Planets in Minecraft would be Mod that allows the player to venture off into space to go to other planets.
Different Planets would contain different mobs, different npcs, and even different materials.

1. To get another planet, the player would have to craft either a portal, using materials like netherrack, glowstone, obsidian etc. Or the player would craft a rocket with given dimensions. The rocket would be extremely difficult to build, not because of size, but because of items needed. Items like Diamond blocks, Obsidian, etc.

2. There would be 1 main planet that is easily accessible. Through portal of course. Whereas the other planets required rockets.

3. The rocket flight would be most likely simulated. For example, you sit in the chair, and after a screen load, you would appear on another planet.

4. Mobs on some planets would be very alien like, whereas other planets, the mobs will be very human like.

5. And finally there will be a planet that is only accessible by going through all the other planets, collecting the proper materials, crafting the right recipes, where you will then fight a boss.