It's called the Nice Model: https://en.wikipedia.org/wiki/Nice_model https://en.wikipedia.org/wiki/Nice_model Edit: Basically, the gas giants started really c

It's called the Nice Model: https://en.wikipedia.org/wiki/Nice_model https://en.wikipedia.org/wiki/Nice_model Edit: Basically, the gas giants started really close into the sun but later moved outwards. We see a lot of other systems with gas giants close into their suns right now. In the Nice model, the larger gas giants had their orbits move slowly outwards, causing havoc in our system. This is possibly when the Moon was formed. It's still a lively debate, as we have not yet found Planet X yet. TLDR: Fund NASA more.