The Solar System
The Milky Way is a A cluster of billions of stars, held together by gravity. containing billions of stars. The Sun is one of these stars.
The Sun
The Sun is the largest object in the Solar System. The Sun's huge gravitational field keeps many other objects - planets, dwarf planets, asteroids and comets - in orbit around it.
Planets
The Earth is one of eight planets in the Solar System. The planets orbit the Sun at different distances.
The different planets have different properties and conditions. In general, as the distance from the Sun increases:
- the temperature decreases, for example, Mercury is 430掳C whereas Neptune is -200掳C
- the time taken to orbit the Sun increases, for example, Mercury orbits once every 88 Earth days, but Neptune orbits once every 165 Earth years
For a planet to form, its own gravity must be strong enough to make it spherical in shape. Its gravitational field must also be strong enough to 'clear the neighbourhood', pulling smaller nearby objects into its orbit.
Moons
Moons are natural Body that orbits a planet. For example, the Moon is a natural satellite of the Earth but communication satellites are artificial satellites of the Earth. that orbit a planet. Many planets have moons, and some planets have many moons - Saturn has more than 50. The Earth has just one moon - the Moon.
Dwarf planets
Pluto is a A planetoid that is too small to be considered to be a planet, eg Pluto.. 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. The Solar System contains hundreds of dwarf planets, including Ceres (the only dwarf planet in the asteroid belt).
Asteroids
The Solar System contains smaller objects called A rock in space. Asteroids orbit the Sun but some may cross the Earth's orbit, producing a small risk of collision.. These orbit the Sun in An orbit that is the shape of an ellipse. An ellipse is similar to a slightly stretched circle., and may take up to millions of years to complete. Asteroids are made of metals and rocky material. There are large numbers of asteroids orbiting the Sun in the asteroid belt between Mars and Jupiter. There are also many in a region beyond Neptune called the Kuiper Belt.
Comets
The Solar System also contains small objects called A ball of icy rock that follows an elliptical orbit around the Sun.. Comets are similar to asteroids, but are made of rocky material, dust and ice. These orbit the Sun in highly elliptical orbits that stretch beyond Neptune and pass inside the orbit of Mercury. Their orbit periods are regular but vary from a few years to thirty million years.
As a comet approaches the Sun, it begins to To turn from a liquid to a gas or a vapour., which means that it turns into a gas. It then produces a distinctive tail.