The planets have fascinated us for millennia.
We've sent dozens of spacecraft to explore them.
Touchdown confirmed...
CHEERING
And now, our exploration of the planets is even taking us out,
beyond our own solar system.
MALE REPORTER: Lift-off for the Delta II rocket with Kepler,
on a search for planets in some way like our own.
We've made discoveries we never imagined.
I have no idea how those mountains could be formed.
Fantastic, to really capture how close Mars was to Earth
in many of its features. I think my jaw hit the floor.
I'm pretty convinced it's really out there.
The Sky At Night has covered every major space science discovery
in more than 60 years of occasionally quirky broadcasting.
Line your finger up with my nose.
It's absolutely tremendous!
I'm going to do just a little mathematics.
Please don't be frightened.
We're going to use this archive
to travel on an epic voyage of discovery.
Good evening. Good evening. Welcome.
Welcome to The Sky At Night's guide to the planets.
For thousands of years,
our only view of the planets was from here on Earth.
But, since the start of the Space Age,
we've sent over 60 probes to visit them.
These probes have led to a transformation in our understanding.
When I was growing up, there were nine of them,
each with its own unique orbit around the sun,
and its own unique characteristics,
unchanged since their formation.
It was even thought possible
that our solar system might be the only one in the galaxy.
The only star with planets in orbit around it.
How wrong we were, about almost everything.
You get some idea of just how far we've come
by looking at the two planets closest to Earth.
The rocky worlds of Venus and Mars.
If there's one planet in the solar system that has fascinated us
more than any other, it's Mars.
Through the years, The Sky At Night has returned to look at Mars
over and over again.
No other planet has been studied more,
and no other planet has revealed
such an extraordinary story of transformation.
PATRICK MOORE: Altair, in the Eagle, you can find Mars quite easily...
As recently as the 1960s,
we thought it might be a living world, with plants on its surface.
But then, in 1969, came Mariner 6, and that changed everything.
And Mariner 6 came down over this direction,
over these so-called deserts.
Well, the question is, of course,
are these dark areas due to organic matter, vegetation if you like,
or are they not?
The answer became clear as soon as Mariner 6 sent back close-up photos.
Nothing but bone-dry desert and craters.
Now, what about this all-important question of life on Mars?
Do you think these dark areas are due to anything organic?
Primitive life, there may be. I don't even think so.
Intelligent life, certainly not.
So, in other words, you think that Mars is a dead planet?
Absolutely, dead as a dodo.
Mars was brutally cold...
..with virtually no atmosphere.
Vegetation didn't stand a chance, as The Sky At Night demonstrated.
Just to show you the fate of higher plants,
I've brought along tonight two cacti.
This cactus has been quite healthily growing under Earth conditions.
And, you see, it's quite a nice, firm-looking sort of cactus.
This one here has spent one night under Martian conditions,
and I think you can see without any doubt
that it's got a distinctly morning after appearance.
Mariner 6 changed our approach to the Red Planet.
There was no complex life now, that was certain.
But had it always been such a harsh environment?
Our focus shifted to the past, and one big question.
Had there ever been liquid water on Mars?
In 1976,
the Viking orbiter took a series of stunning images
that hinted that the surface of Mars
may have once been flowing with water.
We looked back at Viking's striking achievements in 2017.
So, this is from one of the Viking orbiters,
and it's not a single picture. It is lots and lots of pictures.
It's a mosaic that has been put together.
It's glorious. It's absolutely fantastic.
Just looking at these pictures, started to really capture
how close Mars was to Earth in many of its features.
It's got these volcanoes, which are like the volcanoes on Hawaii.
It's got this rift valley, which is like East Africa.
And, it's got its ice caps.
But you've also got the features that, on Earth, we would interpret
as being from rivers.
So you've got something that looks as if
it's going out into a huge delta.
You've got other channels, meandering channels.
The photographs taken by Viking indicated that, in the past,
huge bodies of liquid water may have shaped the landscape.
But, with only aerial images to go on, this could only be a theory.
To get proof, we needed to land on the surface of Mars.
Easier said than done.
The problem is that Mars has an atmospheric pressure
just 1% of that on Earth.
That makes it hard to use the atmosphere to slow a spacecraft down
enough to make a soft landing.
19 spacecraft have been sent to land on Mars.
Only seven have successfully managed it.
The latest, Nasa's Curiosity Rover, was the biggest and most ambitious.
We have ignition, and lift-off of a Delta II rocket
carrying NASA on an odyssey back to Mars.
