Over 2.8 billion kilometres from the sun, two vast and
mysterious worlds patrol the outer reaches of the solar system.
These are the huge icy bodies of Uranus and Neptune.
And they are unlike any of the other planets.
Tonight, the Sky at Night asks why are these Ice Giants so unique?
Welcome to the Institute of Astronomy at the University
of Cambridge, a place that played a key role
in the discovery of the Neptune,
the outer most planet of our solar system.
It's a perfect time of year to view Neptune and its neighbour, Uranus,
in the night sky, so tonight, we're dedicating this programme to them.
Coming Up... We find out how the Ice Giants could provide clues to
help us answer one of our greatest riddles.
Uranus and Neptune capture the very best of why
we want to explore the outer solar system.
And that is to try and understand why our solar system is
so perfect for life to exist here on planet number three.
Dr Chamkaur Ghag reveals how their unique location makes the ice
giants the exotic worlds they are.
By the time we're at the orbit of Uranus, the temperature
is so low that it's not just water turning into ice,
complex gases like Methane and Ammonia are forming ice as well.
And Pete joins award winning astro-photographer,
Damian Peach, for tips on imaging
the solar system's outermost planets.
First, the basics. Since Pluto has been relegated,
Uranus and Neptune are the outer planets of the solar system.
They're a colossal distance from the sun, 19 times further
than the Earth for Uranus and a whopping 30 times for Neptune.
That distance means they take a long time to complete an orbit.
One year on Neptune amounts to 165 Earth years.
It also means they're very cold.
Parts of their atmospheres can reach temperatures
as low as minus 200 degrees Celsius.
Their most obvious feature, their striking colours,
comes from thick atmospheres rich in methane that absorbs red light
but reflects green and blue.
Uranus and Neptune are clearly very different planets
from the others in the solar system, so why are they like that?
And what does it tell us about how the other planets were formed?
Physicist Chamkaur Ghag is seeking answers.
All the planets in the solar system are composed of unique
combinations of chemicals.
Whilst the gas giants are made almost entirely
of hydrogen and helium, the same basic stuff of stars,
Uranus and Neptune are chock full of methane and ammonia ice.
And, of course, these ice giants are completely
different from the rocky inner planets like our own.
So why do we see this variety?
The answer to this question is all to do with heat.
And to understand why, we have to go back in time to the
birth of the solar system itself.
Before we had planets, there was a huge cloud of gas and dust,
known as a Solar Nebula.
It was in the heart of this Nebula that our star, the sun, was born.
But making a star didn't use up all of the gas and dust.
The rest of the cloud continued to spin, flattening into a disk.
Imagine this bench is our disk of stellar dust, all the bits
and pieces that will go into making our brand-new solar system.
Now in the centre the sun pushes the temperatures up by
over 1,700 degrees Celsius.
But as we move away, the temperature drops and down towards the edge
of this disc an incredible nine billion miles away,
the temperatures drop to a chilly minus 250 degrees Celsius.
This temperature gradient from inferno to freezer,
is critical to the way each planet is formed.
All elements and compounds have their own special properties,
their own way of responding to certain environments.
Take water. Cool it until it's very cold and it'll form ice.
But heat it up and it's going to vaporise into a gas.
Now, all matter in the universe does this.
And it's when it's solid rather than liquid or gas,
that's it's most likely to become a building block for a planet,
which is why the temperature gradient is so important.
Different elements change states at different temperatures.
Here, close to the centre of our primordial solar system,
the temperature's upwards of 1000 degrees Celsius.
In this searing heat, only certain rocks and metals can remain solid.
And this is why Mercury, Venus, Earth and Mars
are made up mostly of rocks and silicates.
Most rocky material has an extremely high melting point
of 1,200 degrees Celsius.
So, other than very close to the sun, rock is solid
throughout the solar system.
And this explains why we find rock and metals in all planets,
including the ice giants.
As we move further out into the solar system beyond Mars,
we reach the snow line.
An imaginary line where the temperatures drop to zero degrees
and water ice can form.
Now what's really interesting about this, is that ice sticks
together much more easily than rocks do.
And this explains why gas giants, Jupiter and Saturn,
are like these balloons, filled with hydrogen and helium.
The ice helped the cores of these giant planets to form very
early and suck up vast amounts of gas from the proto-planetary disc.
