The Solar System-1.
The story of the Solar System is the story of the emergence of order out of chaos,
guided by the simplest law of physics: Gravity.
The planets and their moons exists in relatively stable orbits,
because of a delicate interplay between gravity and angular momentum.
This beautiful natural balance is written before our eyes,
in the spinning patterns and rhythms of the heavens.
Travelling at 108,000 kilometres an hour, on a 900 million kilometre journey around the Sun,
our planet Earth completes this epic journey once in every 365.25 days.
The year in the life of our planet is just one of the endless rhythms by which we live our life.
All of these are governed by the seemingly clockwork motion of our planet.
It carries us through cycles of night and day as it turns on its axis,
rotating at 1700 kilometres an hour, every 24 hours.
The length of a day at a particular place is dictated by the precise angle of our planet,
in relation to the Sun.
We have seasons here, due to the fact that the Earth’s axis is tilted by 23 degrees.
As we journey around the Sun, this angle creates the changing dynamic,
that defines the cycles of many of the creatures that live on the land, and in the oceans.
In the Northern hemisphere, the summer months coincide with the north pole,
leaning towards the Sun.
At this time of the year, the angle favours the northern half of our planet,
with extra energy from the Sun.
By winter the north pole is pointing away from the Sun,
and the Southern hemisphere is bathed in additional sunlight.
The seasons are intimately connected to a journey through space,
that catapults us at 108,000 kilometres per hour, around the Sun.
We are completely unaware of this roller coaster ride through the Cosmos.
The whole Solar System is full of these cycles.
Each planet orbits the Sun at its own distinct tempo.
Mercury is the fastest.
It is closest to the Sun.
It reaches speeds of 2000 kilometres per hour, completing its orbit in just 84 days.
Venus rotates so slowly that it takes longer to spin on its axis, 225 days,
then it does to go around the Sun.
On Venus and on Mercury the day is longer than a year.
Further out, the planets orbit more and more slowly.
Mars completes one orbit of the Sun in 687 days.
Jupiter the largest planet, takes 12 Earth years to complete each orbit.
Saturn takes 30 years.
Uranus takes 84 years.
At the very farthest reaches of the Solar System,
four and a half billion kilometres from the Sun, Neptune travels so slowly,
that it takes 165 Earth years to complete one orbit.
The Solar System is driven by these rhythms.
So regular that the whole thing could be run by clockwork.
It seems extraordinary that such a well ordered system could have come into being spontaneously.
This little island of order, that we call our Solar System is in fact a wonderful example,
of the beauty and symmetry that emerges as a result of the action,
of the simple physical laws that govern the Universe.
Studying these laws leads us to understand how that order emerged from the chaos of space.
It also helps us to understand the origins and formation of the Solar System.
The ancient Greeks thought the Earth is the centre of the Universe.
They thought that the Sun and all the Stars orbited around Earth.
The North star Polaris, almost exactly aligned with Earth’s spin axis.
This adds to the illusion that all the stars are rotating through the sky.
It is what we thought for thousands of years.
It is of course wrong.
To understand Earth’s real position we have to look at one set of bodies,
that doesn’t behave as predictably as the stars.
The Greeks named them ‘planets’ which means wandering stars.
The Greeks observed that Mars, rather than travelling in a straight line,
across the background of the stars,
the planet occasionally changes direction and loops back on itself.
This strange movement was observed by the Greeks as early as 1534 BCE, but they couldn’t explain it.
It took over 3000 years to reach the right answer.
For half the time the work of one man, Ptolemaeus dominated our view of Universe.
Around 150 CE Ptolemaeus published his great work, the Almagest.
It was a complete explanation of the complex movement of planets and stars.
For over a thousand years, the Ptolemic view of the Solar System was set in stone.
The Earth was in the centre, and everything revolved around it.
This was science and religion working hand in hand.
With man as the most important of God’s creations,
it was only right that the Earth should be the centre of a perfect and uniform Universe.
Something seemingly inexplicable as the retrograde motion of Mars,
was explained by the presence of smaller orbits, known as epicycles.
This perfectly matched the observed data.
This tendency, perhaps innate in humans, to accept the word of authority,
in earthly and heavenly matters, has being the greatest obstacles to progress through history.