It arrived in 2012, and The Sky At Night reported on the complicated
procedure used to deliver the rover to the surface.
Can you start talking us through what had to happen
to get it safely onto the ground?
Yes, there were a number of things that had to happen.
The first is to enter the atmosphere at the right point,
so that the heat shield could slow the craft down enough,
to slow it down from 13,000 mph, down to about 1000 mph,
by the time the atmosphere had done its job,
and, at which point,
the parachute could then come out for the first time.
Parachute deployed. We are decelerating.
APPLAUSE AND CHEERING
We're going to 90 metres per second,
at an altitude of 6.5km per second.
We even got a photo of Curiosity on its parachute.
From the Mars reconnaissance orbiter.
It's great to be able to see this,
to have a spacecraft take a picture of another spacecraft.
But that wasn't the hard bit. What happened next?
Curiosity dropped down, away from its back shell and parachute...
We are in powered flight.
..and then moved away from the parachute and left it behind
and then started to descend on its own powered rockets then,
so coming down, kind of, eight thrusters slowed it down,
made sure it was all smooth.
This slows down to an almost kind of stop, hovering above the surface.
Sky crane started.
CHEERING
So, at about 20 metres above the surface, then Curiosity,
the rover part, then descended on its own kind of cables
on this sky crane technology.
This was the part that we'd never seen anything like this before.
Ready and stable.
Looks good.
Touchdown confirmed...
CHEERING
Like other Rovers before it,
Curiosity was looking for clues into the past.
But how could we piece together the story of what happened
with photographs of Martian rocks?
The answer was to apply skills we have perfected here on Earth,
as geologist Iain Stewart explained.
This rock face is part of the Jurassic coastline in Devon.
It was formed around 240 million years ago,
and it provides a detailed record of different environmental conditions.
There's a whole set of layers in this cliff,
and, if you know how to read them properly,
then you can create these different environments.
Now, these... These are really distinctive.
You can see how beautifully rounded all those pebbles are.
Look at them, incredible.
So they would have started off as just general angular rocks,
but what's made them all smooth is water.
So this really is evidence that through here, in the past,
once flowed a mighty river.
But just along the coast,
it's a different kind of rock formation
that shows a very different kind of evidence of contact with water.
What we've got in front of us here is just a wall of sand,
layer upon layer of sand dunes, piled on top of each other,
but in amongst it, are these little bands here.
There's another one, and they get a little bit thicker,
and a bit different to the sand above.
In fact, if you look in here, how crumbly this is.
Because what we have got here is a deposit
that's been laid down by a really fine sediment,
settling out of standing water.
Essentially, this is a temporary lake among the sand.
You only get rocks with this incredibly fine-grained structure
if they were formed when water becomes really still,
as it is in lakes and ponds.
So that means that, whenever we see a kind of thin band
of really fine sediment, we know that it must have fallen out
from still water.
Using these geological tricks,
we build up this detailed and compelling picture
of the history of water on Earth.
Using this knowledge, we can look at the rocks on Mars
and reconstruct the planet's watery past.
In early 2018, Sanjeev Gupta from Curiosity's imaging team,
gave the latest update on the search for water.
So where on Mars are we, and what are we looking at here?
So, we are sitting in Gale Crater, this is this crater that's about
150km in diameter that sits on the Martian equator.
What we have got here is this incredible mountain.
This is Mount Sharp.
It's 5km high, and it's made up of beautifully layered,
stratified sedimentary rocks.
You can see that on this image here,
you can see those layers working their way up the mountain.
That's right, these are sedimentary layers.
What these are recording is basically
progressive build-up of sediment on the Martian surface.
OK, so something is depositing regular layers of sediment.
OK, let's have a look, if we can have the next image...
There we go. It's coming up.
It's a big image. OK.
The rock is actually a mudstone, it's a very, very fine-grained rock.
We've got hundreds of thousands, if not millions,
of years of sedimentation in a long-lived lake.
it seems it didn't vanish altogether.
It just went underground.
OK, let's have a look at one of the more recent images.
There we go. And here we are.
This ridge is probably three or four metres high here, in this face,
and just gorgeous detail, in terms of the geology.
We've been able to use the rover's instruments
to actually look at the chemistry of the crack fills,
and actually, they show evidence for calcium sulphate,
so this is actually gypsum that's filling these cracks.
And this tells us an interesting story about Mars.
