The map creation wizard should have a "planet" combo box where the user can choose the planet that the map is being used for.

The combobox would most likely fit onto the same page that is used for entering the name, preview and description.

It should be located below the name and preview block. It should look about like this:

Planet: [Earth]

The default should be "Earth"

Apart from earth the other allowed values that are used in marble/src/lib/Planet.cpp should get provided.

The planet that got selected should get written into the dgml file.