The Royal Society in London, the oldest scientific society in the world, was formed in 1660.
It took the motto ’Nullius in verba’ which means ‘Take nobody’s word for it’.
A true and deep understanding of the Universe is gained,
not by reading the authoritative words of the ancients,
but by careful observations of Nature and original thinking.
To drag the Earth away from the centre of the Solar System,
was the life’s work of the astronomer Copernicus, one of the founding fathers of modern science.
Copernicus quite literally turned our view of the Solar System inside out.
Using the most basic of instruments, he collected the data that would lead him,
to create an entirely new view of the heavens, the heliocentric view.
This view placed the Sun at the centre of the Solar System.
Although completed more than a decade earlier,
Copernicus’ work was published shortly before his death in 1543.
It took apart 1500 years of astronomical thought, replacing it with a new way of thinking.
At its heart was an explanation for the mysterious retrograde motion of a planet like mars.
In his model both Earth and Mars are orbiting around the Sun.
Mars is orbiting at a slower rate of 24 kilometres per second,
whereas Earth is orbiting at 30 kilometres per second.
When observed from Earth, Mars appears to reverse its direction,
due to the difference in the orbit speeds.
Although it appears simple it took millennia to work this out.
Understanding the retrograde loops was one of the major achievements of early astronomy.
It created the concept of the Solar System, and allowed us to build the first accurate maps,
of the planets and their orbit around the Sun.
Building on the work of Copernicus, generation of scientists have explored,
the fundamental workings of our Solar System.
In doing so they were forced to ask profound questions about its origins, such as :
Why is it so ordered?
How did that order emerge from the chaos of the heavens?
To find the answers we must search for clues in what we see.
A good place to start is the circular motions of the planets.
The explanation for this astronomical clockwork lies beyond our Solar System.
It requires an understanding of the physical principles that govern the whole Universe.
One of the remarkable things about the laws of nature, is that they are Universal.
To a physicist, quantities that are conserved are of overwhelming importance.
A conserved quantity is something that never changes.
It is something that can be neither created nor destroyed.
Energy is an example of such a conserved quantity, but there are others.
One of them is linear momentum.
Momentum is the velocity, v, of something multiplied by its mass or ‘mv’.
Imagine a cannon waiting to be fired.
Both the cannon and the cannon ball are still.
This means the momentum of both is zero, because nothing has a speed.
Now fire the cannon.
The cannon ball flies out at high speed and therefore has a momentum,
equal to its velocity times its mass.
But momentum is a conserved quantity, which cannot be created or destroyed.
This means as the cannon ball is flying,
the combined momentum of the cannon and the ball must still be zero.
It is because of this the cannon recoils in the opposite direction to the ball.
The sum of its momentum and the ball’s momentum will be zero.
The cannon doesn’t fly backwards at the same speed because the mass is bigger.
When the cannon ball hits the ground it transfers its momentum to the ground.
The cannon will transfer its momentum to the molecules in the ground through friction.
The momentum of everything combined would always add up to zero.
Linear momentum has a counterpart called angular momentum.
Angular momentum deals with the speed at which something spins.
If something of a mass ‘m’ is flying in a circle at a distance ‘r’ from the centre,
with the tangential velocity ‘v’, the angular momentum about the centre of the circle is ‘mvr’.
Like linear momentum angular momentum is conserved.
A classic example of the conservation of angular momentum in action, is a spinning ice skater.
If the skater starts to spin, then pulls her arms inwards, she will spin faster.
This is because the angular momentum of her hands is bigger,
the further away they are from the centre of the spin, which is her body.
If she pulls her arms in, the angular momentum from her hands is lost,
and this must be compensated by an increase in the spin rate for the rest of her body -
she speed’s up.
How did the skater start to spin, if angular momentum cannot be created.
The skater pushed against the ice to start spinning.
The ice is firmly connected to Earth.
The entire Earth recoils against the spinning skater to conserve angular momentum,
and the Earth spin rate changes.
This is a minuscule effect, because the Earth is many millions of times bigger than the skater.
As the skater slows down through friction with the ice,
the angular momentum is redistributed back to Earth by friction.