After these rocks were lithified, they were turned to stone,
they were buried, and then there was ground water flow
through these rocks, and that ground water was full of calcium ions
and sulfate ions, deposited within those crack fills.
The interesting thing about this is that the chemistry,
the chemical changes, are important to habitability,
and that's crucial, and that's why there's so much interest in this.
We now know that Mars once had rivers and lakes,
and this surface water persisted for millions of years.
What's more, there could still be liquid water
deep under the surface today.
As we've studied Mars, an extraordinary story has emerged.
Mars used to be habitable, like the Earth.
But over time, Mars lost its atmosphere,
stripped away by the solar wind.
And, with no atmosphere, Mars also lost its water to space,
gradually becoming the dry, dusty place we know today.
Comparing Earth and Mars reveals that two planets
can start off with similar conditions,
but end up following very different paths of development.
Neither was this an isolated case.
When The Sky At Night began broadcasting in 1957,
Venus, our closest neighbour, was a planet of mystery...
..still thought to be potentially habitable...
..as Patrick Moore remembered in 1982.
Venus, so like the Earth in size and mass, is a curious world.
All we can see from above is the top of a cloud layer.
According to two very famous American astronomers,
Whipple and Menzel,
it was liable to be covered almost, or completely, with ocean,
with only a few islands poking out here and there.
Life on Earth began in our oceans a long, long time ago,
and Venus, therefore, might have been a world
where life was just beginning, and might evolve.
Such speculation was possible
because Venus was completely covered in cloud.
Anything could be hidden underneath.
But when the first successful lander, Russia's Venera 7,
reached its surface in 1970,
all hopes of habitability were dashed.
It revealed an extreme and harsh world.
The pressure at the surface
was greater than being at the bottom of the ocean.
Temperatures were 462 Celsius,
and the atmosphere was almost entirely carbon dioxide.
The conditions were so brutal
that Venera 7 lasted less than an hour, before falling silent.
Conditions on Venus were obviously hellish,
and that provoked a lot of questions.
Just why do you imagine Venus,
which is so like the Earth in size and mass,
has developed an atmosphere which is so utterly unlike ours?
It's closer to the sun, it receives more heat from the sun,
the surface gets hotter therefore,
and the gases rise from the surface, building up the atmosphere,
and, as the atmosphere builds up in capacity,
so the surface temperature will rise too.
It's a kind of runaway effect. It's the runaway greenhouse effect,
in the same way as heat is trapped in your greenhouse, when you're growing your vegetables,
the same heat is getting trapped up on the Venus atmosphere,
building up the surface temperature.
Do you know, Garry, I remember way back in 1957, 1958,
on one of these Sky At Night programmes,
I remember saying that, on the basis of the information we had then,
Venus appeared a rather more promising site for a colony
than Mars, and how wrong I was. How wrong we all were!
Since the Russian Venera triumph,
we've set a total of 21 successful missions to Venus.
They have shown us a planet covered in volcanoes,
and sulphuric acid clouds.
The latest probe to conclude its mission, Venus Express,
revealed just how dynamic the planet is, as I discovered in 2015.
One of the first things these images revealed
was what a constantly changing world Venus is.
That is dramatically illustrated by what happens in the polar regions.
The poles of Venus have these enormous vortex features,
and it really changes shape quite dramatically from day-to-day.
It's also changing from year-to-year.
After eight years of observations we have seen, for example,
that the mean wind speed at the cloud tops of Venus
have increased by 100kmph over the course of the mission.
But Venus Express also suggested that, just like Mars,
in the past, Venus might have been very different.
I met Andrew Coates, who's been studying the way
the solar wind interacts with the atmosphere of Venus.
Venus lacks a magnetic field. You know, unlike the Earth,
which is protected, to some extent, by a magnetic field,
Venus doesn't have such a thing and so the solar wind,
which is a million tonne per second stream of material
coming out of the sun all the time,
can interact with the atmosphere of Venus, actually pull it away.
So, Venus is losing atmosphere?
300kg of the atmosphere per day is actually escaping.
Taken over billions of years,
this means an awful lot of material has been lost.
So, what has surprised you in terms of the results you have
got from Aspera-4?
Looking at the composition of the material escaping from Venus,
it actually turns out to be water.
The composition is very much like...
So, you've got twice the amount of hydrogen,
compared to the amount of oxygen going off, so that's H2O.
Water! Water, yes, and that was a big surprise
because the atmosphere of Venus is basically carbon dioxide,
but it shows that water is still being lost from the Venus atmosphere
so this dehydration is a process which is still going on,
and has been going on for billions of years.
