I don't have have the game running right now, but I believe it works as follows:
-In galaxy view, hold "x" and right click some empty area
-Left clicking, choose the first option (galaxy view, I think), and then alternate view (or something like that. Just poke around the menu)
-Now exit the menu, and while holding "control", left click and drag planets
If this does not work, let me know and I'll start up the game and figure it out.