Planets Are effected by each others gravitationaly.
This should be part of the Flight Menu, Under infinite fuel.
And depending on how clost planets or moons are to its perant body, Tidal forces are different.
If planets get to close it cause the crust to rise into the air, The Roche limit would play a role in this of the smaller body is orbiting just to close to resist the gravitational forces from its preant planet.
If planets, Astroids, Moons, Stars collide they create debri some flying into space some falling back to the planet.
If this happens the moon or planet can by ripped apart and turn into a ring of mini astroid like moons, As shown in the gif below.
Roche limit
I dont know if the devs will see this but I'd like to see somthing like this in Juno.