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Know the following:
 Gas clouds in space, called nebulae, are mainly made up of Hydrogen gas and some
Helium gas and dust particles.
 Gravitational forces acting between all the particles caused them to pull towards
each other and clump together.
 As the particles are forced to move, due to gravitational forces, they pick up speed.
 As the particles collide together they convert the Kinetic energy into Heat energy.
 A rotating Protostar forms with a dense glowing ball of gas at its centre.
 As more and more gas particles fall into the centre it heats up considerably.
 Eventually the temperature reaches sufficient temperature that the Protostar turns into
a Star, in our case, the Sun.
 The material which does not get pulled into the Protostar forms a flat disc orbiting the
star.
 The swirls and eddies form inside the disc as the particles here also start to clump
together.
 These whirlpools of activity begin to form planetecimals which then go on to form the
planets and asteroids which orbit the Sun or moons which orbit the planets.
 Gravity causes the heavier particles to pull closer to the Sun and so the inner planets
are made of rock and therefore have the highest densities out of all the planets.
 The lighter gas particles are not pulled with such a large gravitational force and so
form the outer gaseous planets.
 After a long period of time the planets settle into definite orbits around the Sun.
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Planets, asteroids and comets all orbit around the Sun. This is because the Sun has
so much mass that it exerts a very large gravitational pull on the other objects.
The shape of the orbit is known as an ellipse.
All the objects which orbit the Sun orbit in the same direction of rotation.
Asteroids are made of rocky material.
Asteroids can be thought of as broken bits of planet i.e. planet debris.
The asteroid belt orbits around the Sun between the orbit of Mars and Jupiter.
Comets have highly elliptical orbits (elongated orbits).
Comets can be described as being dirty snowballs because they are mainly made up
of ice and dust.
As the comets approach the Sun they begin to warm up.
The surface Ice begins to evaporate.
The radiation pressure and solar wind (charged ions/particles) emitted by the Sun
forces the water vapour to stream directly away from the Sun forming the comet’s
characteristic tail.
At much larger distance than the furthest planet is a belt of asteroid known as the
Kuiper belt.
Kuiper belt is thought to be material left over from the formation of the solar system.
The Kuiper belt lies on the same plane as the disc of our solar system.
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Further out again is a vast cloud, called the Oort cloud, which entirely surrounds our
solar system.
The Oort cloud is thought to contain a vast number of comet-like objects. In fact all
our comets are thought to have come from this cloud.
Nearby stars sometimes nudge these comets out of their stable orbits causing them to
fall in towards our Sun.
Gravitational forces act between all matter in the universe.
The size of the gravitational force between two masses depends upon the size of the
mass and the distance between the masses.
The larger the mass the greater the size of the force.
The larger the distance the smaller the gravitational force.
Gravitational forces are responsible for keeping all the objects orbiting around the
Sun.
Similarly, gravitational forces are responsible for keeping the Moon and other manmade objects orbiting around the Earth.
Telescopes connected to computers are now used to detect new bodies in the solar
system.
The apparent movement of objects against the background of the stars indicates the
closeness of objects.