I don't think that PhantomWolf is arguing the existence of the planets themselves. After all, they are most demonstrably there - we've even sent spacecraft past them! What is in question is whether their existence can be explained by the standard model of planetary formation in our solar system.

I haven't seen the studies quoted, but I strongly suspect that the early solar system was a most complex affair that is unlikely to lend itself to a simple model. Just off the top of my head: Could the presence of the proto-Jupiter/Saturn have caused a resonance leading to increased density of the nebula at the appropriate radii, allowing quicker planetary formation? Could the high solar wind of the newborn sun have caused a 'snowplough' effect in the nebula 'boonies'? Did the nebular material at that radius particularly tend to generating electrostatic charge, facilitating accretion? Could the sun have had a stellar neighbour during the formative period of the solar system?

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