It's only when we get further out, much further out, double
the distance from Earth to Saturn, that we start to see the ingredients
of the ice giants appear.
By the time we're at the orbit of Uranus,
the temperature is so low that it's not just water turning into ice.
Complex gases like methane and ammonia are forming ice as well.
Suddenly, now these volatile gasses are locked into solid form,
they could coalesce and they seed what will become Uranus and Neptune.
These chemicals are what made Uranus and Neptune so iconic.
They gave them their striking colour,
and drove their extraordinary weather systems.
And as more and more of the volatile ice clumped together
and mixed with frozen water,
it formed a staggering 60% of the ice giant's mass.
But none of this could have happened without the deep freeze
of the outer solar system.
Any closer and these planets simply couldn't have formed.
And although the ice giants seem exotic to us, recent studies
suggest they could be the most common type of planet in the Galaxy.
To find out why, I'm meeting Dr Helen Fraser
who thinks the clue lies in one particular element
within our ice giants.
The strange thing about Uranus and Neptune is
they have 60 times more carbon in them than we see in the sun.
And that is very exciting because that's more carbon than we have on
Earth and it's certainly more carbon,
ten or 20 times more carbon, than we have in Saturn or Jupiter.
So it's a very big sort of warning beacon to tell us
that the formation of Uranus and Neptune is somewhat different.
And probably, it comes down to the fact that Uranus and Neptune
formed very close to what we call the CO Snow Line,
the carbon monoxide snow line.
This is the point that carbon monoxide freezes,
and that solid ice could have become a key building block
for Uranus and Neptune.
Scientists believed that this band of icy carbon monoxide,
might also exist elsewhere.
And now there's proof.
18 months ago, the ALMA telescope, which is
a very big telescope in Chile, was used for the first time to
image the carbon monoxide snow line in a proto-planetary disk outside
our solar system, a different one which is just forming planets.
And the evidence suggests that most planetary disks would also
have carbon monoxide snow lines.
So in each of these disks, there could be vast
amounts of material to build ice giant type planets.
It's very likely if we've got all these CO snow lines everywhere,
that, that kind of process that formed Uranus and Neptune
is absolutely the dominating process in other solar systems.
In which case, I guess it's very important for us to try
and understand the formation of our own ice giants
if we're going to understand what's going on out there with what
seems to be most of the planets.
Absolutely vital and targets that we should really be sending
probes to in the next few years and trying to get missions to.
In the last few decades, we've had enormous success sending
probes out across the solar system to give us windows on new worlds.
Cassini has been orbiting Saturn since 2004 and returned
astonishing images of the ringed planet and its dynamic moons.
Mars has five spacecraft orbiting it, giving us
an unprecedented view of its varied and complex surface.
And on the ground, two rovers leave no rock unturned.
NASA's probe, JUNO, is on its way to Jupiter right now.
But the ice giants have been forgotten.
It's been more than 20 years since Voyager Two visited
Uranus and Neptune.
But now a group of scientists are trying to change that,
planning a mission that will take us back to the Ice Giants.
Maggie has been talking to Leigh Fletcher to tell us why
we should revisit these enigmatic worlds.
I think that we take the ice giants for granted.
Why do you think they are of interest,
compared with the rest of the solar system?
Uranus and Neptune capture the very best of why we want to
explore the outer solar system and that is to try
and understand why our solar system is
so perfect for life to exist on planet number three.
What makes this the architecture of our solar system,
particularly special?
If you do this mission, what would you expect to find?
One of the big questions we're trying to address,
if you look at Uranus and Neptune, they formed at roughly the
same temperature, they took about the same length of time to form.
You'd expect them to be roughly the same,
the same sort of composition, the same sort of weather.
They look similar colours. They have similar colours and that's
because of the amount of methane that they have in their atmosphere.
Methane absorbs all of the red light,
so what you're left with is the blue to be reflected back at you.
There, the similarities end.
In fact, Neptune, despite being the furthest planet from the sun
is actually one of the most dynamic places in our solar system.
It has these incredibly strong weather patterns
and weather systems with clouds popping up and large cumulus systems
developing and you get sheared apart by the winds and jets
and these can happen on hourly basis,
so Neptune really doesn't look the same each night that we look.
Contrast that with somewhere like Uranus.
Uranus, when Voyager flew past in the '80s, was a very sluggish,
very, dare I say, boring looking planet. So serene. Serene and calm.
