Custom planet/biome types are still frontier territory. There's no running water, and you might get eaten by a Grue. This does not refer to the custom CSG and metal placements recently added to the game. Examples of custom biome types: - Devastated Metal Biome - Junkyard Biome - Water Cliff Planets Challenge #1: you need two mods A client mod enables previewing the planet in the system editor, and may make it possible at all. It may also provide planet icons for use in the system editor. A server mod allows you to play games on the planet. It may provide planet icons for use in the lobby and game. Players who join the game don't need any mods installed, but you have to be very careful about when you select the system (see #3) Challenge #2: mod designer: pick your poison A mod can override or shadow an existing biome type. This dodges some of the challenges of custom planet types, but affects all previously created systems, and the new and old versions can't be used together. Having a client mod is convenient for previewing planets, but not required. A mod can provide a new biome type. This allows the planet type to be used with any other planet type, but it's a lot easier to crash things and you have to provide your own icons (unless you like empty white frames) It is absolutely essential for the host to have a client mod to make the new type available in the system editor. Challenge #3: lobby creation requires great care You are strongly advised to not select a system with custom biome types until all players are in the lobby and have downloaded mods. This goes double for custom biomes, but even a shadow wouldn't want clients generating with the wrong data. If this caution isn't observed, it's a race condition between downloading server mods and generating planets; many players will appear to be taking a long time generating, and then time out of the lobby.