It started as a University project, to create a Planetary System with OpenGL. I now use it as a learning tool, for c++ and OpenGL. If you want to use any part of this project, feel free. If you have questions, feel free to ask, and if I can I will answer.
I use a custom deferred lighting system (just point lights) as well as O'Neil Accurate Atmospheric Scattering Shading. I also render transparent objects in another pass, witch is very simple. The lighting and shading is fully in HDR. It is physically correct.
For the GUI I made a simple GUI system using NanoVG graphics, with basic functionality for text input, sliders and buttons.