The solar system

• The Sun is our nearest star and a relatively small star when compared to other stars in the universe.
• Our Solar System contains the Sun and everything that orbits it.
• The Sun is the largest object in the Solar System and its gravitational field keeps many other objects - planets, dwarf planets, asteroids and comets - in orbit around it.
• The Earth is one of eight planets in the Solar System and they orbit the Sun at different distances.
• The different planets have different properties and conditions, for example, as the distance from the Sun increases: the temperature decreases, with Mercury being 430 °C and Neptune -200 °C, and the time taken to orbit the Sun increases, with Mercury orbiting once every 88 Earth days and Neptune once every 165 Earth years.
• For a planet to form, its own gravity must be strong enough to make it round or spherical in shape and its gravitational field must also be strong enough to ‘clear the neighbourhood’, pulling smaller nearby objects into its orbit.
• Moons are natural satellites and bodies that orbit a planet.
• The Solar System was formed around 4.6 billion years ago from a large cloud of dust and gas, called a nebula.
• If a nebula is massive enough, it can collapse under gravity to form a protostar.
• The core of the nebula began to form a hot, dense protostar.
• When the Sun’s core became hot enough and dense enough, nuclear fusion began.
• Gravity provides the force needed to maintain the stable orbit of both planets around a star and also of moons and artificial satellites around a planet.
• Nuclear fusion is the joining together of two smaller atomic nuclei to produce a larger nucleus, and radiation is released when this happens.
• For an object to remain in a steady, circular orbit it must be travelling at the right speed.
• Nuclear fusion happens in stars like our Sun, and in hydrogen bombs.
• If the satellite is moving too quickly then the gravitational attraction between the Earth and the satellite is too weak to keep it in orbit.
• In nuclear fusion reactions, hydrogen nuclei join together to form helium nuclei, and energy is transferred by radiation.
• A star like the Sun is at equilibrium - gravity tends to pull it inwards, and radiation pressure from the nuclear reactions tends to expand it outwards.
• If the satellite is moving too slowly then the gravitational attraction will be too strong, and the satellite will fall towards the Earth.
• The gravitational collapse of a star is balanced by the expansion due to fusion energy.
• The gravitational field of a dwarf planet is not strong enough to clear the neighbourhood, so there may be other objects in its orbit around the Sun.
• Asteroids orbit the Sun in highly elliptical orbits, which are oval or egg-shaped and may take millions of years to complete.
• As a comet approaches the Sun, it begins to vaporise, turning into a gas.
• Many planets have moons, and some planets have many moons - Saturn has more than 50.
• Comets produce a distinctive tail.
• The Earth has just one moon - the Moon.
• The Solar System contains smaller objects called comets, which are similar to asteroids, but are made of rocky material, dust and ice.
• The Solar System contains hundreds of dwarf planets, including Ceres (the only dwarf planet in the asteroid belt).
• Asteroids are smaller objects that orbit the Sun but some may cross the Earth's orbit, producing a small risk of collision.
• The Moon is a natural satellite of the Earth but communication satellites are artificial satellites of the Earth that orbit a planet.
• A dwarf planet is an object orbiting a star that is massive enough to be rounded by its own gravity but has not 'cleared the neighbourhood' of other objects and is not a satellite.
• Asteroids are made of metals and rocky material.
• A stable orbit is one in which the satellite’s speed is just right - it will not move off into space or spiral into the Earth, but will travel around a fixed path.
• These orbits are much higher than polar orbits (typically 36,000 km), so the satellites travel more slowly (around 3 km/s).
• Gravitational attraction provides the centripetal force needed to keep planets and all types of satellite in orbit.
• Artificial satellites travel in one of two different orbits: polar orbits and geostationary orbits.
• The gravitational attraction between two objects decreases with distance, meaning that the closer the two objects are to each other, the stronger the force of gravity between them.
• A geostationary satellite orbiting Earth has a period of 24 hours.
• The orbital speed of a planet changes with its distance from the Sun.
• The satellites travel very close to the Earth (as low as 200 km above sea level), so they must travel at very high speeds (nearly 8,000 m/s).