I am looking for strange and interesting planetary circumstances for basing a board game around. This can involve any configuration of planetary bodies and their environments, but must be at least a theoretically possibility in general terms. Perhaps it would be best to illustrate with some examples: A planet with an atmosphere gravitationally locked to its sun. One side freezing cold the other side boiling hot with a moderate twilight zone in between, perhaps with Volcanism on the dark side that stops the atmosphere from completely freezing (I saw the recent thread on this). 2 planets in very close proximity orbiting around each other with tidal wave tides. Large Asteroid impact onto a planet excavating a huge and very deep crater, (how deep could it be for a roughly earth sized planet?) providing the potential for unusual conditions (flooding/exploration). I want to keep things broadly realistic but would accept some wriggle room for a really interesting ideas!