I know gravity well and orbital speed are affected by other factors besides planetary mass, but I am going to keep it simple.
I think another rule I will make is that if a space object has less than a 2" radius it has no gravity well, to simplify identifying and using asteroid fields... (I'll need rules for them too I suspect)
Planets have a gravity well equal to their radius. It pulls ships drift markers towards the planet.
OTHER PLANET RULES
Satellite bases obviously orbit planets like an orbiting ship does.
Planets block line of sight.
If a ship's move passes through a planet, is it assumed to have passed over the planet. Move the drift marker back towards the planet. If this causes the ship's move to end inside a planet, see the rules below.
If a ship's move ends inside a planet, then (a) if it's velocity is orbital speed or less it can enter mandatory orbit on the side it entered the gravity well OR (b) if it is travelling faster than orbital speed it will crash and explode