All said and done,
Neptune looks like the much more powerful,
much more active planet than Uranus,
despite being much, much further away, at 30 astronomical
units from the sun, versus 20 astronomical units from the sun.
Generally, we think of the sun powering weather,
and energy input from the sun makes the turbulence, drives the weather.
The further you're away from the sun,
you think the less dynamic it would be? That would be the expectation.
Most of the giant planets, if you were to look at them
with infrared eyes, would be glowing hot. Emitting energy.
Neptune has the biggest heat source of any of the giant planets
and maybe that is contributing to this really powerful
weather that we see on that planet.
But then, contrast that with somewhere like Uranus.
Uranus has almost no appreciable heat source that we can detect.
Maybe all that energy is being locked away in the interior,
it can't get out through the various atmospheric layers,
it can't convex and defuse out to where we can see it,
or maybe the energy isn't there any more.
Maybe something in its distance past, removed all
the energy from Uranus which makes it a rather sluggish world today.
Now, there is evidence to support that view. That something happened?
That something happened because one of the biggest mysteries
about Uranus is why it's tilted over almost completely onto its side.
It spins. It spins that way as opposed to that way
in our solar system.
That must make it very strange. It's the oddball in our solar system.
The big question is, why has that happened,
how did that happen in the first place and what implications does
that have for how the planet itself works?
One pole can spend four decades in the darkness of winter,
versus then four decades in summer sunshine, if you like.
That sets up a real difference in the amount of heating that
each hemisphere gets.
Maybe whatever caused that big tilt on Uranus,
whatever bowled it over deep in its distant path after it formed,
is the same explanation for why Uranus has no appreciable
heat source any more. It's all connected.
Going to both planets in one mission sounds pretty challenging,
so which one would you go to? That's a good question.
We do have to choose between the two
and my choice would be to go to Uranus.
The reason for that is exactly what we said.
It's the missing link and it's the oddball of our solar system.
There's nowhere else quite like it.
Frankly, we need to do this to complete our picture
of the solar system.
The planets, one to six, out of Saturn,
have now been characterised by orbital missions.
Uranus and Neptune have simply been forgotten about.
That's the next step, in my opinion,
in the exploration of our solar system.
Let's get to those missing links.
Thanks very much, you put over a very convincing argument
and good luck with the missions in the future.
Thanks very much, it's been a pleasure.
Neptune was the last planet in our solar system to be found.
For good reason. It is by far the faintest of the major planets.
To find it, required an intellectual leap
and an international space race.
We've been joined by Carolin Crawford who's going to
tell us this extraordinary story.
Ancient people knew about all the planets out to Saturn but it wasn't
until 1781, where William Herschel discovered the planet Uranus,
that there was a realisation that there were new planets out
there just waiting to be found.
We can see with our visible eyes,
beyond that, we need other methods to detect them.
That's right, they are too far away so they are dim and faint
and so any further progress really relied on having telescopes.
Uranus was a surprise in the first place.
Herschel wasn't out doing systematic searches for planets.
What a wonderful surprise it must have been to just discover
the first new planet, completely serendipitously
when you're sweeping the sky with your telescope.
After that, people started observing Uranus and mapping
out its orbit and how it's behaving and that is when they really found
a surprise because Uranus wasn't moving as you might predict, it
was just responding to the gravity of the sun and the other planets.
Ever so often, it would speed up
and then a little bit later it would slow down.
By the 1820s, astronomers realised
maybe there was another planet out beyond Uranus
and the gravity was exerting a tug on Uranus every so often.
This is where the Cambridge connection comes in. That's right.
Neptune is peculiar in that it was the first planet whose
presence was predicted but it wasn't until a Cambridge undergraduate
and then a graduate called John Couch Adams came along.
He was the first one to tackle this problem properly and mathematically.
He took a pot shot at where this extra planet would be,
how far out from the sun, where it's relative to Uranus,
predicted what gravitational tug it would produce on that planet
and then compared that to what was seen in Uranus and the data.
Obviously, he didn't get it right first time.
There is almost an infinite variety of possible planets.
It could be further away, you could make a bigger, do a lot with it.
You can imagine it took him a couple of years to refine his calculations
and by 1845, 1846, he's saying, OK, it's in that part of the sky.
It seems obvious, he's done three years worth of work,
you come down to this observatory,
point the telescope over that position and see what you can see.
