KentuckyFC writes "Astronomers have discovered some 250 planetary systems beyond our own, many of them with curious properties. In particular, our theories of planet formation are challenged by 'hot Jupiters,' gas giants that orbit close to their parent stars. Current thinking is that gas giants can only form far away from stars because gas and dust simply gets blown away from the inner regions. Now astronomers have used computer simulations of the way planetary systems form to understand what is going on (abstract). It looks as if gas giants often form a long way from stars and then migrate inwards. That has implications for us: a migrating gas giant sweeps away all in its path, including rocky planets in the habitable zone. And that means that solar systems like ours are likely to be rare."