There are 18,446,744,073,709,551,616 planets in No Man's Sky.
The features of a planet are determined by procedural generation and concepts of physics, like the day/night cycle that is calculated depending on the position and rotation of it relative to any nearby sun, the color of the atmosphere depends on what elements are in it, and whether a planet has water or not is determined by factors such as how close the planet is to its sun.
Planets vary in size but will always have a surface that will be able to be landed upon and the player will be able to travel on foot, i.e. there are no gas giants. Planets may have one or more moons nominally orbiting them.
Each planet has one prevalent biome/weather type; It is entirely possible for a planet to be entirely covered in water or sand. and the environments will be more likely to be hazardous to the player as they approach the center of the galaxy or in more advanced classes of solar system.
The distance between planets in a solar system are much smaller than in real life, to replicate stereotypical sci-fi depictions of alien sky-scapes.
From space, the player may scan the planet and receive information on the prevalent resources that are available.
Planets receive procedurally generated names but once a player has discovered and landed on a planet, they can re-name it and upload their user-defined name.