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Planetary system formation is a very complex beast and whilst we may know the basics to a certain degree, the nitty gritty is still a long way from resolution. There is still a lot to learn, even when including all the planetary systems we've found over the last 15-20 years. Quite frankly, we still don't know enough to really say what's your average planetary system and how they form, simply because we just don't have enough examples to confidently say anything about them. It's most a matter of "best guesstimates" so far as the theory goes. However, we can rule out some processes simply because of their physical improbability. In other words, given what we do know, the Nebula (Accretion Disk) Theory is the best we have to explain how the process happens...plus it fits the data we do have.
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