I just researched blue science and have started looping through the vanilla planets, is it intentional that they are the same every time? Can you build on them, at the cost of having to defend from the residents or will it just get deleted like the starting random planets?

Last planet that you jumped on will always stay loaded so if for example you jumped to Fulgora last time and you will try to jump there again you will be back. This was intended to prevent you from just jumping between 2 planets, but can be abused in late game to gather more resources faster.