If Venus has been losing water for billions of years,
it might have once been a wet planet.
But then, it got hotter and hotter, thanks to greenhouse gases.
Water turned to steam,
and the solar wind stripped the water vapour away into space.
Seen in the light of what happened to its neighbours,
Earth is the one that is unusual.
Whilst Mars and Venus have both undergone dramatic changes,
Earth alone has remained stable and habitable for billions of years.
Even closer to the sun, we find Mercury.
A planet with massive temperature extremes...
..and virtually no atmosphere.
Today, these four worlds are all very different,
but they all belong to the same category.
The rocky planets.
As we move further out into the solar system,
we come to an entirely different class of planet.
The gas giants of Jupiter and Saturn.
These spectacular planets have led to some amazing discoveries.
Perhaps the most surprising is how they have changed our understanding
of how the rest of the solar system developed, including Earth.
As one of the brightest objects in the sky, Jupiter,
named after the king of the Roman gods,
has enthralled humans since ancient times.
It was one of the first objects to be observed through a telescope,
so large that its moons and the striped features
were easily visible from Earth.
Jupiter is making quite a brave show and you can see it
in the southern part of the sky late at night.
It looks like a very brilliant star, and in fact, it's so bright
that you can't really mistake it.
If you've got a telescope,
you'll be able to see its belts and its moons.
This is a picture of Jupiter, which also shows
that extraordinary feature, the great red spot.
Our exploration of Jupiter began in earnest in 1979
with the two Voyager spacecraft.
571, item one,
NA, item two command...
Good evening. As I think most people will know,
the American probe Voyager 1 has flown past the giant planet Jupiter,
and sent back incredible results.
And this, in fact, is a film made up of the various stills taken
as the probe went in,
and you can actually see Jupiter spinning around.
Don't forget, Jupiter has the shortest day in the solar system,
less than ten hours long.
There it is, there is the great red spot below the centre.
What we see as Jupiter's coloured bands are, in fact,
the tops of giant weather systems.
A layer of swirling cloud tops that go down 3,000 kilometres.
The Voyagers confirmed theories that the great red spot,
observed for hundreds of years,
was a gigantic storm, twice the size of Earth.
But how had it persisted for so long?
Helen Czerski met Professor Peter Read to find out
how temperature differences between the different bands of weather
can create very stable features in the atmosphere.
What we tend to see is that the main temperature differences
that we can measure in the atmosphere
are actually between the bright bands
and the dark bands, so that you'll typically have a bright band that's
relatively warm and then, to the north and to the south of that,
there'll be a dark band that is relatively cold.
With this experiment,
we are trying to create a simulation of what happens
within one of Jupiter's bands.
The water is heated,
but the edges and the centre of the vessel are cooled.
This represents a warm band on Jupiter, surrounded by colder air.
And then the whole experiment is rotated.
After a few minutes, some vortices are created that are so stable
they barely appear to move.
So, what you can see now is a whole chain of these eddies
that are all circulating in the same sense.
So if this was a band on Jupiter,
at the bottom of the band the winds are going this way round,
and then you've got these eddies spinning, like this.
But, at the top, the winds are going the other way.
That's right, so this is just like the bright bands on Jupiter.
So, the south of the jets will be going in one direction,
and to the north they'll be going in the opposite direction,
with these vortices rolling in between them.
These rotating storms are trapped within the bands
that have been created by Jupiter's fast rotation.
Since the Voyager probes, we've sent another five probes to Jupiter...
..and discovered it has northern lights
up to 1000 times brighter than on Earth,
an intense magnetic field,
and deadly radiation zones.
But perhaps the most surprising discovery we've made about Jupiter
is that it plays a key role in keeping Earth habitable.
In 1992,
scientists spotted a comet they called Shoemaker-Levy 9,
which had broken up into pieces,
and was on a collision course with Jupiter.
Telescopes, including the Hubble space telescope,
were trained on Jupiter to record the impact.
The Sky At Night looked back at the event two decades on.
In July 1994,
21 fragments of what was the comet Shoemaker-Levy 9
crashed into the atmosphere of Jupiter,
and Hubble was on hand to witness the event.
The impacts happened just over the limb, on the planet's far side,
though the plumes were detected from Earth.
But the true enormity of what had happened became apparent
slightly afterwards, as Jupiter rotated,
bringing the impact sites into Hubble's view.
