It's an utterly complex interaction of gravity and force.<br /><br />The Solar Wind might be of sufficient force to push individual particles or combinations of particles outward. But when enough particles are grouped together to provide enough mass to find a "balanced position" relative to gravity, they do so.<br /><br />In the early Solar System, It was a relatively random and chaotic process.<br /><br />But the Universe likes Order. It prefers balance. What we observe today is the <b>result</b> of that inclination towards order.<br /><br />The big problem is the time scale involved in how that order is determined. If we had the luxury of observing the next new Sun-like star acting on the same or comparable level as our Sun and solar system did in our ancient past, the answers would be obvious.<br /><br />Unfortunately, we don't live half a billion years. And what makes it worse, is that each unique solar system can be subject to forces we can't quanitfy.<br /><br />IOW, we have no clue as to how many Sol mass (or greater or even lesser) stars had gravitational effects on how our own Solar System came to be. Random "passers by" may define our existence.<br /><br />We only know how things are now. And that's the mechanism by which we formulate our theories. Unfortunately, we can't know the unique circumstances that caused our Solar System to manifest itself as we observe it.<br /><br />All we really can do is describe how our Sun influences and dictates the end result that we see today forward.<br /><br />That's a pretty unsatisfactory answer, but it's the best we can do. There are an infinite number of interactions that caused you and I to exist. <div class="Discussion_UserSignature"> <em>"2012.. Year of the Dragon!! Get on the Dragon Wagon!".</em> </div>