Planets in Starbound are entirely procedurally generated and filled to the brim with unique content. Chucklefish went into the planet generation with one simple goal: no two planets should be the same. As such, even the smallest details are procedurally generated. Not only is the terrain of the planet procedurally generated, but the weather, the gravity, the difficulty level, the plant life, the behavior and appearance of alien creatures, and much more all contain random or procedurally generated elements.
The seed for the procedural generation of a planet is based on its coordinates. This means that two players exploring the same coordinates will encounter the same environment. Many coordinates, however, will not yield a planet, simulating the emptiness of space in reality.
All planets have a finite depth, but not all planets are the same size. Round planets vary in size, and can be circumnavigated: traveling far enough in one direction will bring the Player back to where they started. Other planets have an infinite surface to explore.
 Planetary Core
If the Player digs deep enough, they may reach a planet's core. Many planets have a molten core which kills anyone on contact, but a planet's core can also be made of some other impassable material.