Algodoo

I enjoyed watching the particles become part of planets and planets would sometimes crash together and create bigger planets. Caught myself smiling watching this lol

As noted.

Particles in purple represent particles is the gaseous phase, these particles almost never attract with each other due to their low mass and their low density.

Particles in light blue represent particles in the liquid phase.

Particles in grey represent particles in the solid phase, this is the phase where planets tend to form due to their high densities and masses.

The planets are spherical
The planets are usually bigger than the star

yo this is really cool