The wonderful thing about this event
was that no-one had any real idea what to expect.
We hoped we might see the impact with Hubble, but that was about it.
I remember going outside with my own small telescope,
and being able to see these bruises on Jupiter's surface,
and then to see the Hubble images afterwards
made for a really thrilling event.
The resulting impacts were more profound than anyone had predicted.
Using different wavelengths of light,
Hubble captured striking images of the impact scars,
revealing some to be twice the size of Earth.
At the time, we thought it was a once-in-a-lifetime event.
But we now know differently.
Thanks to the work of amateur astronomers,
who monitor Jupiter constantly.
They've spotted several unusual spots over the years.
In 2009, quite unexpectedly, an amateur, Anthony Wesley,
discovered such a spot on the planet.
This was an image he took two nights earlier,
and then he saw this remarkably black spot appearing,
and realised this might well be an impact.
So, other amateurs immediately started taking images to confirm,
and professional scientists took this image
in a far infrared wavelength.
The Hubble space telescope took this image a few days later,
and so the professional astronomers managed to follow this event
over several months,
while we amateurs were also tracking it over several months.
So, the amateurs keep watch, they keep an eye on Jupiter,
and alert the professionals when something exciting happens?
Yes, indeed. And, more recently,
amateurs have been noticing impacts while they happen.
They're much smaller impacts, they don't leave visible scars,
but they're more frequent, and so now three times since 2009,
amateurs have actually seen fireballs in Jupiter's atmosphere,
which previously, if anyone had seen them, they didn't notice them,
or didn't believe they were seeing them.
But now, we have real webcam videos and it is clear
that amateurs are actually detecting flashes as they occur.
Jupiter's enormous gravity acts like a giant magnet,
sweeping up comets and meteors that might otherwise collide with Earth.
Without Jupiter, Earth might have faced many more meteorite impacts,
like the one that wiped out the dinosaurs.
That would have made it much more difficult
for complex life to develop.
Jupiter may be the biggest planet in the solar system,
but it isn't the most spectacular.
Saturn, with its incredible ring system,
is perhaps one of the solar system's greatest planetary wonders.
It's also the planet that sits furthest away from the sun
that can be seen from Earth with just the naked eye.
And its iconic rings have puzzled astronomers for centuries.
What are they made of?
How are they formed?
And, how many rings are there?
PATRICK MOORE: Saturn.
This is the planet with the rings, a magnificent sight,
there's nothing else like it in the sky, so far as we know.
Saturn sits 1.4 billion kilometres from the sun.
And, so far, we've only sent four missions there.
The first was Pioneer 11 in 1979.
This is Saturn, photographed from close range.
Since our last Sky At Night programme,
the space probe Pioneer 11 has flown past the ringed planet,
and has sent back pictures which are surprisingly good,
as well as a great deal of fascinating general information.
But we had to wait 25 years for the definitive mission to Saturn.
And, lift-off of the Cassini spacecraft
on a billion-mile trek to Saturn.
Cassini, which arrived in orbit in 2004,
spent 13 years orbiting the planet.
It revealed enormous hurricanes,
8,000 kilometres across.
And we finally saw Saturn's rings in all their complex glory.
They're made almost entirely of water ice,
Some particles the size of grains of sand,
and others as big as mountains.
They're 280,000 kilometres across,
but, astonishingly, only ten metres deep.
And they show incredible complexity.
There are seven groups of rings,
and each one has its own unique, intricate structure.
What's more, Cassini discovered remarkable and unexpected
interactions between Saturn's rings and its moons.
Gravity from the moons could clearly be seen to be dragging the rings out
of shape, but the relationship between the moons and the rings
went further than that.
This is the moon Enceladus.
So, this is active.
We think there is liquid water,
possibly even a global ocean of liquid water underneath,
and this material is coming out at pressure,
and going out several hundred kilometres,
and then just sort of wandering around.
If you look at it really closely, you can actually see the plumes
of material coming out of the south polar,
and that has created the entire ring,
so it shows this connection between rings and moons.
Yeah, and it seems to be quite a strong one.
It seems that at least some of Saturn's rings,
are formed from material ejected from Saturn's moons.
But there's a twist to this discovery.
So, moons forming rings, and then sometimes rings forming moons?
Yes. I find that surprising!
I did, too, when it was first proposed,
but we know that material kind of naturally can accrete in the rings.
Supposing you get a large enough mass,
then it could start evolving as it interacts with the ring material
around it, and maybe even escape from the rings,
and then move outwards, and maybe catch up with another one
that had already escaped and get larger.
