The following rules are designed to model the use of planets within a Full Thrust game, both as an obstacle to navigation, and also for scenarios where the planet itself is of importance. There are rules in More Thrust (p. 13) for handling planets, but they are very simplistic and don't fit in well with the vector movement system introduced in the Fleet Book.
All these rules assume vectored movement is being used. If you're using cinematic movement, then these rules won't make much sense.
Units of Measurement
Normally, Full Thrust doesn't care about real world measurements, and simply uses the 'turn' and the 'movement unit' (depicted as " in the rules, and often taken as an inch on the table top). If we start using real world objects though (such as planets), it becomes necessary to translate these abstract terms into real world units.
A turn is taken to be fifteen minutes, and 1" is assumed to represent one thousand kilometres. From this, we can work out that a ship thrust factor of 1 is equivalent to an acceleration of approximately 1.125 ms-2. To keep things simple, I'll assume that 1g is equal to a thrust factor of 8.
Unless you have a very big table, you might want to consider using 1" = 1cm on the table top. This provides a lot more room for manouevre around the planets, and also makes modelling planets easier.
Modelling Planets
The easiest way to model a planet is to cut a circle out of a piece of cloth or paper, and simply place it on the table top. This also makes it very easy to place planetary installations on the planet, since they can be drawn/placed ontop of the planet.
Another easy way is to buy a polystyrene sphere from a craft shop, cut it in half to form a hemisphere and paint it to look planet-like. The largest spheres I've found though are only 10cm in diameter, so this works best for either dwarf planets or if you're using centimetres. See the picture above for an example.