Angular momentum is always conserved.
The Eagle Nebula is a interstellar cloud of dust,
hydrogen and helium and the sprinkling of heavier elements.
It is about 6500 light years from Earth.
It is almost hundred trillion kilometres tall.
Within its towering pillars, stars are made.
The famous photograph ’The Pillars of creation’ was taken by the Hubble telescope in 1995.
It is stunningly beautiful, and also gives us a glimpse of where we came from.
Five billion years ago everything we know and see around us was formed from a nebula just like this.
A giant cloud of gas and dust.
Drifting across light years of space, that cloud remained unchanged for millions of years.
Then something happened that caused it to coalesce into the Solar System we have today.
It is thought to be a supernova, the explosive death of a nearby star.
It sent shock waves through the nebula and caused a clump to form in the heart of the cloud.
The clump would have been denser than the surrounding cloud,
so its gravitational pull would have been stronger.
Very slowly, over millions of years, it would have pulled in more and more gas and dust.
Eventually the whole cloud would have collapsed in on itself faster and faster.
Crucially as it collapsed, it was doing something that would establish a series of events,
that we all experience today:
The cloud was spinning.
The initial spin would have probably have come from the glancing blow,
dealt by the supernova shockwave.
Just like the spinning vortex of a tornado, this spinning cloud of cosmic dust,
had to follow the universal laws of the cosmos.
If something spinning contracts then the universal principle of conservation of angular momentum,
dictates that it must rotate faster.
In the case of a tornado the contraction causes the core to rotate faster and faster.
The column of violently rotating air creates havoc on Earth.
Strangely and wonderfully, the same universal principle responsible for this violence,
is also responsible for creating the stability of the Solar System,
because its angular momentum that stops the Solar System from collapsing completely.
As the cloud collapsed, one of the strangest and least understood objects in the Universe was formed.
A Bok globule is one of the coldest known objects in the Universe.
Its low temperature is critically important, because it means that the gas and dust molecules,
are moving very slowly, and are therefore more easily captured by the weak force of gravity,
and pulled together.
As the collapse continued, one particularly dense area dominated,
and the gas slowly formed a more familiar shape.
Out of the cold a new star was born.
Gravity caused one part of our earlier Solar System to contract further and further,
until nuclear fusion reactions halted the collapse.
The rest of the spinning disc was stabilised by a different mechanism.
It was the conserved spin that balanced the inward pull of gravity.
Imagine a tennis ball thrown in the air.
It forms an arc in a curved path called a parabola.
It lands some distance away from you.
The tennis ball has angular momentum relative to the Earth.
This angular momentum would be conserved, were it not for the fact,
that the ball bumps into the ground.
Now imagine that you could throw the ball very fast.
So fast by the time it moved towards the ground, the Earth had curved away beneath it.
The ball would then be in orbit around the Earth.
If there was no air resistance, no forces would act upon it, other than gravity,
because it would never bump into the ground.
It would be constantly falling towards the Earth, and constantly missing!
Because angular momentum, or spin, is conserved.
This is a completely stable situation.
Gravity ensures that the ball keeps falling, and as long as no other forces act,
the ball keeps missing the ground and stays in orbit.
In exactly the same way, a planet doesn’t fall into the sun,
even though it is feeling the attractive force of gravity.
It is constantly falling towards the Sun, but constantly missing.
This is how our Solar System was born.
Rather than the whole system collapsing into the Sun,
a disk of dust extending billions of kilometres into space formed around the newly born Sun.
In just a few hundred million years, pieces of the cloud collapsed to form planets and moons.
So a star system, our Solar System was formed.
The journey from chaos into order had begun.
Of all the Solar System’s wonders, there is one place we can go to where the processes,
that built the Solar System is still in action today.
It is a place of outstanding beauty and complexity.
It is a place that has entranced astronomers for centuries.
It is the planet Saturn.
1.4 billion kilometres from the Sun, Saturn sits at 6 planets away,
from the centre of the Solar System.
It takes 30 years to orbit the Sun.
A day on Saturn is only ten and half hours.
It is believed to fluctuate more than our stable days on Earth.
Saturn is the second largest planet after Jupiter.