And they originally proposed it for small moons,
and they think now, maybe even the larger moons of Saturn,
and indeed other giant planets, could have formed in this way.
This process helps us understand the early days of the solar system.
You have this sort of, what I might call an accretion disc,
with a large mass in the centre.
It's very similar to what we think the early solar system was like.
That there was a disc of material out of which the planets formed,
in that case, and this is a disc that we can study in detail now,
and we can see how things evolve now,
and then try and extrapolate to what went on in the early solar system.
So, by studying the rings of Saturn,
we can actually look back to the formation of our whole solar system.
When we leave the gas giants behind, everything changes again.
We meet a whole new class of planet,
the ice giants of Uranus and Neptune.
Even as recently as 1975, we had no clear photos of Uranus and Neptune.
Well, I'm afraid you're not going to see very much on Uranus
with any telescope. I can show you a picture of it,
and here is one, taken with a major telescope.
You can see there some of Uranus's satellites as well,
but not even a giant telescope is going to show you much on the disc.
And here is Neptune.
But, as I've said,
I'm afraid no telescope is going to show you very much on it.
We've got, I think, to wait for the era of space probes.
Only one probe has ever visited these outer planets.
Voyager II.
And only by flying past, not going into orbit.
When Voyager arrived at Uranus in 1986,
what it found was far from what we had imagined.
Voyager flew past Uranus and saw a greenish disc
with very little cloud activity taking place over the disc itself.
It was a really boring planet, let's be honest about this.
Lots of scientists looked at it, and were quite disappointed.
But observations over the next quarter century showed us
that first impressions aren't always accurate.
We've got a large dataset of Hubble space telescope,
and ground-based observations of Uranus,
which show that we simply flew past at a rather boring time,
and, in fact, it's become a lot more active now, with white spots,
incredible colours around the north pole and the south pole.
Uranus also has its own ring system,
and a bizarre feature that makes it unique amongst the planets.
It's tilted so far over that it's completely on its side,
giving it extreme seasons.
In contrast to Uranus,
Voyager found Neptune's surface to be turbulent and active,
with the strongest winds in the solar system.
Neptune has its own source of internal heat.
Basically, it emits out more energy than it's receiving from the sun.
That emission of heat is driving a really complicated weather pattern.
We see incredible storms, such as the great dark spot that was present
in Neptune when Voyager II flew past in 1989.
Uranus and Neptune have different characteristics,
but also, they share significant similarities.
They both have a blue-green hue.
And they're both made of similar materials,
including methane, ammonia, and water.
Looking back at all the planets in the solar system,
it's striking to consider how they fall into three distinct groups.
The rocky planets closest to the sun.
Then the gas giants.
And, finally, the ice giants.
The question is, what can explain this pattern?
In 2014, physicist Chamkaur Ghag took on the challenge.
It meant going back to the formation of the solar system...
..when there was a huge cloud of gas and dust,
and a newly formed star in the middle.
Imagine this bench is our disc of stellar dust.
All the bits and pieces
are going to go into making our brand-new solar system.
Now, in the centre,
the sun pushes the temperature up by over 1,700 Celsius.
But as we move away, the temperature drops,
and down towards the edge of this disc,
an incredible 9 billion miles away,
the temperature has dropped to a chilly minus 250 Celsius.
This temperatures gradient, from inferno to freezer,
is critical to the way each planet is formed.
Here, close to the centre of our primordial solar system,
the temperature's upwards of 1,000 Celsius.
In this searing heat, only certain rocks and metals can remain solid,
and that's why Mercury, Venus,
Earth and Mars,
are made up mostly of rocks and silicates.
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 a bit 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 protoplanetary 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 that these volatile gases are locked into solid form,
they can start to coalesce,
and they seed what will become Uranus and Neptune.
Although today, the planets are considered to end at Neptune...
..that hasn't always been the case.
When I was growing up,
Pluto was very definitely considered to be
the solar system's ninth planet.
But not any more.
In 2006, it was demoted, and given dwarf planet status.
But this raised a rather interesting question.
Just what is a planet?
Pluto was demoted,
partly because it's small, but also because it's not something known as
"gravitationally dominant".
Pluto sits in a region of the solar system called the Kuiper belt,
a vast realm of icy bodies of many different sizes.
Its weak gravity means it has not cleared the area around it
of other objects.
Nevertheless, the debate over Pluto's status hasn't gone away.
