Sphere type planets?

Hello, i was wondering if anyone could lead me in a direction to make a sphere type planet.

what I mean is, for example, the game Empyrion - Galactic Survival on Steam

I would like to be able to have a world on a sphere type shape visible in space, and when a player fly down to that planet they can enter the world and play on the terrain.

how would one make this happen?

What have you tried so far?