Re: Are single climate/habitat worlds really possible?
Well, if you take an iceball planet (something like a supersized Europa) and have it migrate inwards (or possibly undergo a merger with a rockball further in), you'll wind up with a planetary ocean some hundred miles deep, with most minerals locked down below a layer of some high pressure/high temperature allotrope of ice, and stellar UV breaking down water should cause a gradual loss of hydrogen into space and a gradual buildup of oxygen. You won't have any land, and you might well wind up with some really nasty permanent storms, since there's no land masses to break up hurricanes, but you might be able to manage a breathable atmosphere and a pleasant enough ocean. You probably also won't have any life; while there's water, most of the other necessary minerals are locked down below the ice.