A planet, often called a world, is a celestial body that is in orbit around a star, is neither a star nor a moon, and has cleared out the area around its orbit as it formed by causing all other smaller bodies in its orbit to accrete with it. As a consequence, it would not share its orbital region with any other bodies of significant size, except for moons or those collected later under its gravitational influence.

Depending on the tilt of the planet's axis, planets also have varied seasons with ranging temperatures and weather. Planets were extremely varied in terms of environmental conditions and landscape, ranging from completely oceanic to thick arboreal rainforests. Some planets, are completely frozen, while other planets, are experiencing an ice age. Some planets, are completely barren and desolate, covered in little more than rock and sand; while others became an ecumenopolis, in which the entire planetary surface is covered by one gigantic city.

Most life-bearing planets have primarily nitrogen and oxygen-based Type I atmospheres; Gas giants, planets made up of primarily gaseous layers, were rarely inhabited, though they were sometimes used for resource collection.