First even though they said it wasn't likely, U can't figure why a rogue planet couldn't be covered in dry ice considering this happens on a much warmer mars. Assuming it was ejected from an early solar system. It would still has it water that wasn't evaporated in the late heavy bombardment. Through out gassing a lot of carbon dioxide gets released ie venus, or even earth. Now because of it being so cold all of the carbon would dioxide come back on the planet as snow or ice. Creating a planet covered in dry ice. Lets say a star adds it to their solar system. This best explains outlying planets in other solar systems that are to far out to of formed there. Now because of it not being used to being so warm would the dry ice evaporate causing it to be an atnosphere. If it did have a thick carbon dioxide atnosphere thicker than venus because of a lack of exosphere that vents carbon dioxide like earth. I don't know, but I imagine Venus has the same thing. It would be a lot warmer than it should be if this happens. What are your opinions.