University of Edinburgh

BSL Astronomy Glossary - planet - definition

Definition: Planets are naturally occurring objects that exist in space where they orbit stars. In our solar system, eight planets orbit around the star at the centre, the Sun. These eight planets are divided into two groups: the four inner planets, closest to the Sun and the four outer planets which are further away from the sun and lie beyond the Asteroid Belt. The four inner planets are mainly solid, made up of rock. The BSL sign for the inner, rocky planets, Mercury, Venus, Earth and Mars is formed using a closed fist to indicate their relatively solid composition. The outer planets are different and although it is unknown whether they contain solid material within them, they are predominantly composed of gases. These massive outer planets, Jupiter, Saturn, Uranus and Neptune are also referred to as the Gas Giants due to their size and composition.