Orbital resonance. I'm working on a simulation of this, where the planets and orbits move. A work-in-progress can be seen on my website: orbit resonance
These are the pieces for a pending animation showing the modelled results of the collision of planetesimals. What may happen to atmospheres and oceans on such planetesimals may be revealed by computer modeling of impacts, by modelling of planetary orbits, and by observations of gas content in the atmospheres of Venus, Earth and Mars.
A illustration of the early formation stage after the sun as coalesced and started clearing out the inner solar system of volatile elements while large planets beyond the snow line accrete quickly.
As with all my presentations, this is a discovery process. I reserve the right to make errors, but will correct all that are called to my attention or that I discover long after the presentation.