Though it dwarfs the Earth in volume, it is only 95 times the mass of the Earth.
This is because of its surprisingly low density.
One of the amusing facts about our Solar System, is that Saturn would float in water,
if we could find a ocean big enough.
Saturn’s low density is a result of its composition.
It is mainly composed of hydrogen and helium, with small amounts of other trace elements.
Along with the other three outer planets, Jupiter, Uranus and Neptune - Saturn is a gas giant.
Humans have known Saturn for a long time, because it appears bright and beautiful.
It is 1.3 billion kilometres from Earth.
It is the furthest planet visible with the naked eye.
It has a special place in ancient mythology for millennia.
It was not until Galileo first trained his telescope on Saturn in 1610, did humans see one of the true wonders of the Solar System: Saturn’s rings.
Galileo thought the rings were two moons nestling on either side of the planet.
It was Hugyens, using a more powerful telescope, that magnified the planet 50 times,
distinguished a ‘thin flat ring’, for the first time.
Hugyens was the first to discover Saturn’s largest moon, Titan.
It was Cassini who unravelled the first details of the rings intimate structure.
In 1675, having already discovered four more of Saturn’s smaller moons.
Cassini discovered the gap in the rings.
It is now known as the Cassini’s division.
The Cassini- Hugyen’s spacecraft mission was launched by NASA.
In 2004, after a 3.5 billion kilometre journey, that included slingshots around Venus,
and flybys past Earth and Jupiter, Cassini became the only spacecraft to orbit around Saturn.
Its purpose is to study Saturn and its rings in great detail.
It takes 84 minutes for the Cassini spacecraft images to be sent by radio waves to Earth.
The images have revealed that the rings are impossibly intricate.
It is made up of thousands and thousands of individual bands and gaps,
and surrounded by a network of moons.
Part of Cassini’s mission is to discover how the rings came to be like this.
There is a deep reason for studying Saturn and its rings.
Their intricate structure is as close as we can get to the disc of dust,
rock and ice that surrounds the primordial Sun, and out of which the planets were formed.
That is why the Saturnian system has so much to teach us.
They are like a miniature Solar System, because the moons are the equivalent of the planets,
and Saturn is the equivalent of the Sun.
The physical processes that go on in the rings, and their interaction with the small moons that are
around them, are probably similar to the early Solar System, after the planets were formed.
Like the planets orbiting the Sun, the rings nearest Saturn are the fastest.
They travel at 80,000 kilometres an hour.
While the rings appear solid, they are also incredibly delicate.
The main disc of the rings is over a hundred thousand kilometres across,
and less than one kilometre thick.
Studying Saturn’s rings we have begun to gain valuable insights,
into the evolution of our Solar System.
Each ring of Saturn is made up of hundreds of ringlets.
Each ringlet is made up of billions of separate pieces.
Captured by Saturn’s gravity the ring particles orbit the planet in an impossibly thin layer.
The reason why Saturn’s rings are so incredibly bright from Earth,
is because they are predominantly made of pure water ice.
They sparkle as they reflect the faint Sunlight.
Most of these pieces are smaller than a centimetre.
Many are micron sized ice crystals.
Some are as big as an iceberg.
Some are over a kilometre across.
The brightness of the ice in Saturn’s rings is a puzzle because our Solar System is a very dirty place.
It is full of dust.
Until recently it was thought that Saturn’s rings must be relative newly formed,
because otherwise their magnificence would have dimmed,
as the surfaces of the ice crystal became coated in interplanetary debris.
It is now thought that they are much older, hundreds of millions are even billions of years old.
The reason the rings shine so brightly is that, they are constantly changing.
Paradoxically, despite their intricate structure, we now know that the rings are a chaotic place,
at least in the small scale of individual ice particles.
As the ring particles orbit Saturn, they crash into each other and collect into giant clusters,
that are endlessly forming and breaking apart.
As they collide, the particles shatter, exposing bright new faces of ice that catch the Sunlight.
It is because of this constant recycling that the rings are able to stay as bright and shiny,
as they were when they were formed.
This dynamism is one of the most remarkable and surprising things about Saturn’s rings.
If they were static, their magnificence would have been long ago dimmed by dust.