With numerous attempts to get this collision working I can only get it to partially work.
Starting by adding an event listener on the planet creation for a collision. Within the collision function, create a new mass by adding both planets masses and removing the smaller planets phys and visual objects from the scene and world.
Being able to get this to work has been a gruelling process. I finally managed to figure out the paired objects by using the indexOf the e.with (collision object) and then using that variable inside the array [ ] brackets to determine which three.js object to remove with the phys object.