Planets, stars and galaxies
The Sun is a star. It is at the centre of the solar system - solar means relating to the Sun - and it lies within a galaxygalaxy: A cluster of billions of stars, held together by gravity. of an immense number of other stars.
The solar system consists of planets, dwarf planets, asteroids and comets in orbit around the Sun. The orbits of the planets are almost circular, but the orbits of comets are ellipses - very squashed circles. Asteroids sometimes hit the Earth and this could have serious consequences.
Galaxies contain billions of stars, and the distances involved are huge. The universe contains at least one billion galaxies.
The conditions in space are hostile to life and spacecraft must be designed to protect their occupants. Space probes can explore other planets without needing astronauts. Scientists are using different methods to see if there is life on planets other than the Earth.
The solar system consists of:
There are eight planets, including the Earth, and smaller dwarf planets, such as Pluto, Ceres and Eris.
The Sun's gravity keeps the planets, dwarf planets, comets and asteroids in orbit. The gravity of a planet keeps its satellites in orbit.
The planets take different amounts of time to go around the Sun. A single orbit is called the planet's year, and the further out a planet is the longer its year takes.
The orbits of the planets in the solar system are almost circular – with the Sun near the centre. Many diagrams - including these here - show the orbits very squashed from top to bottom. This is to give a sense of perspective or to fit the diagram to a page in a book.