When working with an ArcGIS Pro 2.3 Global Scene the horizontal coordinate system cannot be changed. I found out that the horizontal coordinate system (CRS) "must always be WGS 1984 Web Mercator [I think 3857]". It is possible to change coordinate systems for maps and local scenes. Why is it not possible to change the CRS for global scenes? I am exploring options to change a Global Scene so that I might map with a different ellipsoid for example other solar system bodies.
The help document should say that GCS WGS 1984 is the only CS option for global scenes. It looks like this is an error in our 2.3 help, but this will be fixed in the update to 2.4.
As for support for other planets, extraterrestrial datums are supported for maps and local scenes, but not for global scenes. We are investigating the potential support of other planets in global scenes in a future release.