Has a big bug in it. When 2 planets collide, it's like one is made out of matter and the other antimatter. They shoot apart at fantastic speed. The opposite of an inelastic/plastic collision in other words, they get an enormous amount of kinetic energy from nowhere.
It is also hard to get it to stay selected with the parameter I want to change. I try to change the velocity vector of my planet for instance and I click on the second square on the left but no, it keeps changing back to other things. Still, it's good to see a hard physics simulator of some sort here, but it definitely should be improved, its problems fixed.