Ring/Sphere world that you create automatically have your atmosphere type. If you are going to build it I guess you make it like home . It also automatically has your world type also so if you start on a oxygen/rock world your ringworlds will be oxygen/rock also.
hummmm,.. i thought ringworlds where always rock. I built ringworlds and they always where rock even though my starting world was methane/gas,.. i did have all three planet types researched, i guess i will have to try it out.
In my Last game, all my ringworlds started as co2/gas, just like my HW, even though I had all planet colonization techs (who would build RWs without getting those first? )