Didn't quite happen that straightforwardly.
In practice, even though Couch Adams had started on this quite early,
there was a French astronomer, who was making the same predictions
and it wasn't until mid summer 1846 when the Brits realised that there
was a competing prediction, that placed this new planet in the
same part of the sky, that they began to look for it in earnest.
There's a race? There's a race on and so this is
where this telescope came in.
It was used for the search for Neptune here at Cambridge.
To find a new planet, you've got to map that whole area of sky from
one day to the next, from one week to the next and you're looking for
the one object that moves relative to the backdrop of the stars.
There are plenty of difficulties.
He couldn't point directly at one spot, the predictions aren't
that precise and no photography.
No photography, so all the observations are done by eye
and the astronomer who was leading the search, who was
Professor James Challis, wasn't entirely sure
whether he had seen it.
In practice, he had actually observed the planet.
He did detect it but he wasn't sure enough of his calculations
and in the end, he was pipped to the post by the French
and the German astronomers.
So one way of describing this,
this is the telescope that nearly discovered Neptune. Yes.
He saw it, but just didn't realise it.
Yes, it failed to find Neptune, I'm afraid.
Next up, Pete's guide to finding the ice giants,
and then getting the most out of observing them.
It's an excellent time of year for the outer planets of the solar
system, because both Uranus
and Neptune are really well placed at the moment.
And even from a light polluted location like a town or a city,
they are both relatively easy to view.
It's possible to see Uranus as a faint dot with the naked eye
but with a bit of optical assistance,
it's a much easier target.
A small telescope really brings out the character of the planet
because it has a striking emerald green colour.
And it's worth taking the time just to look for that.
It's a little trickier to spot Neptune,
but don't let that put you off.
Neptune is over 29 times further away from the sun than we are.
So that far out, it's quite a dim planet
and it certainly can't be seen with just the naked eye.
Now with a small telescope, the view of Neptune shows it has
character all of its own, compared to its neighbour, Uranus.
It appears as a small disk
and its enchanting blue-green colour helps it stand out.
Both planets do have more to offer, because with the right
equipment, it's possible to make out subtle detail on their disks.
I am meeting multi-award winning amateur photographer, Damien Peach,
who's captured some striking images of these exotic worlds.
Damian, you're well know for taking amazing
pictures of planets including Uranus and Neptune.
Have you got any top tips for getting the best pictures?
Firstly, you need a large aperture telescope,
at least ten inches aperture to stand a chance of getting
some detail on these planets.
Secondly, I would say another important factor is the
use of filters, particularly near infrared and red light filters.
OK, so what's the purpose of the filters?
Now, what they do, is to help contrast those brighter zones
and darker bands much more clearly than you'd otherwise
see in visible light.
I see you've got some fantastic pictures on your laptop.
Can we have a look at them? Yes, certainly.
Oh, wow, that's incredible. That's Uranus.
The first thing that strikes me there is you've got colour
in that shot. Is that colour that's been captured
through the telescope or is that added on afterwards?
The image you have here is actually the real image taken through
a near infrared filter and has been colourised later to give
a more aesthetically pleasing appearance.
That's because it's quite tricky to get the different colour
components with it? Exactly, that's right.
Because these planets are so dim,
it's very difficult to obtain good colour data to make a true colour.
It's worked really well. The detail there is amazing
because you've got these shaded bands running around the planet.
That's right and one of the great things is,
it's only in recent times that it's become possible to start recording
detail on planets such as Uranus and Neptune without a telescope. OK.
Why is that, do you think?
I think it's a combination of the development
of very powerful software processing the data, that we are able
to capture it and the cameras we have now.
They are so much better than they were even five or six years ago.
It's a pretty tiny...
In terms of how they appear in the sky,
they're not dissimilar to the moons of Jupiter.
The moons of Jupiter are slightly smaller.
When you're looking at something like Neptune, it's only very
slightly larger than Jupiter's moon, Ganymede, apparent side.
It's incredible, isn't it? It's a very small object.
Have you got any shots of Neptune on there? We have indeed.
Ah, there's Neptune and its moon, Triton.
That's pretty amazing because there is detail on that planet's disc.
There is some vague markings visible
and again, this was taken through an infrared filter.
Absolutely incredible image. Really is something else.
Thank you very much. Thank you.
No matter what your level of expertise, Uranus and Neptune
are well worth a look, so here's my guide on how to find them
and some other highlights happening this month.
