Re: Hold on a bit...
It's complicated. I'm not an expert but I think the idea is that the gas giant swallows material that is closer to the star than itself, but is pulled closer to the star as it does this. Or, to look at it another way, you start with a gas giant a long way from the star, and a bunch of mass in the planetary disc, and you end up with a fat gas giant containing the planetary disc mass. It stops, mostly, when the star lights up, generates a solar wind, and blows the leftover gas out of the star system. The solar wind itself of course has outwards momentum.
I have previously heard arguments based on claimed computer simulations that this is how planet systems are often built, and that in our system, the gas giants interfered with each other and left the inner system for rocky planets.
It is reasonable to assume that the Solar System is a typical system of star and planets, but it is also reasonable to assume that some star systems have no planets where life can exist, and some star systems have habitable planets, and the Solar System is only typical of the second kind.
There's also an argument that the Earth has an unusually big fat Moon and that this has helped us in some ways. There was a discarded theory that the Moon removes some of Earth's atmosphere and without it, the heatland pressure on Earth's surface would be nearly as much as on Venus. Wrong, apparently, but that's the sort of thing. Current fashionable theory is that Earth version 1.0 formed alongside a Mars-sized Trojan orbital companion which then got unstuck and hit the bigger world, and we're living on Earth 1.1 which contains material from both, especially the iron core - I'm not inventing this but I'm interpreting it creatively - and the Moon is made of ejected crust material, but not right away. One thing we get from that may be t!a particularly good protective magnetic field. Apparently in more simulations all of this is quite a common event too, so there may be more rocky planets with big moons around other stars where we could look for life.
For five-sixths of Earth's existence so far, there was basically nothing more sophisticated than slime living here, so we may be disappointed by what we find.