When Nasa's New Horizons mission arrived at Pluto in 2015,
it turned out to be one of the most exciting,
and surprising space missions of all time.
Stand by for telemetry.
This is good.
For Alice Bowman, it had been a ten-year wait.
OK. Copy that. We're in luck with telemetry with the spacecraft.
CHEERING
I told you it would be fine! I wasn't worried!
These people weren't worried!
I was worried. I was worried!
This is such a culmination of so much work!
They'd travelled three billion miles,
and hit a target that was only 60 by 90 miles,
the equivalent of throwing something across the Atlantic Ocean,
and it landing in my hand.
Pluto astonished everyone.
It had huge nitrogen glaciers...
..and mountains made out of water ice,
completely impossible here on Earth.
Carly Howett explained why the team was so excited.
You have to remember that the surface isn't rock,
this is water ice mountains.
Which sounds crazy. That's crazy, right? We don't...
Water ice in the earth just wouldn't be strong enough
to sustain that sort of structure.
But what you have on Pluto are surface temperatures that are
so, so cold that water ice almost has a sort of rocklike quality,
it's just absolutely solid.
And so it's able to support these massive structures.
I don't remember anyone predicting mountains.
Do we understand how these things could have grown?
I have no idea how those mountains could be formed.
Pluto has quite low gravity, it's a small body.
We don't get winds and water, like you do on the Earth,
so how you get uplift, how does that even happen?
I don't know.
The discoveries reignited the debate about Pluto's status.
In 2017, a team of scientists submitted a proposal
to reclassify Pluto as a planet.
For now, though, Pluto is just one of a vast number of bodies
going round the sun in distant orbits.
All part of the Kuiper belt.
And it's in this distant realm
that our solar system has given us one last planetary surprise.
In 2016 came a dramatic announcement.
The claim that scientists had discovered a new ninth planet.
If true, it will transform our ideas about the solar system.
The strangeness of this proposed Planet Nine
began with its extraordinary orbit about the sun,
as I try to demonstrate.
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 if it exists, the orbit of 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...
..in reality, it would be 180 billion kilometres from the sun.
This discovery was enormous news.
But there was one crucial stumbling block.
No-one had actually seen Planet Nine.
Its existence had only been predicted by computer models
that have been built by Caltech astronomers
Mike Brown and Konstantin Batygin.
In 2003,
Mike discovered the first of six objects with bizarre orbits
in the Kuiper belt.
All of the orbits pointed off in the same direction.
They were all tilted downwards, and off to the side.
And the culmination of those two is what really shows you
that there's something strange going on.
So, it's about a 0.01% chance that this is all just randomly aligned.
So, is that what got your attention, got you working on the problem?
That was the big clue that something's 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.
Hey, how are you doing? Nice to see you.
So, Mike challenged him to build a computer model of the solar system
and replicate the strange orbits of the six Kuiper belt objects.
After many calculations, Batygin came up with a model that worked.
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.
Sort of coming in and out of the blackboard? That's right.
They're on their sides,
they're perpendicular to the plane of the solar system.
And this is just so weird,
the fact that the simulations 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.
So, there was the key moment where we looked at it and said,
"This makes no sense."
Then I looked 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 that are on perpendicular orbits
that I didn't... I remember when one of them was discovered a few years
ago and thinking it was just weird, nobody had any explanation for them.
And I said, "Konstantin, I'm going to go plot these right now,
"and we'll see where they are,
"and if they're sitting right 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.
We just both sat there and stared at that, and my jaw hit the floor.
It really was an honest blind prediction of something
we didn't know was there.
And that... When I look at those objects, and those rings,
and then I see those aligned objects, too...
..I think I really am pretty convinced it's really out there.
So far, Planet Nine is just a theory.
It has yet to be discovered.
The planets of our solar system are amazingly varied,
but they are just one case study orbiting our star, the sun.
But the sun is amongst billions that lie in our galaxy.
For many years, we've thought that there must be other planets
out there in the galaxy beyond our solar system,
the so-called exoplanets.
And, some might even be habitable, like our Earth.
But we've had no way of finding them.
Planets with no light of their own were rendered invisible in the glare
from their parent star, as Patrick explained in 1975.
Now, that's the view from close range.
Now we're going to pull the cameras back, and you will see what happens.
As the sun shrinks from a large disc into a smaller one,
so the little Earth gets closer and closer to it,
and finally disappears.
Now, just imagine what would happen if we could go so far away
from the sun that it looked merely like a point of light.
