Unless you are the wizard of C++ already, start with something achievable I suggest, join the modding community, and if you have the capability, pioneer new functionality in the direction you want to go. Maybe begin by importing an asset, learn to configure and use the script interface, and go from there. There is no SDK yet, the API isn't completely exposed, and additional planets aren't possible yet without extensive understanding of GIS tools and specialized software for generating the world data, and Outerra is not at the stage where full fledged game development is even possible.
Regards,
Uriah