Encyclopedia · Glossary
Planet
A round body that orbits the Sun and has cleared its orbit.
Definition
Under the definition adopted by the International Astronomical Union in 2006, a planet is a body that orbits the Sun, has enough mass for its own gravity to pull it into a nearly round shape, and has cleared the neighborhood around its orbit of other comparable bodies.
In context
By this definition the Solar System has eight planets, from Mercury out to Neptune. Bodies that are round but have not cleared their orbits, such as Pluto and Ceres, are classed as dwarf planets instead.
Related ideas
Planets around other stars are called exoplanets; thousands are now known. Planets shine by reflecting sunlight rather than producing their own light like a star.
Related entries
Encyclopedia
Star
A self-gravitating sphere of plasma powered by fusion.
Encyclopedia
Astronomical Unit
The average Earth–Sun distance, about 149.6 million km.
Encyclopedia
Transit
A smaller body passing across the face of a larger one.
Encyclopedia
Galaxy
A gravitationally bound system of stars, gas, dust, and dark matter.