What would be the chances of our being able to see
a world such as the Earth going round it?
Frankly, none.
But then, scientists came up with a new way to search for exoplanets
that meant they didn't have to see the planet to know it was there.
It involved looking for tiny movements in the stars,
wobbles, caused by the planets pulling them off-centre
due to gravity.
In 1995, two young scientists from Geneva
studying a star called 51 Pegasi
made a breakthrough.
Didier Queloz, who made the discovery, told me what happened.
I was a PhD student at the time,
so the first time I 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 in 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.
This was no data error.
Didier had found the first exoplanet orbiting a stable main sequence star
50 light years away from Earth.
Did you hope that the planet you discovered
was going to be Earth-like?
Well, it was impossible, because the instrument we were using,
the best it could do is detecting something like Jupiter.
It was designed to do that,
that was already a tremendous achievement at that time.
The technology then could only detect the most massive planets,
because they exerted the biggest gravitational pull
on their host star, resulting in a bigger wobble.
51 Pegasi b 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.
And it rewrote our ideas of what a planet could be like.
But there was no chance it might be habitable.
Now, the hunt was on for more exoplanets.
Engines start, one-zero...
And lift-off of the Delta II rocket with Kepler,
on a search for planets in some way like our own.
When Nasa's Kepler telescope was launched in 2009,
it unleashed a planet-hunting gold rush.
Designed to stare at a patch of sky continuously,
Kepler measures any subtle dimming in a star's brightness,
a signal that a planet could have passed in front of a star.
Using this technique,
Kepler has found over 2,600 exoplanets
in many different flavours.
Hot Jupiters, mini Neptunes, and even so-called super-Earths,
just a few times bigger than our own planet.
To explore just how strange some of these planets are,
The Sky At Night decided to compare the planets that science fiction had
imagined in movies with the real planets that had been discovered.
So, how realistic are some of these exoplanets
that we see in the movies?
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.
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 16 b.
We found that in 2011.
And it's the first circumbinary planet.
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... So the stars are orbiting each other in a little
merry dance, and the planet is going round all...
Exactly, yes. Wow, OK. So, here is 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, the other one is a little red.
Almost exactly like in Star Wars.
It is! Almost exactly like in Star Wars.
One of the things about Planet Miller in Interstellar
is that it's orbiting a black hole
and you get into all kinds of exotic physics,
time dilation, all this sort of stuff.
But the fact that it's orbiting a black hole
is a really interesting idea. Is that at all feasible?
Yes, there's no reason why you can't have a planet orbiting a black hole.
In fact, there is some evidence to suggest that we've even seen
a super Jupiter being eaten by a black hole.
We've even found a planet that might be made of diamonds.
There are lots of things we've found that are stranger than things we can imagine.
That's the beauty of the exoplanet world at the moment.
We're discovering things we didn't know were possible,
new physics is being discovered all the time.
Although we were constantly being surprised by new exoplanets,
we still hadn't found anything that seemed potentially habitable.
What we needed was a rocky planet in the Goldilocks zone.
Not too hot, not too cold, but just right.
Hopefully, with liquid water on its surface,
and therefore the potential to support life as we know it.
In 2016, came a breakthrough.
Now, scientists are hailing a major discovery,
a new planet which they've called Proxima b.
The planet was found orbiting Proxima Centauri,
our closest neighbouring star.
At just over four light years away,
it's close enough to imagine that we might, one day,
be able to send a spacecraft there.
And so a French team have tried to work out
the planet's characteristics by using comparisons
with the planets in our own solar system.
If it has a large metallic core and a rocky mantle, like Mercury does,
then it's probably a little bit smaller than the Earth.
But there are other, more exotic possibilities.
It could be much bigger than the Earth, 1.4 times the size,
in which case it would have to have a rocky core,
surrounded by a mantle of ice.
And, excitingly, the model suggests that the entire planet
would then be covered in a vast ocean, 200km deep.
Proxima b was tantalising enough.
But then, in 2017, came the most exciting discovery yet.
A whole system of Earth-like, potentially habitable, planets,
known as TRAPPIST-1.
In just 60 years,
we've gone from one solar system of nine planets,
to a galaxy teeming with them.
So, what's next for planetary exploration?
Well, if there's one lesson that we've learned through over 60 years
of The Sky At Night, it's this.
Every mission that we send out to another world
throws up incredible discoveries,
and provokes many new questions.
Really, we've only just begun.