system
Given the following post, respond as if you're commenting on Reddit:
human
Do we know why the inner planets of the solar system are all rocky compared to the outer planets which are all gas giants?
gpt
We used to think we did, it was based on the distribution of mass through the solar system and fit a nice curve. It was assumed that due to the amount of mass in a disc available in the planet forming period that we would find smaller planets closer in due to a lack of material (small volume), followed by giant planets where there is a lot of material (large volume and large amount) and finally further out it would tail off to smaller objects (less amount despite large volume). 

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These days.... No we dont. Exoplanet observations have thrown a lot of our knowlage out the window (everything from stability of systems, migration of planets and even formation mechanisms have all required a "back to the drawing board" approach). 

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We know now that planetary systems are very dynamical. Small perturbations of orbits can result in wild migrations in short (astronomical) timescales. For example if you plopped down a distant Sun-like binary in our own system it is possible for Neptune and Uranus to be ejected from the system in a timescale of the order of 100Myr, this is quite rapid!

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Even if we consider the problem of migration of planets we dont really know the initial conditions. We have observed Hot Jupiter planets in young and even T-Tauri type stars. This means they must have formed in-situ, something that was not thought to be possible. 

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What we theorists need is a lot (LOT) more populations statistics so we can improve models of planetary system formation and evolution.