Space glossary

Plain-English definitions for the words you see across the site.

The average distance from Earth to the Sun: about 150 million kilometres.

A world that orbits the Sun and is round under its own gravity, but has not cleared its orbital neighbourhood.

A planet that orbits a star other than the Sun.

A vast gravitational system containing stars, gas, dust and dark matter.

The distance light travels in one year, roughly 9.46 trillion kilometres.

A natural satellite that orbits a planet, dwarf planet or other small body.

A cloud of gas and dust in space. Some form stars; others are produced by dying stars.

The curved path one object follows around another because of gravity.

A large body orbiting a star that is massive enough to become round and, under the Solar System definition, clear its orbital neighbourhood.

The Sun and everything gravitationally bound to it, including planets, dwarf planets, moons, asteroids and comets.

A self-gravitating sphere of hot plasma that produces energy through nuclear fusion for most of its life.