There is no evidence that one can do statistical mechanics of the sort Tremaine describes here. For one, there are no super Earths inside of Jupiter which likely indicates that they were lost by Type I migration in a relatively dense nebula. Second, the terrestrial planets are not packed. Third, the same argument could (but shouldn't) be applied to the moons of Jupiter and Saturn independently of the planets. But once again the moons are not packed. The clear message is that gas is playing a key role in shaping planetary systems.
