
while we are waiting for the discovery, what are you two working on next?
We are looking for the planet,
but we are also looking for more of these objects that are aligned
or perpendicular, that aren't aligned or perpendicular, to prove
ourselves wrong. If anyone is going to prove us wrong,
we would rather do it ourselves than to have someone else do it.
We are trying very hard to prove ourselves wrong.
It's an exciting time. It is. Let's hope it gets more exciting.
Thanks a lot. Thank you.
Today's been incredibly exciting. The team's belief in Planet Nine is contagious.
And I've become increasingly convinced that it really is out there
somewhere in the outer wastes of the solar system.
I'm sure I'm not the only one. There will be teams
and telescopes from across the world racing to be the first to see it.
And if it's found, when it's found, it will be THE astronomical
discovery of the century.
That's it for this programme.
Don't forget, check out February's star guide on the website.
Next month, we'll be looking at the top five photographs
of the solar system ever taken.
But in the meantime, get outside and...get looking up.