Uranus is in Pisces just south of the faint star, Delta Piscium.
To find Delta, first locate the great square of Pegasus,
high in the southern part of the sky around 10pm.
Extend the upper right to lower left diagonal of the square
half the distance again.
Using binoculars, place Delta at the top of the field of view.
Uranus is the lower of the two dimmer
but similar brightness dots close to the centre.
To find Neptune, first locate the pattern of stars which looks
like a three pronged steering wheel known as the Water Jar Asterism.
Look for the faint star of Sigma Aquarii, which lies
south of the Water Jar.
In binoculars, Neptune is the brightest dot to the right of Sigma.
Finally, don't forget to keep a lookout for Orionid meteors
which peak on the 21st October.
They appear to come from a region of sky above
and left of Betelgeuse in Orion, another result of Earth
passing through the debris stream of Comet Halley.
Uranus and Neptune don't make their slow
procession across the night sky alone.
Like many other planets, they have moons.
And Neptune's largest moon, Triton, is one of the most fascinating
worlds in our solar system.
Chris is speaking to Geraint Jones.
I guess the first thing to say about it, is that it's big.
It is, yes. It's the seventh largest moon in our solar system
so it's smaller than our moon,
slightly smaller than Jupiter's moon, Europa,
but bigger than Pluto,
which until a few years ago, was considered one of the major planets.
And it's just got an incredible surface.
The one encounter we've had, Voyager Two in 1989,
that spacecraft revealed this amazing world, very young,
very few impact craters, so it hasn't been exposed for very long.
It's got an atmosphere and active geysers on the surface.
That, I think, is the iconic Triton discovery.
Voyager actually saw those geysers shooting up into the sky.
That's right. If you look around the South Pole...
We can zoom in, in fact. There we go.
These dark streaks, the dark material is thought to be
dust maybe contaminated with methane which is
darker than the other ices on the surface.
They've been carried up into the atmosphere
and the larger particles fall down immediately downwind of the geysers.
We're getting an eruption and these almost dirty streaks following.
You said a wind, which means there's an atmosphere,
what do we know about the atmosphere of Triton?
It's incredibly tenuous so it's predominantly nitrogen gas,
so it's thought that the pole that's being warmed
the most by the sun has heated up the nitrogen ice that's there,
so it turns from a solid straight into a gas.
It adds to the atmosphere.
That material moves over to the night side of Triton where
it's really cold and it freezes out.
Its surface is incredibly varied.
There's this South Pole and this other region up towards
the top right of this picture where things look very different.
Yes, when that was first viewed in the Voyager images, the scientist
almost immediately referred to it as the cantaloupe terrain.
I've got a cantaloupe melon here. I'm glad we got to the melon.
If you look closely at the surface of this melon, you can
see these interweaving features very similar to the fractures
we see on the surface of Triton.
It suggests there is a liquid water layer somewhere
under the surface.
There's movement of the water ice crust
and through those cracks, you get water lava breaking through,
maybe mixed in with ammonia to make it more runny.
A water lava would be a sludgy nearly melted form of water, right?
It's still very cold. It is extremely cold.
Liquid water flows out of the cracks and form pairs of ridges
on either side of these fractures and this appears to have occurred
over quite a large area of the surface maps by Voyager Two.
If you look at some of the regions, you can
see the hints of ancient impact craters that have been
split in two by these fractures and have been shifted around.
That means activities are more recent than the impact craters?
This is the kind of thing that Triton is possibly still
doing today?
Yes, from looking at the number of impact craters which show
you how all the surfaces,
some parts may be as young as five or six million years old,
which is incredibly young in geological terms and may indicate
that volcanism of the large-scale type might be occurring even now.
That's it for this month, to find out more about the ice giants and to
see Pete's guide how to find them go to our website...
When we come back next month, we have a very special programme.
We'll be following along as one of the most exciting missions
of recent decades reaches its climax.
ESA's Rosetta probe will attempt to place a Lander
on comet Churyumov-Gerasimenko.
The landing site has been selected, the timetable is
in place, but it looks to be a daunting task.
We'll be at the heart of mission control for the landing itself,
bringing you the first ever images from the surface of a comet,
and hopefully some early science from Lander's instruments.
So join us at a 9pm on the 16th of November.
In the meanwhile, get outside and get looking up. Good night.