Real Scale 1:1 Planet

Hi everyone!
I have a very specific question: how can I create realistic scale planets? Or where I can start to learn how to do it? Some programs, like space engine, have a procedurally generated planets system, and they look great with their continents, oceans, mountain chains and atmosphere! They use several optimization techniques and compromises, but eventually they achieve a wonderful result without a massive performance hit. I have a basic understanding of UE 4 materials/shaders, 3d modelling and landscape creation, but I have no idea of what kind of object should I use to create real-scale planets. I expect that creating such an advanced thing would be hard.
Thanks for reading!

Usually developers spend years building an engine specifically for that purpose
Also, what is the value of that?