A planet is an astronomical body, usually orbiting a star, that is massive enough to be rounded by its own gravity, is not massive enough to cause thermonuclear fusion, and has cleared its neighbouring region of planetesimals. Most life originates on these worlds, with the origin planet of a species being known as a homeworld.
- See also: Category:Planets by class
The Federation classifies planets (and similar astronomical phenomenon) by their atmosphere, temperature, vegetation, and size, denoting each grouping by a letter. For example;
- A Class-M world is an Earth-like planet but includes some extremes such as Andoria and Vulcan where they are still friendly to Humanoid life.
- Class-D is a barren planetoid or moon, such as Havas-Kul or Luna.
- Idran I is a Class-J world, a gas-giant.
- Class-P, such as Priors 5d, is majority water-ice with a breathable atmosphere.
- Class-Y worlds, such as Nukara Prime, have toxic atmospheres, sulfuric deserts and surface temperatures exceeding five hundred Kelvin. They are totally inhospitable to most humanoid life, but are native conditions for Tholians.