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Planets in Starbound are entirely procedurally generated and filled to the brim with unique content. The terrain of the planet decides what biomes are present on each planet. 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 coloration, weather, the gravity, the difficulty level, the flora, 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. (with the exception of contents of naturally generated containers, which is random) Many coordinates, however, will not yield a planet, simulating the emptiness of space in reality.
Terraforming is the steps which one goes though to change things about a planet such as danger, flatness, and cave systems throughout Starbound but this has not been implemented yet.
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 CoreIf the player digs deep enough, they may reach a planet's core.
The temperature of a planet is perhaps the most important thing that early players will have to deal with early on in their exploration. Planets with extremely low or high temperatures often require special equipment to navigate, and if the player stays in these locales for an excess amount of time, the player will start to take damage. Additionally, there are some planets with a lack of atmosphere or a surface composed of dangerous gas that make the surface itself uninhabitable.
The threat level of a planet dictates how dangerous the planet itself is, and is a good indicator of whether or not you are ready to explore said planet. If the threat level exceeds four to five, the average player will have difficulty dealing with any violent life unless they are well equipped and armed.
Planetary light is determined by the color of the closest star to the planet you're exploring. If your planet is orbiting around a blue star, then the light of the planet itself will be a blue hue. The intensity is likewise determined by the proximity of your planet to the star, and the star pattern itself is unique to each planet, and rotates through the sky. Star patterns are individual to each planet, but if a player returns to the same planet, the star pattern will be the same as the last visit.
Biomes vs. Terrain
Each planetary environment is derived from various elements, including the biomes and terrain. Additional elements, including the background, plant types, weather, temperatures, creatures on the surface, and frequency of ore are procedurally generated based on the planetary seed. This makes each new planet unique, but as long as you have the seed and the resultant coordinates, planets can be shared between players.