Sorry for the late response, but I believe you would indeed need to use World Composition and level streaming in the same way DarkS474 does; I haven’t played around with planet systems yet.
My game will go in the direction of a seamless open world with no loading screens between levels and no fast travel etcetera. In the style of Elder Scrolls, although it’ll go multiplayer. I’'ll be doing this by using World Composition and level streaming in order to load out and in unseen levels and assets.
You’ll have to stream the levels in and out using LODs, that way you can have a good amount of assets implemented in your game levels. 
I’ve been working a lot on my world map in world machine the last week, trying to create a map that I could use as a finalized result.