10 months ago
Last edited at 10:01AM on 2/10/2013
Overall Neptune is the coldest. But the side of Mercury that is turned away from the sun gets even colder as I understand. It has no atmosphere to retain heat and its rotation is so slow it has a long time to cool off. Mercury's day is longer than its year.
Well, if we're only talking about planets in the Sol system, then yes, Neptune is the coldest, since Pluto lost its status as a planet. I don't doubt for a second there are colder planets out there in other solar systems, though.