One theory that I know of, has very small particles and, gas left over after our Sun formed, bumping into each other and settling in the middle of the Solar System's dusty and gaseous disk, they then became lumpy or gravitationally bound and grew over time.
This resulted in utter chaos, as they grew, into rubble piles, they accrued even more matter, and then eventually started perturbing each others Orbits as they were growing, and reaching Planetary size, Gravity became a big factor in shaping what we now know as our System,
smashing themselves into bits or sending other large and small objects toward the sun or ejecting them out of the early Solar System.
The evidence the theory uses as support is all of the impact cratering that has been found on all the Rocky Planets and their Moons, from all of the probes that have been sent to study and photograph the Planets.
The fact is, that we really don't know for sure how many mechanisms were involved in the formation of our System, but I would assume it was not attributed to one basic mechanism.
We could have even captured some objects or moons as the planets were forming, also take into account that we really don't know what happened in the past, when the Planets were forming, such as Galactic Orbital Oscillations or, other Galactic debris, Gas, and the Gravitational effects of Passing Stars as we revolved around the Galaxy and evolved into what we see today.
Dennis
