Canon.js example is helpful to an extent, however also very confusing:
From this code I managed to replicate my own version:
This was very confusing to figure out because the examples position was at 0,0,0 within the physics world. So my attempt worked fine until I tried to move or scale the planets and moons. It was also very important having set the correct mass radius and position for the moons physics object.
Once I realised this I was able to figure out a fix, by finding the distance between the moons and planets in the moonsPlanet_to_planetsMoon variable above. Then applying this distance to the moons force (I have also previously set the moons velocity on creation, which indicates the speed at which to travel). I also had to set the force value to 60 instead of figuring out the power of values on the fly.