But the whole thing is tied up with things like the Great Filter and "where are all the aliens" and other questions that bother me. So I keeping poking around and finding things like the Grand Tack, which is a simply fascinating theory about the origin of the Solar System where a resonance between Jupiter and Saturn kept Jupiter from spiraling in and becoming one of those "hot Jupiters" that we see so frequently in other planetary systems. And once a Jupiter-sized object has crashed its way into a close orbit, the chances of putting together an Earth-like planet fall abruptly, I suspect.
All of this leaves open the question of why the terrestrial planets of the system are, well, terrestrial. There should be plenty of hydrogen-rich material to accrete to make one of those "super-Earths" that we see in other planetary systems. Where did all of the hydrogen and all of the extra mass go?
And it turns out that there's a theory for that.
According to this article, the Grand Tack would have bombarded any super-Earths that had formed in the system with planetesimals, destroyed them, and driven most of the mass into the Sun. The terrestrial planets formed out of the hydrogen-depleted leftovers.
Well, that would explain a lot.
We are apparently a very rare Earth.
The Grand Tack: is there anything it can't do? :)